Various Theories, Pile load test, Slope Stability

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Theories:

New Mark— shows that influence chart for stress distribution in irregularly shaped areas. NewMark’s influence chart is based on the concept of the vertical stress below the centre of the circular area.

Westergaard—shows stress distribution due to point load in an anisotropic medium. Westergaard assumed that an elastic soil to be laterally reinforced by numerous, closely spaced, horizontal sheets of negligible thickness but of infinite rigidity, that allow only vertical movement but prevent the mass as a whole from undergoing any lateral strain.

Westergaard’s analysis for stress distribution beneath loaded areas is applicable to stratified soils.

SteinBrenner—shows influence chart for stress distribution in a rectangular area.
Boussinesq theories—shows that stress distribution due to point load in homogeneous isotropic medium, loaded normally on its upper plane surface by a concentrated point load.Boussinesq gave equations to determine the state of stress in subgrade material.

Pile load test is generally used for determining the load carrying capacity of pile. The setup generally made of two anchor piles and an anchor girder or a reaction girder provided at their top. The test pile is provided between the anchors piles in which the foundation piles are to be installed. The test pile should be at least three times of width or 2.5m clear from the anchor piles.

    • The test load is done with the help of hydraulic jack resting on the reaction girder. The pile movement is generally measured with respect to a fixed reference mark.
    • The test is conducted after a rest period of 3 days after the installation in sandy soils and a period of 1 month in silts and soft clays. The load is applied in equal increments of about one-fifth of the estimated allowable load.
    • The settlements are recorded with the help of three dial gauges. Each stage of the loading is maintained till the rate of movement of the pile top is not more than 0.1 mm per hour for sandy soils and 0.02 mm per hour for clay soils.
    • The test piles are loaded until ultimate load is reached. The obtained results are plotted in the form of load-settlement curve. The ultimate load can be clearly obtained by load settlement-curve approaching vertical.

Stability of slopes

The failure of soil mass located beneath the slope is called slide. It consists of a outward and downward movement of the entire mass of soil that participates in the failure. The slope failure generally occurs due to the gravitational forces and seepage forces within the soil mass and also failed due to excavation or undercutting of their foot.

While analysis stability of slope consists of two parts:-

First one—the knowledge of severely stressed internal surface and the magnitude of the shearing stress to which it is subjected and

Second one—the knowledge of shearing strength along this surface.

The unit weight of material and geometry of slope affect the shearing stress. Slopes are of two type’s mainly finite and infinite slope. If the slope is of limited extent is called finite slope(failure occurs along a surface which is curve.)

If a slope represents the boundary surface of a semi-infinite soil mass, and all the properties of soil for all identical depths below the surface are constant so it is called infinite slope(the failure of surface generally involves a sliding of a soil mass along a plane parallel to the slope at some depth.)

 

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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