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 I: Bored Piles and Mini-piles 1. What is the function of introducing cement plug before pouring concrete in bored piles? (BP1) It is a common practice in the construction of bored piles by adding a cement plug before concreting of bored piles takes place. One of the possible explanations is that the cement grout serves as a barrier and protects fresh concrete from being washed away by water inside pile bore. The cement grout helps to set aside water when the first batch of concrete is poured down from the tremie pipe. As such, the quality of fresh concrete is anticipated not to be seriously affected by water and this helps to improve toe imperfection problem. 2. Why are bored piles usually cast higher than the required final level? It appears to be a common construction practice that bored piles are normally cast up to a piling platform much higher than the formation level of pile caps. Then, the excessive pile length has to be cropped and removed to the correct level for the subsequent construction of pile cap. The reasons of such practice are as follows: (i) It is technically sound to employ such practice because laitance, impurities and poorly-compacted concrete should have migrated to the top of the piles. Therefore, additional length at the top of piles is constructed to accommodate these sub-standard concrete, which are subsequently removed and cropped to leave high quality concrete over the entire length of piles. (ii) In actual site practice, the details of construction sequence and access arrangements have potential impact on the possibility of constructing piles to correct level without the need of cropping. 3. What is the reason of adding water to bored piles? (BP2) In water bearing ground, some water head (about 1 m) above the existing ground water table is maintained to stabilize the bore during excavation below casings by pumping water to the pile bore. This balanced head condition is created to minimize the possible drawdown of surrounding 200
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU water table which may affect the stability of nearby structures. Moreover, this helps to limit the possible inflow of water by piping from the base of pile bore. 4. What is the purpose of maintaining excess water head during excavation for bored piles? (BP2) Excavation of bored piles is usually implemented by a hammer grab. The steel casing will be extended by welding or bolting on additional casing and is installed by hydraulic oscillator. Water is pumped into the casing during excavation and excess water head is required to be maintained to prevent any ingress of material at the bottom of casing. Moreover, for excavation below pile casing, the provision of excess water head is essential to maintain bore stability. 5. What is the difference between direct circulation drilling and reverse circulation drilling? (BP3) For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annulus between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem. For reverse circulation drilling, the direction of flow of drilling flow is opposite to that of direct circulation drilling. Drilling fluid flows from the annulus between the drill stem and hole wall to the drill stem. The drilling fluid is pumped to an nearby sump pits where cuttings are dropped and settled. 6. What are the potential problematic areas in bell-out of bored piles? (BP4) Owing to the relatively low presumed bearing value for founding rock (i.e. 5MPa for Grade II rock with total core recovery of 85% and unconfined compressive strength more than 25Mpa), it leads to the necessity to form bell-out in order to spread the pile loads in larger area, thus reducing the bearing stress at pile toes. Firstly, in Hong Kong most of rock belongs to igneous rocks whose 201
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A Self-Learning Manual - Institution of Engineers Mauritius

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