Chapter 6

6.1 Types of Foundations

Types of foundations for buildings covered in this Section are:

a) Shallow foundations (see 7)

1. Pad or spread and strip foundations (see 7.2),

2. Raft foundations (see 7.4),

3. Ring foundations (see 7.5), and

4. Shell foundations (see 7.6).

b) Deep foundations

1. Pile Foundations

i) Driven cast in-situ concrete piles (see 8),

ii) Bored cast in-situ concrete piles (see 8),

iii) Driven precast concrete piles (see 9),

iv) Precast concrete piles in prebored holes (see 10),

v) Under-reamed concrete piles (see 11),

vi) Timber piles (see 12), and

vii) Spun piles.

NOTE — Spun piles are used in deep marshy soils as conventional pile installation beyond 50 m in such soils is very difficult.

c) Other foundations/substructures/foundations for special structure

1. Pier foundations (see 13.1),

2. Diaphragm walls (see 13.2), and

3. Machine foundations (see 13.2).

6.2.1 The depth to which foundations should be carried depends upon the following principal factors:

a) Securing of adequate allowable capacity.

b) In the case of clayey soils, penetration below the zone where shrinkage and swelling due to seasonal weather changes, and due to trees and shrubs are likely to cause appreciable movements.

c) In fine sands and silts, penetration below the zone in which trouble may be expected from frost.

d) Maximum depth of scour, wherever relevant, should also be considered and the foundation should be located sufficiently below this depth.

e) Other factors such as ground movements and heat transmitted from the building to the supporting ground may be important.

6.2.2 All foundations shall extend to a depth of at least 500 mm below natural ground level. On rock or such other weather resisting natural ground, removal of the top soil may be all that is required. In such cases, the surface shall be cleaned and, if necessary, stepped or otherwise prepared so as to provide a suitable bearing and thus prevent slipping or other unwanted movements.

6.2.3 Where there is excavation, ditch pond, water course, filled up ground or similar condition adjoining or adjacent to the subsoil on which the structure is to be erected and which is likely to impair the stability of structure, either the foundation of such structure shall be carried down to a depth beyond the detrimental influence of such conditions, or retaining walls or similar works shall be constructed for the purpose of shielding from their effects.

6.2.4 A foundation in any type of soil shall be below the zone significantly weakened by root holes or cavities produced by burrowing animals or works. The depth shall also be enough to prevent the rainwater scouring below the footings.

6.2.5 Clay soils, like black cotton soils, are seasonally affected by drying, shrinkage and cracking in dry and hot weather, and by swelling in the following wet weather to a depth which will vary according to the nature of the clay and the climatic condition of the region. It is necessary in these soils, either to place the foundation bearing at such a depth where the effects of seasonal changes are not important or to make the foundation capable of eliminating the undesirable effects due to relative movement by providing flexible type of construction or rigid foundations. Adequate load counteracting against swelling pressures also provide satisfactory foundations.

6.3.1 Where footings are adjacent to sloping ground or where the bottoms of the footings of a structure are at different levels or at levels different from those of the footings of adjoining structures, the depth of the footings shall be such that the difference in footing elevations shall be subject to the following limitations:

a) When the ground surface slopes downward adjacent to a footing, the sloping surface shall not intersect a frustum of bearing material under the footing having sides which make an angle of $30^{\circ}$ with the horizontal for soil and horizontal distance from the lower edge of the footing to the sloping surface shall be at least 600 mm for rock and 900 mm for soil (see Fig. 2).

b) In the case of footings in granular soil, a line drawn between the lower adjacent edges of adjacent footings shall not have a steeper slope than one vertical to two horizontal (see Fig. 3).

c) In case of footing in clayey soils a line drawn between the lower adjacent edge of the upper footing and the upper adjacent edge of lower footing shall not have a steeper slope than one vertical to two horizontal (see Fig. 4).

6.3.2 The requirement given in 6.3.1 shall not apply in the condition where adequate provision is made for the lateral support (such as with retaining walls) of the material supporting the higher footing.