Chapter Eight Pile Foundations 8-1 Introduction: Piles are structural members of timber, concrete, and/or steel that are used to transmit surface loads to lower levels in the soil mass. This transfer may be by: • vertical distribution of the load along the pile shaft (friction pile ) • direct application of load to a lower stratum through the pile

point (end-bearing pile) Piles are commonly used for the following purposes: 1. To carry structure loads into or through a soil stratum. 2. To resist uplift or overturning forces. 3. To control settlements when spread footings are on highly

compressible soil. 4. In offshore construction to transmit loads through the water and into

the underlying soil. 5. To control earth movements, such as landslides.

Piles are inserted into the soil via a number of methods: 1. Driving with a steady succession of blows on the top of the pile using

a pile hammer. This produces both considerable noise and local vibrations, which may be disallowed by local codes or environmental agencies and, of course, may damage adjacent property.

2. Driving using a vibratory device attached to the top of the pile. This method is usually relatively quiet, and driving vibrations may not

be excessive. The method is more applicable in deposits with little cohesion.

3. Jetting the pile. This technique is more applicable when the pile needs to penetrate a thin layer of hard soil(sand or gravel) overlying a softer soil; water is discharged at the pile point to loosen hard soil

4. Drilling a hole and either inserting a pile into it or, more commonly, filling the cavity with concrete, which produces a pile upon hardening.

When a pile foundation is decided upon, it is necessary to compute the required pile cross section and length based on the load from the superstructure, allowable stress in the pile material (usually a code value), and the in situ soil properties.

Driven pile by hammering

Bored pile

hammer

vibration

Jetting the pile

Cast-in -situ

8.2 Pile types and characteristics: Different types of piles are used in construction work, depending on: • the type of load to be carried • the subsoil conditions • the presence of water table • the local available materials and construction techniques. 1.Timber piles: Timber piles are tree trunks with the branches carefully trimmed off, usually treated with a preservative and driven with the small end as a point: - Easy to cut and splice, however Splicing to be avoided when pile is

subjected to tensile or lateral forces - The pile length is usually limited to 10-20 m - The minimum pile tip diameter is 15 cm - Timber piles have a limited load-carrying ability(from 10 to 50 tons) - Their ends may splinter under driving loads (“brooming”). - Subject to insect attack and organic decay. - Pressure treated wood used to reduce the piles vulnerability to such

damage, but creates environmental problems by poisoning of ground waters.

- Can stay undamaged if surrounded by fresh water, however in a marine environment they are subject to decay

Driving shoe

2.Steel Piles: Steel piles generally are either pipe piles or rolled steel section 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 pile. However, H section piles are usually preferred because their web flange thicknesses are equal. When necessary, steel piles are spliced by welding or by riveting. Usual length : 15 to 60 m Usual load:30 to 120 tons Advantages: • Support heavy loads. • Can be driven to great depth without damage • Are easily cut and spliced • Sections as H-Piles and Pipe Piles are common • Pipes are filled with concrete for additional strength. Disadvantages: • Relatively costly • High level of noise during pile driving • Subject to corrosion • H-piles may be damaged or deflected during driving through hard

layers or past major obstructions

Driving steel piles

The allowable structural capacity for steel pile is: Q all = As.fs

As : cross-sectional area of the steel fs: allowable stress of steel (≈0.33-0.5 fy)

Pipe pile

(a)Splicing of H-pile by welding;(b)splicing of pipe pile by welding;(c) splicing of H-pile by rivets and bolts

3.Concrete Piles: concrete piles may be precast or cast in place

I-)Precast Concrete Piles: Piles in this category are formed in a central casting yard to the specified length, cured, and then shipped to the construction site. Precast piles may be made using: • ordinary reinforcement • prestressed. Precast piles using ordinary reinforcement are designed to resist bending stresses during pickup and transport to the site and bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving.

Precast piles with ordinary reinforcement

Usual length 10 to 15 m Usual load : 30 to 300 tons

Advantages: • Can be subjected to hard driving • Can be easily combined with a concrete superstructure • Concrete piles are manufactured to almost any desired size or

shape. • high strength and resistant to decay. Disadvantages: • Because of their weight, brittleness, and lack of tensile strength,

special care in handling of the pile is required. • Cutting requires the use of pneumatic hammers, torches or special

saws. Precast piles can be Prestressed by the use of high-strength steel prestressing cables. The ultimate strength of these cables is about 1800MN/m2.During casting of the piles, the cables are pretensioned to about 900 to 1300MN/m2, and concrete is poured around them. After curing, the cables are cut, producing a compressive force on the pile section as the steel pile attempts to return to its unstretched length some general facts are as follows: • Usual length 10 to 45 m, maximum 60 m • Usual load 750 to 850 tons

