Civil and Building Engineering.

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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...

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The design and construction of drilled shafts are discussed in detail in many publications (e.g., Reese and O’Neill, 1988). Drilled shafts (called bored piles in some countries) are constructed by predrilling a cylindrical excavation while keeping the excavation from collapsing by appropriate means, placing
a rebar cage, and then filling the excavation with concrete. A typical drilling rig for constructing drilled shafts is shown in Figure 5.4.

The drilling machine may be mounted on one of three types of carriers: a truck, a crane, or a crawler. The smaller machines are truck-mounted and may move readily along a public road. The soil-filled auger is visible on the truckmounted machine in Figure 5.4, and spinning will dislodge dry soil. Exca- vations for light loads may be made with diameters as small as 12 to 18 in. and to depths of a few feet. Excavations for massive loads may be made with diameters of 15 ft or more and to depths of 200 ft. The drilled-shaft industry
provides technical information and training through the International Association of Foundation Drilling (ADSC) in Dallas, Texas.

The engineer develops an appropriate design by sizing the drilled shaft for a particular application, but significant effort is necessary in preparing of specifications for construction. The engineer who made the design and prepared the specifications should manage the field inspection. Drilled shafts are a popular type of deep foundation, but as with other deep foundations, special care must be taken by the engineer to ensure proper construction.

Drilling machines are fitted with powerful engines to drive a rotary table and kelly. A variety of drilling tools are available. The appropriate drilling tool operating with downward force from cables or with a weighted drill string can be used for drilling into rock. Rock sockets are common to accommodate design requirements, giving the drilled shaft an advantage over some other types of deep foundations.

The following sections describe three types of constructions in common use, but the details of each may vary with the contractor. The specifications for construction prepared by the engineer must be prescriptive in some in-

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The initial step in an offshore investigation is to refer to a detailed map of the topography of the sea bed. Such a survey will be available from the owner of the site. In addition to the location of the structure to be constructed, the topography of the sea bed in the general area should be studied. There may
be evidence of the existence of stream beds that have been refilled with soft material. A further step would be to review geophysical tests that have been performed at the site. In the North Sea, it is customary to use high-power sparker profiling with a spacing of 200 to 1000 m (Andresen et al., 1978).

An investigation of the character of the soils at an offshore site usually requires a special vessel, such as that shown in Figure 4.14. The daily cost for the drilling vessel is substantial, so double crews are necessary to allow the subsurface investigation to continue around the clock. A weather window is necessary to allow the vessel to stay on site for the few to several days required to complete the work on site.

Over the several decades that offshore borings have been made, procedures have been developed to allow the work to proceed with dispatch. Each rig includes a mechanism, as shown in Figure 4.14, to compensate for the heave of the ship. The ship goes to the drill site and is anchored in multiple directions; a casing is then placed on the ocean floor, and soil exploration proceeds.

Drilling is accomplished through the drill string, and drilling fluid may be used to maintain the excavation if necessary. The common procedure is to lower a sampling tool with a wire line; the wire line can raise a weight, and
the tool is driven into position. The end of the drill string and the sampling tool are shown on the left-hand side of Figure 4.15 (Richards and Zuidberg, 1983). The sample is retrieved by lifting the wire line, and the borehole is advanced by drilling to the next position for sampling.

An alternative method of sampling is shown on the right-hand side of Figure 4.15. The wire line lowers a hydraulic piston and sampling tube. The tool can be activated from the drill ship, and the thin-walled sampler is pushed into place. A number of other tools may be lowered; however, the hammer sampler is the most common because of the extra work in latching more complex tools to the bottom of the drill string.
Emrich (1971) reported on a series of studies aimed at comparing the results of the properties of clays obtained by various techniques in borings done to a depth of 91 m at an offshore site near the Mississippi River. Samples were taken using a hammer sampler with a diameter of 57 mm, with a hammer sampler with a diameter of 76 mm, with an open-push sampler with a diameter of 76 mm, and with a fixed-piston sampler with a diameter of 76 mm. Unconfined compression tests were performed on samples from each of the three methods of sampling. In addition, field vane tests were performed, and miniature vane tests were performed at the ends of some of the samples within the sampling tubes.

