Pile Foundation Textbook PDF 2024-2025

Chapter One PILE FOUNDATION Tikrit University Engineering College Civil Engineering Department Chapter ONE PILE Foundations Lecturers Prof. Dr. Farouk m Muhauwiss...

Chapter One PILE FOUNDATION Tikrit University Engineering College Civil Engineering Department Chapter ONE PILE Foundations Lecturers Prof. Dr. Farouk m Muhauwiss Dr. Muhammed KH Ahmed 2024 - 2025 1 Chapter One PILE FOUNDATION 1.1. Introduction Piles are structural members made of steel, concrete, or timber. They are deep foundations where the depth (or length) is significantly larger than the width. Deep foundations require special equipment and skill and hence cost more. Therefore, piles are considered only in situations where shallow foundations prove to be inadequate (e.g., large loads or poor soil conditions). Piles are recommended in the following situations: 1. In weak ground conditions When the soil conditions near the surface are poor, shallow foundations will not be able to carry the building loads, and deep foundations are required. If bedrock is present at reasonable depth, it is possible to drive the pile into the bedrock and transfer the entire load to the bedrock (Figure 1.1a). Generally, when the pile is driven into a thick deposit of soil, the load applied on the pile head is transferred to the soil through the pile tip (or point) and the pile shaft, as shown in Figure 1.1b. 2. For carrying lateral loads Tall buildings, earth-retaining structures, transmission towers, and chimneys can be subjected to large lateral loading due to wind loads, earth pressures, or seismic loads. Unlike shallow foundations, the piles can resist lateral loads very effectively (Figure 1.1c). Sometimes these piles are installed at an angle to the vertical to resist the lateral load and are known as batter piles. 3. In expansive or collapsible soil Expansive soil pose a significant threat to low-rise buildings, roads, and other infrastructure in many parts of the world. Shallow foundations placed in expansive soil can undergo repeated swelling and shrinkage due to seasonal variations, which cause considerable damage to the superstructure. 2 Chapter One PILE FOUNDATION Piles can be driven well beyond the depths where the expansive soil are present, so this problem can be avoided (Figure 1.1d). Collapsible soil such as loess become weaker when saturated and undergo large settlements. Here, too, piles can be driven beyond the depths where such problematic soil are present. 4. For resisting uplift In transmission towers, offshore platforms, and situations where the basement or pump house lies below the water table, the foundations must be able to resist uplift. Piles can be very effective for such situations (Figure 1.1e). Sometimes, the bottoms of the piles are enlarged to provide anchorage against uplift. Such piles are known as “belled” or “underreamed” piles. 5. For bridge abutments Bridge abutments and piers are usually constructed over pile foundations to avoid the loss of bearing capacity that a shallow foundation may suffer because of soil erosion at the ground level (Figure 1.1f). 6. As compaction piles Under certain circumstances, piles are driven into granular soil to achieve proper compaction of soil close to the ground surface. These piles are called compaction piles. The lengths of compaction piles depend on the relative density of the soil before and after the compaction and the required depth of compaction. These piles are generally short; however, some field tests are necessary to determine a reasonable length. 3 Chapter One PILE FOUNDATION Fig. 1.1 Some of the conditions that require pile foundations 6.2 Types of Piles and Their Structural Characteristics  Different types of piles are used in practice, depending on the type of load to be carried, soil conditions, location of the water table, and the installation technique that is required.  Piles can be divided into the following categories with the general descriptions for conventional steel, concrete, timber, and composite piles. 4 Chapter One PILE FOUNDATION 1.2.1 Steel Piles  Steel piles generally are either pipe piles or rolled steel H-section piles. Pipe piles can be driven into the ground with their ends open or closed. Wide-flange and I-section steel beams can also be used as piles. However, H-section piles are usually preferred because their web and flange thicknesses are equal. (In wide-flange and I-section beams, the web thicknesses are smaller than the thicknesses of the flange.). Table 1.1 gives the dimensions of some standard H-section steel piles used in the United States. Table 1.2 shows selected pipe sections frequency used for piling purposes.  