II-)Cast-in-situ:

A cast-in-place pile is formed by drilling a hole in the ground and filling it with concrete. Various types of cast-in-place concrete piles are currently used in construction ,these might be divided into two categories: (a)-cased or shell;(b)-uncased ,both types may have a pedestal at the bottom

A pedestal is an expanded concrete bulb that is formed by dropping a hammer into fresh concrete

Precast piles

The allowable design load for all non-prestressed concrete piles is: Pa = Ac.fc +As.fs

Ac, As = area of concrete and steel shell respectively fc , fs = allowable concrete and steel stresses

Cast-in situ piles

4.Composite Piles: the upper and lower portions of composite piles are made of different materials, for example steel-concrete and timber-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.

8.3 Estimation of Pile length: 1. Point-bearing piles: When bedrock material is present at reasonable depth, piles can be extended to the rock surface; in this case the ultimate capacity of the piles depends entirely on its end-bearing and the necessary length is well established. When a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into hard stratum, and the ultimate pile load may be expressed as: Qu = Qp + Qs

Where: Qp is the load carried at the pile point Qs is the load carried by the skin friction If Qs is very small, then

2. Friction piles: When relatively hard stratum is not present at a reasonable depth, point bearing piles become very long and uneconomical, hence piles are driven through the softer material to specified depth, and for Qp relatively small ,

Qu = Qp

Qu = Qs

8.4 STATIC CAPACITY OF AXIALLY LOADED SINGLE PILE

1.Point bearing capacity, Qp: The ultimate resistance per unit area developed at the pile tip qp may be expressed by an equation similar in form to that given by the general bearing capacity of shallow foundations:

qu = qp: ultimate pile tip resistance/unit area Nc*,Nq*,Nγ* : bearing capacity factors including the necessary shape

and depth factors D: width or diameter of pile However D is relatively small so that γD Nγ* can be neglected, hence

The point bearing of piles is:

Ap: area of pile tip; qp : unit point resistance; q’: effective vertical overburden stress at the pile tip level

QU = QP+QS

A- Meyerhof’s Method 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 = L; However, where a pile has penetrated into a bearing stratum ,Lb< L. Beyond(Lb/D)cr ,the value of qp remains constant (qp=ql) For piles in Sand, C =0 simplifies the equation to:

qp=ql

L/D =Lb/D

The variation of Nq* with soil friction angle Φ is shown in figure(8.4.1), however Qp shouldn’t exceed the limiting value that is:

Where the limiting point resistance is: A’-Meyerhof Method(1976) Based on SPT-value In a homogeneous granular soil (L=Lb)

ql (KN/m2) = 50 Nq*.tanΦ ql (lb/ft2) = 1000 Nq*.tanΦ

Clay: For piles in saturated clay (undrained condition)

For piles in soil with c, Φ

Qp = Ap.qp=Ap(cNc*+qNq*)

qp (KN/m2)= 40N.L/D ≤ 400N qp (lb/ft2)= 800N.L/D ≤ 8000N Where N = Average SPT-value about 10D above

and 4D below the pile point

Fig.8.4.1

Soil friction ,Φ

B- Vesic’s Method:

Qp = Ap.qp = Ap(C Nc* + σ’0 Nσ*)

Where σ’0 =mean effective normal ground stress at the level of the pile point = ( 1+2 K0)q’/3 K0 = earth pressure coefficient at rest = 1-sin Φ Nσ* = bearing capacity factor

= f(Irr)

In case of saturated clay in undrained conditions(Φ=0)

For saturated clay, no volume changes,Δ = 0, Ir =Irr

Vesic

C- Coyle and Castello’s Method

Coyle and castello

2- Frictional Resistance, Qs The frictional or skin resistance of a pile may be written as:

where p = perimeter of the pile section ∆L = incremental pile length over which p and f are taken constant f = unit friction resistance at any depth z

A- Sand 1- Critical depth Method The unit frictional resistance at any depth for a pile is

where K = earth pressure coefficient σ’o = effective vertical stress at the depth under consideration δ = soil–pile friction angle The effective vertical stress σ’o increases with pile depth to a maximum limit at a depth of 15 to 20 pile diameters and remains constant thereafter,

A conservative estimate is to assume that L = 15D The values of δ = 0.5 Φ to 0.8 Φ Qs = Qs1+Qs2

Where Qs1 from z=0 to 15 D (favg is considered) Qs2 from z=15D to L

2- Coyle and Castello:

B-Clay Several methods are available for obtaining the unit frictional resistance of piles in clay. Three of the presently accepted procedures are described briefly.

