Notes On High Horizontal Stress Generation And Post Processing For Deep Excavation Shoring Wall Using Program Plaxis 2D

ABSTRACT

With recent advancement of computer technology and software engineering, numerical modelling is becoming an essential tool to carry out a soil structure interaction analysis of a shoring system for deep excavation. This technical note has been prepared to provide specific approaches for a shoring system design of a deep excavation in Sydney using commercially available software Plaxis. The first part of this note covers a method of generating high in-situ horizontal stresses in Plaxis model, which are usually presented in Sydney Sandstone and Shale formation. The second part presents a useful tool for post-processing of analysis results from Plaxis 2D for soil structure interaction analysis of shoring walls for deep excavation.

1 INTRODUCTION

Numerical modelling using commercially available software Plaxis has been widely used in geotechnical engineer, including the Sydney Sandstone and Shale where high locked-in in-situ horizontal stresses are often present. The default option in Plaxis 2D for high in-situ stress within rock is to apply a homogenous initial stress state in the model with the filed stress option. Where there is a case with horizontal stresses increase with depth it would require the user to physically define the input for each element with depth. This is often found inconvenient, hence there is a need to develop a methodology for auto-generation of the high in-situ horizontal stresses for Plaxis modelling.

During the detailed design of M4 East project (WestConnex Stage 1B) in Sydney and construction process in 2015, the authors developed an approach to generating the in-situ high horizontal stresses in rock using an applied volume strain when program Plaxis 2D and Plaxis 3D are used. Since successful application of this approach, it has subsequently been widely adopted by many practicing geotechnical engineers for Plaxis modelling over the past 10 years, notably Sydney Metro Northwest and Sydney Metro City, Sydney West Metro, Western Sydney Airport Metro, New M5 Motorway, M4-M5 Link and Rozelle Interchange in Sydney and Melbourne Metro in Melbourne. The first part of this note will set out the details of how this methodology can be used for ease of reference.

It is very important to have efficient communications between geotechnical and structural engineers for shoring wall design of a deep excavation. The role of geotechnical engineers is often to carry our soil and structure interaction analysis and then pass the analysis results to structural engineers to carry out the structural design such as reinforced concrete structure or steel members such as steel props and waler beams.

Preparation of the analysis results for the shoring walls sounds simple but could be a very time-consuming process, especially nowadays the construction sequences or stages are becoming increasingly complex under urban development environment. When many construction stages (for example, more than 30 analysis stages) are modelled in the numerical analysis, the load path transfer will become very complex and therefore the critical stages where the design can be focused on will become less obvious. This situation always present technical challenges to the geotechnical engineers who are responsible for providing analysis results such as bending moment, shear force, axial force and deflection within the retaining wall structures which are modelled by the plate elements in Plaxis.

The second part of this note introduces the geotechnical automation works we are currently undertaking within Arcadis to streamline the interaction process between geotechnical and structural engineers for cut and cover structure design. This process is currently being used for multiple infrastructure the projects across Australia and has been proved useful. This note is to share the approach and written scripts with the geotechnical colleagues who may wish to use for their future projects.