The underpinning of foundations is a method done when foundations get damaged or strengthen the foundation to construct furthermore floors on the existing building.
Sometimes, foundations get damaged unexpectedly suddenly after the construction of the buildings. For the remedial or precautionary measure, a method of underpinning came into work.
There are visible signs like cracks in the buildings. Which shows the need for repairs or strengthening the foundation. If the crack size is a quarter of the inch or more then underpinning is needed.
There are several defects like diagonal cracks in the walls and the cracks in the foundations which show the failure of foundations in the structure.
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Work Nature for Selection of Underpinning Methods.
Work nature is not only dependent on the type of work, but also on the selection of the underpinning methods. These methods are useful as per the suitability of the methods on the site.
Underpinning methods basically depend on the construction site. For example, if the site structure is deteriorating and it shows signs like cracks on the walls.
The reason for the crack is the deterioration of the timber piles, which leads to the weakening of the foundations of the building.
Then the underpinning methods are selected which is suitable to rectify the deteriorating timbre pile problem.
Nature of works for the selection of piles are:-
- Remedial Works.
- Conversion works.
- Protection Works.
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Remedial works have done on the pre-existing structures. As the “Precaution Is Better Than Cure“. Remedial work is done on the present structure to strengthen the foundation.
If there are mistakes in the designing the foundation which prevents the building to collapse from subsidence.
If the building is made up of residential purposes then it converted into a commercial or industrial structure. Therefore the underpinning is used in the conversion works.
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As the name signifies, the protection works are basically used for the protection of the structure.
It is used when there is a modification in the buildings under superstructure. It is done when there is a need for a basement in the pre-existing structure of the building.
If the excavation nearby by the building affects its foundation then protection of the building work has to be done.
If the building is not strong enough to withstand the load of the structure.
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When Underpinning of Foundation is Required
The requirement of underpinning is generally based on the physical observation of the structure, whether the structure is showing some defects or not.
We should carefully observe the offsets of the building. The reading must be taken of horizontal level as well as vertical level. Now it shows how severe the defects and settlements in the buildings.
- The underpinning of foundations is required when the soil bearing capacity is not suitable for the building and foundations are not strong enough to hold the building’s load.
- It is required when there is deterioration in the foundation.
For Example:- If the timber pile starts to corrode after the ages, then it has to be rectified as soon as possible.
- The fluctuation of the water table leads to a decrease in the bearing capacity of the soil.
- The bearing capacity of the soil is not suitable for the building, and it starts settling down.
Methods Of Underpinning
Methods of underpinning are as follows:-
- Pile Raft Method.
- Pit Method (Mass Concreting).
- Pier and Beam Underpinning Method.
- Pile Method of Underpinning.
- Pre-Test Method.
Pile Raft Method
Pile Raft Method of Underpinning is used when the underpinning is done in the structure where the foundation is too deep or the soil is too hard for excavation .
This method is suitable for the structure where the structure is weak, but the foundations are too deep. The needle beams are used to penetrate the plinth beam and then adjoin the beam on the pile.
The load of the structure is transferred in to the ground with the help of piles. It also provides lateral and transverse ties in the strructure for proper stability.
It does not damage the drainage systems of the buildings. It is economical for depths of more than 1.5 m.
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Pit Method (Mass Concreting)
Pit Method of underpinning is the method use in the buildings which generally has shallow foundations. It is the very old technique of underpinning use in the civil engineering.
In this method the excavation is done under the foundation of the buildings till the hard strata and support the foundation by the help of pins.
Then a large amount of concrete is poured under the foundation of the building and starts curing. at the time of transfer of load of the building from old foundation to the new foundation.
The layer of dry cement and sand is poured then the load is transferred. This is the most economical method of underpinning.
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Pier and Beam Method
This is an old method of underpinning use in civil engineering. It was to come in use after the second world war. It is used in where the mass concreting or pit method is not suitable for deep foundations.
In this method, the concrete piers are cast under the foundation then reinforced concrete beams cast to distribute the load on the pier and support the building.
The size of piers and the beams are designed as per the structure and load of the building. This method is suitable for the depths of foundations less than 6 m.
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The pile method is used basically in the structures which are constructed on the clayey soil or where the water gets clogged.
In this method the piling is done adjacent on both sides of the foundation then the beam is constructed through the wall which works as the pile cap and transfers a load of the structure to the pile.
Pre-Test method is not suitable for the raft foundations. They are not suitable for more than 10 story buildings.
In this method, new excavation is done and then compressed or compact the soil with the predetermined load on the soil.
Points should keep in mind While Underpinning
- Proper consolidation of soil should be done before underpinning.
- The process of underpinning should be started from the outside and comes to the inward direction of the structure.
- After the excavation, M10 Concrete (1:3:6) must be poured into the excavated area and fill the cavity.
- Underpinning should be done under the load-bearing members of the building.
- Underpinning must be avoided under the non-load-bearing members of the buildings.
- Concrete should be left for two hours to set in the excavated area to gain its initial strength.
- Backfill should be done properly after the underpinning has been completed.
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This is all about the underpinning of foundations. we’ll come back with some more interesting articles on the remodeling of the structures.
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