Practical Guidelines and A Case Study of Minneapolis

Planning the Development of Underground Space

Raymond L. Sterling Susan Nelson

N recent years there have been use in Sweden. The need for planning carried the exercise the next step into many calls for better planning of is almost self-evident; underground effective resource planning. I underground space utilization. excavations permanently alter the sub• The study which forms the basis for Robert Legget has written extensively surface condition, and the subsurface this paper represents an attempt to about the need for consideration of ge• of cities is becoming increasingly clut• provide resource material and a guide ology in city planning (1962, 1973), tered with a variety of services and ser• to developing a planning structure for Birger Jansson and Torbjorn Wind• vice facilities. On top of this, there is subsurface space use. The generalized qvist (1977) have discussed many of the increasing incentive from energy and planning concerns were developed in planning issues and suggested a plan- · environmental concerns and land costs concert with a de ailed case study of ning methodology for subsurface space to use the subsurface for more than Minneapolis, Minnesota, which is in• routine service functions. tended to serve as a useful planning Raymond L. Sterling, principal investigator While the cogency of this rationale document for Minneapolis as well as a of the planning study on which this article may suggest a rush by cities to docu• case example for the resource guide. is based , is senior co-editor of Underground ment their subsurface conditions and The goal of the effort is to have un• Space and director of the Underground Space plan for effective underground space derground facilities, underground Center at the University of Minnesota. Su• utilization in their urban area, this has space use, and subsurface resources in• san Nelson, a land-use planner, was project really not been the case. A number of corporated into city planning proce• coordinator and principal author of the cities have documented their geology dures specifically as a city resource or study. fairly thoroughly , but very few have a category of public or private works.

86 Underground Space, Vol. 7, pp. 086-103, 1982 0362-0565/82/020086-18$03.00/0 Printed in the U.S.A. A ll rights reserved. Copyright © 1982 Pergamon Press Ltd.

Drawing by John Carmody*

At the same time, it is hoped that the Minneapolis' generally favorable ge• partment of Energy were encouraging development of U.S. planning guide• ology for the creation of subsurface but each faced severe budget cutbacks. lines will encourage other cities besides space use was probed again in 1980. At this stage, it was decided to try and Minneapolis to develop their own de• Discussions were held with officials from start the project with the Control Data tailed inventory and planning docu• the City of Minneapolis, Hennepin share of the funding (approximately ment, that with an exemplary study and County (which includes the city), the one-third of the original amount) and a resource guide other city planners U.S. Department of Energy, and Con• . to solicit private support from large city will incorporate underground plan• trol Data Corporation. By this time, the businesses which the city might then ning as an integral part of their com• American Planning Association had match. Although a very significant effort prehensive planning efforts in the same joined forces in declaring the need for was devoted to raising additional funds, manner as they do land use, transpor• a resource guide for subsurface plan• the total raised was only $6,050. The tation, housing, industry, etc. ning. As a result, a $200,000 budget city continued to be very supportive of A brief account of how the Minne• was prepared by the Underground the study, providing data and other as• apolis study evolved illustrates the Space Center and the American Plan• sistance, but never did find funds to difficulties of gaining widespread in• ning Association for joint funding by support subsurface planning. terest in underground planning and the four organizations. The study was The shortcomings of the study are getting support for such a project• to be split into two parts, a Technical most apparent in the Technical Re• which cannot show immediate bene• Resource Guide for subsurface plan• source Guide, which, due to the lack fits-at a time when state and federal ning and a case study for Minneapolis of funding, generalizes the planning governments are in financial straits. The and Hennepin County. concerns found for the Minneapolis seeds of the study were planted in the There followed a protracted process geology but does not examine or illus• early 1970's when underground and of trying to wrest funding from the trate in detail other typical geologic subsurface space use engineering be• organizations. Control Data Corpora• conditions. The Case Study suffers from came a focus of the Civil and Mineral tion, an unlikely candidate perhaps to a lack of detailed investigation of the Engineering Department at the Uni• most readers, but an innovative com• more favorable geologic areas identi• versity of Minnesota. The main thrust pany which sponsors a varied research fied and from the absence of illustra• of this interest was in the urban appli• program, was willing to commit its share tive designs for the most promising de• cations of underground space use. This of the funds at an early stage to help velopment opportunities. interest was not widely shared, how• leverage funds from the government Merely conducting the study with city ever. A conference, held in 1974, deal• agencies. Hennepin County decided participation has had a tangible re• ing specifically with this topic and fea• quite early on that subsurface planning ward, however. The city is now far more turing underground developments in was not something tor which it had as Montreal, Kansas City, and other ur• much responsibility as the city and *Illustration of potential underground de• ban areas, attracted only 20 people. therefore declined to support the study. velopment at the University of Minnesota, The interest in taking advantage of The City of Minneapolis and the De- Minneapolis campus.

September/October 1982 UNDERGROUND SPACE 87 aware of its subsurface potential and shows far greater interest in develop• ing it. The study also came just in time to affect the route selection process for Data must be presented in a form a Great River Road extension through Minneapolis (see below) which could suitable for planners with little background have prevented future use of one of in geology or underground engineering. the few suitable river bluff access points to mined underground space near the city center. While the components of the study• a technical resource guide for under• termine current practice and problems ried out in practice by drawing up a ground space planning and a detailed in the regulation of underground space series of single-scale maps which can planning evaluation for Minneapolis• through zoning ordinances, building be superimposed upon a base map and are presented to stand independently codes, etc. This valuable collection of upon each other to aid in the analysis. (Sterling and Nelson 1982, 1982a*), the information is touched on only lightly The maps to be prepared for an area information is combined here to avoid in this paper and hopefully will be the will depend on its geology and the repetition and to keep a specific focus subject of a future paper on under• agreed upon assessment criteria. For for the planning discussion. The crit• ground space planning and regulation. instance, in an area where the presence ical issues in the planning process are and condition of a sedimentary bed• the geological data assembly, the de• rock formation is judged to be the pri• velopment of criteria for the interpre• PART I. PRACTICAL GUIDELINES mary determinant of the potential for mined space development, overlays tation of the data for planning pur• Three types of underground space poses, and the presentation of which identify the location, thickness, are considered in this study. Mined space and competency of the rock must be · interpreted data in a form suitable for is created in deep bedrock by tunneling planners with little background in ge• prepared. Further analysis must assess or mining through a vertical shaft or the hydrogeology of the area, the lo• ology or underground engineering. other remote access. Earth-sheltered space The Case Study could not have been cation of existing structures, and po• and cut-and-cove·r space are dug from tential access points. attempted at the current level of detail the surface by excavating earth and re• without an excellent and readily acces• In general, however, the geologic map placing it to complete the construction. inventory for underground space de• sible data base compiled by the Min• This similarity in the construction of nesota Geological Survey. Their data velopment should include the follow• earth-sheltered and cut-and-cover space ing maps: and geological mapping, combined with and their consequent similar relation engineering criteria developed for the to the surface make for similarity in the bedrock geology; most suitable soil, rock, and hydrogeo• planning. Consistency with the sur• the condition of the bedrock; logical conditions for underground face, in terms of use and aesthetics, must the bedrock hydrogeology; construction, made possible the gen• be considered for earth-sheltered the potential points of access to mined eralized identification and mapping of structures since they are only partially space; areas in the city suitable for under• surrounded by earth. For cut-and-cover the location and elevation of the ground space development. While the structures, consistency of use is less of floodplain; simplified data can undoubtedly be de• a consideration since the purposes they existing structures in the bedrock; duced from the existing detailed data, serve are likely to be auxiliary to sur• the suitability of soils; most planners are not conversant face uses. Mined space use, on the other areas with slopes of 8% or more (at enough with geologic maps and data hand, need not be related to surface the community level); to do this. Engineering criteria must activity except at access points. An• detail maps of areas with slopes of also be applied to the data to reveal the other significant difference between 8-15%, 15-25%, and 25% or more distribution of favorable geologic strata, these two similar types of under• (at a large scale); existing structures, and accessible zones ground space and mined space is that the hydrogeology of soils; for development. their construction often entails a much existing structures in the soil; The distribution of the under• greater degree of surface disruption areas of hazardous or difficult geo• ground space resource throughout the and inconvenience in urban areas. logic conditions; city and its relation to existing uses and the location of valuable mineral re• planned development form the basis Assessing the Geologic Potential sources including basic construc• for the incorporation of subsurface for Underground Space tion materials such as sand and planning into a comprehensive long• gravel. range plan which treats the subsurface Geological conditions are the pri• as an integral resource of the urban mary factor determining the potential These maps should be housed cen• area. for developing underground space in trally where they will be readily avail• For the Technical Resource Guide, a community, and the first step in plan• able to anyone who needs them. They the American Planning Association ning the space is conducting an assess• should be updated on a regular basis, surveyed 1,000 of its members to de- ment of these conditions. The assess• preferably after each important con• ment should result in a clear idea of struction project. This last point is es• *Planning for Underground Space: A Technical where underground space can be de• pecially important where large struc• Resource Guide and Planning for Underground veloped without difficulty or with mit• tures located in the subsurface and Space: A Case Study for Minneapolis, Minnesota igation, and where it cannot be devel• utility extensions are concerned. are available from the Underground Space oped, as well as where valuable mineral It is worth emphasizing here that, Center, University of Minnesota, 2 Appleby deposits may be located. while geologic data mapped at the Hall, 128 Pleasant Street SE, Minneapolis, The assessment of a community's community scale will be adequate for Minnesota 55455. potential for underground space is car- community land-use planning, de-

