Civil and Building Engineering.

ALLOWABLE SETTLEMENT OF STRUCTURES: Problems in Proving Settlement.

One of the authors was asked to express opinions on two sites where the failure of foundations was questionable. Structural steel was being erected for a high-rise structure in the southern United States when the contractor noticed that the steel members were not ﬁtting together properly. Even though the settlement survey was inconclusive, the drilled-shaft foundations were underpinned without delay. More than ordinary skill is required to make precise measurements of building settlement. Even though settlement was claimed, the proof was lacking and the ensuing lawsuit was expensive, timeconsuming, and perhaps unnecessary.

At a site in Hong Kong, a nearly horizontal crack was discovered in a thick pile cap during the erection of a reinforced-concrete warehouse. Settlement was postulated, even though no strong evidence was present from precise surveying. Coring of a multitude of bored piles was undertaken, and poor contact was found for some piles between their base and the founding stratum.

Such piles were judged to be defective, even though the area of the core was only a fraction of the total area of the base of the pile. Hand-dug piles were being installed in some places as part of a huge program of underpinning when the decision was made to test some of the piles below the cracked cap.

A slot was cut between a bored pile and the cap, ﬂat jacks were installed, the height of the existing reinforced-concrete structure provided sufﬁcient resistance, and load testing proceeded. Three piles were tested with the ﬂat-jack method, and all showed ample capacity. Underpinning was discontinued, and the construction proceeded without a problem. Later, an investigation revealed that the crack in the pile cap was due to a cold joint resulting from a large concrete pour.

ALLOWABLE SETTLEMENT OF STRUCTURES:Exceptional Case of Settlement

The Valley of Mexico, the site of Mexico City, in some ways is a large-scale laboratory for demonstrating principles of soil mechanics. Pumping from deep-seated aquifers over many years caused several feet of subsidence in the Valley, and stresses from the foundations of major structures caused additional settlement due to the compressible nature of the soils. Zeevaert (1972) presents a proﬁle to a depth of 80 m of the highly stratiﬁed subsurface soils in the Valley, with many meters having water contents averaging 200% and some strata with water contents as high as 400%. Settlement due to soil consoli- dation can be signiﬁcant.

Settlement in Mexico City is illustrated by the view of a government building (Figure 6.1). The building remained in use even though the settlement was much more than what might be tolerated at a site
with less extreme soil conditions.

Figure 6.1 Photograph showing settlement of a government building in Mexico City.

Cummings (1941) noted that ‘‘There is no question but that Mexico City is one of the most interesting places in the world for ﬁeld studies in the settlement of structures. Whereas it is often necessary to search carefully for structural settlements with precise leveling and other accurate methods, the observation of structural settlements in Mexico City can be done by the eye without the use of measuring instruments of any kind.’’

ALLOWABLE SETTLEMENT OF STRUCTURES: Tolerance of Buildings to Settlement.

Recommendations have been published on total and differential settlement for various classes of buildings, starting with monumental structures and ending with temporary warehouses. Total settlement is important because connections to underground services can be broken and sidewalks can be distorted and out of place. Differential settlement leads to distortions in the building and cracking of brittle building materials such as glass and ceramics. Both total settlement and differential settlement must be considered by the engineer for
each structure, and published recommendations should be avoided.

The modern approach to the tolerance of buildings to settlement is to perform analyses to predict immediate and time-related movement of the foundation, consistent with the stiffness of the superstructure. Procedures presented here, such as a deformable slab supported by bearing piles, provide guidance to a comprehensive solution. Tools are available for structural engineers to use in predicting the deformation of the superstructure, taking into account nonstructural elements such as partitions and non-load-bearing walls to conform to the time-related deformation of the foundation. Field observations are used to conﬁrm the validity of the models, but analytical tools are at hand. Engineers and owners will be relieved of the burden of complying
with prescriptive requirements.

The design of some structures may be very sensitive to differential settlement. Conferences between the structural engineer and the geotechnical engineer are useful in such cases. Extraordinary methods can be employed to limit differential settlement when necessary.

ALLOWABLE SETTLEMENT OF STRUCTURES.

1 Tolerance of Buildings to Settlement: Recommendations have been published on total and differential settlement for various classes of buildings, starting with monumental structures and ending with temporary warehouses...
2 Exceptional Case of Settlement: The Valley of Mexico, the site of Mexico City, in some ways is a large-scale laboratory for demonstrating principles of soil mechanics...

