Underground ground conditions & subsidence – the underlying causes of structural issues

Cracking walls and uneven floors are often symptoms of a structural issue caused by unstable ground conditions or subsidence. To fix these problems, it’s important to understand not only the structural effects on the property but also what is happening in the ground.

Poor understanding of an area’s soil condition can have significant repercussions for a property. For example, the area alongside the River Teign in Devon is characterised by very shallow and well-drained loamy soils over limestone, often on steep slopes. A few years ago, we remediated a residential property that was built on these conditions and subsequently suffered from severe subsidence. It had been constructed on a sloping site and the reclaimed Made Ground had caused the home’s foundations to shift, most likely due to inadequate fill material or poor compaction. This eventually caused one of the walls to crack and separate from the foundation by approximately 20mm.[1] This could have been avoided with appropriate measures in place to manage the unstable ground conditions during the building’s construction.

Broadly, there are several soil types that we would encounter when conducting ground remediation. They have different characteristics and therefore different solutions are required. In the UK the most common ground conditions include Clay, Sand, Silt, organic material, and Made Ground.

Clays

A large proportion of London has been built on the aptly named London Clay Formation. Clay soil swells when wet and shrinks when dry causing subsidence in drier conditions, or heave in wetter conditions, particularly after heavy rain and storms. Over-saturated clay also loses its bearing capacity causing building footings to settle in the weakened soil, which can sink even further as it dries out. Clays range from High, Medium or Low Plasticity in direct relation to its shrink-swell capacity.

Sand and Silt

Structures on sand and silt soils often experience settlement caused by flowing ground water, a natural underground watercourse or a leaking sewer or storm water pipe. The finer grains are washed away leaving larger grains to settle, and the building’s footings and foundations become unsupported, compromising the structure above.

Fill / Made Ground

Fill is earthen material placed and compacted in a hole or depression. It can consist of soil, and also includes aggregate, rock or crushed construction waste. Structures built on this type of material are susceptible to the same issues as those on sand and silt, especially when water washes away finer material. Settlement is worse if the fill is poorly compacted.

Organic

Organic soils, especially peat, are weak and highly compressible making them prone to settlement, as the weight of any structure above causes compaction. Primary settlement decreases over time, but can total 100s of millimetres, resulting in significant structural issues.

Often a combination of soils and ground conditions are encountered on the same site. Understanding soils is key to finding an effective, sustainable solution to structural issues, so it’s important to get advice from both structural and geotechnical experts, before deciding on the right remediation plan for your situation.

Mainmark’s Teretek® engineered resin injection solution is used to re-level buildings and remediate issues in foundation ground. It is an ideal solution for all types of soils. For smaller discrete areas typically affected by reactive clay soils, Teretek works well to strengthen building foundations affected by clay soil subsidence. Teretek resin injection is also an effective solution for remediating fill soils, binding the fill to provide extra strength and alleviate the problem of poor compaction. Mainmark’s JOG Computer-Controlled Grouting solution also provides a unique alternative for building level correction and stabilising. JOG is well suited for controlled and precise work, especially on unpredictable sand and silty foundations, and for larger, complex structures.

Both, JOG and Teretek can be applied in situ under existing structures and do not require major excavation works, or manual compaction. Both methods are considerably less invasive than traditional underpinning remedies which require extensive excavation, large quantities of concrete and subsequent backfilling.