Underpinning is an effective method of foundation repair in history due to its vast applications in solving major foundation problems. There are different types of underpinning for foundation repair developed in recent times which can be applied in solving critical foundation problems ranging from strengthening to stabilizing an existing foundation to increasing the load capacity to support additional loads or storey as the case may be.
This article is aimed at helping homeowners understand the available options for underpinning foundations so that you can discuss and analyze them with your structural engineer when choosing your foundation solutions.
What is Underpinning For Foundation?
Underpinning For foundation is a technique for foundation repair by reinforcing and strengthening an existing foundation of a building structure or any other structure. This type of foundation repair method is employed in foundation repair to support and strengthen the foundation of an existing building or infrastructure and it is achieved by installing permanent or temporary support to enhance the load-bearing capacity of the foundation and the soil beneath.
In underpinning, the foundation is extended depthwise or expanded breadthwise to distribute the structural loads of the entire building over a larger surface area on the soil strata with a better bearing capacity.
In most cases underpinning structural foundations becomes the last resort when other methods have failed in tackling foundation problems, there are times when you have no other option than to underpin to salvage your home’s foundation. The underpinning techniques in recent times have been developed a great deal, and there are different types of underpinning techniques available for fixing your home’s foundation today.
Structural Conditions that Necessitate Underpinning
The decision to undertake underpinning foundation repair on an existing foundation can be made based on physical observations. When there are visible signs of foundation problems in an existing foundation structure as a result of differential settlement or any kind of distress, it is necessary to establish vertical level readings and likewise the perpendicular level, on a timely basis. The period depends upon how severe is the settlement. Before the excavation for any new project commences, it is highly recommended for professionals to critically examine and determine the soil’s capability to resist the structure that is coming over it. Based on the professional report the need for underpinning can be decided. Proper soil and geotechnical investigations and analysis would prevent underpinning to be undertaken after construction.
In most cases, the option of underpinning your home’s foundation becomes necessary when the original foundation is no longer strong enough to support the building structure. The reason could be a result of the following:
When there are changes in soil properties supporting the foundation or mischaracterization of the soil properties during the original foundation design.
changes in the original usage of the structure following a major renovation and review.
nearby construction that requires the excavation of soil supporting an existing foundation.
Natural mishap such as droughts, floods, or earthquakes has affected a structure making it structurally unstable.
- the need to increase the load capacity of an existing foundation to support the additional storey.
Types of Underpinning
Historically there are two main types of underpinning foundation methods in use. They are mass concrete underpinning (also known as slab or mud jacking) and pile underpinning (also known as pier underpinning). More recently other methods are being used and have been proven effective in fixing complicated foundation problems.
Here are the different types of underpinning foundations used for foundation strengthening:
- Mass concrete underpinning
- Mini-piled underpinning
- Pier and beam underpinning
- Pile underpinning
- Cantilever pile and beam Underpinning
- Pre-test method of underpinning
Irrespective of your choice of the type of underpinning foundation repair method for strengthening your faulty foundation, they all follow the same pattern of extending the existing foundation either depthwise or in breadth to be laid on a stronger soil stratum to distribute the structural load over a wider surface area. Above is the list of the different types of underpinning for foundation repair, but ultimately the choice of the type of foundation underpinning technique depends on the underlying soil conditions and the required depth of the structural foundation.
Mass Concrete Underpinning
Mass concrete underpinning as the name implies is the oldest foundation underpinning method and the traditional type of underpinning, which dates back over a hundred years ago and has not been changed ever since. It requires the extension of an existing foundation to a more stable soil stratum by excavating box-shaped voids to a specified depth beneath the existing foundations and refilling the voids with fresh concrete in a specific manner and controlled order through different stages or pins and the concrete is allowed to cure between 7 to 14 days before the next stage of excavation begins.
The mass concrete underpinning technique is found more suitable for existing shallow structural foundations, it can also be employed in rare cases in deep foundations. In mass concrete underpinning, heavy types of machinery are not usually required, which reduces the costs of installations.
