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1. 4. FOUNDATIONS Introduction Purpose Terminologies Choice of a foundation Types of foundations

2. 1. Introduction A foundation is the lowest part of a building on which it rest. It is the base which is in contact with the ground to which the loads are transmitted. To perform satisfactory, the foundation should transfer loads to the soil or rock that have : - sufficient bearing capacity - suitable settlement characteristics

3. 2. Purpose To spread loads over a sufficiently large area in order to reduce load intensity. To distribute the non-uniform load of the super structure evenly to the subsoil to avoid undue settlement. To provide leveled and hard surface over which the superstructure can be built. To increase lateral stability of the building due to horizontal forces. To provide the structural safety against undermining or scouring due to burrowing animals and flood water

4. 3. Terminologies Backfill Materials excavated from site and if suitable used to fill in around the walls and foundations Bearing capacity T he maximum pressure which the soil can carry Bearing pressure The pressure produced on the ground by loads.

5. 3. Terminologies Made ground Refuse, excavated soil deposited to fill in a depression or to raise the site above the natural level. Settlement Ground movement which may be caused by: Deformation of the soil due to imposed loads Volume changes of the soil due to seasonal conditions Mass movement of the ground in unstable areas.

6. 4. Choice of a foundation Total loads of the building The type and magnitude of loads, depends on the form and type of building to be constructed. In case of low rise building with large span, shallow foundation is preferred. For high-rise building with short span has high loads, deep foundation is required. In case of concentrated loads, the use of pads and piles are common. Where, loads of the buildings are uniformly distributed, rafts or strip foundations are needed.

7. 4. Choice of a foundation The nature and bearing capacity of the soil The type and bearing capacity of soil is normally obtained from planned soil investigation programme. Soil can be of different types such as clayey soil, sandy soil, etc. The settlement in sandy soil is immediate and may occur even before the building construction completed. Clay is the most difficult subsoil to deal with as it is subjected to seasonal movement. Settlement in clays is large and very slow.

8. 5 . Types of foundations Foundations can be considered under two headings: Shallow foundations Deep foundations Shallow foundations transfer loads near the ground surface such as strip, pads and rafts. Deep foundations transfer loads to a suitable subsoil some distance below the ground such as a pile.

9. 5. Types of foundations Based on the form, foundations may be classified as strip, Pad, raft and pile. Strip foundations Provides a continuous ground bearing under the load bearing walls. Suitable for light weight buildings (up to four storeys) where soil bearing capacity is good. The width and depth of the foundation will depend on the nature of ground and building loads. Where the load bearing capacity of the ground is low, wide strip foundation is recommended.

10. Brickwall G.L Wide strip foundations R.C. foundation

11. 5. Types of foundations Strip foundations A deep strip foundation is used to reduce the width of a foundation in shrinkable clay soils, to counteract the effects of seasonal volume changes. Stepped strip foundation is economical to use on sloping site because it reduces amount of excavation, backfill, surplus soil removal and trench timbering.

12. Deep strip R.C foundation Stepped foundation

13. 5. Types of foundations Pad foundation An isolated foundation designed to carry a point load from a column, post or a pier. These include columns of a framed structures, piers of bridges etc. Often square, rectangular or circular in plan. Pad foundations can be made out of stones, bricks, steel or concrete.

14. Pad foundations Structural plan of a framed building

15. 5. Types of foundations Raft or Mat foundations Covers the whole area of the building and extend beyond it. Consists of R.C slab up to 300 mm thick, which is thickened under load bearing walls. Useful where column loads are heavy thus requiring large bases Best suited on light weight building on soils of low bearing capacity and structures having basement storey. Used when uplift is larger than spread footing can accommodate.

16. Slab thickened under walls to increase its structural strength Raft foundation is best suitable for a structure having basement storey

17. 5. Types of foundations Pile foundations Pile foundation is a foundation system that transfer loads to a deeper and competent soil layer. Piles are required where: Deep strip foundation is uneconomical Heavy concentrated loads such as towers, bridges etc. Loads to be transmitted through very weak soil or problematic soils. Construction in the urban areas Offshore constructions

18. 5. Types of foundations Pile foundations Based on the mode of load transfer, piles may be classified as end bearing or friction. End bearing piles are the one in which shaft passes through soft soils and the base rests on firm clay stratum or bedrock. A friction pile embedded in cohesive soil and obtain its support mainly by the adhesion of the surrounding soil.

19. Types of pile foundations

20. R eferences Ivory H Seeley.(1993), ‘’Building Technology’’ Macmillan Building and Surveying Series. W.G Nash. Brickwork 1 R Chudley (1987).‘’Building site works, substructure and plant’’ Longman Group Limited R Chudley (1981), ‘’Construction Technology’’ Longman Group Limited, Volume 4.