Piles can either be driven into the ground (driven piles) or be installed in a predrilled hole (bored piles or drilled shafts). A variety of driving equipment is used in pile installations. The key components are the leads and the hammer. The leads are used to align the hammer to strike the pile squarely (Figure ). Hammers can be simple drop hammers of weights between 2.5 and 15 kN or modern steam/pneumatic hammers. Two popular types of steam/pneumatic hammers are shown in Figure . The single-acting hammer utilizes steam or air to lift the ram and its accessories (cushion, drive caps, etc.). The doubleacting hammer is used to increase the number of blows/minute and utilizes steam or air to lift the ram and force it down.
The method of installation needs careful consideration because it irreversibly changes the soil stress and strain states and can create intolerable noise and vibration during construction. The maximum installation stress for piles driven from the top must not exceed the compressive or tensile strength of the pile material.
Two types of
Utilizing our knowledge of soil mechanics from previous chapters, especially Chapters 9 through 11, we will construct a simplifi ed framework to understand the effects on the soil from pile installation and structural loads. This would help us to interpret pile test results and understand the limitations of pile load capacity calculations presented later in this chapter. The resistance to vertical loads on a pile is provided by friction along the surfaces of the pile (called skin friction) and by resistance at the base, called end bearing or toe or base resistance .
Pile driving imposes impact load. When the hammer strikes the pile, it sets up a stress wave that propagates through the pile into the surrounding soil. Pushing the pile imposes a continuous (static) load. The installation load is applied quickly so that undrained condition (zero volume change) can be
assumed to apply.
The soil fails by imposition of shear stress (interface shear), t, at the interface of the pile and soil, and radial compression to the soil mass adjacent to the pile (Figure . The stresses on a soil element, A, at a radius, r, near the shaft are shown in Figure If the soil mass over the length of the pile is layered, then there will be different stresses for each of the layers. The stresses on the soil element are not simple. Even if we were to solve for these stresses using mechanics, replicating them and the mode of deformation would be diffi cult for practical applications. The mode of deformation near the shaft is similar to simple shear. An element of soil near the interface is dragged down, as shown for element A
For displacement or closed-ended pipe piles, a volume of soil that equals the volume of the pile must be displaced during installation. This volume of soil mass is disturbed or remolded. Let us assume a pile of length L and radius ro. Further, let us assume that a right cylinder of soil is compressed over an annular area of thickness r 2 ro, where r is the external radius of the disturbed soil (Figure . The resulting soil heave is assumed to be DL. The minimum disturbed region or disturbed volume of soil around the shaft is as follows.
The volume of the pile is
The disturbed volume of soil must equal the volume of the pile. That is,
Solving for r, we get