Monday, November 19, 2012

Design Stresses - Deep Foundation: Constraints.

The design of deep foundations is usually determined by limits specified for lateral or vertical displacement of the pile and placement tolerances. Limiting values of allowable stresses for different deep foundations are included in Table 1-4. Structural capacity rarely controls design, except when piles are founded on rock. Driven piles experience maximum stress during driving, while maximum vertical stresses in drilled shafts usually occur under static conditions.

a. Limiting deformations. Vertical and lateral uve load displacements should be limited to 0.5 inch. However, operational requirements may necessitate additional restrictions. Long-term displacements may be larger than computed values due to creep. Cyclic loads and close spacmgs may increase displacements and should be considered in the design. Methods are presented later for the computation of displacements of deep foundations under vertical and lateral loadings.

b. Geometric constraints
(1) Driven piles. Piles are normally spaced three to four times the diameter from center to center. Typical tolerance of lateral deviation from the specified Location at thc butt is not more than 3 to 6 inches horizontally. The siope from vertical alignment is typically not more than 0.25 inch per foot of length for large pile groups. A deviation of ± 1 inch from the specified cutoff elevation is reasonable. Sloping land surfaces may require adjustment of the pile location if the surface vanes from the reference plane used in the plans to depict pile locations. Other geometric constxaints could be related to the following:

(a) Pile spacing. Bearing and lateral resistance of each pile will be reduced if piles are spaced too closely; close spacing might cause foundation heave or damage to other already driven piles. End bearing piles should usually be spaced not less than three pile diameters from center to center, while function piles should be spaced a minimum of three to five pile diameters from center to center. Large groups of fine or more piles may be checked for pile interference using program CPGP (Wolff 1990).

(b) Pile batter. Batter piles are used to support structures subject to large lateral loads or for smaller lateral loads if the upper foundation stratum will not adequately resist lateral movement of vertical piles. Piles may be battered in opposite
directions or used in combmation with vextical piles. The axial load on a batter pile should not exceed the allowable design load for a vertical pile, and batter should not be greater than 1 horizontal to 2 vertical; the driving efficiency of the hammer decreases as the batter increases.

(c) Sweep. Specifications should include initial sweep (camber) limitations, because piles curved as a result of excessive sweep will be driven out of tolerance. Sweep for steel H-piles, for example, may be limited to 2 inches and for H-piles, up to 42 feet in length. Refer to the American Institute of Steel Construction Manual of Steel Construction” (AISC 1989) for further information. The required number and locations of permissible pick-up po ints on the pile should also be clearly indicated in the specifications. Loading and unloading of long steel piles should be done by support at a minimum of two points about one-fourth of the pile length from each end of the pile. Precast concrete piles should be supported at several points.

(2) Drilled shafts. Drilled shafts are normally placed vertically and spaced at relatively large distances cxceeding eight times the shaft diameter. Guidelines for placement toicrances are given in Table 2-1. Greater toicrances can be considered for drilled shafts in difficult subsoils.

Table 1-4 Characteristics of Deep Foundations

Table 2-1 Tolerances in Dilled Shaft Construction.

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