In many cases, the pipe piles are filled with concrete after they have been driven and they become composite piles..  The allowable structural capacity for steel piles is (1.1)  When necessary, steel piles are spliced by welding or by riveting. Figure 1.2a shows a typical splice by welding for an H-pile. A typical splice by welding for a pipe pile is shown in Figure 1.2b. Figure 1.2c is a diagram of a splice of an H-pile by rivets or bolts.  When hard driving conditions are expected, such as driving through dense gravel, shale, or soft rock, steel piles can be fitted with driving points or shoes. Figures 1.2d and 1.2e are diagrams of two types of shoes used for pipe piles. 5 Chapter One PILE FOUNDATION  Steel piles may be subject to corrosion. For example, swamps, peats, and other organic soil are corrosive. Soil that have a pH greater than 7 are not so corrosive. Table 1.1a Common H-Pile Sections Used in the United States (SI Units). Table 6.2 Selected Pipe Pile Sections (SI Units) 6 Chapter One PILE FOUNDATION Table 1.1b Common H-Pile Sections Used in the United States (English Units) Table 1.2 Selected Pipe Pile Sections (SI Units and English Units) 7 Chapter One PILE FOUNDATION Figure 1.2 Steel piles: (a) splicing of H-pile by welding; (b) splicing of pipe pile by welding; (c) splicing of H-pile by rivets and bolts; (d) flat driving point of pipe pile; (e) conical driving point of pipe pile  To offset the effect of corrosion, an additional thickness of steel (over the actual designed cross-sectional area) is generally recommended. In many circumstances, factory-applied epoxy coatings on piles work satisfactorily against corrosion. These coatings are not easily damaged by pile driving. Concrete encasement of steel piles in most corrosive zones also protects against corrosion. 1.2.2 Concrete Piles 8 Chapter One PILE FOUNDATION  Concrete piles may be divided into two basic categories: (a) precast piles and (b) cast-in-situ piles. Precast piles can be prepared by using ordinary reinforcement, and they can be square or octagonal in cross section (see Figure 1.4).  Reinforcement is provided to enable the pile to resist the bending moment developed during pickup and transportation, the vertical load, and the bending moment caused by a lateral load.  The piles are cast to desired lengths and cured before being transported to the work sites. Figure 1.4 Precast piles with ordinary reinforcement: (a) schematic diagram; (b) photograph of 24 m long octagonal piles ready for driving (Courtesy of N. Sivakugan, James Cook University, Australia) Precast piles can also be prestressed by the use of high-strength steel prestressing cables. The ultimate strength of these cables is about 1800 MN/m2. During casting of the piles, the cables are pretensioned to about 900 to 1300 MN/m2, and concrete is poured around them. After curing, the cables are cut, producing a compressive force on the pile section. Table 1.3 gives additional information about prestressed concrete piles with square and octagonal cross sections. 9 Chapter One PILE FOUNDATION Cast-in-situ, or cast-in-place, piles are built by making a hole in the ground and then filling it with concrete. Various types of cast-in-place concrete piles are currently used in construction, and most of them have been patented by their manufacturers. These piles may be divided into two broad categories: (a) cased and (b) uncased. Both types may have a pedestal at the bottom. Cased piles are made by driving a steel casing into the ground with the help of a mandrel placed inside the casing. When the pile reaches the proper depth the mandrel is withdrawn and the casing is filled with concrete. Figures 1.5a, 1.5b, 1.5c, and 1.5d show some examples of cased piles without a pedestal. Figure 1.5e shows a cased pile with a pedestal. The pedestal is an expanded concrete bulb that is formed by dropping a hammer on fresh concrete. Cased Pile (1.2) Uncased piles (1.3)  Figures 1.5f and 1.5g are two types of uncased pile, one with a pedestal and the other without. The uncased piles are made by first driving the casing to the desired depth and then filling it with fresh concrete. The casing is then gradually withdrawn. 