Underpinning is a construction technique used to strengthen and stabilize the foundation of a building or structure. It involves excavating and reinforcing the existing foundation, typically by adding more concrete or steel supports, in order to address issues such as foundation settling, cracking, or other types of damage.
Underpinning is often necessary when a building’s foundation is not able to support the weight of the structure or the soil beneath the foundation is unstable.
- In construction, underpinning is the Process of strengthening and stabilizing the foundation of an existing building or other structure.
- Process of Underpinning: Underpinning Is accomplished by extending the foundation in depth or in breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area.
- Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout. All of these processes are generally expensive and elaborate.
1) 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 a 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 by cracks in the wall.
2) Applications of underpinning
The following situations demand underpinning.
- When defective foundation of wall is to be replaced with new foundation or when existing foundation of a wall is required to be strengthened to enable it to carry more loads.
- To strengthen shallow footings of the existing building when a building with deep foundation is to be constructed adjoining to it.
- To safeguard against the danger of excessive or differential settlement of foundations of existing structure.
- To increase depth of foundation to increase its bearing capacity so as to sustain heavier loads.
- During construction of basement of an existing building structure.
3) Preparation before underpinning
- It is necessary to notify the adjoining or not about the proposed works with details typical action about the use underpinning sorry for tying.
- bus site and its building or restricted areas should your mind and carry out a survey, at that time and the effects of cracks are recorded and notified to the adjoining owner(s).
- before starting any under the reason for settlement and its solutions are determined, it is done in the case where the reason for underpinning is the settlement.
- we have Meri on to reduce the load of the structure by removing imposed loads from the floors, reducing the unwanted dead loads, and is saved by requiring props and/or shoring only after that the underpinning works should start.
- If there is any for a portion of the area which is vulnerable by proposed underpinning works should be supported and protected by identifying, tracking, and marking or exposing.
4) Necessary of underpinning
- Uneven settlement is caused by the unsymmetrical loading of the building, unequal bearing capacity of all the soil beneath the foundation, By the action of tree roots, or primary or secondary settlements consolidation cohesive soil.
- Increase in loading: the loading process of building man changed due to the addition of more story or change in the imposed loading due to change in service loading.
- Lowering of adjacent ground: Nearby Foundation you stop to work available then it is necessary to lower the foundation of the building.
5) Types or Methods of Underpinning
- Pit method – it involves strengthening by excavation and building below the existing foundation.
- Pile method – it involves supporting the building on newly built piles of various types without excavation.
- Pier method – this is a combination of pit and a pile method where first excavation is done below the foundation and a pier is built up for the new foundation.
- Underpinning to walls
- Jack pile underpinning
- Needle and pile underpinning
- ‘Pynford’ Stool method of underpinning
- Root pile or angle Piling Underpinning columns
i) Pit Method
- In this method the length of the foundation to be underpinned is divided into sections of 1.2 to 1.5 m lengths.
- For each section a hole is made on the wall above the plinth level and a needle is inserted in the hole. Bearing plates are placed above the needle to support the masonry above it.
- Needle is supported on either side of the wall on wooden supports and screw jacks.
- The foundation pit is excavated up to the desired level and new foundation is laid. In the round, the work is undertaken in alternate sections. Once the alternate sections are undertaken, the remaining sections are worked out. If the wall for which underpinning is to be carried out is weak, raking shores is provided to support them.
- 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 Figure below.
Precautions to be taken in pit method –
In pit method of underpinning, the following points are noteworthy.
- Alternate sections are taken up in the round. Remaining section should be taken next.
- For long wall the work should be started from the middle and worked outwards.
- If the foundation is deeper, proper timbering of the foundation trench may be done.
- The needle beam should be removed only when the new foundation has gained strength.
- It is desirable to do the new foundation work in concrete.
