Geotechnical Characterisation of Subgrade and Sub-Base Materials for Road Pavement Design in Kasese District, Western Uganda

Roads are more than just strips of asphalt; they are complex structures built upon a living, breathing foundation of earth. In the world of civil engineering, the ground beneath a road is known as the subgrade, and the layer of material placed directly on top of it is called the sub-base. These layers act as the bedrock of the entire pavement system, distributing the heavy weight of vehicles so that the surface does not crack or sink. However, the earth is not uniform. In some places, the soil is dry and strong, while in others, it is wet, soft, and prone to swelling when it gets soaked. If engineers build a road without understanding the specific behavior of the local soil, the road will fail prematurely, turning into a bumpy, dangerous path that requires constant, expensive repairs. This is particularly critical in regions with heavy rainfall and seasonal flooding, where water can turn solid ground into a weak, unstable slurry.

In the Kasese District of Western Uganda, a team of researchers set out to understand exactly what lies beneath the roads that connect this vibrant region. Kasese is a hub of activity, drawing tourists to the nearby Rwenzori Mountains and Queen Elizabeth National Park, while also serving as a vital corridor for mining and cross-border trade with the Democratic Republic of Congo. Despite its importance, the roads here frequently suffer from early failure. The district experiences significant rainfall and is prone to flooding from rivers like the Nyamwamba and Mubuku, which often inundate the road foundations. To solve this, the researchers needed to move beyond guesswork and general assumptions. They conducted a detailed investigation to map the soil profile and measure its strength, aiming to provide a clear, science-based guide for designing roads that can withstand the local environment.

The team began their work by digging thirty test holes across the district, reaching depths of two meters to see what materials lay beneath the surface. They collected soil samples at regular intervals and brought them to a laboratory to run a series of tests. These tests measured how much water the soil held, how heavy it was, and how it behaved when squeezed or soaked. They also looked at the size of the soil particles, distinguishing between fine clay, silt, sand, and gravel. This process allowed them to build a clear picture of the ground's composition. They found that the top layer, extending from the surface down to about 0.45 meters, consisted of dark brown organic clay. This layer is unsuitable for building roads because it is too soft and unstable. Below that, from 0.45 meters to 1.2 meters, they encountered a layer of silty clay with low to medium plasticity. Finally, from 1.2 meters down to 2.0 meters, the soil changed to a poorly graded sand mixed with gravel, which drains water well but lacks the stickiness needed to hold the road together.

The analysis of these materials revealed some concerning details about the subgrade, the layer that directly supports the road. The soil in this area is predominantly silty clay, and it holds a surprisingly high amount of natural moisture, averaging 28.1 percent. For stable road construction, engineers typically prefer moisture levels between 5 and 15 percent. Because the soil is so wet, it is weaker than ideal. The researchers classified the soil using standard engineering systems and found that 62 percent of the samples were rated as poor to fair, meaning they would struggle to support heavy traffic without help. Only 38 percent were rated as good to excellent. A key factor in this weakness is the presence of expansive clays. These are soils that swell significantly when they absorb water and shrink when they dry out, causing the ground to crack and heave. The study found that the soil's ability to swell was quite high, with some samples showing a potential for expansion that could easily damage a road built directly on top of it.

To measure how strong the ground actually is, the team performed a test known as the California Bearing Ratio, which simulates how much pressure the soil can take before failing. They tested the soil both in its natural state and after soaking it for 48 hours to mimic flood conditions. The results showed that the soaked soil was quite weak, with strength values ranging from 5 percent to 32 percent, averaging just 8.6 percent. This is well below the threshold usually required for unpaved roads, indicating that without intervention, the road would likely sink or rut under the weight of vehicles. The researchers also calculated the stiffness of the ground, finding that it was relatively soft, which further confirms the need for a robust design.

However, the story is not entirely one of weakness. The team also looked for materials that could be used to build the sub-base, the layer placed above the weak subgrade to spread the load. They examined soil from approved borrow pits in areas like Kilembe and Hima. These materials turned out to be much better suited for construction. They were composed mostly of sand with some gravel and a moderate amount of fines, classifying them as excellent to good for road building. When tested, these materials showed a soaked strength of 18 to 42 percent, which meets the minimum requirements for a sub-base. Yet, even these good materials had a vulnerability: when soaked, they lost between 12 and 58 percent of their strength. This highlights a crucial point: even the best local materials will fail if water is allowed to sit in them for too long.

Based on these findings, the researchers proposed a specific design for future roads in the district. They recommended a total pavement thickness of between 350 and 400 millimeters. This structure would include a thin layer of asphalt on top, followed by a thick layer of crushed stone, and then a granular sub-base made from the better local materials. Crucially, they advised removing the top 0.45 meters of organic soil entirely and replacing it with a selected fill layer to ensure the road sits on a stable foundation. To handle the high rainfall and flood risk, the design emphasizes drainage. The sub-base should be made of coarse, granular materials that allow water to flow through them quickly rather than getting trapped. The researchers also suggested installing edge drains and cross culverts at regular intervals to keep water away from the road foundation. For the weakest areas where the soil is too soft or too expansive, they recommended treating the ground with lime or cement to strengthen it and reduce its tendency to swell.

The study concludes that the roads in Kasese District have failed in the past not because of a lack of effort, but because of a lack of specific knowledge about the local soil. By characterizing the ground in detail, the researchers have provided a clear path forward. The data shows that while the natural ground is often too weak and wet to support a road directly, it can be managed with the right combination of thickness, drainage, and material selection. The recommended approach of using a thicker pavement with coarse granular layers and enhanced drainage systems offers a practical solution to the region's flooding challenges. This work ensures that future roads will be built on a foundation that understands the ground it stands on, leading to safer, longer-lasting infrastructure for the people of Western Uganda.