VIC – Goulburn-Murray Water – Program for Testing of Post Tensioned Anchors

Author: Jim Wheelhouse.

Within Goulburn-Murray Water’s portfolio of dams, there are five dams with installed post tensioned anchors. The installed post tensioned anchors are summarized in table 1 (below)

Table 1

In 2006 a 38mm diameter high tensile alloy bar anchor on pier 1 at Goulburn Weir failed.  The failure occurred after a stress test (“lift-off” test) showed the anchor had relaxed to within a stress range that necessitated re-tensioning of the anchor. During the re-tensioning of the anchor, a “loud bang” was accompanied by a sudden pressure drop on the stressing jack pressure gauge. Following this, the anchor nut could be turned by hand, indicating the anchor was carrying no load.

Following this incident, stressing and stress testing of anchors on all Goulburn-Murray Water sites ceased pending an investigation into the cause of the failure.

In 2008 Goulburn-Murray Water replaced the 38 mm bar anchors in piers 1, 2 & 3, with 7 strand x 15.2 mm dia strand anchors.

Goulburn-Murray resumed stress testing of anchors in 2015 with a program to test every anchor prior to 2017.

Observations

In the interest of operator safety, procedures were developed to prevent people from being in front of or over the anchor during stress testing. This made it difficult to determine lift-off by inserting a 1 mm feeler under the jack as with previous tests. Loading of the anchor was carried out gradually using a hand pump until lift-off was observed.  Anchor lift-off was observed visually using dial gauges located on the load cell. Lift-off was called when the rate of displacement increased on the dial gauges correlating to the load at which the strands or bar first begin to extent. The recorded lift-off under this procedure was generally a lesser load than previous lift-off results, assuming there was a 1 mm extension.

Cracking of 2 piece strand wedges was not uncommon and considered to have no detrimental effect on the performance of the anchor. Cracking of 3 piece strand wedges was far less prevalent . Uneven seating of wedges was observed on several strand anchors and with the exception of 1 strand was also not considered detrimental to the performance of the anchor.

It was difficult to check for the presents of void filler and top up void filler under strand anchor heads without spare strand holes. On 1 occasion an orifice was drilled through the anchor head chair to enable inspection and injection of void filler. On other anchors molten void filler was successfully injected using an eye wash syringe purchased from the local veterinary clinic.

Testing anchors while suspended from a crane basket and/or from a knuckle boom basket is difficult due to cramped conditions and movement. Goulburn-Murray Water constructed purpose built access systems where appropriate.

The strands on some anchors were cut off too short to enable future de-stressing or re-stressing of the anchor.

At Goulburn Weir corroded strand anchor covers were replaced with HDPE covers which are considered more suitable for long term inundation.

Strand anchor load cell jacks with a separate internal thread insert are easier to handle than those with the thread incorporated into the jack.

Future Considerations and Recommendations

Strand anchor heads should incorporate an orifice for ease of inspection and injection of void filler under the head.

Anchor strands should be cut off at a height suitable for future re-stressing and or de-stressing.

HDPE protective cover should be considered particularly in inundated applications and corrosion prone situations.

Purpose built re-usable access systems should be considered rather than working from EWP’s or crane baskets. In some locations where access is difficult, such as Buffalo Dam, purpose built systems may be cheaper than erecting scaffold.

Strand anchor jacks with a separate thread insert should be considered as they are easier to handle and could be utilized across various sized anchors.

Two way jacks are preferred over single acting jacks as they are easily re-compressed without the need for additional tools and equipment.

Observing anchor lift-off visually using dial gauges located on the load cell is preferred to inserting feelers under the jack.

Future Program for Testing of Anchors

At the completion of this round of testing Goulburn-Murray Water will evaluate the test results and implement an on-going program for inspection, maintenance and testing of anchors across all sites.

Goulburn-Murray Water intends to develop in-house skills and capabilities to reduce the on-going requirement for engagement of specialized contract services.