Determine the required length of a pile Determine the required length (L) of the maximum loaded pile in Figure 10.34, which was found to be 130.5^k. In order to determine the length required, additional information of the proposed pile and the soil strata, where the pile will be located, is needed.

Question

Determine the required length of a pile
Determine the required length (L) of the maximum loaded pile in Figure 10.34, which was found to be [latex]130.5^k[/latex].
In order to determine the length required, additional information of the proposed pile and the soil strata, where the pile will be located, is needed.
Consider the following additional information:
• 12″ diameter concrete piles
• Allowable end-bearing capacity, [latex]q_{tip}[/latex] = 20,000 psf
• Allowable skin function, f = 1200 psf
• Factor of safety = 2

Accepted Answer

The pile capacity, based on the supporting strength of the soil, can be expressed in equation form as
[latex]Q_{ultimate} = Q_{friction} + Q_{tip}[/latex] (10.13)
where:
[latex]Q_{ultimate}[/latex] is ultimate bearing capacity of a single pile
[latex]Q_{friction}[/latex] is bearing capacity due to friction or adhesion between the soil and sides of the pile
[latex]Q_{tip}[/latex] is bearing capacity due to soil just below the pile’s tip
The term [latex]Q_{friction}[/latex] in Equation 10.12 can be evaluated by multiplying the skin function (f) by the pile’s surface (skin) area ([latex]A_{surface}[/latex]).
[latex]I_{pile group}=\sum n_i(d_i)^2[/latex] (10.12)
The term [latex]Q_{tip}[/latex] is simply the area of the pile ([latex]A_{tip}[/latex]) multiplied by the end-bearing capacity ([latex]q_{tip}[/latex]).
Then, Equation 10.12 can be rewritten as follows:
[latex]Q_{ultimate} = f\cdot A_{surface} + q_{tip} \cdot A_{tip}[/latex] (10.14)
Hence, the solution is as follows:
[latex]A_{tip}[/latex], for a 12″ diameter pile [latex]=\frac{\pi d^2}{4}=\frac{\pi(1^{ft})^2}{4}=0.785[/latex] ft²
[latex]A_{surface}[/latex] = 2πr(L) = 2(π)(0.5 ft) = 3.14 ft(L)
Then, using Equation 10.13, we can write
[latex]130.5^k[/latex] (2) = 1.2 k/ft² (3.14 ft) L + 20 k/ft² (0.785 ft²)
Note, we must include the factor of safety by multiplying the pile load by 2.
Then, solving for L,
[latex]L=\frac{130^k(2)-20^{k/ft^2}(0.785^{ft^2})}{1.2^{k/ft^2}(3.14^{ft})}=64.8^\prime\approx 65^\prime[/latex]
The pile length must include the embedment length in the pile cap. Consequently, for a concrete pile, the minimum embedment length is 6″; therefore, the design length of pile must be L = 65′ + 0.5′ = 65.5′.
It must be noted here that the determined length of the pile, based on subsurface soil conditions, is obviously not the structural strength of the pile. A pile’s structural strength is determined by the size and shape and material composition of the pile. The pile must be designed specifically for its design loads. Selection of materials for the pile, such as timber, reinforced concrete, and steel will require different design methods, which are not covered here but can be found in various design aids based on prevailing building codes.

Question 10.8: Determine the required length of a pile Determine the requir...

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