Underpinning is a sensitive construction technique for strengthening an existing foundation or placing a new foundation below the old foundation to a deeper depth.
Underpinning is a sensitive repair project so, we have to choose the correct method for it. For the correct method, we have to understand and judge the soil strata, the current situation, and problems concerning the entire foundation, the required depth and extent of the new foundation.
Purpose of Underpinning
Underpinning is done for the following purposes:
- For the purpose of leading an old shallow foundation to the deeper depth when the adjoining building is constructed with deep foundation.
- Underpinning is done for building a basement in the existing building.
- Underpinning is done to deepen the existing foundation (resting on poor strata) and make it rest on deeper soil strata of higher bearing capacity.
- To strengthen a settled foundation that may be caused cracks in the wall.
Methods of Underpinning
Underpinning can be carried but by the following methods:
- Pit method
- Pile methods
In this method, the entire length of the foundation to be underpinned is divided into sections of 1.2 to 1.5 m lengths. One section is taken up at a time.
First of all, a hole is made in the wall for all divided sections above the plinth level, and needle is inserted in the hole. The needle may be made of a material of stout, timber or steel section.
Bearing plates are placed above the needle to support the masonry above it. The supporting arrangement of the needle is made by crib supports (wooden blocks) on both sides of the wall and screw jacks.
After that, a foundation pit is excavated up to the required level of new foundation. Then the new foundation is laid in the pit. When the work of one section is over, work on next to next section is taken up, i.e., alternate sections are underpinned in the first round, and then the remaining sections are taken up. Fig2. above shows the section.
Some important precautions should be taken during the work, such as raking shores are provided in week wall, floors are also supported.
In this process cantilever needle beams may be used when a strong interior column is there, or if there is a required foundation increasing in only one side, as shown in Fig. 3.
The following points are to be considered in the pit method:
- Alternate sections are taken up in the first round. The remaining intermediate sections are then taken up. Only one section should be taken at a time.
- It is better to start the work from the middle in the case of longwall extended in both directions.
- If the new foundation is deeper, proper timbering of the foundation trench may be done.
- When the foundation has gained full strength
then only all the arrangements like needle beams etc. should be removed slowly.
- The needle holes etc. should be closed in masonry using cement mortar.
In the Pile method of underpinning, as the name suggests Piles are installed by proper driving technique along both sides of the wall to be strengthened. The piling technique normally used are borehole pile on under-reamed piles are used.
After that, concrete or steel needles are penetrated through the wall and are connected to the Pile. These needles function is as beams and is act as pile caps also.
Pile method is suitable for clayey soils, waterlogged area and also in weak bearing strata.