Slope stabilisation techniques at Khao Laem Dam, River Kwae, Thailand

G. Klenowski and T. Thongsiri

The Khao Laem Multipurpose Project is on the River Kwae Noi in Kanchanaburi Province, Western Thailand. The infamous Death Railway was constructed along this river valley during the Second World War. This paper has been prepared as part of the 20 years since impoundment reunion.

The Project comprised a 92 m high, concrete faced, rockfill dam embankment with a crest length of 1019 m and a left abutment spillway and power station of 300 MW capacity. Water released from the Khao Laem and adjacent Srinagarind reservoirs in the dry season irrigate 400 000 ha in the Mae Klong Irrigation Area.

Preliminary investigations of the Quae Noi Basin were commenced by the Electricity Generating Authority of Thailand (EGAT) in 1968. The Snowy Mountains Engineering Corporation (SMEC) became involved in 1973 and completed geotechnical investigations, detailed design and tender documents by 1979. Construction commenced in January 1980 with SMEC retained as Consultant Engineers during construction. Technical representatives of the World Bank (the main Project funding organisation) considered this scheme to be one of the most difficult ever attempted in the world. Reservoir impoundment commenced in 1984.

As well as having major, karstic foundation, cut-off issues, a number of slope stability problems occurred during construction. These were related to extreme differential weathering, karstic features and penetrative weathering along adversely dipping geological structures. Various remedial measures were needed.

Major rotational failures in soil batters of the diversion channel were recut at flatter batter angles, without regularly spaced berms. On soil slopes, saturated berms appear to facilitate slope failure by allowing initial slumping, which progressively continues upwards to form large slip circles.

Sudden, partial failure of the right abutment cliff occurred during access blasting to install rock anchors. To maintain a safe working environment during excavation down the overhanging cliff to the dam plinth, microseismic monitoring was required during and after every blast to ensure an acceptable acoustic emission threshold, prior to access.

Support for the upstream and downstream diversion tunnel portals included 15 m long rock bolts and dowels, shotcrete lining and concrete retaining walls. At the upstream portal, 20 m long drainage holes were drilled to dewater an elevated water table. Maximum recorded flow from these pre-drainage holes was 180 litres/sec.

An excavation of 100 m vertical depth in erosive soil was required to reach sound foundation rock for the dam plinth and upstream rockfill in Area 2. Resistivity depth sounding and systematic diamond drilling were used to determine the depth and extent of differential weathering.

Significant translational movements, which occurred in the spillway crest and intake structure excavations, were related to wedge sliding on adversely dipping clay seams. Stabilising techniques included recutting the upper part of the spillway crest excavation and installing rock bolts, dowels, rock anchors and a concrete toe block.

The spillway chute and stilling basin excavations were redesigned to be excavated parallel to bedding planes. Only spot bolting and local dental concrete were required.

Prior to the excavation of the power station, pseudo elastic, blocky, computer modelling was completed to determine support requirements. Extensive pre-dowelling and rock bolting were required in areas of adversely dipping geological structures. Monitored movements during construction were consistent with normal stress relief.