PILE FOUNDATION

INTRODUCTION

Deep foundations are employed when the soil strata immediately beneath the structure are not capable of supporting

the load with tolerable settlement or adequate safety against shear failure. Merely extending the level of support to the first hard stratum is not sufficient, although this is a common decision that is reached. Instead, the deep foundation

must be engineered in the same way as the shallow foundation so that the soil strata below remain safe and free of

deleterious settlement.

Two general forms of deep foundation are recognised:

1. Pile foundation

2. Pier, caisson or well foundation.

Piles are relatively long, slender members that are driven into the ground or cast-insitu. Piers, caissons or wells are

larger, constructed by excavation and are sunk to the required depth; these usually permit visual examination of the

soil or rock on which they rest. In effect they are deep spread footings or mats. They are normally used to carry very

heavy loads such as those from bridge piers or multi-storeyed buildings. A sharp distinction between piles and piers is impossible because some foundations combine features of both.

Piles have been used since prehistoric times. The Neolithic inhabitants of Switzerland, 12,000 years ago, drove

wooden poles in the soft bottoms of shallow lakes and on them erected their homes, high above marauding animals

and warring neighbours. Pile foundations were used by Romans; Vitruvius (59 A.D.) records the use of such

foundations. Today, pile foundations are much more common than any other type of deep foundation, where the soil

conditions are unfavourable.

CLASSIFICATION OF PILES

Piles may be classified in a number of ways based on different criteria: (a) Function or action

(b) Composition and material

(c) Installation

Classification Based on Function or Action Piles may be classified as follows based on the function or action:

End-bearing piles

Used to transfer load through the pile tip to a suitable bearing stratum, passing soft soil or water.

Friction piles

Used to transfer loads to a depth in a frictional material by means of skin friction along the surface area of the

pile.

Tension or uplift piles

Used to anchor structures subjected to uplift due to hydrostatic pressure or to overturning moment due to

horizontal forces.

Compaction piles

Used to compact loose granular soils in order to increase the bearing capacity. Since they are not required to

carry any load, the material may not be required to be strong; in fact, sand may be used to form the pile. The

pile tube, driven to compact the soil, is gradually taken out and sand is filled in its place thus forming a ‘sand

pile’.

Anchor piles

Used to provide anchorage against horizontal pull from sheetpiling or water.

Fender piles

Used to protect water-front structures against impact from ships or other floating objects.

Sheet piles

Commonly used as bulkheads, or cut-offs to reduce seepage and uplift in hydraulic structures.

Batter piles

Used to resist horizontal and inclined forces, especially in water front structures.

Laterally-loaded piles

Used to support retaining walls, bridges, dams, and wharves and as fenders for harbour construction.

Classification Based on Material and Composition Piles may be classified as follows based on material and composition:

Timber piles

These are made of timber of sound quality. Length may be up to about 8 m; splicing is adopted

for greater lengths. Diameter may be from 30 to 40 cm. Timber piles perform well either in

fully dry condition or submerged condition. Alternate wet and dry conditions reduce the life of

a timber pile; to overcome this, creosoting is adopted. Maximum design load is about 250 kN.

Steel piles

These are usually H-piles (rolled H-shape), pipe piles, or sheet piles (rolled sections of regular

shapes). They may carry loads up to 1000 kN or more.

Concrete piles

These may be ‘precast’ or ‘cast-in-situ’. Precast piles are reinforced to withstand handling

stresses. They require space for casting and storage, more time to cure and heavy equipment

for handling and driving.

Cast-in-situ piles are installed by pre-excavation, thus eliminating vibration due to driving

and handling. The common types are Raymond pile, Mac Arthur pile and Franki pile.

Composite piles

These may be made of either concrete and timber or concrete and steel. These are considered

suitable when the upper part of the pile is to project above the water table. Lower portion may

be of untreated timber and the upper portion of concrete. Otherwise, the lower portion may be

of steel and the upper one of concrete.

Classification Based on Method of Installation Piles may also be classified as follows based on the method of installation:

Driven piles Timber, steel, or precast concrete piles may be driven into position either vertically or at an inclination. If

inclined they are termed ‘batter’ or ‘raking’ piles. Pile hammers and pile-driving equipment are used for

driving piles.

Cast-in-situ piles

Only concrete piles can be cast-in-situ. Holes are drilled and these are filled with concrete. These may be

straight-bored piles or may be ‘under-reamed’ with one or more bulbs at intervals. Reinforcements may be

used according to the requirements.

