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Jacked-in Pipe Reinforcement of a Deep Excavation in Soft Soil

Soil nailing is an in-situ ground improvement method which has gained increasing acceptance in this region, but is not generally recommended for clayey and organic soils. This restriction is empirically based, and little information exists on the performance of soil nails when the method is applied in these soil conditions. To address this gap, a deep excavation for the construction of a 3/12 storey deep basement carried out using a novel technique, which combines Contiguous Bored Piles and Jack-in pipe reinforcements, was instrumented to monitor its performance. This paper will highlight briefly the instrumentation program, which was set up mainly as a safety control and to monitor the interaction of this retaining system with the surrounding soft soil. In addition, numerical analyses were conducted to study the soil-structure interaction of this new construction technique. Data from the field monitoring and FEM analyses will be presented with regard to the effectiveness this system particularly the jacked-in pipes in supporting the deep excavation. It follows from this comparison that the field instrumentation and numerical analyses have proved to be valuable tools in assessing the deformations to be expected at critical stages of the excavation. The project shows that jacked-in pipe in soft clayey soils is possible, at least for short term and can be effectively used as temporary support in deep excavation. 1.0 INTRODUCTION Deep basement construction is becoming increasingly expensive, nullifying the justification for overconservative designs. One of the ways in which construction cost can be reduced is by employing fast and cost-effective deep basement construction techniques. This paper will highlight the use of Jacked-in Pipe Reinforcement in support of Contiguous Bored Pile walls. The field instrumentation has proven to be a valuable tool in assessing the validity of the initial design assumptions The Observational method (Peck, 1969) was practised whereby field results obtained from the adequately instrumented soil nailed retaining system were used in the back analysis of design parameters. The back calculated parameters were implemented in the Finite Element Analyses to predict deformations in the subsequent construction phases. Soil nailing is an in-situ soil reinforcement technique, which in the last three decades has been successfully used in France, United Kingdom, United States and Southeast Asia. The origins of soil nailing partly stems from the Reinforced Earth techniques, rock bolting and multi-anchorages systems (Recommendations Clouterre 1993). Soil nailing has been primarily used for temporary retaining structures for basement excavation and permanent cut-slope stabilisation. Jacked-in pipe retaining system, which is similar to soil nailed system is not common. This technique has gained momentum in Malaysia mainly for the ease and speed of installation and it is also very cost effective. However, with the advent of numerous research programs (Schlosser, 1986; Elias and Juran, 1990), increase in the state-of-understanding and experience, both abroad (Schlosser, 1982;Bruce and Jewell, 1986, Bruce and Jewell 1987, Juran et al., 1990;) and local (Tan, et al., 1998; Luo et al., 1998; Tan et al., 1999) will promote the use of this technique, both as a temporary and permanent structure. 2.0 DESCRIPTION OF PROJECT The condominium project which is located at UEP Subang Jaya of Malaysia consists of three 2 condominium towers of 33 storey and a single 20 storey office tower. Due to the huge demand for parking space, an approximately three storey deep vehicular parking basement will be required. The deep excavation, through a filled layer of very loose silty sand and soft peaty clay varies from 11m to 13m. The presence of very soft soil condition and the fast track requirement of the project, Contiguous Bored Pile (CBP) walls supported by Jack-in Pipes in six to seven rows were utilised to stabilise and control the lateral displacement and surface settlement. The supported face is approximately 6900 m. Photo 1: The Retaining System; Contiguous Bored Piled Walls (CBP) and Jack-in Soil Nails Fig. 1:Typical Subsoil Profile Fig. 2: SPT ‘N’ Values: Scatter Plot 3.0 GEOLOGICAL INFORMATION The general subsurface soil profile of the site, shown in Fig.1 consists in the order of succession a firm clayey SILT, a loose to medium dense SAND followed by firm to hard clayey SILT. The soils are inter-layered by thick deposits of very soft dark peaty CLAY. The plot of SPT’N’ values, illustrated in Fig.2, shows significant scattering which is commonly found in tropical residual soils in the area. However the trend of the scatter plot shows that the SPT ‘N’ values increases with depth. For the underlain soft clayey material, the registered SPT ‘N’ values were zero and Vane Shear strength varies from 5 kPa to 20 kPa. For analysis purposes the subsoil was simplified into representative layers, namely Granular material and Cohesive material. 4.0 THE JACKED-IN PIPES AND CONTIGUOUS BORED PILE SYSTEM Jacked-in Pipes Reinforcement Mild steel pipes which functions as soil nails are installed by hydraulic jacking. This method has proven to be an efficient and effective technique for excavation support, where conventional soil nails and ground anchors have little success in such difficult soft soil conditions. Such conditions are sandy collapsible soil, high water table and in very soft clayey soils where there is a lack of short-term pullout resistance. S P T 'N ' V S D E P T H ( m ) P L O T