3 Problems in Proving Settlement: One of the authors was asked to express opinions on two sites where the
failure of foundations was questionable...

TOTAL AND DIFFERENTIAL SETTLEMENT.

The total settlement of a structure is the maximum amount the structure has settled with respect to its original position. The Palace of Fine Arts in Mexico has settled several feet but still remains usable because the differential settlement is tolerable. Differential settlement causes distortions in a structure, possible cracks in brittle materials, and discomfort to the occupants. The masonry building materials used in constructing the Palace of Fine Arts can tolerate the inevitable differential settlement, but some discomfort is inevitable. One can feel a deﬁnite tilting while sitting in a seat and watching a performance.

The subsequent chapters on computation of the settlement of a foundation show that the maximum settlement should occur under the center of the foundation and the minimum under the edges. Some proposals suggest that the difference between the maximum and minimum, that is, the differential, should be some fraction of the maximum. However, in all but exceptional cases, the engineer should create a design in which total settlement is moderate and differential settlement negligible.
Settlement as a result of subsidence can be important in the design of foundations. Subsidence occurs because water or perhaps oil is removed from the underlying formations and causes an increase in effective stress, triggering settlement that can amount to several feet. Subsidence has occurred in many cities because of dewatering. Houston, Texas, is an example. The movement of the surface usually is relatively uniform and foundation problems are minimal, but faults can occur suddenly and can pass through the foundation of a
home, with serious and perhaps disastrous consequences. A less common cause of subsidence is the yielding of supports for tunnels used in mines that have been abandoned in some regions of the United States.

Areas of subsidence may be more frequent in the future as the need for water increases and possibly leads to more pumping of water from aquifers.

The engineer must be aware of areas where subsidence is occurring or is expected to occur.

DESIGNING STABLE FOUNDATIONS: INTRODUCTION.

The design of every foundation presents a unique challenge to the engineer.

Designs must take into account the nature and cost of the structure, geology and terrain, quality of subsurface investigation, kind of agreement with the owner, loadings over the life of the structure, effects of the proposed construction on buildings near the construction site, sensitivity of the proposed structure to total and differential settlement, requisite building codes, potential environmental effects due to the proposed construction including excessive noise, adequacy of the analytical tools available for making the design, result
of a failure in terms of monetary loss or loss of life, availability of materials for foundations, and competent contractors in the area. A considerable effort will normally be necessary to gather and analyze the large amount of relevant data.

To create foundations that perform properly, the engineer must address carefully the topics in the above list and other relevant factors. Failures will occur if the foundation is not constructed according to plan and in a timely manner, if detrimental settlement occurs at any time after construction, if adjacent buildings are damaged, or if the design is wasteful.

The topic of designing wasteful foundations is of interest. An engineer could cause more than necessary resources to be expended on a foundation in the absence of peer review. An engineer increases the size of a foundation because the extra material gives a measure of ‘‘pillow comfort.’’ An engineer in a ﬁrm was asked to describe the technique being used to design the piles for an important job and replied: ‘‘We use an approximate method and always add a few piles to the design.’’ Such failures may never be seen, but they are failures nevertheless.

Casagrande (1964) wrote of calculated risk in earthwork and foundation engineering and gave several examples of projects where risk was used in the design and construction of major projects. Risk is always involved in geotechnical engineering because of the inability to get good information about the factors listed above and because of that sometimes unusual behavior of soil that cannot be anticipated in spite of diligence. Risk is minimized by prudent engineers who use available tools and techniques and who stay
abreast of technological advances.

Peck (1967) presented a keynote address at a conference where the emphasis was on improved analytical procedures. He gave ﬁve sources of error with regard to the bearing capacity and settlement of foundations: (1) the assumed loading may be incorrect; (2) the soil conditions used in the design may differ from the actual soil conditions; (3) the theory used in the design may be wrong or may not apply; (4) the supporting structure may be more or less tolerant to differential movement; and (5) defects may occur during construction. The technical literature is replete with examples of foundations that have failed, many for reasons noted by Peck.

The following sections present brief discussions of some examples, emphasizing the care to be taken
by the engineer in planning, designing, and specifying methods of construction of foundations.

