The manuscript presents initial results of a potential linkage between the mineral composition and structural strength in geological samples from a hydropower tunnel currently under construction. Changes in rock properties due to processes of both chemical as well as mechanical weathering are very important issues for water tunnels constructed for hydropower. Weathered and weak rock mass have strong slaking and deterioration potential if in contact with water, which may result failures in the unlined / shotcrete lined water tunnels. The slake durability of intact rock is in many cases closely linked to the mineralogy, especially on the content of weak clay minerals. When the weak rock mass is exposed to water it may lead to the degradation caused by the destruction of the intact rock structure. Such phenomenon may also aggravate swelling possibility in the rock mass if clay-bearing rocks are constituted by swelling minerals like montmorillonite (smectite).
The manuscript first evaluates the mineral composition of the rocks collected from the tunnel, and then the initial laboratory assessment on the extent of slaking is presented. Further, the slaking extent is linked to long-term stability issues in tunnels caused by slaking and disintegration.
From an engineering point of view, disintegration and swelling are among the most serious problems in underground rock excavation and are related to the alteration of previously competent rock mass (Wahlstr⊘m; 2012). Some rock types are especially prone to weakening and/or slaking when exposed to short term weathering processes of a wetting and drying cycles (ISRM 1977). As defined by Panthi (2006), slaking is the deterioration, weakening and breakdown of a rock material when subjected to cycles of drying and wetting. The phenomena is a result of shearing produced by volume change associated with wetting and drying, and is frequently observed in clay-bearing rocks as shales, mudstones and siltstones. In hydropower water tunnels, the rock mass is first exposed to a dry condition during construction due to contentious ventilation and use of heavy construction machinery and once construction is completed the tunnels are filled with water for very long period. Thus, knowledge of the interaction between rock mass and water saturation is crucial to prevent long-term failure or tunnel collapses.