The data were analyzed by plotting the results on the same graph, with the results from the fixed-piston sampler being taken as the correct value. Compared to results from the fixed-piston sampler, the following results were obtained: 57-mm hammer sampler, 64%; 76-mm hammer sampler, 71%; open-push sampler, 95%. The results from the vane tests were scattered but were generally higher than those from the fixed-piston sampler. Sampling disturbance can explain the lower values of shear strength for some of the
methods described above. To reduce the effect of sampling disturbance, some investigations subjected the specimens from the hammer sampler in a triaxial apparatus to a confining pressure equal to the overburden pressure. This procedure could be unwise if the soil at the offshore site is underconsolidated, as are many recent offshore deposits. Many offshore investigations are performed at a site where the loads of
the piles have been computed and the borings are to be carried deep enough so that information is available on the required penetration of the piles, employing the guidelines of the appropriate governing authority, such as the American Petroleum Institute (API, 1987). Because offshore platforms are frequently designed to sustain horizontal loads during a severe storm, some piles will be subjected to large tensile loads. The exploration must alert the construction managers if soils at the site may be impossible to penetrate, even
with a very-heavy-impact hammer, so that proper procedures can be in place to allow construction to proceed without undue delay. Factors involving soil properties that are critical to the design of piles under
axial loading are numerous. The reader is referred to Chapters 10, 11, 12, 13, and 14 for further reading.

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The pressuremeter test consists of placing an inflatable cylindrical probe into a predrilled hole and expanding the probe in increments of volume or pressure. A curve is obtained showing the reading of the volume as a function of the pressure at the wall of the borehole. Baguelin et al., (1978) state that Louis Me ´nard was the driving force behind the development of the pressuremeter. Early tests were carried out in 1955 with a pressuremeter designed by Me ´nard, a graduate student at the University of Illinois. Me ´nard later established a firm in France where the pressuremeter was used extensively in design.

The components of the pressuremeter system are shown in Figure 4.12. The probe consists of a measuring cell and two guard cells. The cells are filled with water, and gas is used to expand the cells against the wall of the borehole. The guard cells are subjected independently to the same pressure as the measuring cell, and all three cells are expanded at the same rate, ensuring two-dimensional behavior of the measuring cell. A borehole is dug slightly larger in diameter than the diameter of the pressuremeter probe, with the nature of the soil being noted at the depths of the tests. The test is per-
z= depth below ground surface where test was performed,
y= unit weight of soil,
u= porewater pressure, and
K0 =coefficient of earth pressure at rest.

Use of the pressuremeter that is installed in a predrilled hole results in a pressure that is zero, or lower than the earth pressure at rest if the excavation is filled with water or drilling mud; thus, some creep of the soil could occur inside the borehole. In some instances, the test cannot be performed if the borehole collapses. A self-boring pressuremeter has been developed; it can be installed with a minimum of disturbance of the soil.

The advantage of the self-boring pressuremeter is obvious; the disadvantages are that extra time is
required to perform the test and, in some instances, the pressuremeter cannot be recovered.

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Field vane testing consists of inserting vanes at the ends of rods into soft, saturated soils at the bottom of a borehole and rotating the rods to find the torsion that causes the surface enclosing the vane to be sheared.

The torsion is converted into a unit shearing resistance. Two views of typical vanes are shown in Figure 4.11. If the rod used to insert the vane is in contact with the soil, a correction must be made for the torsion on the rod.

With the vane in position, the first test is performed by rotating the rod attached to the vane at a rate not exceeding 0.1º per second, usually requiring 2 to 5 minutes to achieve the maximum torque, yielding the undisturbed shear strength. Then the vane is rotated rapidly through a minimum of 10 revolutions to remold the soil. Finally, the test is repeated to obtain the remolded shear strength of the soil.

The shear strength, s (lbf / ft2), is found from the following equation:
A problem with no easy solution is one in which the soil has inclusions such as shells. The value of s from the vane would be higher than the actual shear strength and could lead to an unsafe design.

