Cone Penetration Test (CPT)
Schmertmann (1970) and Schmertmann et al. (1978) proposed a methodology to determine settlement from the quasi-static cone test data for sands. They assumed that the sand is a linearly elastic material, and only stress changes within depths of 2B for axisymmetric conditions and 4B for plane strain conditions infl uence the settlement. Settlement is calculated by integrating the vertical strains; that is,
The equation proposed for settlement (mm) by Schmertmann et al. is
b is cone factor [
β =2.5 for square footing (axisymmetric condition), β = 3.5 for strip footing (plane strain condition is the net footing pressure in kPa (applied stress minus soil pressure above the base of footing), s'zo is the original vertical effective stress in kPa at the depth of the footing, t is time in year (t $ 0.1), A is an empirical factor taken as 0.2, Dzi is the thickness of the ith layer, and (Ico)i is the infl uence factor of the ith layer given as
where qc)i is the cone tip resistance for the ith layer; s9zp is the original vertical effective stress at the depth where Icp occurs, which is B 2
for axisymmetric condition and B for plane strain; and n is the number of sublayers. The unit of B is meters. The procedure to determine the settlement from cone data is as follows:
1. Divide the soil below the footing into a number of sublayers. For square footings, the total depth of the sublayers is 2B and a reasonable number of sublayers is four. For strip footing, the total depth is 4B and a reasonable number of sublayers is eight.
2. Determine the average value of (qc)i for each sublayer from the fi eld data of qc versus depth.
3. Find Ico at the center of each sublayer.
4. Estimate r using Equation .
The bearing capacity from the CPT test is estimated by taking a weighted average of the cone resistance over a depth of 2B for axisymmetric condition and 4B for plane strain condition below the bottom of the footing base.