KCE064 – FOUNDATION DESIGN (B.Tech Sem VI)
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KCE064-FOUNDATION-DESIGN
SECTION A
Attempt all questions in brief (2 × 10 = 20 marks)
(a) Information required for planning a site exploration
For planning a site exploration, information such as the location and layout of the proposed structure, type of foundation required, magnitude of loads, previous site history, topography, groundwater conditions, and availability of nearby soil data must be collected. This helps in deciding the depth, extent, and methods of investigation.
(b) Purpose of a soil sampler
A soil sampler is used to obtain soil samples from below the ground surface for laboratory testing. These samples help determine soil properties such as strength, compressibility, permeability, and classification.
(c) General shear failure vs local shear failure
General shear failure occurs in dense or stiff soils and is characterized by a well-defined failure surface and sudden collapse.
Local shear failure occurs in loose or soft soils, where failure develops gradually with large deformations and less distinct failure surfaces.
(d) Effect of water table on bearing capacity
The presence of water table reduces effective stress in soil, thereby reducing shear strength. When the water table is close to or above the foundation level, the bearing capacity of soil decreases significantly.
(e) Types of shallow foundations
Shallow foundations include isolated footing, strip footing, combined footing, raft or mat foundation, and grillage foundation.
(f) Group efficiency of pile
Group efficiency of a pile group is the ratio of the load-carrying capacity of the pile group to the sum of the individual pile capacities. It indicates interaction effects among piles.
(g) Pile driver
A pile driver is a mechanical device used to drive piles into the ground using impact or vibration to achieve the required depth and load-carrying capacity.
(h) Types of retaining structures
Retaining structures include gravity retaining walls, cantilever retaining walls, counterfort retaining walls, sheet pile walls, and anchored retaining walls.
(i) Uses of soil reinforcement
Soil reinforcement is used to improve bearing capacity, slope stability, reduce settlement, control erosion, and construct reinforced earth walls and embankments.
(j) Idealized soils
Idealized soils are simplified representations of real soils, assumed to have uniform and predictable properties, used for theoretical analysis and design.
SECTION B
Attempt any three (10 × 3 = 30 marks)
(a) Steps in site exploration and characterization
Site exploration begins with desk study and reconnaissance survey. This is followed by preliminary exploration using trial pits or borings, detailed subsurface investigation, sampling, laboratory testing, interpretation of soil data, and preparation of soil investigation report.
(b) Net ultimate bearing capacity using BIS formula
Given:
Footing size = 2 m × 2 m
Depth = 1.3 m
γ = 20 kN/m³, ϕ = 30°, c = 0
Nc = 30.1, Nq = 18.4, Nγ = 22.4
For square footing as per BIS:
qu=0.5γBNγ+γDfNqq_u = 0.5 \gamma B N_\gamma + \gamma D_f N_qqu=0.5γBNγ+γDfNq qu=0.5(20)(2)(22.4)+(20)(1.3)(18.4)q_u = 0.5(20)(2)(22.4) + (20)(1.3)(18.4)qu=0.5(20)(2)(22.4)+(20)(1.3)(18.4) qu=448+478.4=926.4 kN/m2q_u = 448 + 478.4 = 926.4 \text{ kN/m}^2qu=448+478.4=926.4 kN/m2
Net ultimate bearing capacity:
qnu=qu−γDf=926.4−26=900.4 kN/m2q_{nu} = q_u - \gamma D_f = 926.4 - 26 = \boxed{900.4 \text{ kN/m}^2}qnu=qu−γDf=926.4−26=900.4 kN/m2
(c) Allowable load on pile group in clay
Given 9 piles, diameter = 0.3 m, length = 10 m,
Unconfined compression strength = 70 kN/m²
Shear strength cu=35 kN/m2c_u = 35 \, \text{kN/m}^2cu=35kN/m2
Ultimate load on group based on shear failure:
Qu=m×perimeter×L×cuQ_u = m \times \text{perimeter} \times L \times c_uQu=m×perimeter×L×cu Qu=0.6×(9×π×0.3)×10×35Q_u = 0.6 \times (9 \times \pi \times 0.3) \times 10 \times 35Qu=0.6×(9×π×0.3)×10×35 Qu≈1780 kNQ_u \approx 1780 \text{ kN}Qu≈1780 kN
Allowable load (FOS = 2.5):
Qall=712 kN (approx)Q_{all} = \boxed{712 \text{ kN (approx)}}Qall=712 kN (approx)
(d) Correction of tilts and shifts during caisson sinking
Tilts and shifts are corrected by eccentric loading, dredging on the higher side, water jetting, placing kentledge, or controlled blasting. Continuous monitoring ensures vertical alignment.
(e) Functions of geosynthetics
Geosynthetics perform functions such as separation, reinforcement, filtration, drainage, and containment. They improve soil strength and durability in geotechnical applications.
SECTION C
Attempt any one (10 marks)
(a) Seismic refraction and resistivity methods
Seismic refraction measures travel time of seismic waves to identify subsurface layers. Resistivity method measures soil resistance to electrical current to determine soil stratification and groundwater level.
(b) Purpose of hammer blows in SPT
Hammer blows drive the sampler into soil to measure resistance. The number of blows indicates soil density and strength, helping in foundation design.
