Trafficability of Snow

T47 nQ. 3- 14 Report no. 1 1W?

no3t -

na. CORPS OF ENGINEERS. U. S. ARMY AMC

TRAFFICABILITY OF SNOW

REPORT NO. 1

VEHICLES IN SNOW: A CRITICAL REVIEW OF THE STATE OF THE ART

10 100 l 101I

TECHNICAL MEMORANDUM NO. 3-414

A CORPS OF ENGINEERS RESEARCH AND DEVELOPMENT REPORT

PREPARED BY C. J. NUTTALL, JR., AND J. P. FINELLI STEVENS INSTITUTE OF TECHNOLOGY HOBOKEN, NEW JERSEY

FOR

OFFICE OF THE CHIEF OF ENGINEERS

AND

WATERWAYS EXPERIMENT STATION VICKSBURG, MISSISSIPPI

ARMY-MRC VICKSBURG. MISS.

AUGUST 1955 COPY

CORPS OF ENGINERS, U. S. ARMY OFFICE OF THE DIRECTOR WATERWAYS EXPER IENT STATIODN VICKSBURG, MISSISSIPPI

WESDR 8 September 1955

SUBJECT: Submission of Report No. 1, "Vehicles in Snow: A Critical Review of the State of the Art," and Report No. 2, "Greenland Studies, 1954," of the T. M. 3-414, "Trafficability of Snow," series.

TO: The Chief of Engineers (ENGNB) Department of the Army Washington 25, D. C.

1. One copy each of Report No. 1, "Vehicles in Snow: A Critical Review of the State of the Art," and Report No. 2, "Greenland Studies, 1954," of the T. M. 3-414, "Trafficability of Snow," series is forwarded herewith for your files. It is planned to distribute these publications in accordance with List A submitted with our letter dated 9 April 1954, subject: "Distribution Lists for Waterways Experiment Station Publica- tions on Trafficability, Landing Mat, and Environmental Studies," and additions thereto that have been approved by your office.

2. Distribution will be accomplished as soon as approval is received from your office.

/5/ 2 Incls A. P. ROLLINS, JR. 1. T. M. 3-414; Report No. 1 Colonel, Corps of Engineers 2. T. M. 3-414; Report No. 2 Director COP!

ENGNB (wEDR 8 Sep 55) 1st Ind SUBJECT: Submission of Report No. 1, "Vehicles in Snow: A Critical Review of the State of the Art," and Report No. 2, "Greenland Studies, 1954," of the T. M. 3-414, "Trafficability of Snow," series.

DA, OCofEngrs, Washington 25, D. C., 15 September 1955

TO: Director, Waterways Experiment Station, P. 0. Box 631, Halls Ferry Road, Vicksburg, Mississippi

Permission is granted to distribute Report No. 1, "Vehicles in Snow: A Critical Review of the State of the Art," and Report No. 2, "Greenland Studies, 1954," of the T. M. 3-414, "Trafficability of Snow," series in accordance with approved List A.

BY COvAND OF LIEUTENANT GENERAL STURGIS:

/s/ 2 Incl R. R. PHILIPPE w/d Chief, Special Engineering Branch Engineer Research and Development Division

2 LIST A - DISTRIBUTDN LIST FOR

TECHNICAL MERANDA ON TRAFFICABILITY STUDIES

Corps of Engineers

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General Staff, U. S. Amy

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Naval Civil Engr Labs 1 Chief, Bur of Yards & Docks, 1 Office of Naval Research 1 Navy Dept, Washington, D. C. Navy Dept, Washington, D. C. Naval Amphibious Base, Coronado, 1 Attn: Amphibious Branch Calif. Room T-3-2703 USN Ordnance Test Station, 1 030, Naval Photographic Inter- 1 Inyokern, California pretation Center, Naval Re- Office of Naval Res, Chicago Br 1 ceiving Sta, Washington, D. C. c/o Northwestern Univ., Attn: Librarian Attn: Dr. Kohn Special AF Staff College, Librarian 1 U. S. Mil Acad, Engr Detach 1i SCEL, Tech Rpts Library 1 Chf Signal Off, Engr & Tech Ser 1 Trans Res & Devel Command, 1 Prof. Parker D. Trask, U of Calif i Fort Eustis, Virginia The Res & Dev Board Armed Services Tech Infor Agency 1 Comm of Geophysics and Geog- 1 Document Service Center raphy, Attn: Comma. Secretariat U. B. Bldg, Dayton, Ohio Ch of Ordnance, Dept of Army The Res & Dev Board 2 The Pentagon, Washington, D. C. 1 Conn on Ordnance Attn: Research & Dev Div CRDTT Panel on Vehicle obility Operations Research Office Research, Members of Chevy Chase, Maryland Working Group E LIST A (Continued

Special (Continued)

The Chief Signal Officer 1 Commanding Gen, Aberdeen Proving Attn: SIGGE-M-3 Grounds, Aberdeen, Maryland Radar & Meteorological Attn: Automotive Div Br, Washington 25, D. C. Commanding General, Dev & Proof 1 Chief, Office of Transport 5 Serv, Ord Corps, Aberdeen Prvg Washington, D. C. Grounds, Aberdeen, Md., Attn: Director, California Forest and 1 Chief, Library & Museum Branch Range Experiment Station, Commanding Officer, Detroit Arsenal 1 P. O. Box 245, Berkeley 1, Cal., (ORDMX-EA), Ordnance Corps, Attn: Jack R. Fisher, Physical Center Line, Michigan, Attn: Scientist Land Locomotion Laboratory U. S. Geological Survey, Military 1 Hqs., US Army Forces, Far East and 1 Geology Branch, Room 4225, 8th US Army, Office of the Engr, GSA Bldg., Washington 25, D. C. APO 343, c/o PM, San Francisco, Commander, 517th Engineer Detach- 1 Cal., Attn: Control No. IB-3 ment (Terrain), Fourth Army Commandant, Command and General 1 Headquarters, Fort San Houston, Staff College, Fort Leavenworth, Texas Kansas, Attn: Archives

U. S. Air Force

Hqs, USAF, AC/S Installations 2 Cdr, Continental Air Command, 1 Director of Facilities Support Nitchel AF Base, New York (AFCIE-AE), Aviation Engr Div Attn: Air Installations Off Washington 25, D. C. Cdr, Air Training Command 1 Hqs, USAF, DC/S Operations 1 Scott AF Base, Illinois Director of Operations Cdr, MATS, Andrews AF Base 1 Operations & Commitments Washington 25, D. C. Division (AFOOP-OC-S) Attn: Air Installations Off Washington 25, D. C. Cdr, Maxwell AF Base, Ala. 1 Hqs, USAF, DC/S Devel 1 Attn: A-2 Library AFDRD - Equip Div Cdr, Maxwell AF Base, Ala. 1 Washington 25, D. C. Attn: Research Section Hqs, USAF, DC/S Devel 1 Hqs. Aviation Engr Force 2 AFDRQ - Connand Support Div Wolters AF Base, Texas Washington 25, D. C. Attn: Deputy for Operations Cdr, Air Res and Dev Command 1 Cdr, Air Prvg Gr Command 1 P. O. Box 1395, Baltimore 3, 11i. Eglin Air Force Base, Florida Attn: RDTDE, Equipment Division Attn: DCS/Operations, Deputy Cdr, Wright Air Devel Center 1 Director of Test Re- Wright-Patterson AF Base, Ohio quirements (DC S/O-TR) Attn: WCXN, Engr Standards Div Cdr, Air Force Operational Test 1 Cdr, Wright Air Devel Center 1 Center, Eglin Air Force Base, Wright-Patterson AF Base, Ohio Florida, Attn: Support Services Attn: WCLS4-5, Aircraft Lab Division Cdr, Continental Air Command, 1 Installations Engr School Mitchel AF Base, New York, USAFET, Wright-Patterson Attn: Aviation Engrs Air Force Base, Ohio

2 LIST A (Continued

Consultants

Prof. D. W. Taylor S Mr. Robert Horonjeff I. Mass. Inst of Tech Lecturer Research Engr Dept of Civil & Sanitary Engr Institute of Transp & Traf Ehgr Cambridge, ass. University of California Dr. A. A. Warlam S Berkeley, California New York University Prof. K. B. Woods College of Engineering Dept. of Civil Engineering University Heights Purdue University New York 53, New York West Lafayette, Indiana Mr. C. J. Nuttall Wilson, Nuttall and Raimond Engineers Chestertown, Maryland

3 Contents Page Objective ...... 1

Introduction " " " ...... " " .. 1

Trafficability and Mobility ...... " " " 2 . . 5 Snow " " and " Snow " Metamorphism ...... " • 5 Characteristics and Properties of Snow ...... S9 Through the Looking Glass: A Table ...... S. 12

Peculiarities of Snow ...... 19 Snow Terrains ...... 24

Vehicles in Snow: The Several Problems ...... S 29 Vehicle Performance as Related to Snow Conditions:

the General State of the Art ...... " " " " . " 35 The State " " of the Art in Some Detail ...... " " " " " 37 Existing Self-propelled Snow Vehicles ...... " . " " " 4o Sleds and Unpowered Trailers ...... " " " " " " 48 Mobility Testing of Vehicles in " " " " " Snow ...... " 49 An Approach to Trafficability Prediction in Snow Terrains . .. 53 Some Assorted Projects Needed ...... 54 "Where Do We Go from Here?"...... 55 Selected References ...... 57 TRAFFICABILITY OF SNOW

VEHICLES IN SNOW: A CRITICAL REVIEW OF THE STATE OF THE ART

Objective

The objectives of this report are to present a critical summary of the state of the art of designing vehicles for travel on snow, and of operating, and predicting the performance of vehicles in snow, and to make recommendations for improving the situation where such appear in order.

Introduction

Early in 1954, the Waterways Experiment Station (WES), Corps of Engineers, Vicksburg, Miss., was charged with the task of developing means for predicting the trafficability of snow-covered terrain in a fashion similar in operational concept to the prediction methods developed and under development by that facility for soil terrains. As part of their approach to this problem, WES contracted with Stevens Institute of Tech- nology (Experimental Towing Tank, Motor Vehicle Research Division -- ETT), Hoboken, N. J., to review published material on over-snow vehicle performance, and from the literature and the ETT off-road vehicle background to: 1) summarize the state of the art of relating snow conditions to vehicle performance 2) conclude as to the suitability of existing vehicles for over-snow use 3) recommend new approaches in the design of over-snow vehicles 4) recommend revisions or additions to present procedures and techniques for tests of vehicle performance in snow. This report purports to do these things. The study, as intended, has been entirely a desk operation; i.e., no new laboratory or field work has been done in connection with it. Ma- terial from all branches of the Armed Services and from numerous civilian sources, dealing both with specific snow-vehicle work and the current state of knowledge of snow characteristics or properties, was reviewed. 2

A selected bibliography of pertinent material will be found at the end of this report. Personal discussions with numerous individuals, in particular at the Snow, Ice, and Permafrost Research Establishment (SIPRE), Corps of Engineers, accounted for some of the information and many of the ideas discussed herein. In a condensed review of this sort it is difficult to be scrupulous about the ancestry of ideas and concepts, particularly when presented almost entirely in interpretive combinations. The informed reader will undoubtedly find some credits which are questionable, and more often, a questionable lack of credits. During the months of this review the writers were constantly impress- ed by the scale of the effort which has gone into study of the snow-vehicle problem over the past ten years or so, mostly without commensurate results. It is hoped that the present modest effort, perhaps by doing no more than clarifying the reasons for some past failures, may prove useful in the planning of future work. In order to insure accurate communication with the reader of this report for the short time he will spend with it, the authors feel it necessary to discuss, in the snow-vehicle context where possible, several concepts which appear to them to be central. This will be undertaken, as briefly as possible, before proceeding to the snow-vehicle problem per se. In the process certain widely used terms will be defined for present purposes and some of the philosophy of vehicle development objectives will be touched upon.

