Civil and Building Engineering.

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A number of authors have made proposals for bearing capacity factors, including Caquot and Ke ´risel (1953), Meyerhof (1963), Hansen and Christensen (1969), Hansen (1970), and Vesic ´ (1973). The basic form of the bearingcapacity equation (Eq. 7.1), proposed by Terzaghi, has been accepted by most subsequent investigators; however, two modifications have been suggested: (1) improved analysis of the model proposed by Terzaghi (Figure 7.2), and (2) an extension of the method to include a number of factors such as inclined loading and a rectangular footing rather than a strip.

In addition, a number of other studies have been undertaken, such as those using FEM, to study the upper- and lower-bound values of the bearing capacity (Ukritchon et al., 2003). Hansen’s method has been selected and is presented in detail because it is comprehensive and because the analytical results agree reasonably well with experimental results.

Figure 7.4 Assumed curves for unit load versus settlement for a footing on dense soil, C1 and on loose soil, C2 (from Terzaghi, 1943, p. 118).

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The differences in settlement for a general shear failure and a local shear failure are shown in Figure 7.4, where the general shear failure is depicted by the solid line. The footing on loose soil is expected to settle a large amount compared to the footing on dense soil, which Terzaghi elected to reflect by reducing the values of the bearing capacity factors for loose soil. A proposed reduction in bearing capacity to deal with excessive settlement is disregarded here because Chapter 9 deals with short-term settlement of shallow foundations. The engineer may reduce the load on the foundation if the immediate settlement is deemed excessive.

Figure 7.2 Model used by Terzaghi in developing valves for bearing-capacity factors (from Terzaghi, 1943, p. 121).

Figure 7.3 Terzaghi’s bearing-capacity factors (from Terzaghi, 1943, p. 125).

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INTRODUCTION

Testing of soil in the laboratory, and perhaps in the field, to obtain properties is required to allow the computation of bearing capacity and settlement. The reader is referred to Chapter 3 for details on the determination of the required parameters of the soil at a site.

The problems to be addressed in this chapter can be illustrated by the shallow foundation of given lateral dimensions resting on soil, as shown in Figure 7.1a. The first of two problems facing the engineer is to find the unit vertical load on a shallow foundation, or on the base of a deep foundation, that will cause the foundation to settle precipitously or to collapse. The unit load at failure is termed the bearing capacity. With regard to bearing capacity, the following comments are pertinent:

1. Any deformation of the foundation itself is negligible or disregarded.

2. The stress-strain curve for the soil is as shown in Figure 7.1b.

3. The base of the foundation may be smooth or rough.

4. The soil is homogeneous through the semi-infinite region along and below the foundation.

5. The loading is increased slowly with no vibration.

 6. There is no interaction with nearby foundations. In spite of the constraints on computing the values, the concept of bearing capacity has been used for the design of foundations for decades and presently remains in extensive use.

Figure 7.1 Example of footing and stress-strain curve used in developing bearingcapacity equations.

Investigators have noted that the bearing capacity equations will vary if the failure is general where symmetrical failure surfaces develop below the base of the foundation; is local where a failure occurs due to excessive settlement of the foundation; or is a punch-through where the foundation punches through a strong surface layer and causes the weak soil below to fail. The equations for general shear failure are most important.

The second of the two problems addressed here is the computation of settlement of a foundation such as that shown in Figure 7.1. Two types of settlement are noted: immediate or short-term settlement and long-term settlement due to the consolidation of saturated clays. The immediate settlement of foundations on sands of loose or medium density is relatively so large that settlement controls, termed a local shear failure, as noted above, and a general bearing capacity failure does not occur. Immediate settlement of foundations on sand and clay is discussed in Chapter 9.

Equations and procedures for dealing with long-term settlement due to consolidation are presented here. The settlement of deep foundations is discussed in chapters that deal specifically with piles and drilled shafts.