10 Chapter One PILE FOUNDATION Table 1.3 Typical Prestressed Concrete Pile in Use (SI Units) Figure 1.5 Cast-in-place concrete piles 11 Chapter One PILE FOUNDATION 1.2.3 Timber Piles Timber piles are tree trunks that have had their branches and bark carefully trimmed off. The maximum length of most timber piles is 10 to 20 m. To qualify for use as a pile, the timber should be straight, sound, and without any defects. The American Society of Civil Engineers’ Manual of Practice, No. 17 (1959), divided timber piles into three classes: 1. Class A piles carry heavy loads. The minimum diameter of the butt should be 356 mm (14 in.). 2. Class B piles are used to carry medium loads. The minimum butt diameter should be 305 to 330 mm. 3. Class C piles are used in temporary construction work. They can be used permanently for structures when the entire pile is below the water table. The minimum butt diameter should be 305 mm.  In any case, a pile tip should not have a diameter less than 150 mm.  Timber piles cannot withstand hard driving stress; therefore, the pile capacity is generally limited. 1.2.4 Composite Piles  The upper and lower portions of composite piles are made of different materials. For example, composite piles may be made of steel and concrete or timber and concrete.  Steel-and-concrete piles consist of a lower portion of steel and an upper portion of cast-in-place concrete. This type of pile is used when the length of the pile required for adequate bearing exceeds the capacity of simple cast-in-place concrete piles.  Timber-and-concrete piles usually consist of a lower portion of timber pile below the permanent water table and an upper portion of concrete. In any case, forming proper joints between two dissimilar materials is difficult, and for that reason, composite piles are not widely used. 12 Chapter One PILE FOUNDATION Nowadays, fiber reinforced polymer (FRP) composite piles are widely used for waterfront structures. Comparison of Pile Types Table 1.4 gives a common of the advantage and disadvantages of the various types of pile based on the pile material. Table 1.4 Comparisons of Pile made of Different Materials 13 Chapter One PILE FOUNDATION 1.3 Estimating Pile Length Selecting the type of pile to be used and estimating its necessary length are fairly difficult tasks that require good judgment. In addition to being broken down into the classification given in Section 6.2, piles can be divided into three major categories, depending on their lengths and the mechanisms of load transfer to the soil: (a) point bearing piles, (b) friction piles, and (c) compaction piles. 1.4 Point Bearing and Friction Piles Piles generally carry the applied column load through skin friction along the pile shaft and the bearing capacity at the pile point (or tip). When the pile carries the ultimate load Qu, the ultimate shaft resistance and ultimate point resistance are denoted by Qs and Qp, respectively. From equilibrium considerations, Qu = QP + Qs (6.5)  In point bearing piles, it is assumed that the entire load is transferred to the soil as through the point, so Qs  0.  In friction piles, it is assumed that the entire load is transferred through the pile shaft in the form of friction or adhesion, with Qp  0. Point Bearing Piles In situations where the soil near the ground surface is weak and cannot support shallow foundations, pile foundations can be used. Especially when there is bedrock or a stiff stratum (e.g., stiff clay or dense sand) located at 14 Chapter One PILE FOUNDATION relatively shallow depths, it is possible to drive the piles through the weak soil and transfer the load to the underlying stiff stratum, as shown in Figure 1.7a. The pile can socket into the stiff stratum by extending a few meters. In point bearing piles, Qu = Qp (1.6) Friction Piles When there is no stiff stratum within reasonable depth, point bearing piles can become expensive. Here it is necessary to rely on the shaft resistance, which comes from skin friction or adhesion. The point resistance becomes insignificant; hence Qp  0 (Figure 1.7b). Therefore, in friction piles, Qu  Qs (1.7) Figure 1.7 (a) Point bearing piles; (b) friction piles 1.5 Installation of Piles 15 Chapter One PILE FOUNDATION  There are different ways of installing piles, depending on the type of pile and the ground conditions. Steel, timber, and precast concrete piles are generally driven into the ground using an impact hammer or a vibratory hammer.  