Trafficability and Mobility

Since early in World War II, considerable effort, by and under the aegis of the military, has gone into the study of terrain trafficability and vehicle mobility. Obviously both have to do with vehicle-terrain interaction and hence are closely related. To date, however, they have been treated in isolation; trafficability primarily as an operational concept, mobility as a vehicle engineering problem. As a result there is far less correspondence between the work on the two than might, and perhaps should, be expected. Most generally and usefully defined, "trafficability" is the ability of a terrain to "sustain the motion of a vehicle" (198)*. Usage has tended to restrict the meaning of the term to the ability of the soil (or snow) of a terrain to sustain the motion of a vehicle for a large nun- ber of passages over the same path. The authors prefer the more general definition, and will use specific, self-explanatory terminology (such as "snow first-pass trafficability") where more restricted meanings are intended. Trafficability is fundamentally a "yes-or-no" proposition in relation to each specific vehicle. It might be quantified for a given terrain by specifying the least mobile vehicle from an empirically ranked list of existing vehicles for which that terrain is still trafficable. In general, as might be expected, the ranking of vehicles which form the scale must have been carried out in similar terrain (199). In principle this has been the approach of the Corps of Engineers to the trafficability prediction problem, and as an operational tool (all it was ever intended to be) the procedures and supporting data implementing it appear to have been satisfactory for the prediction by contact means of the multi-pass trafficability of most natural soils. Such completely empirical quantification, however, offers but little hope, in the present state of development of vehicle-terrain mechanics, of coming to grips with the component behaviors which separately may be influenced by design, and in the aggregate account for the observable performance of the vehicle in the field. The "mobility" of a vehicle in a given terrain is the "ease" with which it moves in that terrain. As was the case with trafficability, usage has tended to limit the scope of the meaning, in this instance to the ease of motion of the vehicle only on the first pass through a virgin terrain. Again the more general meaning is to be preferred. Particu- larizations of meaning can be readily made as needed. Unlike its companion concept, however, the definition of mobility invites (although it does not detail the techniques of) quantification. In many important respects the philosophically most satisfying proposal for

* Numbers in parentheses refer to the references at the end of this report. quantification, in accord with the definition just given, is that by Bekker, who has suggested that the average point-to-point operational speed of a vehicle in a given terrain be taken as the mobility of that vehicle in that terrain (200). A simple trial of this method gives encouraging results. For example, it is apparent that a "jeep" will have greater mobility than a "Weasel" on the highway, and conversely, the "Weasel" will be much the more mobile in particularly soft off-road terrain. Because such clearly demonstrable conclusions are so completely in accord with intuitive reactions to the term mobility, the proposal is especially gratifying. Moreover, in that it admits of ready measurement in "any given terrain," and provides a framework for the comparison of widely different vehicles, it is eminently satisfactory. It fails as a practical research and development tool at this moment, none-the-less, for the same reasons that the WES cone penetrometer is unacceptable in this service. The single number that results from a test, however simply obtained, is presently impossible to analyze in sufficient detail to permit factoring the over-all performance into the component behaviors which, in the usual absence of blinding flashes of revelatory genius, must ultimately be made clear if future designs are to show any steady improvement in performance. Other, more readily interpreted quantitative mobility tests have been based upon measuring the drawbar pull of vehicles in simple terrains of interest, and equating the measured pulls in various ways (mostly implicit) with the ease of motion of the vehicle and hence its degree of mobility. Of such tests, those which measure the steady-state pull-slip relationship, i.e., those which are obtained while the vehicle is in true forward motion, and in which inertia or impact effects are patently eliminated, are considered to be the most valid. Not only do they avoid several large areas for possible test errors, they measure, in the final analysis, the margin by which the ultimate tractive effectiveness of the vehicle in the terrain exceeds the motion resistance offered. This margin, related to the gross weight of the vehicle, is a clear and valid measure of the ease of motion of the vehicle in the particular situation. Although mobility measurements of the drawbar pull-slip type have 5 provren useful in vehicle research and development, in the absence of better and more complete analyses than are now available, they are of little practical use from an operational-viewpoint. The task of bring- ing the engineering and operational concepts of off-road performance together within a common framework, of uniting trafficability and mobility study under a single philosophy (but not necessarily a single roof), remains. It is suggested by the authors that it be considered one of the most important immediate goals of vehicle-terrain mechanics. Within a proper and adequate analytical frame, it should be possible to define the ability of a terrain to sustain traffic entirely in terms of its geometric configuration (including vegetation) and the mechanical properties of its surface material (to an appropriate depth). To meet the test of adequacy, the framework would of course have to provide for the calculation of the performance, and thence mobility, of any vehicle in any terrain for which the complete data were specified. To meet operational needs, the performance computations for each existing vehicle could be made once and for all and graphed as functions of the several terrain parameters in the general manner, but not by the particular method, indicated in the work of Stewart and Weiss (201).

Snow and Snow Metamorphism

The following brief presentation of snow concepts and terms is made for the special purpose of emphasizing those points which seem to the authors to be of particular importance from the viewpoint of vehicle problems. The authors are well aware that several authoritative treatments with good nomenclatures exist, for it is entirely to these that they owe such understanding of the field as is herein displayed (113, 115, 128, 133, 138). Snow is an aggregate of individual crystals which have formed in the atmosphere by sublimation of water vapor into solid nuclei and fallen to earth. The form of each individual snowflake or crystal at the moment of its arrival at the earth's surface is determined by numerous complex atmospheric factors, of which the temperature and accompanying degree of 6

saturation, and to a lesser extent the degree of turbulence, have been shown to be the most important (115, 133, 140, 147). It is readily understood that in falling, a snowflake passes through numerous regions differing markedly one from the other with respect to some or all of the primary and secondary factors, and whose character and extent vary with time. This accounts in a general way for the marvelous diversity of individual crystal forms that are found from snowfall to snowfall, and within even a small sample of a single, apparently homogeneous new fall. It should be noted that the dynamics of snowflake growth are such that the new-fallen crystal is of an unstable form, which cannot long remain in its fresh-fallen stage except under extremely low, static temperatures with particular humidity and air flow conditions which can only be maintained for any appreciable period in the laboratory. The mechanical properties of a layer of new fallen snow are, for a period of a few hours to several days, depending upon atmospheric conditions subsequent to the fall, determined primarily by the form of the crystals as they reach the ground, and hence upon previous extremely complex meteorological conditions. An important secondary, but by no means independent, variable is the surface wind condition at the time of the fall. Despite the wide range of variation in individual crystal patterns, the snowflakes falling in a given area over appreciable continuous periods of time may usually be classified as to their general character. In consequence of this, and of the fact that the ranges in mechanical properties corresponding to the various qualitatively describable new snow types appear to be reasonably small, useful generalization seems possible and will be attempted in a later section of the report. The constant changes in the individual snow crystals, which charac- terize their growth during their fall to earth, continue almost unabated under the same basic influences, external and internal, once the crystal lands, albeit along somewhat different lines. The process of change within the snow pack is referred to as the metamorphism of snow (138). The basic metamorphic process is an essentially irreversible one of reconstituting the individual snow crystals into more stable, more compact forms. In most cases under continuously freezing conditions, favored individual crystals maintain their identity, growing in mass, but not particularly in spatial extent, at the expense of less favored crystals. The first and most pronounced effects of metamorphism under colder than freezing conditions in the days immediately following a snowfall are a distinct settling and densification of the new snow layer. Another facet of the metamorphic process, which continues throughout the existence of a snow cover and is of particular importance in vehicular considerations, is the bonding and unbonding of neighboring crystals by ice bridges. A highly bonded snow mass has appreciable structural strength which can, and often does, play a major role in vehicle performance. The bonding process may proceed within the snow mass simply through the redis- tribution of the water content of the pack under the influence of the laws of crystal stability and the thermodynamics of convection and sublimation that govern the primary metamorphic behavior. More usually, however, changes in the degree of bonding are associated with wind-induced air flow (horizontal as well as vertical) through the snow. Humid air flow will cause or accelerate bonding by depositing water mass in the form of ice bridges between crystals. In similar fashion, dry air flow tends to evapo- rate ice bridges and reduce the structural bonding in a pack already made firm by previous processes. The extent to which intercrystalline bonding may increase the strength of a snow layer is obviously a function of the number and size of such bonds which may be made in a unit volume. In fine- grained snows, bonding takes place through a large number of very light bridges. A coarse snow (the result of continued metamorphic crystal growth, and hence of the calendar age of the snow among many other things) cannot have as many bonds per unit volume, and rarely makes up the deficit through increased individual bond strength. As a result, the degree of bonding is most important in relation to fine-grained snows (113). High temperature gradients within a snow pack, such as are usual in treeline snow areas and in places where radiation between the snow pack and the atmosphere is favored by special orientation, cause considerable air flow within the pack through convection. Such high gradients of course imply large differences in equilibrium vapor pressures at various points in the pack, so that the over-all situation favors rapid transfer of the water mass towards the colder portion of the pack and a resulting rapid change in the crystalline structure of the entire cover. The most usual pattern for high pack gradients is that in which the atmospheric temperature is lower than that of the underlying layers of snow or earth. In this case, usually in the absence of wind effects, the entire snow pack (with the exception of a thin surface layer) may be transformed into depth hoar within a matter of hours. It appears that the hoar crystals are not simply altered snow crystals, so that depth hoar is not, strictly speaking, snow. (Neither are ice sheets, ice lenses, rime crusts, surface hoar, and several other ice forms found often on or within a snow cover.) For vehicular purposes, however, rigor on this point will serve no useful purpose, so unless qualified, the term snow will (throughout this report) mean true snow after it has fallen and any of the forms of ice or ice and water which it subsequently assumes or becomes associated with in the pack through natural processes. It is to be noted that the entire snow cover is involved in the metamorphic processes (as distinct from the essentially individual character of snowflake growth) under the influence principally of the following, not necessarily independent variables: air temperature, humidity, wind, snow pack temperature profile, and radiation effects between the snow and the sky above, and the ground below. The properties of the cover at any given time are a function of the original character of the snow crystals at the time they were deposited, and their subsequent history in terms of the above factors. It is apparent that the constant variation in these pre- cludes the establishment in the top few feet of a snow cover of that equilibrium with its neighbors and its thermodynamic environment which each individual crystal is seeking. The nature of the metamorphic process is such (in sub-freezing conditions) that it brings about the most rapid and marked changes in the hours following the snow's first fall, when the crystals are most unstable in relation to their new environment. (Another period of rapid change in snow character occurs near the end of the snow's life when air temperatures hover about and above freezing.) For this reason, the influence of the original crystal type upon the character of the pack appears to become secondary to its meteorological history after the first 9

relatively short stages of readjustment, the first "settling" of the snow, have taken place.

Characteristics and Properties of Snow

Procedures for accurately describing snow and snow cover for purposes of run-off prediction, construction of snow roads and structures, and avalanche control (in descending order of success) have been rather thoroughly worked out in the past twenty years (122, 129, 133, 138). Ac- cording to these procedures, snow may be described in terms of its grain form and size, density, porosity, temperature, water content, and degree of intercrystalline bonding as measured by various "hardness" tests. Insofar as these qualities, however quantitatively expressed, generally do not in themselves directly determine the behavior or performance of a vehicle in snow, or of a snow under vehicle action, they are purely descriptive. All have been shown to be valid characteristics of interest, though, and experience has indicated that the list is essentially complete; i.e., when all the descriptive characteristics are quantitatively known tor two snows and the values are approximately the same, the two snows are, for practical engineering purposes, the same. These quantities and the techniques for obtaining them appear adequate for descriptive purposes in vehicle-snow study, and should be used to the fullest extent in connection with any future field or laboratory work. However, it is the dimensionally consistent parameters relating the elastic, collapse and flow characteristics of the aggregate to the history of the loads imposed upon it which must be used in any basic, dimensionally correct system of equations that seeks to analytically explain or predict the behavior of a snow mass under vehicle action. These stress- strain-time parameters are usually termed the mechanical properties of snow, and it is implicit in dealing with objects of vehicle size that it is the average behavior and strength of the snow mass, rather than of the individual snow particles, that are of most immediate interest. It is not meant by this last statement to imply that the more basic approaches to the behavior of snow under stress, which will one day explain 10

the mass properties in terms of individual crystal configuration and similar fundaments, are not of vast importance in the long range Pcheme of things. To do so in the face of the recent history of advances in other fields resulting directly from the study of particle dynamics in place of mass behavior would be foolhardy. At the same time, it must be remembered that with minor exceptions, engineering today is still based largely upon the results of earlier, less precise and fundamental theories and understanding. Suffice to say, any understanding, however less-than- pure, leading to a like degree of success in the sphere of snow vehicles would stand as an elegant achievement at the present moment. The mechanical properties of snow, then, are those parameters which describe its behavior under shear, tensile, and compressive stresses applied in various ways and at different rates. It should be noted that much of the existing general elastic and plastic theory that utilizes like parameters in dealing with other materials and problems is based upon convenient assumptions of linear relationships between principal variables, and the properties (as usually thought of) have been correspondingly defined. To the extent that many such assumptions are misleading when dealing with snow and/or vehicle-snow problems, the list of mechanical properties necessary to particularize fully any given homogeneous snow for over-all engineering purposes probably cannot be considered complete as yet. A broad program to study the mechanical properties of snow is under way at the Snow, Ice, and Permafrost Research Establishment (SIPRE) (132, 133). This program, pursued vigorously, will greatly increase understanding of all snow-mechanical questions. The network of problems underlying the program, however, is not primarily concerned with vehicles, with the result that the parameters under study will in large measure be more appropriate for dealing with static or long-term steady loading conditions, than for the dynamic loadings, often well past the failure-point, imposed by vehicle action. The experience of the past ten years in vehicle-soil relationship study has shown that considerations based upon static properties can be misleading where vehicles are concerned. In the present instance, the determinations of snow shear strength 11

which have thus far (with minor exceptions) been published may be cited (132, 133, 138). The usual presentation of such data is in the form of a curve showing the variation with normal stress of the ultimate shear strength of the snow. Each point on the curve is obtained at the moment of failure of the snow structure (whether that structure arises from crystal interlocking or bonding by ice bridges), which in most cases is well in excess of the unit shearing resistance that the material will support if the shearing action continues after the initial failure. Such data is useful and valid, but it is incomplete for vehicle purposes. In order to make even a crude estimate of the tractive effort of a vehicle whose running gear is slipping, the shear forces the material will develop after structural failure must be known (76, 196, 199). Complete direct shear information for vehicle purposes thus consists at least of a family of shear-normal stress-displacement curves for a wide range of normal loads starting at zero in which the displacement encompasses both the elastic range and several inches of slip-failure motion. Some work by Kamm, cited by Bekker (190), indicates that the "after failure" unit shear forces that a typical snow will support may be less than one-half the "at failure" values. While it has not yet been shown to be a first order effect in slow- moving vehicles, there can be little doubt that the viscous properties of snow will also eventually form part of the complete snow data necessary for snow-vehicle research (although they may never be needed for practical operational field work). Lest the picture at this moment look too forbidding, a contrary (but somewhat premature) suggestion might be discussed at this point, also in relation to snow shear generalization. The "typical" "lazy S" shaped shear-normal stress curve for snow, apparently first given by Haefeli (138) and cited by all writers on snow since, is probably characteristic of slowly and gently applied loadings, rather than the dynamic, impact, vi- bratory loads under a vehicle. The accepted explanation for the initial dip in the curve (when it occurs), in terms of increasing collapse under normal stress alone of the snow structure in way of the preferred shear plane, would seem to indicate that this peculiarity is of little 12

significance to most vehicles. If this is true, then some simplification, back perhaps to the well-known Coulomb linearization,