Driven and cast-in-situ piles

This is a combination of both types. Casing or shell may be used. The Franki pile falls in thiscategory.

USE OF PILES

The important ways in which piles are used are as follows:

(i) To carry vertical compressive loads,

(ii) To resist uplift or tensile forces, and

(iii) To resist horizontal or inclined loads.

Bearing piles are used to support vertical loads from the foundations of buildings and bridges. The load is carried

either by transferring to the incompressible soil or rock below through soft strata, or by spreading the load through soft

strata that are incapable of supporting concentrated loads from shallow foootings. The former type is called point-

bearing piles, while the latter are known as friction-piles.

Tension piles are used to resist upward forces in structures subjected to uplift, such asbuildings with basements below

the ground water level, aprons of dams or buried tanks. They are also used to resist overturning of walls and dams and

for anchors of towers, guywires and bulkheads.

Laterally loaded piles support horizontal or inclined forces such as the foundations of retaining walls, bridges, dams,

and wharves and as fenders in harbour construction. In case the lateral loads are of large magnitude they may be more

effectively resisted by batter piles, driven at an inclination. Closely spaced piles or thin sheet piles are used as

cofferdams,seepage cut-offs and retaining walls. Piles may be used to compact loose granular soils and also to

safeguard foundations against scouring. These are illustrated in Fig. 16.1.

PILE DRIVING The operation of forcing a pile into the ground is known as ‘pile driving’. The oldest method and the most widely used

even today is by means of a hammer. The equipment used to lift the hammer and allow it to fall on to the head of the

pile is known as the ‘pile driver’. The Romans used a stone block hoisted by an A-frame derrick with slave or horse

power. While such a simple pile-driving rig is still in use today with mechanical power, the more common equipment

consists of essentially a crawler-mounted crane, shown schematically in Fig. 16.2. Attached to the boom are the

‘leads’, which are just two parallel steel channels fastened together by U-shaped spacers and stiffened by trussing. The

leads are braced against the crane with a stay, which is usually adjustable to permit driving of batter piles. A steam

generator or air compressor is required for steam hammers. The most important feature of the driving rig, from an engineering point of view, is its ability to guide the pile

accurately. It must be rugged and rigid enough to keep the pile and hammer in alignment and plumb inspite of wind,

underground obstructions and the movement of the pile hammer.

PILE CAPACITY The ultimate bearing capacity of a pile is the maximum load which it can carry without failure or excessive settlement of the ground. The allowable load on a pile is the load which can be imposed upon it with an adequate margin of

safety; it may be the ultimate load divided by a suitable factor of safety, or the load at which the settlement reaches the

allowable value. The bearing capacity of a pile depends primarily on the type of soil through which and/or on which it

rests, and on the method of installation. It also depends upon the cross-section and length of the pile. The pile shaft is a

structural column that is fixed at the point and usually restrained at the top. The elastic stability of piles, or their

resistance against buckling, has been investigated both theoretically and by load tests (Bjerrum, 1957).

Both theory and experience demonstrate that buckling rarely occurs because of the effective lateral support of the soil;

it may occur only in extremely slender piles in very soft clays or in piles that extend through open air or water.

Therefore, the ordinary pile in sand or clay may be designed as though it were a short column. The pile transfers the

load into the soil in two ways. Firstly, through the tip-in compression, termed ‘end-bearing’ or ‘point-bearing’; and,

secondly, by shear along the surface, termed ‘skin friction’. If the strata through which the pile is driven are weak, the tip resting on a hard stratum transfers most part of the load by end-bearing; the pile is then said to be an endbearing

pile.

Piles in homogeneous soils transfer the greater part of their load by skin friction, and are then called friction piles;

however, nearly all piles develop both end-bearing and skin friction.

The following is the classification of the methods of determining pile capacity:

(i) Static analysis (ii) Dynamic analysis

(iii) Load tests on pile (iv) Penetration tests The first two are theoretical approaches and the last two are field or practical approaches.

Static Analysis The ultimate bearing load of a pile is considered to be the sum of the end-bearing resistance and the resistance due to

skin friction:

Qup= Qeb + Qsf ...(Eq. 16.1)

where Qup = ultimate bearing load of the pile,

Qeb = end-bearing resistance of the pile, and Qsf = skin-friction resistance of the pile.

