Evaluation of the Performance of Geotextile Reinforced Red Coffee Soil Embankments with Sand Cushions Subject to Rainfall Event

Abstract

Geosynthetic reinforced soil (GRS) structures are in a period of enormous growth in Kenya’s infrastructure projects. This is due to their numerous advantages over conventional retaining structures which include improved stability and reduced base area in the construction of the structures. However, economic and environmental constrains have necessitated the use of locally available fine grained soils as backfill material contrary to design guidelines recommendations. The guidelines provide that backfill material should be non-cohesive sand. The main deterrent in the use of fine grained soil as backfill material is its inability to quickly drain water, leading to build up of pore-water pressures. The use of nonwoven geotextile drains within these backfills has been suggested in the reduction of the pore water pressure. However, it is recognized that the nonwoven geotextile retard water due to capillary barrier effect under unsaturated soil conditions and start to act as a drainage material only when the soil immediately above it became nearly saturated. The aim of this study was to evaluate the performance of red coffee soils embankments when incorporated with sand layers to aid in drainage. Specifically the study evaluated the effect of inclining non-woven geotextile layers on the performance of red coffee soils embankments, it evaluated the effect of the sand layer thickness on the performance of the red coffee soils embankments and looked into the effect of drainage layers spacing on the performance of the red coffee soils embankments. The numerical model SEEP/W that employs the use of the elliptical transient Richard’s equation was adopted in the analysis of infiltration into the unsaturated soil backfill while the Spencer method; a limit equilibrium method of slope stability analysis in SLOPE/W was used in evaluating the stability of the embankments. Findings from the numerical study showed that non-woven geotextile became more effective in drainage when inclined at 3° above the horizontal in red coffee soils embankments. Sand cushion layers improved both the drainage and stability of the red coffee soils embankment in long term rainfall events. It was concluded that sand layers can be safely used to improve on the drainage and stability red coffee soils embankments subject to long duration rainfall events.