New Plymouth – Stratford reconductoring

Groundline New Zealand developed the detailed design for the reconductoring of a section of Transpower’s New Plymouth – Stratford 220kV line.

Groundline developed the detailed design to reconductor a section of line where the conductor was nearing the end of its useful life. Because codes and standards had changed significantly since the line was originally designed and built, and because loadings would increase on the reconductored sections of line, the team carried out a full structural analysis of the towers and foundations.  

The structural analysis involved carrying out Transient Dynamic Response (TDR) inspections on several of the foundations to verify constructed depth and shape and to assess the integrity of the constructed piles. This was supplemented with boreholes drilled alongside the piles to confirm constructed belled foundations. These analyses confirmed that some of the foundations needed strengthening.

Geotechnical investigations identified a soil high in alluvium. This presented Groundline with a challenge because when displaced, alluvial soils tend to liquify and don’t solidify easily. Because of this, any foundation strengthening work had to minimise soil displacement. To mitigate this, the team designed a strengthening collar solution to provide additional uplift capacity for the foundations. For one particular site, this included constructing a retaining wall to enable the excavation of the existing foundations and then construction of the foundation strengthening solution.

Groundline also developed a suite of design solutions for upgrading the insulator attachment points. These attachment points had been operating for many years in a high salt environment.

Groundline provided field support during construction, including construction monitoring of foundation strengthening, providing load calculations and sag verification.

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