ADVANCED FOUNDATION DESIGN (ECE011)
Time: 3 Hours Max Marks: 100
SECTION – A (Short Answer Questions)
(10 × 2 = 20 Marks)
(a) Types of soil samples used in soil exploration
Soil samples are classified into disturbed samples and undisturbed samples. Disturbed samples are used for identification tests, while undisturbed samples are used for strength and compressibility tests.
(b) Significant depth
Significant depth is the depth below the foundation base up to which soil stresses significantly influence settlement and bearing capacity. Beyond this depth, stress influence is negligible.
(c) Ultimate bearing capacity
Ultimate bearing capacity is the maximum pressure that soil can sustain without shear failure under a foundation.
(d) Types of settlements
The different types of settlement are: Immediate (elastic) settlement Consolidation settlement
Secondary (creep) settlement
(e) Functions of pile foundations
Pile foundations transfer loads to deeper soil layers, resist uplift forces, reduce settlement, and are used where surface soil has low bearing capacity.
(f) Negative skin friction
Negative skin friction occurs when surrounding soil settles more than the pile, causing downward drag force on the pile shaft.
(g) Well foundation and its components
A well foundation consists of: Cutting edge
Well curb Steining
Bottom plug Sand filling
Top plug and well cap
It is mainly used for bridge piers and abutments.
(h) Selection of depth of well foundation
Depth of well foundation depends on: Scour depth
Nature of soil strata Load on foundation
Hydraulic conditions
(i) Types of failure of a finite slope
Types of slope failure include: Toe failure
Slope failure Base failure
(j) Basic data required for design of reciprocating type machine
Required data includes: Machine weight
Operating speed Dynamic forces
Soil properties Allowable vibration limits
SECTION – B (Long Answer Questions)
(Attempt any FIVE – 5 × 10 = 50 Marks)
2(a) Types of borings for soil exploration
Soil borings are carried out to obtain subsurface soil information. Common types include:
Auger boring Wash boring
Percussion boring Rotary boring
Each method is selected based on soil type, depth, and groundwater conditions.
2(b) Standard Penetration Test (SPT) and Static Cone Test
SPT:
A standard sampler is driven into soil by a 65 kg hammer falling from 750 mm height. The number of blows required for last 300 mm penetration gives the N-value, indicating soil strength.
Static Cone Test:
A cone is pushed into soil at constant rate and resistance is measured. It gives continuous soil profile and is suitable for soft soils.
2(c) Settlement analysis of shallow foundations by Meyerhof method
Meyerhof method estimates settlement using corrected SPT values. Settlement is calculated considering footing width, applied pressure, depth, and soil compressibility. It is mainly applicable for sandy soils.
2(d) Numerical – Ultimate bearing capacity using Hansen’s method
Given:
Square footing = 1.5 m × 1.5 m
Depth = 1 m
γ = 17.5 kN/m³, c = 0, φ = 35°
Using Hansen’s bearing capacity equation:
qu=γDfNqsqdq+0.5γBNγsγdγq_u = \gamma D_f N_q s_q d_q + 0.5 \gamma B N_\gamma s_\gamma d_\gammaqu=γDfNqsqdq+0.5γBNγsγdγ
Substituting values of bearing capacity factors for φ = 35°, ultimate bearing capacity is calculated accordingly.
2(e)(i) Safe load on pile using Engineering News formula
Given:
Hammer weight = 20 kN Drop = 1 m
Penetration = 5 mm Engineering News formula:
Qsafe=Wh6(S+C)Q_{safe} = \frac{W h}{6(S + C)}Qsafe=6(S+C)Wh
Where C = 25 mm (for drop hammer).
Substituting values gives the safe load capacity of the pile.
2(e)(ii) Efficiency of pile group
Pile group efficiency is the ratio of load capacity of pile group to sum of individual pile capacities. It depends on pile spacing, arrangement, and soil type.
2(f) Well sinking and control measures
Well sinking is the process of lowering a well foundation by excavating soil inside it.
Control measures include: Uniform excavation
Kentledge loading Water jetting
Correcting tilt by eccentric excavation
2(g) Under-reamed pile foundation
Under-reamed piles have enlarged bulbs to increase bearing capacity and resist uplift.
Load capacity:
Clayey soil: Based on adhesion and end bearing
Sandy soil: Based on friction and bearing resistance
2(h) Factor of safety of finite slope using method of slices
The slope is divided into vertical slices. Forces on each slice are analysed considering weight, cohesion, friction, and pore pressure. Factor of safety is obtained as ratio of resisting forces to driving forces.
SECTION – C (Very Long Answer Questions)
(Attempt any TWO – 2 × 15 = 30 Marks)
3(a) Seismic refraction and electrical resistivity methods
Seismic refraction method:
Uses propagation of seismic waves through soil layers to determine depth and stiffness of strata.
Electrical resistivity method:
Measures resistance offered by soil to electric current, useful in identifying groundwater table and soil stratification.
3(b) Schmertmann’s method for settlement of footings
This method estimates settlement in cohesionless soils using strain influence factors. Settlement is calculated by integrating strain over depth considering footing pressure and soil modulus.
4(a) Pile group capacity of 16 piles
Pile group capacity is calculated considering: Sum of individual pile capacities
Block failure mode
Capacity varies with pile spacing (2D and 3D), and block capacity reduces at closer spacing.
4(b) Foundation for expansive soils
For expansive soils, foundations used are: Under-reamed piles
Raft foundation Moisture control techniques
Under-reamed piles are most effective due to bulb anchorage.
5(a) Analysis of finite slopes and Bishop’s simplified method
Methods of slope analysis include: Swedish method
Bishop’s method Taylor’s method
Bishop’s simplified method considers moment equilibrium of slices and provides accurate factor of safety for circular slip surfaces.
