The new Oil Tanking MOGS Saldanha crude oil storage facility at the deep-water port of Saldanha, situated on the west coast of South Africa, is geographically well located between the Atlantic and Indian Oceans to allow quick access to the markets in America, Europe and Asia via global shipping routes.
Once completed, the tank farm will consist of twelve underground crude oil storage tanks, which allow space for a total of 13.2 million barrels of oil. Each tank contains 192 No. of 56 foot (17 m) high, continuously cast reinforced concrete columns to support the roof slab. A total quantity of 5.8 million cubic feet (165,000 m3) of concrete was needed during their construction. The total quantity of fill material required was 45.9 million cubic feet (1,300,000 m3). The size of the project equals 24 football fields.
The sides of the tanks consist of 29.5 foot (9 m) high vertical retaining walls that rest on 26.2 foot (8 m) high sloping embankments. To provide sufficient stability against the slope and bearing failure under consideration of the high loads resulting from the concrete tank side walls together with the fill between adjacent walls, a safe solution was required to increase the stability of the embankment structures.
Slag fill from the nearby Saldanha works iron ore smelter was used in lieu of the original calcrete (carbonated sand gravel mix) was used to construct the embankments. This resulted in a significant cost savings and was also more sustainable.
Geogrid reinforcement in the upper 8.2 feet (2.5 m) of the embankments was considered in the design to achieve the required safety factors. Four layers of geogrid were incorporated at vertical intervals of 23.6 inches (600 mm), each with a tieback length of 23 feet (7 m). A uniaxial laid and welded Secugrid polyester (PET) geogrid with an ultimate tensile strength of 200 kN/m in the longitudinal direction was chosen.
Because of the slag material’s high abrasiveness due to the angular and often sharp edges of the individual pieces of aggregate, combined with the vibratory forces induced during compaction, an exceptionally robust and high-strength geogrid was required to withstand the harsh installation conditions. Secugrid 200/40 R6 met the requirements for geogrid embankment reinforcement and provided the necessary long-term design strength in combination with its robustness resulting from the rigid monolithic bars. The use of more than 1.7 million square feet (161,000 m²) of Secugrid helped increase the embankment stability and provided the required bearing capacity for the 29.5 foot (9 m) high vertical concrete retaining oil tank side walls.