Study on the Settlement Response of Soft Ground Improved by Granular Columns

Abstract

Organic soil is generally encountered at a depth of 10 to 25 ft from the existing ground surface in the Khulna region, the South-west part of Bangladesh. Due to inherent geotechnical properties of this soil, i.e. high moisture content, high compressibility and low shear strength; conventional foundation systems are not suitable for this ground. To overcome such problems, soil improvement techniques are to be adopted to provide safe and economical foundation systems. In many ground improvement techniques, geotextileis used to enhance the performance of the adopted foundation systems. Granular columns overlain by compacted sand bed with geotextile is one of such systems, the behaviour of which needs to be examined experimentally. This dissertation describes the load-settlement behaviour of treated and untreated reconstituted organic grounds constnicted in the laboratory. The reconstituted organic grounds or the test grounds were prepared in a cylindrical tank based on "unit cell" concept. There were mainly two layers in the test grounds. The bottom layer was compacted sand bed performed as a good filter media and the top layer was reconstituted organic soil represented the problematic ground of Khulna region. The reconstituted organic grounds were prepared under pre-set pre-consolidation pressure and different ground improvement techniques were applied on it to find out effective solution for organic soil. The test grounds were treated by four conditions - compacted sand column, compacted sand bed with and without geotextile and compacted sand bed with geotextile in conjunction of compacted sand column. The load-settlement responses of the treated and untreated test grounds were determined by footing load test. Laboratory investigation reveals that the compacted sand bed with and without geotextile improve the bearing capacity of organic grounds 1.5 times than untreated grounds. The results also showed that degree of improvement increases significantly due to the installation of compacted sand column with compacted sand bed with geotextile over it. This system improved the bearing capacity of footing more than two times than that of placed on untreated ground. The use of geotextile not only improves the bearing capacity but also ensures flatter settelement pattern against the increase of load intensity. The study reveals that the available empirical equations are suitable for the prediction of bearing carrying capacity of footing resting on the sand column and compacted sand bed treated grounds but highly over predicts the same in case of grounds treated by compacted sand bed with geotextile and compacted sand bed with geotextile in conjunction of compacted sand column.