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Geological Society, London, Engineering Geology Special Publications; 1998; v. 15; p. 41-48;
DOI: 10.1144/GSL.ENG.1998.015.01.05
© 1998 Geological Society of London

Section 1: Coastal and fluvial geohazards

The construction of a drainage tunnel as part of glacial lake hazard mitigation at Hualcán, Cordillera Blanca, Peru

J. M. Reynolds1, A. Dolecki2 & C. Portocarrero3

1 Reynolds Geo-Sciences Ltd, The Stables, Waen Farm, Nercwys, Mold, Flintshire CH7 4EW, UK
2 Rust Environmental, 29 Cathedral Road, Cardiff, South Glamorgan CF1 9HA, UK
3 Unidad de Glaciología y Recursos Hídricos, Huaraz, Ancash, Peru

At Hualcán in the Cordillera Blanca is a high-altitude glacier lake dammed by a moraine. Local glaciers regularly produce ice avalanches. In 1988 it was confirmed that the moraine was ice-cored. The rate of melting of the ice was sufficiently fast that, unless mitigation measures had been undertaken rapidly, the moraine would have collapsed. This would have resulted in the inundation downstream of Carhuaz, a town with a population of 25000 people. Following the successful installation of siphons in 1988–89 to reduce the water level by 8 m, it was decided to undertake more permanent engineering works to ensure that the lake could never again pose a threat. It was proposed to construct a 2-m diameter tunnel, 155 m long beneath a rock bar below the moraine dam, to lower the lake level by a further 20 m. This would create sufficient freeboard to contain possible displacement waves.

Work on the tunnel was started in May 1993 using compressed air drilling and blasting with hand excavation. The initial tunnel design consisted of a single tunnel drive 135 m long plus a 20-m inclined drive under the lake. A second, near-vertical shaft was constructed for ventilation and access. Had the proposed method of breakthrough from the tunnel to the lake been carried out, it would have resulted in a rockburst leading to the catastrophic discharge of the lake. The tunnel design was changed on site to include three additional inclined drives from the main shaft, reaching the lake at vertical intervals of 5 m. Using this method, the lake level was lowered 20 m safely.

The objective of this paper is to describe in detail the geotechnical engineering aspects of the tunnel construction, the reasons for the change in its design, and the results of the mitigation work.