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Underpinning Method of Foundation

What is Underpinning?
Underpinning is a method used to repair and strengthen the foundation of a building. During underpinning, reinforcements are positioned throughout the length or breadth of an already established base. Because of this, its weight is distributed across a larger area and rests on solid earth layers. Micro-piling and jet grouting are common underlying methods, despite being time-consuming and costly.

Underpinning is excavating the soil that’s pulling away from the structure and eliminating the foundation holding it up. There will be movement in the building when the current foundations become weakened. The property’s foundations and structure are strengthened by installing new, more robust materials and digging deeper footings on top of the more stable growth.

All about Underpinning Foundation Method
Fig 1: Underpinning Method
Courtesy: structural guide.com

The Necessity of Underpinning the Foundation:
By observing, you can figure out what kind of foundation is needed. When a structure changes due to settlement or other damage, take vertical and offset measurements as quickly as possible. It depends on how bad the settlement is on how long it takes. Before starting to dig for a new project, experts have to take a close look at the soil to see how well it can support the structure that will be built on top of it. This report decides if underpinning is needed. Sometimes, these tests would show that the building didn’t need underpinning.

Selection of Underpinning Methods:
Underpinning methods are chosen based on the age of the building and the kinds of work it has. Depending on how old a building is, it is classified as:

i) Ancient structures are older than 150 years.
ii) Recent Structures: Ages between 50 and 150 years.
iii) Modern Structures: Ages under 50 years.

Types of Works for Underpinning Methods Selection:

i) Conversion Works:
The building needs to be changed so it can be used for a different purpose, which means the foundation needs to be made stronger than it is now.

ii) Protection Works:
The following issues with a building require protective measures:

iii) Remedial Works:

Structural Conditions which Require Underpinning:

As a substructure stabilizing strategy, underpinning is often recommended by engineers for a variety of reasons such as:

Methods of Underpinning Foundation:
1. Pit Method or Mass Concrete Underpinning Method:
All about Underpinning Foundation
Fig 2: Pit Method of Underpinning
Courtesy: falconstructural.co.uk

Pit or mass concrete underpinning is one of the earliest techniques. Procedures are carried out to increase the size of the present foundation until a stable layer is attained. Stages or pegs are used to reveal the soil beneath the current foundation. After the desired layers have been reached, the excavation is backfilled with concrete and left to dry before the next removal process can begin.
Adding a dry sand-cement pack as a second linchpin helps shift the weight from the old to the new base. This simple method is effective due to the light foundation. Better options exist to address more intricate issues at the ground level.

2. Pile Method of Underpinning:
The driving technique is utilized to install piles as part of the piling method for underpinning a foundation. Driving piles are placed on either side of the wall to strengthen it. The borehole is normally supported by piles that have been under-reamed. Steel or concrete “needles” are regularly driven into the wall and connected to the foundation piles. These metal or cement piles serve as a support structure and a pile cap. Water-logged soils, clay, and shaky bearing stratum are ideal conditions for installing underpinning piles.

All about Underpinning Foundation
Fig 3: Pile Method of Underpinning
Courtesy: anbeal.co.uk

3. Jack Pile Method of Underpinning:
When the cost of traditional underpinning would be prohibitive due to the depth of good bearing capacity subsoil, Jack Pile Underpinning is an option. As the pile depth may be adjusted to suit the underlying soil, jack pile underpinning is silent, vibration-free, and adaptable. Since the pile caps are cast directly onto the jack pile heads once the hydraulic jacks have been removed, the condition of the existing foundations is critical.

All about Underpinning Foundation2

Fig 4: Jack Pile Method of Underpinning
Courtesy: civil-eng-blog.worldspot.com

4. Root Pile or Angle Piling For Underpinning:

Fig 5: Angle Pile Method of Underpinning
Courtesy: buildersengineer.info

The Root or Angle piling method utilizes cutting-edge concrete drilling technologies to produce concrete, reducing building expenses and time. Line Pairs of reinforced concrete piles are often installed or driven at right angles to one another. Predrilling using an air-flushed percussion drill reinforces the brick wall without disturbing the subfloor. It might be challenging to install angle piling inside a wall. As a result, they are stacked closely together to ensure their stability.

5. Pynford Stool Method for Underpinning:
When the current foundations are in poor form, this method can be employed to underpin the wall in one continuous run without using needles or shoring. The resulting reinforced concrete beam could be sufficient to distribute the weight of the existing wall, or it could be used in conjunction with other underpinning techniques, such as regular piles or jack piles.

6. Underpinning Columns:
All about Underpinning Foundation4
Fig 5: Underpinning Columns
Courtesy: buildersengineer.info

Once the columns’ loads have been removed, they can be supported in the same way as walls are—with conventional or jack piles. When dead beaches are used, beam loads are taken off the columns, and the real column weight is transferred by two beams resting on a collar at the base of the column shaft.

Advantages of Underpinning:

Disadvantages of Underpinning:

Conclusion: 
Underpinning is a technique used to reinforce and restore the foundation of a building. By underpinning, existing buildings and infrastructures can have their weak foundations strengthened. One such method is adding permanent or temporary support to an existing base to increase its depth and load capacity.

References:
1. Discoveries, Engineering. “What Is Underpinning? Uses In Foundation Strengthening and Methods – Engineering Discoveries.” Engineering Discoveries, 1 Aug. 2021, engineeringdiscoveries.com/what-is-underpinning-uses-in-foundation-strengthening-and-methods.
2. CementConcrete. “Underpinning Methods, Procedure, Use in Foundation Strengthening and Repair – Cement Concrete.” Cement Concrete, 22 Mar. 2020, cementconcrete.org/buildings/underpinning-methods-procedure-foundation-strengthening-repair/2092.
3. “Underpinning – Wikipedia.” Underpinning – Wikipedia, 1 Jan. 2012, en.wikipedia.org/wiki/Underpinning.
4. Boss, kumaresh. “(PDF) SHORING AND UNDERPINNING | Kumaresh Boss – Academia.Edu.” (PDF) SHORING AND UNDERPINNING | Kumaresh Boss – Academia.Edu, www.academia.edu/40161849/SHORING_AND_UNDERPINNING. Accessed 26 Sept. 2022..
5. Projects, Engineering. “Underpinning | Underpinning Methods and Purpose.” Engineering Projects, 17 Oct. 2013, bestengineeringprojects.com/underpinning-method-of-underpinning-pit-method-pile-method.
6. LCETED. “WHAT IS UNDERPINNING | USES OF UNDERPINNING | METHODS OF UNDERPINNING | PRECAUTIONARY MEASURES -Lceted LCETED INSTITUTE FOR CIVIL ENGINEERS.” LCETED INSTITUTE FOR CIVIL ENGINEERS, 2 Sept. 2001, www.lceted.com/2021/09/what-is-underpinning.html.

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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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