Wolmar Fellenius ( 1876-1 957)

Chairman and active leader of the Swedish Geotechnical Commission, appointed in 1913 to study the causes of catastrophic landslides on the Swedish State Railways. The report of this Commission, pub- lished in 1924, is justly regarded as a milestone in the application of soil mechanics to practical problems. The investigators were among the first to perform laboratory and field tests for determining the shearing strength of soils, and to recognize the essential differences in the properties of undisturbed and remolded clays. (Photo

courtesy of B. H. Fellenius.)

PLATE 7.

162

Copy

right

ed M

ater

ial

Copyright © 1974 John Wiley & Sons Retrieved from: www.knovel.com

CHAPTER 7

Program of Subsurface Exploration

7.1. Development of Subsurface

Introduction. The preceding chapter demon- strates that very few natural soil deposits are even approximately uniform, and many are extremely erratic. I t is obvious that in an erratic deposit no program of subsurface exploration can lead to more than a rough idea of the average values for the physical properties of the subsurface material and of the probable variations from these values.

The nature of the deposit is an important factor in determining the method of soil exploration that will yield the greatest amount of useful information. If, for ex- ample, the foundation of an important structure is to be established above a fairly homogeneous layer of clay, a considerable amount of testing of undisturbed samples may be justified because the test results permit a relatively accurate forecast of both the amount and the rate of settlement. On the other hand, if the same structure is to be located above a deposit composed of pockets and lenses of sand, silt, and clay, a comprehensive testing program would not be justified because it would provide little more information than could be obtained merely by determining the index properties of representative samples. Much more use- ful information could be obtained at less

Exploratory Program cost by making an adequate number of penetrometer measurements that would dis- close the pattern of the various soft and stiff elements in the subsoil.

The magnitude and character of the ex- ploratory program should also be chosen in consideration of the importance of the proj- ect under construction. If the job involves only a small expenditure, extensive pro- grams of soil exploration cannot be justified economically. I t is cheaper to take ad- vantage of whatever information may al- ready be available and to use a liberal factor of safety in the design.

Finally, the program of soil exploration should develop step by step as information accumulates. By this procedure the maxi- mum amount of information can be ob- tained for a given expenditure and the pro- gram can be terminated as soon as adequate data have been collected. Hence, no definite rules can be established for an exploratory program, and even engineers with con- siderable experience should not attempt to determine the final program before ex- ploration begins.

Preliminary Exploration. The program of sub- surface exploration should be preceded by a fact-finding survey. In such a survey the engineer responsible for the exploration should prepare a digest of all available in-

163

Copy

right

ed M

ater

ial

Copyright © 1974 John Wiley & Sons Retrieved from: www.knovel.com

164 7 / Program of Subsurface Exploration

formation on soil conditions near the site and on the behavior of other structures in the vicinity. In highly developed regions with unfavorable subsurface conditions, use- ful information is likely to be available in technical journals and published reports, but in most rural areas or in areas being newly developed for industry, information concerning structural behavior may be lack- ing. However, the engineer should not over- look the maps and publications of state and federal geological surveys, or reports of soil surveys prepared in connection with agricul- ture or highway construction. The type of information to be obtained from these sources has been discussed in Art. 6.1.

The preliminary exploration procedure is selected on the basis of the information ob- tained from the fact-finding survey. Most soil deposits, however, can be appropriately explored by means of a split-barrel sampler and standard penetration tests (Arts. 5.3 and 5.4) carried out in holes made by augers, rotary drills, or wash-boring tools (Art. 5.2). Other methods of exploration are not usually considered in the preliminary phase unless it is known that the underlying material consists of bedrock on the one hand, or of very soft clays, silts, or highly organic materials on the other. Moreover, for many projects, no further subsurface exploration is necessary. This is likely to be the case if the loads on the subsoil will be small and a large factor of safety can be used without excessive cost, if the structure can be founded on rock or strata of high bearing capacity, or if an ordinary structure is to be erected in an area where much practical experience has been summarized in the form of reliable empirical rules or building codes.