The finite-element method (FEM) discussed in Chapter 5 provides valuable information to the engineer on both bearing capacity and settlement, as demonstrated by the example solution presented. FEM can now be implemented on most personal computers, rather than on large mainframes, and will play a much greater role in geotechnical engineering as methods for modeling the behavior of soil are perfected. Leshchinsky and Marcozzi (1990) performed small-scale experiments with flexible and rigid footings and noted that the
flexible footings performed better than the rigid ones. Rather than use the performance of expensive tests with full-sized footings, FEM can be used to study the flexibility of footings.

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The scour or erosion of soils along streams or at offshore locations can be catastrophic. If soil is eroded around the piles supporting bridge bents, the possible collapse of the structure will cause inconvenience, may be very expensive, and could result in loss of life. Unfortunately, technical literature contains many examples of such failures (ENR, 1962).

Predicting the amount of scour is a complex undertaking. Predictions can be made of the velocity of a stream to put in suspension a particle of granular soil, up to the velocity of a mountain stream that moves boulders. Moore and Masch (1962) developed a laboratory apparatus that could be used to study the scour of cohesive soil. It is vital to predict the increase in velocity when an obstruction, such as a foundation pile, is in the stream.

Simply placing large stones around the foundations to prevent scour will not always suffice because in time the stones can settle into the fine soil beneath. One solution is to employ a reverse filter, in which layers of granular soil of increasing size are placed on the fine natural soil so that the particles at the surface are sufficiently large to remain in place during swift stream flow. The design of the reverse filter follows recommendations of Terzaghi on research at the Waterways Experiment Station at Vicksburg, the so-called TV grading (Posey, 1971).

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Steel pipes exposed to a corrosive environment can be damaged severely.

Designs must address two conditions: corrosive water in natural soil deposits and structures in sea water. A soil investigation must determine the character of the water. If the water is found to be corrosive, the engineer may provide extra wall thickness to allow for an amount of loss of metal throughout the life of the structure or may provide a coating for the piles. Several types of coatings or wraps may be used but, in any case, the engineer must be assured by preparing appropriate specifications that the installation of the piles does not damage the coating or wrap.

Steel piles that support waterfront or offshore structures in the oceans must be protected against corrosion by the use of extra metal in the splash zone or by coatings. Alternatively, the engineer may specify the employment of cathodic protection, in which sacrificial metal ingots are installed in connection with appropriate electrical circuits.

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Excavation near existing structures can cause two problems if the excavation is carried out below the water table. First, the cut, either open or braced, can allow the soil to move toward the excavation, resulting in lateral movement of the foundations of an existing structure. Second, the lowering of the water table will result in the possible drop of the water table at some distance away from the cut. The increase in the effective stress due to the lowered water table can cause settlement in some soils.

With respect to the latter case, an excavation was made in Renton, Washington, for a sewer line. The excavation traversed an old river channel and an aquifer that existed below the bottom of the trench. Dewatering was done with large-diameter deep-well pumps. The Boeing Company claimed that the
dewatering of the excavation caused significant damage to a pile-supported two-story steel-frame structure built at a site that was formerly a peat bog.

Even though the building was 1,300 ft away from the dewatered excavation, settling of the ground floor was observed and cracks appeared in the interior walls (ENR, 1962).

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Driving Piles

The installation of deep foundations obviously affects the properties of the nearby soils. Such effects may be considered in design. In addition, movements of soil from installation of piles must be considered. The driving of a pile will displace an amount of soil that can affect nearby construction. The displacement is greater if a solid pile is driven, such as a reinforced-concrete section, and less if an H-pile or an open-ended-pipe pile is driven. However, in some soils, the pipe pile will plug and the soil between the flanges will move with the pile; these piles can become displacement piles.

When a displacement pile is driven into clay, the ground surface will move upward, or heave, and the heave can cause previously driven piles to move up. Often the heaved piles must be retapped to restore end bearing. The heave and lateral movement could also affect existing structures, depending on the distance to the structures. The volume of the heaved soil has been measured and is a percentage of the volume of the driven piles.

