Underground Space - Elseviermedia.journals.elsevier.com/content/files/planning-the...Underground Space Raymond L. Sterling Susan Nelson N recent years there have been many calls for

I

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 many calls for better planning of underground space utilization.

Robert Legget has written extensively about the need for consideration of ge ology in city planning (1962, 1973), Birger Jansson and Torbjorn Wind qvist (1977) have discussed many of the

planning issues and suggested a plan- · ning methodology for subsurface space

use in Sweden. The need for planning is almost self-evident; underground excavations permanently alter the sub surface condition, and the subsurface of cities is becoming increasingly clut tered with a variety of services and ser vice facilities. On top of this, there is increasing incentive from energy and environmental concerns and land costs to use the subsurface for more than

carried the exercise the next step into effective resource planning.

The study which forms the basis for this paper represents an attempt to provide resource material and a guide to developing a planning structure for subsurface space use. The generalized planning concerns were developed in concert with a de ailed case study of Minneapolis, Minnesota, which is in

routine service functions. tended to serve as a useful planning Raymond L. Sterling, principal investigator

of the planning study on which this article

is based , is senior co-editor of Underground

Space and director of the Underground Space

Center at the University of Minnesota. Su

san Nelson, a land-use planner, was project

coordinator and principal author of the

study.

86

While the cogency of this rationale may suggest a rush by cities to docu ment their subsurface conditions and plan for effective underground space utilization in their urban area, this has really not been the case. A number of cities have documented their geology fairly thoroughly , but very few have

Underground Space, Vol. 7, pp. 086-103, 1982

Printed in the U.S.A. A ll rights reserved.

document for Minneapolis as well as a case example for the resource guide. The goal of the effort is to have un derground facilities, underground space use, and subsurface resources in corporated into city planning proce dures specifically as a city resource or a category of public or private works.

0362-0565/82/020086-18$03.00/0

Copyright © 1982 Pergamon Press Ltd.

Drawing by John Carmody*

At the same time, it is hoped that the development of U.S. planning guide lines will encourage other cities besides Minneapolis to develop their own de tailed inventory and planning docu ment, that with an exemplary study and a resource guide other city planners will incorporate underground plan ning as an integral part of their com prehensive planning efforts in the same manner as they do land use, transpor tation, housing, industry, etc.

A brief account of how the Minne apolis study evolved illustrates the difficulties of gaining widespread in terest in underground planning and getting support for such a project which cannot show immediate bene fits-at a time when state and federal governments are in financial straits. The seeds of the study were planted in the early 1970's when underground and subsurface space use engineering be came a focus of the Civil and Mineral Engineering Department at the Uni versity of Minnesota. The main thrust of this interest was in the urban appli cations of underground space use. This interest was not widely shared, how ever. A conference, held in 1974, deal ing specifically with this topic and fea

Minneapolis' generally favorable ge ology for the creation of subsurface space use was probed again in 1980. Discussions were held with officials from the City of Minneapolis, Hennepin County (which includes the city), the U.S. Department of Energy, and Con trol Data Corporation. By this time, the American Planning Association had joined forces in declaring the need for a resource guide for subsurface plan ning. As a result, a $200,000 budget was prepared by the Underground Space Center and the American Plan ning Association for joint funding by the four organizations. The study was to be split into two parts, a Technical Resource Guide for subsurface plan ning and a case study for Minneapolis and Hennepin County.

There followed a protracted process of trying to wrest funding from the organizations. Control Data Corpora tion, an unlikely candidate perhaps to most readers, but an innovative com pany which sponsors a varied research program, was willing to commit its share of the funds at an early stage to help leverage funds from the government agencies. Hennepin County decided quite early on that subsurface planning

partment of Energy were encouraging but each faced severe budget cutbacks. At this stage, it was decided to try and start the project with the Control Data share of the funding (approximately one-third of the original amount) and

. to solicit private support from large city businesses which the city might then match. Although a very significant effort was devoted to raising additional funds, the total raised was only $6,050. The city continued to be very supportive of the study, providing data and other as sistance, but never did find funds to support subsurface planning.

The shortcomings of the study are most apparent in the Technical Re source Guide, which, due to the lack of funding, generalizes the planning concerns found for the Minneapolis geology but does not examine or illus trate in detail other typical geologic conditions. The Case Study suffers from a lack of detailed investigation of the more favorable geologic areas identi fied and from the absence of illustra tive designs for the most promising de velopment opportunities.

Merely conducting the study with city participation has had a tangible re 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 ban areas, attracted only 20 people.

The interest in taking advantage of

September/October 1982

much responsibility as the city and therefore declined to support the study. The City of Minneapolis and the De-

*Illustration of potential underground de

velopment at the University of Minnesota,

Minneapolis campus.

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 a Great River Road extension through Minneapolis (see below) which could have prevented future use of one of the few suitable river bluff access points to mined underground space near the city center.

While the components of the study

Data must be presented in a form

suitable for planners with little background

in geology or underground engineering.

a technical resource guide for under ground space planning and a detailed planning evaluation for Minneapolis are presented to stand independently (Sterling and Nelson 1982, 1982a*), the information is combined here to avoid repetition and to keep a specific focus for the planning discussion. The crit ical issues in the planning process are the geological data assembly, the de velopment of criteria for the interpre tation of the data for planning pur poses, and the presentation of interpreted data in a form suitable for planners with little background in ge ology or underground engineering.

The Case Study could not have been attempted at the current level of detail without an excellent and readily acces sible data base compiled by the Min nesota Geological Survey. Their data and geological mapping, combined with engineering criteria developed for the most suitable soil, rock, and hydrogeo logical conditions for underground construction, made possible the gen eralized identification and mapping of areas in the city suitable for under ground space development. While the simplified data can undoubtedly be de duced from the existing detailed data, most planners are not conversant enough with geologic maps and data to do this. Engineering criteria must also be applied to the data to reveal the distribution of favorable geologic strata, existing structures, and accessible zones for development.

The distribution of the under ground space resource throughout the city and its relation to existing uses and planned development form the basis for the incorporation of subsurface planning into a comprehensive long range plan which treats the subsurface as an integral resource of the urban area.

For the Technical Resource Guide, the American Planning Association surveyed 1,000 of its members to de-

*Planning for Underground Space: A Technical

Resource Guide and Planning for Underground

Space: A Case Study for Minneapolis, Minnesota

are available from the Underground Space

Center, University of Minnesota, 2 Appleby

Hall, 128 Pleasant Street SE, Minneapolis,

Minnesota 55455.

