Stabilization of unstable slopes - a case study in Cabo Hill near Rajbhavan, Panjim, Goa

摘要

The stabilization of a failing slope needs careful and detailed investigation so as to arrive at effective remedial measures. The part of Cabo Hill, adjacent of Rajbhavan, a 1540-AD historical monument, was affected by instability, which is manifested by development of tensional and longitudinal cracks, movement in the pavements and tilting of trees. Detailed geotechnical investigations including surface geological mapping, evaluating cross sections, drilling, Standard Penetration Tests (SPT) and characterizing crack geometry were carried out to decipher the affected slope boundary, the slope geometry, failure depth, depth to critical failure plane using shear parameters of slope-forming material (C and §).Six bore holes were drilled and samples were collected from the bore holes representing different soil units along depth. The grain size, specific gravity, bulk density, natural moisture content, porosity, compressive strength, cohesion (C) and angle of internal friction (φ)) of the samples were determined. Since the spatial disposition of the critical slip circle happens to be an important decisive factor in suggesting various stabilization methods, an attempt was also made to compute the factor of safety ('F') to the existing unstable slope.From the observed surface and subsurface geological conditions of the unstable slopes adjacent to Rajbhavan, the in-situ characteristics of the subsoil, their physico-mechanical properties relevant to the stability analysis of the slope, the position of the critical slip circle and the computed safety factor contours were determined. These analytical techniques facilitated recommendations of various corrective and control measures to stabilize the slopes viz. pre-stressed cable anchors, inclined bore piles with beam caps, rock bolting/shotcreting, sealing of ground cracks in the upper part of the slope; easing of slope and surface drainages, vertical bore piles with beam caps, vertical sand piles, turfing/terrace gardening in the middle part of the slope; construction of retaining wall and effective shore protection in the lower part of the slope.