Ground Conditions

Soft or compressible ground reduces bearing capacity and can cause excessive settlement or failure of strip foundations. Peat, silt, very soft clay and loose fill are common. A ground investigation identifies the problem; the structural engineer specifies piling, raft or ground improvement.

Waterlogged or saturated soil has reduced strength and can cause instability during excavation. High water table, poor drainage or impermeable layers are common causes. Temporary dewatering or deeper foundations (e.g. piling) are often specified.

A high water table fills excavations and can cause instability and delay. Ground investigation should identify water levels; dewatering or exclusion may be required. Piling through the water-bearing layer is often specified.

Tree roots can cause subsidence by extracting moisture from clay soils and may physically disturb shallow foundations. Species, distance and soil type affect risk. The structural engineer and arboriculturist will advise; piling below root influence is common.

Reactive clay shrinks in dry weather and swells when wet, causing movement. Shallow strip foundations are at risk. Design typically requires deeper foundations, piling or rafts, and sometimes movement joints or heave precautions.

Sites can have mixed strata, pockets of soft ground or variable fill. A single foundation type may not suit the whole site. Ground investigation maps the variation; the design may specify piling, rafts or different solutions in different zones.

Sloping sites can have variable ground, cut and fill, and lateral loads. Proper investigation and design are essential to avoid differential settlement. Solutions include stepped foundations, piling and careful management of fill.

Peat and highly organic soils have very low bearing capacity and compressibility. Strip foundations are not suitable. Ground investigation will recommend piling to competent strata or, in some cases, removal or ground improvement.

Some soils collapse when wetted, causing sudden settlement. Certain fills and loose wind-blown deposits can behave this way. Ground investigation identifies the risk; design may require piling, pre-wetting or avoidance.

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

Former industrial or landfill sites may contain contamination that affects foundation design and construction. A ground investigation and remediation strategy inform the foundation solution. Encapsulation, removal or founding through contamination may be specified.

Water in the ground can freeze and expand, lifting shallow foundations. UK practice is to found below the frost zone (typically 450–750 mm depending on region). In frost-susceptible soils, adequate depth and drainage are essential.

Saturated loose sands and silts can lose strength under vibration or rapid loading (liquefaction). In areas of seismic or dynamic loading, the risk is assessed. Ground improvement or piling may be specified.

Bearing capacity is the ability of the ground to support load without failure or excessive settlement. Ground investigation and lab testing provide parameters for the structural engineer to design strip, pad, raft or piled foundations.

Ground improvement can make marginal ground suitable for shallow foundations or reduce settlement. Vibro compaction, vibro stone columns, lime/cement stabilisation and pre-loading are options. The ground investigation and structural engineer will specify if and which method is appropriate.

Settlement depends on soil stiffness and compressibility. Stiff soils settle less; soft or compressible soils can cause significant settlement. The structural engineer uses the ground investigation to predict settlement and design accordingly — often piling or raft where settlement must be limited.

Strip foundations suit good ground and moderate loads; rafts spread load over a larger area and suit variable or poorer ground. The structural engineer chooses based on ground investigation, loads and settlement criteria. Piling is an alternative where both strip and raft are inadequate.

A ground investigation report describes the strata, groundwater, contamination risk and recommendations for foundation type. The structural engineer uses it to design strip, raft or piling. This guide summarises what to look for and when to involve groundworks contractors.

Trial pits are shallow excavations that allow visual inspection; boreholes go deeper and provide samples for lab testing. The choice depends on depth, access and what the design team need. Both inform foundation design.

Older buildings may have no record of foundation depth or type. Before extending or altering, the structural engineer may need investigation (trial pits, opening up) to assess adequacy. New work may then be designed to match or improve on the existing.

Clay and other reactive soils move with seasonal moisture change — shrinking in summer and swelling in winter. Foundations must be designed for this: often deeper foundations, piling or movement joints. The ground investigation will inform the design.

Sands and gravels are non-cohesive and generally have good drainage and bearing capacity when compact. Loose or saturated granular soils can be problematic. Ground investigation confirms density and water table; the structural engineer designs accordingly.

Sulphates in the ground can attack ordinary concrete. Ground investigation may classify the site as sulphate-bearing; the structural engineer will specify sulphate-resisting cement or other measures. This affects foundation concrete specification.

Retaining walls and steep slopes may require ground anchors or other support. The geotechnical engineer specifies the system; we install to design. This can overlap with foundation works where stability affects the building.