Cotter Dam Remedial Works

Project Location: Cotter Dam
Client: Actew Water Division
Client’s Representative: Mr Javad Tabatabei
Superintendent: SMEC
Superintendent’s Representative: Martin Fox
Project Start Date: March 1999

Description:

The aim of the Cotter Dam Remedial Works project was to strengthen the wall of the mass concrete gravity dam by replacing the top of the existing wall.

Scope of works:

• The spillway section of the dam wall was removed to 1.5m below the original level and the abutments were removed to 1.8m below the original level for the full width of the dam.

• Permanent post tensioned re-stressable ground anchors were placed. The 45 anchors, ranging in length from 20m to 60m and weighing up to 2.5tonne, involved drilling 1800m of holes

• Form work was erected and the concrete for each of the new load distribution beams was placed in two pours.

• Water proof grouting and stressing the anchors.

• Construction of a discharge facility

Discussion:

The Cotter Dam Remedial Works construction is a signature project for Guideline ACT. Completed in 1999, it was an innovative, difficult and dangerous undertaking, involving an inventive approach to construction and a high level of detail and control. The calibre of project Guideline ACT has built its reputation on.

Cotter dam, a mass concrete gravity dam, was constructed in 1912 and was raised in 1949. Concerns about ageing and structural integrity of the dam were broached as early as 1967 and with the revision to the probable maximum flood and earthquake requirements of 1 in 10,000 years, plus the proximity to a popular recreational resort, the decision was made to strengthen the dam.

Guideline ACT  was invited to tender for the Cotter dam project.

Guideline’s engineers developed innovative access and construction procedures, and it was these procedures and the development of the environmental and risk management systems which played an integral role in Guideline ACT being awarded the contract in February 1999.

To reduce the risk of construction being interrupted due to flooding, a slot was formed below excavation level at the southern end of the spillway, which together with the discharge facility cone valve, maintained the water level below the work area.

A bulk head was also constructed to keep water off 70% of the spillway and this also formed an effective barrier for personnel safety and against spillage into the dam impacting the environment.

The spillway section of the dam was excavated to 1.5m below original level and the abutments were excavated to 1.8m below original level, both for the full width of the dam. A total of 620 cubic metres of concrete was removed.

This allowed the construction of a new load distribution beam matching existing levels of the dam.

Hydraulic splitting was the technique used for controlled splitting of the concrete into blocks. The blocks had to break into chunks small enough to be removed by the purpose built grab on the barge mounted excavator.

The system works by drilling a hole to a specified diameter and depth into the concrete. Then the wedge set is inserted into the hole with the wedge in a retracted position. The hydraulic pressure then forces the wedge between the two counter wedges, pressing them against the walls of the drilled hole. The effective splitting force of up to 4000 KN breaks concrete from the inside.

The holes were cored in a pattern so as to break the concrete into blocks less than 2 tonne, so the excavator could then grab them and drop them into the water 15m from the dam face.

Demolishing the top of the dam wall carried a high potential risk of causing structural damage to the rest of the dam structure, a possible high level of impact to the environment, and the potential to cause a blow out in construction time.

These risks, substantial risks for the entire project, were all mitigated by using the splitter system, because the splitter system allowed a high level of control of how and to what depth, the concrete would crack so as not to cause damage to the remaining structure. It was also a system that caused clean cracks so there was no significant fine material produced to pollute downstream waterways.

Another complex and dangerous activity was installing 45 permanent post tensioned ground anchors.

The ground anchors, ranging in length from 20m to 60m, were constructed on site and then placed using a helicopter, believed to be a first in Australia. Due to the unique shape of the load and how it acted when being lifted, the task of placing the anchors in the hole was difficult. Several installations were trialled until a suitable hitching arrangement was found, which involved lifting the anchors from the fabrication area to a clearing in the pine forest and then picking them up again with a different sling to transport them to the dam wall for installation.

Using the helicopter was a significant factor in completing the project on time and it also reduced the risk to the environment, due to the work on the water being significantly reduced.

Drilling the holes for the anchors and the subsequent redrilling of the grouted holes for water proofing proved to be the greatest threat to the environment, due to the quantities of drilling cuttings being generated.

The drill rig was positioned by floating it down the dam on a barge and operated from this platform. Settling containers were also set up in a purpose built pontoon to handle drill cuttings.

Cuttings containing cement were made fluid with water and pumped into sealed trucks at the toe of the dam, so the cuttings could be disposed off site.

Grouting of the anchors was done from a grout station based at the toe of the dam, where mixers and pumps were set up to push the grout to the top of the dam.

The ultimate load range for the anchors varied from 3750 to 8400KN. After the placing and curing of the load distribution beams, the anchors were initially stressed. 48hours after the initial stress the anchors were checked via a lift off test and then rechecked at 28 days.

The design of the heavily reinforced distribution beam allowed the force of the anchors to be spread over the top of the dam. The concrete was placed in 2 pours for each beam. Curved forms used for the spillway sections were designed and placed so that after initial set the form could be stripped and the concrete finished.

To enable the concrete to be fluid enough to pump it from the stilling basin to the top of the dam wall plasticisers were added on site

Despite all the work being carried out within the Cotter reserve and a significant amount of the work being done adjacent to the Cotter River, including work within the stilling basin upstream of the river, no environmental impact was recorded due to the works.

This result can be directly attributed to the Environmental Management Systems being fully implemented and monitored throughout the project. Environmental issues were discussed in the mandatory site induction for all site personnel. Above all this, a true regard for environmental protection by the Guideline ACT site personnel can take credit for the excellent environmental outcome of the project.

This project demonstrates that the Guideline ACT philosophy of “no job too difficult” and its partnering approach, its management style with its clients and subcontractors, together with close monitoring of environment, quality and progress issues, delivered a successful project to its clients. The project was constructed in nine months and completed four months ahead of schedule. The client’s risk was minimised by the construction methods employed, and therefore the client was not exposed to any additional costs due to flooding and there were no contractual claims. No lost time injuries were incurred.

The Cotter dam remedial works project showcases Guideline ACT ability to deliver exceptional civil construction projects.