A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level One (Core FAQs) Part IV: Retaining Walls 1. What is the typical proportioning of a retaining wall? (RW1) The base slab thickness of a cantilever retaining wall is about 10% of the total height of retaining wall. The length of base slab is about 50-70% of the total height of retaining wall. Generally speaking, the thickness of wall stems may vary along the stem provided that its size should not be less than 300mm to facilitate concrete placement. For retaining wall with a total height exceeding 8-12m, it is recommended to adopt counterfort retaining wall. The counterforts in counterfort retaining wall are normally spaced at about 30% to 70% of the total height. The length of base slab is about 40-70% of the total height of retaining wall. 2. What is the function of shear keys in the design of retaining walls? (RW2) In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows: The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance. On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is φ 310
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU instead of 0.8φ in the previous case and the sliding resistance can be enhanced. 3. Where is the best position of shear keys under retaining walls? (RW2) The installation of shears keys helps to increase the sliding resistance of retaining walls without the necessity to widen the their base. The effect of shears keys enhances the deepening of the soil failure plane locally at the keys. The increased sliding resistance comes from the difference between the passive and active forces at the sides of the keys. In case weak soils are encountered at the base level of shear keys, the failure planes along the base of retaining walls due to sliding may be shifted downwards to the base level of the keys. Shear keys are normally designed not to be placed at the front of the retaining wall footing base because of the possible removal of soils by excavation and consequently the lateral resistance of soils can hardly be mobilized for proper functioning of the shear keys [30]. For shear keys located at the back of footings, it poses a potential advantage that higher passive pressures can be mobilized owing to the higher vertical pressure on top of the passive soils. 4. Why is the kicker of reinforced concrete cantilever retaining walls located at the position of largest moment and shear force? (RW3) Normally for reinforced concrete cantilever retaining walls, there is a 75mm kicker at the junction wall stem and base slab to facilitate the fixing of formwork for concreting of wall stems. If a higher kicker (i.e. more than 75mm height) is provided instead, during the concreting of base slab the hydraulic pressure built up at kicker of fresh concrete cause great problem in forming a uniform and level base slab. Despite the fact that the position of kicker in a cantilever retaining wall is the place of largest flexure and shear, there is no option left but to provide the kicker at this position. 311
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A Self-Learning Manual - Institution of Engineers Mauritius

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