Geotechnical Engineering Online Library

Slope failures are a common occurrence in tropical regions with a high intensity of rainfall. Tropical areas such as Singapore are normally covered with residual soils whose behaviour does not follow the principles of classical saturated soil mechanics because these soils are often unsaturated in nature. The negative pore-water pressure in unsaturated soil is highly influenced by the changes in the flux boundary conditions, resulting from the variation in climatic conditions. On the other hand, the negative pore-water pressure contributes additional shear strength to the unsaturated soil. As water infiltrates into the slope, pore-water pressure in the slope increases (matric suction decreases), and the additional shear strength due to matric suction will decrease, causing the slope to be more susceptible to failure. Singapore is a land scarce country with a critical need to optimize land utilization. Steepening slopes or cutting back slopes and supporting them using a retaining structure is one way to create new spaces. In this study, a new type of retaining structure, Geobarrier System (GBS) is proposed. A GBS is a man-made three-layer cover system designed as a vegetative layer combined with a two-layer unsaturated system, which harnesses the distinct difference in unsaturated hydraulic properties between a fine-grained layer and a coarse-grained layer. GBS consists of recycled materials and does not use steel or concrete and is hence more cost effective, thereby making it economical for use in urban areas. Geobag for vegetative layer is supported by specially designed pockets for planting different types of sustainable plant species. The paper presents the design, construction procedures, material selection and field performance of a GBS constructed at an inclination angle of 70o in response to rainfall infiltration. In addition, the results of the finite element seepage and slope stability analyses of the GBS subjected to extreme rainfalls are also presented. The results from field instruments and numerical analyses showed that GBS was able to protect the slope from rainfall infiltration; therefore, the stability of the slope retained by GBS was not affected by the rainfall.

The construction of the steel arched Tsakona Bridge, designed to overpass an active landslide, required several temporary projects. Auxiliary steel towers were used for the erection and welding of the arch segments. Most of them had to be founded in the body of the sliding mass. An automated instrumentation and monitoring system was installed on site providing early warning to the engineers in charge in case large displacements were measured. The real-time recording data provided valuable information about the landslide movements. These monitoring results defined the geotechnical and structural design criteria for the deep foundation of the temporary towers. Several 2D and 3D geotechnical and structural models were created to evaluate the sliding effect on the foundation components, finalize their dimensioning and calculate the required reinforcement. The procedure followed for the design of the temporary deep foundation is described based on the criteria devised using the monitoring system, emphasizing on the continuous feedback between the geotechnical and structural design teams. Several key structural design issues required during construction are also presented.

During the winter of 2001-2002, a landslide of 80.000 m3 was activated and moved downslope at the Northeast outskirts of the Pissouri village in Cyprus (Limassol prefecture), displacing a section of a country road that was crossing the upper part of the sliding mass and destroying three newly built residences. This paper presents the geologic and hydrogeologic regime of the landslide and its implication on the landslide’s mobilization. Geotechnical site investigation and post failure monitoring, in conjunction with slope stability back analysis formed the basis for the study of the slide and subsequently guided the remediation strategy. Various possible interventions to stabilize the slope and allow the safe reconstruction of the displaced road have been investigated and compared considering both economic and environmental criteria. The performance of the remediation measures has been studied in terms of stability safety, using numerical models previously calibrated via back analyses. Observations made during the execution of the stabilization works provided additional data verifying and supplementing the original design.

The paper describes a large scale landslide in a coal mine in Servia area in Kozani, Greece, which occurred on February 2011. The landslide, of ~250m width and ~350m length, took place within marly lacustrine sediments. The main factors for the manifestation of the landslide were a temporary stoppage of works, the existence of a specific weak surface, the clayey nature and the sensitivity of marl to environmental agents, the action of water and the large slope height. Due to the magnitude and the position of the failure, a purely geotechnical solution was proposed that involved rearrangement of ground masses. The proposed solution led to successful rehabilitation and unimpeded continuation of the exploitation process.

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.