Detailed Exploration. When the preliminary exploratory program does not provide suffi- cient information for design or construction, further investigations are required. The methods should be selected to obtain the most pertinent information at the least cost. Frequently, the properties of fairly uniform deposits of soft clay and plastic silt can be investigated most economically by field vane tests or by obtaining continuous samples in

2- or 3-in. thin-walled tubes and performing appropriate laboratory tests (see Chap. 18). Erratic deposits of soft silt and clay can be examined by means of penetration tests combined with enough tube borings to permit interpretation of the penetrometer data, Standard penetration tests or dynamic cone penetrometer tests are appropriate for sands. Rotary or percussion core barrels are normally used to sample rock, and special peat samplers are available for highly organic deposits. Standard load tests are ap- propriate for loess and other collapsible soils.

O n some jobs involving structures of great importance or foundation conditions of ex- ceptional difficulty, additional information may still be required. I t may be advisable to obtain large-diameter undisturbed sam- ples from critical strata, to conduct load tests, to make field pur-ping tests, or to con- duct other special tests. Since such studies are always expensive, they should be under- taken only to investigate specific questions that the cheaper procedures are inadequate to answer.

Number and Depth of Borings. For buildings or structures of ordinary size, it is suitable to plan on making four borings, one at each corner of the structure. Unless bedrock is encountered, the first boring should ordi- narily extend to the maximum depth within which the stress caused by the structure could conceivably produce excessive settle- ment. This depth may have to be established on the basis of approximate stress and settle- ment computations, as indicated in Part C. For a heavily loaded structure the first boring should ordinarily extend to a depth equal to twice the least width of the struc- ture. Beneath a lightly loaded structure with widely spaced columns, the depth of the first boring should not be less than twice the probable width of the largest footing.

The second boring will serve to indicate whether soil conditions are likely to be rela- tively uniform or erratic. If the results are similar to those for the first boring, subse- quent borings may ordinarily be discon- tinued when they have penetrated all the soft or compressible strata.

Copy

right

ed M

ater

ial

Copyright © 1974 John Wiley & Sons Retrieved from: www.knovel.com

Development of Subsurface Exploratory Program 165

If the borings encounter rock and the conditions are such that the structure may be founded on rock, cores should be ob- tained for a depth of 5 to 10 ft to make sure that sound bedrock, rather than a boulder or a piece of detached rock, has been reached. If there is evidence of solution channels or deep weathering, the cores should usually be continued into sound rock.

As the exploratory program develops,the engineer should study the implications of all new information. If additional borings, pen- etration tests, or special investigations are needed, they should be so located and planned that each addition to the program will provide the maximum increase of knowledge at the current stage of the in- vestigation.

Although the program should be de- veloped to provide the information neces- sary for the project at hand, the engineer should allow for the possibility that there may be changes in preliminary structural layouts, including column spacings and loadings. He should therefore obtain suffi- cient data to permit consideration of the various practical foundation types for the revised layout if the changes are not too radical. Moreover, the exploration should not be limited to obtaining the information needed for design of foundations of a type the engineer may initially think most suit- able; otherwise, he may lack information to select or design foundations of another type that may prove more practical or eco- nomical.

Presentation of Results, The arrangement of responsibilities between the owner and the various professional disciplines involved in the design of a project often requires the preparation of a foundation engineering report. In such a report, the foundation engineer is responsible for the presentation of the re- sults of all field and laboratory tests in a format most useful and readily compre- hensible to the owner and to the other de- sign professionals. Therefore, the data ob- tained from the overall exploratory program should be presented in a manner that most

clearly explains and substantiates the rec- ommendations regarding types of founda- tions and their predicted ranges of per- formance and costs. Summary plots of test data should be included and presented in such a way that the behavior of the subsoil and foundation structure as a whole unit can be readily evaluated.

The foundation engineer must be experi- enced with the design procedures discussed in Parts C and D before he is qualified to develop an exploratory program, prepare a foundation engineering report, or use the results of a program of subsurface explora- tion for purposes of design.

