Your Answer : (Not Answered)

Correct Answer : B

The hydraulic gradient across the clay is 0.55.

Take ground level as datum. The gravel is hydrostatic, so the head at the top of the clay equals the water table level, 0.6 m below ground. The artesian sandstone gives a head 6 m above ground at the base of the clay. The flow is upward through the 12 m of clay:

\[ \begin{aligned} \Delta h &= 6 - (-0.6) = 6.6\ \text{m} \\ i &= \frac{\Delta h}{L} = \frac{6.6}{12} = 0.55 \end{aligned} \]

A common slip is to measure the head loss from ground level (6/12 = 0.50) and forget that the water table in the gravel sits 0.6 m lower; 0.04, 0.70 and 0.96 do not follow from the heads given. The answer has no unit because a gradient is a length divided by a length.

Your Answer : (Not Answered)

Correct Answer : C

The total vertical stress is 1200 kPa.

In the original situation the base of the clay is at depth \(z = 20 + 50 = 70\) m. The total vertical stress there is the weight of everything above: 20 m of lake water and 50 m of saturated clay.

\[ \begin{aligned} \sigma_{v} &= 10 \times 20 + 20 \times 50 \\ &= 200 + 1000 \\ &= 1200\ \text{kPa} \end{aligned} \]

Leaving out the water gives 1000 kPa, which is not offered. The effective stress would be smaller (pore water pressure \(10 \times 70 = 700\) kPa, leaving 500 kPa), but the question asks for total stress. The new 2 m sediment layer belongs to the later situation, not the original one.

Your Answer : (Not Answered)

Correct Answer : B

The bearing plate size varies from 300 to 750 mm.

IS 1888, the code for the plate load test, uses square steel plates of 300 mm to 750 mm side (or circular plates of equivalent area), at least 25 mm thick. A 300 mm plate is common for granular soils and larger plates for clayey soils.

Plates of 25 to 275 mm are too small: a small plate stresses only a shallow zone of soil and gives results that do not scale reliably to a real footing.

Your Answer : (Not Answered)

Correct Answer : A

The dilatancy correction is \(N_{e} = 15 + (N_{0} - 15) \times \frac{1}{2}\).

In very fine or silty sands below the water table, rapid driving of the sampler causes dilatancy and a temporary build-up of pore pressure, which raises the observed blow count. Terzaghi and Peck recommended that, when the observed value \(N_0\) exceeds 15, the corrected value be taken as:

\[ N_e = 15 + \frac{1}{2}(N_0 - 15) \]

So only half the excess over 15 is counted. The other formulas, with base values of 20, 25 or 30, are not the standard correction.

Your Answer : (Not Answered)

Correct Answer : C

Coulomb's theory is based on the Wedge theory.

Coulomb (1776) treated the soil behind a retaining wall as a rigid wedge that slides on a plane failure surface. By considering the equilibrium of this wedge, including wall friction, and finding the most critical plane, he obtained the active or passive earth pressure.

Rankine's theory is the one based on the plastic equilibrium of the soil mass, not elasticity. Coulomb's method is a rational analysis, not a set of empirical rules.

Your Answer : (Not Answered)

Correct Answer : D

Greater compactive effort gives an increase in maximum dry density and decrease in OMC.

Heavier compaction, as in the modified Proctor test compared with the standard test, pushes the particles closer together, so the soil reaches a higher maximum dry density. Because the extra energy does part of the work that water does as a lubricant, that peak is reached at a lower water content. The compaction curve moves upward and to the left, along the line of optimums.

So the maximum dry density does change, ruling out option A, and the optimum moisture content falls, ruling out B and C.

Your Answer : (Not Answered)

Correct Answer : B

The vibratory roller is used for Well graded dry sand.

Vibration rearranges the grains of cohesionless soils such as sands and gravels into a denser packing, so vibratory rollers are the most effective compactors for granular material, especially well-graded sand.

Clays and silts are cohesive; their particles are held by water films and are compacted better by kneading and pressure, with sheepsfoot or pneumatic-tyred rollers. Moist silty sand also responds less well to vibration than clean sand.

Your Answer : (Not Answered)

Correct Answer : C

The active thrust is 27.75 kN per metre, acting 2 m from the top.

By Rankine's theory, for a level backfill of cohesionless soil:

\[ \begin{aligned} K_{a} &= \frac{1 - \sin 30^{\circ}}{1 + \sin 30^{\circ}} = \frac{1}{3} \\ P_{a} &= \frac{1}{2}K_{a}\gamma H^{2} \\ &= \frac{1}{2} \times \frac{1}{3} \times 18.5 \times 3^{2} = 27.75 \end{aligned} \]

The pressure increases linearly with depth, so the triangle's resultant acts at \(H/3 = 1\) m above the base, which is 2 m below the top.

The options with 1 m measure the point from the base instead of the top. 249.75 is \(\frac{1}{2} \times 18.5 \times 3^{2} \times 3\), using \(1/K_{a} = 3\), the passive coefficient, instead of \(K_{a}\).

Your Answer : (Not Answered)

Correct Answer : A

Percussion boring (in which a heavy chisel or bit is repeatedly dropped) is the more suitable method in glacial tills that contain boulders, as it can break up and pass through boulders and hard obstructions. Wash boring cannot get through boulders and gravel, since it depends on a water jet. Core drilling and rotary drilling need uniform hard rock or soft soil, and in till with boulders the bit is deflected or jammed.

Your Answer : (Not Answered)

Correct Answer : B

Meyerhof's theory takes into account the shear strength of the soil above the base of the footing, treating the soil there as a surcharge which supplies shear resistance along an extended failure surface. Terzaghi's theory replaces the soil above the footing base by a surcharge of equal weight, ignoring that shear strength. Vesic's and Hansen's methods are later modifications that add shape, depth and inclination factors to the general equation, and the shear above the base is not the distinguishing feature of those.

Your Answer : (Not Answered)

Correct Answer : A

The false statement is that a pile foundation is preferred when there are small fluctuations in the sub-soil water level. Piles are preferred when the water level fluctuates a great deal, because shallow footings would be disturbed by the changing moisture, so it is large fluctuations that favour piles. The other three are genuine conditions for choosing piles: an expansive top soil, a heavy superstructure load that must be carried to deeper, stronger strata or spread over a wider area, and a weak top soil of poor bearing capacity that cannot support the structure.

Your Answer : (Not Answered)

Correct Answer : B

The figure shows a compaction pile: a pile driven into loose granular soil to compact it. The horizontal arrows point outward from the pile along its whole length, showing the soil being pushed sideways and densified, which raises its bearing capacity. An end-bearing pile would show load passing to a firm stratum at the tip, a friction pile would show shear resistance along the shaft carrying the load, and a tension pile (uplift pile) resists upward pull. The arrows here do not show those actions.