Study on Effect of Water on Stability or Instability of the Earth Slopes

Transcription

1 International Research Journal of Applied and Basic Sciences 2014 Available online at ISSN X / Vol, 8 (9): Science Explorer Publications Study on Effect of Water on Stability or Instability of the Earth Slopes Mohammad Hosein Taghizadeh 1, Mahmoud Vafaeiyan 2 1. Department of Engineering, Islamic Azad University, Science and Research Isfahan Branch, Isfahan, Iran 2. Department of Civil Engineering, Isfahan University of Technology, Isfahan, Iran Corresponding author s.taqizadeh@gmail.com ABSTRACT: Water is the origin of the life, but the existence of water in earth slopes can be lead to irreparable damages. Water due to the fluid nature can be easily moved in the soil pores and due to reducing the effective stress and soil strength can be have devastating effects on the stability of earth slopes. Rise of groundwater in fine-grained soils causes swelling of soil seeds and in coarse-grained soils reduces the internal friction angle of the soil and thus lead to loss of ultimate bearing capacity of the soil. Hence, in slopes stability studies, water is a critical issue that ignoring it can be have devastating effects in the short or long term. Therefore by appropriate methods of drainage should be prevented of accumulation of water on landslideprone slopes and during the rainfall and rising groundwater that is the main factors of the presence of water in the soil, before reaching to the soil to collapse mode, this damaging water should be directed to out of the soil. Therefore, in this study, the stability of landslide-prone slopes in the presence and absence of water are determined and the safety factors gained in the two cases are compared. Keywords: Water, Earth Slopes, Effective Stress, Soil Resistance, Drainage, Safety Factor. INTRODUCTION Soil is composed of solid particles and voids between them. The other hand according to gravity law, water can be flows from a high-energy point to points with lower energy. In principle, several parameters in the deformation and displacement of the embankments foundation are effective. Some of these factors can noted to faults, veins and air-stricken loose rock layers, soft clay layers and air-stricken shale and increased pore water pressure (resulting from the consolidation of the layers due to applied load) which reduces its resistance. Hence, immediately after making due to increasing pore pressure and decreasing shear strength, increases the likelihood of Slip of layers. In general, the soil loses its apparent cohesion when becomes saturated and yet the specific gravity of saturated soils severely reduced and almost half the same of the soil which is above the groundwater level has been reduced. Embankments as heavy structures which have a significant dynamic interaction with their underlying substrate must be considered. Construction of a road embankment on soft soil layers with high saturated water level increases the pore pressure in the layers. Due to the applied load without draining, the effective stress is reduced. For this purpose, the investigation of pore pressure variations and settlement of road embankment on soft soil took place [1]. Kastunen et al (2006) examined the behavior of the stress - strain of embankment on flexible foundation with five different models and non-drainage conditions. The results were slightly different in each model [2]. Mellah et al (2000) by finite elements method and study on issues related to soil-structure interaction, found that this method has the ability to determine the mechanical parameters (Interstitial stress, deformation, displacement, pore pressure) [3]. Borges et al found the maximum amount of expected subsidence at the end of embankment operations on the soil with soft tissue is more than any other tissues and this change is due to the reduction of porosity during consolidation. [4]. Sari W Abusharar et al by study on layered embankment and numerical modeling by using finite element method found that in most cases, the maximum subsidence occurs at the toe of the embankment [5]. In this study, first by scheme a earth slope without water are determined the safety factor against soil sliding. Then this values obtained are compared with the saturated earth slope safety factor.

2 The effect of water on the stability of slopes Although water does not directly lead to the slopes displacement, but is an important factor for the following reasons [6, 7, 8, 9, 10]: - Water increases due to rainfall and snow melt will lead to slope weight gain. Water can penetrate into the pores and fractures and water instead of air to be replaced. Because water is heavier than air therefore the soil weight increases. Increase of stress due to increase of the soil weight can lead to instability. - Water can change the angle of slope (Angle of slope is an angle that slope is stable in this angle). The not consolidate dry seeds are formed in a column with an angle that is defined as the inertia angle. Inertia angle is angle of slope that stability of not consolidated seeds is controlled by the friction between them. For dry materials, inertia angle is increased by increasing the seeds size but it is usually between 30 to 45 degrees. With little increase of moisture in dry materials, the inertia angle of slope can be increased greatly. Because the surface tension between water and soil seeds are formed and keeps the soil seeds together. When minerals are saturated by water, so greatly the inertia angle is reduced. Because the seeds have a tendency to become fluid, therefore seeds friction disappears. - Water can be absorbed or excreted by minerals are available in the soil. Absorption causes the electric poles of water molecules bind to the minerals surface and penetrate into their structure. After adding the water, the weight of the rock and soil increases. Furthermore, if absorption occurs, the contact surface of friction between the seeds is low and will lead to reduce the cohesion of soil and resulting in the strength of rock is reduced. In general, the resistance of wet clay is less than dry clay. Therefore, water absorption will decrease the resistance. - Water can dissolve the cement between the seeds. If cement is made of calcite, gypsum and salt is easily dissolved in water and the cohesion between the seeds is lost. - Condensation (liquefaction) occurs when the sediments are saturated by water. The result is a loss of contact between the seeds, because water covers them. - Almost, groundwater exists in all parts of the earth. Groundwater level due to rain, fills the empty space between seeds of rock and the fractures that is available in them. Such changes in water level can be an effective factor in the stability of the slopes. - Another aspect of the influence of water on slopes stability is the pore pressure. The soil in the deep parts of ground is contains seeds with compact structure, and the pore water occupy a little space. Due to the high weight of overburden, water can apply a high pore pressure which led to a reduction in normal effective pressure and this is a contributing factor to the stability of the slopes and as a result the shear strength is reduced [11]. Slope profiles According to fig 1, a slope with angle of 45 degrees relative to the horizon so consider that the length of the upstream and downstream slope and the height difference between these two levels is equal to 10 meters. Since the objective of this study is to investigate the stability of slopes that is composed with sandy soil by water level changes. Therefore, first we study the stability of the slope without the presence of water and then bring the water level up to the level of the slope and stability or instability at saturated conditions will be discussed and the safety factor of the two modes are compared. For design and stability analysis of slope is used from the Flac finite difference software and static analysis that is based on the criterion of Mohr Coulomb is used in this study [12, 13, 14, 15, 16].