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Several types of cone penetrometers are in use, including the mechanical cone, mechanical-friction cone, electric cone, electric-friction cone, and piezometer cone. The mechanical cone has a 60 point angle and a 1.406-in. base diameter. The cone is attached to hollow drill rods and may be pushed down about 3 in. by push rods inside the hollow drill rods while measuring the push force. The mechanical-friction cone has a point of the same size as the mechanical cone and, in addition, includes a sleeve that will be engaged
and pulled down after the cone has been pushed down. Two resistances are measured, the resistance from the cone only and the resistance from the cone and the friction sleeve. Sketches of the friction-cone penetrometer are shown in Figure 4.8.

The electric-cone penetrometer has a cone the same size as the mechanical cone, but the resistance to penetration is measured by a load cell at the top of the penetrometer. Readings are taken by an electrical conduit coming to the ground surface. The electric-friction-cone penetrometer is similar to the
electric-cone penetrometer, but two measurements on resistance are taken as the device is pushed into the soil. Load cells from strain gauges allow the cone resistance and the friction-sleeve resistance to be measured simultaneously. Sketches of the electric-cone penetrometer and the electric frictioncone penetrometer are shown in Figures 4.9 and 4.10, respectively.

The piezometer-cone penetrometer combines the electric cone with a piezometer that can be read electronically. In addition to yielding the cone resistance, the device gives information on the value of the porewater pressure at the depth of the test. The additional information gives the engineer a way to predict more accurately the characteristics of the stratum being investigated.
Figure 4.8 Example of a mechanical friction-cone penetrometer tip (Begemann friction-cone) (from ASTM D 3441).

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1 Cone Penetrometer and Piezometer-Cone Penetrometer: Several types of cone penetrometers are in use, including the mechanical cone, mechanical-friction cone, electric cone, electric-friction cone, and piezometer cone...

2 Vane Shear Device: Field vane testing consists of inserting vanes at the ends of rods into soft, saturated soils at the bottom of a borehole and rotating the rods to find the torsion that causes the surface enclosing the vane to be sheared...

3 Pressuremeter: The pressuremeter test consists of placing an inflatable cylindrical probe into a predrilled hole and expanding the probe in increments of volume or pressure...

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Samples of rock can be taken by core drilling, as shown in Figure 4.7, and the standard practice for diamond core drilling for site investigation is given in ASTM D 2113, as shown in Figure 4.6. The rock quality designation (RQD) should be recorded, and is defined in percentage terms as determined by summing the length of the sound pieces of core that are at least 4 in. long and dividing that length by the length of core drilled. The RQD will vary, of course, through the thickness of a stratum, and the RQD should be recorded
for a specified length of core.

Intact specimens of core can be tested in a triaxial device (ASTM D 2664) or in unconfined compression (ASTM D 2938). Tensile strength and elastic moduli may be tested using intact cores if the values are needed in design.
During the investigation of the site, the face of the rock for the foundation may be exposed. If so, a comprehensive examination of the face of the rock is recommended (Gaich et al., 2003; Lemy and Hadjigeorgiou, 2003).

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Exploratory investigations can be made using a thick-walled sampler that has a split barrel. The sampler can be opened in the field and the contents examined in order to log the stratum being bored. A sketch of the sampler is shown in Figure 4.6, taken from ASTM Standard D 1586, Standard Method for Penetration Test and Split-Barrel Sampling of Soils. After the boring has penetrated to the desired depth, the sampler is fixed to sampling rods and the Standard Penetration Test (SPT) can be performed.

The SPT is performed by dropping a 140-lb weight a distance of 30 in. to impact the top of the sampling rods. The blows required to drive the sampler for each of three 6-in. intervals are counted. The N-value is the sum of the number of blows required to drive the sampler through the second and third intervals. The SPT has been used for many years as an exploratory tool and sometimes to gain information for design. Undisturbed samples of sand cannot be taken except sometimes in the capillary zone or by freezing. As noted in Chapter 3, correlations have been proposed for values of the friction angle as a function of N. Even though the sample of sand is disturbed, it can be examined for grain shape and character, and grain-size distribution curves can be developed.

Samples of clay obtained by the split-spoon sampler can be examined in the field to gain information on the character of the deposit. Specimens can be taken to the laboratory for determination of water content and for determining Atterberg limits. However, correlations between shear strength and N-value for clay soils are not recommended. Most clays are either saturated or partially saturated, and the porewater pressures in the clay below the impacted sampling tool are certainly affected. Clays can be sampled with the thinwalled tube and tested in the laboratory for strength and other characteristics.