If a pile is driven into loose sand, the vibration will cause the sand to become denser and the ground surface will frequently settle. Settlement occurs even though a volume of soil is displaced by the placement of the pile.

As noted earlier, settlement will occur if mats or spread footings on loose sand are subjected to vibratory loadings.

Lacy and Gould (1985) describe a case where fine sand and varved silt from glacial outwash overlie bouldery till at Foley Square, New York City. H-piles were driven for a high-rise structure on 3-ft centers through 80 ft of sand and silt. The vibrations from pile driving caused settlement of adjacent buildings founded on footings above the glacial sand. Even though the con- struction procedures were changed, the vibration of the sand at the adjacent buildings caused settlement of the footings to continue. The adjacent buildings, 6 and 16 stories in height, had to be demolished.

The driving of any type of pile will cause vibrations to be transmitted, with the magnitude of the vibrations dependent on the distance from the construction site. Predicting whether or not such driving will damage an existing structure requires careful attention. The engineer must be cognizant of the possibility of such damage and take necessary precautions. Mohan et al. (1970) present the details of damage to existing buildings in Calcutta due to nearby pile driving.

The near-surface soils in Calcutta consist of silts and clays to a depth of about 15 m. Foundations of relatively small buildings can be placed in the top 5 m, where the clay has medium stiffness. Larger buildings are founded on piles that penetrate the 15 m into stronger soil below. Piles were being driven at a site where the central portion of the site was about 30 m from two existing buildings that were founded on spread footings. After 145 reinforced-concrete piles had been driven, cracks were observed in the exist- ing buildings. While there was no danger of failure of the spread footings, the cracks were unsightly and reduced the quality of the buildings. A study was undertaken before driving the last 96 piles for the new building. Driving at more than 30 m from the site caused no damage to existing buildings, but as the driving moved closer, the height of fall of the pile hammer had to be reduced from 1 m to 15–30 cm.

Construction of a large federal building in New York City was halted when only a third of the piles had been driven. The building had plan dimensions of 165 by 150 ft and was to be 45 stories high. It was to be supported on 2,700 14BP73 piles. The water table was 15 ft below street level, and much of the underlying soil was medium to fine sand. The subsidence due to the pile driving had lowered a nearby street more than 12 in., and sewer lines had been broken on two occasions (ENR, 1963b, p. 20).

Lacy et al. (1994) recommend the use of CFA piles to reduce the impact on adjacent structures—for example, by eliminating densification (‘‘loss of ground’’) of granular soil as a result of pile driving.

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Introduction

All types of deep foundations can be damaged by improper methods of construction. As noted by Lacy and Moskowitz (1993), the engineer has several responsibilities prior to and during construction. Specifications for construction must be prepared that give the methods to be used to achieve foundations of good quality. Also, it is critical for qualified inspectors to be present during construction to ensure compliance with the specifications. The selection of a qualified contractor with experience with the proposed system and with proper equipment is vital, and the owners of the project should allow procedures that ensure a qualified contractor, such as prequalification of bidders. Preconstruction meetings with the successful contractor are almost always useful.

Driven Piles

Piles are sometimes damaged by overdriving. The waveequation method is useful to select an appropriate pile hammer for the pile to be driven. Attention to detail is important concerning such things as the driving equipment, the quality of the pile being driven, and the arrangements of the components of the system. Even with the use of an appropriate hammer and appurtenances, tough piles such as H-piles can be damaged at their ends by overdriving.

Damage of steel piles in unusual ways can involve great expense and loss of time. Open-ended steel pipes were being driven to support an offshore platform when the lower ends were distorted by being banged against the template structure. The distortion caused the lower portion of the piles to buckle inward during driving due to lateral forces from the soil. The collapse prevented drilling through the piles to allow the installation of grouted inserts.

