Every year, about 1.4 million tonnes of construction waste is being produced in Singapore alone. To help achieve circular economy, the built environment can treat unusable marine clay into an alternative construction material.
WHAT IS MARINE CLAY?
Marine clay is soft soil with undrained shear strength values ranging from 10 to 30 kPa. It has been a source of difficulty for property owners as it can cause structural damage in houses or poor drainage. This is because marine clay can shrink and swell during natural changes in soil moisture.
During dry seasons, the soil can lose moisture, which causes the clay to shrink, leaving a void, or gap, under the footings. Bearing support for the footings is lost. The rain then replenishes the soil moisture, causing the clay to swell again. After several cycles, the rebound of the foundation may be progressively less, resulting in cracks.
Uneven building settlement like this leads to broken footings, cracked masonry walls, interior cracks in plaster, as well as warped door and window frames. At construction sites, marine clay needs to be removed. In the construction of Marina Barrage in Singapore, for example, marine clay was found at the site and had to be removed to ensure the structure’s stability.
MARINE CLAY IN SINGAPORE
About 40 per cent of Singapore’s land surface are rock formations containing marine clay. There is a vast abundance and yet we are unable to use it in construction due to its weak soil properties. Some notable problems are its low bearing capacity, instability and high long-term consolidation. These occur as a result of its low shear strength, high sensitivity, low permeability and high plasticity.
The sticky, platy texture of raw marine clay that makes it difficult to handle and transport only exacerbates the situation. Building contractors often dread finding marine clay on site and the most common approach is to excavate it entirely and send for disposal at the two remaining staging grounds in Singapore, namely, the Aviation Park Staging Grounds and the Tanah Merah Staging Grounds. After that, there is a need to procure replacement soil to fill up the existing voids.
The implications involved in this process include transportation and disposal costs, procurement of replacement fill, and the embodied carbon produced in the transportation in and out of site. This is unnecessary and there is actually a way to breathe life into the unwanted marine clay to make it usable.
In brief, these are some of the problems relating to marine clay in Singapore:
- The existing geology
There are large portions of underground marine clay in Singapore such as in Kallang and Old Alluvium Formation. There have been 8.5 million cubic metres of soil being excavated.
- Properties of marine clay
Marine clay has poor soil properties and is hence unsuitable for engineering requirements. It has low strength and permeability, and high moisture content.
- Transportation
Transporting and disposing marine clay to staging grounds is associated with high costs. Besides, they will release high CO2 emissions.
- Limited disposal sites
Stockpiling marine clay takes up land space, while dumping it in sea causes environmental issues. In addition, exportation of marine clay to Indonesia for disposal has been banned.
TREATING MARINE CLAY
Treating marine clay can be done with a two-pronged approach. Firstly, additives are added to the clay to trigger exothermic reactions. The additives could be metal slag from factory processing. When added in correct amounts, the metal slag contributes to the strength of the finished product.
The exothermic reactions serve to dewater the clays and this process is further accelerated by mixing the mixture in a rotatory tank. The mixing ensures that the additive thoroughly coats the lumpy clays. Next, the mixture goes through an extruding process where it is forced through a template to produce pellet-shaped materials. These mixing and extruding stages also help to agglomerate and bind the mixture effectively. The moisture content can be significantly reduced. Under a microscope, the initial sticky and platy texture is transformed into individual particles. The outcomes can then be used as hardcore aggregates and compacted easily to fill up any voids.
Cost saving could be further maximised when the treatment machine is deployed onsite in the event that marine clay is discovered. Time and resources are saved when marine clay is directly fed into the treatment machine instead of stockpiling it somewhere and then sending it for disposal. Due to the high demand, there are often long queues to the staging grounds. This results in idling carbon emissions, which adds to the long list of embodied carbon sources.
Transformed marine clay into a new pellet-like material (PLM) can provide supplies for:
- Replacement for hardcore aggregate
- Reclamation fill material
- Back fill material
- Caisson fill material
Research by Newsoil Technologies Pte Ltd, as presented by marketing engineer, Vemula Jahnavi, during the session entitled Breathing New Life Into Soft Soil by Newsoil Technologies at the International Built Environment Week (IBEW) 2020 shows that:
- PLM consists of individual particles, unlike untreated marine clay that is platy and sticky
- The durability is similar to cementitious material. It undergoes pozzolanic reaction and is durable in the long run.
- The x-ray refraction of PLM shows that there is no trace of heavy metals and foreign or toxic materials.
- The strength as tested by Unconfined Compression Strength (UCS) is 1567.8 kPa (marine clay is 30kPa); and as tested by California Bearing Ratio (CBR) is 23.34 (marine clay is 4.5).
- PLM has the same range of permeability as sand.
THE APPLICATION
The application could be done off-site and on-site, summarised as follows:
| Off-site | On-site |
| Collect marine clay to be disposed from construction sites | Rent machinery to a project site |
| Convert marine clay to PLM using special machinery | Excavated soft soil to be placed in machinery and converted into PLM |
| Stockpile an inventory of PLM to be sold | Filling to be done on-site immediately or up to two days |
Read: Waste Management and Minimisation in Southeast Asia: An Overview
In Singapore, PLM has been used in the building carpark and pavement at the Gardens by the Bay and cluster houses in Paya Lebar. With construction waste piling up at an alarming rate, effective management and minimisation programmes are imperative. The use of sustainable materials and advanced technology are definitely a promising solution to achieve circular economy.
Multiple trials, including the one co-funded by the Building and Construction Authority (BCA) show that this method is highly feasible. Treated marine clay was used to fill up a depth of about 200 millimetres to investigate the material performance as a replacement for hardcore aggregate in subbase layers. Subjected to periodical precipitation and heavy daily traffic, treated marine clay was found to be a satisfactory replacement. In situ test results showed that the consolidation was well within the allowable limits.
Nevertheless, despite the promising outlook and breakthrough in recent years, much work still has to be done before the world is ready for material such as treated marine clay. It is understandable that there will be great inertia for the relevant authorities to accept and adopt something entirely new. Through strong support from the academia, more industry partners will be convinced to work with recycled material and crease a more sustainable construction sector
– Construction+ Online
