Sep 28, 2016 - IN UNDER CONSOLIDATION JAKARTA SOFT CLAY. Aswin Lim 1 ... unbraced shallow excavation project which is located in the North Jakarta.
Proceedings of Soft Soils2016 , September 27-28th 2016
PERFORMANCE AND MODELLING OF UNBRACED SHALLOW EXCAVATION IN UNDER CONSOLIDATION JAKARTA SOFT CLAY Aswin Lim 1, Bondan Widi Anggoro 2 and Paulus Pramono Rahardjo 3
ABSTRACT: This paper highlights the importance of considering the behavior of under consolidation clay in an unbraced shallow excavation project which is located in the North Jakarta. The excavation geometry is very wide, around 160 m, with a diamond shape. The excavation depth is 5.25 m. Due to this wide excavation geometry, the unbraced excavation system was selected as the retaining system. The corrugated sheet piles were installed as the front wall and they tied together with the rectangular concrete piles at the back of the front wall. Several soil investigations have been conducted, especially the CPT-u test, and the wall deformation were well monitored with the inclinometers. According to the CPT-u test results, they detected the residual excess pore pressure in the 10 m thick of soft clay. It is believed that the soft clay is under consolidating and generated the residual excess pore pressure. This residual excess pore pressure caused the extra pore pressure acting on the retaining wall. The 2D finite element analysis was conducted to investigate the effects of the residual excess pore pressure on the performance of excavation. First, the input parameters of soft clay were back analyzed with the consideration of the in-situ of pore pressure, which was interpreted from the CPT-u test. Later, parametric studies were performed to investigate the relationship between the degree of consolidation and the deformations of retaining wall. The results show the larger the degree of consolidation, the smaller the deflections of retaining wall. In addition, from the back analyses results, the undrained shear strength and the effective Young’s modulus of under consolidating Jakarta soft clay are 20 kPa and 2600 kPa, respectively. Keywords: Excavation, FEM modelling, back analysis, under consolidation, soft clay.
1. INTRODUCTION In the traditional soil mechanics, the soils are differentiate between the normally consolidated (NC) and overly consolidated (OC) soils. However, some researchers reported that in some locations, the soil is under consolidating. The under consolidating soils could be found in the bay and/or reclaimed area, such as the Osaka Bay (Tanaka & Sakagami, 1989), the Craney Island reclaimed area (Karakouzian et al, 2003), the North Jakarta (Setionegoro 2013), and the Gwangyang Bay (Lim et al, 2014). The characteristic of the under consolidating soils is the existence of residual pore pressure in the soil layer which is detected from the piezocone penetration test (CPTu). Rahardjo (2008) explained the evidence of under consolidation soil with the dissipation test of CPTu as shown in Fig 1. Fig 1 illustrates the typical dissipation test result of under consolidation clay. The measured pore pressure (U2) consists of the hydrostatic pore 1
pressure and the excess pore pressure. For the under consolidating soils, the excess pore pressure could be differentiated as the excess pore pressure caused by the cone penetration and the residual excess pore pressure. For the normally consolidate soils, the residual excess pore pressure is zero.
Figure 1. Typical dissipation test result of under consolidating soil
Ph.D candidate, Taiwan Tech, Taipei, TAIWAN Lecturer, Universitas Katolik Parahyangan, Bandung, INDONESIA 2 Senior Engineer, Geotechnical Engineering Consultant, Bandung, INDONESIA 3 Professor, Universitas Katolik Parahyangan, Bandung, INDONESIA 1
The main objective of this paper is to investigate the effect of residual excess pore pressure to the performance of excavations by analyzing a case history of excavation. First, the input parameters of soft clay were back analyzed with the consideration of the in-situ of pore pressure, which was interpreted from the CPT-u test. Later, parametric studies were performed to investigate the relationship between the degree of consolidation and the deformations of retaining wall. 2. PROJECT DESCRIPTION The project is a one level basement excavation project which is located in the reclaimed land at North Jakarta, for a 24 stories office building. The typical upper stratum of soils stratification with thickness around 10 m is the very soft to soft sedimentary clays. For the design of the foundation and excavation, 7 bore holes were drilled in conjunction with the Standard Penetration Test (SPT), five locations of mechanic cone penetration test (CPT), 8 locations of piezocone test (CPTu), 8 locations of the Pressuremeter test, and laboratory testing of soil index properties, soil shear strength, and soil compressibility. Fig 2a and 2b shows the location and the excavation plan of the project, respectively.
2.1 Soil Stratification According to the several drilled bore holes, the soils stratification could be classified as follow: First layer was the fill soil with the average thickness of 2.5 m. The consistency was soft to medium with N-SPT was 3 to 9. Second layer is the under consolidating very soft to soft clay. The elevation of the second layer was around -9 to -12 m. In this layer, no record of SPT because the soil was too soft. The third layer was the stiff clay with the elevation around -12 to -22 m. The fourth layer was the lens of cemented sand. The thickness of this lens of cemented sand was around 5 m. Below the lens, it was founded the stiff clays until end of boring at elevation of -80m. The typical result of CPTu test is shown in Fig 3. The CPTu was performed in the fill and the very soft to soft clay layers. The result of CPTu was consistent with the bore hole data. Later, the result of CPTu test was used to estimate the input parameters of the under consolidating clay layer. 2.2 Result of Laboratory Test The water content, plasticity index, and liquidity index are plotted in Fig 4. In general, the water content of upper layer (0 to 10 m) had the water contents close to the liquid limit and/or the liquidity index was larger than unity. The results were consistent with the in-situ testing which classified that layer as the very soft to soft clay. The soil compressibility parameters are plotted in Fig 5. The results were obtained from the series of Oedometer test. For this location, the average coefficient of compression (cc) of soil below the cemented sand layer was around 0.5. The ratio between coefficient of compression and the coefficient of recompression (cr) was around 20 to 30.
38 m
67 m
3. FINITE ELEMENT MODELLING
Figure 2. (a).Location and (b). Excavation plan of the project X1-2
The Plaxis 2D finite element program was used to model the excavation project. Fig 6 depicts the finite element mesh and the model boundary. The excavation depth was 5.25 m, and the excavation width was 80 m. The analysis was followed the cross section A-A. Only a half of the excavation geometry was adopted in the analyses due to symmetry. The soil movements were fixed at the bottom of boundary and restrained at the vertical direction for the both sides. In addition, the distance between the retaining system and the outer boundary of mesh was ensured to be larger than 2He (final excavation depth) to minimize boundary effects. Moreover, fifteen-node triangular elements were used to simulate the soil cluster, 5-node plate elements were used to model the retaining system and 10-node interface element were applied to model the soil-plate element interaction behavior.
Proceedings of Soft Soils2016 , September 27-28th 2016
Soil Behaviour Type after Robertson,1986
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