88 UNDERGROUND SPACE

termine current practice and problems in the regulation of underground space through zoning ordinances, building codes, etc. This valuable collection of information is touched on only lightly in this paper and hopefully will be the subject of a future paper on under ground space planning and regulation. PART I. PRACTICAL GUIDELINES

Three types of underground space are considered in this study. Mined space is created in deep bedrock by tunneling or mining through a vertical shaft or other remote access. Earth-sheltered space and cut-and-cove·r space are dug from the surface by excavating earth and re placing it to complete the construction. This similarity in the construction of earth-sheltered and cut-and-cover space and their consequent similar relation to the surface make for similarity in planning. Consistency with the sur face, in terms of use and aesthetics, must be considered for earth-sheltered structures since they are only partially surrounded by earth. For cut-and-cover structures, consistency of use is less of a consideration since the purposes they serve are likely to be auxiliary to sur face uses. Mined space use, on the other hand, need not be related to surface activity except at access points. An other significant difference between these two similar types of under ground space and mined space is that their construction often entails a much greater degree of surface disruption and inconvenience in urban areas. Assessing the Geologic Potential for Underground Space

Geological conditions are the pri mary factor determining the potential for developing underground space in a community, and the first step in plan ning the space is conducting an assess ment of these conditions. The assess ment should result in a clear idea of where underground space can be de veloped without difficulty or with mit igation, and where it cannot be devel oped, as well as where valuable mineral deposits may be located.

The assessment of a community's potential for underground space is car-

ried out in practice by drawing up a series of single-scale maps which can be superimposed upon a base map and upon each other to aid in the analysis. The maps to be prepared for an area will depend on its geology and the agreed upon assessment criteria. For instance, in an area where the presence and condition of a sedimentary bed rock formation is judged to be the pri mary determinant of the potential for mined space development, overlays which identify the location, thickness, and competency of the rock must be · prepared. Further analysis must assess the hydrogeology of the area, the lo cation of existing structures, and po tential access points.

In general, however, the geologic map inventory for underground space de velopment should include the follow ing maps:

the bedrock geology; the condition of the bedrock; the bedrock hydrogeology; the potential points of access to mined

space; the location and elevation of the

floodplain; existing structures in the bedrock; the suitability of soils; areas with slopes of 8% or more (at

the community level); detail maps of areas with slopes of

8-15%, 15-25%, and 25% or more (at a large scale);

the hydrogeology of soils; existing structures in the soil; areas of hazardous or difficult geo

logic conditions; the location of valuable mineral re

sources including basic construc tion materials such as sand and gravel.

These maps should be housed cen trally where they will be readily avail able to anyone who needs them. They should be updated on a regular basis, preferably after each important con struction project. This last point is es pecially important where large struc tures located in the subsurface and utility extensions are concerned.

It is worth emphasizing here that, while geologic data mapped at the community scale will be adequate for community land-use planning, de-


tailed engineering studies are the only reliable source of detailed information for proposed specific projects.

Mined Space

The main factors in assessing the po tential for mined space development are evaluating the bedrock geology, the hydrogeology of the bedrock and of the overlying soils, the potential points of access to mined space, and existing structures above and in the bedrock.

Bedrock geology. Most mined space will be created within a thick and compe tent bedrock stratum, but it can also be dug in a soft layer beneath a layer of competent bedrock. In either case, the bedrock must be of sufficient thickness to provide the structural support nec essary for mined space development. The essential conditions for mined space development vary with the type of rock, so that geotechnical engineers familiar with the engineering properties of the local geologic formations must be con sulted early in the planning process.

The factors to be considered include the presence and extent of suitable rock strata, the condition of the bedrock, potential clear spans, the depth of the strata, and the potential for access.

Geologic data can be assembled from a number of sources. Most states in the United States have a Geological Sur vey, although the extent and reliability of the data these provide vary greatly. A second important source is geology departments at major universities. Data from these main sources can be coor dinated with additional information

from the U.S. Geological Survey in the Department of Interior when it is avail able.

Besides the state Geological Survey, state agencies such as the Department of Natural Resources, Department of Transportation, Health Department, Historical Society, and architecture and engineering offices in administrative departments often hold relevant rec ords. Regionally, airport and waste control commissions may have useful holdings. At the municipal level, the Department of Public Works, Depart ment of Community Services, Housing Authority, and similar agencies often keep records of geological information obtained during the completion of projects. Mining records are invaluable in regions where mineral resources have played a role in development.

The reliability of these data from various sources with different record ing procedures can be improved through attempting to create a certain degree of redundancy in the source material. Cross-checking data can com pensate in part for sketchy specific in formation from sources usually relied upon to provide more thorough infor mation.

Hydrogeology. The presence of water in bedrock and soil will affect the cost and feasibility of mined space devel opment. The absence of water in bed rock presents the optimum condition for mined space development, since space can be created without the costly mitigation required when water is present.

Water can occur in bedrock in sev eral forms. The least problematic is un confined water, i.e., under water table conditions. The level of this water may vary somewhat within the bedrock due to seasonal conditions, but the water is not confined or under pressure. Gen erally, proper design and construction techniques together with drainage and waterproofing will ensure that the mined space will remain dry and sound if above or near the free-water surface.

Water in bedrock can also be con fined i.e., under artesian conditions, wherein the bedrock stratum is satu rated and the water is confined by im pervious layers above and below. Seep age of surface water into the stratum via exposed and slanted layers of the rock creates pressure. Water under artesian conditions is more difficult to mitigate if the rock permeability is high, and can raise the cost of projects in

mined space substantially. Water in soils above the bedrock

stratum can also affect mined space de velopment. Water retained between or above impervious layers in soil or rock, called perched water tables, can affect mined space if the impervious layers are punctured and the water drains into the bedrock space below.

If it is necessary to collect more data to provide an accurate view of the hy drogeology in an area, it is best to per form repeated testing at the same time of the year, so that data do not vary with seasonal shifts in water conditions.

Access. Access to the subsurface is an 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 subsurface development. The primary criteria for determining the appropri ate type of access for mined space are the use to which the space will be put, the extent of operations, the physical potential for access, and cost. Horizon

tal portal access is the most cost-effective entry. Potential horizontal or bluff ac cess points are best identified by visual inspection in conjunction with geologic mapping, which allows the assessment of such factors as height from the bot tom of "roof" material to adjacent land or road level, the potential for creation of vehicle maneuvering areas, the av erage water level if the site is adjacent to a body of water, and potential flood levels.

Vertical shaft access can be created in a wide range of competent materials. The integration of surface and sub surface uses will be an important factor in determining the location of vertical access points. The main geologic fac tors to consider are the depth from the surface to the level at which mined space will be created and the likelihood of perched water tables at each shaft lo cation.

Existing structures. Mined space can often be developed with little difficulty in regard to surface structures. While surface structures may increase the load on the bedrock, the load can be offset 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

depth from a significant rock stratum to the top and bottom of tunnels and pipes. Elevation and capacity should also be indicated at various points along the facility.

Earth-Sheltered

and Cut-and -Cover Space

The economical construction of earth-sheltered and cut-and-cover space depends primarily on the presence of well-drained, stable soils. The major factors under evalu ation in planning these types of underground space are the quality and distribution of the soil, the hydrogeology of the soil, topog raphy, the potential for the favorable orientation of openings, and existing structures in the soil.

Soils. Information on shallow soils recorded on survey interpretation sheets are available in every region of the United States from the Soil Con servation Service. Additional infor mation is also available through local geologic surveys and universities. Deeper soil borings should also be used to assess soil suitability for subsurface construction, since foundation loads rely on the bearing capacity of soils below an excavation for their support.

Table 1 shows the unified classifica-

tion of soils and their general suitability for construction. Some of these factors take on increased importance in ea rth sheltered and cut-and-cover construc tion, and there are additional factors to be examined.

