Prevention of Landslides

Chapter Seven Prevention of Landslides

Arthur W. Root

Preceding chapters. have been devoted number of years, rather than of cen- to the nature, classification, recognition, turies, might be considered as either and investigation of landslides, all of preventive or correctional. which are of only academic interest un- Any attempt to classify an existing less they are utilized in the prevention slide according to age or degree of quies- or correction of landslides. On the other cence would be confusing. Accordingly, hand, a knowledge and understanding prevention of landslides as discussed in of these subjects will be of invaluable this chapter will apply not only to un- assistance in the selection and design of stable areas and potential landslides, but the most economical and effective meth- also will include all existing landslides ods of preventing or correcting land- which might be disturbed or reactivated slides, which should be the ultimate ob- by proposed construction, either by im- jective of the reader for whom this book posing additional load or by excavation. is primarily intended. The category of slide correction, treated There is no sharp line of demarcation in Chapter Eight, then includes all land- between prevention and control or cor- slides which develop during or subse- rection of landslides; the basic prin- quent to construction. ciples governing them are the same, and As would be expected, most of the many of the general methods of treat- treatment methods for the prevention ment are similar. However, there are of landslides are also used for correction significant differences which justify sep- or control purposes. On the other hand, arate chapters on the two phases of slide some of the. corrective measures are sel- treatment, even though this results in dom if ever applied as preventive treat- some duplication or repetition. ment. Table 4 is a summary of the more The treatment of potential landslides, common methods of treatment for both where there is no evidence of any previ- correction and prevention of landslides. ous slide movement, would clearly be For convenience of reference the numer- preventive in nature. Likewise, there ous methods of treatment have been would be little doubt that treatment of listed under four general types, with a landslides developing during or subse- fifth category for miscellaneous meth- quent to construction should be classi- ods, most of which are used less fre- fied as corrective in nature. Where old quently. In this table there is no refer- landslides are involved, however, treat- ence to the cause of the landslides. This ment might be considered as either pre- omission is feasible only because it is not ventive or correctional - if the landslide always essential to know, the cause or is geologically old and has been quiescent causes of a landslide as such in order to for centuries, treatment could scarcely prescribe treatment. Frequently there be classed as correctional; on the other is no one single cause for a land move- hand, treatment of old landslides which ment, but a combination of two or sev- apparently have been inactive for a eral contributing factors.

113 TABLE 4 - SUMMARY OF METHODS FOR PREVENTION AND CORRECTION OF LANDSLIDES

Effect on Method of Treatment General Use Frequency of Position of Treatment Best Applications and Limitations Stability of Successful-Use' on Landslidez Landslide

Pre- I Cor- yen- rec- Fall Slide Flow tion tion

Not I. Avoidance methods: affected A. Relocation ,, , 2 2 2 Outside slide limits Most positive method if alternate location economical B. Bridging x x 3 3 3 Outside slide limits Primary highway applications for steep, hill- side locations affecting short sections (par- allel to c/L)

Reduces II. Excavation: shearing stresses A. Removal of head x x N 1 N Top and head Deep masses of cohesive material B. Flattening of slopes , x 1 1 i Above road or structure Bedrock; also extensive masses of cohesive C. Benching of slopes x x 1 1 1 Above road or structure material where little material is removed at toe D. Removal of all unstable x x 2 2 2 Entire slide Relatively small shallow masses of moving material material

Reduces shear- III. Drainage: ing stresses and increases A. Surface: shear - resistance 1. Surface ditches x x 1 1 i Above crown Essential for all types 2. Slope treatment x x 3 s a Surface of moving mass Rock facing or pervious blanket to control seepage 3. Regrading surface x x 1 1 1 Surface of moving mass Beneficial for all types 4. Sealing cj'acks x x 2 2 2 Entire, crown to toe Beneficial for all types 5. Sealing joint planes x x 3 3 N Entire, crown to toe Applicable to rock formations and fissures

B. Subdrainage: Located to intercept 1. Horizontal drains x x N 2 Deep extensive soil mass where ground water 2 - and remove subsurface exists 2. Drainage trenches x x N water - 1 3 Relatively shallow soil mass with ground - water present 3. Tunnels x X N 3 N Deep extensive soil mass with some permeability x x N 4. Vertical drain wells 3 3 Deep slide mass, ground water in various strata or lenses 5. Continuous siphon X X N 2 3 Used principally as outlet for trenches or ,frai nll, Increases IV. Restraining Structures: shearing resistance A. Buttresses at foot: I , - Rock fill x x N 1 1 Toe and foot Bedrock or firm soil at reasonable depth Earth fill x x N 1 1 Toe and foot Counterweight at toe provides additional resistance B. Cribs or retaining walls x x 3 3 3 Foot Relatively small moving mass or where removal of support is negligible C. Piling:

Fixed at slip surface x N 3 N Foot Shearing resistance at slip surface increased Not fixed at slip sur- x N 3 N Foot by force required to shear or bend piles face

D. Dowels in rock x x 3 3 N Above road or structure Rock layers fixed together with dowels

E. Tie-rodding slopes x x 3 3 N Above road or structure Weak slope retained by barrier, which in turn is anchored to solid formation

Primarily V. Miscellaneous Methods: increases shearing A. Hardening of slide mass: resistance 1. Cementation or chemi cal treatment

At foot x 3 3 3 Toe and foot Non-cohesive soils Entire slide mass x N 3 N Entire slide mass Non-cohesive soils 2. Freezing x N 3 3 Entire To prevent movement temporarily in relatively large moving mass 3. Electro-osmosis x N 3 3 Entire Effects hardening of soil by reducing moisture content - B. Blasting x N 3 N Lower half of landslide Relatively shallow cohesive mass underlain by - bedrock Slip surface disrupted; blasting may also per- - mit water to drain Out of slide mass C. Partial removal of slide - - N N N Foot and toe Temporary expedient only; usually decreases at toe . stability of slide

1 = frequently; 2 = occasionally; 3 = rarely; N = not considered applicable. Relative to moving or potentially moving mass. 3 Exclusive of drainage methods. 116 LANDSLIDES

