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).

In general, a constant value of modulus of subgrade reaction is normally applied for structure with a rigid superstructure and the rigid foundation. However, a variable modulus of subgrade reaction is adopted instead for non-rigid superstructure and non-dominance of foundation rigidity to account for the effect of pressure bulbs.

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.

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.

Point of virtual fixity refers to a certain dept below ground surface where the piles are fixed without movement under loads. The depth to the point of fixity is useful in assessing the buckling loads of piles. It is obvious that the depth to the point of virtual fixity should be smaller than the critical length of piles.

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 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. For the pile cap to be assumed as rigid the stiffness of pile cap is infinite relative to that of pile/soil system and the deformations within the cap are not considered owing to its rigidity. On the other hand, for the pile cap to be designed as flexible, internal deformations of pile cap would occur.

In some design guidelines, maximum spacing of piles is specified to limit the length between adjacent piles so that the assumption of rigid pile cap can be justified.

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.

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.

Capping beam performs the same functions as pile caps. However, ground beams are structural elements to connect adjacent pile caps to improve the stability of foundation.

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.

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.

(iii) Socket shaft friction is also used for resisting uplifting forces. But only 70% of this capacity should be used because of the effect of negative Poisson ratio.

Note: Rock anchors, which may consist of a high tensile bar or a stranded cable, are provided for tension piles when there are insufficient soil covers to develop the required uplifting resistance.

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.