Soil sampling methods are not applicable if the blow count reaches 50 for a penetration of 1 inch. The material can then be sampled by core drilling.

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The ASTM has published a detailed standard (1587) for thin-walled tube sampling of soils. The specified dimensions of sampling tubes are shown in Figure 4.4.

Two mounting holes are required for 2- to 3.5-in. samplers and four mounting holes for larger sizes. Hardened screws are required for the mounting.

Dimensions for tubes of three diameters are given in Table 4.1, but tubes of intermediate or larger sizes are acceptable. The lengths shown are for illustration; the proper length is to be determined by field conditions.
The preparation of the tip of the sampler is specified, and the clearance of 1 percent is designed to minimize disturbance due to resistance to penetration of the sample. The interior of the sampling tube must be clean, and a coating is sometimes recommended. Procedures for the transportation of thin-walled tubes are specified in ASTM D 4220, Standard Practice for Preserving and Transporting Soil Samples.
Special tools have been developed to eliminate disturbance partially due to the interior resistance of the sampling tool. At the Swedish Geotechnical Institute, a sample more than 2 m long was laid out for examination. The sample had been taken by the Swedish Foil Sampler, which consisted of a short, thin-walled section followed by a thick-walled section in which was embedded a series of rolls of foil. As the sample was pushed into full penetration, the rolls of foil were simultaneously pulled back to eliminate completely resistance due to sample penetration. The description of a similar device is presented in ASTM D 3550, Standard Practice for Ring-Lined Barrel Sampling of Soils. A sketch of the sampling tool is shown in Figure 4.5. The ASTM Book of Standards presents 13 standards, in addition to D1587, related to surface and subsurface characterization of soil and rock. A list of the relevant ASTM standards is presented in Appendix 4.1.

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1 Introduction: In a remarkable effort, the late Dr. M. Juul Hvorslev, working at the Waterways Experiment Station in Vicksburg, Mississippi, and supported by a number of other agencies, presented a comprehensive document on subsurface exploration and sampling of soils for purposes of civil engineering (Hvorslev,
1949). The document has been reprinted by the Waterways Experiment Station and remains a valuable reference.

The term undisturbed is used to designate samples of high quality. TheASTM uses the term relatively undisturbed in describing the use of sampling with thin-walled tubes (ASTM-D 1587). Disturbance of samples is due to a number of factors: change in the state of stress as the sample is retrieved from the soil, especially if the sample contains gas; disturbance due to resistance against the sides of the sample as it enters the sampling tube; disturbance during transportation to the laboratory; and disturbance as the sample
is retrieved from the sampling tube.

Disturbance due to the presence of gas in soils is difficult to overcome.

Methods must be implemented to prevent the specimen from expanding throughout the sampling, transportation, trimming, and testing periods. A most severe problem involves the sampling and testing of hydrates that occur in frozen layers at offshore sites.

Sampling disturbance due to resistance against the sample as it is pushed into the sampling tube can be significant. Hvorslev (1949) includes some remarkable photographs of samples that have been distorted in the sampling process (pp. 95, 96, 98, 104, 106, 112, 115, 116). In accounting for the effects of sampling disturbance, Ladd and Foott (1974) proposed experimental procedures for use in the laboratory to determine the strength of most soft, saturated clays.

2 Sampling with Thin-Walled Tubes: The ASTM has published a detailed standard (1587) for thin-walled tube sampling of soils...

3 Sampling with Thick-Walled Tubes: Exploratory investigations can be made using a thick-walled sampler that has a split barrel...

 4 Sampling Rock: Samples of rock can be taken by core drilling, as shown in Figure 4.7, and the standard practice for diamond core drilling for site investigation is given in ASTM D 2113, as shown in Figure 4.6...

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Borings to limited depths can be made with a continuous-flight auger driven by a powered rotary table. The stem of the auger is hollow, allowing samples to be taken through the stem without removing the auger. Boring and sampling are done rapidly but, even with a powerful rotary table, the penetration of the continuous-flight auger is limited.