Lacy and Moskowitz (1993) made the following suggestions concerning deep foundations: understand the subsurface conditions in detail; select a qualiﬁed contractor; take care in preparing speciﬁcations for construction; provide full-time inspection by a knowledgeable person who has the necessary authority; and monitor adjacent structures as construction progresses.

HYBRID FOUNDATION.

A hybrid foundation consists of both a soil-supported mat and piles and is used principally to support an axial load. In Europe it is commonly called a piled raft because engineers conceived the idea of designing the foundation for high-rise buildings using a mat (raft) resting on the ground with piles supporting the mat (Figure 5.9). The concept was that the combined foundation would be sufﬁcient to support the applied axial loading with an appropriate factor of safety and that the settlement of the combined foundation at working load would be acceptable. The settlement of a mat foundation is dish-shaped, with the largest settlement at the center of the mat. To achieve a more uniform settlement of a structure, it has been suggested that the piles be clustered near the center of the mat.

The analysis of such a system is complicated because the settlement of the raft is affected by the presence of the piles and because a piled raft foundation consists of conventional piles and a rigid raft, as shown in Figure 5.10. Considering each of these foundation elements separately leads to the conclusion that interaction is inevitable. The mat alone is certainly affected by the presence of the piles because the foundation is much stiffer than with the soil alone. The piles alone are affected by the earth pressure from the raft because the increased lateral stresses on the piles affect the capacity for side resistance.

The problem can be solved by using the ﬁnite-element method, where appropriate plate or solid elements can be used for modeling the raft. Beam elements can be used for modeling piles. The soil around the piled raft system can be conveniently modeled as solid elements. Modeling the problem by the ﬁnite-element method, which is widely available, was found to be practical and yielded reasonable agreement between results from analyses and from experiments (Novak et al., 2005).

Micropiles.

Micropiles are deep foundations with a small diameter. They may be installed in a variety of ways and have several purposes. Bedenis et al. (2004) describe the use of micropiles as load-bearing elements to strengthen foundations that were to support a greater load. The piles were installed by drilling, and were employed because pile-installation equipment could not be operated in the available space inside the building. The piles consisted of a high-strength steel bar with a diameter ranging in size from 32 to 63 mm, grouted into place,
and forming a pile 152 mm in diameter. The piles extended through overburden soils to sandstone at a depth of 24.7 m. The capacity was developed principally through side resistance in the sandstone.

Go ´mez et al. (2003) describe the testing of a micropile with a diameter of 219 mm and a length of 7.13 m, consisting of a central casing with a diameter of 178 mm that was embedded in grout. The soil at the test site consisted of 3.0 meters of residual soil above rock. The instrumentation revealed that the bond between the rock and the grout did not fail, but some loss of load transfer in skin friction was noted as the movement of the pile relative to the soil increased above 0.05 mm.

If micropiles are used to sustain compressive loads, the engineer should compute the buckling load by using a computer code. The unfactored lateral load should be applied, and the compressive load is increased in increments until failure occurs by excessive lateral deﬂection. Pile-head ﬁxity must be modeled as well as possible or the analyses should be performed with a range of pile-head ﬁxities that encompass the value to be anticipated.

GeoJet Piles.

The GeoJet pile is described as an example of a special kind of deep foundation. Numerous kinds of special deep foundations have been developed and employed in some projects. The engineer faces the problems of designing for the support of axial and lateral loading and of preparing speciﬁcations for construction that allow for inspection to ensure good quality. With regard to design, the properties of the soil will be affected by the method of installation.

Therefore, the engineer must depend heavily on the results of load tests with the special foundation at the site or as reported in the technical literature where all relevant details are presented.

The GeoJet pile is constructed by rotating a special drilling head into the soil and introducing grout through the drilling head during withdrawal. Figure 5.8 shows a typical drilling head attached to a kelly prior to initiating construction at a site. If construction conditions are favorable, the column of soil cement can be constructed in less than 5 minutes. After the column of ﬂuid soil-cement is created, a steel insert, such as a pipe, is placed to the full depth of the pile by gravity with minor driving. Tests of samples of soil-cement
made with the soil from the construction site lead to the desirable percentage of cement to give the soil-cement the necessary strength and the desirable percentage of water so that the steel insert can fall under its own weight. The concept employed in construction is that sensors in the system will provide data to a computer on the speciﬁc gravity of the grout, the amount of grout being pumped, the measurements of pressures at points in the system, the rate of rotation of the auger, and the force encountered in drilling. A printed record may be made showing relevant data as a function of depth. The record is valuable in evaluating the quality of the construction.

