While shafts are typically encountered as vertical structures in tunnel projects, they can come in various orientations including steeply inclined, or radically sloping, conduits such as for pressure tunnels on hydropower projects. Creating the steeply inclined tunnels can also involve various excavation techniques, including the use of tunnel boring machines (TBMs).

For the hydropower renewal project at Ritom hydropower plant, in Switzerland, the new tunnelling works have been many and varied, performed by the contractor consortium of Marti Tunnel, Mancini & Marti, and Ennio Ferrari.

One of the interesting challenges on Ritom was to use a TBM to bore the two-step, steeply inclined sections of the new pressure tunnel. Before the drive could commence, conventional tunnelling on the shallower low portion of the pressure tunnel met with unexpected hydrogeological setbacks. The knock-on effects included loving the TBM launch cavern deeper into the mountain. When the machine got underway, driving upward, it a fault zone. The contractor successfully overcame the tough challenges and the TBM completed its climb earlier this year.

While opposition to new large hydro development may limit use of some water resources, especially in already regions where there has been major development already, opportunities exist to tap hydro on a mini-scale in environmentally friendly ways. Small TBMs, boring at steep grades compared to drill and blast method, can help to realise those projects, notes Robbins. Benefits can include helping owners to develop marginal projects.

Robbins has supplied a number of small diameter machines for mini hydro development projects in Norway.

RENEWAL AT RITOM

In operation since 1920, the Ritom hydropower plant is located between Ritom reservoir and Piotta village and its Pelton turbines have been powered under a head of 830m for the last hundred years. With a power capacity of 40MW, the plant produces about 150GWh of electricity annually and is a key piece of the energy infrastructure in the area. It is now owned and operated by Ritom SA, itself owned by Ticino canton and Swiss Federal Railways on a 25:75 basis.

The original infrastructure for the hydropower complex included an intake at the high reservoir, an approximately 900m long headrace tunnel, a surge tank and the surface powerhouse at the lower level. But with its age and access to water rights due for review, the owners looked at options to refurbish and enlarge the hydro asset. Considerations included use of the nearby Stalvedro plant, built in the late 1960s by Azienda Elettrica Ticinese (AET). The Ritom SA owner was created as part of this process, in 2015, with AET also representing the canton of Ticino.

Design of the new Ritom plant was developed by a consultancy consortium, which comprises IM Maggia Engineering, Lombardi, and Pagani & Lanfranchi. As the new project would require increased water flow the new infrastructure needed higher conveyance capacity, which was to come in the form of a new headrace waterway.

The construction consortium was awarded the contract in 2018 and construction work of a full package of works, including tunnelling, commenced later that year. The scope of the works included construction of a new intake structure, a 2.3km-long new pressure tunnel, a pressure equalising basin and also a new surface power plant next to the existing facilities.

Key tunnelling activities, working across two sites (Piora, Piotta) included use of both drill and blast method as well as the TBM at the latter site.

The pressure tunnel has a horizontal section at the top, near the reservoir, and a sub-horizontal stretch closest to the powerhouse. In between those shallow sections, the pressure tunnel also has a two-step steeply inclined alignment, drop the height over a relatively short distance.

A TBM was chosen to bore the steeply inclined sections – a 3.20m diameter Herrenknecht open hard rock gripper, fitted with a safety system for handling the slopes of the pressure shaft. The bored tunnel would be completed later to a 2.2m i.d. finish in concrete lining.

Tunnel works for the horizontal and sub-horizontal parts of the pressure shaft would be performed by conventional tunnelling, entering from the reservoir at the top and advancing upstream from the new powerhouse area below, respectively. Any intercepted groundwater was to be treated before discharge.

As explained by Marti to the most recent Swiss Tunnel Congress (STC 2022), earlier this year, ground conditions provided more difficult than expected in sections, adding to the tunnelling challenge deeper into the mountain. The difficulties began with excessive groundwater ingress as the conventional drive blasted into the hillside, calling initially for significant extra capacity to treat the water before discharge. Then a weak zone of rock was encountered, causing tunnelling works to be suspended by agreement.

