Geological, geotechnical, and GPR investigations along the Mansa Devi hill-bypass (MDHB) Road, Uttarakhand, India

In the present work, geological, geotechnical, and ground penetration radar studies were carried out along the Mansa Devi hill bypass (MDHB) road Uttarakhand, India in order to assess the slope instability conditions. Seven unstable zones along the entire MDHB road were identified which are prone to landslides. Geologically, the area is composed of mudstone and sandstone with thick overburden constituting sandy soil. The laboratory tests were carried out in order to evaluate the geo-mechanical characterization of soil, sandstone, and mudstone that constitute the slopes. The laboratory tests reveal that the slope is comprised of soil characterized by high permeability and low strength values. The soil is mainly sandy and non-cohesive. The laboratory tests on sandstone reveal that fresh sandstone exhibits exceptionally higher value of unconfined compressive strength (> 100 MPa) than the weathered sandstone (< 35 MPa). The mudstone is non-durable and contains considerable amount of expanded clay minerals which has a tendency to expand in the presence of water. It can weather easily and facilitate instability at shallow depth. A ground penetrating radar survey confirmed that the road is unstable particularly on those spots where the road either lying over mudstone beds or over thick non-cohesive sandy soil. The infiltration rate of water in this sandy soil is generally rapid which allows fast flow of water into the deeper horizons and thus cracks can be seen in road caused by vertical settlement. Similar results are observed in slope stability analysis where the slope covered with sandy soil exhibits factor of safety less than 1.0. However factor of safety is close to 1 where the slopes consist of alternate beds of sandstone and mudstone.