THEORY EXAMINATION (SEM–VI) 2016-17 ADVANCED FOUNDATION DESIGN

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​=γDf​Nq​sq​dq​+0.5γBNγ​sγ​dγ​

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.