Geotechnical Assessment of Slope Instability Triggered by Solid Waste Dumping in Engineered Landfills and Informal Waste Dumps

Imagine a hillside not as a natural formation of rock and soil, but as a massive, man-made pile of trash. To the casual observer, a garbage dump is an environmental nuisance or a public health hazard, a place where waste goes to rot. But to a geotechnical engineer, it is a physical structure with a slope, a foundation, and a specific way of failing, just like a natural hill or a man-made dam. The material making up this structure is municipal solid waste, a chaotic mix of organic matter, plastics, and debris that changes its strength and weight as it decomposes over time. Unlike the soil beneath it, this waste does not behave predictably; it can trap water, generate gas, and weaken as it settles. When this material is piled too high, too steeply, or without proper drainage, the forces holding it together can be overwhelmed, leading to a catastrophic slide. Understanding why these piles collapse is not just about cleaning up messes; it is about preventing the loss of life and property in rapidly growing cities where formal waste management often cannot keep pace with population growth.

A team of researchers from the University of Engineering & Technology Taxila has set out to understand exactly how and why these waste piles fail. They approached the problem by looking at solid waste not as a sanitation issue, but as a geotechnical one—a question of how a specific type of material holds up under its own weight and the pressure of water. The researchers conducted a systematic review, gathering and analyzing documented cases of slope failures from around the world. They examined five major disasters: the Doña Juana landfill in Colombia, the Payatas dump in the Philippines, the Shenzhen construction waste dump in China, the Meethotamulla open dump in Sri Lanka, and the Koshe landfill in Ethiopia. These sites ranged from formally engineered facilities with liners and drainage systems to uncontrolled, informal dumps where waste was simply piled onto the ground with no engineering oversight. By comparing these events, the team sought to identify the common triggers that turn a stable pile of trash into a deadly landslide.

The investigation revealed a clear and consistent pattern across all five cases. In every instance, water was the primary culprit. Whether it was rain soaking into an unmanaged heap, leachate—the liquid that drains from decomposing waste—trapped inside a landfill, or a rising groundwater table, the accumulation of fluid was the key factor that destabilized the slopes. Water reduces the friction between particles and increases the internal pressure within the pile, effectively turning a solid mass into something that can flow. In the case of the Shenzhen disaster in 2015, which involved 2.73 million cubic meters of construction waste, the failure was triggered by a combination of rapid filling and a rising water table that the underlying soil could not support. Similarly, at the Meethotamulla dump in Sri Lanka, three days of heavy rain immediately preceded the collapse of a 91-meter-high pile of garbage. Even in the Koshe tragedy in Ethiopia, where bulldozers were working on top of the dump, the researchers noted that decades of unmanaged dumping had likely left the waste mass saturated and weak, making it vulnerable to the added weight of construction equipment.

The study also highlighted a stark difference between engineered landfills and informal dumps, though the outcome can be just as deadly in both. Engineered sites, like the one in Shenzhen, often fail due to specific technical flaws, such as a weak interface between the waste and the plastic liner at the bottom, or a groundwater condition that was not fully accounted for in the design. These are technical problems with technical solutions, such as better testing of materials or more conservative assumptions about water levels. Informal dumps, however, lack any formal design entirely. They grow taller and steeper than safety limits would allow, often without any drainage systems or foundation checks. The researchers found that these unregulated sites frequently host communities living directly on or next to the unstable slopes, a situation driven by poverty and a lack of housing options. The Payatas disaster in the Philippines, which killed over 300 people, and the Koshe collapse, which claimed more than 100 lives, both occurred at such informal sites. The researchers noted that while the engineered failures are well-documented in scientific literature, the informal disasters, despite causing far more fatalities, often lack rigorous technical investigation. For instance, nearly a decade after the Koshe collapse, there is still no published geotechnical back-analysis of the event, leaving a significant gap in our understanding of how these specific types of failures occur.

A critical finding of the paper is that the safety margins for these waste slopes are often dangerously thin. The researchers reviewed reliability-based studies that applied standard engineering codes, originally designed for natural soil, to these waste masses. They found that the calculated safety factors for many of these failed slopes were close to the point of failure even before the disaster struck. This suggests that the current design standards, which assume a material behaves like soil, may systematically underestimate the risk posed by waste. Municipal solid waste is far more variable and time-dependent than soil; it changes as it decomposes, and its strength can drop significantly as it becomes wet. Relying on codes meant for natural earth structures may give a false sense of security, leading engineers and officials to believe a slope is stable when it is actually on the verge of collapse.

The paper concludes that preventing future disasters does not require inventing new science, but rather applying existing geotechnical principles with greater rigor and urgency. The solutions are straightforward: manage the water through proper drainage and leachate collection, control the height and steepness of the piles, and monitor the sites continuously for signs of movement. The researchers argue that waste dumps should be treated as geotechnical structures subject to the same stability reviews as any other large earthwork, regardless of whether they are formally regulated or informal. This shift in perspective is crucial, especially in developing regions where informal dumping is common. The study emphasizes that the gap in safety is not primarily a lack of knowledge, but a lack of implementation. The tools to prevent these tragedies—drainage systems, slope limits, and monitoring equipment—already exist. What is missing is the institutional will and resources to apply them, particularly at the informal sites where the human cost of failure has been the highest. By recognizing the dump as a physical structure subject to the laws of physics, rather than just a pile of trash, communities can take the necessary steps to ensure that these slopes remain stable and do not become the next source of a preventable catastrophe.