Moreover, the methods listed in Table Such slipouts usually occur where the 4 and described in this and the follow- roadbed is partially on embankment, and ing chapter can only be applied success- typically do not extend above roadway fully if the nature and history of the grade. However, if the surface of rup- slide are thoroughly understood; whether ture is deep and the highway is on side- or not such understanding is translated hill, cut and fill section, the head of the back into terms of the causes of the slipout may be within the cut slope above slide is immaterial to solution of the the road. problem. All of the landslide treatments In the entire field of landslide preven- which improve the stability of an action and control, no other type of land- tive or potential landslide mass do so slide presents such a challenge to the either by reducing the activating forces soil engineer and geologist, or affords which tend to induce the movement, or such an opportunity for effecting sav- by increasing the shearing resistance or ings in cost. Even though much less other forces that resist the movement. It spectacular than the large landslides in is apparent, therefore, that any treat- slopes above roadway grade, the slipout ment which accomplishes either of these of a large embankment is difficult and two effects will be of some benefit in pre- costly to correct. Often a nominal ex- venting or minimizing landslide move- penditure for treatment during construc- ment. For any particular landslide, how- tion would prevent the subsequent oc- ever, not all types of treatment will be currence of a slipout which might seri- equally effective or economical. The se- ously impair the usefulness of the high- lection of the best method of treatment way and cost tens of thousands of dol- is an engineering problem, requiring lars to correct. the evaluation of many factors which This chapter considers only those em- will be discussed later in this chapter. bankment slipouts in which the surface The prevention of landslides is, in of rupture is wholly or partially in orig- many respects, more difficult than cor- inal ground beneath the fill. Embank- rection, from the standpoint of both ments may fail within themselves due to analysis and design. The limits, type and improper slope design, poor compaction, depth of an existing active slide can or similar causes. Although these fail- usually be determined by exploration ures are true landslides, according to the and investigation; in contrast, the pre- definition used in this book, embankment vention of an incipient or potential land- slope failures above natural ground are slide requires: first, recognition of the not discussed here. Similarly, embank- hazard, which may not be at all evident ments placed on level terrain - that fail from superficial examination; second, an- solely because of displacement of weak ticipation of the character and magni- foundation soil -are not treated here. tude of movement which may occur; It will be noted that throughout this and third, design of suitable treatment book the landslides most frequently cited which will prevent any land movement or discussed are on highways. This is during or following the proposed con- appropriate, not for the reason that the struction. Perhaps a fourth requirement writers are principally highway engi- should be added - decision by those in neers, but because the field of highway control that the hazard is sufficiently real engineering will derive the greatest to justify the expense of treatment. benefit from the application of sound One type of landslide, because it is so engineering to the problem of landslide prevalent and costly to correct, is par- prevention and correction. Most of the ticularly troublesome to highway engi- references to highways would apply also neers; this is the roadway "slipout," a to railroads; however, the mileage of landslide which occurs at or below road- new railroad construction is negligible way grade, with a portion or all of the compared to roads, and the problem of roadbed moving downward and outward. the railroads is primarily control and PREVENTION 117 correction of landslides on existing road- tations of large-scale land movement. It beds. In the design and construction of is seldom that any large-scale landslide, dams and similar large structures tho- however old, does not leave some telltale rough investigation of landslides is more evidence which can be detected by an common practice than is the case with engineer trained to look for the proper highway construction, hence it can be features. Many old landslides can be most presumed that most of the facts con- readily recognized by the proper inter- tained here are well known to the engi- pretation of aerial photOgraphs, as de- neers in those fields. scribed in Chapter Five. Once the slide area is identified by this or other means, Recognition of Existing Landslides a detailed ground study will commonly reveal further evidence of previous land Recognition and investigation of un- movement. Field methods for recogniz- stable areas and the design of preven- ing and identifying old landslides are tive treatment should be considered as described in Chapter Four. essential phases of the preliminary plan- Having identified an existing land- ning and design of any project on which slide, active or latent, the engineer can the proposed construction might induce then determine whether avoidance is eco- land movements. It must be remembered nomically practicable; if the slide can- that landslides may be caused by two not be avoided the necessary investiga- general types of construction activities: tion can be made to determine the ex- (a) the imposing of additional load, such tent and nature of preventive treatment as by embankments, dams, or other required. structures; and (b) the changing of existing ground slope by excavation, ero- Investigation sion, or other causes. It is true that landslides may occur where the existing Recognition, classification and inves- ground is undisturbed by man; this is tigation of landslides have been discussed evidenced by the numerous landslides in previous chapters. All of the previ- which occur in many areas remote from ously described techniques for detect- any construction activity. The possibil- ing old landslides should be utilized dur- ity of such landslides affecting proposed ing the reconnaissance or preliminary facilities should not be overlooked, par- stages of a project in order to recognize ticularly if the new facility is located on and identify any old landslide, whether or crosses an old landslide. There have active or quiescent. been many instances of residential de- Recognition of existing landslides, al- velopments on old quiescent landslides, though important, is not sufficient. A where various conditions, perhaps un- geologically ancient landslide may now related to the residential construction, be quite stable, so far as being affected have caused the landslide to become ac- by proposed construction. On the other tive, resulting in damage to buildings hand, the excavation or loading involved and structures within the slide area. De- in the construction may induce land velopment and construction over a large movement even where there is no evi- area may obliterate all evidence of the dence of previous landslides. Where pre- original landslide, leaving the purchas- ventive measures are to be applied the er blissfully unaware of any hazard. investigation would, in general, be simi- In highway construction the proposed lar to that described in Chapter Six. In- location may cross an old inactive land- vestigation in connection with landslide slide of such areal extent that it is over- prevention does, however, differ in some looked in the usual routine soil survey. respects from that to be applied to an Or, the engineer may recognize and inactive landslide: unstable areas must treat small local unstable areas withoñt be explored, even though no prior slide realizing that they are merely .manifes- movement is suspected, and a study made 118 LANDSLIDES

of the possible effects of the proposed high-standard highways which comprise construction. If the proposed highway the major portion of the current high- or structure will be located upon or way programs, the engineer can afford across, or may be affected by an old only the best available practice in de- landslide, an analysis must be made to tecting and preventing landslides. The determine whether the slide area will cost of controlling one preventable slip- be stable under the conditions which out or major landslide on a project will will be imposed by the construction. In often more than offset the cost of a both cases the limits of the potential or proper preliminary investigation on the incipient landslide are necessarily un- entire project. It may not be possible known, in contrast to slide correction in- or economical to design a highway to vestigations in which an active slide of preclude the possibility of an occasional definite extent already exists. landslide, but this fact should merely In any area of inherently low stability, emphasize rather than minimize the need especially where slides are known to be for better engineering in the recogni- prevalent, the design of any major tion and treatment of unstable areas. structure should be preceded by tho- The investigation aimed at slide pre- rough investigation. Particularly in the vention should be a cooperative project construction of embankments on steep by the geologist and the soil engineer, or slopes in localities of questionable stabil- be made under the jurisdiction of an ity, each such site should be viewed with engineer who is thoroughly familiar with suspicion and thoroughly explored dur- both of these phases of engineering. ing preliminary stages of the project. To be of greatest value, the geologic Similarly, the design of cut slopes in such and soils studies should be both general areas should be carefully scrutinized; in and specific. That is, a general or re- regions where instability is evidenced by gional knowledge of the soils and geol- land movement on existing routes, ex- ogy in the area under investigation will ploration of all proposed major excava- be helpful in estimating the landslide po- tion may be required. One error which tentials on a specific project, but such is all too prevalent in the investigation knowledge cannot take the place of de- of potential landslides is that of basing tailed studies along the proposed right- the analysis and design on data derived of -way. from shallow borings or test pits. All Much of the desirable general informa- too often the material recorded in shal- tion on geology and soils can be obtained low borings as "solid formation" or "bed- from available maps or from geologists rock" may be merely float rock, boulders, or soil specialists who are familiar with or a thin layer of hard material under- the area. In a given geographic region, lain by a dangerously weak horizon. In landslides tend to be more prevalent in excavation areas the borings should ex- one particular geologic formation or soil tend below the proposed grade; the foun- type than in others. Many of these "bad dation for embankments or other struc- actors" are known in various parts of tures should be explored to whatever the country. The questionnaire upon depth might be affected by the proposed which parts of this volume are based loading. elicited a long list of geologic forma- A common justification for failure to tions that are known to be landslide- make thorough exploration of potential susceptible. The list is not reproduced landslides is that such exploration is too here because it is known to be incom- costly and would result in curtailed plete for some parts of the country, a highway construction. It may be true situation that could lead to a false sense that detailed soil surveys and landslide of security in places. Moreover, it is not investigations are not economically prac- safe to label any entire geologic forma- ticable on certain unimportant roads, but tion as landslide-susceptible; landslides for the freeways, toll roads, and other generally result from a combination of PREVENTION 119 conditions or "causes" and stratigraphic or correction. Familiarity with local con- sequence or rock type alone do not neces- ditions and a background of experience sarily presage land movement. Neverthe- in landslide work will, of course, assist less, if the engineer is familiar with the the engineer in exercising the sound geologic formations and soils in his re- judgment which is essential to the solu- gion that are especially susceptible to tion of the specific problem involved. sliding, investigations can be made to Soils are characteristically nonuniform; determine the presence or absence of the stability of an embankment or exca- other contributing factors. vation is influenced by a great many fac- Methods of making detailed studies tors; and the geology and subsurface of the geology and soils of the construc- water conditions are often complex. Be- tion site itself are described in Chapter cause of these conditions, the analysis Six. Both surficial and bedrock geology of landslides and the design of control should be studied, for many of the most treatment cannot be standardized or rou- troublesome slides are confined to the tine; however, an understanding of the mantle soil or weathered zone. A geolo- types, causes, and mechanics of land- gist trained in the study of surficial ma- slides will make possible the application terials - and in the practical applica- of certain basic principles having gener- tion of his results - can aid the soil al validity. engineer in his application of the theories Prevention of all types of landslides of soil mechanics to the analysis of ac- may be accomplished by one or more of tual or potential slides in earthy ma- the following methods: (a) reduction of terials. In the case of bedrock or rocky activating forces, (b) increasing the formations, of course, the geologic sur- forces resisting movement, and (c) vey may provide the most useful in- avoidance or elimination of the slide. formation obtainable. Reduction in the activating forces can be accomplished by two general methods Analysis - removal of material from the portion of the slide which provides the driving Regardless of how comprehensive or force tending to cause movement, and thorough the investigation and explora- subdrainage to eliminate hydrostatic tion may be, the utilization of the data pressure and/or to diminish the weight thus obtained depends on proper inter- of the soil mass by reducing moisture pretation and analysis of those data. content. However, the stabilizing effect Methods of analyzing landslides and de- of subdrainage is generally due primar- termining the effect of control treat- ily to increasing the shear resistance ments are described in detail in Chapter rather than by reduction of the motivat- Nine. Practical applications of analytical ing forces. methods have also been described by Ba- There are a great many methods for ker (1952). In many cases the area be- increasing the forces resisting slide ing investigated is not amenable to the movement, including the following: sub- classical, theoretical methods of analysis; drainage, in order to increase the shear nevertheless, application of the princi- resistance of the soil; elimination of ples of soil mechanics usually makes pos- weak zones or potential surfaces of rup- sible a rational comparison of various ture by stripping or by breaking up or treatments, even though the absolute benching of smooth sloping surfaces; stability cannot be accurately computed. construction of restraining structures Correct interpretation of the geologi- such as piles, walls, cribs, or toe support cal, geophysical, boring and test data fills; and solidification of loose granular derived from the investigation of a po- material by chemical treatment. tential or actual slide area constitutes The most obvious and sometimes the the most difficult phase of the engineer- most economical, but often overlooked, ing pertaining to landslide prevention method of preventing landslides is ly 120 LANDSLIDES avoidance. Methods by which this may structure. In some cases a revision in be accomplished include: relocation of the grade line of a proposed highway the proposed highway or structure to may be equally effective in preventing avoid unstable terrain, complete removal slide movement. For example, where the of an existing slide; or bridging the un- most desirable grade line for new high- stable area. - way construction requires excavation and undercutting of an unstable slope, Evaluation and Comparison of it may be possible to adjust the profile Various Treatments grade of the road so as to avoid any excavation at the toe of the hill, and AVOIDANCE OF POTENTIAL SLIDES instead to provide additional support by construction of an embankment which It is not often feasible to avoid a po- will act as a toe support or "strut." tential slide by changing the location of If there is no way to avoid a potential a proposed highway or structure, but slide and if preventive treatment will not the possibility should not be overlooked. assure stability, it is sometimes neces- In some cases the highway can be shifted sary to construct a bridge across the un- into stable ground by a slight change in stable area. The cost of a bridge is alignment. Even though it may not be usually prohibitive, however, and ex- feasible to avoid an old landslide or an treme care must be exercised to design unstable area completely, it may be pos- a structure which will not itself be dam- sible to so locate the highway that the aged by moderate slide movement. slide area is crossed at the safest loca- Bridging may be done in conjunction tion, and where the construction would with the prevention method listed under be least likely to induce further slide II-D in Table 4, "Removal of all unstable movement. material." The removal of all or a por- Where the proposed excavation will tion of the unstable material may be cross formations that are susceptible to necessary to protect the structure from bedding plane slides, the slide hazard can damage should slide movement occur. sometimes be reduced by adjusting the Figure 63 shows a bridge constructed alignment so that the cut slopes will in- across an active landslide which would tercept the beds at a more favorable not support an embankment and which angle to the bedding planes. In some could not be avoided by change of align- places, for instance, it may be possible ment of the highway. The bridge was to choose the opposite side of a valley so constructed that the superstructure or hill, where the bedding planes of the could be shifted laterally on extended rock will dip into the cut slope rather pile caps, in order that the alignment of than clipping toward the roadway. the bridge could be maintained as the In Chapter Four, there are listed nine substructure moved with the landslide. ways in which proposed construction of By periodically sluicing out the slide cuts or fills may induce landslides. These material above the bridge, the slide factors are not repeated herb, but they movement at the bridge site has been held to a minimum, and the proper posi- should be kept in mind when evaluating tion of the bridge superstructure has the probable effects of the new construc- been maintained. tion; recognition and consideration of A more common application of the these factors are essential in determining bridging method of slide prevention is whether an attempt should be made to the sidehill viaduct. On a sidehill cut forestall a potential landslide by avoid- and fill section with a steep transverse ance. slope the terrain below the highway Prevention of landslides by avoidance grade may be too unstable to support the does not necessarily require a change in heavy embankment required. In such a alignment or location of a highway or case it is sometimes more economical to PREVENTION 121

