A process of placing a new foundation under an existing one or strengthening an existing foundation is known as the underpinning of foundation. Underpinning may be required for many purposes.
Following are the purpose of the underpinning of foundation,
- To strengthen the shallow foundation of an existing building when a building with the deep foundation is to be constructed adjoining it.
- To strengthen the existing foundation which has settled and caused cracks in the wall.
- To deepen the existing foundation ( resting on poor strata) so as to rest it on deeper soil strata of a higher bearing power.
- To construct a basement in an existing building.
Types of Underpinning of foundation
- Pit method.
- Pile method.
1) Pit method:- On this method, the entire length of a foundation to be underpinned is divided into a section of 1.2 to 1.5m lengths. One section is taken up at a time. For each section, a hole is made in the wall, above the plinth level, and the needle is inserted in the hole.
The needle may be either of a stout timber or steel section. Bearing plates are placed above the needle to support the masonry above it. The needle is supported on either side of the wall on crib supports ( wooden blocks) and screw Jack’s. A foundation pit is then excavated up to the desired level and a new foundation is laid.
When the work of one section is over, work on next to next section is taken up, I,e,.., alternate section are underpinned in the first round, and then the remaining sections are taken up. If the wall to be underpinned is weak, taking shores may be provided. Similarly, the floor may also be supposed, if required.
If an interior strong column exists, or if the foundation is to be extended only to one side, cantilever needle beams may be used in the place of the central needle beam. Jack is placed between the column and the wall.
Following points should be noted in the pit method
- The alternate section is taken up in the first round. The remaining intermediate sections are taken then taken up. only one section should be taken at the time.
- If the wall is long, the work is started from the middle and is extended in both the direction.
- If the new foundation is deeper, proper timbering of the foundation trench may be done.
- Needle beams etc should be removed only when the new foundation has gained strength.
- This is desirable to do the new foundation work in concrete.
- A needle holes etc. should be closed in masonry using cement mortar.
2) Pile method:- On this method, piles are driven at regular intervals along both sides of the wall. Generally, borehole piles or under- reamed piles may be used. In this piles are connected by concrete or steel needles, penetrating through the wall.
These beams incidentally act as pile caps also. This method is very much useful in clayey soils, and also in water-logged areas. The existing foundation is very much relieved of the load.
So, friends, I hope this article on the “Underpinning of foundation” remains helpful to you.