Controll Of Landslide By Zero Energy Ground Water Extraction System

Controll Of Landslide By Zero Energy Ground Water Extraction System

Selvendran.R1 , Abinave.K2 , Athul Krishna.P2 , Abdul Rahim .R2

1Associate professor , 2Student

Department of Mechanical Engineering, Hindusthan Institute of Technology Coimbatore 641032

ABSTRACT:

Slope failure is a common issue in tropical countries. The rise of groundwater table due to rainfall is one of the main triggering factors. There are several methods for slope stabilization such as soil nailing, retaining walls, cut and fill, vegetation and so on. Most of those methods are costly and we are in need for stabilizing methods that are more economical and easier to construct. This article introduces a new method for slope stability. This method is examined numerically and experimentally. It is represented in an automatic zero-energy groundwater withdrawal system to enhance slope stability. The system is validated in a pre-fabricated model to ensure that it works on natural soil slope. The numerical simulation is performed in Soilworks software with coupled seepage-slope stability analysis using finite element methods to check the safety factor with and without the system. The effectiveness of this method is investigated with various rainfall intensities and soil permeabilities. The results for slopes with the application of groundwater withdrawal system are compared with the results without the system. The results demonstrate the effectiveness of the proposed method in reducing groundwater table and enhancing slope stability. The factor of safety for the slope with high soil permeability drops from

1.312 before the rainfall to 1.292 and 0.93 after the third rainfall event for the slope with and without pumping groundwater respectively. For soil slope with moderate

soil permeability, the factor of safety deteriorates from 1.314 to 1.157 at the end of the third day, while it remains stable with pumping groundwater. Matric suction is highly increased at the crest of the slope due to pumping.

Slope failures have been identified as one of the most frequent natural disasters. The rise of groundwater due to infiltration of rainwater is a major triggering factor for slope failure . The soil permeability affects the rainfall infiltration within unsaturated soil. Soil with higher permeability allows more rainwater to infiltrate and flow into the soil slope resulting in a quick change of pore-water pressure from negative to positive. The infiltration of rainwater increases water content and decreases matric suction in the soil. During rainfall, soil suction dissipates due to the saturation of the soil. Failure would happen because of positive pore-water pressure development. The factor of safety increases with the increase of matric suction . When the rainfall intensity is greater than the soil permeability, it infiltrates initially by higher infiltration rate than ks, by as much as 3.5 times. Then it declines over time towards the steady state conditions . However, when the rainfall intensity is less than ks , the initial infiltration is low at the crest of the slope but rises gradually towards the steady state conditions

Fig 1. Landslide

This paper focuses on the effect of rainfall induced landslide in which, the effect of the rise of ground water table with respect to rainfall intensity and soil permeability is identified. Pumping groundwater method is examined numerically and experimentally and compared with slope conditions without pumping. The rain is applied at far field from the area having potential slope failure with different rainfall intensities. This study is based on numerical analysis in which a coupled seepage-slope stability analysis is carried out by Soilworks software using Finite Element Method and Limit Equilibrium method.

The soil properties are taken from the lab experiments for soil case studies located in Malaysia and reported by at University Technology Malaysia, Skudai Campus, namely Bukit Cerapan.

Table 1 shows the physical properties of the soil.

The soil has a hydraulic conductivity of 5.00E-7, saturated volumetric water content

is 45%, a residual volumetric water content is 33%, specific gravity 2.63, cohesion equals to 7.6 kpa and friction angle of 32o . The parameters used for predicting hydraulic conductivity functions of soils using Van Genuchten method are

Table 2: Van Genuchten fitting parameters

for predicting hydraulic conductivity function

A homogeneous soil slope model is designed, with 1.00 Km long, 0.200 Km height and 45o slope angel. A ground water table is specified which is 5 m deep at the toe of the slope. The slope model is underlined with an impermeable layer as shown in Fig. 1. The raining zone is assumed to be limited at Zone 1 [300 m] and the slope stability check is allocated at Zone

3. This design enables us to study the effect of surrounding topography on slope stability and to study the infiltration of rainwater through long and deep distance as well as the effect of rain intensity on the rise of deep groundwater table. A ten-meter long sub- horizontal well [45o to the horizontal] is located at the toe of the slope.

Fig 2.1. Soil properties

The procedures of the laboratory experiment are as follow:

1. Soil sample is collected and oven dried for 24 hours

2. The cylinder is filled with soil until point B and the reservoir is filled with water

3. The valve is open to allow the water flow through the sample

4. When the water level reaches point c, it automatically flows and discharges at point D.

5. After that, the valve is closed.

6. The flow at point D and the decrease of water level is monitored

7. The pipe at point D is extended to a level parallel to point A.

8. The steps 4 6 are repeated, the decrease of water level and the flow at the end of the pipe are monitored

Fig 2.2. Equipment and procedure

Fig 2.3. Model

1. The rainfall is specified at Zone 1 along 300 m of the slope model. The rainfall intensity varies from low intensity of 0.2 m/day to high intensity of 0.6 m/day with different soil permeability ranging from very permeable soil with ks = 1E-5 m/s to low permeable soil with ks = 1E-7 m/s. The time stages of rainfall are specified for three days in a rainfall period of 6 hrs/day, which are sufficient for the study of the variation of groundwater level . Figure 1 illustrates the distribution of rain along a homogeneous slope. The maximum negative pore water pressure in the slope model is 20 KN/m2 . A well at the toe of the slope is specified and a negative flux boundary is applied to the well boundary representing pumping rate during the rise of the groundwater table. The negative flux rate is the same value of soil permeability in each scenario to get the lowest seepage through well boundaries. Table 3 shows the rainfall functions for rainfall intensity within the rainfall period

