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<br />OJ2736 <br /> <br />and railroads, and mining operations. These activities can increase slope <br />angle, decrease toe or lateral support, or load the head of a slide. Changes <br />in surface drainage are usually associated with irrigation or.surface runoff <br />and can increase erosion, or contribute to loading a slope or raising the <br />groundwater table. The groundwater table level can be increased by diverted <br />surface drainage, waste-water effluent, leaking swimming pools or ponds, and <br />irrigation or conveyance of irrigation water. A high groundwater level <br />results in increased pore pressure and decreased shear strength, thus <br />facilitating slope failure. Conversely, the lowering of the groundwater table <br />as a result of drawdown by water-supply wells, or the lowering of a lake or <br />reservoir, can also cause slope failure as the buoyancy provided by the water <br />decreases and gradients are steepened. <br /> <br />Man-induced vibrations from manufacturing machinery, blasting, and traffic can <br />also trigger landslides. <br /> <br />Environmental factors: There are a wide range of environmental factors that <br />can cause slope failure. Some of these, such as regional tilting, residual <br />stress, and long-term or cyclic climate changes, are undiscernible without <br />instrumentation and/or long-term record-keeping. <br /> <br />Regional tilting can eventually increase slope angle enough to contribute to <br />slope failure in weak materials. Erosion by streams and rivers, and waves and <br />currents removes toe and lateral slope support. Long-term climate changes can <br />have a significant impact on slope stability. Decreased precipitation results <br />in a lowering of the water table, as well as decreases in the weight of the <br />soil mass, solution of materials, and freeze-thaw activity. An increase in <br />precipitation will raise the level of the groundwater table, increase the <br />weight of the soil mass with rain, hail, and snow, and increase erosion and <br />freeze-thaw activity. <br /> <br />Weathering is the natural process of rock deterioration which produces weak, <br />slide-prone materials. It is caused by the chemcial action of air, rain <br />water, plants and bacteria; and the mechanical action of changes of <br />temperature (expansion and shrinkage), and the freeze-thaw cycle. <br /> <br />Earthquakes have not only triggered landslides, but the fault activity <br />associated with them can create steep and potentially unstable slopes. <br /> <br />- 10 - <br />