Lecture04-General-Bearing-Capacity

Lecture04-General-Bearing-Capacity - Foundation Engineering...

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Unformatted text preview: Foundation Engineering Foundation Engineering Lecture #04 Lecture #04 The Bearing Capacity of Soils- Terzaghis Ultimate Bearing Capacity- Meyerhofs Method- Brinch Hansen Method- Vesics Method Luis A. Prieto-Portar, 2009 These wheat storage silos in Canada are an example of the bearing capacity failure of a large foundation. This failure alerted engineers to the mechanism of how surface loads may exceeded the shear strength of the soils beneath the foundation. Some foundations in congested cities must consider how they transfer loads onto adjacent utilities and the adjacent buildings foundations, in addition to the underlying soil. This is a typical excavation to repair a utility in a New York City street. Torontos Queen and Yonge Street intersection. A shallow foundation must: 1. be safe against an overall shear failure in the soil that supports it. 2. cannot experience excessive displacement (in other words, settlement ). The definitions of bearing capacity are, q o is the actual contact pressure of the soil at the footings invert; q ult is the load per unit area of the foundation at which the shear failure in soil occurs and is called the ultimate bearing capacity of the foundation ; and q all is the load per unit area of the foundation that is supported without an unsafe movement of the soil, and is called the allowable bearing capacity . CONCENTRIC LOAD ( NO ECCENTRICITY) B X L B q Q y x Figure 1. A continuous footing resting on the surface of a dense sand or a stiff cohesive soil is shown in Figure 2a with a width of B. If a load is gradually applied to the footing, its settlement will increase. When the load per unit area equals q ult a sudden failure in the soil supporting the foundation will take place, with the failure surface in the soil extending to the ground surface. This type of sudden failure is called a general shear failure . If the foundation rests on send or clayey soil of medium compaction (Figure 2b), an increase of load on the foundation will increase the settlement and the failure surface will gradually extend outward from the foundation (as shown by the solid line). When the load per unit area on the foundation equals q ult , the foundation movement will be like sudden jerks. A considerable movement of the foundation is required for the failure surface in soil to extend to the ground surface (as shown by the broken lines). The load per unit area at which this happens is the ultimate bearing capacity q ult . Beyond this point, an increase of the load will be accompanied by a large increase of footings settlement. The load per unit area of the footing q ult , is referred to as the first failure load (Vesic 1963). Note that the peak value of q is not realized in this type of failure, which is called the local shear failure in soil....
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Lecture04-General-Bearing-Capacity - Foundation Engineering...

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