30m Deep Excavation in a Congested Urban Area

Our involvement

• Geotechnical site investigation for a proposed 30m deep excavation
• 3D Numerical Analyses including:
  • Seepage analyses
  • Impact assessment for a 900mm diameter CICL water main
  • Predicting the shoring system behaviour and ground movements
  • Estimating the settlement, earth pressures and subgrade reaction modulus across the Basement hydrostatic slab and deeper lift core slab.
• Instrumentation and monitoring during and following the completion of bulk excavation.

Project challenges

• Complex geotechnical conditions.
• Transmission of ground-borne vibrations from construction activity causing settlement of the Heritage-listed St Andrews Church, partly founded in loose alluvial sands.
• Retention of the soil and weathered bedrock profiles
• Limiting seepage inflows during construction and in the long term.
• Presence of a 100 year old, 900mm diameter cast iron cement lined (CICL) water main, which was set back approximately 7.3m from the southern site boundary.
• Stability of a 1.1m to 2.3m wide plinth of soil and rock located between the neighbouring 25m deep basement excavation and the northern side of the proposed 30m deep basement excavation.

Our solutions

A comprehensive site investigation program was carried out to characterise the complex geotechnical conditions of the site, including deep cored boreholes, insitu permeability testing and borehole imaging. Due to the challenging ground conditions encountered, proximity of adjacent structures and shallow groundwater table, the shoring system installed consisted of secant pile walls terminated above bulk excavation level in competent bedrock, below which a grout curtain extended approximately 50m below surface level to limit seepage inflows. An instrumentation and monitoring (I&M) program was implemented to assess actual wall movements against those predicted from the numerical analyses.