A deep foundation is a type of foundation which transfers building loads to the earth farther down from the surface than a shallow foundation does, to a subsurface layer or a range of depths.
A pile is typically a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.
where Q tip,ult = tip resistance; QsL = skin resistance; qtip-ult = bearing stress; Ab = area of pile tip; qsi = skin friction of ith layer; Asi = skin area in contact with ith layer;
There are numerous methods that have been used over the years to estimate shaft resistance. A few of these methods are illustrated in this section.
K and δ are usually estimated based on the type of pile and the characteristics of the soil.
Here are some methods to estimate these two parameters.
► Estimate K:
1- Stas and Kulhawy method:
|Foundation type and methd of installation||Ratio of horizontal soil stress coefficient to in-situ value, K/K0|
|Jetted pile||1/2 to 2/3|
|Drilled shaft, cast-in-place||2/3 to 1|
|Driven pile, small displacement||3/4 to 5/4|
|Driven pile, large displacement||1 to 2|
2- Sowers method:
|Foundation type||Ratio of horizontal soil stress coefficient to in-situ value, K/K0|
|Loose sand (Dr < 30%)||Dense sand (Dr > 70%)|
|Jetted piles||0.5 to 0.75||0.5 to 1.0|
|Drilled piles||0.75 to 1.5||1 to 2|
|Driven piles||2 to 3||3 to 4|
► Estimate δ
1- Stas and Kulhawy method:
|Interface materials||Ratio of interface angle of friction to soil angle of friction δ/φ||Typical field analogy|
|sand/smooth concrete||0.8 to 1.0||precast|
|sand/rough steel||0.7 to 0.9||corrugated|
|sand/smooth steel||0.5 to 0.7||coated|
|sand/timber||0.8 to 0.9||pressure-treated|
obtain α using the following equation from Kulhawy
where Pa = atmospheric pressure; Su = undrained shear strength
where q is the mean effective vertical stress over the embedded pile length and λ is obtained from the figure below (from Kulhawy). According to Kulhawy, the values of λ shown in the figure below are valid only for steel pipe piles. Limited research has shown that λ for drilled shafts (typically less than 20 ft deep) is on the order of about 1/3 to 2/3 of the values shown in the figure below.
Direct Estimates from In Situ Tests:
From the CPT test, pile shaft resistance can be determined from either the sleeve friction (fs) or the tip resistance (qc):
where ρ is the friction ratio. For driven piles, the value of rho can be estimated from either of the following equations:
Values of ρ for drilled shafts are 1/3 to 1/2 the values shown in the two equations above.
Meyerhof recommended the following equations for shaft resistance in high and low
High displacement piles:
Low displacement piles:
For driven piles in sand, Briaud suggested that: