ECE 2311 SOIL MECHANICS II Batch 3 Sept.pdf

# ECE 2311 SOIL MECHANICS II Batch 3 Sept.pdf - ECE2311 Soil...

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ECE2311 Soil Mechanics II Tulatia Mungathia©2013 Page 1 STABILITY OF SLOPES Introduction A slope is an inclined boundary surface between air and the body of an earthwork such as highways, cut or fill, railway cut or fill, earth dams, levees and river training works. The stability of slope is one the most important one in civil engineering practice. A fairly common engineering failure of slope is slipping of an embankment or cutting. The factor leading to instability can generally be classified as 1. Those causing increased stress and 2. Those causing a reduction in strength. Factors causing increased stress include: a) Increased unit weight of soil by wetting b) Added external loads (moving loads, buildings etc) c) Steepened slopes either by excavation or by erosion d) Shock loads Loss of strength may occur by a) Vibration and earthquakes b) Increase in moisture content c) Freezing and thawing action d) Increase in pore pressure e) Loss of cementing pressure Therefore a sensible design of the slope of these structures is very essential since a structural failure may lead to loss of human life and money. The common method of a slope stability analysis of natural slopes and slopes formed by cutting and filling are based on limiting equilibrium. In this type of analysis the factor of safety with regard to the slope stability is estimated by examining the condition of equilibrium when incipient failure is assumed along a predetermined failure plane and then comparing the strength necessary to maintain equilibrium with the available strength of the soil. Stability analysis of slopes is based on the following assumptions:

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ECE2311 Soil Mechanics II Tulatia Mungathia©2013 Page 2 1. Problems are two dimensional 2. Coulomb’s theory can be used to compute shear strength and shear strength is assumed as uniform along the slip surface. 3. The flownet in case of seepage can be drawn and seepage forces evaluated. Granular Materials Soils such as gravel and sand are collectively referred to as granular soils and normally exhibit only a frictional component of strength. A potential slip surface in a slope of granular material will be planar and the analysis of the slope is relatively simple. However, most soils exhibit both cohesive and frictional strength and pure granular soils are fairly infrequent. Nevertheless a study of granular soils affords a useful introduction to the later treatment of soil slopes that exhibit both cohesive and frictional strength. Figure 1 below illustrates an embankment of granular material with an angel of shearing resistances, , and with its surface sloping at angle to the horizontal. Consider an element of the embankment of weight W. Force parallel to slope sin W Force perpendicular to slope cos W For stability, T N W Slope Triangle of forces N W T Figure 1: forces involved in a slope of granular material
ECE2311 Soil Mechanics II Tulatia Mungathia©2013 Page 3 Sliding forces ) ( F fety factorofsa orces retainingf i.e F W W tan cos sin

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