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Piles Engineering

Which of the following stages is noisier, at the ending of pile driving operation or at the end of pile driving operation?

When the piles are progressively driven into the ground, the pile section above the ground declines. As a result, the degree of damping on the piles increases. Moreover, the area of exposure of piling surface reduces, thereby reducing the area generating noise form piles. Hence towards the end of pile driving operation, the noise level shall be reduced accordingly.

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In designing the lateral resistance of piles, should engineers only use the earth pressure against pile caps only?

In some design lateral loads are assumed to be resisted by earth pressure exerted against the side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance of piles, earth pressure exerted on piles should also be taken into consideration.

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Which one is a better choice, a large diameter piles or a system of several smaller piles with the same load capacity?

The choice of a large diameter pile suffers from the disadvantage that serious consequences would occur in case there is setting out error of the pile.

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Do edge piles take up same loadings as central piles in rigid cap?

Due to the effect of interaction of individual piles, the central piles tend to settle more than the edge piles when the pile cap is under a uniform load. For the pile cap to be rigid, the local deformation of central piles would not occur. Instead, the stiff pile cap would transfer the loads from the central piles and redistribute them to the outer piles. Therefore, raking piles at the edge take up a higher fraction of the total loads and are subjected to higher axial and bending loads in case the pile cap is stiff. In the extreme case, the side piles may take up as much as about two to three times the loads in the central piles and this may lead to the failure of these raking edge piles.

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What are the differences between pinned bases and fixed bases?

When structures like portal frames are connected to the base foundation, engineers have to decide the degree of fixity for the connection. In general, the two common design options are pinned bases and fixed bases.

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How should the piles be arranged in a pile cap to reduce bending moment induced in piles?

Consider that piles are designed to intersect at a single common point in a pile cap. The resultant reactions would pass through the point of intersection in the pile cap. This type of arrangement does not involve any bending moment induced if the horizontal loads pass through this point. However, in real life situation, the piling system is expected to resist a combination of vertical loads, horizontal loads and bending moment.

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In some pile design, the settlement of piles are not checked. Is it correct?

The performance of piles mainly consists of the two elements, namely ultimate bearing capacity and settlement. The local practice of pile design is place emphasis on checking if the bearing capacity of piles would be exceeded.

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Which type of pile cap transfers loads equally to piles, flexible pile cap or rigid pile cap?

Loads from columns transferring to pile cap induce tensile forces at the bottom of the cap. For instance, by using truss analogy to analyze a pile cap sitting on two piles with a column at the centre of the pile cap, the tensile force at the bottom is proportional to the pile spacing and is inversely proportional to depth of pile cap. The bottom reinforcement is designed to resist the tensile stressed generated from loads in columns.

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In modeling a nonrigid mat foundation by using elastic springs, should a uniform modulus of subgrade reaction be used along the whole base of mat?

By using a bed of springs to simulate the flexible behaviour of mat subject to loads, care should be taken in selection of the modulus of subgrade reaction. In fact, the modulus of subgrade reaction depends on many factors like the width of the mat, the shape of the mat, the depth of founding level of the mat etc. In particular, the modulus of subgrade reaction is smaller at the center while it is larger near the mat’s edges. If a constant modulus of subgrade reaction is adopted throughout the width of the mat, then a more or less uniform settlement will result when subject to a uniform load. However, the actual behaviour is that settlement in the center is higher than that at side edges. Consequently, it leads to an underestimation of bending moment by 18% to 25% as suggested by Donald P. Coduto (1994).

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What is the difference between point of virtual fixity and critical length of lateral loading for piles?

Some engineers may get confused about the difference between the two terms i.e. point of virtual fixity and critical length used for piles for resisting lateral loads. For critical length of lateral loading for piles, it refers to a certain depth from the ground level where the piles behave as if it were infinitely long. As such, beyond the critical length, the change in lateral response of piles with increase in pile length will be negligible.

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