T = c + a tan 0, where T = shear strength c = apparent cohesion a = normal stress and = apparent angle of internal friction, may be valid for first order vehicle work at least, despite the formidable appearance of the static curve of Haefeli. Only further work on measuring snow shear strengths and unit shearing forces under vehicle type loadings will tell. At the present moment, complete shear-normal- displacement-rate data on several different types of snow is badly needed in order to establish what linearizations might be acceptable, what modes of behavior neglected, and, from both, what tests should be run in the field in connection with research and evaluation of snow vehicles.

Through the Looking Glass: A Table

In the literature, from time to time, numerical values for various mechanical properties of snow are quoted, often without an overly clear presentation of the methods used in obtaining them. (The Russians -- or their translators -- appear to be particular offenders in this matter.) The authors have had the temerity (not unusual in engineers, but unfor- tunately rare in research scientists) to summarize these, and after inter- polating and surmising much, to display the highly speculative table herein- after known as Table I. It is presented not only as a guide to the magnitude of the values and ranges that can be expected for snow, but to indicate the type and form of presentation of snow information that might one day be desired by, and useful to, vehicle designers. Although information on dynamic properties of snow is so scarce as to make any attempt at estimates similar to those on static properties impossible, it should be clear that parameters describing dynamic compaction, shear, and flow behavior are also definitely required.

It will be noted in Table I that two parameters appear, which the authors have called "k" and "n." These are the parameters appearing in the Table I

Some Typical Characteristics and (Estimated) Corresponding Mechanic a.l

Properties of Several Generalized Surface Snow Types

A. DESCRIPTIVE CHARACTERISTICS (rouhly averaged) Normal In Situ Free-water Maximum Specific Content Depth Snow Type Grain Form Grain Size Gravity% Degree of Bonding ft

NEW SNOW 1. Wild Fluffy, dendritic Fine 0.03 0 Some crystal interlocking 2. Ordinary dry Dendritic Fine 0.06 0 Some crystal interlocking 3. Wet Dendritic to amorphous Medium 0.15 10 Some capillary cohesion SETTLING SNOW 4. Powder Compact dendritic Fine 0.2 0 None 3 5. Bonded Compact dendritic Fine 0.2 0 Some ice bridging 3 6. Wet Dendritic to amorphous Medium 0.2 10 Capillary cohesion 2

7. D~E HOAR Cup crystals Medium to large 0.2 0 None 3

8. BARRES "SAND" Saltlike Fine 0.3 0 None 2 SNOW SETTLED SNOW 9. Powder Saltlike Fine to medium 0.4 0 None 2 10. Bonded Saltlike Fine to medium 0.4 0 Ice bridging 2 11. Wet Saltlike to amorphous Medium to large 0.4 10 Some capillary cohesion 2 O SNOW, FIRM, OR

12. Disaggregated Almost spherical Medium to large 0.5 0 None 2 13. Bonded Almost spherical Medium to large 0.5 0 Some ice bridging 2 14. Wet Spherical Large 0.5 15 None 2

15. SPRING, CORN, Spherical Large 0.6 20 None 2 OR SUGAR SNMW

16. ROTEN SPRING Spherical Large 0.7 30 None 2 SNOW OR SLUSH 17. ICE 0.9 0 Full

18. FNOSE NMJS EG 1.1 0 Full

(Continued) Table I (Cont'd)

Some Typical Characteristics and (Estimated) Corresponding Mechanical

Properties of Several Generalized Surface Snow Tes

ATP "' A emmemT*? m^'A A MRI" 1 UK b'RAITi its17" ViAIVVlV4W&" Vil L PROPERTIES (Mostly estimated) Prlncipal Linearized "Open Field" Uniaxial Behavior Sheear Tensile Shear-normal Bernstein Compression "Critical under Temp Striength Strength Stress Parameters Parameters Parameters Density" Cone K N Snow Type Bearing Loads °C psi psi c, psi tan k n pM/ce Index NEW SNOW 1. Wild Compacts -10 0 0 0 0.01 1 2 0.4 0 2. Ordinary dry Compacts -10 0.2 0.3 0.1 0.1 1 6 0.1 0 3. Wet Compacts 0 0.3 0.4 0.1 0.2 1 7 0.5 0

SETTLII G SNOW 4. Powder Compacts -10 0 0 0 1 1 1 30 0.4 0 5. Bonded Compacts -10 0.5 0.7 0.3 1 1 1 35 0.14 10 6. Wet Compacts 0 0.5 0.7 0.1 0.7 2 1.5 40 0.5 10

7. DEPTH HOAR Shear flow -10 0 0.1 0.3 0.2 1 25 3 0.3 0

8. BARRENS "SAND" Shear flow -20 0 0 0.4 0.5 1.5 100 3 0.4 o SNOW

SETTLED SNOW 9. Powder Compacts -10 0 0 0 0.5 5 1.5 500 0.45 10 10. Bonded Compacts -10 1.0 1.2 0.41 0.5 10 1.5 600 0.45 30 11. Wet Compacts 0 0.2 0.2 0.1 0.4 5 2 700 0.55 30

OD,SNOW, FIRN, OR NEVE 12. Disaggregated Shear flow -10 0 0 0 0.4 1 1 500 0.5 10 13. Bonded Hydro flow -10 5 7 2 0.4 20 1.5 600 0.5 50 14. Wet Hydro flow 0 0 0 0 0.3 2 1 60 0.5 10

15. SPRING, CORN, Hydro flow 0 0 0.2 0.1 1 100 2 0.6 OR SUGAR SNOW

16. ROTTEN SPRING Hydro flow 0 0 0.1 0.02 1 -- 2 0.7 SNOW OR SLUSH

17. ICE Elastic -10 100 200 100 0 E = 1.5 x 106 0.9

1.1 18. FRO2 ( I4ISKEG Elastic -10 --- 200 ------300

18. FRO~.EN I43SK~ Elastic -- 1.1 300 15

equation relating the sinkage (Z) of an object in snow to the unit load (p) imposed upon it, p = k Zn which appears to be the only approach to the sinkage question which at this moment has been even partially successful (126, 132, 133, 147). This be- guilingly simple equation appears to have first been suggested in connection with wheels rolling in soil by Grandvoinet in the early nineteenth century (126). It has been widely used in various guises in vehicle-soil work (189, 195), and in recent work by Bekker and ETT has been referred to as the "Bernstein" equation after the German investigator of 1913. The Bernstein equation, in the final analysis, merely states that the curve of sinkage versus load of an object in the material to which it is to be applied is a smooth, parabolic type curve. However, bonded snows do not behave in this way under gently applied loads, but rather permit sinkage by a series of structural "collapses" as load is increased, producing a discontinuous, stepped experimental curve (132, 156). The smooth envelope of maximum sinkage, however, does seem to follow a parabolic curve to a useful degree. Moreover, it is reasonable to expect as Barrett has suggested, that under the impact and vibrations of vehicle action, the collapse-sinkage behavior (at least in the mass of snow most within the pressure field under the vehicle) will generally not materialize (156). In short, despite many reasons why the Bernstein equation might not be usable in snow, there is evidence that it is, and hence, until something better comes along, the effort to measure the necessary parameters should be made wherever possible. "k" and "n" may be determined by induction from the results of tests of a three-dimensional object in a mass of snow of effectively semi- infinite planar extent and usually definitely finite depth. They are obviously functions of the geometry of the loading surface and of the snow mass, as well as of the strength and compactive properties of the snow material. Thus, they are merely curve-fitting parameters with no basic mechanics foundation, and not true snow properties. For this reason they must be used only with great caution. In particular, the size and properties of the loading area, and the range of loads and methods of application used in any experiment to determine "k" and'"n" by an independent test must correspond closely to snow vehicle conditions. Kragelski et al appear unwittingly to have demonstrated this particularly well (126). These investigators assumed "n" equal to one and quoted values for "k" obtained from essentially static tests with a small footing. Somewhat later, in connection with a rolling resistance formula directly derivable from their basic equation,

p =k . Z , values of "k" (otherwise identified) are quoted from analyses of measured towing forces on rollers in the same snow. The two sets of values differ by an order of magnitude. This observation, and the difficulties experi- enced in a recent attempt to apply the Bernstein approach to the analysis of a large amount of consistently obtained snow vehicle data (81), suggests that different values of "k" and "n" will be found under dynamic and static loadings (which would be entirely reasonable). Two other parameters, paralleling the Bernstein numbers and obtain- able from a relatively simple test, may one day replace them. A sample of snow may be confined in a cylinder, and its change in height (related to its original height) under slowly increasing axial loads recorded. Wall friction can be practically eliminated by using a gently heated tube, and errors that might be introduced by this procedure in turn minimized by using a fairly large diameter tube. Experience with this type of test to date has shown that up to some critical density (see below), the experimental curve is approximately of the form, p = K (Z/L)N where "K" and "N" obviously bear a close relationship to the Bernstein parameters, and at the same time are independent of the test geometry. Data available from this type of test show that N = 2 approximately, which is, as would be expected, somewhat larger than the usual values for the Bernstein "n." At the moment, the analytical relationship that must exist between the Bernstein parameters and the numbers "K" and "N" has not yet been developed. However, the latter may be considered as properties of the snow material alone, and the uniaxial compression test by which they are measured is simple and may be readily run in the field 17 without complicated apparatus. Note that snow as found in nature is fundamentally non-isotropic, so that complete snow data require uniaxial tests along at least two, and perhaps more, axes. It seems apparent, however, that the behavior of vertically taken samples will be most important in compacting snows, and probably will suffice for most practical vehicle purposes. The authors feel that this test should be part of all future vehicle-snow test data, against the day when it can be properly interpreted. One other unusual item graces Table I, i.e., "critical density." There are numerous indications throughout the literature that each snow type has a density beyond which it will not further compact appreciably, under the normal order of vehicle loadings. Below this density, a given volume of snow within a pack will absorb normal stresses primarily through compacting; above it, the same snow, when it fails under bearing loads, will fail either by flowing more-or-less hydrodynamically, or by the classical general shear failure to a boundary (79, 113, 147, 178, 206). The latter type of behavior is easily noticeable in a snow pack just after extensive mechanical working (as the first step in preparing a snow road, or airfield, for example). Here the snow has usually been densified well beyond the critical density, and its internal shear strength having been temporarily reduced by the destruction of all internal bonding, fails in the classic general shear mode under the stresses imposed by a man walking. Examination of some of Kragelski's data shows the possibility of a critical density quite clearly (126), and moreover indicates that, as might be expected, it is a function of snow temperature, decreasing as the temperature falls. A column hopefully entitled "normal maximum depth" is also displayed. It seems reasonable to expect that the surface layers of a snow pack (exclusive of drifts) will be deposited and subsequently metamorphose in depths of the order shown. Richter (140) gives data on average maximum level snow cover for USSR which is helpful in indicating the total depths that might be encountered in level, undrifted deposits. His figures indicate that the normal maximum 10-day average total depth is of the order of 30 inches for treeline areas and 15 inches for Arctic areas. 18

