If you have a structural problem with your building, whether it be a house, commercial property or other infrastructure, it is important to understand the underlying cause. Cracking walls and uneven floors are often symptoms of the problem. Many structural issues relate to 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. Different soil types give rise to different problems and require different solutions. Having an expert on hand who knows how soils behave can help ensure that when you solve one issue you don’t cause another.
Getting it wrong can have disastrous results. One residential property in Melbourne was substantially damaged after a trench dug beneath the floor to fix a leaking pipe was back filled with sand. The property was built on reactive clay soil. The sand-filled trench created a channel that funnelled ground water under the home, saturating the clay and heaving the entire building. The pipe fix had caused far more extensive damage to the building than the original burst pipe because the soil type hadn’t been considered.
Broadly, there are five soil types that we would encounter when conducting ground remediation. They have different characteristics and therefore different solutions are required.
The volume of reactive clay varies with moisture levels. It swells when wet and shrinks when dry. In dry areas, visible cracks appear in the ground when it shrinks, and most people are familiar with sticky, slippery wet clay soils that get stuck on your boots. Structural problems often occur when there are varying moisture levels in the soil below, causing subsidence in drier regions, or heave in wetter areas.
An added complication with clay soil is that when it becomes over-saturated it loses strength and its bearing capacity is reduced. Building footings bear down through the weakened soil and settle. This has double the impact when the soil later dries out, with footings sinking even further. Clay soils can often become oversaturated when exposed to additional moisture from broken sewers and storm water pipes, for example, or sometimes from poor overland drainage or burst water mains.
Mainmark’s Teretek® engineered resin injection solution is generally used to re-level buildings and remediate issues in foundation ground. It is ideal for the smaller, discrete areas typically affected by reactive clay soils, and works well to strengthen building foundations. Teretek is applied at very precise and specific injection points, and is a less invasive process than traditional remedies which require extensive excavation and subsequent backfilling. It also avoids the possibility of further problems associated with backfilling such as poor compaction or water permeating the fill material and creating a local wet spot. Teretek expands in the ground on application, both compacting and strengthening the soil. The degree of ground strengthening depends on the levels of compaction and the overburden.
Structures on sand and silt soils are more often subject to settlement due to water flow. Water flowing though the soil washes away finer grains in the soil. Larger grains then settle, leaving building footings and foundations unsupported locally, compromising the structure above. The water can be ground water, a natural underground watercourse or the result of leaking sewer or storm water pipes. Settlement can affect part or the whole of a structure depending on the water’s course.
Permeation Grouting is an effective remediation in these circumstances. It works by binding or ‘cementing’ the soil particles together. This stops the flow of water, halting the migration of fine material, and strengthens the soil so it can support the structure above. Permeation Grouting needs to be carefully applied to manage the flow of water to avoid causing new problems in another areas. Blocking water flow in one area will simply divert it to find another route. If the source of the water can’t be eliminated, by fixing a leaking pipe for example, the grout needs to be applied in a pattern that provides support to the structure, but still allows the water to travel through. This can be achieved by creating columns of grout around the building’s footings. This approach is suitable for situations where the water source can’t be eliminated, for example natural ground water flows.
Alternatively, in areas where the ground is very soft, Teretek resin injection can be used. In Australia, Teretek has been used to re-level buildings on sand and silt soils, particularly on the Gold Coast and in Central Queensland, including beneath homes, mining infrastructure and port facilities.
This solution is also suitable for sand and silt soils in earthquake-prone areas to mitigate soil liquefaction, and has been used in the Christchurch area where the technology is recognised in the MBIE Module 5 in NZ for earthquake strengthening.
JOG Computer-Controlled Grouting is suitable for larger multi-storey or more complex buildings affected by settlement in sand or by soil liquefaction caused by earthquakes. It is capable of lifting an entire building footprint, as was the case in our award-winning work to re-level the Christchurch Art Gallery – a 33,000 tonne building.
Fill is a volume of earthen material that is placed and compacted for the purpose of filling in a hole or depression. Fill can consist of soil, but may also include other materials such as aggregate, rock or crushed construction waste.
Structures built on fill are susceptible to the same issues as those built on sand and silt. Fill is also prone to settlement, especially when water flows through it washing away the finer particles. The settlement can be more significant if the fill is poorly compacted. The level of compaction can vary across a site, so different areas of the building above can be affected to different degrees of settlement, causing structural damage.
Teretek engineered resin injection is an effective solution for remediating fill soils. It binds the fill to provide extra strength and alleviates the problem of poor compaction. In addition, it can be applied in situ under existing structures and does not require major excavation works, or manual compaction. It is non-invasive, quick and cost-effective.
Organic soils, especially peat, are very weak and highly compressible. They are prone to settlement as the weight of any structure causes the soil to compact. The primary settlement decreases over time but it can be up to 100s of millimetres in total, leading to significant structural issues. It’s important to balance the total overburden by using soil substitution.
Terefil®, a very strong yet lightweight cementitious fill can be used to replace the top layer of soil to provide a new-build platform. Terefil can withstand significant pressure, maintaining its integrity even under heavy loads. Lighter than the soil it is replacing, Terefil can support a structure without compacting the soil beneath, as long as the total mass does not exceed the weight of the soil that was removed.
In new builds, it is common to excavate up to 800mm of organic soil, which can be replaced with Terefil before adding a structure – ensuring the combined weight is no greater. Avoiding adding extra load to the ground helps eliminate the problem of poor soil compaction and settlement.
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.
James O’Grady is the Sales and Business Development Manager at Mainmark. He is a civil engineer with 25 years’ experience in structural engineering, construction materials and ground treatment.