FOUNDATIONS Generally there are two types of foundations are available for the Chimneys. The

type of foundation to be adopted depends on the mainly type & nature of foundation.

Solid circular raft is commonly adopted where soil strata is not uniform and has low

bearing capacity i.e less than 350KPa. The solid raft has higher bearing area

compared to annular raft so that load per sq.m on the foundation can be brought

down. Sometimes even friction/bearing piles will have to be used to transfer &

distribute the load safely to foundation substrata.

Rigidity of raft: It is necessary to compute rigidity of any raft in order to ascertain the pressure distribution at the base.

Whether a structure behaves as rigid or flexible, it depends on the relative stiffness

of the structure and the foundation soil. The behaviour of the foundation as rigid or

flexible will also depend upon the rigidity of the super-structure above and properties

of soil below.

In physical terms, a rigid foundation would mean a foundation which is capable of

bridging over pockets of soil with different properties and thus try to even out the

settlements at various points. A rigid foundation would, therefore, have

comparatively lower values of differential settlement but higher values of stresses. A

rigid foundation with a rigid super-structure on a comparatively compressible soil will

result in uniform settlements of structure

A flexible foundation with a flexible super-structures and a comparatively rigid soil

below will behave as a flexible foundation and would result in large differential

settlements and low stresses. Thus:

(i) A rigid member is characterised by high bending moments and relatively

small, uniform deflections. Over all differential settlements are small.

(ii) An intermediate raft, as the term implies, has intermediate bending and

deflection values.

(iii) The flexible raft has comparatively smaller bending moments and

deflection is maximum in vicinity of the loads and small values elsewhere.

Overall differential settlement would be of higher orders.

Rigidity criteria given by IS : 2950 (Part I) is discussed below:

Equation for rigidity of raft is given by

Where, d= raft thickness= Ec=Modulus of elasticity of conc Es=Modulus of elasticity of foundation material d=Thickness of raft Do =outer dia of raft Di=Inner Dia of raft

Annular raft is generally preferred if strata is uniform and is incompressible such as

hard murrum,soft rock, hard rock,etc.,which have also high bearing capacity. There

are

1. Solid raft foundation

2. Annualar foundation

Annular raft:

For the present case as per the analysis & design enclosed vide annexure-…..the

raft is designed as rigid plate & corresponding equations as per IS 11089 are used

for computation of design moments. equations are ised for the Annular raft is

preferably positioned such that under dead loads the centre of gravity of the uniform

soil reaction coincides with the shaft location. The equation for radial moments and

circumferential moments are given by the following equations.

33

3

12 ios

c

DD

d

E

EK

In case the annular circular raft with a ring beam, two conditions are ,generally to be

satisfied.

The maximum pressure under foundation should not exceed the safe allowable

bearing pressure of the soil.

The minimum pressure shall be zero or of a compressive nature. To satisfy these,

the conditions of rafts shall be determined as follows:

where w = total weight of the structure above ground e= eccentricity of the vertical load on base due to overturning moment of lateral loads of wind or earthquake a= outer radius of annular raft or radius of solid raft b = inner radius of annular raft A= area of raft=π( a2 – b2)l qall = net safe bearing capacity of soil at base Po= overburden pressure due to depth of foundation P1 = net pressure desired at base.