TGPSC HMWSSB Manager Geotechnical Engineering previous year questions with answers
Every Geotechnical Engineering question asked in the TGPSC HMWSSB Manager (Engineering) papers on this site, grouped by topic. Answers come from TSPSC's final key; each has a short explanation.
Practise Geotechnical Engineering onlineSoil Mechanics
The unconfined compressive strength of a clay in un-disturbed and disturbed state was found to be 180 kN/sqm and 10 kN/sqm respectively. Based on Sensitivity, the soil may be classified as:
- In-sensitive
- Sensitive
- Quick Clays
- Extra Sensitive Clays
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Answer: (3) Quick Clays
Sensitivity = undisturbed strength/remoulded strength = 180/10 = 18. Clays with sensitivity above 16 are classed as quick clays (8 to 16 is extra-sensitive, 4 to 8 sensitive).
If R_1 and R_2 are the radii of curvature of a non-uniform meniscus in two orthogonal planes, the capillary rise is given by:
- h_c = (σ/γ_w){R_1 + R_2}
- h_c = (σ/γ_w){(1/R_1) + (1/R_2)}
- h_c = (σ/γ_w){R_1 × R_2}
- h_c = (σ/γ_w){R_1/R_2}
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Answer: (2) h_c = (σ/γ_w){(1/R_1) + (1/R_2)}
By Laplace's equation, the pressure difference across a curved meniscus is σ(1/R_1 + 1/R_2); equating it to the weight of the raised column, γ_w h_c, gives h_c = (σ/γ_w)(1/R_1 + 1/R_2). For a spherical meniscus (R_1 = R_2 = R) this reduces to the familiar 2σ/(γ_w R).
The coefficient of permeability of a soil sample having its void ratio as 0.50 and co-efficient of percolation as 3.00 × 10⁻⁴ cm/s is:
- 3.00 × 10⁻⁴ cm/s
- 1.50 × 10⁻⁴ cm/s
- 6.00 × 10⁻⁴ cm/s
- 1.00 × 10⁻⁴ cm/s
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Answer: (4) 1.00 × 10⁻⁴ cm/s
Coefficient of percolation k_p = k/n, where porosity n = e/(1 + e) = 0.50/1.50 = 1/3. So k = k_p × n = 3.00 × 10⁻⁴ × 1/3 = 1.00 × 10⁻⁴ cm/s.
In a Laboratory, to perform IS Heavy Compaction Test, it was required to use a mould of 1400 cc capacity in place of standard 1000 cc capacity mould. All other parameters remaining same, the number of blows to be applied per layer to ensure the designated compaction energy is:
- 56
- 25
- 35
- 50
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Answer: (3) 35
The standard 1000 cc mould receives 25 blows per layer, and to keep the same compaction energy per unit volume the number of blows must increase in proportion to the mould volume. Blows = 25 × 1400/1000 = 35.
The magnitude of total primary consolidation settlement of a 6m thick clay with single drainage was estimated as 96 mm. Later it was found that, the medium has double drainage. Then, the magnitude of total primary consolidation settlement will be:
- 48 mm
- 192 mm
- 384 mm
- 96 mm
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Answer: (4) 96 mm
Final consolidation settlement (m_v Δσ H) depends on the compressibility, stress increase and layer thickness, not on the drainage conditions. Double drainage only shortens the drainage path, so the settlement happens about four times faster but stays 96 mm.
The shear strength of a pure clay specimen when tested in Unconfined compression Test was found to be 100 kPa. If the same specimen was tested in Tri-axial Compression Test, the deviatoric stress at which specimen will undergo failure when the confining stress was 50 kPa, will be:
- 50 kPa
- 100 kPa
- 200 kPa
- 400 kPa
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Answer: (3) 200 kPa
A shear strength of 100 kPa means c_u = 100 kPa, so the unconfined compressive strength is q_u = 2c_u = 200 kPa. For a pure clay tested undrained (φ_u = 0) the deviator stress at failure does not depend on the confining pressure, so at 50 kPa confinement it is again 2c_u = 200 kPa; 100 kPa would be the answer only if 100 kPa were the compressive strength.