-Drainage: Well-drained soils are preferable to poorly-drained soils. - Water table depth: This factor is more important for earth-sheltered a nd cut a nd-cover construction than for con ventional construction because pri mary spaces are dug below grade. A typical earth-sheltered residential foundation is 30-120 in. (76-300 em) deep; enclosed cut-and-cover struc tures extend much deeper. Preferably, the water table should be significantly lower than the foundation. - Bedrock depth: Bedrock within the planned depth of the foundation of the proposed cut-and-cover or earth sheltered structure is undesirable, since excavation is usually more difficult and costly in bedrock. - Bulk density: This property is con sidered together with the soil type in order to determine whether the soil is loose or dense. In general, the denser soils within each soil type are better for construction purposes. - Erosion potential and frost action

the bottom of the structures, and the


Figure 1 . Simplified calculations for the effect of surface structures on mined space.


Soli type

Drainage

Frost heave

potential

Volume change

Backfill

potential

Typical bearing capacity

Range (psi)

General

suitability

Well-graded

gravels &

gravel-sand mixtures, little or no

fines

Excellent Low Low Best 8,000 psi 1,500 psito 20 tonsfft2

Good

Poorly graded gravels &

gravel-sand mix- lures, little or no fines

Excellent Low Low Excellent 6,000 psi 1,500 psito 20 tons!ft2

Good

Silty gravels, gravel-sand silt mixtures

Good Medium Low Good 4,000 psi 1 ,500 psito 20 tons/ft2

Good

Clayey gravels, gravel-sand-clay mixtures

Fair Medium Low Good 3,500 psi 1,500 psito 10 tonsfft2

Good

Well-graded sands & gravelly

sands, little or no fines

Good Low Low Good 5,000 psi 1,500 psito 15 tonsfft2

Good

Poorly graded sand & gravelly sands, little or no fines

Good Low Low Good 4,000 psi 1,500 psito 10 tons!ft2

Good

Silty sands,

sand-silt mixtures

Good Medium Low Fair 3,500 psi 1,500 psito 5 tonsfft2

Good

Clayey sands, sand-clay mixtures

Fair Medium Low Fair 3,000 psi 1,000 psito 8,000 psi

Good

Inorganic silts, very fine sands, rock flour, silty or clayey fine sands

Fair High Low Fair 2,000 psi 1,000 psito 8,000 psi

Fair

Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays

Fair Medium Medium Fair 2,000 psi 500 psito 5,000 psi

Fair

Inorganic silts, micaceous or

diatomaceous fine sands or silts, elastic silts

Poor High High Poor 1,500 psi 500 psito 4,000 psi

Poor

Inorganic clays of medium to high plasticity

Poor Medium High Bad 1,500 psi 500 psito 4,000 psi

Poor

Organic silts and organic silty clays of low plasticity

Poor Medium Medium Poor 400 psi

or remove Generally remove soil

Poor

Organic clays of medium to high

plasticity

No good Medium High No good R emove - Poor

Peat, muck and other highly organic soils

No good - High No good Remove - Poor

poten tial: These factors are considered together to determine the erodibility and stability of the soil types on various degrees of slope. Severe erodibility or frost action potential is a negative fac tor for slopes at or greater than the maximum slope recommended for el evational earth-sheltered designs. Frost action is also important for the design of roads in conjunction with buildings. - Shrink-swell potential: A low shrink swell potential is preferable; a mod erate or high shrink-swell potential could require a special design. - Percent liquid limit: This factor is used to determine the moisture con tent at which silts and clays will pass from a liquid to a plastic state. Higher values of the liquid limit indicate in creasing amounts of fine material in inorganic soils. A liquid limit less than 50% is generally considered good whereas a liquid limit greater than 50% indicates a poorer condition for con struction. - Permeability: A permeability rate of over 4 in. (10 em) per hour is consid ered good; 2-4 in. (5-10 em) per hour, moderate; and less than 2 in. (5 em) per hour, poor. - Corrosiveness on concrete: The cor rosiveness of soil on concrete is a sig nificant factor in assessing the suita bility of soils. Soils with high sulfate contents can be particularly damaging to normal concrete.

Slopes. Elevational earth-sheltered designs are best suited to areas with a slope ofS-15%. Aerial photographs and topographic maps will provide reliable primary information on slopes. They can be obtained from the soil conser vation service, the geologic survey, lo cal universities, or local government agencies.

Existing structures. Building founda tions, sub-basements, tunnels, and util ity services are some of the most ex tensive existing underground structures 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

Table 1. General characteristics of soils and their suitability for construction.

facilities are essentially the same as those for mined space. It is not uncommon to happen upon abandoned structures in the soil for which records do not exist.

Detail Planning for Underground Space

Upon a firm foundation of geolog ical data interpreted with a view toward the development of underground space, planning for community needs can be carried out rationally and confidently.

September/October

1982

But the possession of good data does not ensure the good planning of detail.

To date, planning· for underground space has generally been approached in

an "incremental" fashion rather than comprehensively. One of two ap

proaches is typically taken: consistency in land use, whereby surface zoning

requirements are applied to the details of proposed underground projects; and

discretionary review, whereby a pro ject is reviewed in detail either as an exceptional case or as part of a planned development. Both of these ap 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


One city, Independence, Missouri, has taken

an innovative approach to mined space by

establishing a separate zoning category for it.

and subsurface land are zoned sepa rately, and this may encourage the most creative use of mined space.

Planning Mined Space

Mined space is an appropriate site for the major activities of manufactur ing, warehousing, and secure storage. It is especially well-suited to operations requiring a controlled environment,

such as laboratory work and precision equipment manufacture.

specific language. The goals of this sort of promotion in planning documents are to encourage the development of underground space; to remove bar riers to development; to ensure the best use of the underground resource in harmony with community goals, objec tives, and policies; and to develop reg ulations which will ensure the safe con struction and use of underground space.

The inclusion of specific language which addresses the details of under ground space development in the com prehensive plan, either as a separate plan element or as part of a more gen eral element, will facilitate its devel opment and use and provide support for amendments in appropriate codes and ordinances.

Zoning ordinances are intended to im plement the community land-use plan by identifying districts where particu lar uses are to be located. These or dinances also contain prescriptive re quirements which are intended to ensure consistency in a development.

Most zoning ordinances were de signed to address conventional build ing construction and therefore do not adequately address the issues raised in underground space development. The language of many zoning ordinances inadvertently raises barriers to the use of underground space. For instance, most zoning ordinances prohibit the inclusion of basement or cellar space in the computation of habitable area. If earth-sheltered structures fall within the definition of a basement or cellar in a particular community, their use for habitation may be prohibited. Sim ilarly, extending surface zoning re strictions downward may unreasonably limit the use of mined space for op erations unsuited for the surface.

A study performed for the U.S. De partment of Housing and Urban De velopment, Earth sheltered housing: code,

zoning, and financing issues (Sterling et al. 1980), addresses problems which the language of some zoning ordinances poses for earth-sheltered construction. The major barriers identified by the study include: • Discrimination against earth-shel

tered housing in statewide laws and

92 UNDERGROUND SPA CE

the prohibition of this type of con struction in zoning provisions de fining earth-sheltered dwellings as non-habitable basements or cel lars.