However, at low values of load Qeb will be zero, and the whole load will be carried by skin friction of soil around the

pile. Qeb and Qsf may be analysed separately; both are based upon the state of stress around the pile and on the shear

patterns that develop at failure. Meyerhof (1959) and Vesic (1967) proposed certain failure surfaces for deep

foundations.

According to Vesic, only punching shear failure occurs in deep foundations irrespective of the density index of the

soil, so long as the depth to width ratio is greater than 4 (This is invariably so for pile foundations).

Qeb = qb . Ab ...(Eq. 16.2)

Qsf = fs As ...(Eq. 16.3)

Here, qb = bearing capacity in point-bearing for the pile,

fs = unit skin friction for the pile-soil system,

Ab = bearing area of the base of the pile, and

As = surface area of the pile in contact with the soil.

The general form of the equation for qb presented by various investigators is:

qb = cNc + 1/2γbNɣ+ q . Nq ...(Eq. 16.4)

which is the same form as the bearing capacity of shallow foundations.

For piles in sands:

qb =1/2 γbNɣ + q . Nq ...(Eq. 16.5) (for square or rectangular piles)

qb = 0.3 γDbN ɣ+ q . Nq ...(Eq. 16.6) (for circular piles with diameter D)

With driven piles the term involving the size of the pile is invariably negligible compared with the surcharge term q .

Nq. Thus, for all practical purposes,

qb = q . Nq ...(Eq. 16.7)

The surcharge pressure q is given by

q = γ.z if Z < Zc, and

q = γ.Zc if Z > Zc ...(Eq. 16.8)

Z being the embedded length of pile and Zc the critical depth.

This indicates that the vertical stress at the tip of a long pile tends to reach a constant value and the depth beyond

which the stress does not increase linearly with depth is called the critical depth. This is due to the mechanics of

transfer of load from a driven pile to the surrounding soil. Large-scale tests by Vesic (1967) in the U.S.A. and Kerisel

(1967) in France indicate that the critical depth Zc is a function of density index. For ID < 30%, Zc = 10D ; for ID >

70%, Zc = 30 D ; and, for intermediate values, it is nearly proportional to density index (D isthe dimension of the pile

cross-section).

The bearing capacity factor Nq is related to the angle of internal friction of the sand in the vicinity of the pile tip

(several pile diameters above and below the pile tip), and the ratio of the pile depth to pile width. Values of Nq

presented by different investigators show a wide range of variation because of the assumptions made in defining the

shear zones near the pile tip; for example, while Meyerhof assumes the shear zones to extend back to the pile shaft,

Vesic assumes punching shear in which the shear zones do not extend to the pile shaft. Values of Nq attributed to

Berezantzev et al. (1961), which take into account the effect of z/b ratio, are believed to be the most applicable for the

most commonly encountered field conditions.

The angle of internal friction for the soil in the vicinity of the pile tip is determined from the standard penetration test.

If Dutch cone resistance data are available, these values are correlated directly to the end-bearing resistance of the pile,

qb. Values of Nq given by different investigators are shown in Fig. 16.3. These values of Nq are based on the assumption that the soil above the pile tip is comparable to the soil below the pile tip. If the pile penetrates the compact

layer only slightly, and loose material exists above the compact soil, an Nq value for a shallow foundation will be more

appropriate than a value from Fig. 16.3.

If Eq. 16.5 or 16.6 is to be used, the value of Nɣfor a deep foundation can be conservatively taken as twice the

Nɣvalue used for shallow foundation; otherwise, it may be taken from Fig. 16.3, the values given by Berezantzev et

al. According to Nordlund and Tomlinson (1969), Berezantzev’s values of Nq increase rapidly for high values of ϕ.

Further, the decrease in Nq with increase in z/b also will be significant for high values of ϕ.

Vesic’s equation for qb is:

qb = 3q . Nq ...(Eq. 16.9)

where Nq = e3.8ϕtan ϕ.

Nϕ ...(Eq. 16.10) with the usual notation for Nϕ[= tan2

(45° + ϕ/2)].

Vesic’s values of Nq are given in Table 16.1:

For piles in clays, qb is given by:

qb = cNc + q ...(Eq. 16.11)

since Nq = 1 and Nγ= 0 for ϕ= 0° Nc ranges from 6 to 10 depending upon the stiffness of the clay: a value of 9 is taken for Nc conventionally.