Some data have been collected on load tests with piles constructed by the GeoJet system, but the concept of design is that pile tests are performed at a site with designs of soil-cement based on the tests noted above. Some data have been collected on GeoJet piles that have been tested (Spear et al., 1994; Reavis et al., 1995).

Figure 5.8 Drilling head used in construction of the GeoJet pile.

Augercast Piles.

Augercast piles (also known as augered-cast-in-place piles) are constructed by turning a continuous-ﬂight auger with a hollow stem into the soil. On reaching the desired depth, grout is pumped through the hollow stem as the auger is withdrawn, forming a column of ﬂuid grout. A short rebar cage can be set by gravity or sometimes by the use of vibration. In addition, if desired, a single rebar can be placed in the hollow stem before grout is pumped. These piles are termed continuous-ﬂight-auger (CFA) piles in Europe and have re-
ceived wide acceptance there and elsewhere.

Among the construction problems that have been encountered with the use of augercast piles are the sudden retrieval of the auger, allowing the soil to squeeze in and cause a neck in the pile; the continued rotation of the auger after it reaches the desired depth, with the result that soil is ‘‘mined’’; and upward ﬂow of water around the sides of a pile or through the center opening when installations occur where the water table is high. The mining of soil during drilling by an unqualiﬁed or careless workman can lead to a collapse of the ground surface in the area around the pile. Some problems can be eliminated by the use of instruments on the drilling machine. Such instruments produce a record showing the rate of rotation of the auger as a function of depth and the amount of grout that is pumped as a function of the depth of the auger.

Loss of ground due to mining of subsurface soils is more of a problem when constructing an augercast pile in sand than in clay. Some engineers are reluctant to specify the use of augercast piles in sand unless careful inspection is programmed.

A potential problem with augercast piles is possible eccentricity during the application of loading (Siegel and Mackiewitz, 2003). This problem may arise because of the lack of reinforcement except for the short cage near the top of the pile and a single bar along the axis.

Lacy et al. (1994) have discussed the use of augercast piles to reduce or eliminate the impact on adjacent structures. Driving piles will cause vibration that can affect nearby structures and can also cause the settlement of deposits of loose sand that can potentially cause great damage to structures in an urban
setting. Augercast piles have proved to be very useful in some settings, but construction must be monitored carefully to ensure a sound and competent foundation.

One of the authors has observed the use of CFA piles in the overconsoli- dated clay near London. The drilling rigs are invariably instrumented, and axial-load tests are frequently performed at a site to prove the quality of the construction and to conﬁrm the results of the analyses of axial capacity. The system for testing the pile is often controlled by a computer, and the working load can be conﬁrmed by maintaining the load overnight or longer without the presence of a technician.

Drilled Shafts.

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.

Figure 5.3 Impact hammer atop a driven pile.

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-

Figure 5.4 Construction of a drilled shaft with a truck-mounted unit.

stances (e.g., giving the required slump of concrete for the particular job) but should lean toward the performance desired from the drilled shaft. Prescribing a particular construction method is usually unwise because anomalies in the soil proﬁle could lead to the use of a wet method even though the dry method
appeared feasible at the outset.

Dry Method of Construction The dry method of construction may be employed in soils that will not cave, slump excessively, or deﬂect inward when the hole is drilled to the full depth. A type of soil that meets these requirements is stiff clay. The water table may be located in the stratum of clay, and above the water table the clay may be saturated by capillarity. A problem occurs if the clay below the water table contains fractures that allow water to ﬂow into the excavation. If such fractures were not observed in the soil in- vestigation, the dry method of construction may have been speciﬁed and then may have become impossible to achieve.
The steps in employing the dry method of construction are shown in Figure 5.5. A crane-mounted drilling machine is positioned as shown in Figure 5.5a.

The location has been surveyed and staked to give the contractor precise information on location and on the ﬁnal position of the top of the shaft.