1. The α Method:

According to the α method, the unit skin resistance in clayey soils

can be represented by the equation:

Where α = empirical adhesion factor, cu =undrained cohesion

2. The β Method

the unit frictional resistance for the pile can be determined on the basis of the effective stress parameters of the clay in a remolded state (c =0).

and

• K = 1 - sin ΦR (for normally consolidated clays) • K = (1 - sin ΦR) √OCR (for over consolidated clays) where OCR = σ’p/σ’v over consolidation ratio

where σ’o = mean vertical effective stress β = K tan ΦR ; ΦR= drained friction angle of remolded clay K = earth pressure coefficient

3. The λ Method: the average unit skin resistance is:

where σ’o :mean effective vertical stress for the entire embedment length cu : mean undrained shear strength ( Φ = 0 concept) The value of λ changes with the depth of pile penetration Thus, the total frictional resistance may be calculated as Qs = pLfav

• the mean value of cu is (cu1(L1) + cu2(L2 ) . . . )/L • the mean effective stress is

The λ method

α method λ method

8.5 Allowable Pile Capacity

The ultimate total load carrying capacity of a pile is determined by: Qu = Qp+Qs

• Then Q all = Qu/FS where FS = factor of safety = 2.5 to 4

8.6 Elastic Settlement of Piles

The elastic settlement of a pile under a vertical working load, Qw, is determined by three factors:

S = S1+S2+S3 where S = total pile settlement S1 = settlement of pile shaft S2= settlement of pile caused by the load at the pile point S3= settlement of pile caused by the load transmitted along the pile

shaft

1

1- determination of S1

Ap: area of the pile cross section L : length of the pile Ep: modulus of elasticity of the pile material The magnitude of ξ depends on the nature of the unit friction (skin) resistance distribution along the pile shaft. ξ ≈ 0.6 is acceptable.

where Qwp : load carried at the pile point under working load condition Qws : load carried by frictional (skin) resistance under working load

condition

2- determination of S2

2

where D : width or diameter of the pile qwp : point load per unit area at the pile point= Qwp/Ap Es: modulus of elasticity of soil at or below the pile point μ: Poisson’s ratio of soil

Iwp : influence factor = αr (fig.8.6.1)

Vesic also proposed a semi empirical method to obtain the magnitude of the settlement

Where: qp : ultimate point resistance of the pile Cp :an empirical coefficient in the following table

2

Fig.8.6.1

3-determination of S3:

3

where p : perimeter of the pile L : embedded length of the pile Iws : influence factor

Or Vesic

3

where Cs an empirical constant (=0.93 + 0.16√L/D )Cp The values of Cp may be estimated from Table

8.7 Laterally Loaded Piles

Consider pile of length L subjected to a lateral force Qg and a moment Mg at the ground surface (z=0) as shown in the next figure. According to Winkler’s model, the soil can be replaced by a series of infinitely close independent elastic springs:

From the fundamentals of mechanics of materials: 1-Granular soils: The subgrade modulus for granular soils at a depth z is defined as:

Kz = nh.z

Where nh: constant of modulus of horizontal subgrade reaction

a-Pile deflection at any depth

b- slope of pile at any depth

c-Moment at any depth

d- shear force at any depth

e- soil reaction at any depth

Remark: • When L ≥ 5T, the pile is considered “Long pile” • When L ≤ 2T, the pile is considered “Rigid pile”

Representative values of Nh

Note that Z = z/T

2- cohesive soils For cohesive soil the subgrade reaction may be assumed to be approximately constant with depth according to vesic:

a-Pile deflection at any depth

b- pile moment at any depth

and

8.8 Pile groups

8.8.1Group Efficiency: in most cases, piles are used in groups (min. two or three piles) to transmit the structural load to the soil. A pile cap is constructed over group piles to spread the load to the several piles, it can be in contact with the ground as in most cases, or well above the ground, as in case of offshore platforms.

8.8.2 Determining the load-bearing 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 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 individual piles. • The minimum center- to center pile spacing ”d” or “S”, is 2.5D to

3.5D (D:diameter of pile) or 2H to 3H(H:diagonal of rectangular shape or HP pile) for vertical loads.