88 UNDERGROUND SPACE September/October 1982 tailed engineering studies are the only from the U.S. Geological Survey in the Water can occur in bedrock in sev• reliable source of detailed information Department of Interior when it is avail• eral forms. The least problematic is un• for proposed specific projects. able. confined water, i.e., under water table Besides the state Geological Survey, conditions. The level of this water may Mined Space state agencies such as the Department vary somewhat within the bedrock due The main factors in assessing the po• of Natural Resources, Department of to seasonal conditions, but the water is tential for mined space development Transportation, Health Department, not confined or under pressure. Gen• are evaluating the bedrock geology, the Historical Society, and architecture and erally, proper design and construction hydrogeology of the bedrock and of engineering offices in administrative techniques together with drainage and the overlying soils, the potential points departments often hold relevant rec• waterproofing will ensure that the of access to mined space, and existing ords. Regionally, airport and waste mined space will remain dry and sound structures above and in the bedrock. control commissions may have useful if above or near the free-water surface. Bedrock geology. Most mined space will holdings. At the municipal level, the Water in bedrock can also be con• be created within a thick and compe• Department of Public Works, Depart• fined i.e., under artesian conditions, tent bedrock stratum, but it can also be ment of Community Services, Housing wherein the bedrock stratum is satu• dug in a soft layer beneath a layer of Authority, and similar agencies often rated and the water is confined by im• competent bedrock. In either case, the keep records of geological information pervious layers above and below. Seep• bedrock must be of sufficient thickness obtained during the completion of age of surface water into the stratum to provide the structural support nec• projects. Mining records are invaluable via exposed and slanted layers of the essary for mined space development. in regions where mineral resources have rock creates pressure. Water under The essential conditions for mined space played a role in development. artesian conditions is more difficult to development vary with the type of rock, The reliability of these data from mitigate if the rock permeability is high, so that geotechnical engineers familiar various sources with different record• and can raise the cost of projects in with the engineering properties of the ing procedures can be improved mined space substantially. local geologic formations must be con• through attempting to create a certain Water in soils above the bedrock sulted early in the planning process. degree of redundancy in the source stratum can also affect mined space de• The factors to be considered include material. Cross-checking data can com• velopment. Water retained between or the presence and extent of suitable rock pensate in part for sketchy specific in• above impervious layers in soil or rock, strata, the condition of the bedrock, formation from sources usually relied called perched water tables, can affect potential clear spans, the depth of the upon to provide more thorough infor• mined space if the impervious layers strata, and the potential for access. mation. are punctured and the water drains into Geologic data can be assembled from Hydrogeology. The presence of water the bedrock space below. a number of sources. Most states in the in bedrock and soil will affect the cost If it is necessary to collect more data United States have a Geological Sur• and feasibility of mined space devel• to provide an accurate view of the hy• vey, although the extent and reliability opment. The absence of water in bed• drogeology in an area, it is best to per• of the data these provide vary greatly. rock presents the optimum condition form repeated testing at the same time A second important source is geology for mined space development, since of the year, so that data do not vary departments at major universities. Data space can be created without the costly with seasonal shifts in water conditions. from these main sources can be coor• mitigation required when water is Access. Access to the subsurface is an dinated with additional information present. important factor during both the con-

View of the Mississippi River, near downtown Minneapolis.

September/October 1982 UNDERGROUND SPACE 89 struction and the operational phases of depth from a significant rock stratum tion of soils and their general suitability subsurface development. The primary to the top and bottom of tunnels and for construction. Some of these factors criteria for determining the appropri• pipes. Elevation and capacity should also take on increased importance in ea rth• ate type of access for mined space are be indicated at various points along the sheltered and cut-and-cover construc• the use to which the space will be put, facility. tion, and there are additional factors the extent of operations, the physical to be examined. potential for access, and cost. Horizon• Earth-Sheltered -Drainage: Well-drained soils are tal portal access is the most cost-effective and Cut-and -Cover Space preferable to poorly-drained soils. entry. Potential horizontal or bluff ac• The economical construction of - Water table depth: This factor is cess points are best identified by visual earth-sheltered and cut-and-cover space more inspection in conjunction with geologic depends primarily on the presence of important for earth-sheltered a nd cut• mapping, which allows the assessment well-drained, stable soils. The major a nd-cover construction than for con• of such factors as height from the bot• factors under evalu ation in planning ventional construction because pri• tom of "roof" material to adjacent land these types of underground space are mary spaces are dug below grade. A or road level, the potential for creation the quality and distribution of the soil, typical earth-sheltered residential of vehicle maneuvering areas, the av• the hydrogeology of the soil, topog• foundation is 30-120 in. (76-300 em) erage water level if the site is adjacent raphy, the potential for the favorable deep; enclosed cut-and-cover struc• to a body of water, and potential flood orientation of openings, and existing tures extend much deeper. Preferably, levels. structures in the soil. the water table should be significantly Vertical shaft access can be created in Soils. Information on shallow soils lower than the foundation. a wide range of competent materials. recorded on survey interpretation - Bedrock depth: Bedrock within The integration of surface and sub• sheets are available in every region of the surface uses will be an important factor the United States from the Soil Con• planned depth of the foundation of in determining the location of vertical servation Service. Additional infor• the proposed cut-and-cover or earth• access points. The main geologic fac• mation is also available through local sheltered structure is undesirable, since tors to consider are the depth from the geologic surveys and universities. excavation is usually more difficult and surface to the level at which mined space Deeper soil borings should also be used costly in bedrock. will be created and the likelihood of to assess soil suitability for subsurface - Bulk density: This property is perched water tables at each shaft lo• construction, since foundation loads rely con• sidered together with the soil cation. on the bearing capacity of soils below type in order to determine whether Existing structures. Mined space can an excavation for their support. the soil is loose or dense. In general, often be developed with little difficulty Table 1 shows the unified classifica- the denser soils within each soil type in regard to surface structures. While are better for construction purposes. surface structures may increase the load - Erosion potential and frost on the bedrock, the load can be offset action by reduced loads due to the excavation of material for basement and sub-base• ment structures. Figure 1 demonstrates this point. The calculations are only approximate, but it is clear that, for the three buildings shown, any increase in load on the bed • rock material will not be significant. The only type of structure that would greatly increase the load would be a building much taller than the 19-story building shown , or a building of equivalent size with a smaller basement. Existing structures in the subsur• face, both natural and man - made, present greater potential problems for development. Mined space has tradi• tionally housed utilities which are un• desirable on the surface. These include sewer, fresh water, storm ·sewer, elec• trical, telecommunications, and many other utilities. Abandoned structures and natural structures such as caves or voids in the bedrock may also exist and should be identified . Each type of structure located in the subsurface should be mapped on a sep• arate overlay and a master map should be prepared to indicate the location, elevation, and size of each facility. Im• portant statistics to be keyed on the map include the diameter of pipes and tun• nels, the distance from the surface to the bottom of the structures, and the Figure 1 . Simplified calculations for the effect of surface structures on mined space.