A traditional impact pile driver allows a heavy hammer to slide up and down between guide rails, hitting the pile head and making the tip penetrate the ground by a few millimeters for every blow. The weight is raised by power from compressed air, steam, hydraulics, diesel, or simply by manual labor, as in ancient civilizations (Figures 6.9a and 6.9b).  The vibratory hammer was developed in the Soviet Union during World War II. Vibratory pile drivers produce less noise and are preferred near residential or office buildings. They are effective in medium-dense granular soil and with steel piles. Figure 1.9 shows the schematic diagrams of two impact hammers and a vibratory hammer.  In Figures 1.9a and 1.9b, the pile is driven into the ground by raising and dropping a heavy weight, known as the ram. In the vibratory hammer, two counterrotating weights produce a vertical sinusoidal load that drives the pile into the ground (Figure 1.9c). The horizontal components of the centrifugal forces induced by the two rotating weights cancel each other out.  Cast-in-place (or bored) concrete piles are installed by placing a reinforcement cage into a cylindrical hole in the ground and pouring fresh concrete.  Installation of piles generally causes lateral displacement of the surrounding soil. The extent of displacement depends on the method of installation and other factors. Driven piles generally cause high 16 Chapter One PILE FOUNDATION displacements: the larger the diameter, the larger the displacements. Cast-in-place piles (or bored piles) literally cause no displacement and are known as nondisplacement piles.  H-piles and open-ended pipe piles cause little displacement and are known as low-displacement piles. Precast Figure 1.9 Pile-driving hammers: (a) drop hammer; (b) pneumatic hammer; (c) Vibratory hammer 1.6 Load Transfer Mechanism The load transfer mechanism from a pile to the soil is complicated. To understand it, consider a pile of length L, as shown in Figure 1.10a. The load 17 Chapter One PILE FOUNDATION on the pile is gradually increased from zero to Q(z=0) at the ground surface. Part of this load will be resisted by the side friction developed along the shaft, Q1 , and part by the soil below the tip of the pile, Q2. Now, how are Q1 and Q2 related to the total load? If measurements are made to obtain the load carried by the pile shaft, Q(z) , at any depth z, the nature of the variation found will be like that shown in curve 1 of Figure 1.10b. The frictional resistance per unit area at any depth z may be determined as (1.8) Where p = perimeter of the cross section of the pile. Figure 1.10c shows the variation of f(z) with depth. If the load Q at the ground surface is gradually increased, maximum frictional resistance along the pile shaft will be fully mobilized when the relative displacement between the soil and the pile is about 5 to 10 mm, irrespective of the pile size and length L. However, the maximum point resistance Q2 = Qp will not be mobilized until the tip of the pile has moved about 10 to 25% of the pile width (or diameter). (The lower limit applies to driven piles and the upper limit to bored piles). At ultimate load (Figure 1.10d and curve 2 in Figure 1.10b), Q(z=0) = Qu. Thus, Q1 = Qs and Q2 = Qp The preceding explanation indicates that Qs (or the unit skin friction, f, along the pile shaft) is developed at a much smaller pile displacement compared with the point resistance, Qp. 18 Chapter One PILE FOUNDATION At ultimate load, the failure surface in the soil at the pile tip (a bearing capacity failure caused by Qp) is like that shown in Figure 6.9e. Note that pile foundations are deep foundations and that the soil fails mostly in a punching mode. That is, a triangular zone, I, is developed at the pile tip, which is pushed downward without producing any other visible slip surface. In dense sands and stiff clayey soils, a radial shear zone, II, may partially develop. Figure 1.10 Load transfer mechanism for piles 1.7 Equations for Estimating Pile Capacity The ultimate load-carrying capacity Qu of a pile is given by the equation (1.9) 19 Chapter One PILE FOUNDATION Qp = load-carrying capacity of the pile point Qs = frictional resistance (skin friction) derived from the soil–pile interface (see Figure 1.11) Numerous published studies cover the determination of the values of Qp and Qs. Excellent reviews of many of these investigations have been provided by Vesic (1977), Meyerhof (1976), and Coyle and Castello (1981). These studies afford an insight into the problem of determining the ultimate pile capacity. Point Bearing Capacity, Qp The ultimate bearing capacity of shallow foundations was discussed in Chapter 3. According to Terzaghi’s equations, (1.10) 20 Chapter One PILE FOUNDATION Figure 1.11 Ultimate load-carrying capacity of pile Pile foundations are deep. However, the ultimate resistance per unit area developed at the pile tip, qp , may be expressed by an equation similar in form to Eq. (1.10), although the values of N*c , N*q , and N* will change. The notation used in this chapter for the width of a pile is D. Hence, substituting D for B in Eq. (1.10) gives (1.11) (1.12)  Note that the term q has been replaced by q' in Eq. (1.12), to signify effective vertical stress. Thus, the point bearing of piles is 21 Chapter One PILE FOUNDATION (1.13) Frictional Resistance, Qs The frictional, or skin, resistance of a pile may be written as (1.14) The various methods for estimating Qp and Qs are discussed in the next several sections. It needs to be reemphasized that, in the field, for full mobilization of the point resistance (Qp), the pile tip must go through a displacement of 10 to 25% of the pile width (or diameter). Allowable Load, Qall After the total ultimate load-carrying capacity of a pile has been determined by summing the point bearing capacity and the frictional (or skin) resistance, a reasonable factor of safety should be used to obtain the total allowable load for each pile, or 22 Chapter One PILE FOUNDATION  The factor of safety generally used ranges from 2 to 3, depending on the uncertainties surrounding the calculation of ultimate load. 1.8 Meyerhof’s Method for Estimating Qp Sand The point bearing capacity, qp , of a pile in sand generally increases with the depth of embedment in the bearing stratum and reaches a maximum value at an embedment ratio of Lb/D = (Lb/D)cr. Note that in a homogeneous soil Lb is equal to the actual embedment length of the pile, L. However, where a pile has penetrated into a bearing stratum, Lb  L. Beyond the critical embedment ratio, (Lb/D)cr , the value of qp remains constant (qp = ql). That is, as shown in Figure 1.12 for the case of a homogeneous soil, L = Lb. For piles in sand, c' = 0, and Eq. (1.13) simplifies to (1.15) The variation of N*q with soil friction angle  ' is shown in Figure 1.13. The interpolated values of N*q for various friction angles are also given in Table 1.6. However, Qp should not exceed the limiting value Apql ; that is, (1.16) The limiting point resistance is 23 Chapter One PILE FOUNDATION (1.17) Figure 1.12 Nature of variation of unit point resistance in a homogeneous sand Table 1.6 Interpolated Values of N*q Based on Meyerhof’s Theory Figure 1.13 Variation of the maximum values of N*q with soil friction angle . 24 Chapter One PILE FOUNDATION Clay ( = 0) (1.18) Example 1.1 Example 1.2 25 Chapter One PILE FOUNDATION 1.10 Correlations for Calculating Qp with SPT Results  On the basis of field observations, Meyerhof (1976) also suggested that the ultimate point resistance qp in a homogeneous granular soil (L = Lb) may be obtained from standard penetration numbers as (1.20) (1.21) (1.22) 26 Chapter One PILE FOUNDATION Example 1.3 27 Chapter One PILE FOUNDATION 1.11 Frictional Resistance (Qs) in Sand According to Eq. (1.14), the frictional resistance Qs =  p L f The unit frictional resistance, f, is hard to estimate. In making an estimation of f, several important factors must be kept in mind: 1. The nature of the pile installation. For driven piles in sand, the vibration caused during pile driving helps densify the soil around the pile. The zone of sand densification may be as much as 2.5 times the pile diameter, in the sand surrounding the pile. 2. It has been observed that the nature of variation of f in the field is approximately as shown in Figure 1.16. The unit skin friction increases with depth more or less linearly to a depth of L' and remains constant thereafter. The magnitude of the critical depth L' may be 15 to 20 pile diameters. A conservative estimate would be L' < 15D (1.23) 3. At similar depths, the unit skin friction in loose sand is higher for a high displacement pile, compared with a low-displacement pile. 4. At similar depths, bored, or jetted, piles will have a lower unit skin friction compared with driven piles. Taking into account the preceding factors, we can give the following approximate relationship for f (see Figure 1.16): (1.24) 28 Chapter One PILE FOUNDATION Fig.