1. Why Below-Ground Structures on Sand Still Leak
One of the most common and costly misconceptions in construction is the belief that below-ground structures built on sandy soil do not require robust waterproofing because “sand drains water away.”
At Baleh Consulting, we have designed waterproofing and drainage systems for numerous new and remedial projects built on sandy ground conditions. Almost every time, the same comment comes back:
“There is no water problem here — it’s all sand.”
While sandy soils are generally more permeable than clay, this does not mean the structure will remain dry over its lifespan.
2. The Overlooked Problem: Siltation & Clogging
Water moving through sandy soils often carries fine particles such as silt and clay. Over time, these fines can accumulate within the soil and drainage systems through Clogging, Siltation, and Blinding
This process can progressively reduce permeability and cause water build-up around the structure, increasing hydrostatic pressure over time.
What appears “free-draining” today may not remain that way decades later. Therefore, designers ought to design to address the risk for decades later.
3. Clogging, Bridging & Settling
3.1 Clogging => When inside (b)
Drainage pipes, aggregates, and geofabrics can become blocked over time by sediment and debris.
3.2 Bridging => Blinding if on top (a)
Fine particles and poorly graded materials can create localised blockages that trap water against the structure.
3.3 Settling => Siltation
Sandy backfill can settle due to water migration, poor compaction, or washout of fines, potentially damaging drainage systems and stressing waterproofing systems.
(b) Inner/subsurface clogging, in the subsurface of the water stream
(c) Unimpeded static percolation, deeper into the water stream
4. Designing for the Future — Not Just Today
Designs must not be prepared based only on the conditions observed during excavation or at the time of construction. Sandy soils can easily mislead waterproofing designers, builders, structural engineers, and developers when attempting to control water ingress for the design life of the structure.
Designers must assess:
- Long-term groundwater risks;
- Future changes in site conditions;
- Drainage deterioration over time;
- Structural movement and cracking;
- Durability and accessibility for future repairs.
Because successful waterproofing design is not about predicting today’s weather, it is about anticipating what may occur decades later.
5. Final Thought
Sandy soil is not a waterproofing strategy.
Ground conditions evolve, drainage systems age, and water always finds the weakest path. Proper below-ground waterproofing should be based on long-term performance, risk mitigation, and durability, not assumptions made during excavation.
It is also imperative to design a multi-layered system with Plan A, Plan B, and possibly Plan C in place, as basements and below-ground structures offer one shot due to inaccessibility once concrete is poured and the perimeters are back-filled.
If you are building near the ocean, rivers, estuaries, or alluvial areas, it is highly likely that the structure will be founded on sandy soils. However, if the geotechnical report identifies additional layers such as silt, clay, silty clay, or clayey silt, the risk becomes significantly greater. These lower-permeability layers can trap and retain water, leading to the formation of perched or false groundwater tables, sometimes occurring sporadically due to rainfall, tidal influence, or seasonal groundwater fluctuations.
Baleh Consulting are specialised in Basement and Below-ground waterproofing and remedial. If you require assistance anywhere in Australia, contact us.
Prepared by Hacène Baleh
View Baleh Consulting page on LinkedIn
28 May 2026