90 UNDERGROUND SPACE September/October 1982 poten tial: These factors are considered Table 1. General characteristics of soils and their suitability for construction. together to determine the erodibility Frost Typical and stability of the soil types on various heave Volume Backfill bearing Range General degrees of slope. Severe erodibility or Soli type Drainage potential change potential capacity (psi) suitability frost action potential is a negative fac• Well-graded Excellent Low Low Best 8,000 psi 1,500 psito Good gravels & 20 tonsfft2 tor for slopes at or greater than the gravel-sand mix- maximum slope recommended for el• tures, little or no evational earth-sheltered designs. Frost fines action is also important for the design Poorly graded Excellent Low Low Excellent 6,000 psi 1,500 psito Good gravels & 20 tons!ft2 of roads in conjunction with buildings. gravel-sand mix- - Shrink-swell potential: A low lures, little or no shrink• swell potential is preferable; fines Silty gravels, Good Medium Low Good 4,000 psi 1 ,500 psito Good a mod• erate or high shrink-swell gravel-sand silt 20 tons/ft2 potential could require a special mixtures design. Clayey gravels, Fair Medium Low Good 3,500 psi 1,500 psito Good - Percent liquid limit: This factor gravel-sand-clay 10 tonsfft2 is used to determine the moisture mixtures Well-graded Good Low Low Good 5,000 psi 1,500 psito Good con• tent at which silts and clays will sands & gravelly 15 tonsfft2 pass from a liquid to a plastic state. sands, little or Higher values of the liquid limit no fines indicate in• creasing amounts of fine Poorly graded Good Low Low Good 4,000 psi 1,500 psito Good sand & gravelly 10 tons!ft2 material in inorganic soils. A liquid sands, little or limit less than no fines 50% is generally considered good Silty sands, Good Medium Low Fair 3,500 psi 1,500 psito Good whereas a liquid limit greater than 50% sand-silt mix- 5 tonsfft2 tures indicates a poorer condition for con• Clayey sands, Fair Medium Low Fair 3,000 psi 1,000 psito Good struction. sand-clay mix- 8,000 psi - Permeability: A permeability rate tures of over 4 in. (10 em) per hour is Inorganic silts, Fair High Low Fair 2,000 psi 1,000 psito Fair consid• ered good; 2-4 in. (5-10 em) very fine sands, 8,000 psi rock flour, silty per hour, moderate; and less than 2 or clayey fine in. (5 em) per hour, poor. sands - Corrosiveness on concrete: The Inorganic clays Fair Medium Medium Fair 2,000 psi 500 psito Fair of low to me- 5,000 psi cor• dium plasticity, rosiveness of soil on concrete is a sig• gravelly clays, nificant factor in assessing the suita• sandy clays, silty clays, lean bility of soils. Soils with high sulfate clays contents can be particularly damaging Inorganic silts, Poor High High Poor 1,500 psi 500 psito Poor to normal concrete. micaceous or 4,000 psi diatomaceous Slopes. Elevational earth-sheltered fine sands or designs are best suited to areas with a silts, elastic silts slope ofS-15%. Aerial photographs and Inorganic clays Poor Medium High Bad 1,500 psi 500 psito Poor topographic maps will provide reliable of medium to 4,000 psi primary information on slopes. They high plasticity can be obtained from the soil conser• Organic silts Poor Medium Medium Poor 400 psi Generally re- Poor and organic silty or remove move soil vation service, the geologic survey, lo• clays of low cal universities, or local government plasticity agencies. Organic clays of No good Medium High No good R emove - Poor medium to high Existing structures. Building founda• plasticity tions, sub-basements, tunnels, and util• Peat, muck and No good - High No good Remove - Poor ity services are some of the most ex• other highly or- tensive existing underground structures ganic soils and facilities in soil. Their presence can affect the cost and viability of under• ground construction. The sources of data regarding the location, elevation, and capacity of these facilities are essentially the same as those But the possession of good data does for mined space. It is not uncommon September/October not ensure the good planning of detail. to happen upon abandoned structures 1982 To date, planning· for underground in the soil for which records do not space has generally been approached in exist. an "incremental" fashion rather than comprehensively. One of two ap• proaches is typically taken: consistency in land use, whereby surface zoning Detail Planning for Underground requirements are applied to the details of Space proposed underground projects; and Upon a firm foundation of geolog• discretionary review, whereby a pro• ical data interpreted with a view toward ject is reviewed in detail either as an the development of underground space, exceptional case or as part of a planned planning for community needs can be development. Both of these ap• carried out rationally and confidently. proaches are essentially reactive in na-

ture and may unnecessarily limit the uses to which underground space can be put- and, ultimately, diminish its value. Because of the potential effect of ex• tensive underground development on community growth and surface devel• opment, it is important to consider de• tail planning for underground space in the framework of comprehensive plan• ning and not as a separate planning issue. Some communities actively pro• mote underground space use in their comprehensive plans through the in• clusion of specific plan elements or

UNDERGROUND SPACE 91

and subsurface land are zoned sepa• rately, and this may encourage the most One city, Independence, Missouri, has taken creative use of mined space. Planning Mined Space an innovative approach to mined space by Mined space is an appropriate site for the major activities of manufactur• establishing a separate zoning category for it. ing, warehousing, and secure storage. It is especially well-suited to operations requiring a controlled environment, such as laboratory work and precision equipment manufacture. Mined space generally goes unno• specific language. The goals of this sort the prohibition of this type of con• ticed at the surface due to the buffering of promotion in planning documents struction in zoning provisions de• effect of bedrock and soil between sur• are to encourage the development of fining earth-sheltered dwellings face and subsurface, and this provides underground space; to remove bar• as non-habitable basements or cel• flexibility in the siting of operations in riers to development; to ensure the best lars. the space. The consistency of access to use of the underground resource in • Minimum height and floor-area mined space with existing or planned harmony with community goals, objec• requirements that discriminate surface use may be more important than tives, and policies; and to develop reg• against earth-sheltering or don't the relationship between subsurface and take into account floor-area cal• ulations which will ensure the safe con• surface uses themselves. struction and use of underground culations for habitable earth-shel• Access. The access requirements for tered space. space. mined space are determined by the uses The inclusion of specific language • Minimum and maximum lot-cov• to which it is put. Goods handling, pro• which addresses the details of under• erage requirements which don't duction, and storage will usually in• ground space development in the com• take earth-sheltered construction volve high levels of vehicular activity, prehensive plan, either as a separate into account, since earth shelter• with human access primarily for reach• plan element or as part of a more gen• ing affords different water runoff ing the workplace. On the other hand, eral element, will facilitate its devel• and open space than conventional office space will require primarily hu• opment and use and provide support development. man movement. for amendments in appropriate codes • Setback, garage, and off-street As mentioned previously, the least and ordinances. parking requirements in zoning disruptive and most cost-effective ac• Zoning ordinances are intended to im• ordinances that may pose special cess is horizontal, via a bluff or escarp• plement the community land-use plan administrative and design issues ment, created by tunneling. Handling by identifying districts where particu• for earth-sheltered dwellings. of the excavated material will be more lar uses are to be located. These or• • Problems of uniform administra• efficient than for other kinds of access dinances also contain prescriptive re• tion and objective review where because it can be loaded directly onto quirements which are intended to earth-sheltered development is ensure consistency in a development. approved on a case-by-case basis Most zoning ordinances were de• through discretionary approval. signed to address conventional build• Some areas have ·revised, or intend to ing construction and therefore do not revise, their zoning codes to eliminate adequately address the issues raised in barriers to earth-sheltered structures underground space development. The by rewording definitions and provid• language of many zoning ordinances ing flexibility in prescriptive zoning inadvertently raises barriers to the use standards to accommodate earth shel• of underground space. For instance, tering. most zoning ordinances prohibit the In regard to the development of inclusion of basement or cellar space mined space, the zoning issues are not in the computation of habitable area. generally addressed in ordinances since If earth-sheltered structures fall within its use is relatively infrequent. In most the definition of a basement or cellar cases where mined space is addressed, in a particular community, their use the subsurface is subject to the same for habitation may be prohibited. Sim• restrictions as the surface land directly ilarly, extending surface zoning re• overhead. strictions downward may unreasonably One city, Independence, Missouri, limit the use of mined space for op• has taken an innovative approach to erations unsuited for the surface. mined space by establishing a separate A study performed for the U.S. De• zoning category for the subsurface. partment of Housing and Urban De• Permitted operations are treated sep• velopment, Earth sheltered housing: code, arately from surface operations. Ac• zoning, and financing issues (Sterling et cess to mined space is limited to non• al. 1980), addresses problems which the residential areas, and support equip• language of some zoning ordinances ment on the surface, such as utility or poses for earth-sheltered construction. ventilation mec hanisms, must meet Existing underground structures that could The major barriers identified by the surface architectural standards to en• affect the development of mined space in the study include: sure compatibility with surface struc• study area include abandoned tail race tun• • Discrimination against earth-shel• tures. The significance of the approach nels, once part of the water power system in tered housing in statewide laws and taken in Independence is that surface the mill district in Minneapolis.

