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When to use Deep Foundation 1. upper soils are weak, structural loads are high; required spread footings are too large. 2. Upper soils are subject to scour or undermining 3. Foundation must penetrate through water 4. Need large uplift capacity, such as transmission towers, offshore platforms, and basement mats below the water table 5. Need large lateral load capacity, encountered in the design and construction of earth-retaining structures and foundations of tall structures that are subjected to high wind or to earthquake forces. 6. For expansive and collapsible soils; deep foundations are extended into stable soil layers beyond the zone where moisture will change.

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Types of Piles and Their Structural Characteristics • Different types of piles are used in construction work, depending on the type of load to be carried, the subsoil conditions, and the location of the water table. • Piles can be divided into the following categories with the general descriptions for conventional steel, concrete, timber, and composite piles

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Steel Piles

Usual length: 15 m to 60 m (50 ft to 200 ft) Usual load: 300 kN to 1200 kN (67 kip to 265 kip)

Concrete Piles

• Usual length: 10 m to 15 m (30 ft to 50 ft) • Usual load: 300 kN to 3000 kN (67 kip to 675 kip)

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• precast prestressed piles: • Usual length: 10 m to 45 m (30 ft to 150 ft) • Maximum length: 60 m (200 ft) • Maximum load: 7500 kN to 8500 kN (1700 kip to 1900 kip)

Cast-in-situ, or cast-in-place, piles are built by making a hole in the ground and then filling it with concrete

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• The usual length of wooden piles is 5 m to 15 m (15 ft to 50 ft). • The maximum length is about 30 m to 40 m (100 ft to 130 ft). • The usual load carried by wooden piles is 300 kN to 500 kN (67 kip to 115 kip).

Estimating Pile Length where

Qp = load carried at the pile point Qs = load carried by skin friction developed at the side of the pile (caused by shearing resistance between the soil and the pile)

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Equations for Estimating Pile Capacity

• The ultimate load-carrying capacity Qu of a pile is given by the equation

Frictional Resistance (Qs)

p = perimeter of the pile section L = incremental pile length over which p and f are taken to be constant ƒ = unit friction resistance ( skin friction) at any depth z

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Frictional (Skin) Resistance in Clay α Method the unit skin resistance in clayey soils

α = empirical adhesion factor. cu = undrained shear strength

 Method the unit skin resistance in clayey soils

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 Method

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Frictional Resistance (Qs) in Sand • The unit skin friction increases with depth more or less linearly to a depth of L and remains constant thereafter.

For z = 0 to L

and for z = L to L

• The values of  in the range from 0.5 to 0.8 

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Correlation with Standard Penetration Test Results Meyerhof Method

the average unit frictional resistance, fav , for high- displacement driven piles

For low- displacement driven piles

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Point Bearing Capacity, Qp

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Meyerhof’s Method for Estimating Qp

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

cu = undrained cohesion of the soil below the tip of the pile.

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Point Bearing Capacity of Piles Resting on Rock

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Group Efficiency • The efficiency of the load-bearing capacity of a group pile may be defined as

The efficiency factor, η, shall be 1.0 for a center-to-center pile spacing of 2.5 diameters or greater.

Drilled-Shaft Foundations

• the term drilled shaft is used for a hole drilled or excavated to the bottom of a structure’s foundation and then filled with concrete. • Depending on the soil conditions, casings may be used to prevent the soil around the hole from caving in during construction. The diameter of the shaft is usually large enough for a person to enter for inspection.

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