Pile foundations have become a common solution when the bearing capacity or performance cannot be satisfactorily guaranteed by using shallow foundations. This often occurs in urban areas where increasingly higher structures apply increasingly higher forces to the ground. Pile foundations are also required where ground conditions are poor and provide little capacity or stiffness to ensure short or long term serviceability requirements.
Ground improvement is an alternative solution for piled foundations in poor soil conditions, whereby the properties of the ground are improved to allow structures to be founded on simple, shallow foundations. Keller is the global leader in ground improvement technology and with Franki Africa being part of the Keller Group, a wide variety of ground improvement technologies are now available for the African geotechnical market.
Vibrocompaction and vibro replacement
Vibrocompaction requires a depth vibrator to be inserted into relatively clean sand to increase the density of the sand thereby increasing its bearing capacity and stiffness. The method has been used extensively in the Middle East and Asia on land reclamation and improvement projects, one of the most well-known being the Palm Islands in Dubai.
Soft silts and clays are not suitable for vibrocompaction, but can be improved by vibro replacement. The method utilizes the same depth vibrator used in vibrocompaction to introduce columns of stone, mortar or concrete to improve the properties of the soil mass. Stone columns are often used as foundations for tanks, warehouse structures, embankments, wind turbines or light foundations by increasing the strength and stiffness, and reducing the consolidation time of the soil mass. Franki Africa has completed vibro replacement stone and concrete columns for tank structures, warehouses and water treatment facilities in South Africa and Africa.
Dynamic compaction has been part of Franki Africa’s product range with numerous projects successfully completed over the last two decades. The method involves dropping a heavy steel pounder to compact and densify the founding soil. Dynamic compaction is a versatile ground improvement method that can be utilized in various soil types, ranging from sands to silts to dumps and rubble deposits, above the water table. Over the last 2-3 years, Franki Africa has successfully completed dynamic compaction for structures such as tanks, wind turbines, warehouses, shopping centres and showrooms.
Rigid inclusion is possibly the newest ground improvement technology which involves relatively slender concrete/grout columns to improve the properties of the soil mass. Rigid inclusions can be utilized to improve soils to depths in excess of 40m with significantly higher productions compared to conventional piling methods. Franki Africa is currently providing rigid inclusions on a design and construct basis for the 350 000m² Clairwood Logistics Park in Durban to improve the underlying hippo muds which extend to depths in excess of 35m.
Grouting and soil mixing
Various grouting methods such as jet grouting, compensation grouting and permeation grouting are part of ground improvement product range available from Franki Africa. These methods are particularly versatile for urban areas where additional support or repair work is required around existing structures.
Soil mixing is a ground improvement method whereby the in situ soil is mixed with cement or slurry to improve the properties of the soil mass. Soil mixing can be carried out in column, panels or mass to suit the project requirements.
The debate: pile foundations or ground improvement?
The answer to whether piled foundations or ground improvement should be used lies in the requirements of the structure. Structures with high concentrated loads require pile foundations to meet the capacity, but more importantly the serviceability requirements.
Ground improvement, on the other hand, is a cost-effective solution for structures requiring control on differential settlements. In addition, ground improvement solutions such as vibrocompaction, vibro replacement and dynamic compaction do not require addition of binders to the soil, thereby reducing the carbon consumption for the project.
The debate therefore should not be focused on whether pile foundations or ground improvement are better solutions, but which solution is cost effective for the requirement of the structure. Often ground improvement can be used in conjunction with piled foundations to provide optimized geotechnical solutions for the project. Franki Africa possesses a wide range of piling and ground improvement products, as well as design capabilities to ensure optimized geotechnical solution for the project.
By Dr Nicol Chang – Technical Manager Franki Africa
Published in the Civil Engineering Magazine – November 2017 edition