Approximately one-third of the world’s land surface falls within arid climate zones, where sabkha soils are a prominent feature. With their low shear strength and susceptibility to voids and subsidence, these soils present significant challenges for engineers. Mohamed Wehbi, technical director at geotechnical specialist Geobear, emphasizes the importance of careful consideration and planning when constructing in regions with sabkha soil and explores the stabilization methods necessary to address these challenges.
Sabkha soils pose significant construction challenges. Despite these issues, they are common in rapidly urbanizing regions like the Arabian Gulf, necessitating solutions to mitigate subsidence and ensure safe, durable land use.
Understanding Sabkha Soil
Derived from the Arabic word for “salt flat,” sabkha refers to flat, low-lying areas common in arid regions.
Sabkha soils are characterized by their loose, compressible nature, limited bearing capacity, and extremely high salt content, with brine salinity levels up to six times higher than seawater.
Sabkha soil’s loose structure allows high water infiltration, enabling large volumes to permeate. When wet, the dissolution of salt-binding agents weakens the soil, causing it to collapse. Upon drying, salt crystallization and recrystallization can result in heaving, further destabilizing the ground.
The fragility of the soil structure, combined with the hydration-dehydration cycle, amplifies instability, making specialized engineering solutions essential for construction.
Sabkha Soils in the Arabian Coast
Sabkha soils are widespread along the Arabian Gulf coast, a region housing significant infrastructure such as petrochemical plants, residential complexes, and recreation centers.
Studies reveal that sabkha soil’s collapse potential increases with greater applied loads and higher compaction fluid content. With multi-billion-dollar investments in petrochemical and residential projects, the need for effective soil stabilization methods has never been greater.
Stabilization Methods
Given the prevalence of sabkha soils and the impracticality of avoiding construction on them, soil stabilization is a crucial consideration. Various methods have been explored.
Mechanical stabilization methods include preloading, stone columns, and dynamic compaction.
Preloading is effective for shallow layers but is time-intensive and prone to uncertain settlement. Stone columns, though useful for denser soils, often encounter challenges in very soft sabkha conditions and can lead to differential settlement. Dynamic compaction, on the other hand, can improve deeper soil layers but generates significant noise and vibrations and risks soil liquefaction in saturated conditions.
Chemical stabilization involves the use of traditional binders such as ordinary Portland cement (OPC), lime, or cement kiln dust.
These methods are effective in improving soil stability, but OPC production, in particular, has significant environmental drawbacks, contributing 8% of global CO2 emissions from industrial and energy-related sources. This raises serious concerns, particularly in the current global push for sustainability.
Each of these solutions requires an in-depth knowledge of soil composition. This is critical because it influences the soil’s strength, stability, and response to environmental factors. These, in turn, affect the choice of stabilization method, project cost, sustainability, and the long-term safety of the future infrastructure.
Geopolymer injection offers a more sustainable alternative to these methods. It resists hydration-dehydration cycles and has been shown to maintain stability with minimal weight loss and volume change, even after repeated wetting and drying cycles. While analysis of the soil composition is also required here, it is more flexible and effective in a wider range of soil types than traditional methods.
Real-World Application: Sama Salt Factory
The advantages of geopolymer solutions are vividly demonstrated in the case of the Sama Salt Factory in Abu Dhabi. This facility, which currently produces over 250,000 tons of salt annually and plans to expand to over two million tons, faced significant operational challenges due to sabkha soil.
Settlement issues caused some areas to sink by as much as 100 mm, resulting in uneven concrete floors that disrupted machinery operations. Traditional mechanical or chemical stabilization methods were unsuitable due to the factory’s need to remain fully operational and avoid downtime.
Instead, Geobear implemented its geopolymer solution, completing the project in just seven days while the factory continued running without interruption. The process involves injecting the resin-based material into specific points, where it fills voids, strengthens the soil, and binds it together.
At the Sama Salt Factory, Geobear deployed 65 injection points at a depth of 16 mm, using 1,405 kg of geopolymer resin. While the primary goal was ground improvement and stabilization to prevent further subsidence, Sama Salt also requested a lift of the already subsiding areas.
This presented a significant challenge due to poor soil conditions and a high water table, which further compromised stability.
However, in addition to achieving ground stabilization, Geobear successfully implemented the lift. The rapid stabilization restored operational efficiency, enhanced the long-term stability of the soil, and restored the factory to satisfactory municipal standards within six weeks of initial contact.
While building on sabkha soils is often unavoidable in certain regions, advanced approaches like geopolymer technology offer a strategic way to mitigate associated risks.
As urbanization progresses in areas with difficult soil conditions, adopting more environmentally friendly techniques will play a crucial role in ensuring stable and resilient infrastructure.