Many engineers worked for several months to design a remedial foundation. Damage to precast concrete piles during driving can be vexing and expensive. The authors are aware of a bridge along the coast of Texas where many of the precast piles had been damaged due to overdriving, causing tensile cracks at intervals along the piles. Salt water entered the cracks, corroded the reinforcing steel, and destroyed the integrity of the piles. The contractor worked beneath the deck of the bridge to install a special system of drilled piles.

Selection and proper replacement of cushioning material when driving concrete piles is important (Womack et al., 2003). The authors were asked to review the installation of concrete piles where plywood was used as cushioning material. Driving continued until the plywood actually collapsed and burned due to continued use. The concrete piles exhibited a number of cracks in the portions above the ground.

Drilled Shafts

Drilled shafts or bored piles are becoming increasingly popular for several reasons. The noise of installation is less than that of driven piles, drilling can penetrate soft rock to provide resistance in side resistance and end bearing, and diameters and penetrations are almost without limit.

Drilled shafts are discussed in Chapter 11, and construction methods are described in detail in Chapter 5.
As noted earlier, preparation of detailed specifications for construction of drilled shafts is essential, and a qualified, knowledgeable engineer must provide inspection. Preconstruction meetings are highly desirable to ensure that the specified details of construction are consistent with the contractor’s ability to construct the project.

CFA Piles

As noted earlier, CFA piles have been used for many years and are competitive with respect to the cost per ton of load to be supported when the subsurface conditions are suitable. The CFA pile is constructed by rotating a hollow-stem CFA into the soil to the depth selected in the design. Cement grout is then injected through the hollow stem as the auger is withdrawn.

Two errors in construction with CFA piles must be avoided: (1) the mining of soil when an obstruction is encountered and the auger continues to be rotated and (2) abrupt withdrawal of the auger, allowing the supporting soil to collapse, causing a neck in the pile. To assist in preventing these two construction problems, a data-acquisition system can be utilized to record rotations, grout pressure, and grout volume, all as a function of penetration of the auger. Alternatively, the inspector can obtain such data on each pile as construction progresses.

Other Types of Piles

Many other types of piles for deep foundations are described in Chapter 5. The engineer must understand the function of such piles and the details of the construction methods. Preparation of appropriate specifications for construction and inspection of construction are essential. Field load tests of full-sized piles often must be recommended if such experimental data are unavailable from sites where the subsurface conditions are similar.

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A slide occurred at Fort Peck Dam that allowed impounded water to rush out, with consequent damage and public outcry. A comprehensive investigation followed, and Middlebrooks (1940) reported that the slide occurred in shale with seams of bentonite at the hydraulic-fill dam. The investigation prior to design failed to indicate the extent of weathering in the shale and bentonitic seams; further, the investigation failed to predict the high hydrostatic pressure in the bentonite due to the load from the hydraulic fill.

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The soil for the levee was moved and distributed by earth-moving equipment. When the fill reached the maximum height of 32 ft, a slide caused the levee to fail on the land side. The levee had been instrumented extensively to gain information on present and future construction.

Terzaghi (1944) studied the results of the soil investigation and the data from the instruments and concluded that failure had occurred in a thin horizontal stratum of fine sand or coarse silt in the soft clay that was undetected in the soil investigation. The layer of granular soil could reasonably have occurred during the process of deposition over the centuries as flooding deposited soil in layers. Variations in climatic conditions could have led to the deposition of the cohesionless soil. The fill caused the pore pressures to increase in the thin, cohesionless stratum, and failure occurred along that stratum as its strength was reduced.

Peck (1944) discussed the paper and noted that the identification of such a stratum of fine sand in the clay would require continuous sampling with proper tools. Kjellman et al. (1951) described the use of the Swedish sampler with thin strips of foil along the interior of a sampling tube. The foil is retracted as the sampler penetrates the soil, eliminating the resistance between the soil and the interior of the tube and allowing undisturbed samples to be obtained with lengths of several meters. Such a tool would have been ideal for sampling the soil at the Pendleton Levee but the method has found little use outside of Sweden.

