Erosion Studies on Lithomargic Clays and Slope Stability Studies of Excavated Slopes in Lateritic Formations

Abstract

This study is done in three parts. In the first part, a brief study of the geology of the area is being made, especially that of laterites and lateritic soils. The study area is coastal Karnataka in India. This coastal area receives copious amount of rainfall and a lot of developmental activities are taking place. The soil stratification in lateritic areas consists of lithomargic clays, which are products of laterization, sandwiched between the hard and porous weathered laterite crust at the top and the hard parent rock of granite or granitic gneiss underneath. These lithomargic clays which are extensively used in construction purposes behave as dispersive soils and are found to be highly erosive. In the second part of this research work, erosion characteristics of lithomargic clays are being studied in detail since very few and detailed studies on erosion of lithomargic clays are available in literature. A number of hole erosion tests are conducted on controlled lithomargic clay samples with varying percentage of fines. The influence of degree of compaction, moulding water content, head causing flow, percentage silt content and plasticity index on the erosion rate index and critical shear stress of controlled lithomargic clay samples are being studied. The results of this study indicate that the critical shear stress for soils with higher silt fraction and fine sand content varied from 45 to 125N/m2 whereas for soils with higher clay fraction and fine sand content the critical shear stress varied from 200 to 400N/m2. The erosion rate increased with a decrease in percentage compaction in all the samples and critical shear stress is found to be highest at optimum moisture content conditions. It is generally observed that soils with fines whose plasticity indices are high, are less erodible compared to soils with fines whose plasticity indices are low. Excavated slopes for railway and highway projects in such lateritic formations are posing serious erosion and slope stability problems, especially, due to the presence of these lithomargic clays and seepage pressures from stagnated water at top. In the third part of this study, slope stability analyses of excavated slopes in lateritic formations is being conducted using the software Plaxis 2D. Slope stability analyses is actually a very complex problem which should take into consideration thecombined effect of geotechnical [berm position, height and slope of excavated slope, soil properties etc.], hydrological [precipitation, ponding at top and seepage through the slope etc.] and biological [vegetation (trees at various positions and turfing on slope), wind action on trees etc.] factors in addition to erosion (both surface and internal) problems. In this research work, the influences of these various factors on slope stability are being studied separately. Some of these factors have a positive influence by increasing the factor of safety of slopes, whilst others have a negative influence. When trees are provided at the toe of the slope a percentage increase in factor of safety up to 12% and 6% is observed for drained and undrained conditions respectively. When turfing along with trees are considered, a percentage increase of factor of safety up to 15% and 12% is observed for drained and undrained conditions respectively.