Judgment for contractor over Glendoe collapse 12 Jan 2017

Patrick Reynolds for TunnelTalk

UK power utility SSE is weighing options for its next steps in the Glendoe hydro tunnel collapse saga after a court ruled against claims for compensation against the turnkey contractor JV led by Hochtief of Germany.

TBM excavation of the unlined headrace
TBM excavation of the unlined headrace

The Court of Session in Scotland issued the verdict that the contractor was not liable for the collapse in the upper part of the 6.2km long unlined TBM-excavated headrace, in 2009, and therefore not required to compensate SSE for the subsequent repair costs.

Collapse in the headrace occurred barely months after the 100MW plant had begun operating. Glendoe is the first hydro project built in the UK for decades and once the headrace was repaired and the plant placed back in operation in late 2012, SSE brought a £130 million claim against the turnkey contractor JV. Hearings progressed over several years. In late December 2016 a ruling issued by Lord Woolman found against SSE as the plaintiff.

SSE said in a statement that it was “disappointed” in the Court’s ruling and added, “we will review the decision in more detail and assess our options”.

Glendoe power plant was out of action for three years due to the collapse and recovery works until operations restarted in the third quarter of 2012. Plant equipment in the powerhouse suffered no damage from the headrace problem.

Fig 1. Plan of the Glendoe hydropower scheme
Fig 1. Plan of the Glendoe hydropower scheme

Glendoe is located on the south end of the famous Lock Ness near Fort Augustus in Scotland and is designed to generate 180GWh of electricity annually (Fig 1).

SSE procured the scheme in 2005 as a design-build package from the Hochtief Glendoe JV. Led by Hochtief, the team included Poyry as designer. SSE retained the engineering services of Jacobs to complete the site investigation studies, prepare the client’s design, and supervise the works.

The 6.2km long x 5m diamter headrace is aligned through quartz mica schist and quartzite rock and with the region including a geological feature called the Conagleann Fault Zone.

During the construction phase, SSE acknowledged a degree of geotechnical risk on the project with the contractor responsible for performing the tunnel excavation.

The headrace was excavated by a 5.03m diameter Robbins main-beam hard rock gripper TBM that was refurbished and supplied to the project by Herrenknecht. Anticipated rock conditions were based on the Barton Q system and four classes of support were prescribed and installed, ranging from minimum rock bolts, mesh and shotcrete, should it be required, up to a full ring steel sets in the weakest areas identified during excavation. The TBM began boring on the headrace in early 2007 and completed the following year.

Fig 2.  Plan of the necessary bypass repair works
Fig 2. Plan of the necessary bypass repair works

The collapse in the headrace occurred in the identified Conagleann Fault Zone (CFZ), about 2km from the upstream end and in the TBM bored section. Poorer rock conditions were expected in the CFZ area, however no signs of the anticipated weakness were found during excavation.

Excavation works were being monitored by all contractual sides of the project, and particular when passing through the CFZ. As a result of more favourable conditions met during boring, the area that later collapsed had been judged not to require heavy rock support.

Soon after the plant began operating, problems with diminishing output led to investigations and discovery of a large headrace collapse zone.

Court ruling

In the ruling, Lord Woolman said the collapse occurred because, in short, there was not enough support, stating that, “poor rock conditions coincided with insufficient shotcrete and rockbolts”.

He continued, saying that those involved, on all sides of the contract, had inspected the rock and found no reason for heavy reinforcing support to be required at the section that later collapsed. He was therefore satisfied that the contractor used “reasonable skill and care” during construction of the tunnelling works, and that the utility company’s case depended on data gathered and interpreted with “hindsight” following the collapse.

 Junction of drill+blast bypass with collapsed TBM headrace ahead of bypass plug being installed
Junction of drill+blast bypass with collapsed TBM headrace ahead of bypass plug being installed

Lord Woolman also concluded: “The collapse was not due to a defect that existed at take over [of the completed project by the owner]. Accordingly, it was an employer’s risk event.” He said: “Having regard to the weight of the expert evidence, I incline to the view that it is impossible to determine the cause for the collapse.”

Repair of the headrace involved construction of a drill+blast bypass around the failed section. The contract was awarded to BAM Nuttall as one of four prequalified bidders, including original contractor Hochtief.

Repair works proved more extensive than expected, extending completion from late 2010 into mid-2012. SSE also called for the works to include application of shotcrete lining to other areas of the headrace tunnel, as well as along the tailrace tunnel.

In the ruling, Lord Woolman said he was minded to award a sum, possibly of about £1 million, towards the business disruption suffered by SSE when the plant was out of service and the utility was losing revenue. The figure had been agreed by the parties as a cap on lost income by the utility.


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