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 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