   for low, moderate, and high intensities. Table 4 shows the scenarios of rain intensity, soil permeability, and pumping rate. Nine scenarios are analyzed without pumping to represent the natural condition of the slope and nine scenarios are analyzed with pumping

   Table 1: Rainfall functions

   Table 2: Rainfall intensity, soil permeability, and pumping rate scenarios

   The groundwater table rises symmetrically with all rainfall intensities because the infiltration of rainwater declines with the depth and almost the same quantity of rainwater reaches the groundwater table. The groundwater table rises rapidly with high permeability sil slope; it rises up to

   9.69 m at the end of the first day and 10.61 m above the initial groundwater level at the

   end of the third day at Zone 3 as shown in Fig. 4. The rise of groundwater table causes the factor of safety of the slope at Zone 3 to drop from 1.312 to 0.95 at the end of the first day and 0.93 at the end of the third day. On the other hand, pumping groundwater table at Zone 3 keeps the groundwater as low as the depth of the pumping well. As a result, the factor of safety remains almost constant, it drops slightly from 1.317 to

   1.294 at end of the first day due to the advancement of groundwater from Zone 2 which touches the slip surface at the middle of the slip surface and remains stable till the end of the third day . For the slope with moderate soil permeability, the groundwater table rises 3.28 m at the end of the first day and 5.87 m above the toe of the slope at the end of the third day as shown in Fig. 5. It decreases the factor of safety of the slope from 1.314 to 1.157 at the end of the third day. On the other hand, pumping groundwater systems keeps the groundwater below the depth of the pumping well and the factor of safety remains constant . For the slope with low soil permeability, the rise of groundwater table is slight. It rises 1.22 m above the initial groundwater table for slope without pumping while it remains below the wells depth with pumping system as shown in Fig. 6.

   Fig. 3.1: The rise of GWT after 0.2, 0.4, &

   0.6 m/day rainfall intensity in soil of 1E-5 m/sec permeability

   Fig .3.2: The rise of GWT after 0.2, 0.4, &

   0.6 m/day rainfall intensity in soil of 1E-6 m/sec permeability

   Fig. 3.3: The rise of GWT after 0.2, 0.4, &

   0.6 m/day rainfall intensity in soil of 1E-7 m/sec permeability

   Fig. 3.4: The rise of groundwater for 0.2, 0.4, and 0.6 m/day rain intensity for slope with high soil permeability and Pumping of 0.864 m/day.

   Fig. 3.5: The rise of groundwater for 0.2, 0.4, and 0.6 m/day rain intensity for slope with moderate soil permeability and Pumping of 0.0864 m/day

It is clear that the rise of groundwater table due to the rainfall at far field may trigger landslide as shown in Fig. 9a where the factor of safety drops to 0.95 in one day. The factor of safety drops just after the first rainfall event from 1.312 to 1.239 at the end of the first rainfall event and 0.95 at the end of the first day for slope with high permeability soil. Groundwater pumping enhances the slope stability and the factor of safety decreases slightly at the end of the first day to 1.294 and levels up until the end of the third day as shown in Fig. 9. Figure 10 shows the slip surface and the factor of safety for slope without pumping groundwater [a] and with pumping groundwater at the end of the third day. For the slope with moderate soil permeability [1E-6 m/s], the factor of safety declines from 1.314 to 1.158 at the end of the third while it remains stable with pumping ground water. The factor of safety does not change with low soil permeability slope [1E-7] as the rise of groundwater is slight and does not touch the slip surface in both cases.

Fig. 4.1: The resulted factor of safety for the slope [a] without pumping groundwater and [b] with pumping.

Fig. 4.2: The factor of safety at the end of the third day for high soil permeability soil slope [a] without Pumping groundwater and [b] with pumping groundwater

The groundwater table rises due to the rainwater infiltration, the high permeable soil allows much rainwater to reach the groundwater table and cause a quick rise of groundwater level. The quick rise of groundwater may trigger a landslide even though the rain occurs at far field from the slope. The rise of groundwater may induce landslide just after a few hours of the rain event. Slopes with moderate or low soil permeability may fail due to the rise of groundwater table after one or two days of the rain event because of the time taken for the rainwater to infiltrate and the developed

perched water tables to dissipates to the main groundwater table. The effect of rain intensity on deep groundwater table is symmetrical because the rainwater decreases while infiltration and almost the same quantity reach the groundwater table but a large perched water develops with high rain intensity that causes the groundwater table to rise for days after the rain event. The application of groundwater withdrawal system is effective in enhancing the slope stability and avoiding the slope failure induced by the rise of groundwater table.

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