SLOPE FAILURE OF AN EMBANKMENT ON CLAY SHALE AT KM OF THE CIPULARANG TOLL ROAD AND THE SELECTED SOLUTION

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1 International Symposium on Geotechnical Engineering, Ground Improvement and Geosynthetics for Human Security and Environmental Preservation, Bangkok, Thailand SLOPE FAILURE OF AN EMBANKMENT ON CLAY SHALE AT KM OF THE CIPULARANG TOLL ROAD AND THE SELECTED SOLUTION M. Irsyam Associate Professor Department of Civil Engineering, Bandung Institute of Technology E. Susila Assistant Professor Department of Civil Engineering, Bandung Institute of Technology A. Himawan Geotechnical Engineer LAPI ITB ABSTRACT This paper presents slope failure causes, mechanism and a selected solution of an embankment on clay shale in one of most vital highways in Indonesia, the Cipularang Toll Road. The toll road was constructed in 2004 to 2005 to support economic growth in Jakarta, Bandung and the surrounding area. The toll road had to pass hills and valleys on clay shale of the Miocene Djatiluhur Marl formation. The slope failure occurred in February 2006 at Km and attracted national attention due to its national critical function. This paper also presents the subsurface condition, the clay shale degrading strength due to exposure, back calculated strength parameters of degrading shale, slope movement monitoring result, geotechnical analyses and a group of bored piles as the selected reinforcement method. Geotechnical analyses were performed by utilizing PLAXIS, finite element software. The elastic plastic constitutive model and the Mohr-Coulomb failure criteria were chosen to model soils. Field monitoring during the following several heavy rains and geotechnical analyses show that a group of bored piles with a diameter of one meter is effective to stabilize the slope. INTRODUCTION The Cipularang Toll Road, located in West Java, Indonesia, is one of the most vital highways in Indonesia that supports transportation link among its major cities: Jakarta, Bandung and the surrounding area. The toll road was constructed in 2004 to 2005 and was opened for public transportation in March Due to topographical and geological conditions, the highway had to pass hills and valleys on clay shale of the Miocene Djatiluhur Marl formation. In early February 2006, after 11 months of operation, a road bed embankment at KM failed. Figure 1 illustrates the slope failure of the slope embankment on clay shale layer. Records of slope monitoring indicators which are shown in Figure 2 indicated that failure plane was from toe of embankment to the top of embankment at the median of the highway. 531

Fig. 1 The slope failure at road bed embankment on clay shale at KM 97+500 in February 2006 (Irsyam et al., 2006) ROW ROW Fig. 2 The result of slope indicators monitoring (Irsyam, et al.

2 Fig. 1 The slope failure at road bed embankment on clay shale at KM in February 2006 (Irsyam et al., 2006) ROW ROW Fig. 2 The result of slope indicators monitoring (Irsyam, et al., 2006) The as-built drawing and construction documentation show that construction of the road bed embankment was initiated by stripping the original surfacial soft clay shale with approximate thicknesses of 0.3 to 2.0 m. However, soil investigation conducted after slope failure revealed that a layer of original soft clay shale was found directly below fill embankment, especially below the toe of slope. Back calculation by slope stability analysis verified that the slope failure occurred on the soft clay shale. Shear strength and volume stability of the shale is time dependent. Due to the soil stripping/excavation and weathering, shale looses its shear strength. This paper presents behaviors of clay shales, slope failure mechanism, selection of shear strength parameters, geotechnical analyses and the chosen slope reinforcement of failure slope which is a group of bored piles. 532

