Ground surface deformation adjacent to a deep excavation in shale

Jeremy Toh and Garry Mostyn

This paper presents a case study of a deep basement excavation in Sydney’s Ashfield Shale. The intention of the paper is to present the high quality movement monitoring data that was gathered and analysed, and provide some accompanying interpretive commentary. These data may be of use when designing similar excavations.

Three mechanisms observed to cause ground surface deformation adjacent to the excavation are compared and contrasted:

1 – Lateral soil pressure acting on the shoring system, causing deflection of the shoring system.

2 – Excavation induced reduction in confining stress, leading to inwards movement of the rock mass.

3 – Drilling holes for ground anchor construction.

Mechanisms (1) and (2) above are those that geotechnical engineers normally consider in the design of deep excavations. Geotechnical engineers can usually predict these movements with reasonable accuracy. Lateral soil pressures and resulting deflections of shoring systems can be predicted or modelled using a range of basic and advanced techniques. These predicted deflections of the shoring system can be minimised if necessary by adjusting the design and the construction sequence. Rock mass relaxation resulting from a reduction in confining stress is more or less independent of the shoring design and as far as the authors are concerned cannot be controlled by any practical means.

Proven rules of thumb for common geological conditions in Sydney enable these deformations to be estimated with adequate accuracy for the majority of purposes.

Conversely we cannot usually predict ground deformations caused by anchor drilling using theory, modelling, or rules of thumb, with any real accuracy. Such deformations are highly dependent on the volume of soil removed by drilling, which in turn depends on the combination of geotechnical conditions, anchor hole drilling and construction methodology, and anchor design details including spacing, depth, hole diameter and declination. The volume of soil removed by drilling usually exceeds the theoretical volume, thereby meaning theoretical approaches are not usually particularly useful (at least for predictive purposes). These movements (and other movements due to shoring construction, e.g. pile excavation) are not always considered in a deep excavation design.

The case study highlights the significance of ground movements caused by shoring construction relative to total ground movements. Some conclusions are also drawn with respect to predicting and controlling anchor drilling induced deformations and appropriate types of monitoring for similar excavations.