Sealing and drainage experiences in Swiss tunnels
6 Dec 2018

Cavities cause challenges in tunnel construction but in the operational life of underground assets they play a useful role, such as drainage. Used in a controlled form, it is argued, the cavities become designed space which enable effective drainage and, hopefully, minimise the risk of sintering or scaling when carbonate deposits form in the flows with potential to cause blockage over time.

Stefan Maurhofer, president of the Swiss Tunnelling Society, in his preface to the Colloquium presentations at Swiss Tunnel Congress 2018 said: “The impact of groundwater is key to the serviceability of structures.”

Formation of scaling cavity for a tunnel with vault drainage

Groundwater challenge

Where there is risk in tunnels, the cause is two-fold:

First, a natural process is at play where man-made materials can be susceptible to certain hydrogeological environments. It is about chemistry. Seepage from different rock types and groundwater with varying chemical and temperature properties meets the tunnel air and come into contact with construction materials. The water reacts with concrete, shotcrete and grout, and carbonates gradually precipitate from solution and accumulate.

The second risk is not designing solutions to adequately deal with the chemistry and potential for build-up of precipitate.The challenge of managing drainage in environments where sintering is a risk is vital to the long-term integrity and usability of many tunnels. It was suggested that an important approach could be the deliberate, calculated use of space in the annular gap around the tunnel lining plus further support methods.

Cavities by design

Swiss tunnel drainage consultant Marcel Christian Wegmüller says a primary drainage system can be designed with sufficient calculated voids to convey the seepage to the secondary system, prevent the build up of deposits, and avoid flow blockage and further problems. The designed cavity can be slim-line space around the tunnel lining.

Wegmüller said a cavity is “an infilled space that needs to be big enough to allow for sintering while not obstructing drainage”. He added that there must not be bottlenecks to drainage, even by inappropriate placement of geotextiles.

Formation of the scaling cavity for a tunnel with invert drainage

In his paper to the Swiss conference, he discussed the size of the void, stating, “in the primary draining system, these deposits or scales cannot be removed” and they “must have as much space as necessary to ensure that they can spread unhindered for the structure’s entire lifespan.”

Failure to do so will lead to changes in the groundwater regime, he said. “If the system fails or is closed off, the flow of groundwater leads to a build up of pressure in the area. Depending on the geological conditions, pressure can mount quickly, which may cause damage.”

In his talk, Wegmüller told delegates that old tunnels often have a lot of cavity spaces surrounding their linings and that these are relatively large spaces compared to the design and construction of many modern tunnels. The cavities behind the lining in the old tunnels have provided sufficient volume through which seepage flows continue, despite sintering. “So the old tunnels are still standing,” he said.

Wegmüller calls the space the scaling or sintering cavity volume (this does not required capital letters – why?) (SCV). He said the cavity volume can be calculated by groundwater sampling, giving a value of the Ryznar stability index and a correlating scaling, or deposition, rate for flows through the primary drainage system.

To then calculate the SCV space, other factors need to be known, such as seepage rate, rock density, length of the tunnel zone of concern and the proposed lifespan of the structure. The SCV calculation does not include the secondary draining system, he emphasises.

Simplon Tunnel

Looking at the Simplon Tunnel in south west Switzerland, he asked what SCV space would be needed for such a tunnel to ensure that the functionality of the primary draining system is never put at risk throughout the entire design lifespan.

Tunnel profile in TBM excavation with tubbing lining

The almost 20km long, and more than 100-year old, twin tube tunnel is structurally in good shape. Groundwater ingress varies along the tunnel with the middle having highly mineralised waters and temperatures of more than 40°C, whereas the south end has multiple flows from karst-like springs.

The key zone of most seepage, according to Wegmüller, is the southern end, and the resulting SCV calculation indicates 443 litres of void space per linear metre of tunnel would be required. He observes that while the value appears low it is roughly in line with on-site experience, and the fact that the tunnel has a massive tendency towards forming deposits at this point. He added that the inflows along the leakage zone are not evenly distributed, affecting the SCV calculation performed as an average. Where inflows are high locally, special designs, he said, such as volumetric well collectors are required.