92 UNDERGROUND SPA CE September/October 1982 a truck in the tunnel and carried away. In addition, the heavy equipment nec• essary for construction of the space will reach the site more easily. Horizontal access is also the most suitable for the movement of high-volume goods since vehicular traffic can be accommodated easily. Vertical access to mined space is pos• sible via elevator, escalator, or the lower levels of adjacent buildings. Vertical access is generally very unobtrusive at the surface, and requires minimal amou nts of land. Safety. The greatest potential hazard in mined space is fire. Mined space generally has a low occurrence rate of Figure 3. Layout of mined space development. fire unless hazardous materials are present. Like surface space, it must be provided with fire protection devices such as sprinkling systems, fire walls, and fire doors. Emergency exit in case of fire must be provided for users of the space. Codes generally require that there be two means of exit and that no point in the space be more than 150 ft from an exit (200 ft if there is an automatic sprinkling system). In mined space the initial development space with access portal expansion of space relatively high cost of shafts makes op• timal placement of exits an important Figure 4. Placement of the access points is important to ensure adequate circulation in the concern. evolving mined space development.

Expansion. Mined space cannot be re• turned to the natural state, but with proper planning it can be expanded. The expansion of mined space should be considered an integral part of the original planning and layout. Devel• opment of the space will probably oc• cur in phases over a number of years. Figure 2 illustrates two typical con• DISTRICT HEATING figurations for mined space. If a mod-

rib system room & pillar system Figure 2. Typical configurations for mined space. ular system of development cannot ad• Figure 5. Consolidation of a number of utilities in a single tunnel (utilidor). equately meet special space needs, a skeleton of horizontal circulation (such as a vehicular service loop) may tie to• easy movement of equipment and ve• of service as surface facilities- power, gether several clusters of mined space hicles. Proper placement of the ac• communications, water, sewer, etc. The with other systems. Another possibility cesses to the initial development is im• structural requirements in mined space is a fairly rigid, large-scale system which portant, since these points will evolve will not permit the arbitrary siting of includes areas of distinctive character into a circulation system which will ser• services. Consolidation of as many of within the system (Fig. 3). vice the entire development. Figure 4 these services as possible in a single The initial development of mined illustrates placement in relation to de• tunnel, or "utilidor," should be consid• space may occur in areas where hori• veloped space to ensure adequate cir• ered. This reduces the amount of mined zontal access is most readily available, culation in future development. space required, reduces the cost of such as an escarpment or bluff. This The provision of services. The use of providing service, and makes mainte• facilitates construction by providing for mined space requires the same types nance and repair simpler (Fig. 5).

September/October 1982 UNDERGROUND S!'ACE 93

incorporate passive-solar design fea• tures. The elevational units can be built as free-standing structures or com• bined to form various attached config• urations. Two-level structures are pos• sible, as are rows of attached structures. It is possible to stage units up a hillside and construct a large facility with en• closed corridors and vertical circula• tion. In the atrium design, the outside walls and roof are covered with earth and the rooms of the structure are ar• ranged around a sunken courtyard (Fig. 7). This design, like conventional con• Figure 6. Elevational design of earth-sheltered structure, in which three sides are earth• struction, is best suited for fiat land, bermed and one side is exposed. (The earth-sheltered designs shown here and in Figure 7 but can be sited in areas with slopes up are illustrated by residential structures; these designs are also used for industrial, commercial, to 15%. and educational facilities.) Although currently popular as a housing style, earth sheltering is ame• nable to nearly any use. Earth-shel• tered structures currently house resi• dential, commercial, industrial, and institutional facilities throughout the United States and in several countries abroad. Proper design and layout of earth-sheltered structures can ensure a high degree of integration with the natural environment and surface ac• tivity. As a result, there will be fewer psychological constraints to its utiliza• tion than with mined space. Because earth-sheltered structures are oriented to the surface and surface activity, facilities will most often be ex• pected to conform with existing or planned uses in the area where they Figure 7. The atrium design of earth-sheltered structure, in which the outside walls and are located. Earth-sheltering can pro• roof are covered with earth. In this design, the rooms are arranged around a sunken vide opportunities for discretion in sit• courtyard. ing "incompatible" facilities, however, due to the buffering effect of earth Some existing utility tunnels can be sheltered design, elevational design and berms. retrofitted or expanded to accommo• atrium design. Topography. Whereas conventional date a variety of utility services. Heat• The most widely constructed type of structures are generally limited to areas ing tunnels present a good example, as earth-sheltered structure is the eleva• with 8% slope or less, elevational earth• they can accommodate a variety of other tiona! design, which is earth-bermed sheltered structures are best suited to services, subject of course to the ca• on three sides with one exposed ele• areas where the slope ranges from 8- pacity and location of the tunnel. vation, usually a window wall (Fig. 6). 15%. (Areas with steeper slopes can be This design can have either an earth• employed with special design and en• Planning Earth-Sheltered Space covered roof or a conventional, well• gineering consideration.) The follow• There are two basic types of earth- insulated one. Elevational designs can ing are slope guidelines for earth-shel• tered construction : Slope of One-story earth• w w ">' ">' > w w 8-15%: sheltered con• «W "' it > "« struction. .,«0 2 ;;; 0 :;; i> z "'z Slope of Two- or three• 0 2 "' 2 "' 15-25%: story earth-shel• tered construc• tion. Slope of Earth-sheltered 25% or more: construction with special technol- ogy. and Stratigraphy of the Twin City Region, in Geology It is also possible to site atrium struc• of Minnesota, Sims, P. K., Morey, G. B., eds. A Centiennial Volume, p. 485-497. tures on sloped sites, although they are " more suitable to fiat sites. Figure 8. As much as 1,000 ft of Paleozoic rock was deposited in the Twin Cities basin. Orientation for solar access. Orienta• The formations remain largely undisturbed. tion for solar access is of concern only