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1 Pendleton Levee: A failure occurred as described by Fields and Wells (1944)...

2 Fort Peck Dam: A slide occurred at Fort Peck Dam that allowed impounded water to rush out, with consequent damage and public outcry...

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The model for the design of a pile under axial loading (see Chapter 10) is fairly simple. For transfer of load in side resistance (skin friction) the model employs the distribution of stresses on the pile–soil interface of elements along the length of the pile. The axial load sustained by an element can be computed by integrating the vertical stresses along the face of the element.

For transfer of load at the base of the pile (end bearing), the model employed is similar to that for a footing. If piles are driven close to each other, an allowance must be made for pile–soil–pile interaction.

While the models described above have been in use for many years, a design in 1959 used piles of different lengths along a pier (Figure 6.4) where the axial loading on the deck of the pier was presumably uniform. The shorter piles settled more than the longer ones and caused an unacceptable and uneven settlement of the pier. Long et al. (1983) describe the repair of the pier caused by the unequal settlement of the piles. The soil profile across the site was relatively uniform. The use of the simple models described above should have produced an acceptable design. However, the use of the models for an axially loaded pile is obviously more complex if the soil profile varies across the construction site.

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This grain elevator located near Winnipeg, Canada, was constructed in 1913 on a raft foundation. Failure of the bin house by gradual tilting occurred after 875,000 bushels of wheat had been stored yielding a load of 20,000 tons, distributed uniformly. The bin house had plan dimensions of 77 by 195 ft and was 92 ft high. The elevator was founded on clay that had been deposited in a glacial lake and failed by tilting when the uniform load reached 3.06 tsf.

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Bjerrum and O ¨ verland (1957) studied the failure of an oil tank in Norway.

Soil borings were made at the site of the tank. Properties of the soil were obtained by the use of the in situ vane and by the unconfined compression test. The upper layer of soil was a silty clay to a depth of 7 m with a soft marine clay below. The undrained shear strength was reconstituted to obtain values for the construction time of the tanks. The strength of the soil was almost constant to a depth of 10 m with a value of about 3 tons/m2, but the shear strength was over 4 tons/m2 at a depth of 15 m. The diameter of the tank was 25 m.

If a failure of the entire tank was assumed, the average shear strength along the failure surface yielded a factor of safety of 1.72. On the other hand, if failure was assumed at one edge of the tank, the average shear
strength along the shallower failure surface was less and the factor of safety was computed to be 1.05.

The heave of the soil was near one edge of the tank and indicated a local failure in which the weaker soil was mobilized. The error in the original design was due to the selection of a model for general failure where the
average shear strength along an assumed failure surface was significantly larger than the average shear strength of the near-surface soil related to local failure. Thus, the careful selection of a model for a failure surface that will yield the lowest factor of safety is essential.

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Models of various kinds have been proposed for the solution of every foundation problem. For example, failure surfaces are shown in the soil below a footing, and soil-mechanics theory is used to predict the location and stresses along these surfaces. Integration is then used to compute the load on the footing that generates the failure surfaces. Such a model is difficult to apply when obtaining the bearing capacity of a footing on layered soils, particularly considering three-dimensional behavior. But such models can be used with confidence if proven by full-scale load tests.

While the ultimate load on a footing may be computed with the model described above, another type of model must be used to obtain the movement of the foundation. The goal of analytical techniques is to have a model that may be used both for ultimate capacity and for the nonlinear movement of the foundation. The development of such models is continuing.

1 Oil Tank in Norway: Bjerrum and O ¨ verland (1957) studied the failure of an oil tank in Norway...

2 Transcona Elevator in Canada: This grain elevator located near Winnipeg, Canada, was constructed in 1913 on a raft foundation...

