Use of EPS block geofoam with internal drainage for sandy slopes subjected to seepage flow

Abstract

Subsurface drainage is considered to be an integral part of slope remediation systems where piezometric pressures may threaten slope stability. This is especially true if one considers the use of expanded polystyrene (EPS) block geofoam (geofoam block) for slope remediation. Due to its light weight, geofoam blocks are commonly used to replace the heavy in-situ soil, hence reducing the driving force that can cause global stability slope failure. On the other hand, due to its light weight, geofoam blocks are susceptible to adjacent earth pressures and hydrostatic pressures which may lead to sliding between blocks and/or between EPS/foundation soil interfaces. For this reason, the design precedence requires the use of permanent drainage systems to alleviate hydrostatic pressures and a retention system if the back slope is not stable. In this study, the behaviour of a geofoam block slope system experiencing seepage was investigated through laboratory experiments. For this purpose, a lysimeter with dimensions of 60 cm height, 20 cm width, and 200 cm length was constructed in the laboratory. Geofoam block slope systems were constructed by compacting sand in 2.5 cm lifts to a height of 55 cm. The geofoam blocks (2.5 cm height, 5 cm width, and 15 cm length) were placed on the sandy slope face with an angle of 45 degrees (1:1 horizontal: vertical) in "One Row" and "Two Rows" configurations. The experiments were conducted under water pressure head held constant in the water reservoir located at the opposite end of the lysimeter from the geofoam blocks. In order to facilitate efficient piezometric pressure dissipation within the back slope, an internal drainage system was incorporated by grooving dual drainage channels (weep holes) on the top and bottom side of the geofoam blocks. The lightweight geofoam blocks could not resist earth and hydrostatic pressures under seepage. The back-slope was not self-stable under seepage conditions, and deep-seated global stability failures were observed. For this reason, the internal drainage system was ineffective to dissipate piezometric pressures. This situation was also confirmed by a numerical model which predicted a factor of safety below the critical value for global stability for both geofoam block configurations. More elaborate geofoam block configurations and/or drainage systems should be designed to increase resistance against global stability failure caused by seepage forces.