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Piles Engineering

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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