Raft Foundations

Raft foundation is adopted when isolated footings are not feasible. The whole of foundation area is excavated and RCC raft is placed where the load of the superstructure is high or the safe bearing capacity of the soil is not adequate raft may be lead over piles driven into the soil over the areas. These piles usually are of shorter length and may be end bearing resting on hard stratum/rock or friction piles.

After excavation is done as given in 7.3 upto the required level the surface should be inspected. If any area has been excavated deeper then it should be filled up to the required level with suitable fill material. If any soft or loose soil is encountered then those areas should be excavated deeper and then suitably filled up as per the advice of the Engineer-in-Charge.

After the excavation is complete a levelling course of lean concrete should be laid and cured. Over this the reinforcement is laid as per the structural drawings. The shuttering of the sides should be suitably propped to the adequately strong to withstand the pressure of green concrete and prevent building. Adequate chairs for

supporting top reinforcement should be provided to keep them in position.

Adequate cover should be provided for the reinforcement. Do not provide more cover thickness than specified as it does not increase protection but increases surface crack width. For deep rafts concreting shall be planned carefully to have continuous concreting to  the extent possible to avoid weak construction joints. For Raft resting over piles suitable anchorage of the piles inside the raft should be provided.

In general following points should be checked:

1. Quality and size of the coarse aggregate whether under size. over size, or improperly graded and contains disintegrated or soft material, earth and other organic impurities. 
2. Sand should be checked for grading, silt content, bulk age and foreign material. 
3. Water should be tested for sulphates and chloride and any other impurities. 
4. To ensure designed concrete water cement ratio should be as specified. 
5. Cover blocks should be of proper size and strength and adequate in numbers. 
6. Cement concrete should be as specified. 
7. Form work should be as per the drawings. 
8. Strength of form work should be adequate to withstand pressure of concrete, workman and machinery during casting. The support &me work should be cross braced. Failure on this account is fairly common. 
9. Forms should be easily removable in sections in desired sequence without damaging the concrete. 
10. Use of burnt mobile oil on the shuttering surface instead of proper shuttering oil should be avoid as it leaves stain in the concrete surface. 
11. Form should be water tight. 
12. General quality of line and level should be proper. 
13. Adequate arrangement of curring is very important. 
14. Mixing arrangement of concrete by mechanical mixture should be proper. 
15. Needle and plate vibrates should be proper and in working condition. 
16. Slump of concrete should be as specified to ensure workability and consolidation. 
17. Vibration should be properly done. Over or under vibration both are undesirable. 
18. steel reinforcement should he checked for suitability and strength. 
19. The length of overlap and its spacing should be adequate. As the force is transmitted by bond with concrete there should be adequate gap between bars in the lap length. Tying with binders should not be done. 
20. Use of proper quality and gauge of hinding wire and use in all intersections should be ensured. 
21. Rein cement details should be such as to permit easy placing and vibration of concrete. 
22. Cement should be stored properly. 
23. Reinforcement cage should be properly secured by chairs and hangers to prevent allocations during concreting. 

Some of the defects observed in execution of RCC rafts are given below:

1. The storage of cement was not properly done. The principle of 'first in' and 'first out' was not followed. As a result old cement of lesser strength was used occasionally. 
2. The water for mixing concrete and curing was not tested periodically. Ground water used contained  high quantity of chlorides and sulphates and was not suitable. 
3. The water storage tank was used by labourers and contained algae. The water used for concreting was dirty and contained sand. 
4. The top reinforcement sagged due to government of labour while concreting. 
5. The cover blocks wire made of weak concrete and gave away easily. 
6. Proper cover blocks were not used and many of the blocks were dtached from the steel to which there were tied.