Land Subsidence (Proceedings of the Fourth International Symposium on Land Subsidence, May 1991). IAHS Publ. no. 200,1991.

Detection of Aquifers Susceptibility to Land Subsidence

N. P. PROKOPOVICH 3333 Braeburn Street Sacramento, CA 95821, U.S.A.

ABSTRACT Consolidometer testing of aquifer sediments can be used to determine: (1) the confined aquifer systems susceptible to subsidence due to a piezometric decline; and (2) the amount of such subsidence. "Undisturbed" core samples of aquifer sediments should be preconsolidated using a loading equal to the estimated weight of the overburden minus the supporting pressure of the existing piezometric head. The following second consolidation test under the total weight of overburden (without the subtraction) will indicate (1) the susceptibility of sediments to compaction after a complete or partial decline of piezometric head, and (2) the amount of subsidence. With some modifications, similar testing may be applied also for prediction of subsidence in unconfined aquifers.

INTRODUCTION

Confined and unconfined aquifer systems are an important source of water. In many areas, the development of such aquifers, particularly their overdraft, leads to a costly geological hazard—land subsidence (Bolt et al., 1975). Such subsidence is now known on all continents except Antarctica, and its future spread seems to be unavoidable with the growth of world population and the increasing need for water (Prokopovich, 1972). To be able to predict aquifers which have sediments that are susceptible to compaction that causes such land subsidence is, therefore, an important and challenging task in engineering geology (Prokopovich, 1978).

The following paper summarizes some ideas that were developed gradually by the author during some 30 years of studies of land subsidence for the Federal Bureau of Reclamation. Most of the studies were conducted on the west central portion of the San Joaquin Valley in California, United States of America, and were made in connection with the design and construction of the Federal San Luis Canal (Anonymous; 1974a, 1981), which is also incorporated as one of the reaches of the California Aqueduct (Anonymous, 1974b) (Figure 1). The valley (Figure 1) is one of the world's largest areas affected by man-induced land subsidence (Poland et al., 1975). In places, the amount of subsidence in the valley has approached 10 m (Figure 2), and has caused damage to several existing canals (by flooding of their lining), pipe crossings, bridges, etc. (Figure 3) costing multimillions of dollars.

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N. P. Prokopovich 28

PIG. 1 Land subsidence due to an overdraf t of ground water in Cal i forn ia , U.S.A. (1) Outline of C a l i f o r n i a ' s Central Valley, composed of Sacramento (N) and San Joaquin (S) Val leys; (2) major cana l s ; (3) subsiding a r ea s . Darkened pa t t e rn ind ica tes major subsidence.

Major i r r i g a t i o n c a n a l s : (a) Tehama-Colusa; (b) Folsom South; (c) P e r i p h e r a l (Proposed) Canal; (d) Contra Cos ta ; (e) Delta-Mendota; (f) C a l i f o r n i a Aqueduct; (g) San Lu i s ; (h) San Fe l ipe D i v i s i o n ; ( i ) Coal inga; ( j ) Madera; (k) Fr ian t -Kern Canal ; (m) Colorado Aqueduct; (n) Coachel la and All American Cana l s .

The ideas expressed in the paper are, however, those of the author and may not represent the official views of the Bureau of Reclamation. Unfortunately, the present concept was finalized after construction of the San Luis Canal and, therefore, was not properly field-laboratory tested.

Detection of aquifers susceptibility to land subsidence

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FIG. 2 Past changes of elevation of land surface in the San Joaquin Valley in an area of maximum subsidence (west of Mendota). Signs show the position of land surface in different years (photograph by B. Lofgren).

BASIC CONSIDERATIONS

Subsidence caused by a decline of ground-water levels is a surficial expression of the compaction of aquifer sediments at depth (Poland and Davis, 1969). Such compaction will take place only if these sediments are not already precompacted by the weight of their overburden.

In the case of a confined aquifer, the aquifer sediments will be practically compacted and not susceptible to subsidence if the piezometric head was developed after deposition of the overburden. Decline of piezometric head under such conditions with already preconsolidated sediments will increase the effective loading on the aquifer, but will not cause a major compaction—subsidence. On the contrary: if the piezometric head was developed prior (or during) the deposition of overburden, the head's decline by pumping and the resulting increase in the effective loading will cause subsurface-compaction of sediments and land subsidence (Prokopovich, 1983).

METHODOLOGY OF PREDICTIONS

The consideration discussed above allows to (a) determine theoretically the susceptibility of an aquifer sediment to

N. P. Prokopovich 30

FIG. 3 Impacts of subsidence on the Delta-Mendota Canal, California (see Fig. 1). (a) Concrete canal lining is submerged below water; (b) highway bridge is partially submerged and canal side lining is completely flooded; (c) check structure is partially flooded; side lining is submerged; (d) partially rehabilitated concrete lining; lining on the opposite bank is flooded; (e) rehabilitated highway bridge and its approaches; (f) rehabilitated check structure.

subsidence, and (b) estimate ultimate amounts of such subsidence: using "undisturbed" {Anonymous, 1974c) samples of the overburden and of aquifer sediments obtained by drilling (Figure 4); and following laboratory consolidometer • testing of samples. The following procedure is recommended: (1) The total weight of overburden capping of an aquifer system should

be established using "field" (="wet") densities (Anonymous,

31 Detection of aquifers susceptibility to land subsidence

1974c) of typical overburden sediments obtained at different depth intervals and the thinness of the overburden.