Figures on snow depths in a typical Canadian treeline area show about the same order, with actual maxima approximately 50% greater than the average (81). The figures in Table I reflect the latter fact. Such figures, properly substantiated by research, would be of great importance in vehicle design and operational work. It is one thing to be dealing with a bottom- less sea of wild snow, and conceivably quite another to have to contemplate a "normal maximum depth" of (only?) six feet. Similarly, the flowing corn snow and slush of warmer periods will not normally extend to great depths at any one time (except where there is no drainage) because of self- insulation and the tremendous thermal inertia of deep packs. The very approximate properties for ice and frozen muskeg appearing at the bottom of Table I are largely a reminder that, in many snow terrains, the trafficability of the terrain will be as much or more determined by the properties of the "hard ground" underneath, as by the snow cover on top. In many areas, despite sub-freezing temperatures for long periods, the ground or ice underneath the snow is frozen only to a small depth because of the excellent insulating properties of snow. In such areas, the weakness of the ground makes the terrain untrafficable to heavy vehicles which might cope with only the snow. This point will not be discussed at any length, as it is considered outside the scope of the present report (opening a whole new field of inquiry), but obviously has an important bearing upon many over-snow locomotion problems. The current state of knowledge about the ability of various thicknesses and qualities of ice to withstand vehicle loadings is well summarized in a SIPRE REPORT (133). No discussion of the figures for the several mechanical properties shown in Table I is possible. They are, themselves, no more than a discussion at the present time; a series of reasoned estimates based upon very little data. It bears repeating that Table I was prepared as much to indicate the type of information that is thought to be required, as for any other purpose. It is hoped, however, that the orders of magnitude indicated therein are sufficiently correct to make clear many of the difficulties faced in attempting to design an all-purpose, all-snow vehicle on the one hand, and in trying to evaluate snow vehicles by tests simply in "snow" on the other. 19

Peculiarities of Snow

In addition to the descriptive characteristics and mechanical properties of snow just discussed, snow has, or may have, several other behavior characteristics which are of importance, but which are not yet sufficiently codified to discuss in terms of measurable properties. These peculiarities offer both special problems and special promise in dealing with the design, evaluation, and operation of snow vehicles. In this category the authors include the impact of metamorphic behavior on the evaluation of vehicles, the hardening of snow following mechanical working (in relation to repetitive traffic), and the collapse behavior of snow and its possible structural behavior under different loading regimes, both of which may have important bearing on vehicle design. For the most part, these behaviors have no direct counterpart in vehicle-soil considerations, and serve to emphasize the dangers of treating soil and snow as being alike in any given respect, without careful prior examination. Snow metamorphism has been discussed as a general snow proposition. The extreme and totally uncontrollable variability in the snow material at a given spot over a period of time makes the conduct of field tests and the recording of test-condition data even more difficult in snowv work than it has been in soil studies. At the present moment there is little known (despite Table I) of the relationship between the order of change in descriptive characteristics which obviously are taking place continuously, and the mechanical behavior of the snow to which a vehicle reacts. Even the order of change in a vehicle's performance that might be brought about by a relatively major upheaval in the snow processes, such as partial thaw and subsequent refreezing, is not known. Superficial study of the drawbar pull-slip data from the winter snow vehicle trials held at Kapuskasing, Ontario, during early 1954 (81), which was obtained during a period extending from a new snow condition through the probable formation of depth hoar to considerable depths (139), and through several hours- long periods of above-freezing air temperatures, does not show any significant changes in the performance of the control vehicle, and no greater scatter appears in the data as a whole than can be amply accounted for 20 from other field sources. (It is upon such observations that the extreme extrapolations necessary to fill in Table I were based.) This is encouraging as to the practical accuracy of the snow data needed to adequately complete the field data for any vehicle. On the other hand, Dr. R. W. Gerdel, in a SIPRE report not yet published, covering the results of vehicle "break-traction" tests in the Sierras during the winter of 1951-52, shows that there are significant statistical correlations between measured traction and several snow characteristics. While the "break-traction" type of test is particularly subject to test errors from several sources, Gerdel's tests appear to have been carefully run in a manner to minimize such errors, so that his results are considered entirely pertinent. The apparent contradiction in these two observations epitomizes the present state of the art of relating vehicle performance to snow characteristics. The definitive tests have still to be devised and run. The compaction and hardening of snow for purposes of constructing roads and airdromes have received considerable attention for a number of years in Russia, and since World War II, in Canada and the U. S. (126, 136, 142, 144, 146, 147). The extensive data available will not be reviewed here. For a thorough summary, the reader is referred to a forthcoming SIPRE report seen in chrysalis several months ago. For present purposes it is sufficient to recount a few of the findings of this work. It has been amply demonstrated that by a combination of mechanical working and compacting by various means, a snow cover may be densified and hardened sufficiently to support the traffic of ordinary highway vehicles up to the medium weight class. It appears that the mechanical working is the more potent of the two mechanisms utilized. Its effects are many, but the principal one was perhaps best summarized by Harwood (146) as the abrupt disturbance of the thermodynamic quasi-equilibrium of the snow pacK, which in re-establishing itself, bonds the now more closely spaced individual crystals (by several detailed processes) into a suitably hard, dense mass. The complete success of the method appears to depend largely upon high initial temperature gradients through the snow pack, but there is generally sufficient gradient to give some hardening, even when the working 21 is provided only by vehicle action. Vehicle action, of course, includes some straight compactive working, which is also effective, but to a lesser degree (79). The hardening action begins slowly, perhaps building up by a sort of chain reaction, and dies out again as it approaches a terminal value. It may continue for as much as twelve hours after all active snow working has ceased, and the increase in the hardness (over the initial pack condition) will be greater, the greater the initial temperature gradient, the lower the temperature, the greater the intensity of mechanical working, and the higher the compacting pressures. This effect, and snow metamorphism, of which it appears merely to be a forced extension, make repetitive traffic tests in snow difficult to run, interpret, or even justify. It is evident that the time interval between successive passes must be controlled and specified as part of any test data , and its selection in the first place justified on grounds other than the con- venience of the tester. An interval corresponding to "bumper to bumper" operation might be justifiable on the grounds that it is probably the most severe treatment possible, but the results would have to be clearly understood as valid probably only for this type of traffic. A pilot study to determine the order of variation in results that might be expected in a given snow and meteorological circumstance from varying the interval in various ways would clarify the problem. Some less well recognized peculiarities of potentially great importance (if study bears out their putative existence) have to do with the detailed manner in which some snows react to loads. The fact that under gently applied normal loads a footing will sink in most snows by a series of structural collapses, and with only a minor amount of sinkage under increasing loads between collapses, has already been noted. The current thinking at SIPRE is that to properly treat snow deformation under load will require the development of what Dr. Henri Bader terms the "collapse mechanics" of the material, rather than the application of more classical plasticity concepts which have thus far proved "disappointing." While it is unlikely that any vehicle will be able to load the snow in its immediate vicinity gently enough to take advantage of operating above, rather than below any given collapse level (of loading) therein, 22

it does seem reasonable that the degree of uniformity of loading and gen- tleness of application may determine whether a critical stratum several inches below the contact area holds at a low collapse level, or fails according to the smooth envelope of dynamic sinkage vs load already mentioned. The sinkage, and hence the motion resistance, of two otherwise identical vehicles could well be far less on that vehicle properly designed for even load distribution and gentle load application than could be explained simply by the reduction of dynamic loadings and peak pressures in a continuously plastic medium. The possibility of similar vehicles operating at different collapse levels, which is suggested by the collapse (rather than plastic flow) behavior of most snows, is, at least superficially, at variance with certain ideally plastic properties of snow observed by Barrett (156), although they may, upon closer experimental examination, prove to be not only compatible, but identical. Barrett reports that most snows loaded to a specified amount, and failing sufficiently to carry that load, will subsequently carry lesser loads without further failure. He also suggests that while snow is failing under normal loading, its capacity to carry shear loads imposed by a vehicle's tractive needs is reduced; i.e., in consonance with the usual explanation for the minimum in the "lazy S" shear-normal stress curve by Haefeli (for bonded snows), the shear strength of snow is greater if the shear load is imposed on snow not already actively failing under bearing loads. Barrett's observations, plus the possibilities of operating vehicles at differing collapse levels, suggest some further important possible ex- planations for the unanimously admitted success of the Tucker Sno-Cat (when operating in snow, where it was designed to be) (8, 9, 10, 11, 71, 73, 75, 76, 99), and for several other phenomena whose clarification might help future designs. Everyone who has considered the success of the Sno-Cat has recognized that the uniformity of loading under the individual pontons due to rigid loading areas, the large track area per unit of weight, and perhaps the reduction of stress concentrations in the snow through the use of round-bar grousers, were at least in part responsible. There is another feature of the Tucker design which, in the light of 23

Barrett' s suggestions, is of great importance, but however, has not been previously discussed. That is the fact that each individual ponton, because of the track drive set-up, has a nose-down couple on it at all times, independent of drawbar, resistance, or slope loads (8). This means that the loading conditions under each ponton are precisely those that would be dictated by Barrett's perfectly plastic snow, and it should be noted in this connection that the authors have never seen photographs or movies showing a Tucker Sno-Cat operating with the individual pontons trimmed by the tail in the usual attitude of other tracked running gear in snow. Barrett further reasons from his hypotheses that, regardless of changes in weight distribution over any practical range, a normal tracked vehicle with road wheels must operate in a tail-down attitude in deep snow. It is an unfortunate property of rigid wheels, in the present context at least, that under a fixed load, the harder the surface upon which one is run, the greater is the contact pressure exerted by it upon that surface. Kragelski examines this rather thoroughly in discussing rollers for snow compaction (126). All present road wheels, together with the track wrapped around them, must be considered rigid in comparison to snow, and further, the amount of overloading of the front wheels, relative to the rear ones, possible through practical changes in weight distribution is limited. Thus the action of successive road wheels as they pass over a given section of snow is, according to Barrett, automatically to stress the snow above the pre-compression imparted by preceding wheels, so that each causes additional failure. Converting this history of one point to an instantaneous picture of the entire vehicle and the snow under it, it is obvious that the vehicle will be running in a tail-down attitude. This trim not only reduces the shearing resistance of the snow under the vehicle (if Barrett is correct), but also offers a continuous climbing resistance to the motion of the vehicle (42, 118), further reducing the margin of ultimate tractive ability of the vehicle over its external motion resistance. The obvious methods of avoiding the continuous bearing failure under a tracked vehicle are to keep the center of gravity of the vehicle as far forward as is practicable, and to use either a skid-girderized track, such as is found on many snow freighting-tractors, or the type of roller track found on the Sno-Cat and various tracked marsh machines (194). It is probable that the complete rigidity of loading area offered by the Tucker method and the round bar grousers (by minimizing the working effects of track-link oscillations) both represent optimum solutions within such a framework where snow is concerned. The relatively low average value of friction between snow and the ponton bottom is, of course, also important to the success of the Tucker method. It is also noteworthy that there appear to be no reports of severe icing or freezing-in with the Tucker, although this is a major problem with aircraft skis under the same order of unit loading (130). A completely different method which suggests itself, and which does not appear to have been tried, would be the use of a normal leading road wheel, together with highly flexible road wheels following. The present pneumatic tires in use on numerous Bombardier style vehicles and the Frandee Sno-Shu are obviously not flexible in relation to snows. Something with the flexibility of Rolligon bags, for example, would probably be required. The authors are well aware of the difficulties such road wheels might present in retaining the track, but it does seem to be one possible approach to retaining the high performance of the Sno-Cat in snow (which has made it the object of so much interest), and at the same time widening its scope of successful operation by giving its pontons reasonable suspensions. None of the "improvements" now under way, inspired by the latter goal, will, in the authors' opinion, leave the Sno-Cat's snow mobility in the same good health in which they found it.