It is also considered that q is not significant compared to cNc. Hence, for all practical purposes

qb = 9c ...(Eq. 16.12) (for piles in clay)

The general form for the unit skin friction resistance, fs, is given by

fs = cα + σh tan δ ...(Eq. 16.13)

where cα = adhesion, which is independent of the normal pressure on the contact area. Cohesion c is used if the

shearing is between soil and soil;

σh = average lateral pressure of soil against the pile surface; and δ= angle of wall friction, which depends upon the material of the pile σh is given by

σh = Ks . q ...(Eq. 16.14)

where Ks = coefficient of earth pressure.

For loose sand (ID < 30%), Ks = 1 to 3, and

for dense sand (ID > 70%), Ks = 2 to 5

For piles in sands:

fs = σh tan δ, cα being zero.

The values of tan δmay be determined by direct shear tests in which one half of the shear box is replaced by the same material as the pile surface. Representative values of the coefficient of friction between sand and various pile materials

are shown in Table 16.2.

For piles in clays:

fs = cα, since tan δis zero. The adhesion cα may be expressed as

cα = α. c ...(Eq. 16.15)

where α is called the ‘adhesion factor’, which varies with the consistency of the clay.

When a pile is driven in soft clay, the soil around gets remoulded and loses some of its strength. However, it regains

almost its full strength within a few weeks of driving, through consolidation. Since piles will be usually loaded a few

months after driving, this reduction in strength soon after driving does not pose any problem. However, if piles are to

be loaded soon after driving, the remoulded shear strength is to be considered.

When piles are driven into stiff clays, the soil close to the pile may get remoulded and this may also create a slight gap

between the pile and the soil; consequently the adhesion is always smaller than cohesion and α will be less than unity.

The adhesion factors for different pile materials and consistency of the clay are shown in Table 16.4.

Load Test on Pile

Load test on a pile is one of the best methods of determining the load-carrying capacity of a pile. It may be conducted

on a driven pile or cast-in-situ pile, on a working pile or a test pile, and on a single pile or a group of piles. A working

pile is one which forms part of the foundation, while a test pile is one which is used primarily to check estimated

capacities (as predetermined by other methods).

The aim of a pile load test is invariably to determine the vertical load capacity; however, in certain special cases the

test may be used to obtain the uplift capacity or lateral load capacity. Load test on a pile group is expensive and may

be conducted only in the case of important projects.

Both cohesive and cohesionless soils will have their properties altered by pile driving. In clays, the disturbance causes

remoulding and consequent loss of strength. With passage of time, much of the original strength will be regained. The

effect of pile driving in sand is to create a temporary condition wherein extra resistance is developed, which is lost

later by stress relaxation. Hence, the test should be conducted only after a lapse of a few weeks in clays

and at least a few days in sands, in order that the results obtained be more meaningful for design.

Load may be applied by using a hydraulic jack against a supported platform (Fig. 16.6a), or against a reaction girder

secured to anchor piles (Fig. 16.6b). Sometimes a proving ring is preferred for better accuracy in obtaining the load. Instead of reaction loading, gravity loading may also be used; but the former is given better uniformity in loading.

Measurement for pile settlement is related to a fixed reference mark. The support for the reference mark has to be

located outside the zone that could be affected by pile movements.

The most common procedure is the test in which the load is maintained slowly. About five to eight equal increments

are used until the load reaches about double the design value. Time-settlement data are recorded for each load

increment. Each increment is maintained until the rate of settlement becomes a value less than 0.25 mm per hour. The

final load is maintained for 24 hours.

Another procedure is the constant-strain rate method. In this method, the load is increased such that the settlement

occurs at a predetermined rate such as 0.5 mm per minute. This test is considerably faster than the other approach.

Other procedures include cyclic loading, where each load increment is repeatedly applied and removed. Settlements

are recorded at every increment or decrement of load. These help in separating elastic and plastic settlements, and also

point-bearing and frictional resistances. The load-settlement curve is obtained from the data. Often the definition of ‘failure load’ is arbitrary. It may be taken when a predetermined amount of settlement has occurred or where the load-

settlement plot is no longer a straight line. If the ultimate load could be found, a suitable factor of safety—2 to 3—may

be used to determine the allowable load.

