By Dr Roohollah Kalatehjari, Senior Lecturer; Dr Samira Kakh, Researcher; and Sheeju Kunnumpuram Siby, Graduate, Auckland University of Technology
Giving a second life to construction and demolition waste for a circular economy in infrastructure.
The surge in urban development and construction activities worldwide during the 1990s led to a substantial increase in the volume of construction and demolition (C&D) waste produced. Traditionally destined for landfill, this waste reached uncontrollable levels. With landfill capacity being finite and landfill management costs escalating, the urgent need to adopt measures for recycling or reuse of waste become more evident.
Aotearoa New Zealand’s ongoing development demands a similar approach. While this growth is vital for economic progress, managing C&D waste presents an environmental challenge; however, this challenge can also be seen as an opportunity to reroute Class 2 C&D waste from landfill to give it a second life. Our team at Auckland University of Technology (AUT) conducted a thorough investigation on how the re-utilisation and recycling of these solid waste materials in soil stabilisation applications can conserve valuable resources, and promote a circular economy in infrastructure development.
weak soils in New Zealand
New Zealand’s complex geological landscape features several problematic and weak soil types, including expansive clays, organic peat soils, liquefaction-prone soils and loose volcanic soils. These problematic soils are prone to instability and erosion, posing challenges to construction projects and infrastructure integrity. Understanding the potential of using C&D waste can improve the engineering properties of these soils while reducing landfill waste.
Rising construction waste: a problem or a solution?
C&D waste is a significant contributor to the overall waste stream in New Zealand, with around 1.7 million tonnes disposed of in landfill annually, based on an investigation conducted by researchers at Massey University. Materials like timber, concrete, bricks, glass, plastics and plasterboard account for most of this waste. In addition, the inefficient handling, transportation and storage of materials contribute to waste accumulation. As suburban developments increase – like the Tāmaki Regeneration initiative, which will replace 2500 homes with 10,500 new ones – C&D waste will continue to rise. On the other hand, natural hazards and climate change are among the constant threats destroying existing infrastructure and houses – like the demolition of many structures in Canterbury due to earthquake – resulting in the unexpected and substantial production of C&D debris. Our investigations show that some streams from this waste have the potential to be utilised in soil stabilisation practices to reduce environmental impact and improve the cost‑effectiveness of infrastructure construction.
Problems with weak soils in New Zealand
Problematic soil profiles in New Zealand often consist of expansive clay soils, loose soils, highly compressible organic soils and loose volcanic soils, all of which pose significant challenges in engineering. Expansive clay soils, commonly found in regions like Auckland and Waikato, experience notable changes in volume due to fluctuations in moisture levels, leading to swelling and shrinkage that may cause building and infrastructure damage. These soils prove especially troublesome during periods of increased rainfall as they swell and apply pressure on structures, causing fractures, and upward movement. This issue will become more pressing as climate change alters the rainfall and humidity patterns in areas such as the northern parts of the North Island in the coming years. Highly weathered volcanic soils, widespread in locations such as the Bay of Plenty and Northland, are known for their limited load-bearing capacities, high compressibility, and vulnerability to erosion.
Organic soils, such as peat, form from partly decomposed remains of plants in wetlands or deep accumulations of forest litter, with the largest areas in the Waikato, on the high-rainfall South Island West Coast, and in some moist mountainous areas where soils are often saturated. These soils are usually associated with high compressibility, low shear strength, high moisture content and low bearing capacity.
Liquefaction is an infamous phenomenon in New Zealand due to the adverse impacts of the Canterbury earthquake sequence. While the manifestation of liquefaction and its severity depend on several factors, the loosely compacted, water-saturated sediments, such as silty, clayey, and sandy soils, are among the key factors. This makes the ground vulnerable to losing its bearing capacity or lateral spreading in case of an earthquake, which has happened multiple times in Canterbury during the past decade, and caused severe damage to infrastructure and construction, among others.