94 UNDERGROUND SPACE September/October 1982 with elevational earth-sheltered de• opment. Viewed in cross secton, most medium-grained, well-sorted, very pure signs, since enclosed cut-and-cover of the city is underlain by well-drained quartz sandstone, is a friable, easily ex• construction and atrium structures do soils, a thick layer of strong limestone cavated rock. Since most f this "sand• not rely greatly on passive-solar energy able to span large openings without rock" is only weakly cemented, with techniques. In most parts of the north• structural support, and a layer of soft much of its strength being a result of ern hemisphere, it is desirable to site sandstone which is easily excavated. compaction, much of the tunneling in elevational earth-sheltered structures Minneapolis is also fortunate in that the Twin Cities has been done using so as to take advantage of winter solar information regarding the city's geol• water jets. heat gain. ogy is well documented and readily Due to its varying hardness, the 150- As with other solar building designs, available through the Minnesota Geo• ft-thick St. Peter sandstone is consid• earth-sheltered structures which incor• logical Survey. All available engineer• ered a layered rock- as can be seen porate passive-solar features are elon• ing test-boring and well-log data, some in exposed sections along bluffs gated in an east-west direction for dating back 100 years, have been col• where differential erosion maximum exposure to sunlight. The lected by the Survey and stored in a accentuates the harder, more optimal orientation for these designs is computerized data system with a map• resistant layers. When driving a within a range of 20° either direction ping capability. tunnel by jetting, an occa• stional of due south. Land sloping directly to The study area includes the city of hard layer may need to be blasted to the east or west is less desirable since Minneapolis, the Minneapolis-St. Paul assist the work. winter heat gain is not as substantial International Airport, and the seg• Underlying the Platteville limestone and the large summer heat gain causes ment of the adjacent city of Blooming• and the St. Peter sandstone is the Prai• cooling problems. ton which encompasses the abandoned rie du Chien group, comprised of do• Metropolitan Stadium site (slated for lomite and sandstone members. redevelopment into mixed commer• PART II. A CASE STUDY cial, industrial, and residential devel• OF MINNEAPOLIS opment). Soils and Topography The surface geology of the area is As in many northern cities, the land considerably more complex than the area in Minneapolis is fully developed, bedrock geology and no attempt was Geology of the Study Area leaving little room to accommodate in• made in this report to identify and clas• dustrial development or expansion. The Bedrock Geology sify all the surficial deposits. Extensive result: tax revenues and jobs are lost Many layers of sedimentary rock have information on the surficial deposits in to outlying areas still possessing un• been deposited in the study area over the area, including the implications for developed land. And like other north• time and remain largely undisturbed. engineering, is available from the Min• ern cities located in states which must The bedrock formations of interest to nesota Geological Survey. import all of their fuel, the cost of en• underground space development are The surficial deposits, formed dur• ergy figures largely in the city budget. the Platteville limestone, the St. Peter ing the Quaternary period, are typi• It has been estimated that energy could sandstone, and the Prairie du Chien cally 50 to 100 ft thick but may be nearly cost the city and its residents and busi• group, formations deposited about 400 400 ft thick where they fill preglacial nesses as much as $1 billion annually million years ago in early and mid• valleys, e.g., under the chain of lakes by 1990 (in constant 1980 dollars). Paleozoic time. As much as 1,000 ft of in western Minneapolis. All of these The use of underground space could Paleozoic rock was deposited in the surficial deposits are classified as un• address both the space and the energy Twin Cities basin (Figs. 8 and 9). consolidated soils. problem. The subsurface, by offering The nearly flat-lying layers of lime• Included in the study - is a general a new development layer within city stone and dolomitic limestone that con• classification of soils for engineering boundaries, could provide an ideal site stitute the Platteville formation aver• purposes using the unified soil classi• for many types of commercial, indus• age about 30 ft in thickness, though fication system (Table 1., above). By trial, and warehousing activities. Ap• in some areas the limestone has been combining various classifications of soil proximately 6,000 acres of mined space eroded to thicknesses of less than 10 ft with suitability criteria for earth-shel• could be developed with little difficulty, (Fig. 10). Where it is at least 10 ft thick, tered and cut-and-cover construction, and an additional 14,000 acres could most of the Platteville formation con• it is possible to simplify the soils situ• be developed with dewatering, water• stitutes a strong, competent layer that ation into areas of general degrees of proofing, and special construction can support itself over large openings. suitability for cut-and-cover construc• techniques. Appropriate use of earth• The new Civil and Mineral Engineer• tion (Fig. 11). Although this is a gross sheltering could utilize land unsuitable ing building on the Minneapolis cam• simplification of a complex surficial ge• for conventional techniques due to the pus of the University of Minnesota in• ology, it is instructive for general plan• presence of sloping topography or poor cludes 40,000 sq ft of space excavated ning purposes. As can be seen from the environmental conditions such as ex• directly beneath the Platteville. Spans map, most of the Minneapolis area has cessive noise from freeways or an air• up to 58 ft testify to the structural abil• soil conditions which are either good port. And in a city whose temperature ity of the formation to span large un• or good-to-fair for cut-and-cover con• ranges from -30°F in the winter to derground openings. struction. The soil suitability for tun• , l00°F in the summer, the stable mod• A monoclinal fold in the Platteville neled construction may vary from that erate temperature of an underground formation near downtown is an area of presented in Figure 11 due to the location (at a depth of 25 ft the year• disturbance in the limestone, with in• greater importance of other factors, round temperature is 50-55°F) has a creased frequency of joints and poorer such as the ease of dewatering. ot to offer in the way of energy sav• rock conditions. Construction should The topography of the site area is mgs. be approached with extreme caution another important factor in the suita• Minneapolis is fortunate in that the here since a number of tunnels have bility for earth-sheltered or cut-and• geology of its underground is ideally collapsed in this area. cover buildings because they require suited for underground space devel- The St. Peter sandstone, a fine- to the significant exposure of one or more elevations. The areas of the city with

September/October 1982 UNDERGROUND SPACE 95 Bedrock Geology Condition of Limestone

KILOMETERS KILOMETERS f(f) (f) 0 2 5 10 0 2 5 10 FEET IN THOUSANDS FEET IN THOUSANDS

Limestone Less than Platteville Limestone 10 Feet Thick ;: .

Limestone 10 Feet or [@<1/H St Peter Sandstone Greater _·::

• Prairie Du Chien Group • SCALE 1:140,000 APPROX. Monoclinal Fold SCALE 1:140,000 APPROX.

Limestone Absent D

Source: Geologic data and mapping. Bruce Bloomgren, Minnesota Geological Survey. Presen· Source: Geologic data and mapping: Bruce Bloomgren, Minnesota Geological Survey. Presen• tation and interpretation for planning: Susan Nelson, Underground Space Center. tation and interpretation for planning: Susan Nelson, Underground Space Center.

Notes: For data source information refer to Norvitch, A. F., and Walton, Matt, eds., 1979, Geologic Notes: For data source information refer to Norvitch, A. F., and Walton, Matt, eds. 1979, Geologic and hydrologic aspects of tunneling in the Twin Cities area, Minnesota. U.S. Geological Survey and hydrologic aspects of tunneling in the Twin Cities area, Minnesota. U.S. Geological Survey Miscellaneous Investigations Series 1·1157, 7 plates. Miscellaneous Investigations Series 1-1157,7 plates.

Figure 9. The bedrock formations of interest to underground de• Figure 10. Where it is at least 10ft thick, most of the Platteville velopment are the Platteville limestone, the St. Peter sandstone, limestone constitutes a strong competent layer that can support itself and the Prairie du Chien group. over large openings. Note the monoclinal fold near the downtown area. slopes of 8% or greater (Fig. 15 below) Hydrogeology tions that can be expected in the study are not a large percentage of the total Since the presence of ground water area are described. land area and many of the areas in• can affect the location, design, con• It is not uncommon to find a satu• dicated are devoted to uses which are struction, cost, and feasibility of ·un• rated zone underlain by an imperme• not likely to change in the near future, derground space development, the able layer, followed by unsaturated e.g., parks and cemeteries. types of potential ground water situa- material, and again by a saturated zone.

96 UNDERGROUND SPACE September/October 1982 Soil Suitability for Cut-and-Cover and Hydrogeology Earth-Sheltered Construction of the St. Peter Sandstone ' I l

KILOMEnRS KILOMETERS

0r -- 2 LJ5 --110 FEET IN THOUSANDS r--LJ--1 0 2 $ 10 FEET IN THOUSANDS

Good Artesian Condition Good to Fair • Water Table Condition Fair to Poor SCALE 1:140,000 APPROX• D St. Peter Sandstone Absent Poor SCALE 1:140,000 APPROX. BoundarY. of Artesian and ••• Water Table Conditions in Source: Geological data and mapping: Gary Meyer, Minnesota Geological Survey. Presentation the St. Peter Sandstone and interpretation• for planning: Susan Nelson, Underground Space Center.

