Following duplexing of the Roxburgh-Twizel line between Roxburgh and Clyde, it became necessary to strengthen towers, and upgrade foundations to resist the increased loads due to the new conductor configuration.
Commenced: April 2015
Completed: Projected April 2016
Ground anchors were drilled alongside existing foundations to increase uplift resistance. These were coupled to the existing footings using reinforced concrete corbels and pads.
Knowledge of existing foundations and ground conditions was limited and site specific loads for each leg were unique. Whilst general designs were finalised prior to tendering of the project, it was necessary to maintain open communications with the designers throughout the project due to the necessity to change the design continually throughout works. The Team on site were faced with accessing steep terrain in an alpine climate that experiences a 40 degree variation in temperature within a single day.
Original designs called for excavation works to be closed overnight, and filled in when wind speeds exceeded 46km/h. As this wind speed was exceeded most days, and due to curing requirements of adhesives utilised in the original design, the project would not have been achievable while meeting these conditions.
Tower models were analysed and temporary restraint arrangements determined to allow excavations to remain open without any reduction in the capacity of the foundations to resist the loads applied to the towers. This made it possible to complete the works without any increase in the failure risk during construction.
A specialist drilling contractor utilising plant with a small footprint was engaged to minimise the disturbance to land that would be required by the significantly larger rigs employed by other contractors.
Tower strengthening was completed as a variation to the contract which required installation of supplementary steel members at locations specific to each tower.
Whilst many challenges have been encountered over the course of these works, an experienced team have been able to minimise disruption to the project, deliver a quality outcome and to satisfy the requirements of landowners and restore structural equilibrium to the structures.