Study of geo-mechanical characteristics and stability of half-tunnel at Dharapani along Besisahar—Chame road section, Manang
Cut slope design and excavation is a difficult task in mountainous terrain. In vertical cliffs, for a small length of cut, a huge amount of excavation is needed. In this context, a curve cut slope cutting appears more time and cost effective and are in practice for steep rock walls, thus leaving rock overhangs. Such structure is commonly named as negative cut slope, curve cut slope, half tunnel, half-open and half-hidden cut, etc. This type of slope cutting is essential case of mountainous countries. In this study, stability analysis of such curve-cut constructed in Nepal for the first time, has been carried out. The cut lies at Dharapani of Manang District in the Besisahar—Chame road section that links the headquarter of the district. The area consists of high grade metamorphic rock—gneiss, forming vertical wall in river valley thus creating a barrier for road construction to link Manang with other area. Most commonly used rock mass classifications—RMR, Q and GSI are carried in the area of construction, using line method. Joint properties and rock wall strength of the selected stretch were determined during the field visit whereas, other geotechnical properties have been determined in the laboratory scale. Three sections were selected for analysis and the selected section consists of good quality rock mass with two to three sets of joints. In such rock quality section, slope was cut in 6 to 7 m span and the span roof is planar. The ground is analyzed using FEM numerical modeling software Phase2 and the kinematic analysis is done in DIPS. The maximum and minimum stresses at different parts of an individual section of the half tunnel and the total displacement in the three sections have been compared. The analysis shows that the half tunnels have very little displacement even reducing the joint strength by 10%. The half tunnel is stable that is the reason it is withstanding for 10 years without any failure. This study shows construction of half tunnels for reduced excavation is possible in good quality rock slope.