What is Mass Concreting Underpinning?

 Mass concreting is the commonly used form of underpinning. The method involves excavation a segment of ground below the existing foundation in controlled steps. The depth of excavation is based on suitable bearing strata present in the ground. The excavated pit is filled with concrete and then allowed to cure before the next pit is excavated.

A dry sand and cement packing mortar is rammed between the new and old foundation. This layer helps to transfer the load safely to the new pin. This method is low cost and suitable for shallow depth foundation underpinning.

The new foundation is constructed to act as a retaining structure that can assist in retro-fit of the basement construction. When the base width of the foundation is increases, the imposed stresses are spread uniformly to the weak soil.

Mass Concreting Underpinning; Image Courtsey: Falcon Structural

The method is used in cohesive or granular soil but it works weak in shrinkable clays. The reduce the movement caused due to subsidence and heave.

How to Perform Mass Concreting Underpinning?

The mass concreting involves the following procedure:

1. Individual bases in shorter length not exceeding 1.2m are excavated. The depth of the bases vary based on the profile of stratum.
2. After excavating each base, the supervisor staff and local authority must check whether the excavation performed are correct, free from soft spots , tree roots etc. 
3. After proper excavation, shutters are placed and set in position. After this the bse in backfilled with concrete with the right mix. 
4. During casting a narrow gap is placed between the top of the base and the underside of the footing.r
5. After the concrete has cure and overcome the issues of initial shrinkage, the narrow gap is filled with dry sand and cement. 
6. Always well vibrate the concrete placed to avoid the presence of air pockets in it.
7. The base construction is repeated sequentially until the whole length of the wall underpinning is complete and are supported sufficiently.
8. All the bases are linked together by using "Joggle joints" so as to provide sufficient keys between the adjacent bases.
9. Always provide reinforcement for underpinning considering the health and safety implications for the operations.
10. Polythene sheeting or low density polystyrene can be used to provide anti-heave precautions. This is mainly required when underpinning process is performed on shrinkable clay.

Also Read: How Underpinning Repair Faulty Foundation?