3 Fig 1: Schematic figure of the slope. Investigation of the slope stability without water The studied soil is the coarse sandy soil with unit weight 16 KN/m 3 so that the internal friction angle is 34 degrees and amount of the soil porosity is equal to 0.5. After simulation of slope according to considered dimensions, a coarse mesh grid that is defined by the software for ease and speed of analysis is used and this case for all of discussed slopes in this study is fixed. It should be noted that the smaller size of the mesh grids leads to increase the accuracy of software in analysis and obtained safety factor is more accurate. Looking at fig 2, the fracture created is shown. Fractures are based on shear strain rate contours and direction that occurs in the upper surface slope and its upstream portion. Safety factor in this case is estimated to be 0.72, that due to the minimum necessary safety factor for the stability of the slopes is equal to 1.5, this slope under the weight due to the driving forces is unstable and in fact the driving forces have overcome the resisting forces due to soil shear strength in sliding surface and sliding of slope will occur. Fig 2: Fractures caused by shear strain in the slope with a coarse sandy soil without water. Investigation of the slope stability in the presence of water Now we want to examine the effects of water on stability safety factor of slopes. For this purpose, first the coarse sandy soil were studied in the first model will used for this model and as shown in fig 3, the water level in second model rises to the fracture level. 3848

4 Fig 3: Water level in the slope with a coarse sandy soil. Looking at fig 4 can be seen that the water flow due to exposure below the sliding level is not changed in compared to the previous model and safety factor remains unchanged and is equal to The reason for this is due to the shear strength of the soil. As know, when the soil is under buoyancy forces due to its own weight, if these forces are able to overcome the resistance forces due to soil shear strength occurs a rupture level in soil. Presence of water on this rupture level can have dramatic effects on increasing levels of soil failure and decreasing safety factors. But the absence of water in this area due to the no change in internal friction angle of the soil particles and also the no change in specific weight of the soil in the rupture area is leads to constant safety factor. But by entry of water into the critical areas and prone to landslides, the soil are quickly loses its shear resistance against sliding forces and will be broken. So this rupture level in comparison with the condition of water absence is considerably wider. Fig 4: fractures caused by shear strain in the slope with a coarse sandy soil after the presence of water below the landslide-prone surface. Investigation of the stability of fully saturated slope According to fig 5, this time the water level rises to near of the ground surface. The other soil parameters are according to the previous model and the only difference is the fully water-saturated soil in its critical section and prone to landslides.

5 Fig 5: Water level in the slope with a coarse sandy soil. Looking at fig 6 can be seen that the water is reached to ground level and surrounds the critical slip surface. In this case the value of safety factor is greatly reduced and is reached to The reason for this is that water as a major factor in the reduction of soil shear strength when reaches to the area of prone to landslides, the water will surround the soil particles and aggregates and the friction angle between them decreases. This is leads to the intensification of soil failure and the safety factor for slope stability is reduced. Such that shown in fig 6, in the landslide-prone area of slope with fully saturated soil, the failure surface is increased greatly and instability of the slope is occurred. Fig 6: fractures caused by shear strain in the slope with a coarse sandy soil after the presence of water in the landslide-prone surface. CONCLUSIONS Water in earth slopes due to fluid property can be easily moved in the soil pores and can reach to the slope surface and the soil is fully saturated and it can lead to a reduction in effective stress and thus the soil strength reduces. The slopes composed of coarse-grained soils, water due to reduction of the internal friction angle of soil seeds and more freedom to move relative to each other is leads to decreasing in soil bearing capacity and its resistance to slip and this will reduce the safety factor of slopes stability. Therefore, for study and design of landslide-prone slopes, consideration of the factors of water presence (such as heavy rainfall and rising groundwater) are an important matter. Using from a proper drainage system in the earth slopes and directing the water to out of the soil profile can prevent of slippage and instability of slopes. The used drainage system must be effective against the volume of water in the soil and ensure of the authenticity of the drainage system and lack of obstruction before the operation is necessary.

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