(2) Using these data, laboratory consolidation tests of typical samples of sediments of an aquifer system should be conducted using at least two consecutive loadings for each sample. The first loading should be equal to the total weight of overburden minus the relief of the loading created by the existing piezometric head. Such loading will eliminate possible rebound of sediments during sampling.

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FIG. 4 Preconstruction drilling and sampling along the San Luis Canal alignment. (a) Drillrig of the test hole SE-501. The trailer is used by geologists for logging and field measurements of push samples; (b) geologist trims push sample in a 3-inch diameter Shelby Sampler for determination of field (wet) density and moisture content of sediments.

N. P. Prokopovich 32

(3) The following, second test should be conducted on the already precompacted samples using the total weight of overburden without such subtraction. Additional compaction during this second test will indicate: (1) aquifers susceptible to subsidence due to a decline of piezometric head, and (2) the ultimate amount of possible subsidence. In some cases (for example in California's San Joaquin Valley),

a piezometric head probably was created both prior to and after the deposition of overburden (Prokopovich, 1983). A fractional increase of loading between the two basic tests may provide important data in this field.

In cases of unconfined aquifers, a decline of water levels increases grain-to-grain pressure within aquifers (Poland and Davis, 1969). Susceptibility of sediments to subsidence in such systems may be determined by similar drilling, sampling, and consolidation testing. The initial loading should represent the total weight of overburden, and the following loading or loadings should include the weight of overburden plus the weight of drained aquifer sediments. These weights can be obtained using the wet density of aquifer sediments at the point of their specific retention and the thickness of sediments.

DISCUSSION AND CONCLUSIONS

Land subsidence, including subsidence due to an overdraft of ground water is now a well-recognized geologic hazard causing a multimillion dollar expense. Determination of aquifer systems with sediments capable of creating subsidence, and prediction of its ultimate amounts under different rates of depletion of an aquifer system are essential for the development of new and the rehabilitation of existing engineering and agricultural projects. The method described in this paper provides a theoretically sound approach for such predictions.

The predictions, however, require rather expensive drilling of deep core holes, obtaining numerous "undisturbed" samples of sediments, and a slightly modified laboratory consolidometer testing of samples.

With an application of more than two basic loadings during such consolidometer testing, the method may help to establish a "safe piezometric decline" of an aquifer system (Prokopovich, 1976); i.e., piezometric decline which causes no notable subsidence. Such data may also provide essential information on past tectonic movements (or climatic changes) and their timing.

As stated previously, the method was developed but not tested during studies of land subsidence along the San Luis Canal. The present, record, 4- to 5-year (1987-1991) drought spell in California and the resulting unavoidable rejuvenation of intense ground-water pumping, and the following subsidence could, however, require such testing.

REFERENCES

Anonymous (1974a) San Luis Unit - Technical Report of Design and Construction, Design of Waterways and Detention Dams, Vol. VI.

33 Detection of aquifers susceptibility to land subsidence

United States Bureau of Reclamation: U.S. Government Printing Office, Denver, CO.

Anonymous (1974b) California State Water Project • California Department of Water Resources Bulletin 200, Sacramento, CA, Vol. 1, 173 p, Vol. 2, 349 p.

Anonymous (1974c) Earth Manual • United States Bureau of Reclamation, A Water Resources Technical Publication, Second Edition: U.S. Government Printing Office, Washington, DC, 810 p.

Anonymous (1981) Project Data- United States Water and Power Resources Service (Now Bureau of Reclamation) Technical Publication: U.S. Government Printing Office, Denver, CO, 1463 p.

Bolt, B. A., Horn, W. L. , MacDonald, G. At and Scott, R. F. (1975) Geological Hazards. Springer-Verlag, New York.

Poland, J. F. and Davis, G. H. (1969) Land Subsidence due to withdrawal of Fluids„ Reviews in Engineering Geology II : Geological Society of America, pp. 187-269.

Poland, J. F. , Lofgren, B. E., Ireland, R. L. and Pugh, R. G. 0.975) Land Subsidence in the San Joaquin Valley, California, as of 1972. United States Geological Survey Professional Paper 437-H: U.S. Government Printing Office, Washington, DC .

Prokopovich, N. P. (1972 ) Land Subsidence and Population Growth . Engineering Geology: Proceedings 24th International Geological Congress, Section 13, Montreal, Canada, p. 44-54.

Prokopovich, N. P. (1976) Some Geologic Factors Determining Land Subsidence . Bulletin of the International Association of Engineering Geolog. No. 14, Krefield, West Germany, p. 75-81.

Prokopovich, N. P. (1978) Prediction of Land Subsidence for Irrigation Canals . Evaluation and Prediction of Subsidence, Engineering Foudnation Conference in Pensacola Beach, Florida, Proceedings: American Society of Civil Engineerings, New York, New York, pp. 234-251.

Prokopovich, N. P. (1983) Neotectonic Movements and Subsidence caused by Piezometric Decline. Bulletin of the Association of Engineering Geologists, Vol. XX, No. 4, pp. 393-404.