Cyclic behaviour of compacted black soil-coal wash matrix

Highlights

• Coal wash (CW) mixed with expansive clay, can improve the geotechnical properties such as swell pressure, the monotonic and cyclic strength.

• 30% CW-soil was the optimal mix because it reduced the swell pressure and improved the strength of the mix.

• The shear strength of the CW-soil mixture was higher than the original compacted clay because the CW altered the PSD of the soil matrix, and its overall plasticity and shrinkage.

Abstract

Recent population and industrial growth require innovative approaches for recycling construction materials for civil infrastructure such as railways, roads, and building foundations. This study adopts a combination of expansive black soil blended with coal wash (CW), a by-product of coal mining, to produce a resilient substructural fill for railroads. When considered individually, expansive clays and CW are prone to high swell pressure, shrinking and swelling, excessive degradation, and relatively low bearing capacity, properties that would adversely affect infrastructure. This current study is a laboratory characterisation of soil and CW mixtures in relation to Atterberg Limits, Particle Size Distribution (PSD), and X-ray diffraction spectroscopy. A mini-compaction procedure is used to assess the changes in maximum dry density and optimum moisture content. This is followed by a series of consolidated-undrained monotonic and cyclic triaxial tests of the mixtures. Our findings show that CW can reduce the plasticity, susceptibility to shrinkage, and the swell pressure of clay soil. When clay is blended with CW there is a significant improvement in the undrained shear strength, resilient modulus, and yield response before failure, as well as the number of loading cycles before failure. Furthermore, the inclusion of CW to improve the mechanical properties of soil also reduced the large amount of space needed to store CW.

Introduction

The issues with using expansive soils in geotechnical engineering during construction and post-construction phases of the superstructure have been widely documented, and the rail industry is no exception (Tang et al., 2009; Alazigha et al., 2018; Wang and Wei, 2014). Alternate wetting and drying lead to swelling and shrinkage with subsequent cracking that can lower the bearing capacity of subgrade at different times of the year. In lightweight structures such as rail tracks, this problem is often expressed through an intensified differential settlement (Sánchez et al., 2014) which places rail tracks built on such ground at substantial risk of instability and potential failure. For instance, along the Wellcamp-Charlton alignment in Toowoomba, Australia, 100 km of expansive clays were discovered and 248,600 m³ out of 369,600 m³ needed to be removed (AECOM, 2017). Here, the excavation of expansive clay and the need to quarry appropriate replacement geo-materials significantly increased the cost of construction; moreover, the subsequent cost of maintaining tracks built on expansive soils compared to country rocks almost doubled.

Coal wash (CW) is the waste granular material left after processing/separation of coal (i.e. “washing” in coal processing). It usually consists of about 30% gravel and 70% sand in NSW, Australia. CW requires an enormous area of landfill for storage (Indraratna et al., 2020b; Rujikiatkamjorn et al., 2013). This landfill poses a serious environmental problem because coal is still being produced in many parts of the world. In fact, current predictions indicate a future demand for more than 4500 million tonnes in 2030 (International Energy Agency, 2020), and thus a continual increase in CW tips. The Australian coal mining industry is responsible for producing ever-growing stockpiles of coal wash, mine waste aggregates and washery refuse. Unsightly stockpiles now occupy otherwise usable land in regional suburbs, and the recycling of this waste would reduce the demand for quarried aggregates. The potential effective reuse and recycling of granular wastes such as coal wash as an additive to improve the properties of expansive soil underneath railway track have important advantages, both from economic and environmental perspectives. To be used as a fill, CW is subjected to stringent chemical composition test requirements to ensure its environmental friendliness which is outlined in New South Wales Environmental Protection Authority order NSW (2014). As documented by Wang et al. (2019), the main elements in the coal wash are Quartz, Kaolinite, Illite, and Siderite, which did not react with soil unless lime is added to trigger a pozzolanic reaction. Laboratory and field evidence have proven the potential use of CW in geotechnical engineering (Chiaro et al., 2015; Rujikiatkamjorn et al., 2013; Qi et al., 2019). Rujikiatkamjorn et al. (2013) proved that compacting CW close to its optimum moisture content can produce favourable permeability coefficients and sufficient undrained shear strength for landfills and roadworks. The caveat in this study is that every batch of CW should be investigated because its properties often vary depending on its location (i.e. coal measures) and the intensity with which the ore is processed. A recent study showed that adding rubber crumbs to CW could produce a capping layer for rail substructure that would absorb the energy transmitted from dynamic train loading (Qi et al., 2019).

Different approaches have been used to ameliorate the negative effects that swelling and shrinkage have on expansive black soils. These methods include soil replacement, mechanical and chemical stabilisation, geo-synthetics inclusion, sand cushion, and soil reinforcement (Modarres and Nosoudy, 2015; Firoozi et al., 2017; Indraratna et al., 2020b). In practice, a combination of these methods is generally used based on the complexity of the problems encountered on a particular site. Each method has specific advantages and disadvantages, but there are also other controlling factors such as site geology, project finance, the size of the construction, and duration of the project. The most widely used and most successful chemical stabilisation technique is lime, cement, and fly ash. Depending on the dosage, these chemicals alter the physiochemical setting to produce soil that is suitable for engineering purposes (Firoozi et al., 2017). Chemically treated soils have a reduced volume change capacity, a higher shear strength, and lower plasticity. However, traditional chemical stabilisers such as lime, cement, gypsum, and fly ash have limitations associated with stringent occupational health and safety issues and threats to the soil and groundwater environment, where the increased alkalinity (pH @ 8–9) and significant void reduction affect the scope of certain native vegetation and sub-surface fauna. Kamruzzaman et al. (2009) and Lasledj and Al-Mukhtar (2008) observed that soil treated with alkaline admixtures showed brittle behaviour during shear loading which affected the stability of runways, rail, and road embankments. Alternative sustainable approaches are therefore needed to overcome these issues.

This study has adopted a blend of CW and expansive clay to produce a more resilient and environmentally friendly subgrade material for rail track construction. The role of CW in a composite mixture is to improve the shear strength and suppress the swell potential, so when mixed with expansive soils, CW will alter the particle size distribution and thus enhance the grain interlock and stiffness. Selected blends of soil and CW are characterised as they undertake basic geotechnical tests to assess the changes in plasticity and the shrinkage and swelling potential of the soil. A series of monotonic and cyclic triaxial tests also took place to examine the blended matrix under dynamic loading conditions.