A Closer Look at Prevailing Civil Engineering Practice - European ...

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A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU 22. What is the difference between safety fence and safety barrier A safety fence is intended to absorb some energy caused by hitting vehicles and to realign the vehicles to move parallel to the safety fence. A safety barrier is intended to provide containment instead of energy absorption upon hit by vehicles. Hence, it is anticipated to have little deflection and deformation only. After crashing, it serves to re-direct the vehicles along the line of barrier. 52
A Closer Look at Prevailing Civil Engineering Practice, What, Why and How Vincent T. H. CHU Chapter 7. Slopes and Excavation 1. In braced excavation, why is Rankine’s theory of lateral earth pressure not applicable In braced excavation, sheetpiling is constructed at the first place, followed by the installation of struts as excavation proceeds. Following the installation of first row of struts, the depth of excavation is small so that there is no major yielding of soils. However, as further excavation takes place, soils yield before the installation of n th row of struts. The first row of struts prevents yielding near the ground surface. As such, deformation of wall increases with depth with the smallest at the ground level. Owing to the effect of construction method of braced excavation, it differs from the deformation condition of Rankine’s theory. This is attributed to arching effects in which there exists upward redistribution of loads. The upper part of braced excavation is in the state of elastic equilibrium while the lower part is in the state of plastic equilibrium. 2. How could counterforts in counterfort retaining wall assist in resisting earth pressure Counterforts are used for high walls with height greater than 8 to 12 m. They are also used in situations where there is high lateral pressure, i.e. where the backfill soils are heavily surcharged. The counterforts tie the base slab and wall stem together and they act as tension bracing which strengthen the connection between wall and base slab. The counterforts help to reduce bending moment and shear forces induced by soil pressure to the retaining wall. Moreover, it also serves to increase the self-weight of the retaining wall which adds stability to the retaining wall. 3. What is the basic mechanism of soil nails in improving soil stability Soil nails improve slope stability by: (i) Partial increase the normal force and shear resistance along potential slip surface in frictional soils. (ii) Direct reduction of driving force along potential slip surface for cohesive and frictional soils. 53
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