Geomembranes to Line Surge Shafts

Surge shafts are underground water structures that are usually lined. The lining provides stability and watertightness to the excavation line, avoiding that excessive water infiltration undermines the stability of the slopes and of the structure itself. Traditional lining systems have been reinforced or un-reinforced concrete and steel. The construction of concrete and steel linings is expensive and time consuming, it can be further complicated in locations with difficult access, and it may require expensive maintenance. Over time, or in presence of particularly difficult geological conditions, concrete linings deteriorate, water infiltrates into the soil through fissures, or/and failing joints and increased permeability, increased roughness results in reduced hydraulic efficiency. Similarly, steel liners may be stressed above their yielding point. Loss of watertightness can eventually trigger landslides and ultimately cause partial or total collapse of the structure. Waterproof geomembrane lining systems based on the use of thermoplastic materials can efficiently substitute traditional linings for rehabilitation as well as for new construction. They can be installed in short times, and are a permanent solution to provide watertightness, reduce head losses, and maintain the shaft in its design conditions. The high flexibility and elongation capability of the thermoplastic geomembrane allows bridging construction joints and existing cracks and accommodating the opening of new cracks. In case of difficult geology, a geomembrane system can increase the safety of the shaft by maintaining the watertightness for which it has been designed. The paper discusses the design concepts and components of drained geomembrane lining systems anchored by longitudinal fastening lines and by watertight perimeter seals, through case histories of surge shafts projects recently completed, one in South America and one in the middle East. In both projects the geomembrane system was installed on the existing concrete lining and left exposed to the water.