Foundation Problems

Soft or compressible soils reduce bearing capacity and can cause excessive settlement. Peat, silt, very soft clay or loose fill may not support strip or pad foundations. A ground investigation identifies the problem and the structural engineer will specify piling, raft or ground improvement.

Waterlogged or saturated soil has reduced strength and can cause instability during excavation and construction. High water table, poor drainage or impermeable layers can keep the ground wet. Temporary dewatering or revised foundation design is often needed.

A high water table fills excavations with water and can cause instability, flotation of structures and delay. Ground investigation should identify water levels; in some areas seasonal variation is significant. Dewatering or exclusion may be required.

Tree roots can cause subsidence by extracting moisture from clay soils, leading to shrinkage and movement. Roots may also physically disturb shallow foundations. Species, distance and soil type all matter. A structural engineer and arboriculturist will advise on risk and mitigation.

Sloping sites can have variable ground conditions, fill, and lateral loads that increase settlement risk. Cut and fill may leave soft or loose material under part of the building. Proper investigation and design are essential to avoid differential settlement.

Cracks in new extension foundations can result from inadequate design for the ground conditions, poor construction, differential movement between existing and new build, or reactive clay. Early diagnosis prevents the problem from worsening.

Former industrial or landfill sites may contain contamination that affects foundation design and construction. Contamination can require encapsulation, removal or treatment. Ground investigation and a remediation strategy inform the foundation solution.

Made ground (fill) is often variable, poorly compacted and unsuitable for conventional strip foundations. Bearing capacity may be low and settlement risk high. A ground investigation will recommend piling, raft or ground improvement.

Former foundations, cellars, tanks, services or rubble can be encountered during excavation. They may delay the job, require redesign or need removal. A utility survey and desktop study help, but obstructions are not always known in advance.

Extensions often settle differently from the existing building if foundations are not designed for the ground conditions or if the existing building is on different founding strata. Cracks at the junction and stepped floors are common signs.

Clay soils shrink in dry weather and swell when wet. This can cause cyclical movement and subsidence, especially where trees or vegetation remove moisture. Buildings on shallow foundations in clay are most at risk.

Former mining can leave cavities, shallow workings or compressible strata that cause subsidence. Coal Authority and historical records help, but ground investigation is usually needed. Foundations may need to be designed to accommodate or resist movement.

Heave is upward movement of the ground, often due to clay swelling when moisture increases (e.g. after tree removal) or when soil recovers from previous loading. It can lift foundations and cause cracking. Design and construction must allow for heave where the risk exists.

Foundations that are too shallow for the ground conditions, frost depth or adjacent trees can move or crack. Older buildings were often built to less rigorous standards. Extension or change of use may expose the problem.

Piling, demolition or heavy plant can transmit vibration that damages nearby foundations or structures. Pre-existing weakness may be exposed. Assessment and monitoring may be needed; in some cases mitigation or repair is required.

Leaking or collapsed drains can wash soil from under foundations, causing settlement and cracking. The drain must be repaired and the foundation supported. Ground investigation and drainage survey identify the cause and extent.

Highly reactive (plastic) clay shrinks and swells significantly with moisture change. Conventional strip foundations may crack or move. Design typically requires deeper foundations, piling or rafts, and sometimes movement joints.

Strip foundations may be specified where a raft would have been more appropriate for variable or poor ground, leading to differential settlement and cracking. The reverse can also occur. Correct choice depends on ground investigation and design.

Where an extension is tied into the existing building, differential movement can cause cracks at the junction. The extension may need independent movement joints or foundations designed to match the existing behaviour.

Water in the ground can freeze and expand (frost heave), lifting shallow foundations. Foundations in the UK should be below the frost zone (typically 450–750 mm depending on region). Older or poorly designed foundations may be too shallow.

Adding storeys, heavy plant, or change of use can exceed the capacity of existing foundations. The structural engineer will assess and may specify strengthening, piling or underpinning.

Bulging or leaning walls can indicate foundation movement, often from subsidence, heave or lateral pressure. The cause must be identified before repair. A structural engineer will specify investigation and remedy.

New build foundations can fail due to design error, poor ground investigation, construction defect or unexpected ground conditions. Early signs include cracking, settlement or failed tests. The warranty provider and structural engineer will be involved.