Notes: For data source information refer to Norvitch, A. F., and Walton, Matt, eds. 1979 Geologic and hydrologic aspects of tunneling in the Twin Cities area, Minnesota. U.S. Geological Survey Source: Geologic data and mapping: Roman Kanivetsky, Minnesota Geological Survey. Presen• Miscellaneous Investigations Series 1·1157, 7 plates. tation and interpretation for planning: Susan Nelson, Underground Space Center.

Notes: For data source information refer to Norvitch, A. F., and Walton, Matt, eds., 1979, Geologic and hydrologic aspects of tunneling in the Twin Cities area, Minnesota: U.S. Geological Survey Figure 11. By combining various classificatiom of soil with suit• Miscellaneous investigation Series 1·1157, 7 plates. ability criteria for earth-sheltered and cut-and-cover comtruction, the complex surficial geology can be simplified and a range of Figure 13. Both artesian conditions and water table conditions suitability (good to pom) for construction can be identified. prevail in the St. Peter sandstone.

A typical situation is shown in Figure The upper part of the sandstone below into the river valley exit points. The 12, where a zone of saturated drift or the shale is unsaturated because any deeper strata of the sandstone are sat• soil overlies limestone and shale. The leakage from the stratum above is not urated, creating a second water table. shales are often impermeable and pre• sufficient to fill or recharge the sand• The upper · portions of the St. Peter vent the glacial material from draining. stone with water, and the water drains sandstone which are essentially unsat-

September/October 1982 UNDERGROUND SPACE 97

Occurrence of Water in Geologic Formations

PERCHED WATER TABLE - - - -- (WINTER) 1

PERCHED WATER TABLE--- --, BJ;1IT Ci*t-:.::.::·"'"

(SUMMER) _j! iiii - i -! :: ETONE UNSATURATED SANDSTONE MAIN WATER TABLE - - , SHALE

,···''.':''/::;·.0..:./.::.::C:T-MAIN WATER TABLE f.\;·\' ':/:;'i/ >,: f."·:.:•':. ::_-FULLY SATURATED SANDSTONE (ARTESIAN CONDITIONS)

Source: Potential use of underground space. Department of Civil and Mineral Engineering, University of Minnesota, 1975.

Figure 12. A zone of saturated drift or soil overlying limestone or shale is a common occurrence. urated present the best opportunities stone, and less commonly in the upper Most of the water supply for the rest for mined space development. parts of the stratum where most un• of the Metropolitan Region is obtained derground space development is ex• from the Jordan aquifer, a 90-ft layer Artesian vs. Unconfined Water Conditions pected to take place. These lenses in of sandstone approximately 530 ft be• Since the various rock layers differ the upper portions can cause minor low the surface, separated from the St. in their ability to transmit water, the confined water conditions in the sand• Peter sandstone by the 130-ft - thick Twin Cities basin consists of a series of stone, and reduce the efficiency of de• Prairie du Chien group. The imperme• aquifers separated by aquicludes. The saturating the sandstone via deep well able basal layer (siltstones) of the St. confining impermeable beds severely pumps. Fortunately, their occurrence Peter sandstone would limit the effect restrict the vertical movements of water is not common in the upper 30ft. on underlying rocks of mined space to or from a confined aquifer. Where Unconsolidated materials that cover development in the upper portions of this occurs and where water can enter the bedrock are quite variable in na• the St. Peter sandstone. an aquifer at the eroded edges of the ture both horizontally and vertically. It In brief, the prevailing geologic con• strata, the water will move downslope would be unusual to find a location ditions which make possible cost• along the dip of the permeable layer, where there were not at least two effective mined space development in creating a build- up of pressure in the different geologic units represented in Minneapolis are: aquifer, i.e. artesian conditions. a vertical section. These units occur as l. The presence of a minimum 10- In regard to unconfined water, the discontinuous lenses, are intertongued ft-thick layer of Platteville Limestone level of the water table, i.e., the top of and interlayered, and consist of gravel, to form the strong roof of the exca• the saturated zone, will fluctuate with silt, clay, and alluvial materials. vated space. the seasons due to the varying rate at Where impermeable lenses of clay which the geologic unit is recharged by 2. Suitable hydrogeology in the St. occur in the glacial till, it is common to rainfall or by leakage into it during the Peter sandstone, the bedrock layer to have saturated materials on top of them, year, as well as the varying rate at which be excavated. creating local perched water tables water is drained or removed via wells. The optimal hydrogeological situation which, if not adequately drained, may Where water drains more or less hor• would be a water table well below the cause support problems when tunnels izontally to a river bluff, freezing at the lower level to be excavated , resulting are driven through the saturated area. in a dry space. The next best is one bluff face will block the drainage and It may be less difficult to tunnel be• cause a rise in the water level back from where water is present in the St. Peter neath these impervious clay layers than the valley wall-a common occurrence sandstone, but is unconfined. This lat• to construct tunnels through them. ter situation requires waterproofing and in the study area. No reliable data have been compiled dewatering. The least desirable hydro• The St. Peter sandstone is under both regarding the location of perched water geologic condition would be the pres• artesian conditions and water table tables in soil in the study area though . ence of water under artesian condi• conditions (Fig. 13, on preceding page). some information is available through tions in the St. Peter sandstone. Though In the areas which exhibit unconfined soil testing and engineering data. construction of mined space is possible water, the St. Peter sandstone drains under such conditions, it requires ex• to the Mississippi River. The depth of Domestic Water Supply tensive mitigation and is more costly. unsaturated or dry sandstone in those While the presence of ground water areas ranges from 30 ft or more near• must be considered when considering est the river to the point of saturation Geologic Hazards underground space development, so at the contour where artesian condi• A hazardous geologic feature in the too must the effect of underground tions begin. Those areas where unsat• study area is the occurrence of caves space development on groundwater be urated sandstone occurs beneath com• and voids in the sandstone, usually just considered. petent limestone provide the most below the overlying limestone layer. The major water supply for the city favorable sites for most types of un• Some known natural ca ves occur along of Minneapolis is the Mississippi River. derground space development and use. the river bluffs, where water entering The St. Peter sandstone is not an im• joints or fractures in the limestone mi• Perched Water Tables portant aquifer for domestic water grated down into the friable sandstone Lenses of relatively impermeable supply as it produces only small amounts and moved toward the Mississippi River, siltstones occur regularly in the lower of water and may be contaminated due which is at a lower elevation. Where 50ft of the 150-ft-thick St. Peter Sand- to leakage from near-surface facilities. the disaggregated sand grains escaped

98 UNDERGROUND SPACE September/October 1982 Potential Areas Most Suitable for Cut-and-Cover and for Mined Space Development Earth-Sheltered Construction

KILOMETERS r-i....]--i KILOMETERS 0 2 5 10 r-i....]--i

FEET IN THOUSANDS 0 2 5 10 FEET IN THOUSANOS aJ Most Favorable Areas

Less Favorable Areas Suitable Soil Conditions '; ISil1ll1l1l Monoclinal Fold in lll1llllll!l\!l Limestone' Water Table Slopes Greater than Conditions in Sandstone 8 Percent SCALE 1:140,000 APPROX. SCALE: 1:140,000 APPROX. Monoclinal Fold in Unsuitable Soil Conditions •D Limestone' Artesian Boundary of Artesian and Conditions i,n Sandstone • • • Water Table Conditions in the St. Peter Sandstone DLimestone Absent -Bedrock Contours

Source: Presentation and interpretation for planning: Susan Nelson, Underground Space Center. Source: Presentation and interpretation for planning: Susan Nelson, Underground Space Center.

