Engineering Geological and Geotechnical Assessment for 3-lane Carriageway Tunnel Constructed at Hospet in Hungund-Hospet Section of National Highway-13 (NH-13), India

ABSTRACT In this paper engineering geological and geotechnical assessment for 3-lane carriageways 15.5 m excavated diameter and 14.5 m finished diameter semi-circular shaped RHS tunnel constructed at Hospet of Hungund-Hospet section of National Highway-13 (NH-13) is discussed. The tunnel was excavated through the hill underneath the operating railway track and clear vertical distance from the crown of the tunnel to railway track was 15 m. Before construction, based on surface geological mapping, boreholes data and laboratory investigations, rock mass along the tunnel alignment was characterised and excavation methodology and support system were finalised. Sequential excavation method was adopted due to adverse geological conditions and low cover at certain chainages. The rocks encountered during the excavation of these tunnels are oldest metasedimentary, and metavolcanic rocks of Archaean age of Sandur Group. Prominent three to four joint sets were recorded during 3D geological mapping and rock mass was slightly to moderately weathered. The entire length of the tunnel was falling under poor, fair and good rock mass categories based on assessment of Tunnelling Quality Index (Q-system) rock mass classification. The support systems were estimated based on Q-system values, which were applicable for the stability of ground conditions. Wedge analysis along the tunnel alignment at different chainages was done using Rocscience UNWEDGE v 4.0 software. The stability of the tunnel was evaluated and based on investigation, geotechnical problems were identified and appropriate measures for rock support were suggested and implemented.