Snow Terrains

From the literature it appears that the broad term "snow-covered terrain" is conveniently divisible into several rough categories, each presenting somewhat different challenges to over-snow vehicles; year- round frozen Arctic and glacial ice cap terrain, Antarctic terrain, winter sub-Arctic barrens, northern winter treeline terrain, temperate 25

intermittently snow-covered terrain, and mountain snow terrain. From the point of view of precipitation, the Arctic is a desert. In those areas which remain continually frozen (including areas where there is some surface melting in deep snow packs, but no general thaw) one-pass trafficability appears generally to be good throughout most of the year, i.e., suitable for the operation of most medium and all light- weight, tracked, self-propelled, towing vehicles. One-pass trafficability becomes marginal when surface layers of old snow become partly thawed, and may be poor in drifts, but neither condition is usually found in great depth. Stefansson has characterized the Arctic when frozen as ideal off- road terrain, because of its generally good one-pass trafficability and the relative lack of steep grades and obstacles (165). Preliminary work on the Greenland ice cap by Waterways Experiment Station personnel during the summer of 1954 (106) indicates that while one-pass trafficability of this type of snow-terrain is high, repetitive traffic along a single trail (of the order of fifty passes with a PD-7 tractor, for example) creates swales along the trail which in extreme cases will immobilize a vehicle or a tractor train. The same phenomena were observed with several vehicles, ranging in unit loadings down to the Tucker Sno-Cat (large four-ponton model), by Transportation Corps investigators on the ice cap the preceding year (79, 180). It was concluded by the WES crew that the terrain multi-pass trafficability was poor, which is valid but may be misleading where such wide-open spaces of high one-pass trafficability are concerned. The principal vehicular problems in Arctic terrain, when frozen, are mechanical (due to icing at moderate temperatures, material failures at extremely low temperatures), obstacles in limited areas (crevasses, loads, and pressure ridges in sea ice), and operational difficulties (aris- ing from extreme cold, periods of near-zero visibility, etc.). Those Arctic and sub-Arctic areas that are subject to complete summer thaws (complete except for permafrost thawing, that is) present trafficability problems in summer that in many ways are more extreme than those met during the winter. Any vehicle for year-round Arctic use, including such summer operation, must be designed specifically for the latter, which will largely influence, and may well limit, the approaches to winter mobility 26 problems. A detailed discussion of this phase of the Arctic vehicle problem is (perhaps fortunately) not within the scope of the present report. In general, it may be predicted that snow-going vehicles found to be satisfactory (from a mobility viewpoint) in other snow-terrains will have no difficulty in frozen Arctic snow-terrain, and that many that are wholly inadequate for other snow conditions will function to some extent in the Arctic (180). The problem of predicting trafficability will largely concern the latter marginal vehicles. Antarctic terrain, insofar as the trafficability of its snows is concerned, appears from the literature to be about the same as frozen Arctic terrain. The order of recommended maximum nominal unit ground pressures for Antarctic operation seems to be around 5 psi (92), which, as a gross generalization, would also hold for Arctic terrain. The Byrd-Poulter Snow Cruiser, which was almost completely immobile in this terrain, utilized four tires made in the same molds used for the present Gulf Marsh Buggy tires, but the wheel loadings of the two vehicles differ by a factor of about five (2, 4). The high mobility of the Gulf Buggy in several snow terrains (81, 181, 184) cannot, therefore, be used in juxta- position to the failure of the Snow Cruiser as a guide to relative traf- ficabilities. The sub-Arctic barrens, that vast area between the treeline and permanent ice; i.e., roughly between the 50 F and 32 F isotherms for the warmest month, is apparently similar to permanently frozen areas (from a vehicle standpoint) during the long winter months when it is frozen. The snow cover is light, and although the action of the dry winds tends to keep it powdery and drifting, resulting in a fine, granular powder snow which behaves under vehicle action much like a light, loose sand, it presents little difficulty to vehicles of moderate mobility, such as medium-weight tracked machines. In many areas the average cover, throughout most of the winter, is so shallow that modern military, all-wheel drive trucks equipped with current, large diameter, flexible, off-road tires operate satisfactorily (78). In these cases, the trafficability of the frozen ground underneath the snow determines the trafficability of 27

the terrain for most classes of off-road vehicles. The effect of the snow is to increase the motion resistance of vehicles operating through it, but the load is supported mostly by the largely unyielding ground. Even the motion resistance penalty can be considerably mitigated, for low mobility vehicles for which even a relatively slight increment might be critical, by breaking a trail for them with a machine of adequate first- pass mobility. Problems during the winter months, so far as vehicles in general are concerned, are mainly mechanical and operational. However, snow-ground trafficability deteriorates once the spring thaws set in, and the locomotion problem quickly changes to the more difficult one of the summer Arctic. Trafficability prediction in the Arctic and sub- Arctic barrens will be most difficult in periods of thawing, when conditions are disintegrating rapidly. For all the snow-covered terrains discussed so far, there is a common factor tending to limit the mobility of high speed vehicles. This factor is a varying and sometimes severe degree of surface roughness created by wind action. Wind packing and hardening, and subsequent erosion of the snow surface, create "ground waves" of various forms (sastrugi) which sometimes will limit the speed of potentially fast vehicles because, in the final analysis, of the poor "ride." Northern winter treeline terrain, such as is found in a belt approximately 500 miles wide across Canada, appears in many respects to offer the severest snow problems in vehicle operation. The snow is generally deep enough, and light enough, to require high mobility in vehicles operating in virgin snow. Frequently the pack is almost entirely depth hoar, especially towards the end of the snow season (128, 139, 147). The muskeg under the snow is often frozen only to a limited depth (or sometimes not at all) as a result of very early deep snowfalls. A brief mid- winter thaw (which removes the snow insulation), followed by a period of sub-zero weather before the next snowfall, can change this situation within a few days. The condition of the muskeg will determine the trafficability of such terrain for medium and heavy vehicles, including many which in general would be mobile only when operated on trails already broken by lighter vehicles. The trees, scrub evergreens throughout 28 large areas, are large enough to stand as obstacles to light vehicles, and close enough together to make the process of threading through them tedious at best. For this reason, light vehicles, and to a lesser extent medium and heavy machines, will be operated largely along trails (existing or specially prepared) and frozen stream beds and lakes. The problem of multi-pass trafficability may often be very real in such areas, but as already indicated, it is probably the ground beneath the snow, rather than the snow itself that will mainly govern. The practical necessity to operate for the most part along clearly defined routes offers considerable flexibility to the approach to mass traffic in such areas, however, With only simple procedures, such as sending out trail-breakers about 12 hours before a main convoy, virgin routes of very poor trafficability (both because of the resistance offered by deep snow, and the poor bearing capacity of the little-frozen muskeg underlying) may be converted to routes of sufficient trafficability (through snow hardening and greater freezing of the muskeg because of the reduced insulation offered by the collapsed snow in the broken trail) to accommodate normal off-road vehicle types. Most of southern Canada and the northern United States may be termed temperate intermittently snow-covered terrain. The problems of trafficability prediction throughout most of the year have nothing to do with snow, but when snow is involved, are apparently similar to those for treeline snow areas in that the character of the ground under the snow will usually be of greater importance than the snow properties. However, deep falls of snow (often wet), almost endless circumstances favoring drift accumulation, and temperature conditions ideal for rapid metamorphosis of new snow will all be found in such areas, so that severe trafficability problems directly chargeable to the snow cover only will not be rare. Vehicles satisfactory for treeline area operation will none-the-less generally be entirely adequate for use in this temperate snow-covered terrain. Mountain snow-covered terrain, depending on its altitude and lati- tude, will at various times and places present all the problems of the preceding areas, plus a few of its own. This is one reason why snow vehicles successfully developed for use in mountainous areas have been 29

found to have such outstanding mobility in other snow terrains. Chief among the special mountain conditions affecting trafficability may be mentioned the extreme grades, the avalanche hazard, and numerous extensive, extremely deep, soft snow deposits.

Vehicles in Snow: The Several Problems

As is usual in any complex engineering situation, there are as many solutions to the problems of over-snow vehicles for military purposes as there are statements of it. The same, of course, is true for each of the fragments of the problem. However apparent this may be, it emphasizes the importance of clarifying the objectives of the present large effort in snow-vehicle study before attempting to analyze past data and experi-. ence or setting out to acquire more. There has been a conscious attempt in the writing of this report to suggest means for integrating all vehicle-snow work more closely on a technical level, and the ultimate treatment of the entire field within a single framework of methods and technical viewpoints has been discussed. It has not been meant to imply, however, that there is or should be only one legitimate objective for the present large effort. The facts are quite the contrary; there are many objectives, all related, but all different. It is this very profusion of goals that excuses the present attempt to compartment them. For the most part the task is not too taxing. There is the operational trafficability problem, "Will this (or these) vehicle(s) get through here?"; a specific question about a specific number and type of vehicle in a specific terrain which seemingly has only one correct answer: yes, or no. However, in most Arctic and sub- Arctic terrain, there will usually be sufficient room to "move" a trail a few feet each time that repetitive traffic has reduced its trafficability, or, in a mass movement, to operate on numerous parallel trails si- multaneously, so that each has time to "recuperate" between repetitions. Data on which a yes-or-no answer can be reliably based can only be developed in an experimental program realistically reflecting these possibilities. 30

The vehicle evaluator's trafficability problem, similarly simplified, is principally, "How do these test conditions compare with those under which I tested the last vehicle?" and the researcher's might be, "What are the conditions, in terms that I can analyze?" The evaluator's question is best answered by stating the mechanical properties of the medium in which the vehicle is running; the researcher's must be so answered. Finally, there is the designer's trafficability problem (shared at some points with the evaluator), "What conditions must I design for?" His answer must be in terms which are usable in his design procedures without extensive analysis; again ultimately, the mechanical properties, measured in ways consistent with the analytic methods given him. The designer will be interested in distributions, ranges, and average properties (the sort of thing attempted in Table I plus indications of the macro- and microgeographic and t me-wise extents and distributions of the various qualitatively describable snow types) rather than in the pin- point accuracy about conditions at an exact time and place needed by the evaluator (in some phases of his job) and the researcher. On the vehicle side, the mobility question, from an operational standpoint, is embodied in the original query, "Will this vehicle get through here?" and still invites an unequivocable answer. The researcher's interest is entirely in quantitative performance measurements in terrains that present some problem to vehicles. It is the designer's question, and the problems it raises for the evaluator and the trafficability student, which are complex, for he has numerous possible mobility objectives. These must be understood by the evaluator if the latter is properly to judge the degree of success with which a given vehicle solves a situation, or if he is to be competent to place new technical concepts in their correct relationship to fundamental off-road vehicle goals. The impact of vehicle design objectives and operational procedures on trafficability work has already been touched upon. The discussion which follows will perhaps serve to clarify these last several points. Everyone who has had much to do with vehicles, or with similarly complex machinery, realizes that there can be no such thing as a completely 31

practical "all purpose, all terrain" vehicle. ("All weather" must be add- ed, if aircraft are to be considered within the one framework.) It does appear possible that if first cost were no object (whether measured in dollars or man-hours, or critical materials), a vehicle with positive mobility in almost any terrain could be built. However, unless decades of engineering experience are completely misleading, it might be jack-of- all trades but surely would be master of none. On the one hand, it would have less mobility in any given condition than a vehicle designed specifically for that condition, and on the other, would have little or no useful payload capacity. Comparison of the Tucker Sno-Cat, the Weasel (very nearly an all-terrain vehicle), and a five-axle commercial semi- trailer rig, first in deep, soft snow, and then on the highway, will clearly demonstrate this obvious but fundamental fact. The authors will not apologize for having raised so primary a point, however, because most vehicle tests, in or out of snow, appear to be conducted in complete ignorance of it; i.e., every off-road military vehicle (probably not excepting Hannibal's elephants) is sooner or later tested as though it were the "White Hope" for an all-purpose machine, and eventually in this process provokes a scathing, and sometimes damaging, deficiency report from some board or another which finds that the vehicle cannot do something it never was intended to do. One of the first distinctions that must be made in order to establish either a rational design viewpoint or a like evaluation outlook, is that between transport and combat vehicles. If present indications are not entirely misleading, combat vehicles, in all but the temperate snow areas and the fringes of Arctic areas, will be limited for the most part to the small, light, reconnaissance type capable of carrying little or no armor or armament. They will require the highest possible mobility in virgin snow, but not necessarily high repeat mobility. It might not matter particularly whether or not they leave a highly reusable trail. (A World War II LVT is an example of a vehicle which in some conditions has acceptable mobility on its first pass, but low repeat mobility because, in this case, of the destruction wrought by its deep grousers. Spaced-link track 32 vehicles, despite their high potential first-pass mobility in many adverse circumstances, might also have poor repeat mobility on occasion, for the same reason.) While the combat snow vehicle, then, will be essentially a "lone wolf" (and perhaps not highly important), the picture for the transport vehicle is quite different. Modern warfare, even in the Arctic, will require vast quantities of supplies, and transport, therefore, must be looked upon primarily as a mass operation. Vehicle types for this mass transport problem in snow terrains will, in general, require reasonably good mobility, but most of them probably need not have exceptional mobility in virgin snows. To illustrate the type of design viewpoint which might be profitably adopted, and also the type of concept that can grow out of in- telligent evaluation of technical concepts already in hand, consider first two general problems in the economics of transport: the optimum size for transport units, and the optimum distribution of effort between travel route preparation and vehicle building and operation. The pure economics of transport, unsullied by military, transport- within-transport, and similar practical matters, will always show that the minimum transport cost will be achieved by the use of the largest practical vehicle unit that will provide the required delivery rate in usable increments. This conclusion is mitigated to a greater or lesser degree by considerations of system flexibility, dispersion of loading and delivery points, and further still by factors such as those first mentioned. How- ever, it stands as a fundamental trend which cannot safely be ignored. Similarly, calculations can be made showing, for any given cargo volume and stability of route, an economic optimum distribution of effort between route improvement and the construction of rolling stock of correspondingly decreasing unit initial and operating costs, and perhaps numbers. This latter type of calculation would show that the optimum expenditure on route preparation and maintenance, in relation to that for vehicle construction and operation, increases as cargo volume and/or route stability increase. (Amortization, at any rate realistically suiting the situation, would not alter these qualitative conclusions.) While such calculations might profitably be made on a high-speed 33 computer for an important tactical decision, the myriad detailed data necessary to arrive at any accurate estimate (which is then only correct for the one situation) preclude their direct use as a general design procedure. For design purposes, knowledge of the trends they reveal is about all that can be digested at the present. This much, however, taken with other facets of the snow vehicle picture, suggests that a general solution to the mass over-snow transport problem might lie in consideration of groups of vehicles (perhaps trains, but not necessarily so) as the units for which trafficability should be determined, and into which mobility should be designed. A conceivable (nontrain) group might consist of one nonload-carrying lead vehicle of the highest possible mobility functioning as a trail-breaker, command post, and wrecker, followed by a number of self-propelled vehicles of lesser first-pass mobility, but good repeat mobility (for example, Rolligon type vehicles). A group (or train) concept, if carefully executed, could provide a highly flexible and economic off-road transport system, fully consonant with the underlying basic trends and at the same time raising no insuperable difficulties in transporting elements to any scene of operations. The number and cbmposition of the unit could not, of course, simply be pulled out of a hat (6-7/8 would be an inconvenient answer). The whole system would have to be studied from the ground up. In relation to trafficability requirements, however, the concept does not need further development. In open, Arctic terrain, the multi-pass trafficability of the snow or snow and ground need only be sufficient to support passes by the "mother" vehicle and a stated number of her "pups." The next group, if it finds the previous trail deteriorated (following only by as much as an hour, it is more likely to find it improved), has only to move over another twenty feet. By the same reasoning, insofar as multi-pass trafficability in this type of terrain is concerned, it is even less important in relation to more conventional concepts than it is in terms of the group-unit approach. If every vehicle in use has sufficient mobility to break a new trail for itself, then only one-pass trafficability is really critical. (It is recognized that there are apparent advantages in staying on a single, marked trail when visibility is poor, 34 but the trail marking system appears far from adequate in its present state. Vehicular concepts probably need not be limited by considerations of present patently inadequate situations of this sort.) The degree of necessary dispersal of vehicles (in a routing sense) will be determined by the dispersal of loading and delivery points, and possibly by enemy action. It will determine to some extent the optimum size of the group-unit or train, and hence the least degree of multi- pass trafficability necessary for successful operation. In the same way, the necessary concentration of traffic dictated by terrain conditions (treeline area trails and marked safe paths through crevassed areas, for example), weather, and concentration of supplies at pick-up and delivery points, provides an upper limit to the multi-pass trafficability required. Such concentration will have different effects upon the snow, and where applicable, the ground beneath, depending upon the degree of concentration, in time as well as space, atmospheric temperatures, and the type and environment of the snow involved. As already noted, trafficability along treeline trails will probably be determined by the ground rather than the snow, once a suitable trail-breaker has passed. Traffic concentrations around supply dumps and camps, while probably presenting the worst problem from a multi-pass trafficability viewpoint, cannot be taken as a justification for extensive study of the multi-pass trafficability of various vehicles. In such areas the concentration will generally be so high that some terrain modification may be warranted, such as the delib- erate stabilization of the snow by snow-road building techniques, the laying of mats, etc. It should be clear from this discussion that the objectives of the vehicle designer, from the point of view of mobility, may be quite different when dealing with transport vehicles for snow terrain operation, from those necessary in considering the requirements of combat and reconnaissance machines. The solutions to the two design problems require different outlooks in many ways. For transport, vehicles of low initial and operating cost, and only moderate first-pass mobility but good repeat mobility, are proper design objectives. In trail-breakers and combat 35 vehicles, highest possible mobility should be the main goal. Both objectives are obviously not going to be achieved in the same package, although they may result in the end from common approaches, studies of general, fundamental vehicle-terrain interaction and snow research. It should also be clear that the objectives and viewpoints of operational trafficability studies in snow conditions must be somewhat different from those that have proved rational and reasonably successful in soils trafficability prediction work, and that have, in the pilot stages, been carried bodily over into the snow program. Insofar as the evaluator is concerned, ideas for vehicle types and running gear designs, which often do not appear to fit any existing need, are constantly being put forward by inventors and other highly imaginative persons. The evaluator must, if he is to really earn his keep, judge such ideas not only against any existing formal structure of "needs," but also in relation to new approaches to old problems, which the ideas often aug- gest or make possible. His testing and critique of vehicles embodying new ideas must constantly be alert to such possibilities.

Vehicle Performance as Related to Snow Conditions: the General State of the Art

Understanding of vehicle-snow relationships represents a highly particularized branch of applied mechanics. Its state of advancement ultimately depends in large measure upon the advancement of the more basic disciplines upon which it relies; the more general mechanics of vehicle- terrain interaction (itself underlain by still more fundamental divisions of applied mechanics - dynamics, elasticity, plasticity, soil and snow mechanics), the physics of snow (its formation, metamorphism and mechanical behavior), and automotive engineering (in the sense that it is the science of creating practical machines to meet reasonable functional specifications, however developed). The mechanics of vehicle-terrain interaction, or vehicle mechanics, is relatively the newest of the three cornerstones, having its beginnings early in the 1940's. Its slow but reasonably steady development, best 36 measured by the extent to which some of the fundamental ideas first pro- mulgated by it have become part of the everyday thinking in the off-road vehicle field, has been sponsored almost entirely by the military. While still far from elegantly completed, vehicle mechanics has explained many hitherto puzzling vehicle-soil phenomena, given some little practical guidance to designers of off-road vehicles, and provided considerable positive, potentially fruitful direction for off-road vehicle research, development and evaluation. This modicum of success is probably in large measure the result of the (classic engineering) semi-empirical approach which has been adopted. In the (temporary) absence of a complete and detailed analytical picture of the dynamic stress-strain-flow relationships in a soil mass under vehicle action, those fundamental factors that are known (or can be safely presumed from experience in related fields) to be important and rational are fully exploited, with empirical constants introduced only in the final stage, when the limits of the analysis are reached. This procedure, quite usual in engineering, quickly leads to first order understanding and quantitative information which can be of great use both in design and research, and provides a framework for rapidly and continuously integrating subsequently developed refinements, data, etc. It is the approach the authors suggest for developing vehicle- snow understanding, which is, after all, a part of vehicle mechanics. The alternatives are to await in suspended animation the glorious day of the unveiling of a complete and fully validated theoretical presentation, or to accept a complete empiricism which ignores what little is known and may solve a particular problem, but will provide no basis for generalization to wider fields or for future growth. Insofar as the state of understanding of vehicle-snow relationships depends upon the state of vehicle mechanics, it is clear that it is still largely an art rather than a science. Vehicle-snow theory will advance in the same general degree as vehicle mechanics, although because of some useful properties of snow it could as readily be leading as following. That which furthers the one will benefit the other. To a lesser extent, the same may be said about the support which snow physics is presently able to give the vehicle-snow effort, despite the 37 fact that snow physics has a longer history by at least ten years, and has had the services of many more talented people during that time. The reason for this is undoubtedly that vehicle-snow problems are a far less important part of snow physics than they are of vehicle mechanics. Prac- tical applications of snow science have been largely in the fields of run- off predictions, construction of snow roads and airfields, and avalanche control. Concern within the realm of snow science with vehicular problems is apparently less than ten years old. While the present state of snow physics does not fully embrace the viewpoints that vehicle mechanics indicate are necessary for solution of the problem, the understanding and techniques underlying that science are already turning to vehicular viewpoints, so that the future is bright. The oldest of the supporting fields, automotive engineering, is also, as defined at the beginning of this section, the strongest. The state of development of more-or-less standard automotive components and assemblages thereof is high. However, snow vehicle development has to date emphasized a requirement for light but highly reliable components which has in general not been successfully met, particularly as regards running gear. This report is not concerned with such problems unless there is reason to believe that the considerations of vehicle mechanics or snow physics preclude their proper solution in conventional terms. Suffice it to say here, that in some important respects, the support which automotive engineering is able to give snow vehicles is less than seems possible in the light of the general rate of technological advance.

The State of the Art in Some Detail

It has been demonstrated extensively in vehicle-soil research, and to a lesser but still impressive extent in vehicle-snow work, that the mechanical properties of a deep, homogeneous, soft medium that primarily influence the performance of a vehicle operating in it are its unit shearing resistance, its density, and the parameters describing its behavior under compactive loads. These have already been discussed. It is evident, however, that in horizontally stratified materials, such as snow in nature, 38 the variation in properties with vertical location must also be specified. Profiles of the descriptive characteristics of snow in a cover are usual in snow work but, except for snow density, similar profiles in terms of mechanical parameters appear only rarely to have been made. A good deal remains to be done to ascertain in detail the necessary parameters and to develop instrumentation to measure them in the field with suitable accuracy. Simple hardness or penetration tests utilizing various small tips give only indices which cannot presently be interpreted in terms of the desired parameters. Some pioneering in the measurement of mechanical parameters in vehicular context was done by Mark during the OSRD Weasel development program (42, 118). In 1942 he conducted in situ shear tests similar to those later proposed by Wilson and Nuttall for soils (197). The most recent device for making similar measurements is the "soil truss" designed by Weiss and Stewart (192, 193). This device has been used in snow, where it appears to measure only the structural or static values for snow shear strength, and in its present form, can give no indication of the unit shearing resistance after structural failure of the snow (75, 135). Apparently because of lack of this information, correlation of shear data and vehicle drawbar measurements was not good (76, 81). This experience with the soil truss in snow parallels experience with it in various soils. A few measurements of the variation of shearing resistance with continuing shear motion appear to have been made in Germany by Kanm during the late stages of World War II (108, 190), but nothing in the way of data has been published on the subject since. As noted earlier, this and work on the compactive and/or collapse parameters of snow require early study. Kamm's work on snow vehicles led directly to the construction of "wedge" tracks for a few German military vehicles, which, by his reports, subsequently showed improvements in slope-climbing ability of the order of 30-100% (94, 153). This work has never been properly followed up in this country, and every effort to do so should be made. Kamm is now on the staff at ETT and would be available to work closely with any agency that might wish to resume his work. One of the major reasons for the temporary eclipse of the Kanr wedge 39 track was the proposal by Bekker of the spaced-link track which pur- ported on the basis of some observations in sand to embody the essence of the Kamm proposal in a more thoroughly developed and rational technical form (155). There are good reasons to believe (six years later) that, particularly in snow, the Kamm wedges behave in a manner sufficiently different in some details to make critical re-examination of this assumption profitable. The spaced-link track has been applied to three very light, "belly- less" vehicles (29, 81), two of which have shown remarkable drawbar performance and general mobility in soft virgin snow. (Test results on the third and latest, the Oliver T60, are not available.) This excellent performance is now thought to spring from markedly reduced motion resistance, large "entrained" snow weight, and easy penetration (without excessively increased motion resistance) to denser and stronger strata for the development of tractive shear, rather than from any intrinsic difference in the mode of shear as suggested by the original theory, Indications are that the order of improvements thus far demonstrated can be practically achieved only on very light vehicles (up to about 10,000 lb gross), so that 0 the place of this development in the over-all snow vehicle scheme is not at the moment clear. The remarkable development of the Tucker Sno-Cat, which began about 20 years ago to meet Rocky Mountain snow terrain conditions, appears to have come about from practical observation and a close knowledge of snow, but without any particularly erudite analysis. Some of the possible reasons for its success have been mentioned in various connections previously. The design feature seized upon by the rash of Sno-Cat modifiers, who have appeared on the scene since the Corps of Engineers first "discovered" the vehicle (9, 11), to explain its success has been the "ladder" track. Several experimental and theoretical studies have indicated that the "ladder" track is considerably less than the whole story (8, 82). Despite an almost complete ignorance of what design and operational features account for the success of the original, programs are now under way for Sno-Cat running gear modification to improve its hard-ground mobility. Success in snow by any of the "improved" versions will, in this 40 state of affairs, be the merest accident. Suggestions by Bader about the collapse mechanisms in snow and the several interesting observations of Barrett have both been reviewed earlier, and their possible relationships to the Sno-Cat mobility pointed out. They are merely mentioned again at this point to round out the picture of the present state of understanding of vehicles in snow. Both deserve considerably more study. Despite this closer inspection, it is still evident that the understanding in the militarily important sphere of snow vehicles is only just dawning. Because of snow's differences from soils, however, the promise for improvements that complete understanding will offer are brighter than any on the vehicle-soil horizon.

