Integrating structural geology and geotechnics, Los Caracoles Dam

The Los Caracoles is a 140m high hydropower water dam, located in San Juan Province, Argentina, in an arid region of high seismic activity. The spillway system consists of two large parallel tunnels while the gateway structure is formed by two twin boxes, each 30m high, excavated in the upstream slope of the abutment.

Natural stability relies in the continuity of the rock layers along the entire slope and into the foundation levels but this continuity would be altered by the gateway excavation. Design countermeasures were necessary to keep the slope stable.

Soon after the excavation started, fractures appeared on the shotcreted slope surface, the excavation face, and the rock buttress. The job was halted and SRK was called in. The photo below shows the excavation and buttress status at that time.

SRK performed a thorough site inspection and detailed analysis of all conceivable failure modes and determined that, due to the shape of the buttress and lack of lateral confinement, the secondary joints or fractures had been activated so the rock mass slipped along them. The outcome was that secondary joints, other than the bedding planes, had been activated due to the shape of the buttress and lack of lateral confinement. A second reinforcing bolt system (passive, grouted rockbolts made with rebars) perpendicular to the original one, was designed and installed inmediately, and the excavation was continued and completed successfully.

Another challenge was the design of the gateway. The gateway is a critical component of the spillway, and must remain operative after an earthquake. It was feared that the slope would displace further during an earthquake, deform the concrete structure and block the gateway.

When dynamic slope stability analyses were performed to address this concern, SRK found that two large blocks were potentially unstable. For the first block, a step-by-step dynamic Newmark analysis showed that the support system would need to carry 650 MN to assure that its displacement during the shake was compatible with operating the gates. Two large concrete buttresses were built near the rock buttress, both confining it laterally and carrying part of the huge load.

Unfortunately, the 250 MN second block does not contact either the rock or the concrete buttress. Twenty prestressed permanent anchors were designed and installed to reduce displacement. After this design change, the project was completed and works successfully.

Alejo Sfriso: asfriso@srk.com.ar

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