• Minimum height and floor-area requirements that discriminate against earth-sheltering or don't take into account floor-area cal culations for habitable earth-shel tered space. • Minimum and maximum lot-cov

erage requirements which don't take earth-sheltered construction into account, since earth shelter ing affords different water runoff and open space than conventional development.

• Setback, garage, and off-street parking requirements in zoning ordinances that may pose special administrative and design issues for earth-sheltered dwellings.

• Problems of uniform administra tion and objective review where earth-sheltered development is approved on a case-by-case basis through discretionary approval.

Some areas have ·revised, or intend to revise, their zoning codes to eliminate barriers to earth-sheltered structures by rewording definitions and provid ing flexibility in prescriptive zoning standards to accommodate earth shel tering.

In regard to the development of mined space, the zoning issues are not generally addressed in ordinances since its use is relatively infrequent. In most cases where mined space is addressed, the subsurface is subject to the same restrictions as the surface land directly overhead.

One city, Independence, Missouri, has taken an innovative approach to mined space by establishing a separate zoning category for the subsurface. Permitted operations are treated sep arately from surface operations. Ac cess to mined space is limited to non residential areas, and support equip ment on the surface, such as utility or ventilation mec hanisms, must meet surface architectural standards to en sure compatibility with surface struc tures. The significance of the approach taken in Independence is that surface

Mined space generally goes unno ticed at the surface due to the buffering effect of bedrock and soil between sur face and subsurface, and this provides flexibility in the siting of operations in the space. The consistency of access to mined space with existing or planned surface use may be more important than the relationship between subsurface and surface uses themselves.

Access. The access requirements for mined space are determined by the uses to which it is put. Goods handling, pro duction, and storage will usually in volve high levels of vehicular activity, with human access primarily for reach ing the workplace. On the other hand, office space will require primarily hu man movement.

As mentioned previously, the least disruptive and most cost-effective ac cess is horizontal, via a bluff or escarp ment, created by tunneling. Handling of the excavated material will be more efficient than for other kinds of access because it can be loaded directly onto

Existing underground structures that could

affect the development of mined space in the

study area include abandoned tail race tun

nels, once part of the water power system in

the mill district in Minneapolis.


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 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 relatively high cost of shafts makes op timal placement of exits an important concern.

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 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 equately meet special space needs, a skeleton of horizontal circulation (such

Figure 3. Layout of mined space development. initial development space with access portal expansion of space

Figure 4. Placement of the access points is important to ensure adequate circulation in the

evolving mined space development.

DISTRICT HEATING

Figure 5. Consolidation of a number of utilities in a single tunnel (utilidor).

as a vehicular service loop) may tie to gether several clusters of mined space with other systems. Another possibility is a fairly rigid, large-scale system which includes areas of distinctive character within the system (Fig. 3).

The initial development of mined space may occur in areas where hori zontal access is most readily available, such as an escarpment or bluff. This facilitates construction by providing for


easy movement of equipment and ve hicles. Proper placement of the ac cesses to the initial development is im portant, since these points will evolve into a circulation system which will ser vice the entire development. Figure 4 illustrates placement in relation to de veloped space to ensure adequate cir culation in future development.

The provision of services. The use of mined space requires the same types

of service as surface facilities- power, communications, water, sewer, etc. The structural requirements in mined space will not permit the arbitrary siting of services. Consolidation of as many of these services as possible in a single tunnel, or "utilidor," should be consid ered. This reduces the amount of mined space required, reduces the cost of providing service, and makes mainte nance and repair simpler (Fig. 5).

UNDERGROUND S!'ACE 93

> w

>

i> z

2

z 0

Figure 6. Elevational design of earth-sheltered structure, in which three sides are earth bermed and one side is exposed. (The earth-sheltered designs shown here and in Figure 7 are illustrated by residential structures; these designs are also used for industrial, commercial, and educational facilities.)

Figure 7. The atrium design of earth-sheltered structure, in which the outside walls and roof are covered with earth. In this design, the rooms are arranged around a sunken courtyard.

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 struction, is best suited for fiat land, but can be sited in areas with slopes up to 15%.

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 are located. Earth-sheltering can pro vide opportunities for discretion in sit ing "incompatible" facilities, however, due to the buffering effect of earth

Some existing utility tunnels can be retrofitted or expanded to accommo date a variety of utility services. Heat ing tunnels present a good example, as they can accommodate a variety of other services, subject of course to the ca pacity and location of the tunnel.

Planning Earth-Sheltered Space

There are two basic types of earth-

sheltered design, elevational design and atrium design.

The most widely constructed type of earth-sheltered structure is the eleva tiona! design, which is earth-bermed on three sides with one exposed ele vation, usually a window wall (Fig. 6). This design can have either an earth covered roof or a conventional, well insulated one. Elevational designs can

berms. Topography. Whereas conventional

structures are generally limited to areas with 8% slope or less, elevational earth sheltered structures are best suited to areas where the slope ranges from 8- 15%. (Areas with steeper slopes can be employed with special design and en gineering consideration.) The follow ing are slope guidelines for earth-shel tered construction :

"w' "w' > "'

w

Slope of

8-15%: One-story earth

sheltered con «W 2 it > "« struction. 0.,«

2

;;; 0"'

"' "'

:;; Slope of 15-25%: Slope of 25% or more:

Two- or three story earth-shel tered construc tion. Earth-sheltered construction with special technol- ogy.

and Stratigraphy of the Twin City Region, in Geology

of Minnesota, Sims, P. K., Morey, G. B., eds. A

Centiennial Volume, p. 485-497.

Figure 8. As much as 1,000 ft of Paleozoic rock was deposited in the Twin Cities basin. The formations remain largely undisturbed.


It is also possible to site atrium struc

tures on sloped sites, although they are " more suitable to fiat sites.

Orientation for solar access. Orienta tion for solar access is of concern only


with elevational earth-sheltered de signs, since enclosed cut-and-cover construction and atrium structures do not rely greatly on passive-solar energy techniques. In most parts of the north ern hemisphere, it is desirable to site elevational earth-sheltered structures so as to take advantage of winter solar heat gain.

As with other solar building designs, earth-sheltered structures which incor porate passive-solar features are elon gated in an east-west direction for maximum exposure to sunlight. The optimal orientation for these designs is within a range of 20° either direction of due south. Land sloping directly to the east or west is less desirable since winter heat gain is not as substantial and the large summer heat gain causes cooling problems.

PART II. A CASE STUDY OF MINNEAPOLIS

As in many northern cities, the land area in Minneapolis is fully developed, leaving little room to accommodate in dustrial development or expansion. The result: tax revenues and jobs are lost to outlying areas still possessing un developed land. And like other north ern cities located in states which must import all of their fuel, the cost of en ergy figures largely in the city budget. It has been estimated that energy could cost the city and its residents and busi nesses as much as $1 billion annually by 1990 (in constant 1980 dollars).

