Coal Ash-hardened Marine Soil for Backfilling

What it does

Traditionally, sand is used in reclamation backfilling but it's becoming more costly. Now, a new geomaterial is invented. Coal ash-hardened marine soil can achieve an unconfined compressive strength of 2MPa. It's suitable for future urban development works.

Your inspiration

Extensive reclamation projects are proposed in Hong Kong. However, from some on-going reclamation works, the cost of purchasing backfilling material has seriously risen. As a civil engineering student and a former intern at a coal-fired power plant, I understood the basics of concrete production and the properties of coal ashes. The idea of developing a new geomaterial by combining the theories then came to my mind. Since coal ash and cement are both pozzolanic materials, soil particle linkages can be formed during the hydration and curing process. Loose soil particles hence become solid chunks of stiff material with high load capacity.

How it works

Marine soil is typically soft and therefore subjected to consolidation settlement when a load is applied. The process takes months or even years to complete the thus causes trouble for rapid construction work in a reclamation site. In this project, coal ash and a minor proportion of cement were used to harden soil particles by a series of chemical reactions, mainly hydration. Calcium-silicate-hydrate gels are formed when a soil-ash-water mix is allowed to settle. Such gel glues the particles together and therefore strengthen the soil. By analysing the unconfined compressive strength and stiffness of mixes composing of different water and ash ratios. It was found that greater strength can be obtained when the water-to-stabilised ratio is reduced and when more time was given to cure the specimen. The result proved that coal ash is an effective yet economic choice of stabilising agent for marine soil. It also reduced the wastage from coal-fired power plants.

Design process

In the beginning, I went through several studies regarding stabilised marine soil using different types of stabilisers. The objectives were then set 1) To assess the feasibility of using coal ash-hardened marine soil as a backfilling material to replace sand, 2) To examine the effect of water-to-stabiliser ratio, curing time and seawater as a curing medium on the unconfined compressive strength. The target strength was designed according to a guideline for earth remediation works. A systematic test programme was designed after performing two trials testing. Variables including Raw Pulverised Fly Ash (PFA) contents ranging from 10% to 40% were tested. The amounts of water added were revised to 30%, 35%, 40% & 50%. Meanwhile, identical specimens were allowed to cure for 3, 7 & 28 days. To ensure test representativeness, Raw PFA was collected from the Castle Peak Power Station, Hong Kong. The marine soils were collected from North Lantau Island waters and ordinary Portland cement conforming to the UK & EU-standards were purchased from Green Island Cement Company. Cylindrical specimens of 50mm diameter, 100mm height, were then produced from these raw ingredients. A series of unconfined compressive test was then carried out and results are corrected per the ASTM C42-03 standard.

How it is different

Stabilised marine soil is an idea widely tested. In most circumstances, cement or specially treated ash were used as the stabiliser agents. Despite having a very promising hardening effect, cement is an industrial product with very high carbon emission. In fact, it is often the reason for associating construction activities with global warming. Therefore, in this project, coal ash was used to replace the majority of cement proportion. Coal ash is the by-product of coal-fired power plants. Those without further classification (i.e. Raw PFA) even has a zero carbon footprint. Thus, this idea prevents sending useful material into landfills. Furthermore, marine soil was used as it is often a problematic soil in reclamation works. Dredging and dumping, despite polluting surrounding waters, are common procedures for eliminating this trouble natural geomaterial. The idea of coal ash-stabilised marine soil is, therefore, an innovative way of solving multiple issues.

Future plans

In the future, the heavy metal leaching potential of the stabilised marine soil can be studied. This further address the environmental concerns of adopting this idea. It is also planned to perform similar tests on specimens without cement added and use freshwater instead of seawater in mixing and curing. These tests allow the standalone effect of coal ash hydration and water salinity to be studied. I look forward to witnessing this new geomaterial to be used in an actual reclamation project.

Awards