Liquefaction of Soils Questions and Answers

Answer: A

Cyclic shearing makes a loose saturated sand try to contract, and because the water cannot escape quickly the pore pressure rises instead. Effective stress falls towards zero and with it the frictional strength on which the soil depends.

Answer: B

Terzaghi's relation \( \sigma' = \sigma - u \) is the whole of the explanation: the soil skeleton carries only what the water does not. Raising \( u \) towards \( \sigma \) therefore drives \( \sigma' \) and the strength to zero.

Answer: C

At that point the effective stress vanishes and the deposit has no frictional strength left, behaving as a heavy liquid. Partial pore pressure rise reduces the strength without destroying it, which is the more common condition.

Answer: D

Excess pore water pressure is the mechanism, so there must be water in the pores with no quick escape route. Deposits above the water table are therefore not susceptible, which is why the depth to groundwater is one of the first things checked.

Answer: A

Denser soils dilate as they shear and regain strength, so deformation builds up in steps rather than running away. Flow liquefaction needs a loose soil in which the static shear stress alone exceeds the residual strength that remains.

Answer: B

Uniform loose sand below the water table contracts readily under cyclic shear and drains too slowly for the pressure to dissipate. Plastic clays resist because of their cohesion, gravels because they drain, and dry soils because there is no pore water to pressurise.

Answer: C

Correct answer: drain quickly enough to dissipate the pore pressure

Liquefaction requires the pore pressure to build up faster than it can escape, which is why it is essentially an undrained phenomenon. Gravel is permeable enough that the pressure generated by each cycle drains away as it forms. The exception worth knowing is a gravel confined between impermeable layers, which cannot drain and can liquefy.

Answer: D

Correct answer: pack more densely and contract less on shearing

A well-graded soil has small particles filling the voids between larger ones, so it is already close to its densest packing and has little room to contract further when sheared. Since the pore pressure rise comes from that tendency to contract, a well-graded deposit generates less of it than a uniform one of the same relative density.

Answer: A

Dilation generates negative pore pressure that increases effective stress and strength, which is the opposite of what happens in a loose deposit. Densification is therefore the most direct of all the mitigation measures.

Answer: B

Effective overburden stress grows with depth, and the cyclic stress needed to liquefy the soil grows with it, so deep deposits are rarely affected. The figure is a rule of thumb rather than a limit, and evaluation is normally carried out to that order of depth.