Mohale Dam is Solid; Confirms the Panel of Experts

The panel of experts that was contracted by the Lesotho Highlands Water Commission (LHWC) to assess developments at the Mohale Dam following the spillage on February 12, 2006, has issued a report that contains observations, review, evaluation, as well as recommendations.

On account of the foregoing, LHWC extended invitation to the LHDA Phase 1B Engineering Panel of Experts (POE) to inspect the said compression joint failure between slabs 17 and 18, and to subsequently offer their independent, professional advice. The POE that comprised two members, namely Dr.Alfred J. Hendron Jr. (USA) and Mr. Bayardo Materon ( Brazil ), did a three-day inspection work on March 5-7, 2006, that culminated in the report under discussion.

In analyzing deformations of the face slab ((joint 17/18) for the central portion of the dam, calculating the normal component to the face of the settlement due to the reservoir filling, the panel has concluded that deflections of the slab were typical of uniform compacted, basalt. Barra Grande (185m high) and Campos Novos (202m high), both found in Brazil, have been cited as a reference for the dams, which presented similar distress by compression.

Besides the compression spalling, observed between slabs 17 and 18, the panel has pronounced that it did not expect any further distresses in other joints, adding that it was however, important, during the Remote Operated Vehicles (ROV) inspection, -the equivalent of the use of under-water-cameras, in plain English- to check the adjacent slabs and joints, and to develop a detailed record of the findings with depth.

On evaluation of the leakage, the panel has maintained that at the rate of 370 l/s, Mohale Dam presented no cause for serious concern, indicating that there were many dams of similar design with immensely larger leakage quantities, which had not experienced any stability problems. The two dams in Brazil , Barra Grande and Campos Novos, that have leakage of about 13001/s were once again quoted as typical examples.

The panel further noted that, New Exchequer CFRD Dam in the USA experienced leakage of 13,900 l/s on first filling in 1967.  This dam that remains in service today, commented the panel, did not experience stability problems, but was fixed so that the long-term leakage was reduced to about 2,000 l/s.  “Thus it is our considered opinion that the present leakage of 370 l/s at Mohale Dam is not a safety problem.  There is considerable precedent to support this judgment” , so said the panel.

In conclusion, the panel has recommended that, much as they have stated that the current leakage at Mohale Dam did not represent a dam safety problem, in their judgment it was prudent in the long-term to design and construct a high quality repair of joint 17/18 during dry seasons.  The process, according to the panel, could be done very deliberately without needlessly throwing water away with the bottom outlet, because the current situation did not require an immediate high quality repair.  

Futhermore, the panel has recommended that joint 17/18 should be temporarily fixed to reduce leakage, either by dumping soils on top of the open joint, or by repairing the joint with a membrane, if the leakage quantity continued to rise. Much as both alternatives could be quickly pursued, the panel has advised that the urgency of action should depend on the behaviour of the leakage; whether it is stable at constant reservoir level, or is found to be increasing at constant reservoir level.