Stability Analysis of Rock Slopes Near the Khlong Tha Dan Dam - Term Paper Example

EXECUTIVE SUMMARY The project seeks to investigate the slope stability of surrounding slopes of The Khlong Tha Dan dam which have been observed to experience a progressive creep landslide after the construction of the hydro dam. Boreholes were dug into the identified area and the rock stratum noted as well as the samples taken for further laboratory analysis. Where there was insufficient information, it was obtained from previous works. The geo-slope program was used to generate the cross-section of the area. Analysis were then undertaken and factor of safety under different water table levels on the fully weathered sandstone were determined. Results obtained are a representation of the condition at the ground for the area covered along line A. occurrence of a landslide on the slopes of this dam would result in the damage of roads and the irrigation systems along these slopes. Mitigation process that can be undertaken have been proposed in this paper. 1. Introduction 1.1 Project description The project was undertaken on the slopes along the Khlong Tha Dan dam in Thailand after some slope instabilities began to be experienced especially when it rained. The investigation involved the assessment and analysis of this slopes. The various failure modes of the slopes were identified using the Roc science computer program. There possibility of damage of a road leading to the dam if the slopes failed and thus reason for assessment and mitigation solutions to reduce such losses. It was found out that the slope is made of weathered sandstone, andesite and tuff. The boreholes were dug 30m deep the soil profile obtained. The sandstone is heavily weathered and subject to instability when the water level rises. Geological mapping and geological and borehole surveys were used for data collection accompanied with shear tests on the samples to find out the strength characteristics and the physical characteristics of the soil. The cross-section was obtained from the borehole logs collected along the line A. The boreholes positions is as shown in the Figure 1. Figure 1: The boreholes positions 1.2 Location The dam is located on the Hin Tang district in the province of Nakhon Nayok. It is accessible through highway 3049 or highway 3239. Figure 2 Figure 2: Real Location The area prone to landslide has been for a long period by the locals for irrigation purposes and if the landslides continue would lead to a lot of economic losses. When it rains the water level rises very high which increases the unit weight of the soil resulting to the soil becoming heavier and consequently increase chances of occurrence of the landslide. 1.3 Modes of failure of slope From the data collected, the degree of weathering of the sandstone is high. This indicates the presence of many joints which makes the rocks weak. Rising groundwater results in the decrease in the friction between the rock surfaces resulting in the instability. It was found out that the slopes failed by either planar, toppling or wedge failure. The slope instabilities however were more evident during the rainy seasons. Planar failure occurs when a rock mass slips along the lines of weakness due to gravity. The slip plane inclination is larger than the friction angle for the planar failure to occur. It is evident only when the supporting block of rock mass material is removed. Common in layered rocks. Toppling failure is the turning over of soil layers. The layers bend due to own weight under gravity. The forces are transferred downstream. When there is occurrence of flexural cracks, large masses of rocks disconnect from main mass as a result of the toe overturning or sliding. Wedge failure occurs when two lines of weaknesses meet. The mass of rock moves as a block along the two weakness planes. 2. Site conditions 2.1 Landslide classification Landslide can be described as the downward movement of soil masses due to gravity. There are various types of landslide depending on the speed of downward movement as well as the manner of movement. In the area investigated it was found that there are three major types discussed above as planar, toppling and wedge failure. Some landslide can be a combination of two or more modes of failure. Observations show that there is creep landslide taking place. Creep is slow and cannot be easily realised. It is only evident by the curving of the trees, electricity poles and cracked roads. Creep landslide can be shown as in the figure below. Figure 3: Creep Landslide 2.2 Field investigation The properties of the material were obtained from boreholes dug in the investigated area along the line A. information on the rock strata was obtained as well as soil condition. The borehole logs were analysed and the information used to generate a model giving a better view about the soil stratums depth, shape and thickness. The level of the water table can also be determined. For the location of the failure plane the inclinometer was used and the stability of the soil layers assessed and classified as stable or unstable. In our case the analysis was done considering four boreholes BH1, BH2, BH3 and BH4 aligned parallel to the direction of failure. The program was then able to produce the two-dimensional cross-section which identified the natural water table level and critical sliding plane. The cross-section was then used to find the factor of safety along the line A. From the data collected from the boreholes, the following material attributes table was constructed Material Dry Weight (kN/m3) Ground Water RQD RMR Heavily weathered sandstone 20-22 3 31 30 Andesite 27-28 2 70 73 Tuff 27-29 2 94 87 3. Analysis 3.1 Methodology and geo-slope model The analysis for slope stability and the factors of safety was carried out using geo-slope program. The factors of safety are calculated based on the material properties of the soil underlying the region being investigated and the slope geometry as is imputed by the user. The model needed is to portray the insitu conditions at site. For this to be achieved various assumptions require to be made for accurate assessment of the factors of safety and slope stability. The boundary conditions and any assumed parameters are explained as; At some instances the soil layers were difficult to differentiate and where