Figure 14. This composite map of the bedrock geology, the condition Figure 15. This composite map of soil suitability and slopes greater of the limestone, and the hydrogeology of the study area shows the than 8% shows the areas most suitable for earth-sheltered and cut• potential areas for mined space development. and-cover construction.

at a bluff face, openings formed and count when future underground de• nellinings has caused the sandstone to gradually widened and extended to• velopment is planned. break down. Where sand has escaped, ward the water sources. A few large Man-made caves also occur, as are• voids have formed, some of substantial natural caves are also known to exist sult of mining in the sandstone or as a size. beneath the downtown area and their side effect of other activities, e.g., where The location of most mined caves and presence will have to be taken into ac- leakage of water along pipes and tun- some natural caves are known and have

September/October 1982 UNDERGROUND SPACE 99 Redevelopment and Redirection Areas, Potential Transitways and Closed-School Sites and Bedrock Geology Composite Soils Suitability Composite

KILOMETERS r--t.J--i 0 .G 1.5 3 0 2 5 10 FEET IN THOUSANDS n r--i 2 ' 10 FEET IN THOUSANDS

1!1!1!!1!1!1!11 Redevelopment/Redirection l!l!l!lll!!!l Areas wh Good to Fair • • • Potential Transitways Soil Conditions

Redevelopment/Redirection IP'Ta Areas with Unsuitable Platteville Limestone Soil Conditions Good to Fair Soil Conditions [);(?;j St. Peter Sandstone SCALE 1:140,000 APPROX. Outside Redevelopment! SCALE 1:140,000 APPROX. Redirection Areas • Prairie Du Chien Group DUnsuitable Soil Cond ions e Closed School Sites

Sources: Minneapolis Plan for the '80's, July 1981. Susan Nelson, Underground Space Center Source: Minneapolis Plan for the '80's, July 1981. Minneapolis School Board.

Figure 16 . Large parcels of land may become available for earth• Figure 17. Owing to the presence of the Platteville limestone, much sheltered and cut-and-cover structures as a result of redevelopment. of the study area is suitable for construction of a subway in mined space. been mapped. Site studies should de• ing the ice age. Later, as drainage was apolis occur as a series of depressions tect the presence of these geologic fea• blocked and changed by the advance along a major buried valley. Develop• tures. An important geologic feature and retreat of glaciers, they were filled ment of underground space is not con• in the study area is the existence of a and buried by water-laid deposits and templated for such areas. number of "buried valleys." These are glacial debris. Commonly there is little Although much of Minneapolis ap• river valleys cut down into the Prairie or no surface evidence of their exis• pears generally suitable for cut-and• du Chien formation prior to and dur- tence. The chain of lakes in Minne- cover construction in soil there are a

100 UNDERGROUND SPACE September/October 1982

number of potential construction cavations in potential mined space, and eliminated by the routing of the Great problems which will require attention some portions of the water system and River Road, a proposed parkway to be in the early stages of a particular pro• abandoned power structures are found constructed by the federal government ject. in the St. Anthony Falls milling district. along the Mississippi River from its Pollution of the groundwater should The likelv candidates for utilidor source at Lake Itasca in northern Min• present a problem for subsurface con• conversion in the study area are the nesota to New Orleans, Louisiana. Pro• struction only if the groundwater has heating distribution tunnels at the U ni• tection of a potential access to mined become corrosive or toxic. A final versity of Minnesota, and the aban• space at that location is now accepted problem in many otherwise suitable doned street car power tunnels con• as one of the criteria for Great River areas of the city is the presence of boul• structed in the early 1900s. Road planning. ders in the glacial drift. Although boul• It may be desirable or necessary to ders are more prevalent in some areas serve mined space with vertical access than others, they can be considered a The Geologic Potential for at certain points of the city. Since the random hazard in that they interfere Underground Space Development cost of access depends to some extent with drilling operations and can in• on its depth, the depth from the sur• Mined Space crease excavation costs, particularly in face to the bottom of the limestone was small excavations. Potential areas for mined space de• also mapped. velopment in the study area are shown In much of the study area, the depth Valuable Mineral Deposits in Figure 14, a composite map of bed• from the surface to the bottom of the The Metropolitan Council is cur• rock geology, the condition of lime• limestone ranges between 50 and 100 rently preparing a study of the location stone, and hydrogeology in the study ft. The above-mentioned Civil and and extent of potential aggregate re• area. Mineral Engineering building on the sources throughout the seven-county As mentioned above, the most fa• University of Minnesota campus is being Metropolitan Area. Generalized maps vorable areas for development are those constructed in such an area. Excava• at a scale of 1:24,000 and 1:100,000, where water in the St. Peter sandstone tion was carried out via a vertical shaft now being prepared under contract by is under water table conditions (Figure at a depth of 75 to 100 ft. Once the the Minnesota Geological Survey, in• 13 above). The level of the water table access shafts were provided it became dicate the potential significance of ag• drops steadily as the bluffs along the economical to add mined space within gregate resource areas based on the Mississippi River are approached, re• the building and reduce space in the amount of data available, the percent• sulting in substantial areas of the upper cut-and-cover section of the structure. age of gravel versus sand, the thickness portion of the St. Peter sandstone re• of the deposit, and the amount of cover maining dry year- round . Most of the Earth-Sheltered and or waste materials over the resource. areas immediately adjoining the bluffs Cut-and-Cover Space have approximately 20-30 ft of dry The most suitable areas for earth• sandstone beneath the Platteville lime• Existing Underground Structures sheltered and cut-and -cover construc• stone. These areas are prime locations tion are shown in Figure 15, a com• Data regarding ex1stmg under• foro two-story mined space develop• posite map combining soil suitability and ground structures in the study area were ment, or for uses which benefit from slopes greater than 8%. gathered from the following sources: a very high ceiling, such as warehous• Earth-sheltered construction is es• Minneapolis Department of Public ing or manufacturing plants. The por• pecially suited to sloped areas of the Works tions which are considered to be most city because access transitions can be Minneapolis Water Works favorable for development in the study easily made. Enclosed cut-and-cover Minnesota Gas Company area comprise approximately 380 mil• construction will often have access (Minnegasco) lion sq ft. through adjacent conventional build• Northern States Power Company The Mississippi River bluffs have ings, in which case a sloped site is not Northwestern Bell Telephone been assessed for potential horizontal necessarily preferable. Company portal access, and the possibilities are Since there is little vacant surface land Minneapolis City Coordinator's Office surprisingly quite few. Besides the top• within the city, earth-sheltered con• Northern Cable Vision Company ographic and geologic factors neces• struction will probably occur primarily Minnesota Historical Society sary for horizontal access, other con• on an infill basis as individual parcels Pillsbury Company siderations that must be taken into of land become available. Larger de• Every effort was made to gather and account are the types of land use in the velopment areas likely to become avail• map all data on existing underground surrounding surface area and its access able are some of the former school sites structures, both abandoned structures to a major transportation network. where the buildings have been or are and those currently in use. Structures Much of the river area with potential scheduled to be demolished (see be• found in the subsurface include water access is zoned for residential use, and low). These larger parcels ofland would mains and tunnels, sanitary sewers, only in two locations are other uses allow construction of earth-sheltered storm sewers, electrical cables, tele• found . An area near Interstate 35W is housing developments or commercial phone lines, district heating pipes, hy• currently used for a variety of activities areas. dro power "raceways," abandoned tun• including residential, commercial, and nels, and caves. some manufacturing. This site pro• Underground Space and the Existing structures that would affect vides excellent access to the interstate, Plan for the 1980's the development of mined space in the regional, and local transportation net• study area are those located in the St. work. The other site, at Lake Street A comprehensive plan for the dec• Peter sandstone beneath the limestone and West River Road, is not located ade of the 1980's has recently been layer or in the limestone itself. Rela• near the major interstate and regional completed for the city of Minneapolis. tively few structures are found in the transportation network. This plan is intencied to guide city de• St. Peter sandstone. Sanitary and storm As mentioned above, one of the few cisions on housing, economic devel• sewer systems comprise the bulk of ex- suitable access points could have been opment, physical environment, trans-