Finally, loose volcanic soils, such as pumiceous soils, can be found in a large area of New Zealand, especially on the North Island along the lower Waikato River valley and in parts of the Bay of Plenty. These soils usually show a low bearing capacity and are prone to erosion when loose, making them potential causes for infrastructure failure. Necessary specialised engineering methods, such as mechanical and chemical soil stabilisation techniques, are commonly employed to ensure the durability and long-term stability of infrastructure and structures when dealing with these soils.
Waste utilisation for soil stabilisation: a closer look
Reports on building practices in New Zealand and conventional designs for residential buildings indicate that concrete is the predominant material utilised in the construction industry, with timber, plasterboard and metals following suit. The material consumption in the construction sector for the year 2021 was approximately 2.07 million tonnes. Estimated waste production in residential construction in 2021 amounted to 347,000 tonnes, with a projected 267,000 tonnes destined for landfill, 33,000 tonnes for potential reuse, and 46,000 tonnes for recycling.
This waste was from various sources, including the building envelope (147,000 tonnes), other construction activities (49,000 tonnes), residential building alterations (25,000 tonnes) and the demolition of residential structures (126,000 tonnes). It should be mentioned that these figures are only an estimation of the waste produced by the residential construction sector and do not represent the total mass of the waste in each category produced in New Zealand.
Our research team provides a broad picture of how some of these waste materials could be, and have been, reused in soil stabilisation practice, from a review of the experimental and case studies conducted around the world. This could provide a roadmap for further investigation and feasibility studies for carrying out similar approaches in New Zealand. Construction and demolition waste materials available in New Zealand present diverse properties that contribute to soil stabilisation. Identification and harnessing the efficacy of those properties can improve the effectiveness of soil stabilisation methods, therefore mitigating environmental impacts, reducing waste disposal, and promoting a circular economy model for sustainable development.
One of the crucial properties represented by construction waste materials is particle size distribution. This factor controls the binding of the waste with the soil particles and its proper distribution within the soil mass, helping to enhance the strength, durability, and resilience of the soil. Construction waste materials like concrete demonstrate characteristics of being crushed to fine particles and dust that may adhere to the soil, thereby enhancing binding with the soil. Research conducted at the University of Saskatchewan indicated that the quantity of dust produced during the crushing operation and the leachate treatment process do not pose a threat to the ecosystem. Construction waste materials like concrete and brick waste can also be easily modified to achieve versatile particle size distributions, as desired, for different applications.
Like concrete and bricks, recycled glass waste also exhibits excellent particle size distribution properties as it can be finely ground into various sizes ranging from coarse aggregate to micron-sized particles. This diverse particle size promotes soil stabilisation solutions, with smaller particles improving soil compaction or contributing to chemical reactions, and larger particles enhancing structural stability. Researchers at CJS Technology Inc., Canada, illustrate that the crushing or grinding process of glass controls the particle size distribution and shape, and is therefore key in achieving desirable soil stabilisation outcomes. Research at RMIT University, in Melbourne, Australia, shows that concrete waste also contains cementitious materials that can react with soil to form stable compounds, improving stability and long‑term performance.
Another key property demonstrated by construction waste materials is their pozzolanic activity, as it facilitates chemical reactions when exposed to moisture, forming cementitious compounds. This helps increase the soil strength, improve stability and reduce permeability. Research conducted at the Université de Toulouse demonstrated that the pozzolanic activity of waste glass in mortar was effective and could be increased by further grinding.
Equally important, chemical stability is another key property that a stabilising agent should deliver in soil stabilisation methods. The chemical stability of the stabilised soil is an important factor for improved stability, strength, and long-term performance in diverse environmental and climatic conditions. Many construction and demolition wastes in New Zealand are organic or carbon-based. Other construction waste materials like concrete, mortar, and plaster are primary examples of calcium-based construction waste. Studies to determine the chemical stability of stabilised soil by calcium-based stabiliser materials were carried out using X-ray fluorescence, X-ray diffraction, electron microscopy, and thermogravimetric analysis, and the results show a promising increase in the chemical stability of expansive soils treated with these materials.
