Alternative design for Snowy 2.0 25 Jun 2020

Construction of the Snowy 2.0 hydroelectric scheme in the Snowy Mountains of New South Wales, Australia, has started with new innovative design alternatives offered by the engaged Future Generation consortium for the 2GW project. With a high head differential of more than 700m, the construction JV, led by Salini Impregilo, alongside Lane Construction and Clough, has proposed an inclined pressure tunnel in place of the vertical pressure shaft of the design-build contract design (Fig 1).

Fig 1. Alternative design changes a vertical pressure shaft to an inclined alignment
Credit: Snowy Hydro Ltd

The project features a total of about 27km of precast segmentally lined waterways, a deep underground power house, and 430m of vertical shafts. When complete, the pumped storage scheme will link the existing reservoirs Tantangara at an elevation of 1,231m, and Talbingo at 546m. According to project owner, Snowy Hydro Ltd, the project takes hydro design and construction to a new level, involving several geological and hydromechanical challenges for the waterways.

Three TBMs of 11m diameter, two supplied by Herrenknecht and one by CREG, are preparing for delivery to site to start excavation later in 2020 with drill+blast of the powerhouse caverns to start once access is afforded (Fig 1). The CREG machine is the first Chinese TBM to be used in Australia.

Alternative design

From large water intake structures of up to 90m high in both reservoirs, the 9.8m i.d. x 17km long headrace will run from Tantangara to the 25m o.d. x 250m high headrace surge tank. In the contractor’s design alternative to the traditional vertical pressure shaft of the initial design, water will then flow in a 1.6km long x 25° inclined TBM excavated pressure shaft. This leads to three penstocks that bifurcate to feed six 340MW Francis pump-turbines in the 30m x 55m x 250m machine hall at about 800m below ground. A 20m x 50 x 200m transformer hall is located downstream of the machine hall (Fig 1). Three of the single-stage, reversible Francis pump-turbines will be conventional synchronous machines, while the other three will be variable-speed asynchronous.

Construction works at the main access portal site
Credit: Snowy Hydro Ltd

On the downstream side of the machine hall, the six draft tubes combine into three collectors that meet at the bottom of the almost 200m-high tailrace surge tank, from which the 9.8m i.d. x 6km tailrace tunnel connects to the Talbingo intake structure. Maximum rock cover is almost 450m above the headrace and 800m above the tailrace.

There are an additional 11.5km of access and construction adits, as well as a 670m ventilation shaft. Primary access to the power station is provided by the main access and the ECTV, emergency egress, cable and ventilation tunnel, both of which are about 2.5km long.

The three TBMs will include two open mode and one dual-mode machine. The CREG machine will drive the tailrace and main access tunnel with the Herrenknecht machines excavating the headrace and the inclined pressure shaft and ECTV, emergency egress, cable and ventilation tunnel.

Two Herrenknecht TBMs will bore the headrace, inclined pressure shaft and ECVT

A AUS$55 million factory for production of the lining segments is being established for the project and is expected to start producing segments by the end of the year. Operated by Future Generation, it will manufacture more than 130,000 segments over the course of the project and will employ 125 people. A steel lining will also be installed at locations where leakage or confinement issues are identified.

With the access tunnels and adits and the underground powerhouse caverns excavated by drill+blast, the shafts will be excavated by blind shaft sinking.

Difficult ground excavation

The project faces challenging geological and hydrogeological conditions with the alignment passing through several highly-variable alpine formations with a wide range of rock types and geological structures. According to project information, excavation is likely to face frequently changing mixed ground conditions with regular transitions through different lithologies, faults and weak zones. High groundwater inflow may also impact construction and ground stability, requiring pre-excavation grouting in critical areas.

A CREG TBM will excavate the tailrace and main access adit

Although a geotechnical baseline has been defined, a more thorough understanding of the ground conditions will only become evident when construction begins. As a result, a contractual risk-sharing mechanism has been developed to allow for a balanced distribution of risk between the project owner and contractor, as well as for working out fair compensation for time and costs during actual construction.

Outlined in a technical paper presented at the World Tunnel Congress in Naples in 2019¹, this risk-sharing mechanism is based on the new Emerald Book prepared specifically by FIDIC and the ITA for underground excavation contracts and on the principal that risks should be managed by the party best able to do so. In this case, risks posed by the subsurface conditions will be carried by the project owner, while those related to production rates and performance are with the contractor. Underpinning this is a comprehensive GBR, Geotechnical Baseline Report, which forms part of the civil works contract and gives a baseline of ground types, ground behaviour types and tunnelling classes along the alignment. By defining the tunnelling classes, the GBR aims to accurately reflect the amount of effort required for construction under the agreed method.

Current work and coping with Covid-19

At the start of its work on site, the project has had to manage the bushfires in the area of 2019/2020 and the Covid-19 coronavirus pandemic of early 2020. In a statement from Snowy Hydro Ltd, both were managed with “minimal disruption to operation and minimal impact on the project timetable”.

Although the bushfires came through the construction site, damage was limited “as a result of good preparation, including heat barrier fencing around the main compound” and contractors were able to return within weeks. Work has also continued through Covid-19 with “strict safety protocols in place and schedule and shipping adjustments made to minimise any impact on the overall project schedule”.

In addition to construction of the Polo Flat precast segment factory, exploratory works have been underway since March 2019 at the Lobs Hole construction site, including the construction of access roads and support infrastructure, including two bridges and temporary worker camps. During 2020, a range of earthworks have also been completed to prepare the portal for launch of the first TBM later this year to drive the main 2.5km power station access tunnel.

Future Generation subcontractor, GHD, is also currently conducting another site investigation programme at key locations to provide information for design of the underground power station and the inclined pressure shaft. This follows initial geotechnical drilling along the project alignment that began in 2017 and has so far collected more than 30,000m of cores over three years. One current borehole at the inclined pressure shaft site has reached a depth of more than 1,300m and is planned to extend more than 2,000m to the area of the penstock manifold.