Underpinning Design

Underpinning is a type of remedial work employed to strengthen and stabilise the foundation of an existing building that transfers the load of a building onto more stable ground.

Where existing foundations are inadequate, they can be repaired or improved by a process known as underpinning. This involves providing new foundations or extending the defective foundations down to more stable ground at a lower level. The use of underpinning as a solution for subsidence has increased greatly since the 1970s.

Foundation underpinning solutions are generally sought where the use or safety of a building is or is likely to be compromised by excessive ground movement (Subsidence), usually as a result of changes to the supporting soil.

The type of house underpinning required will depend on the type of soil present and the existing foundations, and should only be specified by a suitably qualified engineer. PKT Consulting Engineers can provide a specific underpinning design to address the movement identified by the investigation.

Where investigation identifies a need for foundation strengthening, the timing and scope should be planned around the condition of the building and any immediate stabilisation measures required.

• Underpinning is an adaptable solution for buildings suffering from subsidence. Many different underpinning options can be utilised to solve the specific issues that can vary for each property in distress.
• An appropriately designed underpinning scheme can strengthen the foundations and address the mechanism of movement identified by the investigation. Its suitability depends on the building, ground conditions, load transfer, construction and inspection requirements. Foundation work does not remove the need to address other causes of damage or guarantee against every form of future movement.
• Once a property has been underpinned and certified by a chartered engineer, this will generally raise its value compared to its value before the required structural works.
• Properties suffering subsidence that have not been underpinned have a less available pool of buyers as banks may be reluctant to fund the property purchase.

A cost-effective underpinning system is decided upon when the ground conditions are assessed through ground investigations. Factors like access, age of building, superstructure loads, depth to stable ground, etc., are some of the factors that will be considered when designing and pricing an underpinning solution.

Underpinning has several applications, including increasing the bearing resistance of the foundations before refurbishment, increasing the loading on a building by adding an additional floor, and preventing the undermining of existing foundations by nearby excavations. The most common method is to repair buildings affected by downward foundational movement.

The main five underpinning types are:

• Mini-piling
• Pile and beam
• Piled raft
• Mass concrete
• Pier and beam

This involves excavating adjoining bays beneath the existing foundation in a sequence, filling the excavations with concrete, and then filling the small gap between the old and new foundation with non-shrinkable material (See Figure 2). The sequence is indicated by the numbers in Figure 1 below.

Vertical piles are installed on either side of the wall, their heads are connected by casting a concrete needle beam that passes through the wall (see Figure 3). An alternative variation of this type is a cantilever beam used instead of the needle beam, which eliminates the need for piling inside the building (see Figure 4). Alternatively, inclined mini-piles can be installed through holes drilled in existing foundations (see Figure 5).

Mini-piles are installed within the property, and a reinforced concrete raft is cast integrally with the piles. Needle beams extend out from the slab into the walls below ground level to support them (See Figure 6).

Resin injection is one possible ground-treatment method. Its suitability depends on the cause of movement, ground conditions, depth and extent of treatment, and the means of verifying performance. Obtain independent engineering advice before choosing resin injection or an underpinning system, and review the proposed scope, limitations and guarantee terms.

Find out more: Resin Injection vs Mini Piled Underpinning.

Sometimes mitigating the cause of the movement can be an alternative, e.g., using gabions to stabilise a slope or repairing leaking drains.

Before putting a building on a site which has known poor ground, the following can sometimes be options:

• Excavating the poor soil and refilling
• Vibration stone columns
• Drainage removal or addition based on ground conditions
• Soil mixing, e.g., admixtures
• Grouting