What causes foundation movement?
When there is downward movement of the soil strata beneath a building, there is downward movement of the foundations, also known as subsidence. The most common indicator of foundation movement is cracking in a structure. Typically, subsidence related fractures are tapered diagonal cracks, often found around doors and openings.
The leading causes of subsidence include:
- Leaking drains and blocked soakaways can cause fine soil particles beneath foundations to be washed away by ground water causing instability. Granular soils are most at risk.
- Tree roots can take away the moisture from soil, resulting in shrinkage that causes the foundation to contract. This occurs in clay soils only.
- Shallow foundation on extensions – consolidation of soil under the additional weight of a new structure can take a number of years, differing soils over a site can lead to varied rates of settlement leading to differential movement.
- Concentrated loading – where structural load exceeds the bearing capacity of the foundation, for example where building alterations have increased loads leading to stress concentrations. Some building designs may have inherent load concentrations such as Georgian Houses.
Subsidence Repair Techniques
In many cases damage caused by subsidence can be repaired easily and effectively without having to undertake foundation stabilisation work. The first step is to work out what is specifically causing the damage. The defect can sometimes be simply resolved by tree removal or repairing a leaking drain. Here we discuss some of the strategies that are used when the cause of subsidence cannot be remedied and foundation stabilisation work is necessary.
Mass Concrete Underpinning
This foundation repair technique involves excavating a section of the soil beneath the existing building foundation. The excavation is then filled with concrete and allowed to cure. Excavations commonly have to be done in stages and their depth is dependent on finding suitable bearing strata. To keep the building stable during the different stages, a dry sand cement packing mortar is inserted between the new and existing foundation.
– Excavations can cause disruption and will have an added cost of spoil disposal
– Curing times between the different sections can be time consuming and therefore cause project delays
– Not cost effective for deep bearing applications
– Deep excavations leads to serious health and safety implications
In simple terms, a pile is a deep vertical structural column made in a strong material such as concrete, wood or steel. They overcome foundation settlement by being driven into the ground, bypassing the weak layer of soil to a stronger layer of soil or rock, to support the weight of the building on top of it.
– Some installation techniques are noisy and are therefore disruptive for applications like schools and hospitals
– Pile driving causes vibrations through the soil that can cause damage to neighbouring buildings
– Large rigs are often needed which can cause access issues
– Spoil removal may cause issues in contaminated soils
A more cost effective alternative to traditional underpinning or piling is the use of helical micro-piles, otherwise known as circular hollow section piles or pipe piles. Micro-piles are typically 4-5 metres long and consist of a lead section which features two or three helices, extension sections which provide sufficient depth to reach load bearing strata and a bracket which is seated under and mechanically fixed to the foundations.
Micro-piles are ‘screwed’ into the ground, the head bracket is then positioned beneath the foundation to fully support the affected area and prevent further subsidence. Each pile is jacked up to mobilise the weight of the building and can be tested during installation for an instant check. Watch the video for step by step installation guidance.
Due to the lack of required spoil removal, the use of micro-piles has allowed many brownfield sites to become viable locations for building. They are also an ideal foundation system for supporting modular and temporary buildings.
Benefits of Helical Micro-piles
– Don’t require mass excavations or spoil removal resulting in cost savings.
– The use of lightweight hand-held equipment makes the system ideal for areas with restricted access.
– Not having to wait for concrete to cure results in rapid contract times.
– Work can be usually be undertaken outside the building and produces low levels of noise, keeping disruption to a minimum. In certain structures, micro-pile installation from the inside only may offer a considerable advantage.
– They can screw into almost any soil type with no spoil removal or vibration.
– Can be used on project depths of up to 20 metres.
Helibeam – how to repair settlement cracks in brickwork
To increase the spacing of micro-piles and therefore reduce the number required, masonry beams may be created in the supported structure to span between the pile positions and remedy any cracking in the brickwork. By reducing the number of piles the cost is often also reduced.
To create masonry beams, pairs of stainless steel helical reinforcing bars are chemically bonded into channelled-out mortar beds at predetermined spacing’s. Helifix Helibars combine great axial strength with flexibility, enabling them to follow irregular mortar beds and be bent around corners while accommodating normal building movement.
If you are concerned that your home is experiencing foundation movement, please contact the Helifix team on 020 8735 5200.