The interpretation of standard penetration tests in geologically aged sands

Abstract

AbstractThe standard penetration test represents a traditional and convenient method for the assessment of the engineering properties of cohesionless soils for the purpose of foundation design. This test, however, is not a direct measure of the static load/displacement characteristics of a sand and hence it is an empirical one in which the blow count is correlated firstly with the in situ density and shear strength obtained by other methods and, secondly, with case records of foundation performance. Most, if not all, of the existing correlation charts are based either on measurements of density and shear strength in remoulded sands or on the performance of fills or recent sands. The unique properties of sands which have geologically aged (that is been subjected to a variety of diagenetic processes) are often ignored with the erroneous assumption that the correlation charts will apply equally to all sands.The paper examines the penetration resistance of uncemented sands in Southern Britain ranging in age back to the Jurassic. It is seen that there is a marked difference between tests carried out well above the groundwater table (dry tests) and those either below or in close proximity to it (wet or saturated tests). Only dry tests give the high resistances expected from the very dense and strong geologically aged sands. These results show an exponential increase in blow count versus depth and vertical effective pressure. Reasons for the reduced resistances measured in water filled boreholes are discussed. Of significant influence is the markedly moisture sensitive strength behaviour of ancient sands. In addition, the conditions of the test are such that other adverse effects, inevitable with conventional boring operations, can be operative and hence weaker foundation characteristics may be assumed than is actually the case in the undisturbed sand.