p •? 4 3 , £

-. 4* -Y• 7i —1 . ., - / - Tit

Figure 63. Landslide avoidance by bridging. Bridge on piling constructed across font of active landslide near Hopland, Calif. Bridge incorporates provision for realigning superstrurturc if further sliding should cause shifting of piles. (Photograph courtesy of California Division of llighways)

support the outer half of the roadway al), thereby reducing the embankment on a viaduct, rather than to stabilize the load sufficiently to provide a satisfactory foundation to support an embankment. factor of safety against sliding of the Figure 64 illustrates the sidehill via- embankment. If the engineer has an duct type of bridging. understanding of the nature and me- In addition to such actual avoidance chanics of landslides, there will be less methods, many precautions may be ob- likelihood of necessary design consider- served which will minimize the possibil- ations being overlooked. ity of land movement as a result of the In deciding whether to avoid an un- proposed construction. Many of these stable area or to adopt preventive treat- precautionary measures are phases of ment. an economic comparison of the al- design which should be considered when- ternate locations will often supply the ever a structure is proposed in a lo- answer. Such cost comparisons should, cation where ground movement might however, consider the total cost rather occur. Use of lightweight embankment than the cost of construction only. Prop- material might be mentioned as an cx- er consideration should be given to such ample of a design method to l)reVeflt factors as maintenance costs, probable landslides. If tests of the foundation service efficiency of the facility, and pos- soil in a proposed embankment area in- sible interruption in service or structural dicate that the soil will not support the damage by land movement. It is true load with the desired factor of safety, it that an accurate appraisal of the last is sometimes possible to substitute light- factor may be difficult; nevertheless, a weight embankment material (such as rational comparison of alternates is im- cinders, volcanic tuff, or similar materi- possible without consideration of the 122 LANDSLIDES

_.j

FIgure 64. Landslide avoidance by bridging near Santa Cruz, Calif. Sidehill viaduct constructed across short unstable area (Photograph by Bruce Ott. courtesy of California Division of Highways)

risks involved. A choice between alter- EXCAVATION nate locations based only on construction costs may be fallacious; frequently, Preventive measures in excavation a large initial investment may be most areas consist primarily of proper slope economical if the total costs for the life design and drainage. It is usually more of the structure are considered. In l)ac- economical to design the excavation with tice, however, the method of financing a slopes which will minimize sliding, rath- project may be such that immediate er than to excavate steep slopes and then availability of funds will control the de- flatten the slopes after sliding has oc- sign. As a result, the selection of the curred. This is especially true if the most economical alternate may not be slides are of the slump type; for the re- feasible, and the adoption of a less safe worked soil may have only a fraction of or less economical design with lower '' the strength of the in-place soil. For ex- tial cost must be accepted as a compro- ample, a material which would be stable mise. In such a case, the engineer in charge if excavated on 2:1 (2 horizontal: 1 ver- would probably be well-advised to see tical) slopes may, after sliding has oc- that a complete record of the investiga- curred on a steeper slope, require 4:1 tion and predictions, as well as the rea- slopes to prevent further sliding. SOnS for the compromise, be placed on In dealing with a homogeneous soil, permanent file. the strength of which can be determined PREVENTION 123

Figure 6. Multiple benching of cut slope to prevent landslides by unloading, by providing cntchment areas for debris and by surface drainage. Inclined benches are used here to provide roadways for construction and maintenance equipment. I Photograph courtesy of l'ennsvlvania Department of Highways)

with reasonable reliability by laboratory soils engineer can estimate the improve- tests, the slopes required for stable cuts ment in stability effected by flattening can be computed with considerable ac- the proposed cut slope or by removing curacy by applying the theories of soil material which might induce slide move- mechanics (Chapter Nine). It is seldom, ment. Such analyses are helpful, even however, that these ideal conditions pre- where the true shear strength.- of the vail - homogeneity of large soil masses soil cannot be accurately determined by is rare, and the effective average laboratory tests. Obviously, these analy- strength of rocks can seldom be deter- ses are impossible without it knowledge mined by borings and laboratory tests. of the charactei' and strength of the Moreover, it is almost impossible to pre- soil throughout the cut area. Preferably dict accurately what the hydrostatic such information is obtained from bor- pressures will be in the future or at time ings, as well as from geologic or geo- of failure. Nevertheless, a knowledge of physical data. the charactei' of the soil, and of subsur- A study of existing cut slopes of simi- face water conditions, is no less impor- lar material in the region is helpful, but tant because it cannot always be ap- extreme care is required in comparing plied directly. Proper slope design i'e- existing and pioposed cuts. Existing cuts quires that this information be as corn- commonly are much shallower than the plete as possible. With this knowledge, excavation on an improved locationfor and by applying the methods of stability example, 1: 1 slopes in a given formation analysis outlined in Chapter Nine, the might be stable for a height of 50 ft. but 121 LANDSLIDES

