Using small-strain stiffness to predict the settlement of shallow foundations on sand

Abstract

Settlement prediction of shallow foundations is an essential part for the design of a structure. Accuracy is important, and to increase the accuracy, reliable input parameters are required. However, reliable input parameters do not necessarily render accurate results since the parameters used in design are only as good as the prediction method. Foundation design on sands consists of two aspects: bearing capacity and settlement, with settlement being the governing factor in almost all cases. Settlement prediction is not always an easy task. The main reason being that engineers are unable to measure stiffness accurately as well as how to use the stiffness value measured in certain settlement methods. An important soil stiffness value is the initial small-strain shear modulus (G0) which can be obtained relatively easy with in-situ test methods and the values obtained are generally reliable. With in-situ testing becoming increasingly popular, settlement prediction methods utilising the initial small-strain shear stiffness are also becoming more common. The main objective of this project is to determine whether the load-settlement behaviour of a shallow foundation can be estimated accurately using only the small-strain shear modulus of the granular soil below the foundation. Centrifuge tests were conducted on an equivalent 5m circular shallow foundation at three different density sands to establish if the stress-settlement behaviour can be predicted and to what level of accuracy. The different density sands were loose, medium dense and dense sands. Bender- and extender elements were used to determine the small-strain stiffness data which were used for the analysis. The main conclusion drawn from the study is divided into two parts; Full-range load-settlement behaviour and Practical-range load-settlement behaviour. The full-range results relate to full stress-settlement curve with the practical-scale results up to settlements of 0.1D. The method proposed for the load-settlement prediction is a non-linear stepwise method. The proposed method requires utilising a stiffness degradation curve and it was found that curves presented by Oztoprak & Bolton (2013) and Bolton & Whittle (1999) produced the best results. For the full-range load-settlement results, accurate predictions were found between the predicted and measured load-settlement curves for low density sand. However accuracy decreases with increase in density. For settlements up to 0.1D the load-settlement behaviour was predicted with reasonably good accuracy. The study showed that the settlement of a shallow foundation can be predicted with reasonable accuracy up to 0.1D settlement using only the small-strain shear stiffness value of the soil below the foundation and that there is merit in the proposed method for future use.