Speciﬁcations will inform the contractor about tolerance in placement and in deviation from the vertical as the shaft is advanced. A temporary surface

casing is frequently placed after the excavation is advanced a few feet. The surface casing prevents raveling of the soil at the surface and provides a positive guide for inserting the auger as drilling proceeds.
Figure. 5.5b shows concrete being placed in the bottom of the excavation, where the computed stresses in the drilled shaft show that no reinforcing steel is needed. Speciﬁcations almost always state that the concrete must be poured without striking the sides of the excavation or any obstruction to prevent segregation during placement, and the drop chute serves to guide the concrete in free fall. Research has shown that concrete may fall great distances without segregation if no obstruction is encountered during falling.

The placement of the rebar cage is shown in Figure 5.5c, and the ﬁnal concrete is placed by use of a tremie or by pumping. Guides are placed on the rebar cage to ensure centering. A service crane is required to hold the tremie or the pump line, as shown in the ﬁgure. Alternatively, the crane with the drilling machine could do the work, but further drilling would be delayed.

Speciﬁcations frequently require that the concrete be placed the same day the excavation is completed to prevent time-dependent movement of the soil around the excavation. The completed shaft is shown in Figure 5.5d. Casing Method of Construction The casing method of construction may be employed where caving soils are encountered, as shown in Figure 5.6a.

Slurry, either from bentonite or polymer, is introduced when the caving soil is encountered and when drilling proceeds through the caving layer and into cohesive soil below. The casing is placed, and the bottom is sealed into the cohesive soil. Prior to placing the casing, the slurry is treated to remove excessive amounts of inclusions and to ensure that speciﬁcations are met for properties of slurry before placing concrete. The contractor twists and pushes the casing to make a seal that prevents the slurry from entering the excavation
below the casing. Figure 5.6b shows a crane-mounted drilling unit inserting a drill through the casing and drilling into the cohesive soil below. The excavation below the casing is smaller than that used in the initial drilling, and the difference in size, not as great as indicated in the ﬁgure, must be taken into account in computing the geotechnical capacity of the shaft. Figure 5.6c shows that an underream has been excavated at the base of the excavation and that the casing is in the process of being retracted. The ﬂuidity of the concrete and the retained slurry are very important. The concrete must be sufﬁciently ﬂuid that the excavation will be completely ﬁlled and the slurry will be ejectedfrom the excavation. The cleaned slurry must be free of inclusions and easily displaced from the excavation by the ﬂuid concrete. Speciﬁcations address the desirable slump of concrete and the characteristics of the slurry.

Figure 5.6c shows the slurry at the ground surface. Preferably the slurry is directed to a sump, where a pump sends the slurry to a tank for cleaning and reprocessing. The disposal of the slurry must meet environmental standards. Slurry from polymers is usually much more easily disposed of than slurry from bentonite. The completed shaft is shown in Figure 5.6d.

Wet Method of Construction The wet method of construction permits the rebar cage to be placed into a drilled hole ﬁlled with ﬂuid. As with the casing method, the drilling ﬂuid is slurry, as shown in Figure 5.7a. The excavation is made to the full depth with slurry, with the contractor exercising care to keep the height of the column of ﬂuid in the excavation above the water table. The slurry acts to create a membrane at the wall of the excavation in the caving soil, usually a granular material, and any ﬂow of ﬂuid will be from the excavation into the natural soil.

The rebar cage may be placed directly into the ﬂuid column, as shown in Figure 5.7b. Prior to placing the cage, samples of the slurry are taken from the excavation, with most samples taken from the bottom, where suspended particles collect. Not shown in the ﬁgure is the system for pumping the slurry from the excavation, directing the pumped ﬂuid to a container, usually a tank, where the slurry can be cleaned with screens and centrifuges if necessary.

Speciﬁcations are available from the owner of the project or from standard speciﬁcations regarding such characteristics as the sand content of the slurry, the pH, and the viscosity. The aim of the speciﬁcations is to ensure that no debris will collect at the bottom of the excavation to interfere with load trans- fer in end bearing and that no slurry remains along the sides of the excavation to interfere with load transfer in skin friction.

The placing of the concrete is shown in Figure 5.7c using a tremie and a concrete bucket. The bottom of the tremie is sealed with a plate that detaches when the tremie is charged fully with concrete. Alternatively, a plug is placed in the tremie to separate the concrete for the slurry and moves downward as the concrete is placed in the tremie. The plug, perhaps made of foam rubber, is compressed and remains in the concrete or ﬂoats upward in the column of ﬂuid concrete. The completed shaft is shown in Figure 5.7d.

