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 spacing to the reinforcement cage. After concreting barrettes/bored piles, a small volume of water is injected under high pressure into these pipes to crack the concrete. The cracking process should be carried out within 24 hours after concreting. The purpose of cracking is to create a path for grout to go through. About a week after concreting of barrette, grouting is then carried out in these pipes to improve the friction between the foundation and the surrounding soils. 10. Why is sleeving applied in piles constructed on slopes? For high-rise buildings constructed on steep cut slopes, these buildings are usually supported by large diameter piles. Though the piles are founded at some depth below the slopes, lateral load arising from wind on buildings may induce loads on the slop and causes slope failures. For shallow depths of slope which is marginally stable, it is more vulnerable to slope failure. Hence, an annulus of compressible material called sleeving is introduced in piles so as to reduce the transfer of lateral loads from buildings to slopes. 11. Should compaction be carried out to freshly-placed concrete piles? (PC1) In normal practice, reliance is placed on the self-compaction of specially designed concrete mixes to achieve adequate compaction. The use of vibrating devices like poker vibrators is seldom adopted for the compaction of concrete piles. In fact, other than the consideration of the impracticality in using vibrating device in long piles, there is serious concern about the possible occurrence of aggregate interlock which poses difficulty during casing extraction [64]. In the worst scenarios, the temporary casings together with reinforcement cages are extracted during the lifting up of pile casings. This is another reason which accounts for not using vibrating machines for piles with casing extraction. 12. Is the quality of concrete impaired by free-falling concrete placement method in bored piles? (PC2) Based on the research by STS Consultants Ltd. [57], it was found that concrete placed by free falling below 120 feet would not suffer from the problem of segregation and the strength of concrete would not be detrimentally impaired provided that the piles’ bore and base are dry and 238
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU free of debris. Moreover, it is presumed in the past that during free falling of fresh concrete into the pile bores the hitting of falling concrete in the reinforcement cage causes segregation. However, in accordance with the experimental results of STS Consultants Ltd. [57], the striking of reinforcement cage by fresh concrete does not have significant effect on the strength of concrete In addition, for long bored piles, it is impractical to conduct vibration to concrete. For concrete placed by free falling method, the impact action arising from free falling is assumed to induce adequate vibration. On the other hand, concrete placed by tremie method appears to be lack of vibration and this may affect the strength and integrity of concrete. The research results showed that the strength of vibrated concrete was slightly higher than unvibrated concrete. Vibration proved to have added advantage to concrete strength but not essential to achieve the design pile strength. 13. Is pile tip cover necessary for rock-socketed H-piles? In current practice concrete cover is usually provided at the pile tips of pre-bored H-piles socketed in rock. The purpose of such arrangement is to avoid the potential occurrence of corrosion to H-piles in case concrete cover is not designed at pile tips. However, recent field and laboratory observations had reservation of this viewpoint [45]. In case H-piles are designed to be placed directly on top of rock surface, it provides the tip resistance to limit the pile movement in the event of bond rupture between grout and H-piles. As such, some contractors may choose to tamp the H-piles by using drop hammers to ensure the H-piles are founded directly on top of rock surface. Practically speaking, it poses difficulties during the process of tamping because there is a chance of possible buckling of long H-piles when too much energy is provided to the piles. 14. In deep excavation, adjacent ground water table is drawn down which may affect the settlement of nearby buildings. What is the remedial proposal to rectify the situation? (DE1) One of the methods to control settlement of nearby buildings due to excavation work is by recharging. Water collected in wells in deep excavation is put back to the top of excavation in order to raise the drawn-down water table. The location of recharge should be properly selected to ensure the soil is sufficiently permeable to transfer the pumped water back near the affected buildings. 239
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A Self-Learning Manual - Institution of Engineers Mauritius

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