Nov 16, 2015
Background
A Megawatt thermal power plant is under construction near Gandhidham, Gujarat India by OPG Power Ventures. A raw water reservoir was created with a storage capacity of 50,000 cubic meters, to stockpile and ensure constant supply of cooling water. It was lined with 22mil thick geomembrane to render the bottom and side slopes impervious. The geomembrane-lined slopes and bottom required protection from external damage and uplift pressures. Ideally, a reinforced concrete lining is installed above the geomembrane. However, due to steep slopes combined with the weight of concrete tiles or poured concrete as the case may be, the liner would slide down causing system instability. Using StrataWeb® geocells filled with lean concrete proved to be a technically superior and cost effective solution.
Problem and Objective
Strata Geosystems (India) Pvt Ltd was appointed in May 2013 to take up the work of protecting the reservoir slope and liner protection with concrete in-filled StrataWeb®. The side slopes of the reservoir were steep, at a gradient of 1V : 1.5H. Anchoring using stakes or pins was not an option due to the requirement for an impermeable geomembrane lining. Thus the weight of the entire mass of concrete-filled geocells was designed to be suspended at the top from anchors in trenches. Importantly, the reservoir was required to be completed quickly to fill up with rain water during the fast approaching monsoons, requiring urgency in execution.
Construction Overview
StrataWeb® SW660 with a 75mm depth was used in the design to provide adequate concrete thickness to prevent uplift of the geomembrane. From the bottom to the top of the slope, high strength polypropylene tendons were installed through pre-drilled holes in the cells at regular horizontal spacing to suspend the in-filled geocells from anchors at the crest. Tendons were also intertwined between two adjoining panels along the row to hold the panels together. Over the base of the reservoir, the panels were fully spread to cover the entire base. Adjoining panels were joined with staples and cable ties before being filled with concrete. Concrete of grade M15 with 100mm slump was placed with a concrete pump. Concrete placement was done first in the anchor trenches to secure the StrataWeb®panels at the top. Concrete was then poured from top to bottom and made to flow down the slopes. Final concrete dressing was done manually using spades and floats. Thickness of the finished concrete was 100mm and the concrete in-fill was later installed over the base of the reservoir with an overfill of 25mm above the top of the geocells.
Benefits to Clients
Wet concrete normally cannot be placed directly over sloping geomembranes due to inadequate friction between the geomembranes and the concrete. StrataWeb®offered a technical solution by providing cellular pockets to hold the concrete in place until it cured. Since the concrete is not cast in a continuous sheet, structural cracks in concrete are avoided by using the geocells. Also, since no steel reinforcement is needed in the concrete, the project is constructed very quickly without the cost of the steel or the need for specialized contractors.
The StrataWeb® solution is a fast and all-weather installation solution. An estimated costs savings of 40% was achieved, and the product was completed before the monsoons arrived, something no other geomembranes protection scheme could have offered.