Figure 66. Prevention of landslides by flattening cut slopes near Waldo. Calif. Top of lint cut slope is at skyline in left background. Note bench construction of high embankment in foreground. (Photograph by Bruce Utt. courtesy of California Division of Highwaynl the same slope might be much too steep tudinal drainage grade, and with suit- for a 150-ft cut. Another pitfall to le able catch basins and flumes or l)il)es to avoided in basing slope (lesign on exist- carry the water down the slopes. Pav- ing slopes is the assumption that the ing of the gutters or ditches may be soils and rocks, as well as the ground necessary to reduce erosion or to pre- water conditions, will be identical in vent percolation of water into pervious the proposed cut area as in an existing areas on the benches. The benches serve cut. Even though the distance between two purposes: to intercept and remove the two is small, conditions may be quite surface water or seepage from the cut ci issimilar. With proper consideration of face; and to prevent rocks, debris or these factors, the study of existing slopes sloughed material from falling on the can be a valuable guide, but slope de- roadway. The benches should be so con- sign should be the responsibility of the structed that they are accessible to soil engineer or geologist rather than maintenance equipment subsequent to the locating engineer. A background of construction, in order that any small experience in the same region and famil- slides may be removed and the drain- iarity with local conditions are always age system may be properly maintained. helpful to the geologist or soil engineer. Figure 65 illustrates the construction of In general, cut slopes constructed with benched cut slopes; Figures 66 and 104 benches or "berms" are considered pref- illustrate slope flattening. erable to equivalent uniform straight The first type of excavation listed in slopes. The benches should be constructed Table 4 - "A. Removal of head" -- ap- with a V or gutter section, with a longi- plies only to treatment of an existing PREVENTION 125

landslide; the fourth one - "D. Removal Economic COnsiderations of all unstable material" - would usually be practicable only in the case of an In the design of cut slopes to pre- existing slide. However, these two tech- vent landslides, economic considerations niques have also been applied in some cannot be disregarded. If failure of a parts of the country for controlling po- structure might result in loss of life tential slides in talus material. In the and irreparable damage, as in the case case of an existing landslide, removal of a dam or bridge, a high factor of of material from the head of the slide safety is warranted, and may indeed be reduces the activating force, and thus essential. It is seldom economical to de- has a stabilizing effect. But such unload- sign cut slopes sufficiently flat to pre- ing is usually proposed only where the clude the possibility of landslides, and it slide is to be undercut at or near the is often better to remove or correct a toe. The amount of unloading at the few slides during construction or as a head should be sufficient to compensate maintenance operation than to design for any reduction in support caused by with excessively flat slopes. Many sec- excavation elsewhere. Here again, the ondary highways that traverse rough application of the principles of soil me- terrain could not be constructed with chanics, as described in Chapter Nine, available funds except by accepting some will enable the engineer to estimate the risk of landslides. effects of the proposed excavation, and The fact that a calculated risk of slide to design the unloading and cut slopes movement must be accepted at times is, to provide the required stability. In cut however, no justification for. lack of areas the removal of all unstable ma- thorough investigation and adoption of terial is usually not necessary, and is all economical means of slide prevention. seldom economical except for very small With adequate information on geology, masses. Even this type of treatment re- soils and ground water conditions in a quires sufficient investigation to deter- proposed cut area, the probability of mine the depth and areal extent of the landslides can be estimated and the con- weak material. If the yardage involved sequences of possible slides •appraised; is small it may be desirable to remove only after such an evaluation can the all of the weak material; otherwise, the most economical design be selected. Any strength of the weak soil should be de- risk should be a calculated one, rather termined, and the cut slopes designed to than a mere gamble arising from lack provide stable excavation. of investigation and analysis. Where embankment is to be constructed over an old landslide area or DRAINAGE other material having inadequate strength to support the proposed load- Surface Drainage ing, removal of the weak material should be considered. If the weak soil layer is Every precaution should be taken to only a few feet thick, stripping of the prevent surface runoff water from en- weak material is usually more economical tering a potentially unstable area. Any than other methods of treatment. If sags, depressions or ponds above the there is evidence of seepage - and the unstable condition is often caused by slope line of either an enbankment or a ground water - suitable drainage should cut should be drained to minimize the be provided before placing the embank- possibility of surface water percolating ment. A blanket of pervious material, into a weak or unstable area. If the new together with necessary underdrains, construction crosses an old landslide its may be required to prevent reduction of surface should be reshaped as necessary shear strength and/or development of to provide good surface drainage, but pore pressure due to subsurface water. unnecessary removal of vegetation 126 LANDSLIDES should be avoided lest excessive erosion prevention and correction of both em- may occur. Sealing of all surface cracks bankment slipouts and landslides in ex- in any type of slide will be of benefit, cavation areas, the differences in meth- both by preventing entrance of surface ods are considered of sufficient impor- water into the slide mass and by re- tance to justify separate discussion of ducing frost action in areas subject to subdrainage treatments applied to these freezing and thawing. two general types of landslides. Although surface drainage alone will Drainage in Embankment Areas. - seldom correct an active landslide, any Slipouts may occur whenever the im- improvement in surface drainage will posed embankment load results in shear be beneficial. In the case of potential stresses that exceed the shear strength landslides, where no movement has oc- of the foundation soil; or where the curred prior to construction, surface construction of the embankment inter- drainage may result in greater returns feres with the natural movement of from the investment than any other ground water, and results in the devel- type of preventive treatment, even opment of pore pressure or hydrostatic though other preventive measures may pressures. Two factors must, therefore, be required in conjunction with the sur- be considered in the investigation of face drainage. Surface runoff or the wa- possible slipouts: weak zones in the ter flowing from springs or seeps should foundation soil, which may be over- never be allowed to drain into or across stressed by the proposed embankment an unstable area or potential landslide. load, and subsurface water, which may Methods of improving surface drainage either result in the development of hy- include reshaping of slopes, construction drostatic pressure or may reduce the of paved ditches, installation of flumes shear strength of the soil sufficiently to or conduits, and paving or bituminous induce slide movement. Careful explora- treatment of slopes. tion will usually reveal these conditions before construction, but the investiga- Subdrainage tor must be of a suspicious and inquisi- tive nature, as there may be no readily If the preliminary investigation re- apparent surface indications of the un- veals the presence of ground water which stable conditions. Some of the methods may induce slide movement, adequate of preventing roadway slipouts are subdrainage should be included in the listed and discussed hereinafter. plans. Such subdrainage is equally im- As previously noted, if a surface lay- portant in cut areas and under proposed er of weak soil is relatively shallow and embankments. The effectiveness and fre- is underlain by stable rock or soil, the quency of use of the various types of most economical treatment is usually drainage treatment vary according to that of stripping and wasting the un- geologic formation and climatic condi- suitable material; as illustrated by Fig- tions; they probably are influenced by ures 67 and 68. If seepage is evident local custom also. It is generally agreed, after stripping or if there is a pos- however, that for the majority of land- sibility that it may develop during wet slides ground water constitutes the most cycles, a layer of pervious material important single contributory cause; and should be placed before the embank- in many areas of the country the most ment is constructed. This may consist generally used successful methods for of clean pit run gravel, free-draining both prevention and correction of land- sand, or other suitable local materials. slides consist entirely or partially of If springs or concentrated flows are en- ground water control. This is especially countered, drain pipe may be required true of the Pacific Coastal region. also. Although most of the types of sub- Where subsurface water or soil of drainage treatment are applicable to the questionable strength is found at such PREVENTION 127

FILL VS

ORIGINAL

UNSTABLE tM AL

OUTLET SCAt, C PROVIDED AT LOW po,sr

Figure 67. stripping as a slide prevention measure. 'lypiral cross-section of Itedwood Highway in Hum- boldt County. Calif.. showing stripping of unstable material before ronstrurting embankment. (I)rasving furnished by C. P. Sweet, eourtes of California Division of Highways)

great depths that stripping is uneco with an undeidrain pipe in the bottom nomical, deep drainage or stabilization then the trench is hacklilled and the em- trenches have been used successfully to bankment constructed. Figtt re 70 ill us- prevent slipouts. SUCh stabilization trates combined use of stripping and trenches are usually excavated with pow- drainage trenches. er equipment with the steepest side If the unstable area is in a natural slopes that will be stable for the mini- draw- or depression and of limited areal mum construction period; they should extent, one trench normal to the cen- extend below any water-bearing layers terline of the toad may be sufficient; in and into firm material. A layer of per- the case of large areas, an extensive sys- vious backfill material is placed on the tem of stabilization trenches may be bottom and side slopes (see Fig. 69), necessary. fl'equent]y in a herringbone

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- 'sq l",gure 68. Stripping wet unstable material before placing embankment near Orick. Calif. Blanket of perV visus tIter material will he spread over stripped area. 3 Photograph courtesy of California Division of II ighways) 128 LANDSLIDES

Figure 09. l'lari;ig titer material in jet p ir:tiitage trench near Oriek, Calif. Filter material being ,lumped over side slope and spread in bottom of drainage trench with dozer. (Photograph courtesy of Caiifornia Division of Highways) pattern. The trenches, in addition to pro- of slipouts has been used successfully viding subdrainage, add considerable on numerous projects. An early example structural strength to the foundation, was reported by Root (1938). On a re- This type of treatment for prevention cent highway construction project 4.9

Typical Gross Section of Drourioge Ditch Ii

Plan of Drainage Trench

a• so. to.

Figure 70. Slide prevention near Willits. Calif. by combination of stripping and drainage trench. Plan and cross-section of preventive treatment consisting of stripping unsuitable soil and constructing drainage trenches. (Sketch furnished by C. I'. Sweet, courtesy of California I)ivlsion of Highways) Apç'ros.aats nrsle sIoo rrtstniur 'DI

Figure 71. l.arge slide in the fall of 1032 northwest of Santa Monica. Calif. The highway was blocked by a sliding mass of 100.000 cubic yards. and a valuable estate was damaged through loss of approximately 100 feet by 200 feet of land. (Individual slides outlined in white, with dates.) Geologic studies indicated that movement began along slickensides in a nearly horizontal 8tratum of clay lying approximately 10 feet above the highway. Two exploratory tunnels were dug to drain water, believed to be lying on top of the clay stratum, and to determine the extent of the slickensides. No free water was encountered. It was decided that the most economical solution was to dry out the clay. Therefore, additional tunnels were drilled and a gas furnace was installed with blowers to circulate hot air. It was estimated that 3,000 lb of water per day were evaporated during the first six months. The furnace was in operation from August 1933 until approximately 1939, by which time movement was negligible. (Photograph by Fairchild Aerial Surveys, Inc.. courtesy of Harry H. Johnson, Consulting Geologist)

130 LANDSLIDES

150

—H,gfrIWo "N IX Pond \ /77oo

Springs j. Sho ft . Tunnel Nc&2

Rock Bottle Springsff Shof!11 £ O,i- Open Cu! I/Shaft Tunnel No. /

Figure 72. Drainage tunnels to prevent landslides. System of drainage tunnels installed during construction of new highway, designated as "Existing State Highway" on sketch, near Crockett, Calif. (Courtesy of California Division of Righways)

mi in length, the installation of stabiliza- drainage tunnels are sometimes Con- tion trenches for slipout prevention re- structed as a preventive measure. The quired 65,000 cu yd of trench excava- use of drainage tunnels was fairly com- tion; 107,000 Cu yd of filter material mon at one time, both by railroads and were placed in drainage trenches and by some highway departments; but at stripped areas; and more than 20,000 present this method is used rather in- liii ft of perforated metal drain pipe frequently, due largely to the relatively were installed. Stabilization trenches of high cost. An elaborate installation of this kind have been constructed as deep drainage tunnels, together with an in- as 40 or 50 ft. Although the cost ingenious hot-air furnace for drying out creases rapidly with depth, this method the soil, was used to control a large of slipout prevention is often more eco- slide near Santa Monica, Calif. (see Fig. nomical than any other type of treat- 71; Hill, 1934). Use of drainage tun- ment which might be equally effective. nels in Oregon has also been described Where the depth to subsurface water (Roads & Streets, 1947). These tunnels, is so great that the cost of stripping or usually about 4 ft by 6 ft in cross-sec- drainage trenches becomes prohibitive, tion, must be excavated by manual meth- drainage tunnels are sometimes used. ods; skilled tunnel workers are not nor- Although originally and more common- mally employed on usual construction ly used as a correctional treatment, projects; and, of course, other methods PREVENTION 131 of treatment which permit the use of length of these drains may be as great construction equipment are likely to be as 200 to 300 ft or more. less costly than the tunnels. Figure 72 The origin of the horizontal drain is shows an installation of drainage tunnels somewhat obscure ; however, much of on a highway project. the early work in developing equipment Horizontal drains have, since their de- and methods was done by the California velopment during the past few years, i)ivision of Highways beginning about supplanted drainage tunnels in many 1939. There are numerous installations cases. As was the case with drainage of such drains in California, as well as in tunnels, they were first installed as a Oregon, \Vash ington, and several other corrective treatment. Although they are states. Equipment and techniques for still used principally for this purpose, installing horizontal drains have been they have been installed at a number of described by Stanton (1948) and by locations as a preventive treatment (see others. An example of the extensive in- Fig. 73). Horizontal drains usually con- stallation of horizontal drains in slide sist of perforated metal pipe, often 2 in. control work is the Ventura Avenue oil in diameter, forced into a predrilled hole field in California. where hundreds of (generally 3 to 4 in. in diameter) at it horizontal drains, totaling more than slight angle to the horizontal; the gradi- 40 mi. in length, have been installed in ent of horizontal drains may range from the large landslides within this oil field 5 to 25 percent (see Fig. 105). The (Mineral Information Service, 1954).

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Figure 73. Horizontal drains used to stabilize cut in bedrock. The drains were placed 50 to 100 feet apart beneath permeable sandstones. One set is at the baso of the Ames shale, beneath the Grafton standstone; the other at the top of a lens of indurated clay in the Saltsburg standstone. Note on skyline that cut slopes range from 1:1 (horizontal :vertical) to 1:1. depending on the character of each layer of rock. Spiliway of Ynughiogheny River reservoir, Pennsylvania and Maryland. (l'hotograph courtesy of Corps of Engineers> 132 LANDSLIDES

Figures 76 and 77 show a system of provided by means of a horizontal drain; horizontal drains that were installed as such an installation is illustrated by a slide prevention measure. Similar in- Figure 74. stallations are frequently used as cor- 2. Vertical drain wells have also been rective treatment (Figs. 73, 104, and installed under embankments to accel- 106). erate the consolidation, through removal If wet areas or seepage zones which of water, of weak compressible founda- cannot be corrected by stripping and tion soil. Such drains, usually 15 to 24 in,. surface drains exist in the foundation in diameter, are drilled or driven through of a proposed fill area, horizontal drains and to the bottom of the saturated, com- are likely to be the most effective means pressible soil layers, then backfilled with of removing subsurface water which coarse sand or other suitable filter ma- might otherwise cause slipout movement. terial. A layer of filter material is placed Preferably, horizontal drains should be over the area in which the vertical drains installed in such a way that they can are installed, with outlets leading be- be inspected and maintained after the yond the embankment slope line. embankment is completed; this may necessitate placing, a tunnel or large Design of this latter type of vertical pipe to provide access to the drains. drain well should be based on labora- Thorough investigation of the unstable tory tests of undisturbed soil samples, area, including test borings, will furnish from which the consolidation and the necessary information from which strength characteristics of the soil are the elevation, gradient, and spacing of determined. the horizontal drains may be determined. The continuous siphon is an ingenious Vertical drain wells for slipout pre- method devised in the State of Wash- vention may be used for two purposes, ington for providing a drainage outlet as follows: for drainage wells or sumps (see Fig. 75). This siphon arrangement can be 1. In conjunction with horizontal used to drain trenches, wells or sumps drains the vertical drain wells may pro- by siphoning instead of installing more vide a drainage path between lenses or costly tunnels, drilled-in pipes, or simi- strata of water-bearing matérial which lar conventional outlet systems, and per- are separated by impervious strata. If mits installation of subdrainage systems installed under an embankment, an out- in areas not having readily accessible let for the vertical drain well can be outlets. This continuous siphon method

- - —srtv -

Figure 74. Slide treatment consisting of horizontal drains and vertical drain wells. This was corrective treatment of an active landslide at San Marcos Pass near Santa Barbara, Calif.; however, similar drainage treatment has been used as a preventive measure. (Courtesy of California Division of Highways) PREVENTION 133

