An investigation into ground improvement using geogrid encased stone columns
thesisposted on 19.01.2017, 02:53 by Gniel, Joel Robert
Conventional stone columns are commonly used as a form of ground improvement in soft soils, for the support of lightly and moderately loaded structures such as embankments. However, their use in very soft and extremely soft soils is limited by the low stiffness and minimal confinement provided by the soft soil. To extend their use to such soft soils, a method of geotextile encasement has recently been developed, providing additional circumferential confinement. The technique has been used on numerous projects throughout Europe and more recently in South America. Although geotextile encasement provides a practical form of ground improvement, its use can be limited in some cases by excessive settlements, resulting from the adopted materials and installation practices. To investigate the potential benefits of using a stiffer encasement than geotextile (and to broaden the appeal of geosynthetics in ground improvement), the use of geogrid encasement is investigated. The research presented in this thesis was used to investigate practical aspects of geogrid encasement including developing effective and efficient methods of encasement construction and assessment of encased column performance. The research was undertaken using a four-stage approach comprising small-scale laboratory testing, numerical simulation of small-scale tests, medium-scale laboratory testing and scaled-up numerical modelling of full-scale columns. Small-scale testing was undertaken on isolated and simulated group columns to investigate whether the full-length of the column needed to be encased, the impact of geogrid stiffness and methods of constructing the encasement. Numerical modelling was undertaken using the PLAXIS software package and was initially used to reproduce the small-scale test results. Following this, the models were scaled up to investigate the impact of different parameters on full-scale encased column behaviour. Medium-scale testing of unconfined columns was used to investigate methods of encasement construction including the suitability of different geogrids and stone column aggregates. The research indicates that geogrid encasement can be constructed at relatively low cost and most effectively by constructing sleeves with a full circumference of overlap, fixed in position using cable ties. The technique relies on interlock between the overlapped section of geogrid and protruding aggregate to provide a level of fixity similar to welding. Biaxial geogrids provide the stiffest and most reliable encasement material, particularly when used with typical stone column aggregates. Based on the results of modelling and testing, geogrid encased columns are expected to reduce untreated settlements by between 50% and 95%, depending on properties such as geogrid stiffness, column density, soil stiffness and encased length. By progressively increasing the replacement ratio, geogrid stiffness and the encased length of a column, the stiffness of the treated soil mass may be steadily increased. Although the research indicates that geogrid encasement is likely to provide a stiffer alternative to geotextile, site testing is recommended to confirm some aspects of performance, including installation techniques.