Predicting Settlements of Shallow Foundations on Cohesionless, Granular Soil Using Dynamic Soil Measurements

Download or read book Predicting Settlements of Shallow Foundations on Cohesionless, Granular Soil Using Dynamic Soil Measurements written by Jeffrey Donovan Van Pelt and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The design of shallow foundations is typically governed by the limiting factors of bearing capacity and permissible settlement. However, in many cases of freely draining granular soils, the foundation design is controlled by the permissible settlement. The three main components needed to predict footing settlements in granular material are; (1) the size of the footing, (2) the anticipated applied footing pressure (or range of pressures), and (3) the stiffness of the soil beneath the footing. Current methods used in engineering practice to predict settlements in granular soils are typically quite conservative and can result in unnecessarily expensive foundations. The primary goal of this study was to investigate the use of moduli that are assessed by in-situ seismic tests in the prediction of settlements of shallow foundations on cohesionless, granular soils. These settlement predictions are made in the stress ranges typically encountered over the design life of a footing. Two reinforced concrete footings, with diameters of 36 and 18 in., were constructed at a field test site on soil generally classified as loose to medium dense, nonplastic sandy silt according to the Unified Soil Classification System. In-situ field testing was performed at the project site. These tests included the spectral-analysis-of-surface-waves and crosshole seismic tests, and traditional cone penetration testing. The measured shear wave velocities and cone tip resistances were then used to predict the footing settlements using the analytical methods proposed by Schmertmann and Burland and Burbidge. Load tests were performed on the two footings, and the measured settlements of the footings, as well as of the soil mass beneath the 36-in. diameter footing, were recorded and compared with the predicted settlements. The results of the load-settlement comparisons indicate that, at low applied footing pressures in the working stress range, the seismic-based in-situ tests provide a more accurate estimate of the soil stiffness and predicted settlements with Schmertmann's method than conventional SPT or CPT tests. The settlements predicted using the in-situ CPT test for the two footings were found to be consistently larger than the measured settlements in the working stress range, however the overall performance of the method was very good, particularly at applied stress near the upper bound of the working stress range and measured settlements of approximately 1 in. Also, a framework to use empirical, back-calculated modulus reduction factors to someday predict settlements in granular soils was developed. The measured load-settlement results were compared with the results from a previous study and potential relationships were found to exist. Further studies with varying footing sizes on different types of granular soil will determine if these relationships are applicable to practicing engineers.