A Hybrid Geophysical–Geotechnical Approach for Landslide Stability Assessment

In the highlands of Ethiopia, where the earth rises steeply and the roads cut through the mountains, the ground is not always as solid as it appears. For engineers and geologists, the stability of a hillside is a delicate balance between two opposing forces: the weight of the soil and rock pulling downward, and the internal strength of that same material holding it together. This strength is not a fixed number; it changes with the weather. When heavy rains soak the ground, water fills the tiny spaces between soil particles, pushing them apart and making the earth heavier and weaker. This is the fundamental challenge of landslide science: understanding how water, soil type, and the shape of a slope interact to either hold firm or give way. When a road is built into a hill, it often removes the natural support at the bottom of the slope, and if the soil is already weak or the water table is high, the entire hillside can slide, blocking traffic and threatening lives.

A team of researchers set out to solve a persistent problem on a stretch of road in Bonga Town, in southwestern Ethiopia. This section of the highway, which connects Addis Ababa to the town of Mizan, has suffered from repeated landslides, particularly during the rainy season from June to September. Despite previous attempts to fix the road using rock fill, the ground continued to move. The team, led by engineers from Bahir Dar University and collaborators from India and Poland, decided to look beneath the surface to understand exactly what was happening. They combined three different ways of looking at the earth: they drilled deep holes to pull up soil samples, they used sound waves and electrical currents to map the ground without digging, and they built a digital model of the slope to test how it would behave under stress. Their goal was not just to see why the road failed, but to find a solution that would keep it safe for the future.

The investigation began by listening to the earth and feeling its electrical resistance. The researchers used a technique called seismic refraction, which involves sending sound waves through the ground to measure how fast they travel. Faster waves usually mean harder rock, while slower waves indicate soft soil or fractured rock. They also used electrical resistivity tomography, which measures how easily electricity flows through the ground. Wet soil and clay conduct electricity much better than dry rock, so this method helped them locate pockets of water and weak, saturated layers. These non-invasive tests were combined with data from four deep boreholes drilled along the road. From these holes, they extracted soil samples to test in a laboratory, measuring how much the soil weighed, how much water it held, and how much force was needed to make it slide.

The results painted a clear picture of a vulnerable landscape. The ground beneath the road is a complex mix of layers. At the top, there is a layer of imported fill material used to build the road, sitting on top of stiff, reddish-brown silt. Below that lies a layer of clay mixed with fragments of trachyte rock, and at the bottom, a layer of weathered basalt. The critical finding was that the groundwater table sits very close to the surface, often just one meter below the ground. During the rainy season, this water level rises, soaking the soil and turning the upper layers into a heavy, slippery mass. The laboratory tests confirmed that the soil has a high natural moisture content, especially during the summer rains, which significantly reduces its ability to hold together.

To see how these conditions affected the road's safety, the researchers built a computer simulation of the slope. They fed the real-world data—the soil weights, the friction angles, and the water levels—into specialized software to calculate a "factor of safety." This number is a simple ratio that tells engineers how stable a slope is. A number of 1.5 or higher is generally considered safe for a road, meaning the forces holding the hill up are one and a half times stronger than the forces trying to pull it down. The simulation showed that under the current wet conditions, the factor of safety for this road was only 1.347. This is below the required safety threshold, confirming that the slope is unstable and prone to sliding. The model also showed that the most likely place for a slide to start is about 5.5 meters below the road surface, cutting through the weak, wet soil layers.

The researchers then tested different ways to fix the problem within their computer model. They looked at the idea of building a retaining wall, a structure designed to hold back the earth. The simulation showed that adding a wall would significantly increase the factor of safety, providing the extra resistance needed to stop the soil from sliding. They also explored the use of geosynthetic materials, such as geocells and geotextiles. These are mesh-like fabrics that can be placed within the soil to reinforce it, acting like a net that holds the particles together and prevents them from separating. The study suggests that a combination of these measures, along with better drainage systems to lower the water table, offers the best path forward. By removing the excess water and reinforcing the soil, the road could be stabilized without needing to rebuild the entire hillside.

This study highlights that the recurring landslides on the Bonga-Mizan road are not just a result of the steep terrain, but a specific interaction between the soil's properties and the seasonal rains. The research rules out the idea that the current rock fill repairs are sufficient, as the movement continues beneath them. Instead, the evidence points to the need for a more comprehensive approach that addresses the water and the soil strength directly. While the study focused on a specific section of the road, the methods used provide a blueprint for understanding similar problems in other mountainous regions of Ethiopia. The authors acknowledge that time and budget constraints meant they could not investigate the entire 100-kilometer stretch of the highway, but their findings offer a clear, evidence-based guide for engineers to rehabilitate this critical piece of infrastructure and ensure it remains open for travel, even when the rains come.