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Aussies claim anchor record

ALMOST 13km of what are claimed to be the world's largest capacity and longest permanent rock anchors have been installed on a dam safety upgrade in Perth, Western Australia.

Australian firm Structural Systems installed the anchors at Canning Dam which at 467m long and 70m high is a key water source for the coastal city.

A safety inspection identified several problems, including inadequate earthquake resistance, significant cracking and insufficient spillway capacity for the design maximum rainfall, which could lead to the crest over-topping and dam failure with the potential loss of 100,000 lives.

A laboratory test anchor was used to determine the suitability of a 91 strand anchor. A 356mm diameter hole was drilled along the length of a specially prepared 1m square, 18m long concrete section. The hole had a rough finish to provide a greater rate of bond force transfer than expected on site, to test the sheathing ability of the anchor.

During stressing 92 strain gauges were used to monitor the section. The concrete section was set vertically to allow for vertical installation of the tendon and a 20m long permanent anchor, incorporating a 10m bond length, was installed and grouted.

The anchor was stressed 22 days after grouting to a test load of 18,526kN which was reduced to 17,101kN and monitored daily for 21 days. Load creep occurred over the first 14 days and then stabilised. The average rate of load transfer at the sheath was 7.3MPa.

After the force on the anchor was released, the concrete section was sawn mid depth to expose 1m of the free length, 2.5m of the fixed/free transition, and 2.5m of the bond zone. Although the surrounding grout was cracked, no failure had occurred in the corrugated sheath.

Structural Systems was awarded the A-12m (£4.4M) subcontract to stabilise the dam in April 1999. Site work started in July 1999 with three test anchors at the northern abutment, which included a 45m long, 91strand anchor. Some 85% of the equipment used was developed and manufactured specifically for the project.

Anchor lengths ranged from 18.6m to 139.2m and were made up from 8, 27, 55, 65 and 91 strands. A total of 12,846m of anchors were used with 45% of them made from 91 strands.

The anchors had bond lengths between 5m and 12m and free lengths ranging from 13.6m to 127.19m with corrugated sheathing for the bonded zone and smooth sheath over the free length.

The anchorage comprised a galvanized bearing plate up to 0.9m square and 0.2m thick for the 91 strand anchors with externally threaded anchor heads and wedges fitted on top of the bearing plates. Anchor strands were left sticking up above the anchor head to allow for future re-stressing or load adjustment. All the anchors could be monitored, re-stressed or detensioned.

Grout was mixed on site and pumped to the inner and outer sections of the anchors, with the largest anchors taking as much as 15t. Stressing was carried out after the grout cured for 21 days, or 28 days for the 91 strand anchors. This was done using hydraulic jacks; a 20,000kN jack was specially built to stress the 91 strand anchors to the test load of 18,526kN.

Stressing was carried out in five steps up to a 78% of the minimum breaking load (MBL) and held for 10 minutes. The load was then reduced to 72% of the MBL and locked off. The load was checked 28 days later for losses and, providing this did not exceed 2% of MBL, the anchor was accepted. Design was satisfied when 60% of MBL was present.

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