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Lecture24-RConcrete-Ret-Walls

Lecture24-RConcrete-Ret-Walls - Foundation Engineering...

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Foundation Engineering Lecture # Lecture #24 24 Reinforced Concrete Walls L. Prieto-Portar 2008
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This New York City reinforced concrete retaining wall unexpectedly collapsed in May, 2005.
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The Stability of Retaining Walls. The preliminary design of most retaining walls is based on the experience of previously successful projects. Figure 1 and 1a shows the most common proportions used for rigid and semi-rigid reinforced concrete walls. Other types of walls, such as slurry walls, are developed for specific tasks that require project tailored dimensions. Figure 1. Typical proportions of rigid retaining walls.
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Figure 1a. Typical proportions of semi-rigid retaining walls.
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Wall Planes of Action. For simplicity, the lateral earth pressure upon a wall is assumed to act upon an imaginary vertical plane of action AB as shown in Figure 2. This model is used for both the Rankine and Coulomb analyses. Figure 2. Simplified plane of lateral pressure loading.
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Note that the soil block Ws above the heel helps stabilize the wall. The assumption that the lateral pressure is indeed exerted along the plane AB is theoretically correct if the shear zone bounded by AC is not obstructed by the wall stem, i.e. that the angle ° (CAB) is found from, Most walls are designed by ignoring the presence of a hydrostatic head on the wall. This is accomplished with drainage systems, either behind the wall or below it. On the other hand, marine walls typically experience equal, or almost equal pressures, on both sides of the wall. Types of Stability Analysis for walls. ) ( 1 - 2 - 2 + 45 = - φ α φ α η sin sin sin ° There are seven basic types of stability checks required for routing walls: 1) Overturning about the wall toe; 2) Sliding failure along its base; 3) Bearing capacity failure of the base; 4) Settlement failure; 5) Deep and shallow shear failures; 6) Bottom heave in clays; 7) Piping during dewatering.
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1. Overturning Failure. Consider Figure 3 for conditions for overturning: Figure 3
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The factor of safety against overturning (FS) O about the toe (point O) is expressed by, (FS) O = Sum of resisting moments = ° M R Sum of overturning moments ° M O As an example, the resisting moments M R can be summarized on the table shown on the next slide, based on Figure 3b. Note that the contribution of the passive force F p (in front of the wall toe) is neglected; this choice essentially acts as an additional factor of safety. On the other hand, the overturning moment M O is, ° M O = F a cos ± (H’ / 3) Therefore, in order to attain a minimum factor of safety of 1.5 for temporary walls, or at least 2 for permanent walls, the following equation is generally used, cos i i v 1 O a + M M (FS = ) H F ( ) 3 α °
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Table: Procedure for the calculation of ² M R Weight/Unit Moment arm length of measured Moment Section Area Wall from C about C (1) (2) (3) (4) (5) 1 A 1 W 1 = ° 1 x A 1 X 1 M 1 2 A 2 W 2 = ° 2 x A 2 X 2 M 2 3 A 3 W 2 = ° c x A 3 X 3 M 3 4 A 4 W 2 = ° c x A 4 X 4 M 4 5 A 5 W 2 = ° c x A 5 X 5 M 5 6 A 6 W 2 = ° c x A 6 X 6 M 6 P V ______ B M V ____ ± V ± M R Note ± 1 = unit weight of backfill ± c
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