The ultimate load may be determined as the abscissa of the point where the curved part of the load-settlement curve

changes to a steep straight line (Fig. 16.7a). Alternatively, the ultimate load is the abscissa of the point of intersection

of initial and final tangents of the load settlement curve (Fig. 16.7b).

Pile Capacity from Penetration Tests

Results of static cone penetration test and standard penetration test are also used to determine pile load capacity. In the

case of a static cone penetration test, a 60° cone with a base area of 100 mm2 attached to one end of a rod housed in a

pipe and the pipe itself are pushed down alternately at a slow constant rate and the resistance encountered by each is

recorded by means of pressure gauges. The pressure offered by the cone is recorded as penetration resistance

qc and that offered by the pipe as skin friction resistance fc. The values of qc may also be obtained indirectly from the

Standard Penetration Number N, through correlations between N-value and the static cone penetration resistance qc-

value.

Negative Skin Friction

‘Negative skin friction’ or ‘down drag’ is a phenomenon which occurs when a soil layer surrounding a portion of the

pile shaft settles more than the pile. This condition can develop where a soft or loose soil stratum located anywhere

above the pile tip is subjected to new compressive loading. If a soft or loose layer settles after the pile has been

installed, the skin-friction-adhesion developing in this zone is in the direction of the soil movement, pulling the pile

downward, as shown in Fig. 16.11. Extra loading is thus imposed on the pile.

Negative skin friction may also occur by the lowering of ground water which increases the effective stress inducing

consolidation and consequent settlement of the soil surrounding the pile.

It is necessary to subtract negative skin friction force from the total load that the pile can support. In such a case the

factor of safety will be modified as follows:

Factor of safety = Ultimate pile load capacity / [Working load Negative skin friction force]

Sometimes this may also be written as

Factor of safety = [Ultimate pile load capacity - Negative skin friction force] / Working load

Values of negative skin force are computed in just the same way as positive skin friction.

For cohesive soils:

Qnf = P . Dn . c ...(Eq. 16.39)

where Qnf = negative skin friction force on the pile,

P = perimeter of the pile section,

Dn = depth of compressible layer settling in relation to the pile

and c = unit cohesion of soil layer which is setting.

For cohesionless soils:

Qnf = ½ PDn2 . γK tan δ ...(Eq. 16.40)

where γ= unit weight of soil in the compressible zone, K = earth pressure coefficient (Ka < K < Kp), and

δ= angle of wall friction (ϕ/2 < δ< ϕ)

Sometimes negative skin friction may develop even in the zone of the fill, if the fill itself is settling under its self-

weight. When a large magnitude of negative skin friction force is anticipated, a protective sleeve or coating may be

provided for the section that is embedded in the settling soil. Skin friction is thus eliminated for this section of the pile

and a down drag is prevented. Negative skin force may be computed even for pile groups.

Factor of Safety

Where load tests are not performed, it is usual practice to apply a factor of safety of two to determine the design load.

Piles subject to uplift develop resistance to pull-out only by skin friction. Point-bearing resistance does not apply, but

the weight of the pile may be included in the uplift capacity.

Generally, a larger factor of safety is employed for uplift than for conventional downward loading. The strength of pile

to pile-cap connection becomes critical in the case of uplift forces, since tensile force at this location negates any pull-

out resistance of the pile.

Qap = (Qsf Wp) /η ...(Eq. 16.41)

where Wp is the weight of the pile and factor of safety ηis to be not less than 2.

PILE GROUPS

A structure is never founded on a single pile. Piles are ordinarily closely spaced beneath structures; consequently, the

action of the entire pile group must be considered. This is particularly important when purely friction piles are used.

The bearing capacity of a pile group is not necessarily the capacity of the individual pile multiplied by the number of

piles in the group; the phenomenon by virtue of which this discrepancy occurs is known as ‘Group action of piles’.

Number of Piles and Spacing Usually driven piles are not used singly beneath a column or a wall because of the tendency of the pile to wander

laterally during driving and consequent uncertainty with regard to centering the pile beneath the foundation. In cases

where unplanned eccentricities result, failure may occur either at the connection between the pile and column or within

the pile itself. Hence, piles for walls are commonly installed in a staggered arrangement to both sides of the centreline

of the wall. For a column, at least three piles are used in a triangular pattern, even for small loads. When more than

three piles are required in order to obtain adequate capacity, the arrangement of piles is symmetrical about the point or area of load application. Representative pile group patterns for wall and column loads are indicated in Fig. 16.12.