3 MINERALOGY OF SHALE Stark and Duncan (1991) investigated the cause of a slide at the upstream slope of Californias s San Luis Dam which is supported by stiff and desiccated clay. They showed that shear strength of the clay decreases rapidly to the fully softened strength when the clay is soaked. When the clay is subjected to a cyclic loading, the strength decreases gradually from the fully softened to its residual value. Skempton (1977) concludes that heavily over consolidated clay is usually firm and stable and has comparatively high shear strength in its original condition. Chemical changes along exposed fissured (weathering) cause reduction of the shear strength. Depending on the progress of weathering, weak zones are developed in a potentially unstable slope. In the most stressed areas, small movements begin to develop that leads to further reduction of shear strength and slope instability. Shale is a fine grained sedimentary rock formed from clays compacted together by pressure. Shale is generally characterized by thin laminae breaking with an irregular curve fracture, often splintery and usually parallel to the often indistinguishable bedding plane (Wikipedia, 2007). Shales are typically deposited in very slow moving water and are often found in lake and lagoonal deposits, in river deltas, on floodplains and offshore of beach sands (Wikipedia, 2007). The main engineering behaviors of shale is that it is very hard, however, once it is exposed to sunrays, air, and water within a relatively short time it will become soft clays (mud). SHEAR STRENGTH OF SHALE Gartung (1986) observes that when a clay, that is originally dry and hard with high shear strength, absorbes water during the process of unloading, it turns rapidly to stiff or even to soft clay with an extremely low shear strength. After opening a cut, the state of weathering does not only vary locally but also vary with time. Zones that are relatively unweathered before excavation will form a new surface and the clay being unloaded and exposed to the atmosphere will start to weather and change properties very quickly. As the weathering effects proceed, the shear strength distribution of the soil profile changes with time. Gartung (1986) divides the clay into 4 zones according to the degree of weathering as shown in Figure 3. Zone I comprises the unweathered clay stone while zone IV refers to the final stage of the weathering process. Using slow triaxial tests the residual angles of internal friction were determined on samples of the 4 zones of weathering. Gartung (1986) showed that the average residual angles of internal friction, φr is 8.6 o. Gartung (1986) also showed that φr is independent of the degree of weathering. To use the residual strength parameters would be too conservative. On the other hand it appeared unsafe to analyze the stability with peak strength parameters, even of the weathered soil of zone IV. For design, Gartung (1986) suggested to use the reduced parameters for the long term condition: φ = 20 o and c = 20 kn/m 2. In a partially saturated shale, water within the voids would be concentrated in tiny menisci around the particle contacts and would have the effect of bonding of the particles together. In this condition a dry clay exhibits a pseudo - preconsolidation pressure that is related to the strength of these bonds between particles. Soaking the clay eliminates the negative pore pressure, destroying the bonds. Figure 4 presents the effect of soaking on the shear strength of clays as suggested by Stark and Duncan (1991). 533

4 Fig. 3 The shear strength of shale proposed by Gartung (1986) Fig. 4 The time-dependent shear strength of shale (Stark and Duncan, 1991) Skempton (1977) observed that slope failures generally occur some time after excavation. He concludes that the main reason for the delay is the slow rate of equilibrium of pore pressure fluctuation, despite the fissured structure of the clay. He performed shear and triaxial tests on brown London Clay. He chose a 6 cm shear box and 38 mm diameter triaxial tests. He found the peak strength parameters of the clay are: c = 14 kn/m 2 φ = 20 o 534

5 Sandroni (1977) performed triaxial tests of a London Clay with the largest triaxial samples (250 mm diameter) to include a representative assemblage of fissures. He found that the cohesion of the larger diameter samples is smaller, with the following parameters: c = 7 kn/m 2 φ = 20 o Figure 5 shows strength parameters of the London Clay for both diameters of 38 mm and 250 mm. Our back calculations on the slope failure at KM shows that even with the larger diameter triaxial tests, the strength parameters are still larger than the field values obtained from back analysis. Fig. 5 Shear strength of shale for several various sizes of samples (Skempton, 1977) Based on the results of soil investigation before construction and deep coring after construction, the slope failure at Km of the Cipularang Toll Road was predictably caused by strength degradation of the surfacial clay shale due to the stripping works. To obtain a suitable solution, appropriate prediction of shear strength parameters of the degraded clay shale becomes very crucial. Unfortunately, there was no laboratory test on the exposed clay shale before. The monitoring result of slope indicators which is shown in Figure 2 becomes critical since failure plane could be predicted. Based on the predicted failure plane the shear strength parameters of clay shale at failure condition could be evaluated by performing back calculation of failure condition. Soil profile (Figure 6) and shear strength parameters of other layers are developed based on as-built drawing and the result of deep coring after slope failure. The analysis was performed by a parametric study through the slope stability analysis to examine several predicted shear strength parameters of clay shale. The shear strength parameters of clay shale which cause slope failure with the most similar failure plane as interpreted failure plane based on records of slope indicators were selected as the field parameters. Slope failure is indicated by a safety factor value of

FG A B E D C No A B C D E Layer Description Fill Material (Road Bed) Fill Material (Side slope) Uncompacted material Silty Clay dan Weathered clay shale Hard clay shale Fig.

As shown in the figures, the slope failure plane occurs at Layer D which is the soft silty clay and weathered clay shale.

6 FG A B E D C No A B C D E Layer Description Fill Material (Road Bed) Fill Material (Side slope) Uncompacted material Silty Clay dan Weathered clay shale Hard clay shale Fig. 6 Soil profile Figures 7a and 7b present results of back analysis using a finite element software of PLAXIS (Brinkgreve and Vermeer, 1998). As shown in the figures, the slope failure plane occurs at Layer D which is the soft silty clay and weathered clay shale. Result of back analysis shows that the calculated shear strength parameters of the degraded soil layer at failure were c = 5.0 kpa and φ = 13 o. These values are comparable with the parameters of soaked residual and remolded residual suggested by Stark and Duncan (1991) and residual soil by Skempton (1977) but smaller than design shear strength parameters recommended by Gartung (1986) and residual parameters suggested by Gartung (1986). (a) (b) Fig. 7 The predicted slope failure mechanism based on slope stability analysis: (a) deformation vectors (b) deformation contours 536

