Reliability of CPT measurements in sand – influence of spacing

Geotechnical methods, such as cone penetration tests (CPTs), standard penetration tests (SPTs) and seismic cone penetration tests (sCPTs), are widely used to characterise the in situ properties of soil. In studies where more than one in situ method in close spacing is used, it is important for geotechnical engineers to balance between soil heterogeneity and the artefacts of disturbed testing zones. The same applies if detailed information about the subsoil is required and one in situ method is used in close spacing. There is no consensus on how to define the minimum spacing between testing zones and no study on the effect of disturbance caused by in situ tests. In this study, 33 CPTs were performed in natural sediments in northern Germany and a spacing threshold was defined at which cone resistance is affected by soil disturbance from previously performed CPTs. The CPTs were performed sequentially by successively refining the grid spacing, starting with a spacing of 119 cone diameters between CPTs, to a fine grid spacing of seven cone diameters. The cone resistance is affected by previous CPT measurements below a spacing threshold of 24 cone diameters in medium-dense sands. Silt and clay layers showed no reduction in the cone resistance for the minimum grid spacing of seven cone diameters.