Reversed Circulation Drill (RCD) is normally used for forming large diameter rock socket. The method involves the exertion of a download force of roller cutter bits on rock, together with the action of rotation and
grinding of bits on rock. The cuttings are then removed by reverse circulation. The water and cuttings are airlifted through a central drill pipe, which is also used for rotating the drill bits.

To facilitate the grinding action on the rock, about 15 tons of force is used for each cutter. With such a high bit force, the drill frame has to be stationed by attaching to pile casing of bored piles. Therefore, during the
drilling operation, the pile casing is prevented from lifting up by the weight of drill rig and pile casing and the frictional forces developed between the ground and pile casing. Hence, in shallow rock conditions with short length of pile casings, it may affect the stability of RCD drill rig.

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.

For unstable grout, particles will come out of the grout suspension leading to incomplete grouting and clogging of pipes. The stability of grout can be improved by adding additives such as bentonite. However, bentonite
should not be used with very fine cements because its grain size is bigger than that of fine cements. Tests conducted previously confirm that a grout with bentonite is less stable under pressure.

It is commonly accepted that a fissure may be penetrated by grout with the grain size about 3-5 times smaller than the aperture of fissure. Hence, OPC cement may penetrate fissures of aperture greater than 0.4mm while microfine cement and ultrafine cement may penetrate fissures of aperture greater than 0.1mm and 0.03mm respectively.

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.

Compression of rockfill is normally caused by a reduction in dimension of fill and by rearrangement of particles into closer packing.

When the rockfill are saturated, the strength of rock would be reduced accordingly. In fact, wetting of rock surfaces does not reduce the coefficient of sliding friction between rockfills. Considerable settlement may result not from the lubricating effect of water but from a reduction of rock strength at its point of contact. The contact points would then be crushed under intergranular force and the contact area increases until contact pressure is less than the strength of rockfill.

Rockfill with sharp corners proved to be more liable to settlement than those of well-rounded.

To minimize settlement of rockfill, the intergranular force should be reduced and this is achieved by grading the size of rock particles such that there is minimum amount of voids and hence a maximum amount of particle contacts. To avoid particle rearrangement under future loading, the rockfill should be properly compacted with earth-moving machinery.

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.

Most of design codes assume that pile fails during strong earthquake by lateral spreading. Lateral spreading is based on bending mechanism where the inertia and slope movement causes bending in piles. In essence, piles are considered as beams which are subjected to lateral loads such as slope movement leading to pile failure.

Piles are slender columns with lateral support from foundation soils. When the length of pile increases, the buckling loads decrease with the square of pile length. For buckling failure, soils around the piles lose the confining stress during earthquake and can hardly provide lateral support to piles. As such, the pile serves as an unsupported column with axial instability. It will buckle sideways in the direction of least bending stiffness under axial load.

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 use of prestressed tiebacks gets rid of the need of interior bracing. Prestressed tiebacks are anchored into rock or granular soils and excavation can be conducted by using powerful shovel instead of using hand excavation or other small excavators. It provides less restraint and allows free movement for excavation.

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.