The factor of safety of a slope of given inclination of 6 m high constructed using a soil with c=60 kPa, γ=20 kN/cum and its Taylor's stability number=0.20, will be:
- 2.50
- 5.00
- 1.00
- 2.00
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Answer: (1) 2.50
Mobilised cohesion c_m = S_n γ H = 0.20 × 20 × 6 = 24 kPa. Factor of safety = c/c_m = 60/24 = 2.50.
According to Boussinesque's theory, under the application of a point load of 100 kN on the surface, the pressure bulb corresponding to an increment in vertical stress of 47.75 kN/sqm will extend to a depth of:
- 0.50 m
- 1.00 m
- 2.00 m
- 4.00 m
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Answer: (2) 1.00 m
Directly below the load, Boussinesq gives σ_z = (3/2π)Q/z² = 0.4775 Q/z². Setting 47.75 = 0.4775 × 100/z² gives z² = 1, so the bulb extends to z = 1.00 m.
Foundation Engineering
The ultimate bearing capacity of a shallow foundation laid on a cohesion-less soil medium was estimated as 200 kN/sqm, when the water table was far below. All other conditions remaining the same, the ultimate bearing capacity of the foundation, when the water table was risen to ground level, is:
- 400 kN/sqm
- 200 kN/sqm
- 100 kN/sqm
- 50 kN/sqm
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Answer: (3) 100 kN/sqm
For a cohesionless soil both terms of the bearing capacity equation (γD_f N_q and 0.5γBN_γ) contain the unit weight, which becomes the submerged unit weight (about half) when the water table rises to ground level. The capacity is therefore halved: 200/2 = 100 kN/sqm.
The observations of a Standard Penetration Test are reported as 10/15/20. Assuming correction for overburden is not required and correction for dilatancy only is required, the corrected N-value is:
- 45
- 35
- 25
- 15
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Answer: (3) 25
The first 150 mm is the seating drive and is ignored, so observed N = 15 + 20 = 35. Since N > 15, the dilatancy correction gives N' = 15 + (N − 15)/2 = 15 + (35 − 15)/2 = 25.
The ultimate load carrying capacity of an end bearing type of bored cast-in-situ circular pile is estimated as 100 kN. All other parameters remaining the same, the capacity of the Pile if the diameter is doubled, is:
- 100 kN
- 200 kN
- 300 kN
- 400 kN
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Answer: (4) 400 kN
An end bearing pile carries its load through the base, Q = q_b × (π/4)d², so the capacity varies with d². Doubling the diameter makes it 2² = 4 times, i.e. 4 × 100 = 400 kN.
Which of the following type of Piles is more appropriate as foundation of structures constructed on Expansive Clays:
- Batter Piles
- Sheet Piles
- Under-reamed Piles
- Compaction Piles
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Answer: (3) Under-reamed Piles
Under-reamed piles have one or more bulbs that anchor them in stable soil below the zone of seasonal moisture change, so they resist the uplift caused when expansive (black cotton) clays swell. They are the standard foundation for light structures on such soils.
The type of Caissons preferred in sites where high upward seepage exists, is:
- Pneumatic Caissons
- Open Caissons
- Box Caissons
- Open caissons and Box caissons
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Answer: (1) Pneumatic Caissons
A pneumatic caisson has a closed working chamber at its bottom kept under compressed air, which holds back the water and prevents soil from boiling or flowing in, so excavation can be done in the dry. Open caissons, dredged through water, are prone to blows and loss of ground where strong upward seepage exists.
Coffer Dams are :
- Permanent structures meant for storage of water
- Structures built across drains to act as check dams
- Temporary structures build to reserve water in side to cure foundation concrete
- Temporary structures build to reserve water outside to facilitate construction of foundation
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Answer: (4) Temporary structures build to reserve water outside to facilitate construction of foundation
A cofferdam is a temporary enclosure built in water or water-bearing soil to keep water out of the working area, so that foundations can be laid in the dry. It is removed after the work is complete.