3 Bearing Piles in China: The model for the design of a pile under axial loading (see Chapter 10) is
fairly simple...

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The engineer may encounter an unusual soil, especially if working in an area where little is known about the soil. Several years ago, one of the authors attended a preconstruction meeting prior to building an offshore platform on the Northwest Shelf of Australia. A sample of sand was shown, and the results of laboratory testing were presented. The friction angle was consistent with the relative density, and the decision was made to design the piles using available equations even though the sand was calcareous due to the nature of the geologic deposition.

The template was set on the ocean floor, and piles were to be stabbed and driven with the legs of the template as guides (Jewell and Andrews, 1988).

However, after the first open-ended pipe pile was placed into the template, the pile fell suddenly about 100 m and came to rest on a dense stratum. The calcareous grains had crushed under the walls of the pile, and natural cementation in the deposit prevented the calcareous sand from exerting any significant lateral stress against the walls of the pile. A stress-strain curve from the laboratory exhibited severe strain softening. Thus, skin friction was low to nonexistent as a result of the large deformation during installation, and the allowable end bearing on the piles was insufficient to provide adequate safety during the design storm. The result was that special strengthening was designed after an intensive study. The required construction was expensive, complex, and time-consuming.

Very Soft Clay The design of stable foundations must address the presence of soft clay at the construction site. Several of the chapters of this book deals with aspects of soft clay, including identification, strength, deformational characteristics, and design of foundations. Two procedures are noted below.

In addition, the engineer must take special care if an excavation must be made in soft clay. Analytical techniques must be employed to investigate the possibility of sliding that would affect the site and possibly nearby structures as well.

Preloading. If the construction can be delayed for a period of time, the site may be preloaded with a temporary fill. A drainage layer can be placed on the surface of the soft clay, and boring can be used to install vertical drains at appropriate spacing. Analysis can predict the time required for the clay to drain to an appropriate amount, with a consequent increase in shear strength.

Settlement plates can be installed to provide data to confirm the analytical predictions or to allow modification of the predictions.

Load-Bearing Piles. Another method of providing foundations where there is a stratum of very soft clay is to install piles through the clay to a bearing stratum below. Usually piles are driven, causing modification of the properties of the clay. The engineer must be aware that soft clay can settle, subjecting the piles to downdrag. Possible problems related to the buckling of piles in soft clay can be investigated by methods presented in Chapters 12 and 14.

Expansive Clay Expansive clay at a construction site, if not recognized, can sometimes lead to disastrous results. Chapters 2, 3, 6, and 9 will discuss the identification of expansive clay and the design of shallow and deep foundations. Expansive clay increases the cost of the foundation for a low-rise structure, and inadequate foundations are being built in spite of current knowledge and some building codes. Thus, the problem is faced more by homeowners than by the owners of commercial buildings.

Clay expands as it becomes wet and shrinks as it dries. Furthermore, moisture will collect when evaporation is cut off. In addition to swelling of the clay, other important factors are the nature of the foundation, weather, and transmission of moisture through the clay. A foundation on expansive clay may show no distress for perhaps years and then experience severe movement.

On the other hand, one of the authors was asked to visit a site where a church building had cracks in the walls so wide that people in the congregation could see children in a playground. However, a rain had occurred the night before, and when the author arrived, the cracks were virtually closed! Figure 6.2 shows the cracking of a structure on expansive clay.

The severe differential movement of the foundations of homes on expansive clay can sometimes be devastating. In a home belonging to an assistant sports coach at a major university, doors did not close properly, the wallpaper was wrinkled, and the floors were very uneven. A portion of the home was on a slab, and another portion was supported by piers and beams. Repairs could be made, but the expense would be heavy. Later, the coach was divorced, and he moved away.

Some years ago, a nonprofit agency was asked to host a series of seminars on expansive clay. The plan was to encourage potential homeowners to look for cracks in the ground, damage to nearby homes, and elementary methods of identifying expansive clay. Geologists, geotechnical engineers, structural engineers, developers, and local officials were to be invited, along with the potential homeowners. The agency declined to host the seminars, perhaps due to fear of being sued by builders or developers who had tracts where expansive clay existed.