Plain water can sometimes be employed as the drilling ﬂuid. One of the authors worked at a site in Puerto Rico where the founding stratum was a soft rock with joints and cracks. The water table was high, and drilling dry was impossible. Water was employed as the drilling ﬂuid, and the level of water in the excavation was kept above the water table to prevent inward ﬂow and possible weakening of the founding stratum.

Driven Piles with Impact Hammer.

The engineer frequently makes an extensive and thorough investigation prior to selecting of the type and conﬁguration of a pile for a particular project.

The pile must sustain the expected loadings with appropriate safety, and con- struction must be accomplished in a timely manner while complying with local regulations. In some cities, for example, noise is a major concern and regulations may preclude the use of many driven piles. The types of piles and the factors affecting the selection of a particular pile are noted in the following discussion.

Several later chapters address methods for computing the axial and lateral capacity of a pile. The piles may be timber, reinforced concrete, prestressed concrete, structural-steel shapes, steel pipe, or a tapered-steel pipe. The engineer selects the pile type and hammer on the basis of (1) loads to be supported, (2) tolerance of the superstructure to differential settlement, (3) expected life of the project, (4) availability of materials and construction machinery, (5) length of time required for installation, (6) difﬁculty of construction, (7) ability to make a proper inspection, (8) noise during construction, and (9) cost.

Environmental effects on the material composing the pile must be considered. Steel in some environments will be subjected to corrosion. In offshore practice, some piles may extend through the splash zone, where corrosion must be considered. Two procedures are common: extra-thick steel may be provided to account for progressive loss of the steel with time or a form of cathodic protection may be provided. Timber piles can be treated with creosote to prevent attack from insects, but this does not prevent damage from certain species of marine borers (Grand, 1970). And, as discussed below, special care must be employed in driving reinforced concrete piles to prevent cracking that could lead later to failure due to corrosion of the rebars.
The engineer may need to consider the drift of driven piles during installation. The tips of the piles in a group may move close to each other at full penetration and may actually touch each other during driving. The bending stiffness of the piles relative to the stiffness of the soil and the position of the head of the pile are factors that affect the drift. The closeness of the piles after installation may or may not affect their ability to sustain loading. If the ﬁnal positions of the piles in a group are important, the engineer may stipulate that preboring be employed.

A large variety of impact hammers are available up to the size required for driving very long piles and down to the smallest that can be driven.

Hammers may be drop-weight, diesel, single-acting steam, or double-acting steam

The structural engineer and the geotechnical engineer may cooperate in studying the relative advantage of a small number of larger piles and a large number of smaller piles. The selection may be dictated by local construction practice.

With the preliminary selection of a pile, an appropriate computer code should be employed in matching the hammer and cushioning to the pile. The cushioning material may be some form of plastic, or wood of various sorts.

Plywood sheets are frequently used as cushioning. The computer code models a compressive wave yielding stress versus time, generated by the impact of the hammer, taking the cushioning into account. The wave travels down the pile and is reﬂected at the pile tip. For driving against bedrock, a compressive wave is reﬂected. For driving against very weak soil at the tip, the wave is reﬂected as a tensile wave. Tension in a reinforced concrete pile can cause cracking that could result in penetration of water, leading to corrosion. Some reinforced concrete piles under bridges have been so damaged due to tensile cracks as to need replacement.

The proper cushion is important. The authors were asked to review a project where the cushioning was provided by sheets of plywood. The plywood was replaced irregularly and actually caught ﬁre on occasion. The result was cracking of some of the reinforced concrete piles due to compressive forces.

One of the authors participated in driving an instrumented, closed-ended steel pipe with a diameter of 6 in. A blow from a drop hammer caused the pile to move down in soft clay but with very little permanent set. Persistent driving was damaging the instrumentation, and the decision was made to use a coiled spring from a rail car as the cushioning. The pile drove readily, with no damage to the instrumentation. The decision to use the soft cushion was made by judgment, but the implementation of a computer code to model the
pile driving would have been useful.

