Challenging Excavation Stabilization using DSI: The new Old Mutual Head Office in Sandton

The Old Mutual insurance company is currently building a new head office in the center of Sandton north of Johannesburg, South Africa. The building is directly located at the subterranean station of the Gautrain regional train. The new building includes 8 basement floors and offers space for additional offices and shops.

The highly unpredictable geology represented a significant challenge. The excavation support design had to be altered to suit the changing rock levels in the course of the project. The anchor design was only concluded after the soldier pile installation along the complete excavation had been carried out.

The eastern face of the project is situated directly over the Gautrain tunnel, which made blasting and foundation work difficult. At the northern face, the common boundary wall with the adjacent building had to be kept intact and therefore stabilized for the new basement. To complicate matters, the new proposed basement depth exceeded that of the neighboring building’s existing basement. Movement had to be restricted to a bare minimum here and a comprehensive stabilization design was successfully implemented.

Due to the close proximity to the Gautrain station and the adjacent properties, construction tolerances and methods were closely monitored to ensure the stability of the excavation walls. Furthermore, the teams for earthwork, blasting and geotechnical work had to cooperate closely.

A dead-man anchor solution was used that was combined with localized gunite-thickening for retaining and stabilizing the gunite and pile wall in the north. Precision drilling had to be carried out for the dead-man anchor installation. Although drilling “blind” and despite the fact that very little room for error could be afforded, an extremely successful outcome was achieved. The existing wall was successfully retained without any damage to property.

For stabilizing the different excavation walls, Rocbolt supplied a total of 860 600kN and 750kN capacity DSI Strand Anchors as well as 920 DSI Soil Nails in individual lengths of 3 to 12m.