Probing up to almost 150m ahead was performed, with some difficulty, by Stump AG, part of Marti AG group. The probing revealed even greater water pressure and flow difficulties ahead and the ongoing measurements did not decrease.

A large programme of grout injection was initiated, further helped by placing a concrete plug as a counterweight against high pressures behind the excavated face. But in addition to grouting ahead, the contractor also performed peripheral grouting in the form of injection umbrellas around the circumference, aiming to limit any potential for pressurised water seepage and then stability problems creeping back along the length of the excavated tunnel. Renesco GmbH, part of Marti Geotechnik, helped with a variety of grouting measures in the hybrid sealing system developed for the conditions.

Handling the hydrogeological problems led to a realignment of the middle sections of the pressure shaft, as the sub-horizontal tunnel was longer than planned. The assembly chamber for the TBM was moved 165m farther into the mountain, from where to drive the twostep steeply inclined middle sections of the pressure tunnel. At almost 100m-long, the assembly of the TBM and back-up gantries was prepared for launch. The machine included a double-fall arrest safety device to operate on the initial 45% (almost 23°) incline. It also had facility to replace cutterheads (23 x 14”) from inside the machine. The geology ahead was gneiss.

However, the excavation challenge was not over for the contractor. The TBM encountered a fault zone, the weakness of the ground complicating progress with a collapse in the crown area.

Without equipment fitted for probe drilling or injections, a serious of hand mining and investigatory drilling works plus self-drilling anchors and low pressure placement of a cement mixture improved the ground and partly filled the crown void. Dry shotcrete helped to secure the profile. Use of water was limited. The TBM bore resumed – and then, with those proven stabilisation tools in reserve, moved onto the next, even steeper incline (90%, or almost 42°) of the pressure tunnel. The TBM successfully completed its drive in February.

Marti noted that the client has continued to investigate the origins of the large, pressurised earlier inflows and no conclusive answers have been found. The entire upgrade and refurbishment project at Ritom is expected to be completed in 2024.

STEEP OPPORTUNITIES

In helping owners to look at the potential to develop what may have been viewed as marginal projects, the tunnelling sector can offer technology capable of efficiently boring long narrow shafts needed to convey water from far distances upstream, at pressure.

Robbins notes that small diameter TBMs can operate at steep grades compared to drill and blast method, and also their minimum economic cross section is approx 7m2 – 10m2 or about half of that alternative tunnelling method. A further consequence is that there is less material to be excavated and that challenging topography can be overcome.

Set up of TBMs, of course, is a challenge but compact machine and launch areas are possible for the small diameter systems that can bore up to 45° in some cases.

Examples of the small diameter TBM drives successfully undertaken on steep grades for smaller hydropower investments in Norway.

For Holen project, owned by Smaakraft AS, Robbins supplied a 1.89m diameter, newly developed double shield Rockhead TBM that was ordered by contractor Hardanger Maskin AS. It was based on Robbins’ small boring unit system (SBU, a trenchless type line of equipment and machinery) equipped with a gripped system, discs (15 x 14”) and 210kW cutterhead power. Two tunnels were performed as upward drives, starting at shallow angles. The refurbished TBM is working on a third tunnel – Blindtarmen – elsewhere in the country.

The following year, Robbins supplied more standard TBM technology in the form of a 2.8m diameter small Main Beam machine to Norwegian hydro developer Norsk Grønnkraft for a number of its projects. The first project was Salvasskardelva, the tunnel for which had a modest grade and the machine has discs (19 x 17”). A reusable launch frame was used instead of a starter tunnel. A continuous, 450mm-wide conveyor removed spoil – the smallest ever supplied by Robbins – also negotiated curves. The small job site also used a double stack belt storage cassette

The developer started up its own contracting arm, NGK Boring, working alongside Entreprenørservice AS to build the tunnels.

After the 2.85km-long Salvasskardelva drive on an incline (up to 10% grade), follow-on projects have included 3km-long tunnel for Mork hydro project (up to 15% grade), and the TBM is currently boring the 2.1kmlong tunnel for Ovre project (up to 15% grade).

Robbins also has a 2.6m diameter double shield machine on the Tokagjelet hydro project, which will include steeper driving.