A comparison of in-situ instruments to assess settlement in windswept sands

Abstract

Some residential homes in the Cape Peninsula, built on windblown (Aeolian) sand, have suffered structural failure to varying degrees. Severe consolidation settlement has been blamed for the damage. To assess the strength parameters and other influential properties of the sand, a suitable instrument must be calibrated and used in a controlled environment, as results for consolidation settlement can differ vastly. The instrument chosen must also be able to predict unfavourable internal stress conditions. To solve the problem of varying results, two in-situ testing instruments were chosen that have been used before to predict settlement. Samples were collected from a construction site where settlement problems have been known to exist. Dynamic Cone Penetrometer (DCP) tests were done in the bottom of exposed trial holes and sand samples were removed and exported to the calibration chamber, built at the Cape Peninsula University of Technology in Cape Town. Flat Plate Dilatometer (DMT) test results under controlled conditions were then compared to the results obtained by the DCP. Pilot study findings have determined that these sands (in their natural insitu density and level of contamination) pose a risk to dwellings if the necessary precautions are ignored. Consolidation settlement will occur if low levels of in-situ lateral stresses are present and when relative densities, (DR), drop below 60%.