The use of underground space could address both the space and the energy problem. The subsurface, by offering a new development layer within city boundaries, could provide an ideal site for many types of commercial, indus trial, and warehousing activities. Ap proximately 6,000 acres of mined space could be developed with little difficulty, and an additional 14,000 acres could be developed with dewatering, water proofing, and special construction techniques. Appropriate use of earth sheltering could utilize land unsuitable for conventional techniques due to the presence of sloping topography or poor environmental conditions such as ex cessive noise from freeways or an air port. And in a city whose temperature ranges from -30°F in the winter to

, l00°F in the summer, the stable mod erate temperature of an underground location (at a depth of 25 ft the year round temperature is 50-55°F) has a ot to offer in the way of energy sav mgs.

Minneapolis is fortunate in that the geology of its underground is ideally suited for underground space devel-


opment. Viewed in cross secton, most of the city is underlain by well-drained soils, a thick layer of strong limestone able to span large openings without structural support, and a layer of soft sandstone which is easily excavated. Minneapolis is also fortunate in that information regarding the city's geol ogy is well documented and readily available through the Minnesota Geo logical Survey. All available engineer ing test-boring and well-log data, some dating back 100 years, have been col lected by the Survey and stored in a computerized data system with a map ping capability.

The study area includes the city of Minneapolis, the Minneapolis-St. Paul International Airport, and the seg ment of the adjacent city of Blooming ton which encompasses the abandoned Metropolitan Stadium site (slated for redevelopment into mixed commer cial, industrial, and residential devel opment). Geology of the Study Area

Bedrock Geology Many layers of sedimentary rock have

been deposited in the study area over time and remain largely undisturbed. The bedrock formations of interest to underground space development are the Platteville limestone, the St. Peter sandstone, and the Prairie du Chien group, formations deposited about 400 million years ago in early and mid Paleozoic time. As much as 1,000 ft of Paleozoic rock was deposited in the Twin Cities basin (Figs. 8 and 9).

The nearly flat-lying layers of lime stone and dolomitic limestone that con stitute the Platteville formation aver age about 30 ft in thickness, though in some areas the limestone has been eroded to thicknesses of less than 10 ft (Fig. 10). Where it is at least 10 ft thick, most of the Platteville formation con stitutes a strong, competent layer that can support itself over large openings. The new Civil and Mineral Engineer ing building on the Minneapolis cam pus of the University of Minnesota in cludes 40,000 sq ft of space excavated directly beneath the Platteville. Spans up to 58 ft testify to the structural abil ity of the formation to span large un derground openings.

A monoclinal fold in the Platteville formation near downtown is an area of disturbance in the limestone, with in creased frequency of joints and poorer rock conditions. Construction should be approached with extreme caution here since a number of tunnels have collapsed in this area.

The St. Peter sandstone, a fine- to

medium-grained, well-sorted, very pure quartz sandstone, is a friable, easily ex cavated rock. Since most f this "sand rock" is only weakly cemented, with much of its strength being a result of compaction, much of the tunneling in the Twin Cities has been done using water jets.

Due to its varying hardness, the 150- ft-thick St. Peter sandstone is consid ered a layered rock- as can be seen in exposed sections along bluffs where differential erosion accentuates the harder, more resistant layers. When driving a tunnel by jetting, an occa stional hard layer may need to be blasted to assist the work.

Underlying the Platteville limestone and the St. Peter sandstone is the Prai rie du Chien group, comprised of do lomite and sandstone members.

Soils and Topography

The surface geology of the area is considerably more complex than the bedrock geology and no attempt was made in this report to identify and clas sify all the surficial deposits. Extensive information on the surficial deposits in the area, including the implications for engineering, is available from the Min nesota Geological Survey.

The surficial deposits, formed dur ing the Quaternary period, are typi cally 50 to 100 ft thick but may be nearly 400 ft thick where they fill preglacial valleys, e.g., under the chain of lakes in western Minneapolis. All of these surficial deposits are classified as un consolidated soils.

Included in the study - is a general classification of soils for engineering purposes using the unified soil classi fication system (Table 1., above). By combining various classifications of soil with suitability criteria for earth-shel tered and cut-and-cover construction, it is possible to simplify the soils situ ation into areas of general degrees of suitability for cut-and-cover construc tion (Fig. 11). Although this is a gross simplification of a complex surficial ge ology, it is instructive for general plan ning purposes. As can be seen from the map, most of the Minneapolis area has soil conditions which are either good or good-to-fair for cut-and-cover con struction. The soil suitability for tun neled construction may vary from that presented in Figure 11 due to the greater importance of other factors, such as the ease of dewatering.

The topography of the site area is another important factor in the suita bility for earth-sheltered or cut-and cover buildings because they require the significant exposure of one or more elevations. The areas of the city with


•

Bedrock Geology Condition of Limestone

KILOMETERS

f(f)

KILOMETERS

(f) 0 2 5 10

FEET IN THOUSANDS

0 2 5 10

FEET IN THOUSANDS

Platteville Limestone

[@<1/H St Peter Sandstone

• Prairie Du Chien Group

Limestone Less than 10 Feet Thick

Limestone 10 Feet or Greater

;: .

_·::

SCALE 1:140,000 APPROX. Monoclinal Fold

D Limestone Absent

SCALE 1:140,000 APPROX.

Source: Geologic data and mapping. Bruce Bloomgren, Minnesota Geological Survey. Presen·

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 Miscellaneous Investigations Series 1·1157, 7 plates.

Figure 9. The bedrock formations of interest to underground de

velopment are the Platteville limestone, the St. Peter sandstone,

and the Prairie du Chien group.

Source: Geologic data and mapping: Bruce Bloomgren, Minnesota Geological Survey. Presen

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 Miscellaneous Investigations Series 1-1157,7 plates.

Figure 10. Where it is at least 10ft thick, most of the Platteville

limestone constitutes a strong competent layer that can support itself

over large openings. Note the monoclinal fold near the downtown

area.

slopes of 8% or greater (Fig. 15 below) are not a large percentage of the total land area and many of the areas in dicated are devoted to uses which are not likely to change in the near future, e.g., parks and cemeteries.

Hydrogeology

Since the presence of ground water can affect the location, design, con struction, cost, and feasibility of ·un derground space development, the types of potential ground water situa-

tions that can be expected in the study area are described.

It is not uncommon to find a satu rated zone underlain by an imperme able layer, followed by unsaturated material, and again by a saturated zone.

96 UNDERGROUND SPACE September/October 1982

0 2 5 10

0 2 $ 10

•

Soil Suitability for Cut-and-Cover and Earth-Sheltered Construction

' I l

KILOMEnRS

r--LJ--1 FEET IN THOUSANDS

Hydrogeology of the St. Peter Sandstone

KILOMETERS

r--LJ--1 FEET IN THOUSANDS

Good

Good to Fair

Fair to Poor

Poor

SCALE 1:140,000 APPROX•

Artesian Condition

• Water Table Condition

D St. Peter Sandstone Absent


Source: Geological data and mapping: Gary Meyer, Minnesota Geological Survey. Presentation

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

••• BoundarY. of Artesian and Water Table Conditions in the St. Peter Sandstone

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 Miscellaneous Investigations Series 1·1157, 7 plates.