An impact hammer resting on a driven pile is shown in Figure 5.3. A continuous-ﬂight auger is shown that can be used to predrill a small-diameter hole to a given distance to ease the driving into some formations. The photograph shows disturbance to the soil in the vicinity of the pile, which must be considered when computing pile capacity. Vibrations and soil movement due to pile driving can cause severe damage to existing structures and has been investigated in detail (Lacy and Gould, 1985; Lacy and Moskowitz,
1993; Lacy, et al, 1994; Lacy, 1998).

Some piles are driven with vibratory hammers, principally steel sheet piles used for retaining structures. Bearing piles are installed with vibratory techniques in some soils, and vibratory methods are frequently used to pull casings that were installed for the construction of drilled shafts. Vibratory hammers are less noisy than impact hammers and can result in less movement of the soil when driving into some sands.

DEEP FOUNDATIONS.

1 Introduction: Typical types of deep foundations are discussed in the following paragraphs. Entrepreneurs have developed several special and innovative types of deep foundations, and more will continue to be offered by the construction industry...

2 Driven Piles with Impact Hammer: The engineer frequently makes an extensive and thorough investigation prior to selecting of the type and conﬁguration of a pile for a particular project...

3 Drilled Shafts: The design and construction of drilled shafts are discussed in detail in many publications (e.g., Reese and O’Neill, 1988)...

4 Augercast Piles: Augercast piles (also known as augered-cast-in-place piles) are constructed by turning a continuous-ﬂight auger with a hollow stem into the soil...

5 GeoJet Piles: The GeoJet pile is described as an example of a special kind of deep foundation...

6 Micropiles: Micropiles are deep foundations with a small diameter. They may be installed in a variety of ways and have several purposes...

PRINCIPAL TYPES OF FOUNDATIONS.

SHALLOW FOUNDATIONS

A shallow foundation generally is deﬁned as a foundation that bears at a depth less than about two times its width. There is a wide variety of shallow foundations. The most commonly used ones are isolated spread footings, continuous strip footings, and mat foundations.

Many shallow foundations are placed on reinforced concrete pads or mats, with the bottom of the foundation only a few feet below the ground surface.

The engineer will select the relatively inexpensive shallow foundation for support of the applied loads if analyses show that the near-surface soils can sustain the loads with an appropriate factory of safety and with acceptable short-term and long-term movement. A shallow excavation can be made by earth-moving equipment, and many soils allow vertical cuts so that formwork is unnecessary. Construction in progress of a shallow foundation is shown in Figure 5.1. The steel seen in the ﬁgure may be dictated by the building code
controlling construction in the local area.

Shallow foundations of moderate size will be so stiff that bending will not cause much internal deformation, and such foundations are considered rigid in analyses. The distribution of stress for eccentric loading is shown in Figure 5.2a, and bearing-capacity equations can be used to show that the bearing
stress at failure, qult, provides an appropriate factor of safety with respect to qmax. The equations for the computation of bearing values are presented in Chapter 7. Shallow foundations can also be designed to support horizontal loads, as shown in Figure 5.2b. Passive pressure on the resisting face of the footing
and on the surface of a key, along with horizontal resistance along the base

Figure 5.1 Construction of a shallow foundation in progress.

of the footing, can be designed to resist the horizontal load. Active pressure would occur on faces moving away from the soil, but these may be ignored as being too small to make any difference in the solution.
Factors that inﬂuence the selection of a shallow foundation are discussed in Chapter 1. Usually shallow foundations are less expensive than deep foundations, but designs become more complicated as the foundation becomes larger in plan. Signiﬁcant stress for a mat or larger shallow foundation reaches deeper soils, and the computation of deformation becomes more complicated than for the foundation of moderate size. Not only will the vertical movement

be larger than for a footing, but the deformation of the mat must be considered as well as the deformation of the supporting soil.

The principal problem with shallow foundations under light to moderate loading concerns expansive clay, discussed in Chapter 6. The problem is widespread and can be devastating to homeowners. Engineers must be especially diligent in identifying expansive clay at a building site and taking appropriate actions if such soil is present.

SUBSURFACE INVESTIGATIONS FOR OFFSHORE STRUCTURES.

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.

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. 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-

Figure 4.12 Components of the pressuremeter system (from Baguelin et al., 1978).

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.

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.

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.

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. 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).

IN SITU TESTING OF SOIL.

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...

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. 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).

Labels