Tharwa Bridge Upgrade Stage 2 – Substructure and Approach Spans Strengthening

Project Location: Tharwa, ACT
Client: ACT Procurement Solutions
Client’s Representative: Jock McLean
Superintendent: Aurecon
Superintendent’s Representative: Max Chapman
Project Start Date: September 2009

Description:

This project involved strengthening the piers and approach decks of the Tharwa Bridge as part of a greater restoration project to increase its load bearing capacity without affecting its heritage value.

Scope of works:

• Strengthening of 5 existing concrete pier foundations via way of jet grouting 1.2m diameter columns, in total approximately 180 columns and over 1000 lineal metres.
• Installation of ground anchors to piers 5 – 9, 40 Number approx 568m
• Strengthening 6 piers via carbon fibre straps to the head stock.
• Replacement of both abutments including foundations.
• Supply and place precast concrete abutment headstocks and approach slabs approx 32 m³
• 900 diameter reinforced bored piles, for abutments, approx 41 lineal m, strengthening the approach spans by installing additional steel girders

• Demolition and installation of new plinths on the top of piers 5 and 9.
• Construct in-situ parapets to abutments approx 5m³
• Prepare and apply protective coating to existing steelwork
• Anti-carbonation treatment to existing concrete
• Supply and installation of various new bearing pads, bearing strips and expansion joints.

Discussion:

In undertaking Tharwa Bridge restoration Guideline ACT seeked to ensure as little disruption on the surrounding community and project through careful management of its site. The project involved very tight dead lines in road closures and sequences in strengthening to ensure bridge stability and most important inflicting little environmental impact on the Murrumbidgee River flowing below. Frequent monitoring of the river and surrounding environment controls was an ongoing task.

The strengthening of the foundation through jet grouting created concerns that with the permeable soil strata contamination of the river could occur. By installing coffer dams the work area was fully enclosed lowering the risk of any contamination. Grouting had to be injected down 5 to 15m and although some slight movement of the piers was accommodated within the design, it was important that the placement of the grout was staggered to maintain the equilibrium of the soil . This created equal pressures on the piers which ensured they did not move out of tolerances. Excess grout was deposited as hardstand area which reduced truck movement and transport costs, this area would ultimately be developed as a picnic area for the Tharwa community.

Tharwa Bridge is the most direct route in and out of Tharwa, the project was restricted to a maximum bridge closure of three weeks to minimise inconvenience to residents. Therefore tasks which rendered the bridge impassable had to be strictly programmed to ensure they would be completed within the allocated time. To help keep the program to a tight timetable, the process of diamond saw cutting was used as a fast efficient option to demolish concrete plinths between the deck and pier headstocks, allowing them to be removed and replaced quickly.

Heavy flooding occurred late in the project but any potential environmental risks were controlled and the work was completed on schedule and on budget despite several weeks delay due to the water level. A fire management plan was developed and implemented after close consultation with the Department of Parks and Conservation and representatives from ACT Roads to ensure the appropriate controls were in place and hot works were scheduled to be done at the safest possible times.