The use of the cone penetration test to derive parameters for shallow foundation design

In uncemented sands, the second part of Equation 1 reduces to zero, and the non geometrical variables are the bearing capacity factors. As noted by Randolph et. al. (2003), the accuracy with which bearing capacity factors for a given soil can be determined has increased substantially in recent years (See Martin 2003), and the main uncertainty in the implementation of equation 1, lies in the choice of an appropriate friction angle. The primary complication being the difficulty in sampling cohesionless soils to allow strength testing on representative samples, and the stress-dependent effects on choice of friction angle. To overcome some of these problems greater reliance is being placed on in-situ tests such as the Cone Penetration Test (CPT) to derive engineering parameters such as the friction angle, or through the use of empirical formulae to estimate the bearing capacity of the soil. Randolph et al. (2003) summarise the results of tests performed on shallow foundations, and buried piles in the laboratory and field, and reported ratios of the bearing pressure (q) at displacements of 5 and 10% of the foundation diameter, normalized by the CPT end resistance (qc). The data suggest q/qc ratios of 0.09–0.16 at displacements of 5% of the foundation diameter, increasing to 0.13-0.21 at 10%. Due to the relatively large ultimate bearing resistance of sand, actual foundations are usually remote from failure and the designer is primarily concerned with estimating the settlement of the footing under working stress conditions. This is typically achieved by assuming that in this stress range, the foundation response can be assumed to be linear-elastic. Once again the key difficulty for the designer is the choice ABSTRACT: