Improving the Performance of the Square Shallow Foundations using Inferior Skirts

Development of the new type of improvement methods of shallow foundations in the geotechnical engineering, it seems necessary to study the performance of vertical plates beneath the periphery of shallow foundations known as skirted foundations. Skirted foundations are steel or concrete foundations, which have a top raft and a relatively thin plate constructed beneath the periphery. The skirts penetrate the soil beneath foundation which encompass and confine the soil very firmly. The skirts and confined soil behave as a unit to transmit loads to the soil at the level of skirt tip. The benefit of skirted foundations compared with deep foundations such as piers and piles lies in their ease and short time of installations. The skirted foundations can be used as an appropriate alternative for shallow foundations, pier and deep foundations in applications such as oil and gas storage tanks, wind turbines, oil drilling platforms, harbor, and offshore and jacket structures. The behavior of square shallow foundations resting on confined sand by vertical plates as skirt, was studied using small scale physical modeling in the laboratory. The effects of parameters including ratio of skirt length to foundation width, foundation size, the shear strength of sand, and roughness of skirt and foundation surfaces on skirted foundations behavior in terms of increasing bearing capacity, the settlement reduction, and improvement of subgrade reaction modulus were assessed under compression loading, and results compared with the performance of shallow and pier foundations. Modeling test analyses revealed the overall improvement of square skirted foundations performance compared to shallow and pier foundations. Results of this study showed that the values of bearing capacity and settlement of skirted foundations are almost close to those of pier foundations of the same width and depth. Also, it was observed that the existence of skirt, increases the bearing capacity and modulus of subgrade reaction, together with decrease of settlement of shallow foundation. The enhancement in the bearing capacity, as well as reduction in the settlement of shallow foundations increases with increasing skirt depth and decreasing the shear strength of sand. So that, bearing capacity ratio (BCR) of skirted foundations to surface foundations was observed in the range of about 2.4 to 5.1 times for the different values of L/B. Furthermore, the value of skirted foundation settlement decreased up to 91% of that a surface foundation in the case of having skirt depth/ foundation width of 2.0. The modulus of subgrade reaction improved in the range of about 1.5 to 4.23 times duo to skirt existence. From the accomplished laboratory tests, it was found that skirted foundations resting on loose sand, are more beneficial than in case of resting on medium and dense sand. In cases where structures are very sensitive to settlement values, the skirted foundations can be used to gain the same allowable bearing capacity a much lower settlement. In this paper, based on the analysis of the obtained results, charts and equations are presented to estimate bearing capacity and settlement and subgrade reaction modulus of skirted shallow foundations in terms of those of surface foundation and sand relative density, skirt depth to foundation width ratio and its roughness.