In situ testing in low-medium density structured chalk

Chalk, a soft, white, variable, high porosity, rock has been the focus of recent research into pile behaviour following the rapid expansion of offshore windfarms in Northern Europe and the advancement of other major infrastructure projects in areas where foundations are installed in chalk. An overview of recent in-situ testing (cone penetration tests and geophysical tests) at low-medium density chalk sites is presented. Typical cone penetration test profiles are discussed, which illustrate that while cone resistance and friction ratio in chalk lie within the typical ranges for sands and clays, pore pressures are remarkably high and dissipate quickly, leading to partial drainage occurring during cone penetration tests conducted at standard velocities. The CPT cone resistance increases markedly with reducing penetration rate due to increasing degrees of drainage developing around the advancing cone tip. While both cone resistance and friction ratio have been shown to increase with improving chalk grade, a consistent method to classify chalk grade using CPT parameters has not yet been developed. Shear wave velocities measured in the field through geophysical testing fall below the trends interpreted from laboratory tests on intact samples, which may be attributable to the presence of fractures in the chalk mass or differences in applied effective stress levels. However, the cone resistance profiles developed in structured chalk provide sensitive indicators of local variations in key factors that affect mass shear strength and density. This observation has encouraged the authors to propose CPT based design methods for piles driven in chalk.