Kandy is registered as a UNESCO World Heritage Site and is the second largest city in Sri Lanka after the capital Colombo. Kandy is both an administrative and religious city housing the Temple of Tooth which is one of the most sacred places of worship in the world. The city lies on the Kandy plateau which is mountainous and thickly forested. With the population growth of Kandy it was necessary to construct a modern wastewater treatment plant in order for clean effluent water to be discharged into the city’s waterways, and to prevent the existing practice of discharging untreated wastewater directly into these waterways. The wastewater treatment plant was located in a hilly area on the outskirts of Kandy.
Thus, to provide the necessary flat platform areas reinforced slopes were utilized on site. A reinforced slope of two tiers, consisting of gabion facing units with layers of geocomposite reinforcement, was used to enable the required level platform areas for the wastewater treatment plant and sludge drying beds. The reinforced slope was designed in accordance with BS8006:2010, a British Standard Code of Practice for Reinforced Soil. Rock filled gabion facing units, measuring 1 m wide by 1 m high by 1 m deep, were chosen for their hard, durable facing and aesthetic appearance. The slope facing angle was created by stepping each gabion layer half-way back at each level thus creating a slope angle of 65 degrees (2:1).
The main slope reinforcement was MIRAFI® Polyfelt PEC geocomposite reinforcement placed at vertical spacings of 0.5 m. MIRAFI Polyfelt PEC geocomposite reinforcement has well-defined long term engineering properties which makes it ideal for reinforced soil applications. Site preparation works involved excavation of the hill slope to form the foundation platform for the reinforced soil structure. At the base of the slope below the first reinforced slope tier a reinforced platform, 1.5 m deep, was constructed using reinforced fill and three layers of MIRAFI Polyfelt PEC100 geocomposite reinforcement. This was to provide a stable base for the construction of the two-tier reinforced slope. At the base of each tier a subsurface drainage blanket was constructed consisting of granular drainage material wrapped in a MIRAFI Polyfelt TS nonwoven geotextile filter. This drainage blanket was continued up the rear of the two reinforced soil zones to intercept any seepage water coming from the rear of the slope. A drainage pipe was installed to enable groundwater flows from the subsurface drainage blankets to pass into external drainage channels. The reinforced fill used for the reinforced slope was a good quality residual soil. This was laid in lifts and compacted to 95% Standard Proctor density. The compacted reinforced fill lifts were 0.25 m in thickness. The reinforcements used in the reinforced slope were MIRAFI Polyfelt PEC100 in the bottom tier and MIRAFI Polyfelt PEC in the upper tier. The reinforcements were installed at 0.5 m vertical spacings. Where a layer of MIRAFI Polyfelt PEC coincided with the top and base of a gabion facing unit, it was continued between the gabion units thus creating a frictional connection between the reinforcement and gabion units. Where a layer of MIRAFI Polyfelt PEC coincided half-way up a gabion facing unit, it was returned through the gabion unit and brought back into the reinforced fill. After laying the MIRAFI Polyfelt PEC geocomposite reinforcement it is pulled taunt and reinforced fill placed on top and compacted. A MIRAFI Polyfelt TS nonwoven geotextile filter was placed behind the gabion facing units to intercept any seepage water emanating from the reinforced fill zone. Following completion of the reinforced slope, a road pavement was constructed on top of the setback platform to enable access to the wastewater treatment and drying beds area.
Compaction of reinforced fill
Gabion faced reinforced slope partially completed
Laying MIRAFI Polyfelt TS geotextile filter for the drainage blankets
Placing reinforced fill on top of MIRAFI Polyfelt PEC geocomposite reinforcement
Typical cross section through the gabion faced reinforced slope
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