The following text was taken from "Guide to cone penetration testing" by P. K. Robertson and K. L. Robertson. You may obtain a copy of the guide if you visit Dr. Robertson's personal web site at www.cpt-robertson.com or you may contact us directly and we will send you a copy.
Research has shown (Robertson et al., 1988; Briaud and Tucker, 1988; Tand and Funegard, 1989; Sharp et al., 1988) that CPT methods generally give superior predictions of axial pile capacity compared to most conventional methods. The main reason for this is that the CPT provides a continuous profile of soil response. Almost all CPT methods use reduction factors to measured CPT values. The need for such reduction factors is due to a combination of the following influences: scale effect, rate of loading effects, difference of insertion technique, position of the CPT friction sleeve and differences in horizontal soil displacements. The early work by DeBeer (1963) identified the importance of scale effects. Despite these differences, the CPT is still the test that gives the closest simulation to a pile. Superiority of CPT methods over non CPT methods has been confirmed in other studies (e.g. O'Neill, 1986). The main CPT method by Bustamante and Gianeselli (1982 - LCPC Method) is outlined below. The LCPC CPT method is recommended since it provides simple guidance to account for different pile installation methods and provides good estimates of axial capacity of single piles.
The method by Bustamante and Gianeselli was based on the analysis of 197 pile load (and extraction) tests with a wide range of foundation and soil types, which may partly explain the good results obtained with the method. The method, also known as the LCPC method, is summarized in the following figures.
Figure 1. Bearing capacity factors, kc
Figure 2. Friction coefficient, alpha
The pile unit end bearing, qp, is calculated from the calculated equivalent average cone resistance, qca, multiplied by an end bearing coefficient, kc (Figure 1). The pile unit side friction, fp, is calculated from measured qc values divided by a friction coefficient, αLCPC (Figure 2).
Maximum fp values are also recommended based on pile and soil type. Only the measured CPT qc is used for the calculation of both side friction and pile end bearing resistance. This is considered an advantage by many due to the difficulties associated in interpreting sleeve friction (fs) in CPT data.
The equivalent average cone resistance, qca, at the base of the pile used to compute the pile unit end bearing, qp, is the mean qc value measured along two fixed distances, a, (a = 1.5D, where D is the pile diameter) above (-a) and below (+a) the pile tip. The authors suggest that qca be calculated in three steps, as shown in Figure 3. The first step is to calculate q'ca, the mean qc between -a and +a. The second step is to eliminate values higher than 1.3q'ca along the length -a to +a, and the values lower than 0.7q'ca along the length -a, which generates the thick curve shown in Figure 3. The third step is to calculate qca, the mean value of the thick curve.
Figure 3. Procedure for qca calculation
