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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.
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.
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.
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).
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.
One of the factors that affect the distribution of loads from the structures to each pile is the assumption of flexibility of the pile caps in design. A pile cap can be modeled as a flexible or a rigid element based on their relative stiffness.
Capping beams for piles aim at transferring loads from closely spaced columns or walls into a row of piles. On the other hand, ground beams are beams provided between adjacent pile caps and they perform as compression struts or ties in an attempt to prevent lateral displacement or buckling of piles under uneven distribution of loads on pile caps. Both of them have to be specially designed to cater for differential settlement of piles.
Rock anchors are used primarily for resisting uplift forces. On the contrary, rock sockets serve three main purposes:
(i) Rock socket friction and end bearing to resist vertical load.
(ii) Passive resistance of rock sockets contribute to resistance of lateral load.
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Plate load test is carried out to check the bearing capacity of foundation soils.
The limitations of plate load test are:
(i) It has limited depth of influence. It could only give the bearing capacity of soils with depth up to two times the diameter of plate.
(ii) It may not provide information on the potential for long term consolidation of foundation soils.
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Pile Drive Analyser is a high-strain dynamic test to determine the force and velocity response of a pile to an impact force applied axially by a driving hammer at the pile top. It is applicable to driven piles or small diameter bored piles. The operation measures the elastic deformation of a pile after a hammer blow and is mainly used to check the ultimate capacity of piles. However, it may also be adopted to detect damages in pile body and obtain the friction profile along the pile shaft.
