Design Practice of Bored Piles in Nubian Formation Case Study: Foundation and Bridges in Khartoum

Abstract

The term Nubian Sandstone Formation or Nubian Formation (NF) is applied in the Sudan to those bedded and usually flat-lying conglomerates, grits, sandstones, sandy mudstones and mudstones that rest unconformably on the Basement Complex and the Paleozoic sandstones. These formations are either exposed or covered by the recent quaternary formations. They cover large areas in Northern, Central Western and Eastern Sudan. Several important heavy structures such as bridges across the rivers and high rise buildings in Khartoum are supported on these formations. The factors controlling their performance as foundation materials are the type and amount of cementing material for the sandstones; the heterogeneity, degree of weathering and the type and consistency of the mudstones and the characteristics of the formations covering them. Different approaches were used for the design of piles resting or penetrating NF. This paper summarizes the geotechnical characteristics of five bridge sites in Khartoum and the approaches used by the designers for estimating the bearing capacity of the piles supporting these bridges. The piles were all socketed into the NF. The designs were compared with the results from pile load tests carried out in the bridge sites. The analysis has shown that the approaches used for estimating the pile capacities in the NF are very conservative and un-realistic. Alternative design approaches or improvements of the currently used designs are needed