Geotechnical Engineering Online Library

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.

Three areas in Egypt and Saudi Arabia were selected as pilot sites for studying karst foundation problems. Globally, the dissolution of carbonate rocks results in extensive karst landforms that can pose significant challenges to civil engineering projects. Most karst caves and sinkholes are structurally controlled by major faults and joints. Detection of karst limestone foundation bedrock (caves, sinkholes and open fractures) in the three studied sites has been conducted using geological and geophysical studies. The geological studies are based on review of published geologic maps for the three sites. The geophysical studies included a 2D electrical resistivity survey and ground-penetrating radar (GPR). Karst limestone foundation bedrock treatments have been performed in several projects worldwide and include engineering fill, engineering fill with geosynthetics, concrete filling and cement grout low pressure injection. Upper Egypt’s El Minia-Maghagha caves, underlying newly constructed settlement areas have been treated using high-slump concrete filling. Engineers treated foundations in northwestern Riyadh City and in eastern Al Hofuf City, Saudi Arabia, using engineering fill, high slump concrete filling and low pressure cement grout injection. This article summarizes the classification of encountered karst foundation bedrock according to Ford and Williams (1989) engineering classification, introduces the optimum methods for detecting karst features and presents engineering treatments of karst limestone landforms.