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The use of wash borings is the most common method for advancing a boring because the technique is applicable to any soil, the depth is limited only by the equipment employed, samples can be taken with a variety of tools, and in situ tests can be performed as the borehole is advanced. A typical drilling
machine and its associated equipment are shown in Figure 4.2. The derrick is for handling the hollow drill pile that passes through a rotary table, powered by an engine with the necessary power. The hollow drill pipe carries an appropriate cutting tool. A surface casing is set with a T-section above the ground to direct the drilling water to a holding tank. A pump will drive water down the drill pile to raise the cuttings, which may be examined to gain an idea of the formation being drilled. The water is pumped from the top of the tank, with additional water provided as necessary. Drilling fluid can replace the water if caving occurs.

The system can be scaled up for drilling deeper holes and scaled down for hand operation. The Raymond Concrete Pile Company, now out of existence, distributed a movie for the classroom showing the use of a tripod, assembled on site, for raising and lowering the drill pipe by use of a pulley. A small gasoline engine rotated a capstan head used to apply tension to a rope that passed through the pulley to the drill pipe. A small gasoline pump picked up the drilling water from a tank or from an excavation on site. The entire system
could be transported with a light truck and assembled and operated by two workmen. The components of such a system are shown in Figure 4.3.

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1 Wash-Boring Technique: The use of wash borings is the most common method for advancing a boring
because the technique is applicable to any soil, the depth is limited only by the equipment employed, samples can be taken with a variety of tools, and in situ tests can be performed as the borehole is advanced...

2 Continuous-Flight Auger with Hollow Core: Borings to limited depths can be made with a continuous-flight auger driven by a powered rotary table...

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INTRODUCTION

An adequate and appropriate investigation of the subsurface is critical in the design of the foundation for most projects. On occasion, information on the site can lead to a simple and straightforward approach. In parts of the world, soft soil overlies the founding stratum. Thus, the necessary investigation involves sounding to determine the thickness of the weak deposit so that the length of axially loaded piles can be selected. The situation changes, however, if the piles must sustain lateral loading, requiring detailed information on the
soft soil.

For most subsurface investigations, three preliminary activities are proper: (1) gaining information on the geology at the site, as discussed in Chapter 2; (2) a field trip to the site to get specific information related to the design and construction of the foundations (this could be combined with the field trip to obtain geologic information); and (3) meetings with the architect and structural engineers to gain information on the requirements of the foundation.

Meetings with the owner and relevant professionals may lead to an early definition of the general nature of the foundation at the site, whether the foundation is shallow or deep. If the soil is soft, and if the design will require settlement and stability analyses, as for shallow foundations, Ladd (2003) presents a detailed discussion on necessary procedures for subsurface investigation and laboratory testing. If saturated clay exists at the site, the timedependent behavior of all foundations must be considered. Soil properties are
strongly influenced by the installation of deep foundations, but the prediction of the effects must start with a well-designed and effective soil investigation.

A field trip will determine the nature of other structures near the proposed site. If possible, information should be gained on the kinds of foundations employed in nearby buildings, any problems encountered during construction, response of the foundations since construction, and if any undesired movements have occurred. On occasion, the results will be available for nearby structures from previous subsurface investigations. Municipal agencies will provide drawings showing the location of underground lines, and power lines
should be noted, along with any obstructions that would limit access by soilboring equipment.

The condition of the site with respect to the operation of machinery for the investigation of the subsurface is important. With regard to movement of construction equipment, countries in Europe are providing guidelines for what is termed the working platform (European Foundations, 2004). The purpose of such guidelines is to provide a clear statement on the safety of personnel and machinery on the site. If the site is unsuitable for the operation of boring machines for soil sampling, and later for the operation of construction equipment, such guidelines will inform the owner of the site about improvements required. For example, drainage may be needed, as well as treatment of the surface of the site by an appropriate form of soil stabilization.

Meetings with other professionals on the project are essential. The tolerance of the proposed structure to movement, both vertical and lateral, should be established. The requirements of the requisite building code should be reviewed. Of most importance is the magnitude and nature of the expected loadings, whether short-term, sustained, cyclic, and/or seismic. In some instances, the probability of certain loads may be considered. The maximum loads on the foundations of many offshore structures occur during storms, whose frequency must be estimated on the basis of historical information.

