What sabkha is
Sabkha forms where saline groundwater lies close to the surface in a hot, dry climate. Evaporation leaves salts in the upper soil, often with a thin crust. Below it, layers of loose sand, silt, soft clay and cemented material can alternate over short distances.
Why it is difficult ground
- Low and variable strength: a dry crust can look firm but lose strength when wetted.
- Settlement or collapse when fresh water dissolves the salts that bind the soil.
- Groundwater within a metre or two of the surface, which complicates excavation.
- High chloride and sulfate levels that attack concrete and reinforcement.
- Conditions that change over short distances, so a sparse investigation can miss weak zones.
What the site investigation should establish
- The layer sequence and its variation across the site, using boreholes together with CPT where it can penetrate.
- Groundwater level and its seasonal or tidal variation.
- Salt content of the soil and chemistry of the groundwater.
- Compressibility and behaviour on wetting of the weak layers.
Methods usually considered
| Situation | Methods usually considered |
|---|---|
| Loose sandy layers needing density | Dynamic Compaction, Rapid Impact Compaction or Vibro Compaction |
| Soft silt or clay layers under lighter structures | Stone columns, or rigid inclusions where the soil is too soft |
| Heavy loads or tight settlement limits | Bored or CFA piles to a firmer stratum |
| Roads and working platforms on weak surface soil | Geogrid or geocell reinforced granular layers |
| Excavation below the water table | Dewatering with suitable discharge arrangements |
Which method fits depends on the investigation and the performance required. Combinations are common.
Protecting concrete and steel from salts
Whatever the foundation method, concrete and steel in sabkha need protection from salts. This normally means a suitable cement type and concrete mix, adequate cover and protective measures specified by the designer. Grout and concrete used in ground engineering elements are designed with the same exposure in mind.