Figure 75. The Washington siphon. This system of vertical collector pipes and siphon arrangement has been successfully used by the State Highway Commission of Washington for lowering the water level and stabilizing landslides. has the usual limitation of depth that is from one or more benches in the cut true of all siphons, but it is very useful slope. Numerous cut slopes drained by where applicable. this method have remained stable in Drainage in Excavation Areas. - All spite of unfavorable soil formations and of the subdrainage methods discussed in the presence of large amounts of sub- connection with slipout prevention could surface water. It should be, emphasized as well be applied to prevention of land- that if the treatment is delayed until slides in excavation areas. Drainage after a landslide has developed, the cost trenches are sometimes installed as in- of correcting the slide is likely to be terceptors of subsurface water above the much greater than the cost of installing limits of the extavations, too often with drainage which would have prevented the indifferent success. There is seldom any sliding. And it is equally important to assurance that such intercepting trenches note that the need for such preventive will effectively cut off all ground water treatment can be anticipated only if ' a which might contribute to slope failure. thorough soil investigation is made be- If deep trenches are required the cost fore designing the project. In most cases frequently becomes prohibitive, consid- test borings are required in addition to ering the probable effectiveness of the geologic studies or superficial inspection. drainage trenches. The most widely used successful meth- RESTRAINING STRUCTURES od of subdrainage for preventing slides in cut slopes is probably the horizontal Retaining Walls and Bulkheads - drain treatment. These horizontal drains are the same as previously described for Crib walls, piling, bulkheads, and other slipout prevention. In excavation areas restraining devices are most commonly the drains are installed as the cut is ex- used as corrective measures after slide cavated (see Figs. 76 and 77), often movement develops, too often with dubi- 134 LANDSLIDES

Figure 76. Horizontal drains for prevention of landslides. Horizontal drains installed from roadway grade during construction near Vallejo. Calif. Note flow of water from outlet of drain in foreground. (Photo. graph by A. D. Hirsch, courtesy of California Division of Highways)

ous success. rrhee structures are more should be recognized. The increased re- likely to be effective if installed as pre- sistance to sliding provided by any of ventive treatment, before the soil mass these restraining structures is some- has become weakened by slide action. The what limited, and is dependent on the limitations of this type of treatment ability of the structure to resist (a)

0 100 200 PT.

Figure 77. Plan of horizontal drains shown in Figure 76. PREVENTION 135

1 I kii

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Figure78. Log crib to prevent sliding near Napa. Calif. Note evidence of old landslides above cribbing and also near right skyline. )I'hotograph by Bruce lit, courtesy of California Division of highways)

shear action, I U) overturning, and (C lateral clearance for a roadbed or struc- sliding on or below the base of the structure. Strictly speaking, such walls or ture. If the forces tending to cause slide cribs (10 not constitute treatment for movement exceed the resisting forces by slide prevention, but are actually a phase only a small amount, construction of of the slope design. The limitations of some type of restraining structure may these structures as slide preventive treat- provide sufficient additional strength to ment should be recognized. produce stability and thus prevent slide Unless the soil is free-draining, or it movement. is known that no subsurface water will Retaining walls and bulkheads of va- ever he present, the design and construc- rious types have been used: Figure 78 tion of any crib wall or retaining wall shows a redwood log crib; a bin-tyne should include adequate provisions for concrete crib wall is illustrated by Fig- drainage, including pervious backfill, ure 79; Figure 80 is an example of the drain pipes, and weep holes. The use of metal bin-type crib wall; and Figure 81 retaining structure-, is one of the earli- shows a rubble masonry retaining wall est methods used for controlling land- installed to prevent a landslide. A unique slides, but the results of this method, at restraining structure is shown in Fig- least in the earlier attempts, were not ure 82, a modified slope paving as slide encouraging. In 1928. Ladd reported nu- prevention; Figure 83 illustrates the merous failures of retaining walls and use of coarse rock for slope paving. Fig- suggested use of other methods, particu- ore 84 shows a masonry wall used to sup- larly drainage (LacIcI. 1928). Figures 85 port overhanging rocks. and 86 show striking examples of un- The principal use of crib walls or re- successful attempts to correct landslides taining walls is at the toe of an embank- by means of piles and bulkheads. These ment slope where the normal fill slope landslides were subsequently controlled would not "catch," or at the toe of a cut by extensive subdrainage installations slope which must be undercut to provide and by other means. 136 LANDSLII)ES

Figure 7t1. Restraining structure; concrete crib wall installed during construction to prevent land move. ment which might jeopardize dwellings located above top of cut slope at Arcnta. Calif. Note gravel back. fill. (Photograph by T. W. Smith, courtesy of California Division of Highways)

l'igurt .(.. It-,-tr:tin,ing sirtiettire flu-tat ent, ,satI near Snr,ta (ru,. ( uhf. I Phot— graph courtesy of California Division of ltighways) PREVENTION 137

- __'i'•..' - —,

Fjgure SI. Rubble masonry retaining a all installed to prevent land nun ement in old landslide area near Brisbane. Calif. Photograph by Bruce Utt, courtesy of California l)ivision of Highways)

Figure 82. Hestraining structure; concrete slope paving placed monolithically with an underlying grid of reinforced concrete beams, to prevent slide movement of unstable cut slope near Valona, Calif. (Photograph by E. W. flerlinger. courtesy of California flivision of Highways)

138 LANDSLIDES

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- - 1 5, - . ., , ..'. •. .. '- c' -- ...,.t,. '' :1 .. f'4.&,,...... J' .

Figure s:t. Coarse slope paving as used along Route 2, Sheiburne tails, Mass. the batik to he retained is composed of tine sand and silt, probably 50 percent of it less than 200 mesh. The paving, 1 1/2 or more feet thick, is placed on a shaped bed . .tccording to specifications of Ma,.saehusetts l)epartment of I'ublic Works, "stone for slope paving shall consist of field stones, boulders, quarry stone, or rock fragments. The stone shall have at least one reasonably flat face and a thickness perpendicular to the face of not less than 6 ineies. .t least 75 percent of the stone shall he 2 cubic feet or more in volume." Because rock was available from a nearby subgrade excavation the job shown here cost about $9 per square yard usual costs are $12 to $15 per square %ard. (I'hi,tograpii b) ('. B. Tuttle, 37. S. (ieological Survey)

Timber bu1klieals, when constructed buttresses, consisting of either i'oekfill with pervious backfill and drain pipes, or earthfill, generally are used in con- have been utilized successfully to pre- nection with embankment construction, vent the sliding of wet soil whei'e the and seldom, if ever, to restrain slopes transverse slope is relatively flat. Most in excavation. The term buttress, as used such bulkheads have little structural here, includes earth or rock dikes in- strength, and furnish oniy slightly installed for either of two purposes: (a) creased restraint against sliding. Their to l)rovide weight at the toe of a land- success, in many cases. appears to be due slide, such as "toe support" or "strut" to the drainage layer l)rovided at the toe fills; (b) to increase the shear strength of the satui'ated slope.,;. of the soil by construction of a dike or buttress of material having substantially Buttresses highei' shear strength than the native soil. Buttresses at the foot of active or po- In the typical slump type landslide the tential landslides are commonly used for ground at the toe usually moves upward, prevention as well as correction. Such forming a bulge or pushup. By adding

I'REVE N TI ( ) N 139

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Figuro 84. Masonry wall inset beneath overhanging layer- of rock. 1 his iall. plus bench on upper part of c-ut, serves to prevent Or minimi7e rockfall. Note rocks in ditch, hos ever, '- hich represent a small but Constant maintenance Cpense. (Photograph cottrtes of PcnnsIvania Department of Highways) 140 LANDSLII)ES

l,gure . Failure of piles and hulkheak.,t ..tsoa, Calif. Various types of piles, isalls and bulkheads failed to correct this landslide, which was sul,sequcntiv controlled by extensive suhdrninnge treatment. State highway crosses the slide near its head, at extreme top of photo. (Photograph by F. W. Herlinger. courtesy of California I)ivision of Highways) weight in the form of a toe support or strut fill acts to resist slide movement, strut fill where this upheaval would whereas part of the weight added in normally occur, the resistance against flattening the fill slope contributes to sliding is increased. This is one means the driving force causing slide move- of improving the stability of an embankment. Figure 88 shows a toe support ment, but the strut fill must be carefully embankment used as a slide preventive. designed in order to utilize the weight A highly specialized form of toe sup- most effectively and to assure that the port to prevent slides is shown by Fig- toe support fill will not in itself be un- ure 4. Groins were built out into the stable. Unless a careful investigation and water beneath an unstable slope; the analysis is made there is always the groins catlsedl shore currents to build danger that the additional load imposed up sand beaches at the base of the slope, by the toe support fill may increase the thus adding weight and support to the driving force rather than provide added toe. The upper slopes were also treated resistance against sliding. Such fills are to retard erosion. safest if the toe fill extends between the The rock buttress has been used as a embankment and it natural stable bank slipout prevention measure with consid- or hill. Figure 87 illustrates this type of earth buttress. A properly designed erable success. If the rock buttress ex- toe support fill is more effective than tends down to firm material, and is suffi- merely flattening the embankment slope. ciellUy massive, the resistance against because all of the added weight of the sliding is appreciably increased by the