(6.16) (1.25) In reality, the magnitude of K varies with depth; it is approximately equal to the Rankine passive earth pressure coefficient, Kp , at the top of the pile and may be less than the at-rest pressure coefficient, Ko , at a greater depth. Based on presently available results, the following average values of K are recommended for use in Eq. (1.25): 29 Chapter One PILE FOUNDATION Correlation with Standard Penetration Test Results  Meyerhof (1976) indicated that the average unit frictional resistance, fav , for high-displacement driven piles may be obtained from average standard penetration resistance values as (1.28) (2.29) (1.30) (1.31) Example 6.4 Refer to the pile described in Example 6.3. Estimate the magnitude of Qs for the pile. a. Use Eq. (1.28). b. Use Eq. (1.30). c. Considering the results in Example 6.3, determine the allowable load-carrying capacity of the pile based on Meyerhof’s method and Briaud’s method. Use a factor of safety, FS = 3. 30 Chapter One PILE FOUNDATION Example 6.5 Refer to Example 1.1. For the pile, estimate the frictional resistance Qs. a. Based on Eqs. (1.24) and (1.25). Use K = 1.3 and =0.8. 31 Chapter One PILE FOUNDATION 1.12 Frictional (Skin) Resistance in Clay  Estimating the frictional (or skin) resistance of piles in clay is almost as difficult a task as estimating that in sand, due to the presence of several variables that cannot easily be quantified. Several methods for obtaining the unit frictional resistance of piles are described in the literature. We examine some of them next. 1.  Method This method, proposed by Vijayvergiya and Focht (1972), is based on the assumption that the displacement of soil caused by pile driving results in a passive lateral pressure at any depth and that the average unit skin resistance is 32 Chapter One PILE FOUNDATION  The value of  changes with the depth of penetration of the pile. (See Table 6.9.) Thus, the total frictional resistance may be calculated as 33 Chapter One PILE FOUNDATION 2.  Method According to the  method, the unit skin resistance in clayey soils can be represented by the equation where  = empirical adhesion factor. The approximate variation of the value of a is shown in Table 6.10. The ultimate side resistance can thus be given as 34 Chapter One PILE FOUNDATION Example 1.7 35 Chapter One PILE FOUNDATION 1.13 Elastic Settlement of Piles The total settlement of a pile under a vertical working load Qw is given by: The magnitude of  varies between 0.5 and 0.67 and will depend on the nature of the distribution of the unit friction (skin) resistance f along the pile shaft. 36 Chapter One PILE FOUNDATION The settlement of a pile caused by the load carried at the pile point may be expressed in the form: The settlement of a pile caused by the load carried by the pile shaft is given by a relation similar to Eq. (9.82), namely, 37 Chapter One PILE FOUNDATION 38 Chapter One PILE FOUNDATION 1.14 Pile Load Tests  High-rise buildings often require several piles to support the building loads. Varying soil conditions, unreliable soil parameters, and the assumptions and simplifications in the theoretical model used in the prediction contribute to the variability in the ultimate load Qu. The pile load test is a good way to verify the load-carrying capacity of a pile.  Figure 1.21 shows schematic diagrams of two different pile load test arrangements for testing axial compression in the field. The main difference between the two is the way the horizontal reaction beam is held in place. In Figure 1.21a, a kent-ledge, consisting of heavy weights, is required to hold the reaction beam in place , and the hydraulic jack is used to jack against the beam and hence apply the pile load.  In Figure 1.21b, two reaction piles, located far away from the test pile, anchor the horizontal reaction beam to the ground.  The loads are applied in increments as specified by the relevant standards ,with sufficient time between the load increments. Figure 1.21 Pile load test: (a) using kentledge, (b) using reaction pile, (c) load vs. total settlement plots, and (d) load vs. net settlement 39 Chapter One PILE FOUNDATION  Generally, the piles are loaded well beyond their working loads (e.g., 2 times). On reaching the maximum load for the test, the pile is unloaded in steps. The dial gauges measure the settlement of the pile head where they are mounted.  