(SEM VI) THEORY EXAMINATION 2018-19 FOUNDATION DESIGN

FOUNDATION DESIGN (RCE-061)

B.Tech – Semester VI | Section-Wise Important Answers

SECTION A

(Attempt all – 2 marks × 7)

a) Justify: “Seismic refraction method is better than seismic reflection method”

The seismic refraction method is generally preferred in foundation investigations because it is simpler, economical, and more suitable for shallow depth exploration. It gives reliable information about soil strata boundaries and rock depth. Reflection methods are complex, costly, and require sophisticated interpretation, making them less practical for routine foundation design.

b) Classification of piles according to mode of load transfer

Piles are classified into:

End bearing piles – transfer load to hard strata at the bottom.

Friction piles – transfer load through skin friction along the pile surface.

Combined end bearing and friction piles – load is shared by both mechanisms.

c) Define negative skin friction

Negative skin friction occurs when surrounding soil settles more than the pile, exerting a downward drag force on the pile. This additional load must be considered in pile design as it increases the effective load on the pile.

d) Types of settlements in shallow foundations

Shallow foundations undergo:

Immediate settlement

Consolidation settlement

Secondary settlement

e) Define differential settlement

Differential settlement is the uneven settlement of different parts of a structure, which may lead to cracking, tilting, and structural distress.

f) Double reamed pile

A double reamed pile has two enlarged bulbs along its length, increasing bearing capacity and resistance to uplift. It is commonly used in expansive soils.

g) Local shear failure

Local shear failure occurs in loose or soft soils where partial shear zones develop, and soil fails gradually without a clear rupture surface.

 SECTION B

(Attempt any three – descriptive answers)

1) Methods of boring and CPT Test

Boring methods include auger boring, wash boring, percussion boring, and rotary drilling.
The Cone Penetration Test (CPT) involves pushing a cone into the soil at a constant rate and measuring resistance. It provides continuous soil profile and is useful for determining soil strength and stratification.

2) Bearing capacity of square footing on sand

The bearing capacity is calculated using Terzaghi’s bearing capacity equation considering unit weight, depth, angle of shearing resistance, and bearing capacity factors. Total load is obtained by multiplying safe bearing pressure with footing area.

3) Pile group spacing for group efficiency = 1

Pile spacing is calculated considering adhesion factor, pile perimeter, and soil strength. When group efficiency equals unity, the pile group capacity equals the sum of individual pile capacities.

4) Rectification of tilts and shifts in well foundation

Tilts are corrected by dredging soil on the higher side, applying kentledge, or using water jetting. Shifts are rectified by eccentric loading and controlled excavation.

5) Reinforced soil – properties and uses

Reinforced soil is a composite material formed by mixing soil with reinforcing elements like geotextiles. It improves shear strength, reduces settlement, and is used in retaining walls, embankments, and pavements.

SECTION C

Q3 (a) Standard Penetration Test (SPT)

SPT is an in-situ test used to determine soil strength. A split spoon sampler is driven into soil using a 63.5 kg hammer falling from 750 mm height.
Corrections include:             Overburden correction

Dilatancy correction             Energy correction
These corrections improve accuracy of N-values.

Q4 (a) Components of settlement of shallow foundation

Settlement consists of immediate settlement, consolidation settlement, and secondary settlement.
Immediate settlement in cohesive soil is calculated using elastic theory involving modulus of elasticity, Poisson’s ratio, and applied pressure.

Q5 (a) Principles of footing design

Footing design ensures safety against bearing capacity failure, excessive settlement, and structural failure. It must satisfy shear, bending, and serviceability criteria while being economical.

Q6 (a) Well foundation – shapes and components

Well foundations may be circular, rectangular, or double-D shaped. Components include cutting edge, curb, steining, bottom plug, sand filling, and top plug. They are widely used for bridge piers.

Q7 (a) Shallow foundation on reinforced soil

Reinforced soil foundations use geosynthetics to improve load distribution, reduce settlement, and increase bearing capacity. Design considers reinforcement spacing, depth, and tensile strength.