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 city of Adelaide in South Australia is built on a clay plain running north-south between the Adelaide Hills and the sea. Adelaide has a Mediterranean, i.e. semi-arid, climate causing this clay to be unsaturated and stiff to very stiff when above the watertable. To improve traffic flow on the western side of Adelaide an expressway-standard road, the North-South Corridor, is being built. The 4.5km section of the North-South Corridor from the River Torrens in the south to Torrens Road in the north is referred to as the T2T project. For 2 kilometres the expressway-standard road of T2T runs in a cutting up to 9m deep. The amount of material used in conventional retaining walls, and hence their cost, could be greatly reduced if the shear strength of the unsaturated stiff clay could be estimated with sufficient accuracy for engineering design. Recent developments in unsaturated soil mechanics allow the shear strength of the clay to be determined with sufficient accuracy as long as the suction can be estimated with confidence for the various design conditions, e.g. leaking pipes. Geotechnical engineers in Adelaide have over 50 years of experience in measuring soil suction and estimating suction profiles for varying conditions of drainage. This paper shows how that experience, combined with field measurements, allowed for the design of a cost-effective lightweight soil nail/shotcrete walls for T2T by the successful consortium bidding for this job. The walls have now been under construction for a year and will be completed by late 2018. In this paper the geotechnical investigation for the T2T project and the design of the lightweight soil nail/shotcrete wall are described briefly. The geotechnical investigation showed significant sand lenses occurring in the clay in the southernmost part of the project near the River Torrens. A “toolbox” of soil nail/shotcrete wall designs was developed to cope with the variety of sand lenses. During construction the observational method was used to deal with this soil variability. This required inspection of every section of soil exposed during excavation and selection of an appropriate wall design from the design toolbox. Implementing the observational method proved difficult. This, and other problems prolonged construction, resulting in the walls costing more than originally estimated but still considerably less than conventional walls. This paper concludes with a discussion of the difficulties of implementing the observational method in this project, how these difficulties were overcome, and how they could be avoided in the future.

The method of increasing the capacity and avoiding excavation of waste materials as part of the final closure of uncontrolled landfills with the use of a reinforced earth embankment of adequate height is described. The case of the Syros uncontrolled landfill (Cyclades islands, Greece) is presented, where the method was applied. The design principles are detailed along with the method of analysis, the analysis results, the construction specifications and finally the construction method that was used.

A high cantilever seawall was constructed in soft clay of 15 m to 32 m thickness and the enclosed water between the shoreline and the seawall was reclaimed. The area was 7 hectare and the reclamation height ranged from 10 m to 20 m. The reclamation methods were controlled and not intended to cause additional incremental deflection to the already high design deflection of -385 mm or 0.89% of H (H = 44 m is defined as rotating and not cantilever height). However the different methods of reclamation had a significant influence on the slurry clay flows and thus caused additional incremental deflection. The slurry clay flows pushed the deflection of seawall to an additional maximum magnitude of -353 mm or 0.80% of H. The maximum seawall deflection of -526 mm or 1.20% of H was based on inclinometer IW1903 readings. At such high seawall movement, the inclinometer deflection profiles indicate safety in the design and the long term deflection is stable. This reclamation experience indicates that it is difficult to control slurry clay flows in future high reclamation filling of 10 m to 20 m height above seabed with underlying thick soft clay of 15 m to 32 m. The deflection is far exceeding BS8002 guideline of 0.5% of H, which stipulates the geo-structural stability of the seawall. This long term stability at such a high deflection is attributed to the robust turning back capacity of the strong box. The turning back capacity of the strong box against the active force above the rotating point is from the integration of vertical bending capacity of the pipe piles, the rigid connection of the capping beam, and utilisation of the P-P interlock to produce the box action. Below the rotating point the adequate passive resistance into Old Alluvium maintains the external equilibrium.

The excavation system performance and ground movement behavior for the cut-and-cover excavation for the Downtown Line 2 Cashew Station in Singapore is presented. The information presented includes ground settlement profiles, wall deflection profiles, strut loads, and ground water monitoring behavior. Construction activities and remedial measures that were undertaken are also presented. Comparisons of the measured wall deflections and the ground surface settlements are also performed against the empirical methods/charts from the literature. It is hoped that this case study will provide useful reference and insights for future projects involving excavation in the Bukit Timah Granite residual soils.