Column and wall loads are usually transferred to the pile group through a pile cap, which is typically a reinforced

concrete slab structurally connected to the pile heads to help the group act as a unit (Fig. 16.13). The requirement for group arrangement of driven piles does not apply to bored piles. Drilled shafts can be installed quite accurately. A

single large-diameter drilled shaft pile is commonly used to support columns in residential buildings. This may be used

when the three pile configuration yields unnecessarily extra load carrying capacity in the case of driven piles.

The spacing of piles in a group depends upon a number of factors such as the overlapping of stresses of adjacent piles,

cost of foundation and the desired efficiency of the pile group.

The stress isobars of a single pile carrying a concentrated load will be somewhat as shown in Fig. 16.14(a). When piles

are driven in a group, there is a possibility of stress isobars of adjacent piles overlapping each other as shown in Fig.

16.14(b). Since, the overlapping might cause failure either in shear or by excessive settlement, this possibility may be

averted by increasing the spacing as shown in Fig. 16.14(c). Large spacing are not advantageous since a bigger size of pile cap would increase the overall cost of the foundation.

In the case of driven piles there will be greater overlap of stresses due to the displacement of soil. If piles are driven in

loose sands, compaction takes place and hence, the spacing may be small. However, if piles are driven in saturated silt

or clay, compaction does not take place but the piles may experience uplift. To avoid this, greater spacing may be

adopted. Smaller spacings may be used for cast-in-situ piles in view of less disturbance.

Point-bearing piles may be more closely spaced than friction piles. The minimum spacing of piles is usually specified

in building codes. The spacing may vary from 2d to 6d for straight uniform cylindrical piles, d being the diameter of

the pile. For friction piles, the recommended minimum spacing is 3d. For point-bearing piles passing through

relatively compressible strata, the minimum spacing is 2.5d when the piles rest in compact sand or gravel; this should

be 3.5d when the piles rest in stiff clay. The minimum spacing may be 2d for compaction piles. Piles should be, in general, driven proceeding outward from the centre, except in soft clay or very soft soil; in the latter

case, the pile driving proceeds from the periphery of the foundation to the centre to prevent the lateral flow of soil

during driving.

Group Capacity of Piles

The capacity of a pile group is not necessarily the capacity of the individual pile multiplied by the number of

individual piles in the group. Disturbance of soil during the installation of the pile and overlap of stresses between the

adjacent piles, may cause the group capacity to be less than the sum of the individual capacities.

Conversely, the soil between individual piles may become ‘‘locked in’’ due to densification from driving and the

group may tend to behave as a unit or an equivalent single large pile. Densification and improvement of the soil

surrounding the group can also occur. These factors tend to provide the group a capacity greater than the sum of the

capacities of individual piles.

The capacity of the equivalent large pile is analysed by determining the skin friction resistance around the embedded

perimeter of the group and calculating the end-bearing resistance by assuming a tip area formed by this block, as in

Fig. 16.15.

To determine the capacity of a pile group, the sum of the capacities of the individual piles is compared with the

capacity of the single large equivalent (block) pile; the smaller of the two values is taken. Applying an appropriate

factor of safety to this chosen value, the design load of the pile group is obtained.

The skin friction resistance of the single large equivalent pile (block) is obtained by multiplying the surface area of the

group by the shear strength of the soil around the group. The end-bearing resistance is computed by using the general

bearing capacity equation of Terzaghi. The bearing capacity factors for deep foundations are used when the length of the pile is at least ten times the width of the group; otherwise, the factors for shallow foundations

are used.

Obviously, the capacity of the equivalent large pile is affected by soil type and properties, besides spacing of piles.

Generally speaking, there is a greater tendency for the group to act as a block or large single unit when the piles are

close and the soil is firm or compact.

Pile groups in cohesionless soils

For driven piles embedded in cohesionless soils, the capacity of the large equivalent pile (block) will be almost always

greater than the sum of the capacities of individual piles, in view of the densification that occurs during driving.

Consequently, for design, the group capacity is taken as the sum of the individual pile capacities or the product of the

number of piles in the group and the capacity of the individual pile.

