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  National Geofest 2014 1 SLOPE FAILURE ANALYSIS: FAILURE DUE TO WATER SEEPAGE Mr. Mohhammad Nasir Bin Mohammad Taher Affiliation, Faculty Of Civil and Environmental Engineering, University Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor.   ABSTRACT   The issue of slope stability is a core concern to geotechnical engineers. Traditional methods of slope analysis have potentially ignored the effects of surface cracks and vegetation. It is also known that higher seasonal rainfall and seepage through surface crack are closely associated with slope failure. In this study two different rainfall intensities are analysed for rainfall induced surface cracked slope failures. Surface cracks can change an existing seepage pattern and increase the soil moisture content into deeper layers at wet season. Therefore, a surface crack is likely to decrease the stability of the soil and increases the tendency of a slope to fail. INTRODUCTION Permeability of a soil is a measured the capacity of the fluid or gas flow ,which in Geotechnical Engineering more concern in liquid rather than gas. Void in soil particles are interconnected which enables of water to pass through in term of the larger the pore space the more permeable the material. This usually happen in strongly aggregate soils like sand grains. Thousand people are killed by slope failure. Slope failure is a phenomenon that are slope collapse due to weakened self-retain ability of the earth under the influence of a rainfall infiltration through the soil cracks and increases moisture content in deeper layer through the seepage. The slope fail in different way, depend on the angle of slope the water content, types of earth material involved and the local environmental factors. Example of this phenomenon is happen in china on 2012. When more water move into the deeper layer of soil this will reduce the soil shear strength and cause slope failure. Slope failure factors:    Gravity can be divided into components acting parallel to a slope and perpendicular to the slope.    Failure is more likely to occur if the effect of friction on the potential sliding surface is reduced.    The physical properties of the slope materials such as cohesion between grains may reduce the potential for slope failure.    The angle of repose is the maximum slope generated when loose unconsolidated material is formed into a pile.    The addition of excess water may destabilize slopes by adding weight, destroying cohesion between grains, and reducing friction. OBJECTIVE I.   To identify the permeability occur at the slope failure II.   To identify the permeability in different soil III.   To identify the slope failure on clay soil APPLICATION   2.1 Introduction  The relationship between  permeability and range properties of soil has interested researchers from the  beginning of modern geotechnic. Which is how to design and modelling permeability equation and the behaviour of it due to the slope stabilization. In terms of how sediments deform and pore fluid flows during the shearing process has not been  precisely determined for the variety of styles of landslides. While some failures may occur at a stratigraphic discontinuity, others may occur within the sediment column. When failure is within the sediment, to what extent that failure is localized is uncertain. Not knowing when and where failure may start happed field observations of failure initiation and  progression. 2.2 Permeability In different soil. Hydraulic Conductivity (K) depends on the properties of the porous medium as well as the fluid moving through it. K depends on the size of the  National Geofest 2014 2 grains and their subsequent distribution as well as the shape, packing and orientation of the sediment particles. This determines the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient. Darcy Formula K = v/ i  (relationship  between discharge velocity and the hydraulic gradient) Water can permeable between granular void or pore spaces, and fractures  between rocks. The larger the pore space, the more permeable the material. However, the more poorly sorted a sample (mixed grain sizes), the lower the  permeability because the smaller grains fill the openings created by the larger grains. The most rapid water and air movement is in sands and strongly aggregated soils, whose aggregates act like sand grains and  pack to form many large pores . The type of soil consist different value of K where it is classify by the texture of particles show below:- Figure 1 : Permeability Chart for Typical Sediments 2.2.1 Permeability Laboratory Measured The permeability of saturated soils can be measured in either the laboratory or the field. Because of the very large range of  permeability in soils, no single method is suitable for all cases. The types of equipment most generally used are: i.   The constant head permeameter, for permeability’s above 10 -4 m/s, (gravel and sand ) ii.   The falling head values of k  between 10-4 and 10-7 m/s (clay, silt and fine sand) 2.3 Permeability occurs in slope. A smaller but more frequently occurring landslide category is the surficial slump. This is a form of slope instability in which a relatively thin zone of soil, generally parallel to the slope face, slides down the slopes. The surficial slump is the most common form of land sliding, but is not typically referred to as land sliding. In natural slopes, this form of instability is generally limited to topsoil  profiles and occasionally a weathered  portion of the underlying formational material. I graded slopes; the problem is typically manifested in the weathered soil zone near the surface. According, the  problem in graded slopes is most prevalent in compacted fill slopes composed of expansive clayey soil. When the outer face of an expansive fill material swells even slightly, soil particles in the swelling zone move apart ever so slightly. Nonetheless, as the soil porosity increases even slightly, the permeability of soil parallel to the slope face increases significantly. Once permeability differences develop between the outer zone of the slope and the compacted inner core, seepage occurs parallel to the slope face I response to prolonged heavy rainfall. When this seepage does occur, the buoyant effect is triggered, the soil strength is roughly cut I half, and the slumping  begins. Occasionally, granular fill slopes and bedrock slopes may be impacted by surficial slumping. The failure mechanism is controlled by the steepess of the slope,  permeability differences, and the soil or rock strength characteristics. In order to initiate a failure, sufficient water is required to saturate the soil and cause seepage to develop essentially parallel to the slope surface. Figure 2 : Permeability Chart for Typical Sediments  National Geofest 2014 3 Table 1 : Relationship permeability between porosity and shear stress due to slope failure 2.4 The Advantages and Disadvantages of High and Low Permeability in slope stabilization 2.4.1 High Permeability Advantages In hill construction, high  permeability is the main slope stabilization from failure. Where the evidence show that more construction in high soil that the soil content coarse soil and granular are more long time from slope to failure. In addition, high  permeability also is good for influence water flow drainage from slope soil to the main drainage ground. Disadvantages High permeability also have their weakness where at desert there are rock formation  become high show that rock with no combination between small  particles and high porosity will easier the slope rock collapse. 