PILE LOAD CAPACITY OF DRIVEN PILES BASED ON SPT AND CPT RESULTS
Various correlations have been proposed among results from SPT and CPT and pile load capacity. We will use only a few of these correlations in this textbook. These correlations and similar ones should be used with caution. Experience is required to successful apply them to practice.
SPT:
Driven piles in coarse-grained soils (Meyerhof, 1956, correlation for sand)
Skin friction: Qf =fs X perimeter X length
End bearing: Qb = fbAb
where Ls is the length of pile driven in sand.
CPT:
The cone penetrometer was originally developed to estimate the end bearing capacity of piles. The cone resistance, qc, is a measure of the end bearing capacity and the sleeve resistance, qs1, is a measure of the skin or shaft friction. The ultimate end bearing capacity of a single pile (Xu and Lehane, 2005) is estimated from
where Cb 5 0.6 for closed-ended driven pipe piles in sand and Cb 5 0.9 for jacked piles in sand; qc–av is the average cone tip resistance over a distance 1.5 times the pile diameter above and the same distance below the pile base, and Ab is the area of the pile base. For open-ended pipe piles in siliceous sand (Lehane and Randolph, 2002),
and D is the external diameter, D* is the effective diameter, Di is the internal diameter, and min denotes minimum value of the quantity within the braces. Equation (13.29) is based on fi eld tests on piles up to 1.5 m in diameter with lengths greater than 5 times the internal diameter in siliceous sand. The maximum expected settlement from Equation is about 10% of the pile diameter. Several other empirical equations are used in practice. For example, Fleming and Thorburn (1983) suggested that for Equation , Cb 5 1 and qc–av is the average cone value over an infl uence zone of 8 pile diameters above the pile base and 2 pile diameters below the pile base, calculated as follows:
where qc1 is the arithmetic average of cone resistance values over 2 pile diameters below the pile base, qc2 is the minimum cone resistance value over 2 pile diameters below the pile base, and qc3 is the arithmetic average of minimum cone resistance values below qc2 over 8 pile diameters above the pile base.
Where the sleeve resistance is measured, either an arithmetic or a geometric mean value of cone resistance over the embedded pile depth is used as the shaft or skin friction. Cone sleeve friction is affected by the relative density of the soil and soil compressibility, while skin friction on a pile is affected by the relative density of the soil, method of installation, soil compressibility, pile geometry, and roughness. In the case of fi ne-grained soils, the cone sleeve friction value is strongly infl uenced by soil consolidation around the sleeve. Thus, signifi cant differences between short-term and long-term load capacity can be expected using cone penetrometer results. Pile test results show that the short-term load capacity estimated from cone penetrometer results can be as low as 20% of the long-term load capacity. If the cone sleeve resistance is not measured, an estimate can be made from one of several equations proposed in the literature. Some of these are as follows: For both open-ended and closed-ended driven pipe piles, the skin frictional stress (Lehane et al., 2005) is given as
where Cs is a constant (0.03 for compression piles and 0.0225 for tension piles), h is the distance of the pile section under consideration above the pile base, dcv is the soil–pile interface friction angle correlated to the mean particle size (<23° for D50 5 1 mm, increasing to 28.88 for D50 5 2 mm for sand on steel) and Ars is the effective area ratio of the pile shaft given as
where qcs is the cone resistance after adjustments for porewater pressure measured at the cone shoulder, and Cs is a coefficient that depends on soil type, as
where s9rc is the radial effective stress on the shaft and is empirically related to the cone resistance asvertical effective stress at a depth z where the shaft friction is
considered, pa 5 atmospheric pressure, h 5 depth from pile base to the depth at which the shaft friction is considered, fi is the interface friction angle (fi # fcs), R* 5 Ro for closed-ended pipe piles, Ro is outer radius,for open-ended pipe piles, and Ri is inner radius.
Tumay and Fakhroo 119842—stiff clays: fs = 0.5qc
The ultimate skin friction is
You have to exercise caution in using these empirical equations, as they were derived from pile load tests and cone penetrometer data in particular soil types and locations. For example, Equations were obtained from piles and cone penetrometer data in siliceous sand in an offshore environment in Western Australia.