PREVENTION 141

high shear resistance of the rock but- principle has been found effective in pre- tress. Many rock buttresses have failed venting sloughing or flowing of wet cut because they (lid not extend to sufficient slopes. This method, which is really a depth; as a result, a surface of rupture combination of drainage and buttress, passed below the bottom of the buttress, consists of placing over an excavated which then moved as part of the slide. slope a heavy blanket of clean coarse Assuming that the necessary boring and gravel or similar pervious material. If test data are available, the improvement the cut slope is excavated on a 1:1 slope, in stability effected by construction of for example. the gravel blanket would either type of buttress can be estimated be placed to a 1½ :1 (1½ horizontal :1 with reasonable accuracy by app! ication vertical) or 2:1 slope, thus providing a of the principles of soil mechanics, as wedge-shaped buttress of gravel which described in Chapter Nine. allows free drainage of seepage from the A modified application of the buttress slope and, at the same time, furnishes

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F'igurc 86. Failed piling in s ars'cd clas, 1 mile south of Springfiekt Mass. This slum,-ear1h)low. tOO feet wide, 75 feet high, and 6)) feet deep, took place in 1954 in varved lake clays that dip 120 to 150 downslope. The summer la ers are high in silt, hence provide much water for absorption by the winter lovers. Slump was due to high water content, triggered by removal of toe and vibration of construction equipment. Piling having failed, the slide was corrected by a combinahion of rock buttress, partial removal and drainage at hoe. reshaping of slope, and partial removal of head. Photograph courtesy of Massachusetts 1)e' partnlent of l'uhli,' \\orks 142 l.ANlsLil)F:s

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44 AK

liguro 57. Earth buttress fillLI) Ores-cot sliding. Earthfill coactruc ted as buttress bets, cm highwa, em- biinkment and opposite stable salley wall; culvert placed in creek vhanncl under bull ress. This particular installation (I miles west of Pierre. S. Dak.) was for slide cOrreclion, but similar buttress fills are fre- qilenlly conatrurted for prevention of slides. The horizontally bedded soft shale on far bill was moving dt,wn slowly and causing displacement of the highway fill toward the observer. (Photograph by D. J. Varnos, U. S. Geologirnl Survey)

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-___ij l-'igurc. 88. Rcstraining strut-tore: ' Ii. rr 0i.1 fill i stroilrii In., un,table uiu;triniil;i...itu - slippol I or earth buttress fill was constructed to Ore, rot stipouts. lIst ire., fill is rid. r middle of photo at extreme right. s, here right-of-way fence rurs es ouls, ard. Freeway near San Francisco, Calif. (Photograph by Bruce I t I. coil rte-sy of California Division of Ill ghwa. PREVENTION 143 resistance against sliding. This type of Pe,vioos preventive treatment is applicable only to relatively low-cut slopes, and would be nd economical only in locations where a Limit of plentiful supply of cheap gravel is avail- Excovofioft able. The method is illustrated by the cross-section in Figure 89.

Figure 89. Modified buttress. Pervious blanket of Piling gravel or rock prevents sliding of wet cut slope by providing drainage of slope and also by increasing Driving of piles to prevent or retard strength of slope by buttress action. Typical of pre- slide movement appears to have great ventive treatment used near Boonville, Calif. (Cour- appeal to the layman, and to some engi- tesy of California Division of Highways) neers, even though the majority of such installations have been far from success- is relatively steep, the sliding is likely ful. In general, the pile treatment is to be progressive -movement of one more likely to be effective as a preven- block of rock removes the support from tive measure than in controlling an ac- the rock in the face above, and the slid- tive landslide. The shearing resistance ing progresses up the slope. The weak- of a soil mass often can be considerably ness along the joint planes may be aggra- increased by driving piles, and the re- vated due to percolation of water and sulting increase in shear strength may weathering along the contact, and also be sufficient to prevent slide movement. by freezing in frost areas. The piles may, however, be ineffective In many cases a small additional re- because of (a) movement of soil be- straint will prevent the initial move- tween and around the piles, (b) over- ment, although once a block of rock has turning of the piles, (c) shear failure started moving restraint would be most of the piles, or (d) development of a difficult. Dowels have been used success- surface of rupture in the soil below the fully for preventing this type of slide. pile tips. The reasons for and means of In the installation of the dowels, holes preventing each type of failure are ap- are drilled into the rock normal to and parent. Too few engineers have a true across the weak planes, then heavy steel conception of the magnitude of the dowels are grouted in the holes. The forces which may be exerted in a land- spacing and length of the dowels would slide movement, and assume that a few depend, of course, on the degree of joint- piles will control or prevent sliding. 'ing and the dip of the bedding planes In most cases a thorough analysis dur- or joint surfaces. Control of a slide by ing the design stage would indicate such installation of dowels has been described deficiencies in the design of the treat- and illustrated by Laurence (1951). The ment. Of course, piles should not be use of dowels is seldom practicable un- driven in soils that become "quick" unless the rock is durable and free from der vibration. fragmentation. Rock bolts, a modification of the dowel Dowels principle, are now used extensively to prevent movement of rock; these con- Rockslides are not uncommon in cuts through hard durable rock, if bedding sist of heavy bolts with a wedge or ex- planes or joint planes are prevalent in pansion device at the lower end, which the rock. Sliding is likely to be especial- are installed in holes drilled in the rock ly troublesome if the joint planes or (see Fig. 90). A large washer or plate bedding planes slope toward the excava- is provided under the nut at the face tion. If the cut slope is of great height, of the rock. Because the tightening of or if the rock face above the slope line the nut actuates the expansion device 144 LANDSLIDES

Figure 90. Reck anchor bolts used to prevent slippage and fall of bedded rock in a railroad cut in north- eastern Pennsylvania. The bolt consists of shank threaded at one end on which a nut and retainer plate are attached. At the end which is embedded in the rock the bolt has a forged slot. A steel wedge is forced into the slot to hold the bolt securely in the drilled hole. In some cases, slippage along very steeply inclined beds, such as those shown in Figure 20. can be prevented by means of rock bolts. (Photograph courtesy of Bethlehem Steel Company) in the hole, no grouting is required. Such ture. Retaining walls frequently must rock bolts, which have been commonly be founded on such weak material that used in tunnel construction and in mines. the unit pressure at the toe of the foot- are now frequently installed in slope ing exceeds the bearing capacity of the faces of rock excavation, to anchor slabs foundation soil. When the restraining or fragments of jointed rock before any structure consists of piles, the material movement occurs. These rock bolts, if l)enetrated by the piles may not have properly placed, will often anchor key sufficient shear strength to prevent tip- slabs of rock and thus prevent rock- ping of the piles due to lateral thrust slides or rockfalls which might other- of the soil mass. When such conditions wise develop into large-scaie movements. prevail, the use of tie rods may provide the required additional resistance against Tie Rods overturning. When tie rods are employed for this purpose they consist of heavy One of the causes listed for failure of teel rods or wire rope securely, fasteneds retaining walls, cribs and piles was over- to rigid wales, piles, or vertical members turning or tilting of the retaining st rue- of the restraining structure; the tie rods PREVENTION 145 are anchored to deadmen placed in the Herticoing of Slide Mass. - - By means most stable accessible material back of of artificial cementation the individual the structure, frequently in firm ground soil grains are cemented together, thus on the upper side of the roadway. This increasing the shear strength of the soil. type of treatment is best adapted to the Cementation may be accomplished by in- prevention of embankment slipouts. as jection of chemicals or by grouting with it is seldom feasible to install the tie portland cement. rods where the restraining structure Most of the chemical injection proc- protects a cut slope. Tie rods have been esses use sodium silicate, in combina- used to anchor log cribs. pile and plank tion with one or more other chemicals, bulkheads, and similar restraining struc- which react with the sodium silicate to tures (see Fig. 91. form a silica-gel in the interstices of the soil. Several of these injection processes, ljiseellaneous i'vlethods such as the Joosten and K.L.M. methods, are proprietary. The injection meth- Many other methods of slide preven- ods are commonly applicable only to tion have been tried with varying de- sandy soils with an effective grain size grees of success. Most of these treat- of at least 0.1 mm. The treatment has ments have limited application and are been used successfully to effect tempo- effective only under certain combinations rary stabilization of sands during the of conditions. Under "Miscellaneous" in construction period in excavation of tun- Table 4 are listed a few of these less nels and trenches ; as a preventive treat- frequently used methods of slide preven- ment against large-scale slides, it has tion. Because most of them are still been used to a very limited extent. somewhat experimental or have very lim- Portland cement grout injections have ited application, they are not discussed been used successfully for cementing here in detail. coarse sands and gravel, but are general-

