How poor sub-soil drainage can lead to structural damage.
Poor sub-soil drainage is a leading cause of progressive structural failure. Sub-soil drainage – the removal of excess water from the soil beneath structures – is an often-overlooked but critical factor in maintaining the stability of buildings, roads and other infrastructure. This press release explores how inadequate sub-soil water management can contribute to damage, and includes insights from peer-reviewed research and real-world case studies.
Durban’s geotechnical and climatic vulnerabilities
Durban’s subtropical climate is characterised by high annual rainfall (over 1 000mm/year) concentrated in the summer months. Its undulating topography, expansive clay soils (notably the Berea Red Sands) and rapid urbanisation further exacerbate drainage challenges. These factors create a natural predisposition for water accumulation around and beneath structures, particularly in low-lying or poorly graded areas.
According to Brink and Bruynzeel (2018) in the South African Journal of Geotechnical Engineering, the high plasticity of Durban’s clay-rich soils means they are especially sensitive to moisture fluctuations. When wet, these soils swell; when dry, they shrink – leading to differential settlement under structures. Over time, this can cause cracks, slab deformation and even foundational failure.
Mechanisms of structural damage from poor sub-soil drainage
Hydrostatic pressure on foundations and retaining walls
When sub-soil water is not drained effectively, hydrostatic pressure builds against subsurface structures. In Durban’s Glenwood area, several residential retaining walls have failed following sustained rainfall, where poor drainage led to lateral water buildup. This pressure can displace retaining walls, push basement walls inward and cause foundational uplift – especially in structures without weep holes or sub-soil drainage mats.
Research insight: A 2019 paper in the Journal of Performance of Constructed Facilities found that 42% of retaining wall failures in wet climates are due to inadequate drainage systems.
Capillary rise and rising damp
In areas like Berea and Musgrave, where older homes were built with shallow foundations, poor sub-soil drainage has led to widespread rising damp. This occurs when groundwater moves upward through the masonry via capillary action. Without damp-proof membranes or adequate drainage, internal wall surfaces become saturated, leading to blistering paint, mould growth and timber decay.
This is particularly problematic in historical Durban properties, where retrofitting for damp-proofing is complicated by preservation requirements.
Soil erosion and piping
In high-slope developments in areas such as Kloof and Westville, water that flows uncontrolled beneath pavements or retaining structures can initiate soil piping – an internal erosion mechanism where fine particles are carried away by flowing water. Eventually this can lead to sinkholes, cracking and the collapse of surface structures.
Expansive soil movement
Expansive clays found in parts of Umlazi and Ntuzuma expand and contract with moisture content. In dry conditions, these soils shrink, causing structures to settle unevenly. In wet conditions, swelling puts upward pressure on foundations. Without sub-soil drainage systems such as perimeter drains, moisture control becomes nearly impossible.
Study reference: Van der Merwe and Weeks (2015) demonstrated that in KwaZulu-Natal, 60% of foundation problems in low-cost housing were linked to expansive soil movement, exacerbated by poor site drainage.
Real-world example: Durban North apartment collapse (2021)
In March 2021, an apartment block in Durban North experienced partial structural failure after several weeks of heavy rain. Post-incident investigations by the eThekwini Municipality revealed that sub-soil water had accumulated behind a retaining wall with no weep holes or French drain system. The hydrostatic pressure eventually caused wall failure, which undermined the building’s foundation.
This highlighted a broader issue in eThekwini: the lack of enforced regulations requiring engineered sub-soil drainage systems in older developments.
Preventative engineering solutions
1. Sub-soil drainage systems
These include:
- Perforated PVC piping systems (French drains)
- Geotextile-wrapped drainage aggregates
- Vertical drainage composite sheets
- Subsurface soakaways or attenuation tanks
In Durban, municipal infrastructure upgrades have begun to include such systems under new roads and retaining structures to extend service life and prevent washouts.
2. Site-specific geotechnical investigation
A pre-construction geotechnical survey can identify drainage risks. For instance, areas with high clay content or a shallow water table can be engineered using reinforced foundations, controlled fill and proper drainage.
3. Regular maintenance of existing systems
Blocked drainage pipes, clogged weep holes or poorly sloped landscaping can render even well-designed systems ineffective. In several cases across Hillcrest, scheduled maintenance of stormwater systems has prevented structural water ingress in high-rainfall months.
South African legal framework and guidelines
In South Africa, SANS 10400 Part H (Foundations) and Part R (Stormwater Disposal) regulate aspects of drainage. Compliance is often inconsistent, especially in informal or older developments. Durban’s building inspectors have noted that poor drainage detailing is one of the top five compliance failures during foundation inspections.
The National Home Builders Registration Council (NHBRC) requires that new dwellings incorporate effective drainage systems and that geotechnical conditions be considered in the foundation design. Non-compliance could void insurance claims in case of structural failure.
Conclusion
Poor sub-soil drainage is not just a nuisance – it is a serious structural hazard. In water-rich regions like Durban, where rainfall and clay soil predominate, inadequate drainage is directly linked to costly failures. Engineers, architects and developers must prioritise site-specific water management strategies to prevent structural damage.
Through proper geotechnical assessment, installation of robust drainage systems and adherence to legal frameworks, many of these failures can be avoided. As Durban continues to urbanise, sustainable drainage planning must form the cornerstone of resilient infrastructure design.