In addition to presenting information on identification of expansive clay, this book will give recommendations for the construction of shallow and deep foundations.

Loess In the Mississippi Valley of the United States, in Romania, in Russia, and in many other parts of the world, a soil exists called loess. It was created by the transport by wind over long periods of fine grains ranging in size from about 0.01 to 0.05 mm. Grass or other vegetation grew during the deposition, so loess has a pronounced vertical structure with cementation associated with the vegetation.

Cuts in loess will stand almost vertically to considerable heights, but the soil will collapse under load when saturated. Loess is capable of sustaining a considerable load from a spread footing, but the design of foundations must consider the possibility of saturation. For relatively light structures, loess may be treated to a depth of up to2mbyprewetting, compaction, and/or chemicals. Appropriate drainage is critical. Pile foundations extending through the loess are frequently recommended for major structures.

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Cambefort (1965) wrote of experiences with unusual soils and remarked: ‘‘There is no known ‘recipe.’ But the most dangerous thing is to think that the known formulas can explain everything. Only reasoned observation can lead to satisfactory results.’’ The observational method should always be used, particularly on major projects, and some engineers have employed this method extensively. The thesis is that not all the features of the behavior of a foundation can be predicted but that strengthening can be done if excessive movement is observed.

The following sections describe several of types of soil that can cause problems in construction and deserve special attention from the engineer.

Other sections of the book present methods of designing foundations for a number of soils noted below.

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At a number of locations in the United States and elsewhere, the use of bearing piles is dictated by the nature of the soil overlying the founding stratum. The soil investigation is aimed mainly at determining the thickness
of the surface stratum in order to find the required length of the piles. Penetration tests can be used with confidence, and experience may show that the piles need only be driven to refusal into the bearing stratum.

For many low-rise buildings supported on a thick surface stratum of sand, the SPT (Chapter 4) can be used to determine that the sand is not loose or very loose and to ascertain the position of the water table. Spread footings or a raft can be designed with confidence.

Other soil profiles exist that are well known to local engineers, and the type of foundation can be selected without exploratory borings. The soil investigation can be made with methods that are locally acceptable and lead to a standard design. The geotechnical engineer must exercise caution in all cases to identify soils with special characteristics as discussed below. Nature is anything but predictable, and the engineer must be alert when characterizing soils.

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Many factors affect the plans for a proper investigation of the subsurface soils for a project. A cooperative effort is desirable in which the owner conveys to the architect and structural engineer the requirements for the proposed structure, the structural engineer and architect make a preliminary plan that dictates the foundation loads, and the geotechnical engineer describes the geology and suggests a type of foundation. Unfortunately, the geotechnical engineer is often selected later, after a plan is proposed for the subsurface investigation where price could be an important consideration. A cursory study of the soils could result, with the geotechnical engineer recommending design parameters on the basis of limited information. Such limited soil studies have resulted in a large number of claims by the contractor of a ‘‘change of conditions’’ when the soils were not as presented in the soils report.

The ideal plan, unless substantial information on subsurface conditions is available, is to perform exploratory borings for classification and to get data that will guide the borings for design. As noted in Chapter 4, a wide range of techniques are available to the geotechnical engineer. The data will allow the type of foundation to be selected and will provide numerical values for the relevant soil properties.

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1 Planning: Many factors affect the plans for a proper investigation of the subsurface soils for a project...

2 Favorable Profiles: At a number of locations in the United States and elsewhere, the use of bearing piles is dictated by the nature of the soil overlying the founding stratum...

3 Soils with Special Characteristics: Cambefort (1965) wrote of experiences with unusual soils and remarked: ‘‘There is no known ‘recipe.’..

4 Calcareous Soil: The engineer may encounter an unusual soil, especially if working in an area where little is known about the soil. Several years ago, one of the authors attended a preconstruction meeting prior to building an offshore platform on the Northwest Shelf of Australia...