A High Rock Cut Stabilization in Muscat City, Oman

Abstract

A large road box cut was constructed in hilly terrain of Muscat city, Oman, in an area underlain by the geologically complex Sahtan and Mahil formations comprising of limestone, dolomite, sandstone and siltstone with interbedded soft calcareous mudstone. The rockmass contains a large number of small scale faults, shear zones, minor folds and solution cavities. The maximum height of the cut was 50 m with slope angle of 1V:0.25H, and intermediate benches generally 3 m wide. To control vibrations in the urban neighborhood located nearby and avoid damage to the rockmass, a very cautious and time consuming excavation approach was adopted using rock breakers. The excavation work was completed in 2010. Oman has a hyper-arid environment with scarce rainfall for many months. However, occasionally the area is hit by intense rainstorms. During Cyclone Phet and accompanying rains in June 2010, part of the cut slope failed with a downslope movement of about 800 m3 of massive rock. This necessitated a post-excavation re-evaluation of the cut slope, which culminated in design modifications, and construction of long term slope stabilization measures that included high capacity rock anchors, rock bolts and reinforced shotcrete for stabilizing potential shallow and deep seated failures. This case history describes the initial design of the cut slope, causes of slope failure during the cyclone, field and laboratory testing carried out for determination of rock engineering parameters, analytical techniques used for design modification and construction details of the designed remedial works.