Existing Self-propelled Snow Vehicles

There have been an astonishing number of self-propelled machines built for oversnow travel, which may be divided into five general classes according to their running gear: tracked, half-tracked (including motor toboggans), wheeled, propeller-ski, and snow-screw. Snow-screw machines have been tried on numerous occasions (42) but have never proved practical. A similar type of machine has also been tried unsuccessfully in marsh (203). Ski-borne, air screw-propelled machines have had a spotty record of success, and have certain inherent drawbacks that limit their range of application to services where extremely light machines can be used or to areas where snow conditions and terrain levelness limit the maximum thrust- weight ratio required to values of the order of 0.2. (They are a regular part of the winter transport system around Leningrad, for example, where large units operate passenger services on the Neva when it is frozen.) However, for all-round service, reasonable slope-climbing ability, and power to cope with "stick-down" problems, etc., Kamm has recommended (on the basis of extensive tests of this type of machine in the Bavarian Alps) a minimum thrust weight coefficient of 0.5, further suggesting that 0.7 would be more desirable (94, 108). While such coefficients may be 41. achieved on sports and small personnel rigs, on machines designed for greater load carrying this would require the installation of approximately 350 hp per ton of gross weight (about the same as a helicopter or a Grand Prix racing car), which is not practical from either a fuel consumption or payload to gross weight ratio viewpoint. A ski vehicle with such a high horsepower-weight ratio would, moreover, have an extremely high speed potential under favorable conditions, and hence would have to be carefully designed as to its aerodynamic stability and suspension characteristics. The marriage of the ski and the track in several small half-tracked machines such as the M-7, the two-ponton Tucker Sno-Cat and the Frandee Sno-Shu, and in motor toboggans such as the Eliason (62), and the simple home-built units used by Canadian trappers has been fairly successful, and may be thought of as an attempt to utilize the ski without the high horsepower-weight ratio penalty of air-screw propulsion. The motor toboggans consist of light, load-carrying bobsled toboggans propelled by a small, floating track unit mounted through an opening in the rear sliding surface and carrying about 20% of the gross load. Their success depends on keeping entirely on top of a snow cover, so that motion resistance, and hence tractive requirements, are kept to a minimum. The extremely low unit pressures necessary for this in light snows do not make their expansion to true load-carrying machines promising, and no outstanding machines of this type of military load potential can be cited. In the successful ski-track machines, on the other hand, only enough weight is carried on the ski to provide steering (about 20-30%, depending on the relative moment arms of skis and tracks), so that they are in the final analysis essentially tracked vehicles, with ski steering substituted for skid steering. This last, if properly exploited, is an advantage, providing more stable directional performance (very desirable on long runs) and reducing the digging-in of vehicles in soft-snow which is associated with skid steering, and in the critical cases can cause loss of steering control and/or complete immobilization. Provision for some skid steering to assist in close maneuvering, and to give control on hard-packed snow and ice surfaces, should always be provided, however, so that little 42 mechanical simplification is ultimately gained. Barrett discusses the possibilities for improved half-track snow vehicles at some length (156) and there seems little doubt that better ones than in the past could be built, using all knowledge that is now available on ski design, weight distribution, etc. (130, 156), but the trend is definitely away from this style of vehicle, probably because of its ski-imposed mobility limitations in other than snow terrains. As in other off-road terrain, the most successful class of self- propelled snow vehicles consists of those entirely on tracks. In Table II are listed six tracked machines of current interest, together with three wheeled machines which have demonstrated promise in one direction or another. Some simple characteristics of each vehicle are also shown. The performance of three tracked vehicles stands out: the Tucker Sno-Cat, the so-called Peterson D-7 (PD-7), and the World War II Weasel (M-29). The Otter (M-76) represents the Bombardier type of track and suspension widely used on Canadian and U. S. military vehicles, but which as a group have not been outstanding for their mobility. In fact, there are some indications in the analysis of the Defence Research Board (Canada) snow vehicle trials held early in 1954 that the Bombardier type of track and suspension in some odd fashion is particularly unsuited for use in light, virgin, treeline snow (81). The Groundhog and Beetle, which have already been discussed, are both equipped with spaced-link tracks. Suffice it to say at this point that these vehicles have demonstrated the possibility of constructing extremely light tractors to handle in snow gross loads as much as twice those handled by more conventional tractors. The challenge of trans- lating this concept into practical machines has thus far been more than the art of automotive engineering has been able to cope with. The authors do not feel that there are any inherent reasons why light spaced-link track tractors (about 10,000 lb), capable of towing gross sled loads of the order of 25 tons in most snow terrains, and specifically designed for Arctic use, cannot be developed. By all odds, the Tucker Sno-Cat is the most mobile of the oversnow vehicles in the most difficult conditions. Its top speed is limited Table II

Some Characteristics of Various Existing Self-propelled Snow Vehicles

Successful in Themselves and/or Representing Potentially Useful Concepts

Full Load Status from Nominal Unit Maximum Speed, mph Gross Weight Snow Ground Pressure Firm-snow Vehicle tons Form Viewpoint Running Gear psi h/t Cover Higna

Peterson D-7 19 Slow-speed Prototype Standard Caterpillar 10 5 5 Caterpillar tractor with wide track-plates Conversion

Le Tourneau 15.5 Tractor Concept Four Le Tourneau flat- (5) 20 15 15 Snow Buggy 4 x 4 experimental top tires 10 ft OD by 48 in. wide at 5 psi

Rolligon 3 x 3 9.5 1-1/2-ton cargo Concept Three 42 in. by 60 in. (3) 10 10 10 experimental Rolligon bags at 3 psi

Gulf Marsh 1-ton cargo, Concept Four highly flexible (5) 20 20 20 Buggy 4 x 4 personnel experimental pneumatic tires, 10 ft OD by 33-1/2 in. vide at 5 psi

Otter (M-76) 6 1-1/2-ton cargo Production Double Bombardier type 15 20 20 band track and suspension, four sets pneumatic road wheels each side

Tucker Four 4.5 1-ton Comercial Quad-track, rigid pon- 40 20 Ponton Sno-Cat personnel standard tons, "Ladder" track

Weasel (M.29) 1/2-ton cargo, Production Band track on eight 1.5 20 20 30 personnel (obsolete) sets of small road wheels per side

Groadhog 2.5 Tractor Concept Spaced-link track on 0.7 25 10 (Canada) experimental Weasel suspension

Beetle Slow-speed Experimental Spaced-link track on 1.1 10 5 (Canla) tractor slow-speed tractor suspension 44 by mechanical considerations to about 20 mph, and this can only be used where the snow cover is soft enough to compensate for its lack of suspension. It must be operated slowly and with great care in hard, rocky, and rough terrain, but it has shown some promise as a muskeg-going machine (10, 11). It has a nominal unit ground pressure of the order of 1 psi, which probably very nearly equals its true effective pressure because of the absolute rigidity of its snow-contacting load-bearing elements. Each ponton, as a result of the design of the drive, has on it at all times, independent of drawbar, slope or resistance loadings, a slight nose-down moment which may account for much of the really phe- nomenal performance in snows so soft and deep that a man cannot get about, even on snow shoes (9, 11). The Sno-Cat in its present form is basically suitable for reconnaissance and light support missions in most Arctic, barrens, and treeline snow areas. When truly understood, the principles underlying the Sno-Cat's success may prove applicable to a number of different types of vehicles that meet the Army's legitimate demands for better mobility and reliability in non-snow terrain than the present machines afford. At this stage, however, it appears that any further developments in any cargo transport should be made within the framework of a complete concept of its probable employment, rather than as just another isolated vehicle which may prove useful. The World War II Weasel still appears to be the best of the more conventional type, small, high-speed tracked snow vehicles, and because of its ubiquity, has been a convenient yardstick by which the performance of other small snow-going vehicles could be judged. It has good mobility in most snows and in a wide range of other terrains as well. However, it is too small for most military purposes although it will handle one or two two-ton sleds in many Arctic conditions, and has been the backbone of several recent small expeditions in Greenland (167) and an important part of several larger ones. The success of the Caterpillar D-7 as modified by Peterson shows the order of changes that appear desirable in attempting to utilize conventional, highly developed tractors for hauling jobs in Arctic snow areas. The PD-7 has two-thirds the nominal unit ground pressure of its 45

standard counterpart, with about twice the power. In many detailed respects, it still is not entirely satisfactory (180), but is close enough to give a valid demonstration of the potentialities of this type of equipment and of sled hauling as a mass transport possibility in snow-terrain. On the Greenland ice cap it regularly handled gross towed loads of the order of 40 tons at average speeds of about 3 mph, with a fuel expenditure of less than 0.5 lb per ton-mile. For comparison purposes, D-8 tractors (same horsepower) operating on ice haulways in Canada regularly handle about 120 gross tons at the same speeds (147). On the other end of the scale, a dog team on a long trip will haul a gross of about 1-1/2 to 2 times its own weight (80-120 lb per dog) at an average speed of about 3-4 mph, depending on numerous factors such as snow conditions, skill of the team driver, method of har- nessing, etc. (165). The mobility of heavier tracked machines, such as tanks, is not so much limited in snow areas by the depth of light snows, which up to depths of about 3 feet can usually be bulldozed out of the way (140), as it is by the strength of the ultimate load bearing stratum in the terrain; ground, muskeg, ice, or highly bonded, compact, old snow (in permanent Arctic regions). Experience in Greenland and the Antarctic indicates that on glacial snows, nominal unit pressures of .the order of 5 psi in medium and heavy equipment represent a practical statement of the upper limits of loading (80, 82, 92, 180). In areas where the snow cover is seasonal, the strength of the underlying frozen or unfrozen ground or muskeg, or ice, which controls, is highly variable. It varies from place to place, and from time to time in almost any given place, from adequate to support heavy tanks (64) to almost zero. Such heavy equipment thus is frequently immobilized in snow, and hence is widely and perhaps cor- rectly thought of as unsuitable for operation in many snow terrains. The limiting factor, however, is generally not the snow cover per se. The three wheeled vehicles listed represent the concept of flexible low-pressure tires (extremely successful in low compaction, high friction soils such as sand) in three forms: large diameter, large diameter and more width, and small diameter with large width. All three are of great 46 interest because any one, if successful and fully developed, would theo- retically offer material advantages in first cost, maintenance and perhaps average operating economy, over tracked vehicles for the same service. Experience to date (81, 82, 181, 184) has indicated that, of the three, the Gulf Buggy has highest mobility in snows, whether on the first pass or on a trail, but in its present form it is an extremely large, cumbersome vehicle, with little payload capacity. Prospects for reducing its size, without self-defeating loss of over-all mobility, are small. The possibilities of increasing load capacity are better. The unit wheel loading on the Gulf Buggy with one ton aboard is about 3,500 lb. That of the Le Tourneau 4x4 (with tires of the same diameter, but 45% more width), which successfully came through preliminary testing on Greenland during the summer of 1954, is about 8,000 lb, and the loading of the Byrd-Poulter Snow Cruiser, which proved too great (in the Antarctic on tires of the Gulf dimensions), was 17,500 lb. From these experiences it would appear that all-round snow work unit loadings of about 6,000 lb per wheel might be tolerated on highly flexible tires of the Le Tourneau 48x68 size while in permanent Arctic terrain permissible loads might be as high as 10,000 lb per wheel. Where the ground, muskeg or ice under other level snow terrains will support such a load, purely geometric considerations (ratio of probable snow depths to wheel diameter) argue that a vehicle on 48 x 68 tires loaded to as much as 10,000 lb might still have acceptable mobility. The reason for persisting in presenting the Rolligon as a possibly useful snow-vehicle concept, despite its consistently poor performance in deep, soft snows (81, 180, 181, 182), is because it has shown acceptable ability on minimum trails and appears to be very easy on such trails. Further development of the Rolligon, or of more conventional but highly flexible, extremely low pressure tires (in somewhat smaller diameters than the Gulf example), appears to offer the basis for the design and construction of cheap, simple vehicles for use as the "following" vehicles in the group-unit concept discussed earlier as an approach to mass tonnage transport in snow terrains. It is unfortunate that the testing of the Rolligon to date has not reflected this possibility, so that its good performance in "trailing" work has to be inferred largely from chance observations. It is suggested that any future testing with existing Rolligon machines be concentrated on developing data with which to evaluate this possibility. The more-or-less existing large wheel concept (Gulf or Le Tourneau), or various possible developments of the Tucker Sno-Cat ideas, might form the basis for the high-speed trail-breaker, command-post, wrecker vehicle to lead a group-unit. Further work on the Le Tourneau Snow Buggy, the PD-7, spaced-link track tractors and perhaps the Tucker configuration as applied to a tractor, or combinations of them, will develop the train concept for mass transport utilizing sleds or unpowered, tracked and wheeled trailers, while the Le Tourneau Snow Buggy, in context of that firm's interesting Tourna-Train (all-units electric-powered, cargo train on tires) (202), offers possibilities for a highly flexible all-wheel drive train. The possible approach to mass transport (or the necessary approach to reconnaissance vehicles) of utilizing a number of individual units all of highest possible mobility, will probably be furthered most by continued development of the Tucker Sno-Cat, although, as noted previously such development can hardly be expected to be successful if the principles responsible for the success of the original are not clearly understood. Some possibly important points have been suggested in the present report. It should be the first order of further Sno-Cat business to examine these possibilities, and any others that might come up, in a careful experimental program whose objectives and methods are clearly thought out and stated. No hit-or-miss test program is likely to demonstrate conclusively the importance (or otherwise) of the several items mentioned herein. Several proposals for alternatives to the skid-steering of tracked vehicles have been made over a number of years, some of which have been reiterated in several reports (190, 194, 200, 204). The most promising of these involve articulation of the vehicle's track elements in one way or another. The method already successfully adopted by Tucker on the Sno-Cat is one. Others propose the actual jointing of the vehicle chassis. This method has been incorporated recently into the North King, a machine built in Calgary (Canada) for geophysical survey work in that area. It is not listed in Table II because its snow performance has not yet been reliably reported, but its adoption of articulated chassis steering is of more than passing interest. It should also be noted that this machine also demonstrates that the desirable nose-down moment of individual track units under all conditions may be maintained in this configuration, although the other design features of the North King's bogies are not such as to fully capitalize on this feature.

