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 jack the pile into ground. The hydraulic ram is part of machine called “Silent Piler” which uses jacked pile to provide reaction force for jacking. The use of driving shoes could reduce the jacking load. The presence of driving shoes change the flow of soils around pile tip and this leads to reduced effective horizontal stress on internal pile shaft. As a result, this enhances a reduction in shaft friction which is the major element for making up the jacking load. Fig. Soil flow along driving shoes 20. What is the significance of driving sequence of driven piles? (DP9) For basement construction, if piles are driven from the centre to the perimeter, there is a tendency of soils to move outwards. Such lateral movement of soil may cause damage to nearby structures and utilities. However, if piles are driven from the outside perimeter inwards, there are little soil lateral movements. This results in a well-compacted centre with an excess pore water pressure built up to resist the loading of piles. Consequently, shorter pile lengths than the original designed ones may result. However, some time after the pile driving operation, the excess pore water pressure is dissipated and the shorter driven piles may not be able to take up the original design loads. In this situation redriving is required to drive the piles to deeper depths after dissipation of excess pore water pressure. 216
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 21. How does false set occur in pile driving? (DP10) For pile driving in certain soils like dense silt and weathered rock, the occurrence of false set phenomenon is not uncommon. During the driving process, negative pore water pressure is developed and the driven piles appear to have sufficient capacity during pile driving as the built-up of negative pore water pressure leads to an apparent temporary increase in driving resistance and strength. However, some time after the pile driving, the dissipation of this negative pore water pressure would reduce the bearing strength in resisting the design loads. Sometimes, the presence of cracks along pile sections may bring about the problem of false set by the dampening effect of stress waves by these cracks. To avoid the problem of false set, a certain percentage of constructed piles should be chosen to perform re-driving to check for the false set phenomenon [26]. 22. Why are holes present in steel plates connecting to H-piles? There are two kinds of holes present in the steel plate connected to H-piles in the pile cap. The first kind of holes is designed to be filled with welding for better connection with H-piles. The second kind of holes is present to facilitate concreting works of the pile caps. In fact, the void space underneath the steel plate is hardly to be accessed by concrete and these holes provide alternative paths to gain entry into these hidden void. 23. How can piles be driven through steeply dipping karst surfaces? The steep dipping and variable nature of karst surfaces poses problems for installation of driven piles. Very often, the consequences of hard driving piles over steeply-inclined karst are slipping and buckling of piles. To tackle these problems, the following two options are mostly adopted: (i) Pre-boring is carried out in steep dipping karst surface as this method could penetrate hard layers; (ii) Reinforcing the end section of driven piles by welding stiffening plates 24. In the installation of strain gages in driven H-piles to measure loads, why should they be normally used in pairs? Strain gages are often installed in driven piles to measure the load distribution along the piles. They have to be protected from being removed as the pile is driven into the ground. Protection of strain gages is achieved 217
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A Self-Learning Manual - Institution of Engineers Mauritius

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