Figure 11. By combining various classificatiom of soil with suit ability criteria for earth-sheltered and cut-and-cover comtruction, the complex surficial geology can be simplified and a range of suitability (good to pom) for construction can be identified.

Source: Geologic data and mapping: Roman Kanivetsky, Minnesota Geological Survey. Presen 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 Miscellaneous investigation Series 1·1157, 7 plates.

Figure 13. Both artesian conditions and water table conditions prevail in the St. Peter sandstone.

A typical situation is shown in Figure 12, where a zone of saturated drift or soil overlies limestone and shale. The shales are often impermeable and pre vent the glacial material from draining.


The upper part of the sandstone below the shale is unsaturated because any leakage from the stratum above is not sufficient to fill or recharge the sand stone with water, and the water drains

into the river valley exit points. The deeper strata of the sandstone are sat urated, creating a second water table. The upper · portions of the St. Peter sandstone which are essentially unsat-


_j! iiii

Occurrence of Water in Geologic Formations

PERCHED WATER TABLE - - --- (WINTER)

PERCHED WATER TABLE--- --,

(SUMMER)

1BJ;1IT Ci*t-:.::.::·"'" - 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 for mined space development.

Artesian vs. Unconfined Water Conditions

Since the various rock layers differ in their ability to transmit water, the Twin Cities basin consists of a series of aquifers separated by aquicludes. The confining impermeable beds severely restrict the vertical movements of water to or from a confined aquifer. Where this occurs and where water can enter an aquifer at the eroded edges of the strata, the water will move downslope along the dip of the permeable layer, creating a build- up of pressure in the aquifer, i.e. artesian conditions.

In regard to unconfined water, the level of the water table, i.e., the top of the saturated zone, will fluctuate with the seasons due to the varying rate at which the geologic unit is recharged by rainfall or by leakage into it during the year, as well as the varying rate at which water is drained or removed via wells. Where water drains more or less hor izontally to a river bluff, freezing at the bluff face will block the drainage and cause a rise in the water level back from the valley wall-a common occurrence in the study area.

The St. Peter sandstone is under both artesian conditions and water table conditions (Fig. 13, on preceding page). In the areas which exhibit unconfined water, the St. Peter sandstone drains to the Mississippi River. The depth of unsaturated or dry sandstone in those areas ranges from 30 ft or more near est the river to the point of saturation at the contour where artesian condi tions begin. Those areas where unsat urated sandstone occurs beneath com petent limestone provide the most favorable sites for most types of un derground space development and use.

Perched Water Tables

Lenses of relatively impermeable siltstones occur regularly in the lower 50ft of the 150-ft-thick St. Peter Sand-


stone, and less commonly in the upper parts of the stratum where most un derground space development is ex pected to take place. These lenses in the upper portions can cause minor confined water conditions in the sand stone, and reduce the efficiency of de saturating the sandstone via deep well pumps. Fortunately, their occurrence is not common in the upper 30ft.

Unconsolidated materials that cover the bedrock are quite variable in na ture both horizontally and vertically. It would be unusual to find a location where there were not at least two different geologic units represented in a vertical section. These units occur as discontinuous lenses, are intertongued and interlayered, and consist of gravel, silt, clay, and alluvial materials.

Where impermeable lenses of clay occur in the glacial till, it is common to have saturated materials on top of them, creating local perched water tables which, if not adequately drained, may cause support problems when tunnels are driven through the saturated area. It may be less difficult to tunnel be neath these impervious clay layers than to construct tunnels through them.

No reliable data have been compiled regarding the location of perched water tables in soil in the study area though some information is available through soil testing and engineering data. Domestic Water Supply

While the presence of ground water must be considered when considering underground space development, so too must the effect of underground space development on groundwater be considered.

The major water supply for the city of Minneapolis is the Mississippi River. The St. Peter sandstone is not an im portant aquifer for domestic water supply as it produces only small amounts of water and may be contaminated due to leakage from near-surface facilities.

Most of the water supply for the rest

of the Metropolitan Region is obtained from the Jordan aquifer, a 90-ft layer of sandstone approximately 530 ft be low the surface, separated from the St. Peter sandstone by the 130-ft - thick Prairie du Chien group. The imperme able basal layer (siltstones) of the St. Peter sandstone would limit the effect on underlying rocks of mined space development in the upper portions of the St. Peter sandstone.

In brief, the prevailing geologic con ditions which make possible cost effective mined space development in Minneapolis are:

l. The presence of a minimum 10- ft-thick layer of Platteville Limestone to form the strong roof of the exca vated space.

2. Suitable hydrogeology in the St. Peter sandstone, the bedrock layer to be excavated. The optimal hydrogeological situation would be a water table well below the lower level to be excavated , resulting in a dry space. The next best is one where water is present in the St. Peter sandstone, but is unconfined. This lat ter situation requires waterproofing and dewatering. The least desirable hydro geologic condition would be the pres

. ence of water under artesian condi tions in the St. Peter sandstone. Though construction of mined space is possible under such conditions, it requires ex tensive mitigation and is more costly.

Geologic Hazards

A hazardous geologic feature in the study area is the occurrence of caves and voids in the sandstone, usually just below the overlying limestone layer. Some known natural ca ves occur along the river bluffs, where water entering joints or fractures in the limestone mi grated down into the friable sandstone and moved toward the Mississippi River, which is at a lower elevation. Where the disaggregated sand grains escaped


•

Potential Areas for Mined Space Development

KILOMETERS

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

FEET IN THOUSANDS

aJ Most Favorable Areas

Most Suitable for Cut-and-Cover and

Earth-Sheltered Construction

KILOMETERS

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

FEET IN THOUSANOS

Less Favorable Areas

ISil1ll1l1l Monoclinal Fold in lll1llllll!l\!l Limestone' Water Table

Conditions in Sandstone

Monoclinal Fold in

';


Suitable Soil Conditions

Slopes Greater than 8 Percent

DUnsuitable Soil Conditions

SCALE: 1:140,000 APPROX.

Limestone' Artesian Conditions i,n Sandstone

Boundary of Artesian and • • • Water Table Conditions in

the St. Peter Sandstone

DLimestone Absent -Bedrock Contours

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

Figure 14. This composite map of the bedrock geology, the condition

of the limestone, and the hydrogeology of the study area shows the

potential areas for mined space development.

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

Figure 15. This composite map of soil suitability and slopes greater

than 8% shows the areas most suitable for earth-sheltered and cut

and-cover construction.

at a bluff face, openings formed and gradually widened and extended to ward the water sources. A few large natural caves are also known to exist beneath the downtown area and their presence will have to be taken into ac-

count when future underground de velopment is planned.

Man-made caves also occur, as are sult of mining in the sandstone or as a side effect of other activities, e.g., where leakage of water along pipes and tun-

nellinings has caused the sandstone to break down. Where sand has escaped, voids have formed, some of substantial size.

The location of most mined caves and some natural caves are known and have

September/October 1982 UNDERGROUND SPACE 99

0 2 5 10

Redevelopment and Redirection Areas, Closed-School Sites and Soils Suitability Composite

r--t.J--i FEET IN THOUSANDS

Potential Transitways and Bedrock Geology Composite

KILOMETERS

0 .G 1.5 3

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

Soil Conditions

Redevelopment/Redirection IP'Ta Areas with Unsuitable

Soil Conditions

Good to Fair Soil Conditions Outside Redevelopment! Redirection Areas

DUnsuitable Soil Cond ions


e Closed School Sites

• • • Potential Transitways

Platteville Limestone

[);(?;j St. Peter Sandstone

• Prairie Du Chien Group


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

Minneapolis School Board.