Sleds and Unpowered Trailers

The use of tractor-powered sled trains in the Arctic appears to have derived from winter logging and mining practice in northern Canada and Alaska. Considerable development work on the design of appropriate sled runners, suspensions, drawbars, and hook-up methods seems still to be needed (180). According to McConica, runner loadings should be of the order of only 3 psi for minimum friction drag in sub-zero snows (130). At this loading, the total ski drag coefficient will be of the order of 1/8th, which agrees closely with the field measurements of others (106, 180). The major problems with sleds are due to "freezing down," whether because of long or short stops (124, 130, 180), or slush-on-ice-under-snow, such as is found quite often on treeline lakes (124). Freeze-down tendencies appear to be aggravated by low runner pressures, although this may merely be a reflection of the higher possible areas for ice bonding which are implicit in the low pressure runners. A frozen-down sled train may sometimes be started by the sort of starting "jerk" usual in starting rail- road freight trains (if the sled train was backed before freezing down to make the necessary slack available). This method, operationally preferable to uncoupling when possible, obviously places great shock loads on drawbar couplings and frames of the sleds. Some work is required on drawbar designs, to make coupling and uncoupling easier and safer, and (perhaps more important) to eliminate intermittent bulldozing of soft snow into piles along the trail, which appears (along with poor sled "9 runner approach and track angles of attack) to contribute to the development of swales along trails under repetitive traffic (79, 106, 180). Bobsleds are usually preferable to simpler, single runner types, because of better conformance to terrain irregularities, as well as easier steering. The order of the normal sled drag coefficient is not prohibitive, and the train concept (in theory, at least) allows for a maximum adjust- ment of the gross load to the changing traction-resistance character of the terrain. The biggest drawback is the obvious one, that sleds cannot be operated efficiently except in ice and snow. This has led to considerable interest in tracked and wheeled trailers (including Rolligon trailers) in connection with various train ideas. To date, none of the experience with trailers has been especially encouraging. The flexibility in terms of range of mobility is bought at the price of increased towing resistance, increased deadweight and first cost, and increased maintenance.

Mobility Testing of Vehicles in Snow

From all of the foregoing, some direct experience with vehicle testing in soils and snow, and discussions with the authors' associate, C. W. Wilson, who has conducted many such tests over the past twelve years, some brief and for the most part obvious comments on test procedure may be made. First, it is apparent from the diversity of objectives in vehicle testing which were discussed earlier, that no one practical test regimen can be set up that will accomplish all possible objectives. It is therefore important that the objectives be clearly set forth in detail, and the distinctions between tests on production prototypes and on apparatuses for the demonstration and/or exploration of new concepts be recognized. It is important also to distinguish between a vehicle whose function and probable mode of employment can be stated, and one for which the problem is first to find how it might be useful. It has been evident in some of the material reviewed that inflexible test and evaluation viewpoints, 50 which treat every machine as a prototype to fit a (prematurely) standardized problem, overlook many important potentialities where new technical ideas are concerned. Over the past few years the steady-state drawbar pull-slip test has become nearly a standard one for mobility determinations. Exper- ience has shown that a number of similar vehicles of different mobility (as measured by the least trafficable soil or snow in which they retain any mobility) can be accurately ranked for a given type of terrain by drawbar pull-slip determinations in the same general, but less than criti- cally untrafficable, conditions. The results, expressed as fractions of the gross vehicle weights, show the "margins" by which the various vehicles negotiate the test course. The shape of a given curve gives some idea as to the stability of load-slip relationships, which has been observed to parallel field operations. (The progress of a wheeled vehicle up a sand slope, for example, on which at one moment it is operating with very nearly zero slip and the next is immobilized at 100% slip, is reflected in the negative slope of the usual drawbar-slip curve for pneumatic-tired vehicles at low slips). Moreover, the pull-slip test procedure, when properly followed, gives results practically free of inertia effects and the almost imperceptible differences in driving techniques known to make the difference between "go" and "no-go" in critical conditions. As is the case with wheeled vehicles in sand, some snow-vehicles such as the Sno-Cat may bear careful testing at very low slips in the neighborhood of zero. This last is difficult to do in the field, and has not been done in a fully acceptable manner to date. The effort to do so will be well repaid in some cases, however. A further important part of any quantitative mobility test data consists of accurate measurements of the sinkage and trim of the test vehicle under steady-state conditions. All mobility data must be complemented by as complete snow data as is possible. As a very minimum the depth of snow should be characterized by profiles of grain nature, hardness and wetness according to the simplified Field Classification proposed by SIPRE in December 1952 (205). More precise descriptive determinations and measurements of mechanical 51 properties (with methods used carefully described until such time as standardized procedure and apparatus are available) should be made where possible. The need for tests to measure the Bernstein compaction parameters "k" and "n," the uniaxial compression parameters "K" and "N," and shear-normal stress-strain characteristics has already been discussed. Until some yet undeterminable time when the mechanical properties of snow shall be uniformly measurable and interpretable, it is also im- perative that a control vehicle be included in any vehicle field test program, to be retested on frequent occasions throughout the program as a practical check on the uniformity (area and time-wise) of conditions, and as a common denominator between tests at different times and places. Lack of such information throughout most of the vehicular test data reviewed made most of it of limited value. The Weasel has been the most widely tested vehicle in soils and snow for the past ten years, and has, therefore, been adopted as a control in several recent test series. It is the authors' suggestion that it be continued in this role. It cannot be over- emphasized, however, that if a Weasel is unavailable, some other vehicle (preferably one which has been extensively tested at other places).be used as a control. Any control is better than none. With the Weasel now obsolete, it might be wise (to preserve a measure of continuity with the most pertinent past work) for each agency involved with this type of work to obtain and carefully preserve one specifically for this purpose. While the drawbar-pull tests have been run mostly in virgin conditions (usually with special care to find or obtain conditions of homo- geniety for depths in excess of expected vehicle sinkage), they may be also run in distinctly stratified materials (although interpretation of results will be greatly complicated) or in broken trails, etc. In the case of operation on a trail, pulls at very low slips will have more practical meaning than those at high slips. These may be obtained by holding drawbar loads constant at each of several levels and measuring corresponding slips. (At high slips it is necessary to use the reverse procedure; i.e., hold a constant slip and measure drawbar pull.) Two methods for studying the displacement of anow under load which are particularly applicable to vehicle-snow studies have been employed by 52

Nakaya for ski studies (115), but only sporadically attempted in the field since (139). They are both essentially simple, and deserve wide use in future vehicle-snow tests. These are Nakaya's soot-plane and "roaring bonfire" techniques, by which snow displacements in vertical planes along and normal to the direction of travel of an object over and through the snow, respectively, can be revealed following a run. The soot-plane procedure involves merely the placing of soot in a vertical slit normal to the intended direction of the test run and longitudinally sectioning afterwards; obviously a simple technique. The "roaring bonfire" technique is more subtle. Nakaya discovered that even the most apparently homogeneous snow layer is minutely stratified (by minor variations in wind direction and velocity at time of deposit, snowflake history from moment to moment during its fall, etc.) and that this stratification is revealed by differences in mean effective capillarity, and perhaps reflectivity, which can be made visible by exposing a plane (vertical), in situ wall to the effects of a roaring bonfire. Nakaya was able, in ski tests, to trace by this method (the test wall of his snow pit being selected normal to the track left by the ski) the disturbance in the snow structure caused by the ski's passage. No complicated apparatus or preparation of the test site prior to test is required. (SIPRE is planning to do much the same job by means of a field X-ray laboratory (132).) In closing this section, one self-evident item will be briefly mentioned. Every vehicle-snow project should, for the sake of both logic and economy, be coordinated with all similar activities, at least throughout the U. S. and Canadian governments. Many establishments and persons have something to contribute, both in the planning and in the detailed conduct of the work. Use of the talents of qualified people tb the largest extent possible, regardless of inter-service rivalries, is the stated pol- icy of the Department of Defense (and may function at the highest levels). The same approach at a lower level to vehicle-snow work in the next few years could do much to assure the funds, personnel and continuity to the work which it requires and deserves. 53

An Approach to Trafficability Prediction in Snow Terrains

It seems apparent from the preceding material that the objectives and viewpoints of operational trafficability studies in snow terrains should be somewhat different from those which have been rational and reasonably successful in soils trafficability prediction, and have thus far been carried over bodily into the pilot phases of the snow program. The validity of the repetitive traffic viewpoint in wide-open terrain, for example, needs re-examination. It is quite possible that in such areas, snow that will bear perhaps only ten passes in a given lane should be accounted fully trafficable for most operational purposes. Also, in many types of terrain, trafficability after the first pass will be much higher than that of the virgin snow cover. In such areas, means for establishing the first-pass trafficability, or the degree of mobility in a vehicle (expressed, perhaps, simply as a specific vehicle) necessary for it to get through on the first pass, may be required. Considerable experience with hardness testers (very similar to the cone penetrometer) in snow has indicated that many types of snow giving identical readings vary widely as to vehicular support and traction properties (because of differing behavior under compactive loads). The picture, however, appears far from hopeless. First, because snow under continuously freezing conditions is always found in stratified deposits, and because it undergoes its initial "settling" within a matter of days, the depth of surface snows to a well bonded layer is probably, for vehicle purposes, the most critical snow property in Arctic terrain. In the treeline areas, few, if any vehicles can stay on top of the new snow or depth hoar to which it often changes, neither of which have strength enough to register on any but low-scale penetrometers. The facts of interest for vehicles in these circumstances are that the material is (say) depth hoar, lies in a depth of "X" inches, and is underlain by muskeg, or ice, or something else, frozen to a depth of "Y" inches. These facts can readily be developed with a penetrometer type of device. Again, in a rotten snow, the fact that it is rotten snow to a depth of "X" inches with a well bonded layer (cone index cited) of a depth "L"' inches beneath should describe the one-pass trafficability of the terrain for most operational purposes. Multipass trafficability, where of interest, may be a problem simply of knowing the weak snow type, and perhaps its temperature or moisture content, and/or the properties of the frozen soil or ice which forms the ultimate load-carrying base in many situations. (The "remolding" concept is another that will require re-evaluation in relation to snow.) The instrumentation for prediction and the procedure might again turn out to be simple. It is the authors' opinion that a penetrometer of some sort, properly supplemented by other simple instruments and backed by a large accumulation of empirical data of the type that has made soil prediction techniques a relative success, can be made to do the operational trafficability prediction job in snow-terrains. It is hoped, however, that a major vehicle test program will not be embarked upon without in- viting the participation, in both its planning and execution, of all agencies interested in snow vehicles so that the effort involved may have as broad a usefulness as possible.

Some Assorted Projects Needed

Before concluding this discourse, several suggestions for areas in need of careful examination, some already mentioned and some not, will be briefly charted.

In the field of snow science, more emphasis must be placed on measuring the dynamic properties of snow, and upon the accumulation of data relating quantitative descriptive characteristics and mechanical properties. It is possible that a specially instrumented snow survey vehicle, one which, at the outset at least, surveyed the snow in obvious vehicular terms as well as in the usual descriptive terms, might be a useful approach to these problems. Study of the "collapse" mechanics of snow, particularly under vehicle-type loadings, should be begun, and the numerous highly pertinent suggestions of Barrett (amply 55

covered earlier) thoroughly investigated. On the development side, Bekker's spaced-link track should be incorporated into an operational Arctic tractor, and Kamm's wedge tracks given a careful trial on a vehicle, in a manner determined in consultation with him. If it is not too late, some part of the Sno- Cat "improvement" program should be re-oriented along more rational lines. The many possible operational concepts for transport in snow- terrains must be studied to the extent possible with present data, in order to define the objectives of future exercises, design and development more clearly. It is likely that much of the type of information that will be required for this can only be obtained within reasonable time and fund limits by laboratory means. The possibility of adapting scale-model techniques to snow for this purpose, perhaps in a portable, out-door rig, should be investigated.

"Where Do We Go from Here?"

In winding up a review of this nature it is usual to list some conclusions and recommendations. In terms of the objective of this report the conclusions are clear: 1) The art of relating snow conditions to vehicle performance is young but robust. 2) There are several vehicles and vehicle concepts extant capable of providing good over-snow transport, but it is time the requirements were spelled out more clearly, so that the proper techniques and concepts can be nourished. Numerous detailed suggestions have been made throughout the preceding pages that are thought to constitute recommendations for "new approaches" to vehicle design and "revisions or additions" to test procedures and techniques. These will not be recatalogued here. The most important can be summarized and interpreted rather simply, however. Vehicle-snow interaction is a large (but by no means overwhelming) 56 problem, which will require for its practical solution the close cooperation of those working in vehicle trafficability and mobility, in the snow sciences, and in automotive engineering. Attempts to solve it on the basis of any one viewpoint alone are not only unlikely to succeed, but will, in the process of failing, squander a part of the definitely limited time, funds, and talent that can be effectively brought to bear on the problem. There must, therefore, be closer co-ordination of all the efforts in the field, both at the planning and the execution stages. Proper co-ordination of the work for the next few years could make it highly significant. Alternatively, continued repetition of massive, costly, and poorly thought-out field programs, of the construction of full-size experimental vehicles before their technical basis is even approximately understood, and of tests of all sorts without reference to the activities, needs, or recommendations of other interested and competent parties, can only result in more and bigger effort "without commensurate results." 57

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