Conclusion. No matter how complete the program of soil exploration and testing may be, there always remains a large margin of uncertainty concerning the exact nature of subsurface conditions at a given site. This fact is of outstanding practical importance. It makes foundation design fundamentally different in its basic concepts from all other branches of structural design. The engineer cannot proceed as he would with materials having well-defined properties, such as steel, concrete, or timber. Although the latter ma- terials are not perfectly uniform, they can almost always be considered so in design. The foundation engineer must often wait to obtain his final data concerning soil condi- tions until he can observe what happens in the field, and he should always make use of every fragment of evidence. Soil tests per- formed on a few samples taken from an er- ratic deposit do not provide a satisfactory basis for design because the engineer is in- terested in the behavior of the deposit as a whole rather than of a few specimens taken from it.

In the discussions of foundation design procedures that follow in Part Cy naturd deposits are grouped according to their general characteristics as clay and plastic silt (Chap. 18), sand and nonplastic silt (Chap. 19), collapsing and swelling soils (Chap. 20), nonuniform soils (Chap. 21), and rock (Chap. 22). In the consideration of each group, particular attention is given to the uncertainties involved in evaluating

Copy

right

ed M

ater

ial

Copyright © 1974 John Wiley & Sons Retrieved from: www.knovel.com

166 7 / Program of Subsurface Exploration

the probable behavior of the deposit with respect to the bearing capacity and settle- ment of foundations. The importance of these uncertainties in dictating the appro- priate type of foundation for a given type of deposit is emphasized. Furthermore, meth- ods are presented that permit the selection and design of foundations on a rational and economical basis, in spite of the handicap arising from the inevitable uncertainties about subsurface conditions.

SUGGESTED READING Many papers contain excellent sum-

maries of the results of subsurface explora- tions, such as those listed at the end of Chaps. 18 through 22, but surprisingly few describe the steps in the development of the exploratory procedure on specific projects. Hence, some of the following references deal with projects carried out several decades ago. The methods are, accordingly, less im- portant than the principles illustrated.

T. L. Condron and E. R. Math (1932), “Investigating a Foundation in Soft Soil,” Ciu. Eng. ASCE, 2, 4, 237-241. The tech- niques and equipment are somewhat out- dated, but the approach remains valid. It was undoubtedly derived from the practice of the senior author who, at the time the article was written, was nearing the end of a long and distinguished career as a founda- tion and structural engineer.

E. W. Scott, Jr. (1948), “Philadelphia Conducts Extensive Subsurface Explora- tion Pr?or to Airport Expansion,” Civ. Ens. ASCE, 18, 2, 44-46. Brief account of an in- vestigation modified to suit the findings.

J. W. Hunter (1W8), “Site Exploration

for Foundations at Portsmouth,” Proc. 2nd Znt. Conf. Soil Mech., Rotterdam, 2, pp. 159- 162. Program included observation of settle- ments of existing buildings.

K. M. Gammon and G. F. Pedgrift (1962), “The Selection and Investigation of Potential Nuclear Power Station Sites in Suffolk,” Proc. Znst. Civil Engrs., 21, pp. 139-160. Description of a well-planned site evaluation for a nuclear reactor that could tolerate only limited differential settlements under gross pressures up to 4 tons/sq ft. Preliminary surveys of seven sites included geology, airphoto interpreta- tion, and a minimum number of borings. Detailed foundation exploration required a number of techniques.

R. B. Peck (1969a). “Advantages and Limitations of the Observational Method in Applied Soil Mechanics,” Ciotechnique, 79, 2, 171-187. Although this paper is not directed toward the usual techniques or programs of soil exploration, it describes the use of field observations during construc- tion as a means for improving knowledge of subsurface conditions at a time when altera- tions in design are still possible, and it points out the rewards and pitfalls associated with the use of observational data.

A manual on subsurface investigations for design of buildings is in preparation by the American Society of Civil Engineers. I t has been published for discussion as “Sub- surface Investigation for Design and Con- struction of Foundations of Buildings, Task Committee for Foundation Design Manual, Part I, ASCE J. Soil Mech., 98, SM5, 481-490; Part 11, SM6, pp. 557-578; Parts I11 and IV, SM7, pp. 749-764.

,

Copy

right

ed M

ater

ial

Copyright © 1974 John Wiley & Sons Retrieved from: www.knovel.com