• For lateral loads larger spacings are usually more efficient. The efficiency of the load-bearing capacity of a group Of piles is defined as :

We may write it as:

Where: Pg: perimeter of group cross section= 2(n1+n2-2)d+4D D: diameter of a pile; n1,n2 = number of piles Favg: average unit frictional resistance(skin friction)

Pg

Hence,

Case when d>>

Converse –labarre Equation:

A- Piles in sand 1.Driven piles: with d≥ 3D Qg(u) = ∑ Qu

2. Bored piles: with d=3D Qg(u) =(2/3 to ¾) ∑ Qu

B- Piles in clay The ultimate load bearing capacity of pile groups in clay may be estimated as follows: Step1: Determine

Where:

Step2: Determine the ultimate capacity of the pile group block with dimensions LgxBgxL

The skin resistance,

Point-bearing capacity,

Step3 compare the two obtained values of ∑Qu, the lower value is Qg(u)

8.9 Elastic Settlement of Pile Groups

For pile groups in sand and gravel, Meyerhof (1976) suggested the following empirical relation for elastic settlement:

where q (kN/m2)= Qg/(Lg .Bg) N: average corrected standard penetration number within (Bg) below

the pile tip deep below the tip of the piles) I : influence factor = 1 - L/8Bg ≥ 0.5 L : length of embedment of piles Similarly, the pile group settlement is related to the cone penetration resistance as:

Where qc is the average cone penetration resistance within Bg below pile tip

8.10 Consolidation Settlement of Group Piles

The consolidation settlement of a pile group can be estimated by assuming an approximate distribution method that is commonly referred to as the 2:1 method. The calculation procedure involves the following steps: 1. let Qg be The group net total load of. If the pile cap is below the

original ground surface, Qg equals the total load of the superstructure on the piles minus the effective weight of soil above the pile group removed by excavation: Qgnet = Qg(gross)-γ.Df

2. Assume that the load Qg is transmitted to the soil beginning at a depth of 2L/3 from the top of the pile (as shown in Figure), The load Qg spreads out along 2:1 method 3. Calculate the effective stress increase caused at the middle of each soil layer by the load Qg:

4.Calculate the settlement of each layer caused by the increased stress:

5. Calculate the total consolidation settlement of the pile group by:

8.11 Design of PILE CAPS

• Unless a single pile is used, a cap is necessary to spread the vertical and horizontal loads and any overturning moments to all the piles in the group.

• The cap is usually of reinforced concrete, poured on the ground unless the soil is expansive.

• Caps for offshore structures are often fabricated from steel shapes. • The pile cap has a reaction that is a series of concentrated loads (the piles); and the design considers the column loads and

moments, any soil overlying the cap (if it is below the ground surface), and the weight of the cap.

1-Load distribution on the piles below cap • For a concentric axial load on the cap, each pile carries an equal

amount of the load. For n piles carrying a total load Q, the load Pp per pile is:

• If pile cap is noncentrally loaded or loaded with a load Q and a moment:

where Mx, My = moments about x and y axes, respectively x, y = distances from y and x axes to any pile ∑x2, ∑ y2 = moment of inertia of the group, computed as: I = Io + Ad2

2- Cap Design by Circulage Method In this method, it is assumed that the column load is resisted by a truss or number of trusses developed inside the concrete cap. The forces in the compression members are taken by concrete and those of tension are resisted by steel reinforcement that tie or circulage the pile heads in order to prevent their lateral movements.

2.1 Conditions No interior piles should exist , the distance from all piles to the center of pile cap should be equal.

2.2 Procedure 1- Determination of the effective depth This effective depth of the pile cap is the largest of: a- Minimum effective depth dmin = 40Φ pile steel bars

b- Punching shear d = Q/qp.b0 Q= column service load qp= allowable concrete punching stress = 8-10 kg/cm2 b0 = punching perimeter taken at d/3 distance from each column

face

C- Angle α such that 45 ≤α ≤60

Two corrections must be carried out: a- the column load is not concentrated but distributed as shown in the

figure b- yct equals 8/9 d For the new triangle force: d = 9/8 d’ S’ = x –Dequivalent/4 ; where b is the column width; Dequiv: equivalent diameter of column tan α = d’/S’

Then dmin corresponds to α = 45

Plan

In case of 2-pile cap

X = S/2

In case of 3-pile cap

In case of 3-pile cap

Use 1/3 of circulage steel as an additional reinforcement to be placed around the cap in depth Shrinkage steel and As’ Bottom mesh of 0.15% Ac Vertical side reinforcement 4 Φ 16/m Top mesh 0.1% Ac

4Φ16/m’

2- Circulage Steel

T = Rp/tan α where Rp is the reaction transmitted under the pile Ts = T/2cos (β/2) As = Ts/fs ; As is the circulage steel