The Design And Construction Of Very Deep Excavations – Recent Developments

Technical advancements in construction plant, materials and numerical analysis tools have made possible a step change in the achievable depth of excavations required for infrastructure, building and mining projects. This has been in response to an increased complexity in such projects particularly in connection with rail, water and power infrastructure sectors around the globe. Such advances do not come without some risks and a clear understanding of the limitations of the techniques, capabilities of construction monitoring and the benefits of practical design details are key to successful execution. In addition, a sound knowledge of the behaviour and testing of materials particularly fresh concrete and support fluids is essential in the minimisation of defects in deep earth retaining structures, which can be extremely costly to remediate.

This paper considers the state of the art in the construction of very deep and complicated excavations by making reference to a number of recent case histories, where records have been broken and new technologies have been deployed. The construction of diaphragm walls to depths well in excess of 100 m and with wall thickness of 1800 mm and using concrete with a 28-day cube strength in excess of 60 MPa are now possible, provided great care is taken. Improved verticality tolerances of better than 1 in 400, coupled with precise monitoring and advanced design techniques, means that the structural capacity of earth retaining walls in shaft construction have increased significantly which has led to the realisation of deeper excavations, together with deep openings which may be necessary for associated tunnels.

The author will also include the presentation of recent improvements in safety both in cage lifting, handling and splicing as well as around open diaphragm wall excavations. A better understanding of the causation of defects in concrete which has been placed under support fluid via a tremie, has been gained through painful experience and has greatly benefitted from the recent publication of useful guidance in Australia, UK and by the EFFC (European Federation of Foundation Contractors). This has led to a number of new site tests on fresh concrete for mix stability and bleed potential which are gaining increasing traction in the industry. In addition, the introduction of more stringent testing on support fluid such as bentonite during excavation means that instances of defects including leaks, inclusions and areas of poor concrete cover can be reduced. However, despite the availability of extensive guidance on good reinforcement cage detailing for diaphragm cages, examples of poor practice still remain, with great potential to lead to extensive defects such as mattressing which may compromise the durability of permanent works. The author will highlight examples of good and bad practice.