سيدعلي هاشمي امام زاده

Deep soil mixing columns (DSM) is an effective method for improving loose soils. In the past three decades, various researches were done studying the load-settlement behavior of clayey soils improvement with DSM columns denoting the importance of this research area. This study eva‎luates the load-settlement behavior of an improved loose sandy base with DSM columns. Moreover, the effects of the cushion presence on bearing capacity, settlements, load distribution between soil and columns, and stress concentration ratio are determined in this study. The bearing capacity ratio (BCR) for different cushion ratio conditions was calculated and compared too. According to the high significance of the soil density, its constant value in all of the experiments, in addition to the negligible effect of walls and boundary conditions on experiment results, a large dimension, 1.5 × 1.5 × 1.1 m3 rigid box with a mechanized air pluviation and a loading system has been built for physical modeling. A square metal sheet with a side dimension of 40 cm and a thickness of 2 cm was used as a rigid raft. Loading was induced in strain-controlled mode, and the settlement of 10% after the rigid raft was taken as a criterion for finishing the experiment of untreated soil. The base soil was pluviated with 23% density, and DSM columns were cast in nine columns with an 18% improved area ratio. Seven experiments, including untreated soil, improved soil without cushion, and with 1.6, 3.2, 4.5, 5, and 5.5 cm cushions were conducted, respectively. The cushion with 1.6 cm thickness has increased bearing capacity and reduced the settlements of the system. An increase in the cushion thickness would result in asymmetric settlements and decrease the bearing capacity. Therefore, the optimum and the greatest BCR is for the 1.6 cm thick cushion. The testing of the model with a 1.6 cm thick cushion has a bearing capacity of 60% and 19% more than the untreated soil and the model without any cushions, respectively. Additionally, the settlement result of the model with a 1.6 cm thick cushion is 17 times less than the untreated soil model. The highest stress concentration ratio is reported for the model with a 1.6 cm thick cushion. The study results also demonstrate that according to reducing the share of columns in bearing loads and increasing the soil stresses, increasing the cushion thickness would lower the stress concentration ratio. Furthermore, a formula is developed relating the cushion thickness and stress concentration ratio.

Ali Hashemi Emamzade

Dr. Saber Mahani