Effective soil testing is an integral first step for construction projects, as it can prevent future structural issues and identify the presence of harmful contaminants.
Soil can be contaminated from a wide range of activities, such as mining, fuel storage, steel works, landfill, and demolition works, as well as from leaks of human, agricultural, and industrial waste.
Moreover, different types of waste – such as metals, coal slags, and contaminated dredge spoil – have been used historically as backfill in low-lying urban areas, which can pose problems when the waste is disturbed by modern-day construction projects.
Even just close proximity to contaminated sites can cause issues for development, as contaminated soil can be transferred through flooding or groundwater flow.
A study from 2022 published in the International Journal of Environmental Research and Public Health explained that construction activities often led to a deterioration of the physico-chemical properties in and around construction sites.
The researchers said: “Through loss and compaction of topsoil, mixing of topsoil and subsoil, and occupation of the land by residual materials and waste materials, construction activities have potential long-term effects on the physico-chemical properties of the construction site soil, ultimately influencing land use and productivity.”
They noted the physical and chemical parameters of soil differed significantly in pre- and post-construction, with the very fine sand content of soil in post construction at 6.91 per cent significantly higher than pre-construction at 0.82 per cent.
Furthermore, the soil organic matter of pre- construction road was 3.57 times higher than post-construction, while the soil bulk density of post-construction soil was 10.3 per cent higher than pre-construction.
The researchers added: “Topsoil stripping, a current method used in international practice to protect topsoil resources, involves removing topsoil from the construction site just before construction, stockpiling it in a fixed location, and reapplying it to the site when construction is complete.
“Topsoil application promotes the establishment of a persistent vegetative cover and improves revegetation success.”
Pre-construction soil analysis comprises an assessment of various characteristics, such as moisture content, load-bearing capacity, chemical composition, compaction strength, and permeability.
The soil testing is critical because if done effectively, it can ensure the structural stability of the upcoming development by safeguarding against potential geotechnical risks.
What type of foundation that can be safely installed is determined by the bearing capacity of the soil, while its reaction to moisture can lead to specific risks that could compromise structural integrity over time, such as swelling, shrinkage, or erosion.
Moisture content analysis allows an understanding of soil compaction, settlement, and overall stability of the site.
Remediation methods for contaminated soil
There are a broad range of biological and physiochemical remediation methods that can be used to reduce soil contamination, both in-situ and ex-situ.
In-situ techniques, which mainly involve some manipulation of the soil to introduce substances that stimulate remediation, can also consist of separating or concentrating contaminants so the pollution can be extracted and treated on-site or disposed of elsewhere.
These methods can be advantageous as they allow soil treatment without having to excavate and transport the soil, reducing costs and environmental impacts.
Importantly, in-situ methods can retain the soil’s structure, organic matter, and biodiversity, which are typically difficult to restore after more invasive ex-situ methods.
A low-cost, energy-efficient, and environmentally- friendly method advocated by researchers is phytoremediation, which uses the transpiration process of plants to sequester essential elements and nutrients from the soil into their biomass.
Phytoremediation plays two principal roles in remediating polluted soils: stabilising contaminants so they are less mobile and less available, and removing them through degradation or transferring them to other media.
A limitation of phytoremediation is that it has a relatively slow rate of heavy metals removal.
However, the method was advanced recently by a research team at the University of South Australia, which developed a process that greatly accelerated this rate.
The remediation technique uses a super-efficient solar evaporation surface to draw water from the soil through a sponge-like filter that traps contaminants, mimicking the process of transpiration.
Dr Gary Owens, who worked on the research, said plants naturally drew mineral components out of the soil when they move water from their roots into their stems, leaves and flowers, where those mineral components are trapped.
Owens continued: “This means plants can be used to extract contaminants from soil, but the process is very, very slow, often taking multiple growing seasons, particularly in heavily contaminated situations where the soil toxicity means the plants struggle to grow and often die.
“We have created a system that mimics this process – a form of biometric plant – but one that does so at a much faster rate and without any of the problems caused by toxicity.”
The new process can remove contaminants in as little as two weeks, followed by a relatively simple process which removes the captured contaminants from the biometric plant body.
This allows the plant materials to be harvested for reuse, and the adsorption material – having a very high saturation point – can be reused multiple times.
The solar evaporator used in the system was a variation of technology they had developed for many purposes, including desalination and wastewater purification.
Both the evaporator and contaminant-capture component are made from low-cost and abundantly- available materials with long operational lives, and the system has minimal maintenance, setup, and running costs.
