2. GEOLOGICAL CONDITIONS
The geological conditions for the headrace tunnel are defined as shown in
Figure 2. The general geology and structural geology can be described as follows;
2.1 General Geology
The headrace tunnel is located in quartz–rich sandstone and siltstone of
Khorat Group, Jurassic to Cretaceous ages. The quartz-rich sandstone forms a
prominently and nearly vertical cliff. It is typically light grey in fresh and light to
yellowish-brown when it is weathered. There is a medium bedded to massive, very
fine to medium grain and hard to very hard. In addition, there is a few laminated grey
siltstone located at intermediate. The siltstone (interbedded layer) is typically purple,
thin bedded to massive, weak in general, locally medium hard when graded to sandy
siltstone. It is highly weathered to decompose where exposed.