Figure 16 . Large parcels of land may become available for earth

sheltered and cut-and-cover structures as a result of redevelopment.

Source: Minneapolis Plan for the '80's, July 1981.

Figure 17. Owing to the presence of the Platteville limestone, much

of the study area is suitable for construction of a subway in mined space.

been mapped. Site studies should de tect the presence of these geologic fea tures. An important geologic feature in the study area is the existence of a number of "buried valleys." These are river valleys cut down into the Prairie du Chien formation prior to and dur-

ing the ice age. Later, as drainage was blocked and changed by the advance and retreat of glaciers, they were filled and buried by water-laid deposits and glacial debris. Commonly there is little or no surface evidence of their exis tence. The chain of lakes in Minne-

apolis occur as a series of depressions along a major buried valley. Develop ment of underground space is not con templated for such areas.

Although much of Minneapolis ap pears generally suitable for cut-and cover construction in soil there are a

100 UNDERGROUND SPACE September/October 1982

number of potential construction problems which will require attention in the early stages of a particular pro ject.

Pollution of the groundwater should present a problem for subsurface con struction only if the groundwater has become corrosive or toxic. A final problem in many otherwise suitable areas of the city is the presence of boul ders in the glacial drift. Although boul ders are more prevalent in some areas than others, they can be considered a random hazard in that they interfere with drilling operations and can in crease excavation costs, particularly in small excavations.

Valuable Mineral Deposits

The Metropolitan Council is cur rently preparing a study of the location and extent of potential aggregate re sources throughout the seven-county Metropolitan Area. Generalized maps at a scale of 1:24,000 and 1:100,000, now being prepared under contract by the Minnesota Geological Survey, in

dicate the potential significance of ag gregate resource areas based on the amount of data available, the percent age of gravel versus sand, the thickness of the deposit, and the amount of cover or waste materials over the resource.

Existing Underground Structures

Data regarding ex1stmg under ground structures in the study area were gathered from the following sources:

Minneapolis Department of Public Works

Minneapolis Water Works Minnesota Gas Company

(Minnegasco) Northern States Power Company Northwestern Bell Telephone

Company Minneapolis City Coordinator's Office Northern Cable Vision Company Minnesota Historical Society Pillsbury Company Every effort was made to gather and

map all data on existing underground structures, both abandoned structures and those currently in use. Structures found in the subsurface include water mains and tunnels, sanitary sewers, storm sewers, electrical cables, tele phone lines, district heating pipes, hy dro power "raceways," abandoned tun nels, and caves.

Existing structures that would affect the development of mined space in the study area are those located in the St. Peter sandstone beneath the limestone layer or in the limestone itself. Rela tively few structures are found in the St. Peter sandstone. Sanitary and storm sewer systems comprise the bulk of ex-


cavations in potential mined space, and some portions of the water system and abandoned power structures are found in the St. Anthony Falls milling district.

The likelv candidates for utilidor conversion in the study area are the heating distribution tunnels at the U ni versity of Minnesota, and the aban doned street car power tunnels con structed in the early 1900s. The Geologic Potential for

Underground Space Development

Mined Space

Potential areas for mined space de velopment in the study area are shown in Figure 14, a composite map of bed rock geology, the condition of lime stone, and hydrogeology in the study area.

As mentioned above, the most fa vorable areas for development are those where water in the St. Peter sandstone is under water table conditions (Figure 13 above). The level of the water table drops steadily as the bluffs along the Mississippi River are approached, re sulting in substantial areas of the upper portion of the St. Peter sandstone re maining dry year- round . Most of the areas immediately adjoining the bluffs have approximately 20-30 ft of dry sandstone beneath the Platteville lime stone. These areas are prime locations foro two-story mined space develop ment, or for uses which benefit from a very high ceiling, such as warehous ing or manufacturing plants. The por tions which are considered to be most favorable for development in the study area comprise approximately 380 mil lion sq ft.

The Mississippi River bluffs have been assessed for potential horizontal portal access, and the possibilities are surprisingly quite few. Besides the top ographic and geologic factors neces sary for horizontal access, other con siderations that must be taken into account are the types of land use in the surrounding surface area and its access to a major transportation network.

Much of the river area with potential access is zoned for residential use, and only in two locations are other uses found . An area near Interstate 35W is currently used for a variety of activities including residential, commercial, and some manufacturing. This site pro vides excellent access to the interstate, regional, and local transportation net work. The other site, at Lake Street and West River Road, is not located near the major interstate and regional transportation network.

As mentioned above, one of the few suitable access points could have been

eliminated by the routing of the Great River Road, a proposed parkway to be constructed by the federal government along the Mississippi River from its source at Lake Itasca in northern Min nesota to New Orleans, Louisiana. Pro tection of a potential access to mined space at that location is now accepted as one of the criteria for Great River Road planning.

It may be desirable or necessary to serve mined space with vertical access at certain points of the city. Since the cost of access depends to some extent on its depth, the depth from the sur face to the bottom of the limestone was also mapped.

In much of the study area, the depth from the surface to the bottom of the limestone ranges between 50 and 100 ft. The above-mentioned Civil and Mineral Engineering building on the University of Minnesota campus is being constructed in such an area. Excava tion was carried out via a vertical shaft at a depth of 75 to 100 ft. Once the access shafts were provided it became economical to add mined space within the building and reduce space in the cut-and-cover section of the structure. Earth-Sheltered and

Cut-and-Cover Space

The most suitable areas for earth sheltered and cut-and -cover construc tion are shown in Figure 15, a com posite map combining soil suitability and slopes greater than 8%.

Earth-sheltered construction is es pecially suited to sloped areas of the city because access transitions can be easily made. Enclosed cut-and-cover construction will often have access through adjacent conventional build ings, in which case a sloped site is not necessarily preferable.

Since there is little vacant surface land within the city, earth-sheltered con struction will probably occur primarily on an infill basis as individual parcels of land become available. Larger de velopment areas likely to become avail able are some of the former school sites where the buildings have been or are scheduled to be demolished (see be low). These larger parcels ofland would allow construction of earth-sheltered housing developments or commercial areas.

Underground Space and the Plan for the 1980's

A comprehensive plan for the dec ade of the 1980's has recently been completed for the city of Minneapolis. This plan is intencied to guide city de cisions on housing, economic devel opment, physical environment, trans-


.r---------------------- ------ -------- --- ------ ------ -------- ----------- --- ----

The Mississippi River bluffs at this location could provide access to mined space development.

The site is near an industrial area and is served by the interstate freeway (center).

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 its op era tion.

As shown in Figure 17, much of the study area is suitable for construction

portation, property services, human development, and health and safety. Because Minneapolis is a fully devel oped city- only 818 acres, or approx imately 2%, of the city's land area is vacant- the plan is primarily con cerned with the redirection of blighted areas, the redevelopment of under-uti lized areas, and the preservation of the city's best features. It is hoped that knowledge of the availability of a vast amount of developable underground space, as well as the advantages in en ergy efficiency that it could provide, will enable the city to think in more expansive terms.

