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3 Findings INTRODUCTION Efforts in three general areas are needed to mitigate the nation's subsidence problem. Fug, a broad range of earth-science data, the data base, must be collected in order to assess locally and regionally the incidence and potential impact from each type of subsidence. Although some of these data are already available, they must be converted to the appropriate scale and format to be useful. This often will require collection of additional field data S=Qn4, teCh~G~ research is needed bow to improve the capability to predict the dine, rate or magnitude, and place of subsidence and to develop engineering designs that are resistant to subsidence damage. And third, institutional methods of subsidence mitigation need to be evaluated for their cost-effectiveness and suitability for each type of subsidence. Although many different approaches have been tried in the United States, few studies of the methods themselves hake been conducted. Our greatest concern is that efforts to reduce-the federal deficit, which have jeopardized the small but effective subsidence research programs disseminated throughout the federal government, will halt scientific and technical progress at the federal level. Data and research results from these federal programs historically have complemented state and local efforts by establishing much of the technical basis for the recognition A subsidence potential, as well as its prevention or mitigation. Basic earth-science data collection and research on subsidence processes are legitimate roles for the federal government. Without funding for this work, agencies such as the Bureau of Mines, U.S. Geological Survey, and Agricultural Research Service are unable to provide the data and techniques required by state and local government and industry to mitigate subsidence in a timely and cost-effective manner. DATA BASE Mitigation of subsidence requires many types of basic earth-science data, including ge- ologic, hydrogeologic, soils, hydrologic, geodetic, and land-use information as well as infor- mation on the incidence and areal distribution of the subsidence itself. Many of these data are rc,~tinekr collected in map form by the many federal, state, and local agencies. However, 45

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46 they rarely are collected pnmanly for subsidence mitigation. Consequently, without furler interpretation and field work, their usefulness for subsidence mitigation is limited. Findings In areas of the nation where geologic or hydrologic conditions conducive to subsidence exist and activities can trigger subsidence, detailed mapping programs to identify potential subsidence areas are needed. These should be associated with a data base developed to iden- tify the areas of potential subsidence that includes an inventory of sizes, shapes, frequency, predictability, and triggering mechanisms. These maps should be accompanied by the neces- sary graphics and text to be understandable by the general public, legislators, planners, and regulators. The information should be presented in a manner that will be useful to: provide guidelines for building design and codes, provide a basis for development of land-use planning maps as an instrumental resource in land-use management, develop actuarial data for insurance purposes, and guide location, management, and monitoring of landfills for municipal, hazardous, and toxic wastes. Implementation Primary data collection and mapping should be carried out by state geological surveys, the U.S. Geological Survey, and the Soil Conservation Service. Special research to define mapping criteria and identify and solve complex causes of subsidence should be the responsibility of federal agencies such as the U.S. Geological Survey, Bureau of Mines, and Agricultural Research Service. TECHNICAL RESEARCH NEEDS Technical research in two areas is needed to improve our capability to mitigate subsi- dence: (~) prediction of specific subsidence occurrences and (2) subsidence-resistant design of structures. Subsidence Prediction Although the mechanism for each of the six types of subsidence considered in this report is generally understood, the ability to predict time, place, and magnitude or rate of subsidence is frequently impaired by either inadequate mapping or incomplete understanding of details of the subsidence process and difficulty in determining the parameters that control the process. Findings To improve our predictive capability of subsidence, the following research needs were identified: Mining- monitoring of the extent of subsidence and subsurface deformation in coal fields to provide data for the development of predictive models for subsurface conditions peculiar to the United States Sinkholes remote sensing and geophysical methods to identify subsurface voids and provide better understanding of triggering factors

