SOIL IMPROVEMENT USING REINFORCED GRANULAR COLUMNS

Abstract

Stone column is an effective and economical form of ground improvement technique employed for soft and weak soil deposit which meets high bearing capacity requirements. Stone columns occupy an important place and have a major role in ground treatment methods. Their use in reinforcing soft soils has demonstrated their usefulness and makes them one of the most attractive methods in soil improvement techniques. Grouping encased stone columns are columnar structures constructed below the ground that serve as foundations in weak soils and are used to improve their bearing capacity. There is an increase use of group of stone columns in sites with poor soils to support structures like oil tank, earthen embankments, raft foundations….etc, where a relatively large settlement can be tolerated by the structure. ESC's had been used to minimize post construction settlements under different loaded structures. The Finite element model for grouping of ESC is used to study the parameters that fulfilling a limiting displacement criterion under -182- maximum stress and a good bearing resistance of weak soil. The Finite element models are conducted on a footing supported by single, four, five- and nine-stone columns in group of ordinary and encased stone column. Different parameters are investigated to clarify their effect on the settlement and the bearing capacity using 3D Finite element. i) Number of stone column in the group (1, 5, 9 SC’s). ii) Spacing between stone columns in grouping medium. iii) Loading area on stone column. iv) Footing thickness. v) Studying the influence of type of loading applied on footing, (point and distributed loads). vi) Arrangement of ESC and OSC in the load-carrying capacity of the composite medium. vii) Position of stone column in the group. The above parameters are investigated to indicate their effect on settlement and the bearing capacity using 3D FE analysis considering the ESC interface model. -183- 5.2. Numerical Modeling 5.2.1 Material Models Soft soil is modeled as modified cam clay materials. The modified cam clay model requires five material parameters for soil type, as shown Table 4-1. The stone column and the bearing layer are modeled using a linear elastic-perfectly plastic model with Mohr– Coulomb failure criterion. The encased geosynthetic material is modeled as an isotropic linear elastic material with a tensile stiffness (J) of 3000 kN/m (Eg=600Mpa) and a Poisson’s ratio of 0.3. Twenty node stress-pore pressure coupled brick elements with reduced integration (designated C3D8RP) are used to represent the clay layer , the stone column and the bearing layer, while 20 node brick elements with reduced integration (designated C3D8R) are used for the footing . The geosynthetic is modeled using 4-node quadrilateral, reduced integration membrane elements (M3D4R). The 3D FE mesh model shown in Fig. (5-1) consists of 20835 nodes and 16640 elements. The vertical stress is applied on the raft supported by stone columns incrementally