Questions
5–8 MCQs per paper
Difficulty
Medium-Hard
Importance
High yield for HPCL/NTPC
Overview
Geotechnical Engineering forms the backbone of civil engineering competitive exams, focusing on the mechanical behavior of soil under load. Mastering this topic is essential because it bridges theoretical soil mechanics with practical foundation design, appearing heavily in all major PSU technical assessments.
Soil Classification & Phase Relations
This subtopic deals with the fundamental physical properties of soil, focusing on the three-phase system of solids, water, and air. Understanding the interrelationship between void ratio, porosity, and degree of saturation is critical for solving basic numericals.
- Se = wG (Degree of saturation * Void ratio = water content * specific gravity)
- Gamma_bulk = (G + eSr) * Gamma_w / (1 + e)
- IS Classification system (IS 1498-1970)
- A-Line equation: Ip = 0.73(WL - 20)
- Density Index (ID) for granular soils
Permeability & Seepage
Focuses on the flow of water through porous media and the potential for piping failure. In PSU exams, you must know how to calculate equivalent permeability for stratified soil layers and utilize flow nets effectively.
- Darcy's Law: v = ki
- Average permeability (Horizontal): kh = (k1z1 + k2z2) / (z1 + z2)
- Average permeability (Vertical): kv = (z1 + z2) / (z1/k1 + z2/k2)
- Seepage discharge: q = kh(Nf/Nd)
- Exit gradient: ic = (G-1) / (1+e)
Consolidation & Shear Strength
Consolidation measures the time-dependent settlement of saturated soils, while shear strength determines the failure criteria. These areas are high-weightage numerical zones involving Terzaghi’s one-dimensional consolidation theory and Mohr-Coulomb failure envelopes.
- Coefficient of consolidation: Cv = Tv * d^2 / t
- Mohr-Coulomb equation: Tau = c + sigma*tan(phi)
- Direct Shear vs Triaxial Test conditions
- Normally vs Over-Consolidated clay (OCR)
- Skempton's pore pressure parameters
Bearing Capacity
This section covers the ultimate load-carrying capacity of foundations. Terzaghi’s general bearing capacity equation is the most frequently tested concept for shallow foundations.
- Terzaghi Bearing Capacity: qu = cNc + qNq + 0.5*gamma*B*Ngamma
- Effect of water table depth on bearing capacity
- Local vs General shear failure
- Safe Bearing Capacity (SBC) definition
- Factor of Safety considerations
Formula Sheet
e = Vv / Vs
n = Vv / V
S = Vw / Vv
k = (gamma_w / eta) * K
Tv = (pi/4) * U^2 (for U < 60%)
Exam Tip
Always verify the drainage condition (single vs double) before plugging values into the consolidation time-factor formula, as this is the most common trap in numerical problems.
Common Mistakes
- Confusing dry unit weight with bulk unit weight during phase relation calculations.
- Neglecting the drainage path 'd' value in consolidation time-factor calculations (d = H/2 for double drainage).
- Applying General Shear Failure formulas when Local Shear Failure coefficients should be used for loose/soft soils.
More Revision Notes
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