Project Overview

Geological Make-up in the LHWP

The overall geological structure in the LHWP area consists of layers of nearly horizontal sedimentary rocks of the RSA Karoo sequence, capped by basalt flows and intruded by dolerite dykes and sills. The geology has an important influence on tunnelling operations, foundation conditions and availability of construction materials.

Basaltic conditions the underlying sandstone of the Clarens will provide favourable tunnelling conditions.

Poorer tunnelling may occur in the vicinity of the dykes where some minor seepage is expected. In the lower formations of the Karoo sequence, weaker sedimentary rocks occurt and tunnelling conditions are more variable.

Groundwater inflow is not likely to be significant, except locally at dykes, but some siltstones and claystones tend to disintegrate rapidly with changes in moisture content and have realatively low strength.

The LHWP’s dams are founded on either the basalt ( Katse, Mohale, Matsoku) or the underlying competent sandstone (‘Muela and future phase dams). Foundation conditions are good at all sites with adequate strength, low permeability and limited jointing.

Dolerite from dykes and dense doleritic basalt can be crushed to produce coarse and fine concrete. This was actually done in the construction of Katse and Mohale dams. Natural sources of sand and gravel are limited in the higher regions of the Phase I but will become significant in lower reaches of Senqu river for further phases.

The project is located in area of low tectonic activity and low seismic risk. Only one significant incidence of seisimic activity occurrred in 1996? during the filling up of Katse dam at a place called Mapeleng.

The activity caused damage to houses tearing them apart and causing panic in the area. The affected households were received replacement housing of superior quality and standard as compensation from the incidence. No further seismic occurrence has happened since.