Vegetated retaining walls – a sustainable bio-engineering method to stabilise slopes

Abstract

Stabilization of natural and man-made slopes across different regions in Sri Lanka is a major engineering problem. The conventional approach of constructing retaining structures like masonry, concrete, or gabion walls has traditionally been adopted for enhancing slope stability. However, the cost of building and sustaining these traditional systems keeps rising with each passing year. Therefore, soil bioengineering has been demonstrated to be a cost-effective and sustainable option where vegetation is supplemented with structural elements to enhance slope stability. One such bioengineering application is the vegetated crib wall, a gravity retaining structure with a special form of structural support with live vegetation. A crib wall is typically composed of a framework of various materials filled with in-place soil or other fill material. In the current study, bamboo was used as the primary structural material due to its renewable nature, strength, cost-effectiveness, and ecological sustainability. The walls of the bamboo cribs were incorporated with live plant cuttings or vegetation roots, constituting a living system that not only provides immediate structural strength but also assists in long-term slope strengthening through root growth. In this research, numerical modelling is employed in PLAXIS 2D software to determine the stability of bamboo crib walls. For two most vital stages: (1) instant stage following the construction when there is limited vegetation contribution and (2) after that bamboo-growing stage, the root system strengthens slope stability through natural processes, two approaches of assessment are based on factor of safety (FOS), serving as a chief measure of stability of slope. Comparative analysis between concrete crib and bamboo walls indicated that while bamboo walls offer slightly lower FOS initially, they enhance long-term slope stability considerably because of vegetation growth. Further studies were carried out to investigate the effect of important design parameters on FOS, including wall width, embedment length, water table level, and the type of fill material used. Observations indicate that wall width, embedment depth, and elevation of the water table are good indicators of the overall stability of bamboo crib walls. Vegetation, including bamboo, was found to extend functional life of the structure and long-term slope stability. The study analyzed how water table levels influence the factor of safety (FOS) using wall widths of 1.5 m, 2.0 m, and 2.2 m. Two water table shapes; horizontal and inclined were examined. Bamboo was considered non-porous, while both existing and fill soils were treated as drained materials Also, design parameters such as base slope, the angle of the crib wall was found to be crucial parameters that needed more analysis. Incorporating drainage in the crib wall was also suggested to enhance overall performance. Finally, a detailed cross-sectional design of the bamboo crib wall with structural and vegetative components is illustrated based on the findings of this research.