2.4.2 Low Permeability Advantages In submarine soil, slope happen give benefits to geology oil researcher where slope failure in base soil submarine pumping the oil or petroleum and gas. Disadvantages Seem that disadvantages of low permeability already know that it is enemy to due slope failure happen. Which is the infiltration of water below the ground surface are slow and when heavy rain occur the water will floating at above make the  pressure from water above push the soil and the shear stress  between particle become faster. Show that, the slope failure happen faster in heavy rain when the water cannot permeable faster.  CASE STUDY   3.1 Study Case 1: Slope Failure at Highland Tower, Ulu Klang, Malaysia The Highland Towers ollapse was an apartment building collapse that occurred on 11 December 1993 in Taman Hillview, Ulu Klang Selangor, Malaysia. The collapse of Block One of the apartments caused the deaths of 48 people and led to the complete evacuation of the other two blocks due to safety concerns. The Highland Towers consist of three 12-storey blocks, built in phases between 1975 and 1982 at the western base of a steeply sloped hill which was later terraced extensively in the early 1980s. Each block was respectively named: Block 1 (built 1977, southern-most), Block 2 (built 1979, north-northwest of block 1, slightly elevated than the other two, closer in to the hill) and Block 3 (built 1981, northwest of  block 1, west of block 2). A swimming  pool was located between northwest side of Block 2 and northeast rear of Block 3. Block 1 collapsed when 10 continuous days of rainfall led to a landslide after the  National Geofest 2014 4 retaining wall behind the Tower's car park failed. Behind the Towers was a small stream of water known as 'East Creek'  . East Creek flowed into the site of the Towers before the Towers' construction. Later, a pipe system was built to divert the stream to bypass the Towers. In 1991, a new housing development project known as Bukit Antarabangsa Development Project commenced on the hilltop located  behind the Towers. As a result, the hill was cleared of trees and other land-covering plants, exposing the soil to land erosion that will cause land slide. The water from the construction site was diverted into the same pipe system used to divert the flow of East Creek. Eventually, the pipe system became overly pressurized with the water, sand and silt from both East Creek and the construction site. The  pipes burst at various locations on the hill, and the soil had to absorb the excessive water. The monsoon rainfall in December 1993 further worsened the situation. The water content in the soil had exceeded a dangerous level, and the soil had literally turned into mud. By October 1992, the hill slope had been almost flooded with water, and it was reported that water was seen flowing down the hill slopes and the retaining walls. Soon after, a landslip took  place and destroyed the poorly-constructed retaining walls. The landslide contained an estimated 100,000 square meters of mud, a mass equivalent to 200 Boeing 747  jets.   The soil rammed onto the foundation of Block One, pushing it forward for a while  before causing it to snap and bringing down the apartment block. A month before the building fell, in November 1993, residents began to see cracks forming and widening on the road around the Highland Towers, a sign of collapse but unfortunately, no further investigation was carried out. The official death toll released by the authorities was 48, though other sources gave a number greater than 55. The victims are mainly Malaysian, with 12 foreigners who are includes a Briton, a Japanese, 2 Indians, 2 Koreans, 3 Filipino and 3 Indonesian. Rescuers heard knocking and voices right up to the seventh day after the collapse. Only three  people, including an infant, were pulled out from the rubble alive, and only within the first 24 hours. One of them died later in hospital. There were plans to repair the two remaining blocks and re-occupy them  back in 1995, but unfortunately, researches revealed that the blocks were no longer structurally safe and the only thing that could be done is demolish them. But, after the tragedy, The Highland Towers memorial stone was placed at the site of Block 1, but sadly it was a victim of much vandalism. Block 2 and Block 3 of the Highland Towers still remain stand until today although they are now abandoned and almost completely overgrown by the dense jungle. Figure 4 : Slope Failure at Highland Tower   3.2 Case Study 2: Landslide at Pos Selim, Cameron Highland Highway, Malaysia  The Pos Selim, Cameron Highland is one (package 2) of the 8  packages of road that link Simpang Pulai in Perak & Kuala Berang in Terengganu; the 3rd East-West Highway in Peninsular Malaysia, after the KL-Karak and Gerik-Jeli Highway. The construction of the 35 km highway was started in 1997 and was scheduled to be completed on Apr 2000,  but having confronted with slope failures, the opening of the highway to the public had been delayed to 2004. One of the major failures was the deep-seated, structurally-controlled massive cut slope failure between CH23000  –   CH24500. A series of failures on this slope was first reported in April 2000 after a long period of intense and heavy rainfall which triggered further retrogressive slips in the upper catchment. The deep-seated failure mode exhibited by these failures had  prohibited the adoption of stabilization measures using active anchors, passive soil nails, sprayed concrete etc. Eventually, the Main Contractor had no alternative but to adopt the rehabilitation scheme involving slope re-

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