Modulus to SPT Blow Count Correlation for Settlement of Footings on Sand

Generally, the first choice in the selection of a foundation for buildings and other structures is a shallow foundation because it is typically more economical and easier to construct than a deep foundation. However, shallow foundations are usually considered to settle more than deep foundations. This is a fallacy as well-designed shallow foundations do not settle more than well designed deep foundations. To calculate the settlement of shallow foundations on sands, the most critical parameter is the soil modulus. This parameter is influenced by a number of factors and the range of variation is large. Comparison between observed settlement and measured settlement can help to calibrate the soil modulus which best matches the settlement measurements. In this article, a probability analysis based on a database of 315 shallow foundation settlement records on sand “TAMU-SHAL-SAND” is presented. The foundation width in the database varies from 0.30 m to 135 m with a mean of 7.76 m and a standard deviation of 9.42 m. The SPT N values are between 2 blows per foot (bpf) to 60 bpf with an average of 23.68 bpf and a standard deviation of 13.26 bpf. Using the database, scatter plots of predicted vs measured settlement are presented. In addition, a probability plot is presented with the probability that the predicted settlement will be less than the measured settlement on the vertical axis and a prediction multiplier on the horizontal axis. The results can help the practitioner decide at which level of probability to operate. The data show that using E (kPa) = 1,000 N(bpf) give a settlement which has a 90% probability of being larger than the measured settlement.