Discussions among the principals should address the possible effects of a foundation failure, whether a minor monetary loss, a major monetary loss, or a catastrophic failure with loss of life.

The details of the site investigation should be addressed in meetings of the principals for the project. Field and laboratory testing can be done that will have little relevance to the success of the foundation. On the other hand, evidence is clear that a thorough and proper soil investigation will affect favorably the initial and final costs of the structure. Some contracts include a clause placing responsibility on the general contractor for the subsurface investigation, possibly leading to problems as construction progresses. A better solution is to employ the phased method, noted below, and then to require the geotechnical contractor to comply with specifications tailored for the structure and site conditions.

The importance of investing appropriately in the soil investigation is illustrated in Figure 4.1. If no money is spent, the structure may collapse. Spending a small amount could lead to later expenditures to correct for unequal settlement of the foundation. As shown in Figure 4.1, an optimum amount spent on the soil investigation leads to a minimum cost of the structure. If more than the optimum amount is spent, the cost of the structure increases by the cost of the soil investigation, but because the cost of the soil study normally is minor compared to the cost of the structure, the final cost of the structure increases only slightly. If owners and their representatives are aware of the facts presented in the figure, less emphasis will be put on price competition for making a soil investigation.

The ideal procedure for the subsurface investigation for a major structure is to perform exploratory borings to identify the various strata at the site and to determine whether or not the strata are tilted. The final design would then become available, and the nature of the foundation system would be evident.

Borings could then be undertaken to obtain the required specimens for laboratory testing and to perform in situ tests if needed. Field loading tests could also be done if needed. This two-stage process would result in the acquisition of precise data for the design of the foundation.

The two-stage process is not possible in some instances—for example, in performing borings for the design of piles for a fixed offshore platform (see Section 4.6). Also, price competition on many projects can lead to a singlestage investigation with a limited number of borings and reduced laboratory testing. Price competition is plainly unwise when specifying a soil investigation.

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The foundation engineer must tell the testing laboratory how to test the soil so that the necessary soil properties are measured for the problem at hand.

Often the engineer must select soil tests while considering how much money is available for testing. Additionally, the engineer must recognize that meaningful testing can be accomplished only on soil samples of good quality and that testing of disturbed soil samples may obtain results that are not representative of field conditions.
Figure 3.63 Computer-controlled triaxial testing equipment (photograph courtesy of Trautwein Geotac).

that testing of disturbed soil samples may obtain results that are not representative of field conditions.
In light of these considerations, the foundation engineer understands the following limitations and capabilities of the consolidation test, direct shear test, and triaxial test:

1. The consolidation test measures the consolidation properties of the soil needed for calculations of total settlement and time rate of settlement.
2. The direct shear test can be used economically to measure the angle of internal friction of sand. The shortcoming of this test is that it is not possible to measure a stress-strain curve for sand.
3. The direct shear test on clayey soils can be used to measure the drained friction angle of clays more economically than S-type triaxial tests.
Again, as with direct shear tests on sand, the shortcoming of this test is that it is not possible to measure a stress-strain curve for the soil.
4. UU-type triaxial tests are the best tests to measure the undrained shear strength of cohesive soils. While these tests can also be used to test sands, this requires that a specimen be built for testing. This usually
involves more work to set up the specimen for testing than for a specimen of cohesive soil. One limitation of the UU-type triaxial test is that the effects of consolidation or rebound of vertical stress cannot be ad-
justed for in the test because the soil specimen is not subjected to consolidation pressures. If the effects of consolidation are important, a shearing test that permits consolidation must be used.
5. The CD triaxial test is the best and most expensive test used to measure the fully drained shearing properties of soils. The test results include the consolidation properties of the soil, the angle of friction under fully drained conditions, and the stress-strain curve. Unfortunately, time and budget constraints often eliminate the use of CD triaxial test on many projects. If only the fully drained angle of friction is required, a drained
direct shear test is a practical alternative.
6. The CU triaxial test without measurement of porewater pressures during shear is used when undrained shear strength values are needed for soils that are subject to consolidation in the field.
7. The CU triaxial test with measurement of porewater pressures during shear can be used to measure the effective stress shearing parameters of cohesive soils more economically than the CD triaxial test.