This type of underpinning for foundation repair is suitable where the underlying soil conditions vary in nature, where environmental pollution problems exist, or where access to the existing structural foundations is limited. Mini-piled underpinning helps in the transfer of the structural loads to a stable soil stratum at depths usually greater than 5m. The technical installation process of mini-piles underpinning is more involving compared to mass concrete underpinning which will require specialist types of machinery and professional skills for installation.
Pier and Beam Underpinning
This type of underpinning for foundation repair also referred to as base and beam underpinning is an improvement of the mass concrete underpinning foundation method which was first used after the second world war. Pier and beam foundation underpinning method are found suitable for most soil conditions and it requires the placement of a reinforced concrete beam either above or below the existing structural footing to transfer the structural loads of the existing structure on the newly introduced mass concrete piers or bases placed at particular positions, the placement and the sizes of the newly introduced piers or bases depend on the underlying soil conditions and the size of the affected structure.f
In using the pier and beam underpinning technique the beam placement construction method usually depends on the type of the existing structure and the position where the structural loads are applied. In locations endangered by expansive clay soils after the measures are implemented, anti-heave precautions are usually recommended to be included in the design of the foundation, and over time it has been discovered to be cost-effective for foundation depths less than 6m.
The Pile foundation underpinning method usually requires driving two adjacent piles by the sides of the existing wall that supports the affected foundation. A needle or pin pile cap connecting the adjacent piles penetrates through the existing wall of the affected foundation to transfer the structural loads of the building to a more stable soil stratum that can support the building via the installed piles. This type of foundation underpinning with a needle or pin pile cap can be used where other traditional types of foundation underpinning have failed or are practically impossible to implement because of the excessive deep excavation works required during piling in fixing the affected foundation and in places where there is limited access in getting a piling machine close to the affected structure.
Pile foundation underpinning method in fixing a failing foundation can be carried with much more speed than the regular traditional mass concrete techniques, thereby reducing substantially the costs of installations and the inconveniences encountered during installations.
Cantilever Pile and Beam Underpinning
The technical process of installing cantilever pile and beam foundation underpinning is similar to the pile and beam underpinning, the only difference is that all of the works in cantilever pile and beam are carried out in the external surroundings of the affected property. When using cantilever pile and beam techniques for foundation underpinning, the removal of internal floors of the affected building and other fixtures is unnecessary therefore reducing drastically the costs and inconvenience of the installation works. Cantilever pile and beam underpinning can be implemented in conjunction with pile foundation underpinning.
Advantages of Cantilever Pile and Beam Underpinning
- They are easy and faster to install when compared to other traditional foundation underpinning types.
- Can be undertaken on one side access only
- Cantilever pile and beam underpinning has a high load-bearing capacity compared to other types of underpinning techniques
Disadvantages of Cantilever Pile and Beam Underpinning
- Due to the excessive excavation works required for cantilever pile and beam underpinning they appear to be uneconomical when the existing foundation is deep.
- In locations where there is limited access to the existing building, it discourages the use of needle beams
This type of underpinning for foundation repair is commonly employed for structures built on strip or pad foundations. They can be installed for foundation structures with more than five storeys. When implementing this type of foundation underpinning you must ensure that the subsoil is well compacted and compressed within the new excavated level which gives predetermined structural loads to the soil underneath the existing foundation. This has to be done before the commencement of the actual pre-test underpinning. This type of underpinning technique cannot be applied when it is a raft foundation.
Pile and Beam Underpinning
The pile and beam type of underpinning foundation technique can be installed vis-a-viz the mini-piling processes. This will require the installation of the mini-piles on both sides of the affected foundation structure, and a subterranean pocket of brickwork is then removed to install a pre-fabricated steel cage that cuts across the two mini-piles and then fresh concrete is poured in to complete the underpinning.Un