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47 Underground fluid withdrawalmethods to reliably determine the spatial distribution of compressibility of underground fluid reservoirs before they are developed Natural compaction methods to predict compaction, particularly its rate Hydrocompaction methods to reliably identify collapsible soils and to predict mag- nitude of settlement, particularly for routine engineering investigations Organic soil better methods to identify carbon content and temperature within soil profiles; development of truck and field crops suited for high water tables Implementation in many cases, federal and state agencies already have the expertise and mandates to conduct this research. However, reduced funding, particularly at the federal level, has jeopar- dized fundamental research on subsidence processes. Before seeking additional funding, these agencies need to establish priorities collectively and seek to improve coordination of Heir research effort on ground failure, both with other federal agencies and with state agencies. The U.S. Bureau of Mines has been primarily responsible for research on mining; the U.S. Geo- logical Survey for underground fluid withdrawal, natural compaction, sinkholes, and geologic aspects of mining; the U.S. Bureau of Reclamation for hydrocompaction; and the Agricultural Research Service for organic soils. Agencies concerned with construction, such as the Army Corps of Engineers, Soil Conservation Service, ant! Federal Highway Administration, should be involved in establishing these priorities, since they frequently spend large amounts on very specific subsidence problems. Engineering Design and Construction A number of engineering solutions are available to prevent or minimize subsidence damage to structures, transportation facilities, and utilities. However, the movement and resulting damage to structures in response to subsidence ground movements needs to be better documented. Methods also exist to backfill underground mines and voids in soluble rock. Little is known of the long-term effectiveness of void backfilling methods and materials. Findings Mining and sinkholes. Studies of building movements and the resulting damage should be conducted in areas subject to local colIapse, particularly mining. This would improve our ability to design subsidence-resistant structures. Techniques and costs of presubsidence support of buildings should be evaluated against postsubsidence damage repair. Use of geofabncs and earn reinforcement for construction of slabs, roads, and dikes with more resistance to subsidence damage should be examined. The effectiveness of mine backfilling techniques and materials should be evaluated. Core drilling, sampling, and downhole inspection using TV cameras would provide data for eval- uation and improving procedures. Partial mine filling can reduce subsidence by reducing the size of voids and providing lateral support for existing mine pillars. The effectiveness and cost of this procedure should be evaluated relative to total void filling. Research on less-costly materials handling systems, such as pumped slurry, could result in improved, cost-effective void-filling procedures. Fiber-reinforced grout columns should be tested. Hydrocompaction. Cost-effective methods to stabilize colBapsible soils are needed. Such methods need to be developed for small structures, for example, houses, as well as large ~ engineering projects.

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48 Organic soils. In agricultural areas, new crop varieties Hat tolerate shallow water tables are needed. Chemical treatment can inhibit organic soil degradation. Implementation Although research on construction and void backfiHine can be conducted or funded bY ~ ~ _ _ ~ _ _ O a number of agencies and Industries, it appears appropriate to assign responslullltles tor this type of subsidence research to the U.S. Bureau of Mines. Necessary technology-transfer inputs could be obtained by assistance from appropriate professionals and the National Institute of Standards and Technology. Agencies concerned with roads, canals, and dams are the ones most frequently confronted by hydrocompaction. The primary agencies are the Federal Highway A~ninistration and Bureau of Reclamation, although there is also a role for the Federal Housing Administration because of the explosive growth in the western part of the Sun Belt. The Agricultural Research Service historically has had responsibility for organic soil research. MITIGATION MEASURES RESEARCH AND IMPLEMENTATION NEEDS Because of He diverse causes of and impacts from land subsidence, as well as the need to consider the status quo when mitigation is necessary, mitigation measures must be, in general, designed for each situation. General approaches to mitigation of subsidence rarely will apply to aU types of subsidence. Another consequence of the diversity is that governmental responsibilities for mitigation measures primarily fall to state and local governments. The appropriate role of the federal government is to provide national coordination and information, provide technical assistance to state and local governments, and fund research on mitigation techniques. Regulation of Resource Development Introduction When development of natural resources causes subsidence, governments sometimes find it necessary to exercise their power either to prohibit the resource development or to control it in ways that minimize subsidence damage. Findings Legislative regulation of resource development reflects a decision that the benefits of unregulated resource development are outweighed by the harm caused. Therefore, such reg- ulation is most appropriately considered when there is sufficient information regarding He relationships between resource development and subsidence costs. Sufficient information of this kind still needs to be developed. Implementation The federal government can encourage the development and dissemination of technolo- gies for evaluating these relationships through programs of the appropriate agencies of the Deparunents of Agriculture, Housing and Urban Development, and Interior. Where the federal gove'Tunent is itself involved in resource development, it can take steps to ensure that subsi- dence is one of He factors considered as part of the development process. Where states are responsible for the regulation of resource development, He appropriate state agency should consider whether procedures for addressing subsidence should be integrated win the resource

