Monitoring and Numerical Modelling of a Deep Circular Excavation

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• Tina Schwamb
• Thursday 28 November 2013, 16:00-17:00
• Engineering Department - Lecture Room 6.

If you have a question about this talk, please contact Jen Fusiello.

The high uncertainty in current design practice of deep circular excavations may lead to overdesign or underdesign. For the estimation of ground movements New and Bowers’ (1994) empirical formula is used – this has not been confirmed by other studies and is based on specific ground conditions and geometry. Circular excavations rely on the beneficial hoop forces and are therefore structurally stable. However, very deep shafts are often built with diaphragm walls which are of discontinuous nature – this may reduce the circumferential stiffness and hence influence the excavation’s overall performance.

To extend the data base, learn about ground movements and structural forces, an extensive monitoring scheme was implemented at the Thames Water Shaft F at Abbey Mills which is approximately 30 m in diameter and 70 m deep – one of the largest every constructed shafts in the UK. The scheme included innovative fibre optic sensors, conventional soil instrumentation (manual inclinometers, magnet and rod extensometers) and optical surveys.

When comparing the measurements with the initial PLAXIS design some discrepancies are oberserved. To investigate these a parametric study is conducted with FLAC2D . The influence of several parameters (soil models, wall stiffness/anisotropy, wall thickness/verticality and chalk stiffness) was investigated.

The monitoring and numerical results show the following: (1)​ ​Small vertical ground movement during shaft excavation (≈ 1 mm) – the majority of movement occurs during diaphragm wall construction (≈ 6 mm) (2)​ ​Small lateral wall movements during shaft excavation (< 4 mm) (3) ​ ​Large bending moments and hoop forces are measured in the wall during excavation – the measurements are significantly larger than the PLAXIS design suggests (4) ​ ​It appears that the cohesion of the chalk was assumed very low (20 kPa) which was ‘unsafe’ for the diaphragm wall

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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