Capacity of continuous flight auger piles in the Johannesburg CBD

Abstract

Continuous Flight Auger (CFA) piles were installed in residual igneous soil for a structure built in the Johannesburg CBD. The residual soils are low density, fine grained clayey silts derived from the weathering of diabase and andesite but have appreciable consistency below 12 m depth. The CFA piles, which relied on shaft adhesion capacity, were designed mainly on the basis of SPT related undrained shear strengths and employing an appropriate adhesion factor to reflect the anticipated degree of smear during the installation process. The piles were installed using conventional design and piling techniques. However, the failure of a test pile to meet performance specifications resulted in the downgrading of the pile capacity. This paper covers the geological setting of the site and the soil engineering properties. The design approach is discussed as well as the subsequent behaviour of the test piles and the possible reasons for the unexpected behaviour of the test piles. The assumptions made when designing CFA piles are critically examined and the value of pile tests in advance of pile construction are discussed.