Underpinning is the process of strengthening supporting and stabilizing the existing structure of the foundation. This Underpinning process is done by extending reinforcement in a breadth of the existing foundation or in-depth of an existing foundation.
So that it is resting on highly supportive soil strata and distributing the load on the surrounding greater area of soil. Commonly micro pilling process and the jet grouting process are used for underpinning but these processes are expansive and elaborate.
Methods of Underpinning
The following methods of underpinning foundation are generally used,
1. Pit Method for Underpinning Foundation
In-Pit underpinning foundation method the entire length of the foundation is to be divided into sections 1.2 to 9.5 m in length. The first stage is making a hole in the wall for all divided sections above the plinth level of the structure and the needle is penetrated into the hole.
This needle is made up of steel, timber, and stout material. For supporting masonry, the bearing plates are placed immediately above the needle.
This needle-supporting arrangement is made by wooden blocks or crib supports on both sides of the masonry wall and screw jacks. Then after the foundation pit is excavated up to the desired depth of the new foundation and the new foundation is built inside the pit.
Once this whole process is done again the whole process is repeated for the next alternative pit. Some important precautions should be followed during the construction work going on, such as raking shores or supports provided for supporting weak walls and floors.
In case a strong interior column is there and if increasing the existing foundation is on only one side, at that time cantilever needle beams are used.
2. Pile Method for Underpinning
Pile method for underpinning the foundation as the name suggests piles are driven or installed by the driving technique. These driving piles are placed along both sides of the wall for strengthening purposes.
Commonly reamed piles are used for underpinning the borehole. The steel needles or concrete needles are penetrated throughout the wall and connected with the piles. These steel needles or concrete needles work as a beam and act as a pile cap. The Underpinning pile method is suitable for the waterlogged area, clayey soils, and weak-bearing strata.
3. Jack Pile for Underpinning
Jack pile underpinning is used where the traditional underpinning is uneconomical. The biggest advantage of this method is, it is a flexible and vibration-free method. Because the depth of the pile can be easily adjustable according to the subsoil condition.
The existing foundation is in good condition since it will have to span over the head of the pile cap. This pile cap is cast on to head off the jack pile then after hydraulic jacks have been removed.
4. Pynford Stool Underpinning             Â
Pynford stool underpinning foundation method is used, where the existing foundation soil-bearing capacity is weak. This underpinning foundation technique makes the needle run continuously to the masonry.
The following steps are considered for underpinning
Stage-1: Holes are formed inside the wall for receiving the precast concrete stools or steel stools.
Stage 2: The masonry work between the pined tools was removed to leave support on pinned stools.
Stage 3: Used fabricated reinforcement and placed it around the pinned stools.
Stage-4: Then after formwork is erected and the casting beam.
Stage-5: Remove the formwork
5. Root Pile Or Angle Piling
Root piling or Angle piling is a method used by modern concrete drilling applications for achieving fabricated concrete which is economical and also saves the time of construction.
Line Reinforced concrete piles are installed or driven in pairs at the opposite angle. This makes the masonry wall stronger and the existing floors and walls are predrilled with the use of air flushed percussion auger.
Sometimes is very difficult to apply angle piling on both sides of the wall. For greater stability piles are placed relatively close to each other.
Read More: What Is Foundation Repair | 8 Types of House Foundation Repair Methods
6. Underpinning Columns
In this column underpinning foundation method, all the loads are relieved and then it can be underpinned in the same manner using jack piling or traditional methods.
Dead shores are used for transferring the beam load from the column and the column is transferred by the pair of columns.
Advantages of Underpinning Foundation
- Unde pinning foundation improved the structural stability.
- The underpinning foundation gives more natural light.
- Underpinning increases the value of a property.
- The underpinning foundation provides easy access to plumbing lines, water lines, gas lines, etc.
Disadvantages of Underpinning
- The underpinning foundation method is expansive.
- Underpinning construction is very difficult when the unstable and water-logged ground is there.
- During excavation large amount of excavated material is disposed of on the working site.
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