Foundation Guidelines Types, Requirements, and Depth Criteria (IS 1904 Explained)

Guidelines for selecting a foundation

Type of soil and Loading condition	Suitable Foundation
1. If the structural load is less and the soil is medium to dense	SHALLOW
2. If the structural load is heavy, loose, and weak	RAFT or DEEP
3. If swelling pressure is high and the differential free swell value is more than 35%	RAFT or DEEP
4. If the footing area is more than 40% of the plinth area	RAFT or COMBINED
5. If the structural load is heavy and the foundation is to be placed in running water	WELL FOUNDATION
6. In running water (river, sea, canal)	DEEP FOUNDATION (well)
7. If the soil is loose, saturated sand, and is prone to liquefaction	COMPACTION PILE
8. If the structural load is less (2-3 storey building)	ISOLATED FOOTING
9. If soil is expansive and has high swelling and shrinkage characteristics, it is bound	FLOATING/BALANCING FOUNDATION or UNDER-REAMED PILE

General requirements of Foundations

For a satisfactory performance of a foundation must satisfy the following three conditions :
- Location and depth criteria.
- Shear failure criteria (bearing capacity criteria).
- Settlement criteria.

(i) A foundation must be properly located and founded at such a depth that its performance is not affected by factors such as lateral expulsion of soil beneath the foundation, seasonal volume change caused by freezing and thawing, and the presence of moisture and adjoining structure.

(ii) A foundation must be safe against shear failure or soil rupture.

(iii) The settlement of the foundation, especially differential settlement, must be within the permissible limit.

Excessive settlement may affect the utility of the structure and also cause its damage and decrease its aesthetic value.

NOTE: The three requirements are independent of each other and must be satisfied separately.

Location And Depth Criteria:– (IS 1904)

As a general rule, any foundation should be placed at a depth where soil strata are adequate from the point of view of bearing capacity and settlement criteria.
However, it must be placed at a minimum depth of 50 cm below natural G.L.
Further foundation must be placed below the zone of volume change.
In expansive soils, the depth of the foundation must be placed below the depth of the moist zone.
The zone of seasonal variation in water content varies in thickness from (1.5 to 3.5 m) Black cotton soil.
Foundations for structures in rivers have to be protected from the scouring action of the water. The depth of foundation, in such a case, must be below the deepest scour level.
When footings are adjacent to sloping ground or where bases of the footing are nearby at different levels or at a level different from that of the footing of the adjoining structure, the following recommendations are adopted:–

(i) When the G.L slopes downward adjacent to a footing, the sloping surface should not encroach upon a minimum of bearing material under the footing having sides which make an angle of 60° with the horizontal for rock and 30° for soil and the horizontal distance from the lower edge of footing to the sloping surface shall be at least 60 cm for rock and 90 cm for soil.

Foundation Guidelines Types, Requirements, and Depth Criteria (IS 1904 Explained)

(ii) For footing in granular soil, the line joining the lower adjacent edges of adjacent footings should not have a slope steeper than 2H:1V.

(iii) In clayey soil, the slope of the line joining the lower adjacent edge of the upper footing and the upper adjacent edge of the lower footing should not be steeper than 2H:1V.

(iv) To avoid damage to an existing structure, the foundation for a new structure at an adjacent site should be located at a suitable distance such that –

The adjacent edge of the new footing must be at least a distance “S” from the edge of the existing footing, where “S” is the width of the larger footing.

(v) The line from the edge of the new footing to the edge of the existing footing should make an angle of 30° or more with the horizontal plane. i.e., the distance “S” should be greater than the difference in elevation b/w the adjacent footings.

If even the position of the new and existing footing were to be interchanged, the recommendation would hold good or remain the same.
(This provision ensures that the stress overlap due to adjacent footing doesn’t assume any significant proportion).

(vi) When a new footing is placed lower than an old footing, the existing structure may be in danger because of the lateral flow of soil from beneath the existing footing. Minimum “S” should be there.

The excavation must not, therefore, be too close to the existing footing.
If excavation is done, proper provision of bracing must be made.

Foundation Guidelines Types, Requirements, and Depth Criteria (IS 1904 Explained)

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