The entire region is tectonically folded with syncline-anticline sequences, indicating orogenetic stresses along a West-East direction. The project area, including the reservoir and the dam sites, is characterised by the presence of three main rock formations:

Basement metamorphic rocks Crystalline and semi-crystalline limestones Plutonic and Metamorphic acid rocks

It is important to note that chemical weathering is predominant because of the constantly high temperature, heavy rainfalls and dense tropical vegetation which fully cover the entire project area. This has an impact on the design of structures, including the dam, tunnel portals, etc.

There exist two fault systems, namely the Bentong fault system and the Lebir fault system. Therefore, due consideration should be taken when deciding the locations and orientations of the dam, powerhouse and tunnel portals.

One of the main features of the geology is the presence of karstic features at the right abutment of the saddle dam area, upstream and downstream of the dam axis. This could affect the impermeability of the reservoir and result in leakage of water from the reservoir. The karst network has been developed in a marble formation which is intensely folded and faulted. The fold axes and the fault planes are oriented mainly in the N-NE direction; the rest of the fault planes are oriented towards the Nenggiri River in the N-NW and E-SE directions with subvertical dips.

In the case of seismicity, it appears from the report that there is almost no possibility of an earthquake with a magnitude sufficient to cause damage to the dam and other structures occurring.