Improvement of soft soils using geogrid encased stone columns

Topics: Tensile strength, Column, Sand Pages: 21 (3904 words) Published: May 4, 2014
Geotextiles and Geomembranes 27 (2009) 167–175

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Geotextiles and Geomembranes
journal homepage: www.elsevier.com/locate/geotexmem

Improvement of soft soils using geogrid encased stone columns Joel Gniel 1, Abdelmalek Bouazza*
Monash University, Department of Civil Engineering, Building 60, Melbourne, Victoria 3800, Australia

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 16 May 2008
Received in revised form
4 November 2008
Accepted 5 November 2008
Available online 19 January 2009

In recent years, geotextile encasement has been used to extend the use of stone columns to extremely soft soils. Although the technique is now well established, little research has been undertaken on the use of other encasement materials such as geogrid. This paper discusses the results of a series of small-scale model column tests that were undertaken to investigate the behaviour of geogrid encased columns. The tests focused on studying the effect of varying the length of encasement and investigating whether a column that was partially encased with geogrid would behave similarly to a fully-encased column. In addition, isolated column behaviour was compared to group column behaviour. The results of partially encased column tests indicated a steady reduction in vertical strain with increasing encased length for both isolated columns and group columns. Bulging of the column was observed to occur directly beneath the base of the encasement. A significant increase in column stiffness and further reduction in column strain was observed for fully-encased columns, with strain reductions in the order of 80%. This range of performance may lend the techniques of partial and full geogrid encasement to a series of potential site applications.

Ó 2008 Elsevier Ltd. All rights reserved.

Keywords:
Geogrid encasement
Model tests
Stone columns
Reinforcement

1. Introduction
The increasing infrastructure growth in urban and metropolitan areas has resulted in a dramatic rise in land prices and lack of suitable sites for development. These factors have forced the building industry to look for cheaper land for construction. As a result, construction is now carried out on sites which, due to poor ground conditions, would not previously have been considered economic to develop. These types of land include low-lying areas of marine and estuarine Quaternary deposits which are characterized by very poor geotechnical properties due to their low strength. Various ground improvement techniques have been employed in

order to artificially improve the soil properties in these sites (Bouazza et al., 2006; Abuel-Naga et al., 2006; Chu et al., 2006; Rowe and Taechakumthorn, 2008; Liu et al., 2008; Bergado and Teerawattanasuk, 2008; Chen et al., 2008; Li and Rowe, 2008). One of the techniques extensively used in soft soils is vibroreplacement, which consists of replacing some of the soft soil with crushed rock or gravel to form an array of stone columns beneath the foundation. Although the use of conventional stone columns in soft soil deposits was found to benefit foundations in many respects (Madhav and Miura, 1994; Priebe, 1995; Muir-Wood et al., 2000);

* Corresponding author. Tel.: þ61 3 9905 4956; fax: þ61 3 9905 4944. E-mail addresses: Joel.Gniel@eng.monash.edu.au (J. Gniel), Malek.Bouazza@ eng.monash.edu.au (A. Bouazza).
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Tel.: þ61 3 9905 5549; fax: þ61 3 9905 4944.
0266-1144/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2008.11.001

they are generally suited to soils with an undrained cohesion (cu) above 5–10 kPa. Below this strength, the lateral support provided by surrounding soil may be insufficient to prevent column failure through excessive radial expansion. In the last twenty years, this limitation has prompted investigation into geosynthetic column encasement to provide the required lateral support to columns installed in...
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