Experimental and Numerical Modeling of Soft Soil Improvement using Compacted Lime Mortar Columns

Construction of roads, railways and other engineering structures over soft soils usually involves consideration of excessive settlements, deformations, and stability problems. To avoid or reduce such problems, several ground improvement techniques are available including the use of column type elements. Stone columns represent the most known column-type techniques for improving soft soils. It is well established that the stone columns derive their load carrying capacity from the lateral confining pressure provided by surrounding soil. Since intermediate layers consisting of organic or very soft to soft soils can not provide proper lateral support to the stone columns, the external lateral support can be replaced in these columns by an internal bond between the stones. This can be done by applying binders such as lime during the installation of the stone/gravel columns, which bind the gravels together and form a solid column. An alternative method is suggested in the present study to improve the performance of stone columns in soft soils by using compacted lime mortar columns. Laboratory tests are carried out on large scale composite specimens composing of compacted lime mortar columns and soft clay surrounding it. Experiments were carried out on the both end bearing and floating columns with dimensions of 100 mm diameter, and 600 mm height. The columns were surrounded by soft soil in cylindrical tanks of 600 and 800 mm height, and a diameter varying from 230 to 320 mm to represent the required unit cell area of soft soil around each column assuming triangular pattern of installation of columns. In the field, the entire of the column treated ground will be subjected to loading from the superstructure. The same was simulated in the laboratory by loading the entire area of the specimen to study the stiffness of improved ground with regard to the influence of different parameters such as: area ratio (as the ratio of the area of the column (AS) to the total area within the unit cell (A)), and shear strength of the clay. Finite element analyses have also been performed with the software package PLAXIS V 8.2 using 15-noded triangular elements. Tests results show that compacted lime mortar columns exhibit a stiffer and stronger response compared to conventional stone columns installed in soft soils. These columns improve the load carrying capacity and decrease the settlement of soft soils. The performance of these columns is significantly enhanced by increasing the area ratio. However, when the area ratio exceeds 15%, the rate of increment of the load carrying capacity is negligible. When the entire area of the composite specimens is loaded, because of the confining effect of the boundary of the unit cell, failure does not take place even for settlement as high as 15 mm. Comparison of numerical and experimental data shows a good agreement between the results.