This procedure is not applicable, if the pile tips rest on compressible soils such as clays; in such cases, the pile group capacity is governed by the shear strength and compressibility of clay soil, rather than on the characteristics of the

cohesionless soil. Bored piles or cast-in-situ concrete piles are constructed by boring a hole of required

diameter and depth and pouring in of concrete. Boring is accompanied invariably by some degree of loosening of the

soil. In view of this, the group capacity of such piles will be somewhat less than the sum of individual pile capacities

typically—about two-thirds of it. It may also be taken as the sum of individual pile capacities approximately.

Pile groups in cohesive soils

When piles are driven into clay soils, there will be considerable remoulding especially when the soil is soft and

sensitive. The soil between the piles may also heave since compaction cannot be easily achieved in soils of such low

permeability. Bored piles are generally preferred to driven piles in such soils. However, if driven piles are to be used,

spacing of piles must be relatively large and the driving so adjusted as to minimise the development of pore pressure.

The mode of failure of pile groups in cohesive soils depends primarily upon the spacing of piles. For smaller spacings,

‘block failure’ may occur, in other words, the group capacity as a block will be less than the sum of individual pile

capacities. For larger spacings, failure of individual piles may occur; or, it is to say that the group capacity is given by

the sum of the individual pile capacities, which will be smaller than the strength of the group acting as a unit or a

block. The limiting value of the spacing for which the group capacities obtained from the

two criteria—block failure and individual pile failure—are equal is usually considered to be about 3 pile-diameters.

Negative skin friction Qng may be computed for a group in cohesive soils as follows:

Individual pile action:

Qng = nQnf = nP . Dn . c ...(Eq. 16.42) Notations are the same as for Eq. 16.39.

Block action:

Qng = c . Dn . Pg + γ. Dn . Ag ...(Eq. 16.43)

Here, Pg and Ag are the perimeter and area of the pile block.

[Pg = 4B and Ag = B2, where B is the overall width of the block].

The larger of the two values for Qng is chosen as the negative skin friction.

Pile Group Efficiency

The ‘efficiency’, ηg, of a pile group is defined as the ratio of the group capacity, Qg , to the sum of the capacities of the

number of piles, n, in the group:

ηg = Qg / (n.Qp ) or (Σ Qp) ...(Eq. 16.44)

where Qp = capacity of individual pile.

Obviously, the group efficiency depends upon parameters such as the types of soil in which the piles are embedded and on which they rest, method of installation, and spacing of piles. Vesic (1967) has shown that end-bearing

resistance is virtually unaffected by group action. However, skin friction resistance increases with increase in spacing

for pile groups in sands. For pile groups in clay, the skin friction component of the resistance decreases for

certain pile spacings. Thus, in general, efficiencies of pile groups in clay tend to be less than unity. Interestingly,

Vesic’s experimental investigations on pile groups in sands indicate group efficiencies greater than unity.

Sowers et al. (1961) have shown that the optimum spacing at which the group efficiency is unity for long friction piles

in clay is given by

So = 1.1 + 0.4n0.4 ...(Eq. 16.45)

where So = optimum spacing in terms of pile diameters; So is 2 to 3 pile diameters centre to

centre, and n = number of piles in the group.

The actual efficiency, ηg, at the theoretical optimum spacing is ηg = 0.5 + 0 40 / (n- 0.9) 0.1 ...(Eq. 16.46)

This has been found to be 0.85 to 0.90, rather than unity. Since a factor of safety is used

for design, the error in assuming the real efficiency to be 1 at optimum spacing is inconsequential.

A number of empirical equations for pile group efficiency are available. There is no acceptable formula and these

should be used with caution as they may be no better than a goodguess. These formulae yield efficiency values less

than unity, and as such, will not be applicable to closely spaced friction piles in cohesionless soils and to piles through

soft material resting on a firm stratum.

The Converse-Labarre formula, the Feld’s rule and the Seiler-Keeny formula are given here:

Converse-Labarre formula

ηg = 1 – [ϕ/90[m( n – 1) + n (m – 1)/ mn]] ...(Eq. 16.47)

where ηg = efficiency of pile group,

ϕ= tan–1 d/s in degrees, d and s being the diameter and spacing of piles, m = number of rows of piles, and

n = number of piles in a row (interchangeable)

Seiler-Keeney formula

ηg = [1 – 0.479(s/ s2 – 0.093)(m + n - 2 / m + n – 1)] + 0.3/( m+ n) ...(Eq. 16.48)

Here m, n and s stand for the number of rows of piles, number of piles in a row and pile

spacing, respectively.

ILLUSTRATIVE EXAMPLES