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Fgure 91. TimI,r retaining wall with Iic-rods. Vnstable foundation p rer udeI use of conventional re- taming wall. Lateral support provided by steel cables anchored to deadmen in tirm material in slope above the wall. Embankment constructed above this wall near Cuerneville, Calif.. has been stable since con. strurtion in 1939. (Courtesy of California Division of highways) 146 LANDSLIDES

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l'igu ro 92. liii ,Oa hi ized o t Ii cement grout. Ce nent grunting ii as accomplished in 1949 in the Westgate Ill near Wes(gate. Va., by the Virginian Railroad A highly micaceous soil was used in the embankment construction and compaction was ser. diflicult. The inelinvd drain pipes shown at midslope were placed crier. but were ineffective and were later abandoned. Injections were made along the slope with holes on a 10-foot gridwork. The grout was mixed to it proportion of one Part cement to four parts of sand. The section was 5.10 feet long and the slnps were approximately 65 feet high. The cost of the correction amounted to $6.0 II) SmitIi. 1950). (l5Iiuligrcpli by R,ekwell Smith , Association of American Railroads)

ly considered less effective with liner increase in shear strength to stabilize grained soils. However, some of the rail- the slide mass. The grout injection meth- roads have been applying grout mice- cd might he less effective as a preven- tiorts as a slide corrective treatment, tion treatment because of the absence and report encouraging results even of cracks and fissures, most of which when the soil mass was heterogeneous or tend to develol) after movement occurs. comprised largely of elayey materials Nevertheless, the method may be ap- (see Figs. 92 and 93). The grout was plicable to a greater variety of soil types apparently dispersed through seams, than is generally believed. cracks and fissures in the fine-grained Hituminous emulsions, having a low- soil, as well as into the interstices of er viscosity than cement grout, will the coarse sand and gravel. Even though l)enetrate into the pore spaces of finer the grout was not un iforniy distributed grained soils than will cement grout. The throughout the soil mass. the treatment cost is greater than for cement grout has, in many cases. e l'ect e .1 sufficient treatment, and the emulsion is not suit- PRE\'ENTION 147 able for use where ground water might an example of an unusual means of ef- remove the emulsion before setting oc- fecting stability of a slope by an artifi- curs. There is also some question as to cial hardening process. the permanence of the hardening, es- Another method of increasing the pecially if ground water is present. The shear resistance of a soil mass is by asphaltic emulsion treatment has not electro-osmosis, in which migration of been used extensively as a landslide pre- water out of the soil pores is induced ventive treatment. by causing an electric current to flow Freezing of soil to prevent sliding between electrodes driven into the soil. during construction is a unique method The moisture content of fine-grained that has been used on at least one large soils can be reduced appreciably by the project. A description of the operation electro-osmosis process, with a concomi- has been published (Gordon. 1937). The tant increase in shear strength. The freezing process is slow and relatively method has been used successfully on costly; obviously, it woud be applicable full-scale slope treatments in Europe, only as a temporary treatment for slide but most of the work (lone thus far in prevention, and is mentioned merely as the United States has been of an experi-

Li. iijf(ticiff of ii nil it i.riiil I 't.i}inIi,i iiia1I IIi in Siiiitlii r,i ItahI\fai in noytliern hentucki. The method used uns simiInr to t hut ulriuetzhud u nil r Figure 92. (I hotogrn phi h 1t,,ek. eli Smith) 148 LANDSLIDES mental nature. The process has been de- ment during the construction treatment. scribed by Casagrande (1948), Karpoff Partial Removal at Toe. - Partial re- (1951), and others. Available informa- moval at the toe of a landslide has been tion indicates that, because a rather included under "Miscellaneous Methods" strong electric current is required over a of slide prevention and correction in considerable period of time, the cost of Table 4, although such excavation usual- electric power for this treatment may be ly neither prevents nor corrects a land- prohibitive. slide; on the contrary, it more often ag- Another process, which is similar to gravates the sliding. Excavation at the electro-osmosis, is the electro-chemical toe of a landslide is sometimes neces- hardening of clays. If aluminum elec- sary as an expedient to protect a struc- trodes are used in the electro-osmosis ture temporarily, until the structure can treatment, there is, in addition to the be relocated or more permanent correc- reduction in moisture content adjacent tive treatment provided. This type of to the anode, a further hardening of the treatment is usually attempted after oc- soil resulting from base exchange - the currence of a landslide, and.could seldom positive ions of the clay minerals are re- be considered as even an attempt at placed by aluminum ions from the elec- prevention. The fact that a landslide trode, with a loss of metal from the sometimes remains quiescent for a con- electrode. The hardening effected by this siderable period of time after slide ma- process is apparently permanent. As terial is excavated from the toe, is mere- with electro-osmosis, the power require- ly evidence that the factors which acti- ments are high, making the treatment vated the original slide were no longer costly. present after the toe was excavated. For Blasting. - Where a relatively shal- example, a large rockslide, involving low mass of cohesive soils is underlain more than a quarter of a million cubic by bedrock or other hard material, the yards of slide material, completely contact between the two is sometimes blocked a mountain highway a few years a smooth sloping surface; such a con- ago. Conditions were such that neither tact plane is a potential surface of slid- complete removal nor large-scale correc- ing, especially in the presence of sub- tive treatment was feasible; sufficient surface water or if there is a thin layer material was removed at the toe of the of plastic material along the contact slide, in this case at roadway grade, to surface. Blasting is sometimes used to permit opening the road to traffic as break up such a contact surface, thus quickly as possible. Although no correc- providing a mechanical bond between tive measures were taken there has been the two surfaces. In effect the shear no further slide movement. The slide oc- strength along the weak zone is in- curred during a mild earthquake in the .ciased by the shooting and breakup of region, and it is probable that the slide the hard material. It is probable that will remain quiescent until there is an- this method has been most successful other earthquake or some other changed where the hard layer was underlain by condition reactivates the slide. a pervious formation, and the •blasting provided drainage into the underlying Conclusion pervious layer. The permanence of the blasting method has frequently been The number and variety of slide pre- questioned, and there is evidence that vention methods discussed in the fore- in some cases a weak zone later devel- going are evidence that there can be no oped along the original contact, due to rule-of-thumb system of prescribing migration of fine soil and "healing" of treatment; and for a particular landslide the fractured zone. There is, of course, or potential landslide there is seldom one the risk that the blasting, unless care- and only one "correct" method of treat- fully handled, may induce slide movement. Frequently, the most economical PREVENTION 149 effective means of prevention consists Dam." Reclamation Era, v. 27, p. of a combination of two or more of the 12-16, 1937. general preventive measures described Hill, R A., "Clay Stratum Dried Out to Prevent Landslips." Civil Eng., v. 4, in this chapter. p. 403, 407, 1934. For most landslides, a majority of the Karpoff, P., "Laboratory Investigations of possible preventive treatments can be Electrical Drainage and Electro- eliminated at the outset, and only a few chemical Stabilization of Soils." of the many methods need be considered. Structural Research Sec. Rept., No. Frequently, the final selection can be SP-29, U. S. Bureau of Reclamation, made only after careful comparison of 1951. two or more alternate methods. But in Ladd, G. E., "Methods of Controlling Land- slides." Engineering and Contract- spite of the complexity of landslides and ing, v. 67, p. 599-608, 1928. the wide variety of control methods, the Laurence, R. A., "Stabilization of Some problem of landslide prevention and cor- Rockslides in Grainger County, Ten- rection is amenable to a rational engi- nessee." Econ. Geol., v. 46, p. 329-336, neering approach, by proper utilization 1951. of available knowledge on the classifica- Mineral Information Service, "Landslides tion, recognition, and analysis of land- in Ventura Avenue Oil. Field." Min- slides. eral Information Service, Calif. Div. of Mines, v. 7, no. 5, p. 1, 1954. References Anon., "Curing Slides with Drainage Tun- nels." Roads and Streets, v. 90, no. 4, Baker, R. F., "Determining the Corrective p. 72, 14, 76, Apr. 1947. Action for Highway Landslide Prob- Root, A. W., "Problem of Slipouts Studied lems." Highway Research Board by State Highway Engineers." Cali- Bull. 49, p. 1-27, 1952. fornia Highways and Public Works, Casagrande, L., "Electro-Osmosis." Proc. Mar. 1938. 2nd Tnt. Conf. on Soil Mechanics and Stanton, T. E., "Stabilizing Earth Slopes Foundation Eng., v. 1, p.. 218-223, Through Installation of Horizontal 1948. Gordon, Grant, "Freezing Arch Acr.oss Toe Drains." California Highways and of East Forebay Sid, Grand Coulee Public Works, Jam-Feb.. 1948.