Location

Arlington, Virginia

Session Start Date

8-11-2008

Session End Date

8-16-2008

Abstract

The paper discusses major design aspects related to deep excavations in soft clays including; bottom heave stability; deformations and loads on the supporting structure; methods to improve stability and limit displacements; lessons learned from failures; and fi-nally design principles and safety aspects. The various issues are illustrated by both parametric finite element studies and experiences gained from specific case histories. The results show a strong correlation between bottom heave stability, loads and dis-placements, and significant arching effects when the bottom heave safety factor is low. 2 and 3D FEM analyses confirm the applicability of traditional limit equilibrium bottom heave stability analyses, provided a search for critical failure surface is made and toe penetration of the supporting wall is accounted for. A concept based on using diaphragm wall with cross walls below the base is documented to be particularly effective in improving stability and limiting displacements. Ground improvement by deep mixing or jet-grouting has also been extensively used for this purpose and provides versatile design options. Some lessons learned from failures are highlighted and measures to avoid failures discussed. It is recommended to use continuum type FEM programs for de-sign, but their use require a good understanding of soil models to be used in the analyses. It is observed that soil parameters for use in design are often based on rather poor and rudimentary soil investigations, an issue which it is of prime importance for the geotechnical profession to face up to. When using ULS safety principles in design, the use of factored strengths may lead to un-reasonable design loads. FEM based design analyses should therefore be based on using slightly conservative characteristic strength and stiffness values. The resulting characteristic loads in the support structure must then be multiplied with an appropriate load factor to arrive at the design loads.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Sixth Conference

Publisher

Missouri University of Science and Technology

Publication Date

8-11-2008

Document Version

Final Version

Rights

© 2008 Missouri University of Science and Technology, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

Share

 
COinS
 
Aug 11th, 12:00 AM Aug 16th, 12:00 AM

Design and Performance of Deep Excavations in Soft Clays

Arlington, Virginia

The paper discusses major design aspects related to deep excavations in soft clays including; bottom heave stability; deformations and loads on the supporting structure; methods to improve stability and limit displacements; lessons learned from failures; and fi-nally design principles and safety aspects. The various issues are illustrated by both parametric finite element studies and experiences gained from specific case histories. The results show a strong correlation between bottom heave stability, loads and dis-placements, and significant arching effects when the bottom heave safety factor is low. 2 and 3D FEM analyses confirm the applicability of traditional limit equilibrium bottom heave stability analyses, provided a search for critical failure surface is made and toe penetration of the supporting wall is accounted for. A concept based on using diaphragm wall with cross walls below the base is documented to be particularly effective in improving stability and limiting displacements. Ground improvement by deep mixing or jet-grouting has also been extensively used for this purpose and provides versatile design options. Some lessons learned from failures are highlighted and measures to avoid failures discussed. It is recommended to use continuum type FEM programs for de-sign, but their use require a good understanding of soil models to be used in the analyses. It is observed that soil parameters for use in design are often based on rather poor and rudimentary soil investigations, an issue which it is of prime importance for the geotechnical profession to face up to. When using ULS safety principles in design, the use of factored strengths may lead to un-reasonable design loads. FEM based design analyses should therefore be based on using slightly conservative characteristic strength and stiffness values. The resulting characteristic loads in the support structure must then be multiplied with an appropriate load factor to arrive at the design loads.