The ground under your house: expansive soils, sabkha and the soil investigation report before you build

Many cracks we see in houses don't come from the concrete itself but from the soil beneath it. A foundation can't be designed properly without knowing the soil.

1) Expansive soils

These are clay soils that swell when wetted and shrink when they dry. A regional review published in 2025 shows they cover about 37% of Saudi Arabia (around 800,000 km²), documented in 9 of the 13 provinces, with affected areas including Al-Ghatt, Tayma, Tabuk, Al-Qatif, Al-Hofuf, Madinah and Sharurah1.

The same source reports damage ranging from minor cracking in pavements and building finishes to significant displacement of footings and superstructure elements that is hard to repair, and that some light structures failed within a few years of construction because the soil movement was not anticipated1.

2) Sabkha

Sabkha (salt flat) is one of the most unpredictable soils in hot arid climates, spread along the Red Sea and Arabian Gulf coasts. On wetting it may collapse as its cementing materials dissolve; it is also corrosive, and it may heave through salt crystallisation and recrystallisation2.

Buildings on it therefore need ground treatment or special foundations, and protection of concrete and steel from salts. See rebar corrosion.

3) Soil chemistry: sulphates and chlorides

The problem isn't only mechanical. A study of Makkah found sulphate and chloride contents higher in soils within the top 5 m, to be considered when selecting the cement type for foundation concrete and reinforcement, and recommended continuous evaluation of geotechnical and chemical properties for projects3. See cement types (sulphate-resisting cement).

4) What does a soil report give you?

Typical contents of a geotechnical report; details are set by the engineering office for each project.

• Soil layers from boreholes and the groundwater level.
• Bearing capacity and expected settlement.
• Presence of expansive, collapsible or sabkha soils.
• Chemical analysis for sulphates and chlorides.
• Recommendations on foundation type and depth, cement type and protection.

5) The site itself: water, fill and neighbours

Practical questions for the owner to put to the engineering office; the technical decision is the engineer's, based on the soil report.

• Water: the groundwater level in the report, and the route of rain and irrigation water around the plot. Wetting moves expansive soils and threatens sabkha12. See water around the building.
• Fill: is there old fill or debris on the plot? Uncompacted fill is not built on without treatment specified by the engineer.
• Neighbours: are the neighbour's foundations and walls close to the excavation? Excavating next to them needs a design to support the sides, and photos of the neighbour's wall condition before starting.
• No general figures: there is no excavation depth or footing size that suits every building; that comes from the structural design and soil report.

6) Hold points before pouring

1. The engineer signs off the excavation base (level, no standing water or soft soil).
2. Waterproofing under and around foundations where the design requires it.
3. Sign-off of footing and column reinforcement and concrete cover. See rebar.
4. Concrete class and cement type per the report's recommendation. See cement types and ready-mix concrete.

The full list for every stage: construction quality checks.

7) Advice for owners

1. Don't start digging before the soil report, and don't rely on a neighbouring plot's report.
2. Make sure the structural designer has read the report and applied its recommendations.
3. Control drainage around the building and keep irrigation away from the walls.
4. Fix any leak in the water or sewer network immediately. See leak detection.
5. If diagonal cracks or settlement appear, call an engineer. See cracks.