Stability of the temporary excavation between Soldier Piles in Unsaturated Residual Granites in Sandton CBD

F.H van der Merwe and J.H. Engelbrecht. October 2019.

AYGE Conference, Cape Town.

The design of multi-anchored tieback soldier pile walls in Sandton is typically based on either empirical methods or two-dimensional finite element modelling, ignoring the effects of matric suction in unsaturated residual granites. The designer, however, often relies on previous experience in a similar subsurface profile rather than established theory, to assess the ability of the soil to arch between soldier piles. Using experience rather than a sound theoretical basis for design could potentially be problematic. Excavations typically proceed in intervals between 1.5m and 2.5m before a reinforced shotcrete liner is applied. This differs from the way in which timber lagging is typically installed in smaller excavation lifts in the USA. In this study a typical Sandton residual granite profile is considered, and a matric suction profile estimated. 

Soldier pile walls in    

Unsaturated Residual Granites

Most of the northern suburbs of Johannesburg are underlain by basement granites of Johannesburg Dome. These granites are variably weathered giving rise to an undulating bedrock profile which in Sandton results in deeply weathered soils (typically 10 – 15m). These residual granites typically have a high shearing resistance with effective friction angles in the range of 33-38° and negligible effective cohesion. 

Key Words: 

Soldier Piles, Basement design, Deep excavation design, Johannesburg, Sandton, Ground anchors, Shotcrete Lagging, Soil-water retention properties, Excavation stability, Lateral support design, Embedded Retaining Walls.

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