September/October 1982 UNDERGROUND SPACE 101 .r------

continue to generate interest. Previous analyses have found that costs would be too high and the ridership too low to justify construction of a subway or light rail transit (LRT) system. How• ever, this situation may be changing due to the higher price of gas, the ex• pected large number of new employees in the central business district, and the recently construc ted domed stadium in the downtown area which will draw pa• trons from the entire region. Although LRT is attractive in many respects (low initial capital investment, low operating costs) it has disadvan• tages. The difficulty of right-of-way ac• quisition, and the density of develop• ment and the congestion of the existing road network in the downtown area may be obstacles to its operation. Moreover, LRT (like other surface transportation systems) is subject to ad• verse weather- blizzards, ice, heavy winds, and rainstorms can affect The Mississippi River bluffs at this location could provide access to mined space development. its op• era tion. The site is near an industrial area and is served by the interstate freeway (center). As shown in Figure 17, much of the study area is suitable for construction portation, property services, human closing of 18 city schools in June of of a subway in mined space. In those development, and health and safety. 1982 (Fig. 16), but, overall, earth-shel• areas where the Platteville limestone is Because Minneapolis is a fully devel• tered housing is not likely to figure not present, soft-ground construction oped city- only 818 acres, or heavily in the city's future plans. methods could be used , but may be approx• imately 2%, of the city's land prohibitively expensive (except for short area is vacant- the plan is Industrial Development A recent survey indicates that ap• distances) in comparison to a surface primarily con• cerned with the system. The critical factor is knowing redirection of blighted areas, the proximately 25% of all distribution firms in the city plan to expand their facilities where the Platteville is present so that redevelopment of under-uti• lized construction estimates for both time and areas, and the preservation of the city's in the near future, but that fewer than half of these firms have sufficient space expense can be prepared accuratel y. A best features. It is hoped that subway system would become substan• knowledge of the availability of a vast to do so in their present location. Sim• ilarly, 55% of the manufacturing firms tially more competitive in cost if it could amount of developable underground be tied into underground industrial and space, as well as the advantages in en• surveyed in 1980 wanted to expand at their present location, but fewer than commercial space along the transit cor• ergy efficiency that it could provide, ridor. will enable the city to think in more half of those surveyed have space to do so. In addition, many warehousing op• expansive terms. Commercial Development erations are being displaced due to the conversion or removal of warehouses The central business district in downtown Minneapolis is the major The Housing Plan at the edge of the downtown area. A Housing in Minneapolis will serve a recently completed report on the eco• commercial center. The main principle population mix in the 1980's and be• nomic situation in Minneapolis esti• guiding its development is compact• yond different from that in previous mates that as many as 3,000 to 4,000 ness. All major buildings are to be lo• years. Despite a 28% decline in pop• marketing and distribution jobs and cated inside an area the boundaries of w hich are within reasonable walking ulation since 1950, the number of 5,000 to 10,000 manufacturing jobs distance from the center. By providing households has remained essentially could be lost to the suburbs in the next room to accommodate commercial and unchanged due to a decline in the five years unless suitable expansion retail expansion, the use of under• number of families with children and space is found. an increase in the number of one- and ground space could facilitate the prin• two-person households. A major goal The subsurface could provide the ciple of a compact downtown. space for this industrial expansion• of the city's housing policies in the 1980's At present, a pedestrian skyway sys• and a consequent increase in the city is to shift small households into smaller tem linking the major buildings in the housing units, thus freeing much of tax base. In many locations along the central business district at the second• the existing stock oflarge single-family Mississippi River bluffs where the St. floor level might appear to obviate the homes for families with larger space Peter sandstone is dry, facilities with need for a pedestrian tunnel system. demands. To this end, the plan for the ceilings as high as 30 ft could be con• However, as a result of constr uction structed to accommodate many types 1980's recommends the construction currently under way, an estimated of industrial and warehousing facili• of 6,000 new housing units by 1990. 15,000 to 20,000 new employees will The best opportunity for construct• ties. be added to the downtown workforce ing earth-sheltered housing will be on Transportation by 1990 (City of Minneapolis Planning large parcels of land, a square block or Mass transit in Minneapolis is cur• Department 1978). Additionally, it is more, where several units can be con• rently provided by an all-bus system, estimated that the population in the structed as a group. Such large parcels though other forms of mass transit downtown area may increase by ap- may become available as a result of the

September/October 1982 102 UNDERGROUND SPACE I

proximately 13,200 persons by 1990 as a result of new housing construction along the riverfront (City of Minne• Substantial support for the general concepts apolis Planning Department 1981). A contained in the case study has been shown pedestrian tunnel system in the central business district and its immediate vi• in a broad range of city offices and agencies. cinity would be convenient to office workers, shoppers, and residents, and could create a natural link to an un• derground subway system and under• ground commercial centers as it has in the city planning department. Whether ment of Civil and Mineral Engineering at the University of Minnesota, for his work Montreal and Toronto, for instance. this increased awareness of under• ground potential and interest in its de• on the geology and hydrogeology of Min• Not only the downtown area, but also velopment will result in more effective neapolis and for providing technical assis• neighborhood and comm unity com• utilization of underground space in the tance throughout the study. Dr. C harles mercial centers could benefit from un• Nelson, a private geotechnical engineering future is still an open question, al• derground commercial development, consultant, also provided technical assis• though it appears reasonable to con• especially in conjunction with the de• tance and reviewed draft material. clude that the outlook is substantially velopment of medium- and high-den• more promising now than it was before sity housing complexes surrounding References the study was done. 0 these centers. Increased density could City of Minneapolis Planning Department. be accommodated with very little visi• 1978. Minneapolis metro center forecasts ... 1990. Minneapolis, Minnesota. ble impact on the surface environment. ---. 1981. Central riverfront urban design Community centers could retain a scale Acknowledgment guidelines. Minneapolis, Minnesota. appropriate to their surroundings while Jansson, B., and Winqvist, T. 1977. Plan• providing the goods and services nec• Many individuals and institutions were ning of subsurface use. Swedish Council for essary to an expanded community. Pe• involved in the preparation of this study. Building Research. destrian tunnels linking the centers to The generous assistance they have provided Legget, R. F. 1962. Geology and engineering. nearby apartment complexes would during the whole course of the study is New York: McGraw Hill. provide residents the same protection greatly appreciated. The authors wish to - - -. 1973. Cities and geology. New from climate and traffic as is enjoyed thank Control Data Corporation for pro• York: McGraw Hill. viding the major funding which made this McManis and Associates, Inc. 1980. City of by skyway users downtown. study possible. The study was performed in Minneapolis neighborhood economic develop• collaboration with the American Planning ment program strategy. Minneapolis Min• The City's Reception Association (APA), which provided much in• nesota. of Underground Planning formation on current planning practice. Minneapolis Energy Futures Committee. Martin Jaffe, senior research associate with 1981. Charting our energy future. Report of Since the planning study was neither APA, was a major contributor to that part of the Energy Futures Committee to the conducted nor funded by the city, there the study dealing with the regulation of un• Minneapolis City Council and Mayor are obvious questions as to the extent derground space. Donald M. Fraser, April 1981. to which the planning data or the rec• The study could not have been under• Sterling, R., Aiken, R., Carmody, J. 1980. ommendations will actually be incor• taken without the valuable resource data and Earth-sheltered housing: code, zoning and fi• porated in the city's long-range plan• technical assistance of the Minnesota Geo• nancing issues. Washington, D.C.: U.S. ning. The initial experience, since logical Survey. All geologic mapping is based Government Printing Office. completing the study, has been very on current published and unpublished data Sterling, R. L., and Nelson, S. 1982. Plan• positive. Substantial support for the made available to the authors by the Geo• ning for underground space: a technical re• logical Survey. Bruce Bloomgren, Gary source guide. Minneapolis, Minnesota: Un• general concepts contained in the case Meyer, Roman Kanivetsky, and Betty Kee• derground Space Center. study has been shown in a broad range ler, Geological Survey staff members, gen• - - . 1982a. Planning for underground space: of city offices and agencies, including erously lent their time and assistance to the a case study of Minneapolis, Minnesota. Min• the mayor's office, the city council, the project. A special note of thanks is extended neapolis, Minnesota: Underground Space community development agency, and to Professor Donald H. Yardley, Depart- Center.

September/October 1982 UNDERGROUND SPACE 103