Ground water flow into excavations constructed by sheet pile walls should be minimized in order to save the cost of the provision of pumping systems or well points to lower the water table inside the excavation. In case a layer of impermeable material like clay is located slightly below the excavation, it may be desirable to drive the sheet piles further into this layer and the cost of further driving may be less than the cost of the provision of continuous pumping in the excavation. On the other hand, if there is no impermeable layer beneath the excavation, engineers may consider driving the sheet piles further so as to increase the flow path of groundwater into the excavation and this helps to reduce the amount of water flow into the excavation. Similarly, a cost benefit analysis has to be carried out to compare the extra cost of driving further the sheet piles with the reduced pumping costs.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The ground conditions in hillsides of Hong Kong normally consist of colluvium overlying weathered and fresh rock. The groundwater flow in sloping ground may involve both perched and main water flow. As permeability of soil tends to decrease with depth, groundwater flow may take place in the upper aquifer which contains a perched water table.

The presence of pile groups below groundwater level hinders the flow of groundwater, leading to an increase on groundwater level. For major development in hillsides with plenty of deep foundations, the effect of rise in water table is sever. The increase in water table owing to damming effects of pile groups decreases the stability of slopes by reduction of shear strength of soils and may eventually cause slope failures.

The rise in groundwater levels tend to increase with slope angle and the
depth of groundwater flow.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The origin of Daido piles comes from Japan where these prestressed concrete piles are used as replacement plies. Holes are pre-formed in the ground and Daido piles are placed inside these pre-formed holes with subsequent grouting of void space between the piles and adjacent ground. However, in Hong Kong Daido piles are constructed by driving into ground by hammers instead of the originally designed replacement method. Since the installation method of Daido piles is changed, construction problems like deformation of pile tip shoes, crushing of concrete at pile tip etc. occur. Reference is made to B. W. Choy (1993).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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. 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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.