The load test procedure just described requires the application of step loads on the piles and the measurement of settlement and is called a load-controlled test.  Another technique used for a pile load test is the constant- rate-of- penetration test ,wherein the load on the pile is continuously increased to maintain a constant rate of penetration, which can vary from 0.25 to 2.5 mm/min. This test gives a loadsettlement plot similar to that obtained from the load-controlled test. Another type of pile load test is cyclic loading, in which an incremental load is repeatedly applied and removed. 6.14 Negative Skin Friction  Negative skin friction is a downward drag force exerted on a pile by the soil surrounding it. Such a force can exist under the following conditions, among others: 1. If a fill of clay soil is placed over a granular soil layer into which a pile is driven, the fill will gradually consolidate. The consolidation process will exert a downward drag force on the pile (see Figure 1.24a) during the period of consolidation. 2. If a fill of granular soil is placed over a layer of soft clay, as shown in Figure 2.24b, it will induce the process of consolidation in the clay layer and thus exert a downward drag on the pile. 40 Chapter One PILE FOUNDATION 3. Lowering of the water table will increase the vertical effective stress on the soil at any depth, which will induce consolidation settlement in clay. If a pile is located in the clay layer, it will be subjected to a downward drag force.  In some cases, the downward drag force may be excessive and cause foundation failure. This section outlines two tentative methods for the calculation of negative skin friction. Figure 1.24 Negative skin friction 1.15 Tension Piles  Tension piles may be used beneath buildings to resist uplift from hydrostatic pressure. They also may be used to support structures over expansive soils. Overturning caused by wind, ice loads, and broken wires may produce large tension forces for power transmission towers. In this type of situation the piles or piers supporting the tower legs must be designed for both compressive and tension forces.  41 Chapter One PILE FOUNDATION Group Piles 1.16 Group Efficiency In most cases, piles are used in groups, as shown in Figure 1.44, to transmit the structural load to the soil. A pile cap is constructed over group piles. The cap can be in contact with the ground, as in most cases (see Figure 1.44a), or well above the ground, as in the case of offshore platforms (see Figure 1.44b). Determining the load-bearing capacity of group piles is extremely complicated and has not yet been fully resolved. When the piles are placed close to each other, a reasonable assumption is that the stresses transmitted by the piles to the soil will overlap (see Figure 1.44c), reducing the load- bearing capacity of the piles. Ideally, the piles in a group should be spaced so that the load-bearing capacity of the group is not less than the sum of the bearing capacity of the individual piles. In practice, the minimum center-to- center pile spacing, d, is 2.5D and, in ordinary situations, is actually about 3 to 3.5D. The efficiency of the load-bearing capacity of a group pile may be defined as 42 Chapter One PILE FOUNDATION 43 Chapter One PILE FOUNDATION Piles in Sand Many structural engineers use a simplified analysis to obtain the group efficiency for friction piles, particularly in sand. This type of analysis can be explained with the aid of Figure 1.44a. Depending on their spacing within the group, the piles may act in one of two ways: (2) as individual piles. If the piles act as a block, the frictional capacity is pg = perimeter of the cross section of block= Similarly, for each pile acting individually, Qu < pLfav. (Note: p = perimeter of the cross section of each pile.) Thus,  There are several other equations like Eq. (9.129) for calculating the group efficiency of friction piles. 44 Chapter One PILE FOUNDATION Ultimate Capacity of Group Piles in Saturated Clay Figure 6.47 shows a group pile in saturated clay. Using the figure, one can estimate the ultimate load-bearing capacity of group piles in the following manner: 45 Chapter One PILE FOUNDATION 46 Chapter One PILE FOUNDATION Example 1.14 47 Chapter One PILE FOUNDATION 1.17 Elastic Settlement of Group Piles 48 Chapter One PILE FOUNDATION Example 6.17 49 Chapter One PILE FOUNDATION 1.18 Pile groups Subjected to moment 50