Ground Remediation Works

Ground remediation works to mitigate the risk of soil liquefaction triggering  – residential property Trafalgar Street, Christchurch

A geotechnical investigation was recently undertaken by Fraser Thomas Ltd for a property at Trafalgar Street, St Albans. The subject site comprised several buildings with connected units.  The eastern part of the site was adversely affected by a shallow seated landslide that occurred adjacent to a stream that abuts the eastern site boundary, that was inferred to have been caused by liquefaction of the soils in this area, in response to seismic loading associated with the February 2011 earthquake event.

The geotechnical investigation addressed the following issues for the site:

  • The nature and consistency of the subsoils underlying the site
  • The suitability of the existing foundation system, particularly in relation to mitigating the risk of damage resulting from earthquake related ground deformations
  • The risk of further liquefaction of the underlying subsoils
  • The risk of the site being adversely affected by lateral ground spread
  • The performance of the land during earthquake events.

The results of our site specific geotechnical and appraisal works indicated that the foundations supporting the buildings located in the eastern part of the site, were unlikely to comply with the Ultimate Limit State (ULS) performance criteria of the relevant New Zealand Standard Code of Practice, and that these foundations were therefore not suitable for the site conditions.

It was recommended that ground remediation be undertaken in the eastern part of the site, prior to the reconstruction of any new foundation systems in this area, in order to mitigate the risk of liquefaction occurring in the underlying soil layers in response to a future large earthquake event, and to subsequently mitigate the risk of future lateral ground movement occurring.

It was decided to install shallow stone columns in order to achieve the required ground improvement. Detailed design of these works was then undertaken, following demolition of existing structures.  These works involved:

  • Further CPT probe testing at the site, in order to more reliably determine the “pre-ground remediation” subsoil conditions and to provide information for design purposes.  CPT testing comprised 4 x CPT probes, approximately 7 m deep, with their locations being surveyed
  • Collation of subsoil information and provision of design details/sketches
  • Review of detailed design drawings
  • Provision of a design report and necessary Producer Statement- Design (PS1) documentation for the design of the ground remediation works.

Construction observations were undertaken by Fraser Thomas Ltd during installation of the stone column elements in order to confirm that the ground improvement works were undertaken in accordance with the design.  Subsoil testing was also undertaken following the completion of the ground remediation works, in order to confirm that the ground remediation achieved the required ground improvement.  These works involved:

  • Inspection/observation of stone column construction
  • CPT testing comprising 3 x CPT probes, approximately 7 m deep, and survey of their locations
  • Collation and appraisal of construction observation results and subsoil testing
  • Reporting on the results of our construction observations and ground improvement confirmation testing, and provision of a Producer Statement-Construction Review (PS4) Certificate.

We found that the ground remediation works sufficiently improved the subsoils beneath the eastern part of the site, and were suitable in mitigating the risk of liquefaction triggering in the treated soils, in response to a future large earthquake event. The ground remediation works provided a suitable building platform in the treated area for the construction of a shallow concrete waffle slab foundation system. The new foundation system is expected to meet the performance requirements of the NZ Building Code, under static and seismic load conditions.

Expertise provided:

  • Geotechnical
  • Survey.

Project cycle:

  • Geotechnical investigations
  • Detailed concept design
  • Construction observation
  • Construction review.