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49 development process. Such an agency may be able to accomplish this directly through ad- ministrative regulation, or it may be necessary for state legislatures to take action through the legislative process. Land-Use Management and Construction Codes Introduction Land-use management in the United States is primarily the responsibility of state and local governments operating within the constraints of state enabling legislation. The federal role is limited to the collection and dissemination of information, although the federal government may require land-use regulation as a condition of participation in a federal program, as in the case of flood insurance. Because of variations in subsidence characteristics, there is no general zoning approach to the prevention or mitigation of subsidence damage. Zoning is most efficiently used where the risk and potential cost of subsidence are high enough to offset the impacts of controlling the use of property. Zoning can be used to restrict the use of land to levels consistent with the hazard, but short of prohibition of all uses. Zoning win be most effective where risk is very high or where the value of the surface land and improvements is low. Moreover, where the zoning prohibits any use of property in a subsidence-risk area, the question will arise of whether a taking in violation of the Fifth Amendment of the U.S. Constitution has occurred. Where subsidence risk is very high, the most cost-effective approach may be direct acquisition of the land affected or of the development rights of the land. Construction techniques sometimes can be used effectively to eliminate or limit subsidence damage, but these are now seldom used in connection with subsidence hazards in the United States. Findings Research needs to be done specifically on local codes and ordinances that can be used to reduce subsidence damage. Since the subsidence-damage problem, except in the case of increased flooding in areas of lower elevation, is primarily economic, the costs to the community of subsidence damage compared with those of zoning restrictions must be examined. The relative costs of prohibition of development, limits on development, and outright purchase of land in subsidence-nsk areas need to be examined. An optional zoning approach that should be studied is the imposition in high-risk areas of a requirement that subsurface investigations be undertaken and appropriate engineering measures be taken to prevent or reduce subsidence damage. Studies also need to be made of building code provisions that can reduce subsidence damage. These could be applied in connection with the zoning approach suggested above. Imp lementatz on Because land-use and building regulation is a local responsibility, the studies recommended above win have to be earned out by local planning agencies and by related professional groups such as Me Planning Advisory Service of He American Planning Association. Building codes are generally a local government responsibility, but mocle] codes are prepared by such organizations as the Intemational Conference of Building Officials (ICBO), the Building Officials and Codes Administrators, Inc. (BOCA), and the Sou~em Building Code Congress International (SBCCI). These organizations should consider the inclusion of subsidence-related

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so provisions in mode! codes, similar to ICBO's Uniform Building Code, which has provisions for earthquake damage reduction. Federal involvement should consist of He dissemination of information and research on building technology by the National Institute of Standards and Technology, National Science Foundation, and Department of Housing and Urban Development; and research of land-use regulation by the National Science Foundation. Market-Based Methods Introduction There are a few examples of effective systems for market-based methods to internalize subsidence costs. The Surface Mining Control and Reclamation Act of 1977 requires mine operators to reimburse surface owners for damage in some cases, and a tax on groundwater in the Santa Clara Valley eliminates the cost advantage of groundwater over surface water. Subsidence costs are also internalized through litigation by parties suffering damage and by payments negotiated by resource extractors facing possible litigation. Findings Studies need to be ca'Tied out on the relative costs and benefits of different approaches to resource taxation for different types of extraction that could result in subsidence. Studies should also be made of the overall costs of resource taxation compared with leaving He problem of cost distribution to litigation. An approach that could effectively internalize the cost of subsidence resulting from resource extraction would be state legislative action requiring that extractors take measures to . . . . . . . ~ . . ~ . . . prevent subsidence and be responsible under specified circumstances for subsidence damage on the surface resulting from extraction. This approach is applied by the federal government to some coal-mine subsidence, but its application is currently very limited. Implementation Cost-benefit studies could be carried out by university researchers and consultants and funded by industry groups for whom subsidence-related litigation is potentially a significant cost, and possibly by the National Science Foundation. Action to require resource extractors to bear subsidence mitigation and damage costs must be taken by state legislatures. Insurance Introduction Insurance is a viable form of relief for the types of subsidence Hat meet He following criteria. An "insurable risk" is generally defined as presenting itself to a sufficiently large and homogeneous group of exposure units to permit reasonably accurate prediction of average losses, where He loss is definite but unpredictable in time and place and where there is no threat of a major catastrophe of unknown proportions. Experience with coal-mine and sinkhole subsidence, and with flooding, suggests that insurance, if properly structured and executed, may equitably, expeditiously, and predictably provide relief for property damaged by subsidence or by flooding aggravated by subsidence.

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51 Findings 1. Information regarding the incidence of insurable types of subsidence needs to be developed in conjunction with property insurers in states where those types of subsidence are widespread. 2. Basic questions regarding underwnting/coverage, claims/value, and adm~stradon/ program structure for subsidence-insurance programs should be addressed in conjunction with any state legislative efforts to authorize such insurance. Early decisions should be made regarding whether subsidence-insurance availability wiD be mandatory or voluntary, the type and doBar amount of coverage to be available, and whether insurance should or should not be interrelated with land-use controls or other mitigation measures. 3. In flood-prone rivenne and coastal areas, methodologies are needed to determine the incremental contribution of subsidence to flooding costs, and criteria need to be developed for establishing priorities for remapping flood zones in subsidence areas. Implementation The following state and federal agencies are identified to develop the information and methodologies given above: Finding I. state geological surveys in conjunction with the U.S. Geological Survey Finding 2. state insurance administrators, property insurance companies doing business in each affected state, and appropriate staff of state legislatures Finding 3. We Federal Emergency Management Agency

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