this was a limitation, the geological information such as particle size, shape and feel were used to differentiate the soils. However, where the differences in these parameters was not large the soils were grouped together. Such as fully weathered sandstone and partially weathered sandstone. For the model some of the reduced values were not easily found and the cross-section of the slope was used and some values could seem to be off the limits For better analysis and representation of the boundaries, there was an extension of approximately 15m of the slope. This allow for better view of the behaviour of the slope Based on the limit data given, the water table level had to be approximated and changed accordingly. This enabled the view of the influence of water table level to the stability of the slope. The procedure followed in the generation of the computer model in Geo-slope and determination of the slope stability; 1. Set the scale, gridlines, axes dimensions to be used and the units for the model. 2. By using polylines the slope geometry is drawn and the layers of soils and ground water table location is also indicated. 3. Definition of the regions. This is the illustration of the soil layers position and their intersection. Their stratum and interfaces are also shown in the model. 4. The material properties collected from the field are then inputted. Each region was assigned its respective properties depending on the material samples used to collect data. 5. Sketching of the water table position by the polylines. 6. Identification of the slip-surface. The slip surfaces act as the failure planes along which landslides occur. The program can then determine the possible beginning and ending of the failure plane. 7. Check for any errors in the model generated 8. Determination of the factor of safety. The factor of safety is determined from the analysis of the model. 9. For the various stability and factors of safety checks the procedure 5-8 are repeated and the information require obtained in cases of a rise or lower in the water table level. Figure 4: Map of the studied area, and your line (A) for a cross-section. 3.2 Analysis and Results Analysis undertaken to investigate the stability of the slopes when dry and when the slopes are wet. Several analysis were further undertaken when the water table level was only in the sand stone, when the water table was 1m below the ground surface and when the sandstone was fully saturated. The factor of safety was 1.23 which indicates that the model is stable when dry. Dryness of the soil stratum increases the stability of the slope as the friction between the surfaces is high and the unit weight of the soil is less. The possible lines of weakness on such a slope is the weak planes where weathering has taken place. The slopes pose an insignificant threat during the dry season The second model which was analysed when the water level was 1m below the ground surface gave a factor of safety of 1.02. These indicates that the slopes are safe. However, a slight increase in the water table would lead to instability of the slope ‘A’ being investigated. As the water table rises the friction between the soil and rock surfaces reduces causing instabilities. The sandstone was subjected to triaxial test and found to have a greater shear strength as compared to tuff. Fully saturated sandstone resulted in a low factor of safety of 0.915. Presence of water increases the friction and the weight of the soil. Wedge and toppling failure are more evident in such situation. The factor of safety is less than one making the slopes be risky as they are highly unstable and are more likely to experience landslides. The water reduces the shear strength of the soil and may result to the downhill movement of the rock mass. The landslide in this case will be fast compared to the creep landslide that would occur during the dry season. From the analysis done it is clear that the level of the water table affects the shear strength of soil and its factor of safety. The slopes are more stable during the dry season. 4. Conclusion The slope stability analysis and factor of safety determination process was a success. It was found that the slope are more dangerous during the rainy seasons when the water table rises and the slopes become unstable. To mitigate any losses that might occur due to landslides, supports made of steel can be provided to the surface of the slopes. This increases the shear strength of the slopes and reduce chances landslides. Post tensioned rock anchors can also be used which are screwed to the rock surfaces. Along the failure planes, is where the wedge failure is likely to occur To reduce this failure drains can be erected on the slopes to divert excess rain and ground water away from the slopes. REFERENCES Aurocon Australia (2009), ‘Geotechnical investigation and foundations report,’ Aurecon Australia Pty Ltd Goodman, R.E, (1989), Introduction to rock mechanics, 2nd edition, John Wiley and Sons, USA Ch.8- applications of rock mechanics to rock slope engineering Griffith school of engineering (2017) project 1a 2017. Rock mechanics notes, Griffith University, Gold Coast, Australia Borehole logs Borehole BH-1. Elevation is 85 m. a pth, m Rock Type w Degree of eathering Rock mass properties RQD % Unit weight, kN/m3 Ground water RMR Heavily weathered sandstone 33 20-22 3 32 6.7 Andesite 67 26-27 2 75 18.2 Tuff 91 29-30 2 90 End of borehole 30.0 I II III IV V VI 1:completely dry 2: dry 3: moist only c I : Fresh ~ VI ompletely weathered Depth, m Rock Type w Degree of eathering Rock mass properties RQD % Unit weight, kN/m3 Ground water RMR Heavily weathered sandstone 31 20-22 3 30 7.3 Andesite 70 27-28 2 73 19.0 Tuff 94 27-29 2 87 End of borehole 30.0 I II III IV V VI 1:completely dry 2: dry 3: moist only c I : Fresh ~ VI ompletely weathered Depth, m Rock Type w Degree of eathering Rock mass properties RQD % Unit weight, kN/m3 Ground water RMR Heavily weathered sandstone 30 20-22 3 28 8.3 Andesite 69 27-28 2 74 18.8 Tuff 90 27-29 2 87 End of borehole 30.0 I II III IV V VI 1:completely dry 2: dry 3: moist only c I : Fresh ~ VI ompletely weathered Table 1. Results of triaxial tests on samples of sandstone. 3f, MPa 0 2 5 7 1f, MPa 17.2 32.1 45.3 56.0 Table 2. Results o f shear box tests on samples of tuff. , MPa 0 2 5 7 , MPa 7.2 12.5 15.8 19.2 Read More