7 Table 1 The calculated parameters based on the parametric study Layer No Identification γ c φ [kn/m 3 ] [kn/m 2A ] [ ] A Fill Material (Road Bed) B Fill Material (Side Slope) C Uncompacted material D Silty Clay dan Weathered clay shale E Hard clay shale SLOPE REINFORCEMENT USING BORED PILE A group of bored piles was selected as the most suitable solution to overcome the slope failure due to time and space constrains and topographic condition at the site while the cost was still considered effective (Irsyam, et al., 2006). This type of reinforcement has been successfully overcome slope failure in a valve chamber of a power plant (Irsyam et al., 1999). The length of bored piles has to be able to cut failure plane and the passive resistance of soil to bored piles below the failure plane has to be large enough to resist failure. Figure 8 presents the chosen arrangement of group of bored piles for this project. As shown in the figure the group of bored piles was consisted of 2 layers of 18 m length of bored piles with a diameter of 1.0 m and pile spacing (center to center) of 2.0 m, arranged in a zigzag pattern. Another finite element analysis was performed to confirm the slope stability, the sufficiency number of bored piles to resist bending moment, and adequacy of bore pile length The bored piles were modeled as elastic plastic beam elements. Bending and axial stiffness parameters are summarized in Table 2. Figure 9 presents result of finite element analysis after installation of the group of bored pile. The figure shows that failure plane is on top (left side) of bored piles. The analysis result also shows that bored pile has increased the factor of safety to a value of more than 1.3 and the capacity of bending moment of group of bored piles is more that required. Bored piles were constructed around October in Figure 10 illustrates site condition during and after construction of bored piles. During last year rainy season, around November 2006 to January 2007, with several times of hard rains, records of slope indicators showed that there was no significant soil movement. Table 2 The stiffness parameters of bored pile as beam element EA EI w ν Number Identification Type [kn/m 2 ] [kn/m 2 ] [kn/m 2 ] [ - ] 1 Bored Pile D=1000 Elastic

8 Fig. 8 The cross section of slope reinforcement and bored pile arrangement Fig. 9 Result of slope stability analysis after installation of bored piles (SF>1.3) 538

(a) (b) Fig. 10 Slope conditions: (a) during construction of bored piles and (b) after construction completion of soldier-bored piles CONCLUSIONS 1.

The backcalculated shear strength parameters at failure condition were c = 5 kpa and φ = 13 o. 2.

9 (a) (b) Fig. 10 Slope conditions: (a) during construction of bored piles and (b) after construction completion of soldier-bored piles CONCLUSIONS 1. The slope failure at Km of the Cipularang Toll Road was mostly led by the existence of low shear strength layer of silty clay and weathered clay shale. The backcalculated shear strength parameters at failure condition were c = 5 kpa and φ = 13 o. 2. A group of bored piles was selected as the most suitable solution to overcome the slope failure due to time and space constrains and topographic condition at the site while the cost was still considered effective. The calculation result and records of slope indicator during the following several heavy rains indicate that the group of bored pile is effective to stabilize the slope. REFERENCES Brinkgreve, R. B. J. and Vermeer, P. A. (1998). PLAXIS: Finite Element Code for Soil and Rock Analyses, A. A. Balkema, Netherlands. Gartung, E. (1986). Excavation in hard clays of the Keuper Formation, Proceedings of Symposium, Geotechnical Engineering Division, Seattle, Washington. 539

10 Irsyam, M., Surono, A., Himawan, A. and Nugroho, A. (2006). Laporan Disain Penanganan Kelongsoran Timbunan Badan Jalan KM Tol Cipularang, LAPI ITB-PT. JASA MARGA (persero). Irsyam, M., Tami, D., Sadisun, I. A., Karyasuparta, S. R. and Tatang, A. H. (1999). Solving landslide problem in shale cut slope in the construction of the Valve Chamber of the Tulis Hydro Electric Power, Journal of 99 Japan-Korea Joint Symposium on Rock Engineering, ISSN , Fukuoka, Japan. LAPI ITB (2006). Penelitian dan Penyelidikan STA Jalur B Pada Proyek Pembangunan Jalan Tol Cipularang Tahap II Paket 3.1 Ruas Plered - Cikalong Wetan Kabupaten Purwakarta, Jawa Barat Sandroni, S. S. (1977). The Strength of London Clay in Total and Effective Stress Term, Ph.D. Thesis, University of London. Skempton, A. W. (1997). Slope Stability of Cuttings in Brown Clay, Tokyo. Stark, T. D. and Duncan, J. M. (1991). Mechanism of strength loss in stiff clays, Journal of Geotechnical Engineering, Vol. 117, No. 1. Wikipedia (2007). The Free Encyclopedia, Florida, U.S.A. 540

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