Retaining walls are designed to protect homes and roads from unstable slopes. However, when poorly designed or constructed without proper soil testing and drainage, they can become a major hazard, especially during the rainy season, engineers warn.

Figures from the Ministry in charge of Emergency Management (MINEMA) show that March to May rainy season we are approaching accounts for 52.2 per cent of the 948 hydrometeorological disaster-related deaths recorded since 2020, as well as 64.5 per cent of the 27,462 houses damaged or destroyed over the same period.

Landslides, many of which could be prevented through proper slope protection, are responsible for the largest share of fatalities compared to the other disasters, with more than 200 deaths out of the 498 recorded in the period. They have also caused the largest number of house damage and destruction with about 5,000.

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With heavy rains expected to continue through the rest of February and into the March-to-May rainy season, The New Times spoke with Jean Pierre Niyibizi, a PhD researcher in geotechnical engineering and a member of the Institute of Engineering Rwanda, to explain what goes into constructing safe retaining walls.

Niyibizi, who owns a geotechnical laboratory in Kamonyi District, said effective retaining wall design begins with understanding the soil.

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"The same retaining wall with the same dimensions will not behave the same way in different areas because soil properties vary,” he said. "For example, a three-metre-high retaining wall in the Northern Province will not behave the same as one of the same height in the Eastern Province.”

He identified two critical soil parameters—the internal angle of friction and soil cohesion, which are determined through geotechnical tests such as direct shear or triaxial tests.

Engineers must also assess the bearing capacity of the soil using penetration tests to ensure the ground can support the wall’s weight without excessive settlement. Shear tests further help evaluate lateral earth pressure, the force exerted by soil that can push a retaining wall backward.

"The type of soil directly affects the size and design of the wall,” Niyibizi said. "Lateritic or gravelly soils, which have higher internal friction, may require smaller walls. Loose soils such as sandy or silty soils are more unstable and need wider structures.”

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Retaining walls can be constructed using reinforced concrete, masonry, or gabions—steel cages filled with stones. Other slope-stabilisation methods include soil nailing and geogrids.

Niyibizi noted that reinforced concrete walls are often preferred in constrained spaces because they provide stability without requiring a large footprint, while masonry walls rely on mass for stability.

Once soil properties are established through geotechnical investigations, engineers can design retaining walls using specialised software or manual calculations. Designs must be checked for sliding, overturning, and settlement to ensure long-term stability.

However, even a well-designed wall can fail if drainage is neglected.

"The weight of water significantly increases pressure on a retaining wall,” Niyibizi said. "Without proper drainage—such as weep holes or drainage pipes—the wall can collapse. These systems are inexpensive but essential.”

He added that all soil types require drainage, although pipe spacing depends on whether the soil drains water naturally or retains it.

Niyibizi also pointed to common construction flaws. Some walls are built without any design or geotechnical investigation, based on the assumption that similar structures nearby are safe.

Others fail because soil properties are wrongly estimated, resulting in under-designed walls that crack, settle, or slide. Poor workmanship, substandard materials, and improper backfilling also contribute to failures.

"Proper design, soil investigation, and drainage are non-negotiable,” he said. "Even a strong wall can fail if these basics are ignored.”

President of the Institute of Engineering Rwanda (IER) Gentil Kangaho, urged the public to rely on certified professionals when undertaking construction works.

"Whenever you are constructing, use certified engineers,” he said, noting that IER currently has more than 4,000 registered engineers, with over 100 new members joining each year. Among them are geotechnical engineers who assess and monitor land conditions to ensure that buildings are properly designed and constructed.

Kangaho warned that hiring uncertified practitioners makes it difficult to enforce accountability when errors occur.

"Some people opt for non-certified engineers because they are cheaper, but stressed that such decisions put lives at risk and undermine the durability of structures.”

He further emphasised the importance of complying with government procedures. "Any planned construction should be reported to the relevant authorities so that approval is granted and inspections are carried out,” he said.

Emma-Claudine Ntireganya, the Director General in charge of Communication and Community Education at the City of Kigali, said poorly constructed retaining walls are often linked to illegal housing developments.

She explained that builders who bypass permit processes tend to rush construction and ignore technical standards.

"When construction is illegal, it is often rushed, without standard equipment or allowing materials such as cement to cure properly,” she said. "Anyone planning to build should first obtain the required permits and work with certified engineers to ensure construction meets approved standards.”

Ntireganya added that during the rainy season, MINEMA and local government authorities regularly issue public safety advisories.

"We are increasing awareness among the population,” she said.

She urged residents—particularly those in high-risk areas to heed official warnings.

"People should listen to advisories, relocate when necessary, and repair their homes before they become dangerous,” she said. "They should not wait for official classification when visible warning signs, such as cracks, are already present.”