The Housing Plan

Housing in Minneapolis will serve a population mix in the 1980's and be yond different from that in previous years. Despite a 28% decline in pop ulation since 1950, the number of households has remained essentially unchanged due to a decline in the number of families with children and an increase in the number of one- and two-person households. A major goal of the city's housing policies in the 1980's is to shift small households into smaller housing units, thus freeing much of the existing stock oflarge single-family homes for families with larger space demands. To this end, the plan for the 1980's recommends the construction of 6,000 new housing units by 1990.

The best opportunity for construct ing earth-sheltered housing will be on large parcels of land, a square block or more, where several units can be con structed as a group. Such large parcels may become available as a result of the


closing of 18 city schools in June of 1982 (Fig. 16), but, overall, earth-shel tered housing is not likely to figure heavily in the city's future plans.

Industrial Development

A recent survey indicates that ap proximately 25% of all distribution firms in the city plan to expand their facilities in the near future, but that fewer than half of these firms have sufficient space to do so in their present location. Sim ilarly, 55% of the manufacturing firms surveyed in 1980 wanted to expand at their present location, but fewer than half of those surveyed have space to do so. In addition, many warehousing op erations are being displaced due to the conversion or removal of warehouses at the edge of the downtown area. A recently completed report on the eco nomic situation in Minneapolis esti mates that as many as 3,000 to 4,000 marketing and distribution jobs and 5,000 to 10,000 manufacturing jobs could be lost to the suburbs in the next five years unless suitable expansion space is found.

The subsurface could provide the space for this industrial expansion and a consequent increase in the city tax base. In many locations along the Mississippi River bluffs where the St. Peter sandstone is dry, facilities with ceilings as high as 30 ft could be con structed to accommodate many types of industrial and warehousing facili ties.

Transportation

Mass transit in Minneapolis is cur rently provided by an all-bus system, though other forms of mass transit

of a subway in mined space. In those areas where the Platteville limestone is not present, soft-ground construction methods could be used , but may be prohibitively expensive (except for short distances) in comparison to a surface system. The critical factor is knowing where the Platteville is present so that construction estimates for both time and expense can be prepared accuratel y. A subway system would become substan tially more competitive in cost if it could be tied into underground industrial and commercial space along the transit cor ridor.

Commercial Development

The central business district in downtown Minneapolis is the major commercial center. The main principle guiding its development is compact ness. All major buildings are to be lo cated inside an area the boundaries of w hich are within reasonable walking distance from the center. By providing room to accommodate commercial and retail expansion, the use of under ground space could facilitate the prin ciple of a compact downtown.

At present, a pedestrian skyway sys tem linking the major buildings in the central business district at the second floor level might appear to obviate the need for a pedestrian tunnel system. However, as a result of constr uction currently under way, an estimated 15,000 to 20,000 new employees will be added to the downtown workforce by 1990 (City of Minneapolis Planning Department 1978). Additionally, it is estimated that the population in the downtown area may increase by ap-


I

proximately 13,200 persons by 1990 as a result of new housing construction along the riverfront (City of Minne apolis Planning Department 1981). A pedestrian tunnel system in the central business district and its immediate vi cinity would be convenient to office

Substantial support for the general concepts

contained in the case study has been shown

in a broad range of city offices and agencies.

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

Montreal and Toronto, for instance.

Not only the downtown area, but also neighborhood and comm unity com mercial centers could benefit from un derground commercial development, especially in conjunction with the de velopment of medium- and high-den sity housing complexes surrounding these centers. Increased density could be accommodated with very little visi ble impact on the surface environment. Community centers could retain a scale appropriate to their surroundings while providing the goods and services nec essary to an expanded community. Pe destrian tunnels linking the centers to nearby apartment complexes would provide residents the same protection from climate and traffic as is enjoyed by skyway users downtown.

The City's Reception of Underground Planning

Since the planning study was neither conducted nor funded by the city, there are obvious questions as to the extent to which the planning data or the rec ommendations will actually be incor porated in the city's long-range plan ning. The initial experience, since completing the study, has been very positive. Substantial support for the general concepts contained in the case study has been shown in a broad range of city offices and agencies, including the mayor's office, the city council, the community development agency, and


the city planning department. Whether this increased awareness of under ground potential and interest in its de velopment will result in more effective utilization of underground space in the future is still an open question, al though it appears reasonable to con clude that the outlook is substantially more promising now than it was before the study was done. 0 Acknowledgment

Many individuals and institutions were involved in the preparation of this study. The generous assistance they have provided during the whole course of the study is greatly appreciated. The authors wish to thank Control Data Corporation for pro viding the major funding which made this study possible. The study was performed in collaboration with the American Planning Association (APA), which provided much in formation on current planning practice. Martin Jaffe, senior research associate with APA, was a major contributor to that part of the study dealing with the regulation of un derground space.

The study could not have been under taken without the valuable resource data and technical assistance of the Minnesota Geo logical Survey. All geologic mapping is based on current published and unpublished data made available to the authors by the Geo logical Survey. Bruce Bloomgren, Gary Meyer, Roman Kanivetsky, and Betty Kee ler, Geological Survey staff members, gen erously lent their time and assistance to the project. A special note of thanks is extended to Professor Donald H. Yardley, Depart-

ment of Civil and Mineral Engineering at the University of Minnesota, for his work on the geology and hydrogeology of Min neapolis and for providing technical assis tance throughout the study. Dr. C harles Nelson, a private geotechnical engineering consultant, also provided technical assis tance and reviewed draft material.

References

City of Minneapolis Planning Department. 1978. Minneapolis metro center forecasts ...

1990. Minneapolis, Minnesota. ---. 1981. Central riverfront urban design

guidelines. Minneapolis, Minnesota. Jansson, B., and Winqvist, T. 1977. Plan

ning of subsurface use. Swedish Council for Building Research.

Legget, R. F. 1962. Geology and engineering.

New York: McGraw Hill.

- - -. 1973. Cities and geology. New York: McGraw Hill.

McManis and Associates, Inc. 1980. City of

Minneapolis neighborhood economic develop

ment program strategy. Minneapolis Min nesota.

Minneapolis Energy Futures Committee. 1981. Charting our energy future. Report of the Energy Futures Committee to the Minneapolis City Council and Mayor Donald M. Fraser, April 1981.

Sterling, R., Aiken, R., Carmody, J. 1980. Earth-sheltered housing: code, zoning and fi nancing issues. Washington, D.C.: U.S. Government Printing Office.

Sterling, R. L., and Nelson, S. 1982. Plan

ning for underground space: a technical re

source guide. Minneapolis, Minnesota: Un derground Space Center.

- - . 1982a. Planning for underground space:

a case study of Minneapolis, Minnesota. Min neapolis, Minnesota: Underground Space Center.


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Underground Space - Elseviermedia.journals.elsevier.com/content/files/planning-the...Underground Space Raymond L. Sterling Susan Nelson N recent years there have been many calls for