- The needle should be closed in masonry using cement mortar.
ii) Pile Method
- In this method, piles are driven at regular interval along both sides of the wall. The piles are connected by concrete or steel needles, which penetrate through the walls. These beams also act as pile caps. This method is effective in clayey soil and in waterlogged areas. The existing foundation is very much relieved of the load. Fig. illustrates the pile method of underpinning.
iii) Pier Method
- It is also termed as base and beam method which was implemented after the second world war.
iv) Underpinning to walls
For underpinning work in walls, the wall should be divided into legs for bays and is bay is treated individually that prevents fracture, damage, or settlement of the walls.
Following are the factors that affect the length of the legs and bays:
- The overall length of the wall.
- The dead and live loads on walls.
- The bearing capacity and types of soil under the existing Foundation.
- The strength and stability of the walls and foundation of the wall which is going to be underpinned.
- The estimated differential settlement and spinning ability of the existing wall footing.
For mass concrete strip foundations supporting walls of traditional construction, the suitable bay is 1 to 1.5 m, and for moderate loading walls supported by reinforced concrete strip foundations, the bay length is 1.5 To 3 m.
Note: In all cases, the sum of the unsupported length of wall should not exceed 25% of the total length of the wall
v) Jack pile underpinning
- Jack pile underpinning is done where the traditional underpinning is uneconomical because of the depth of suitable bearing capacity of subsoil.
- The main advantage of Jack Pile and defining is it is vibration-free and flexible because the pile depth can be adjusted to suitable subsoil conditions encountered.
- In this system, the existing Foundation is span over the heads of the pipe caps which are cast in onto the Jack pile heads after the hydraulic jacks had been removed that Makes the Foundation in good condition.
vi) Needle and pile underpinning
- Where the traditional or Jack pile underpinning techniques are unsuitable for the existing Foundation condition, then the needle and pile underpinning method can be used for the best result.
- As shown in the figure below the big work in this method above the existing Foundation should we in a sound condition. And the piles used are generally in small diameter bored piles.
vii) ‘Pynford’ Stool method of underpinning
Pynford underpinning is used, when the existing foundation’s soil is poor bearing capacity this method of underpinning is suitable, and this method makes the needle run continuously to the walls.
Following are the detailed of this method of underpinning:
Stage1. Holes formed in the wall to receive steel or precast concrete stools.
Stage II. Stools inserted and pinned to the soffit of brickwork over the opening.
Stage II. Brickwork between pined tools removed to leave wall supported on pined stools.
Stage IV. Reinforcement fabricated and placed around pinned stools.
Stage V. Formwork erected and beam cast.
Stage VI. Formwork removed; beam allowed to cure before being pinned to the underside of the wall.
viii) Root pile or angle Piling
- In the root pile underpinning method, there is an application of modern concrete drilling equipment to achieve fabricate concrete that is economical through timesaving.
- Due to the above reason, this is a simple alternative to traditional underpinning techniques.
- There is no need for a large volume of excavation, show this message is not a disruptive bulk work comparatively.
- Lined reinforced concrete piles installed in pairs at opposite angles make the wall stable in where the sound building starter is located not more than 1 to 2 m. In this process the existing floor, wall information is pre-drilled with air flushed percussion auger. Figure below details.
- Through this drill hole, steel lining is driving to the low grade/clay subsoil until it impacts with firm strata. In many conditions, it is very difficult to apply angle piling to both sides of a wall.
- As for the subsoil condition sometimes, it is down with the remedial treatment from one side only. For more stability piles are placed relativity with close space.
ix) Underpinning columns
- In the column underpinning first of all the loadings are relieved then it can be underpinned in the same manner as walls using traditional or Jack pile methods.
- Dead shores are used to transfer the beam load from the column and the column is transferred by means of a pair of beams.
Underpinning can be costly and time-consuming, but it is often a necessary solution to prevent further damage to a building or structure. A qualified structural engineer or foundation specialist should be consulted to assess the foundation issues and determine if underpinning is the right solution for a particular case.