A recent review of the literature confirms that construction wastes like glass powder and plastics have excellent chemical inertness, making them suitable for use in various weather conditions due to their resistance to chemical and physical actions. Another key property of construction waste materials for soil stabilisation is the durability of the stabilising agent and the stabilised soil. Better durability allows greater resistance to environmental factors such as weathering, corrosion, erosion, and traffic loads over an extended period. It also helps in maintaining the structural integrity of roads and infrastructure developed on the stabilised soil, reducing maintenance costs, and extending their lifespan.
Researchers at Xi’an Jiaotong University discovered that construction materials like fireplace bricks and ceramics have excellent erosion resistance and mechanical properties. Fireplace bricks have excellent corrosion resistance, as they are exposed to high temperatures during their manufacturing process. Ceramic materials exhibit higher erosion resistance compared to various metals.
On the other hand, construction wastes like sawdust have a rough texture that can generate high friction between the fibre and soil matrix, leading to an increase in the shear strength of the soil. This has been confirmed by research conducted at Tecnologico Nacional de Mexico. Sawdust fibres distributed randomly within the soil can create a reinforcing effect, similar to the natural phenomenon of grass and plants stabilising slopes, helping to resist sliding and deformation, and enhancing durability. Carbon sequestration is the process of capturing and storing carbon dioxide from the atmosphere to mitigate its accumulation and reduce its impact on climate change. Construction waste materials like timber waste promote carbon sequestration, as wood chips and sawdust contain carbon captured from the atmosphere during the tree’s growth.
Finally, stabilisers with flocculation properties help create a more stable soil structure, enhancing load‑bearing capacity and water resistance. Flocculation, the process where particles clump together, can improve soil stabilisation by binding soil particles together and increasing soil strength. This key property also helps in reducing soil erosion. Researchers at Kagoshima and Saga universities, Japan, demonstrated that the addition of gypsum to clay soil impacts its pH and consolidation rate, which suggests potential for soil stabilisation.
All the above-mentioned key properties are present in most of the construction waste materials in New Zealand, like concrete, glass, ceramics, bricks, and timber, which offer diverse properties and applications for soil stabilisation. By harnessing the inherent properties of these materials through research and innovation, more sustainable soil stabilisation practices can be developed and employed to mitigate environmental impacts, reduce waste generation in the construction sector, and promote a circular economy model for sustainable infrastructure development.
The reuse of construction waste in soil stabilisation offers multiple advantages. Diverting waste from landfills conserves space and reduces disposal costs, introduces stabilisers with lower carbon emissions compared with the production of traditional materials like cement, reduces the demand for natural aggregates, offers cost savings, and promotes sustainable construction practices. The innovative use of waste in infrastructure projects could also lead to new markets and industries focused on recycling and material repurposing.
Despite the potential benefits, there are challenges in implementing construction waste–based soil stabilisation to be addressed through research. These include the variability in both chemical and physical material properties, regulatory and standards compliance for construction and environmental protection to be met when repurposing waste materials, and assessing the economic feasibility of the solutions for the cost‑effectiveness of recycling and reusing waste for soil stabilisation for each project.
The utilisation of construction waste for soil stabilisation presents a promising solution to address both waste management and infrastructure development challenges in New Zealand. By exploring innovative ways to repurpose materials like glass, gypsum, sawdust, ceramics, concrete, and plastics, the construction industry can move towards more sustainable and cost-effective practices, ultimately contributing to the enhancement of soil stability and the longevity of infrastructure. //
About the authors
Dr Roohollah Kalatehjari is a Senior Lecturer at Auckland University of Technology specialising in sustainable geotechnical and mining engineering. His research focuses on innovative soil stabilisation methods that enhance infrastructure safety and reliability while minimising environmental impact. By exploring the use of natural fibres and recycled materials – including construction and demolition waste – as sustainable alternatives to traditional techniques, he contributes to creating more resilient and eco-friendly geotechnical practices.
Dr Samira Kakh is a sustainability phenomenologist and qualitative research professional.
Sheeju Kunnumpuram Siby is a Master of Engineering graduate from Auckland University of Technology.