The Simplon Tunnel was modified and upgraded in the 1990s to carry larger rail freight. The floor and track bed was lowered and there was the opportunity to work also on the secondary drainage system, replacing pipelines and fixing damage to the vault caused by pressure???.

A further round of drainage intervention was undertaken in recent years, replacing 200m diameter HDPE pipes with pronounced sintering and clogging in parts – in spite of maintenance and hardness stabilisation. As some upgrade works would place electrical ducts over the drainage (SW NB dead give away of copy-paste from some other text and lots through the rest of this text) pathways, a total of approximately 24km of plastic pipe was replaced with 315mm diameter pipes topped with filter concrete to provide favourable condition for maintenance and a long service life. In addition, a line of gabionbaskets was located above to act as a filter media and a barrier to prevent broken pieces of cement from entering the drainage pipe.

At the 2018 Swiss Congress , Wegmüller provided another example of SCV calculation, looking at the Isla Bella road tunnel, built in the 1980s in in Switzerland. He said seepage is distributed evenly over the length of concern and the acidic groundwater tends to form lots of deposits in combination with lime sediments and concrete contact. A suitable SCV, he calculated, would be 623 litres per linear metre.

In the congress question and answer session, Wegmüller responded to a question asking about the practical aspects of pursuing designs that include cavities to manage sintering, saying, “It depends on the willingness of the client.” He added that if he were to build a tunnel then he would seek to have much larger seepage packages and considers it feasible to provide 400-500 litres of SCV space per linear metre.

Cavity Fill

The arrangement of the SCV should run from the tunnel crown with the space increasing in cross-section continually down the outside of the walls and reaching to the seepage lines, or invert, said Wegmüller. The shape and profile of the space will be designed to suit particular tunnel geometry and the location of openings to the secondary drainage lines.

Vitally, the structural stability of the space must be safe-guarded, Wegmüller adds. Different possibilities are suggested to fill the void space. But to build redundancy into the design and ensure longevity of the void space to support its core function, in his paper he suggests em-ploying combinations of fill options.

Marcel Christian Wegmuller

The suggested options are:

• gravel backfill - ensuring connection to the secondary drainage system
• gravel packing in drill+blast excavation in the invert/abutment
• pressure-resistant drainage mats or dimpled membrane sheets
• seepage packing/seepage layers of all kinds
• drilling and cavities in rock
• leaving recesses open
• porous inserts in concrete
• widened joints to function as flow channels
• drainage channels in the invert

With note to TBM driven tunnels, and again in addressing redundancy in aid of longevity, he says space fill could consider combinations of:

• a layer of gravel between excavated rock wall and segment ring
• leaving segment recesses open
• little grout in the invert
• extremely strong fleeces and/or drainage mats between outer and inner lining

The combinations can include also water conditioning stones, helping to prevent the precipitates forming.

Whichever combinations are chosen, Wegmüller said the opening of the secondary drainage pipe needs to match the maximum cross-section of the space. He addedthat openings of current pipes are too small. A sample calculation in his paper, for a particular arrangement, indicated a useful maximum cross-section of the space of approximately 320cm2 for each side of the tunnel where it meets the mouth of the secondary drainage pipe.

Other supporting preventative measures to help reduce problems of sintering include installing water conditioning in either liquid or solid forms, and return feedback systems,.

Sustained-supply stones

Wegmüller explained that supply stones can be laid almost anywhere in the drainage system, helping to prevent scaling around slot openings. They are usefully also to manage water conditioning of small and sporadic seepage flows.

The SCV space can also contain supply stones, in tablet or rod forms, he said, as part of the combination of solutions to build in redundancy and aid longevity of the passive primary drainage system. To that purpose, Wegmüller notes that for groundwater with high lime content and smoothly flowing seepage, providing 1kg of water conditioning stone can offset the need for approximately 60-100 litres per linear metre of SCV space. He added, however, that the conditioning stones have a life span of only about 10 years, and so while noting the various benefits it should be considered as part of the combination of systems.

He adds, however, that the conditioning stones do not have a 100-year life span, only about 10 years, and so while noting the various benefits it should be considered as part of the com-bination of systems.

A further supporting system is a return feedback system to recirculate previously conditioned seepage water, or enhancing the water conditioning capability of the void space environment to help prevent carbonate deposits from lingering over the long term.

The flows and chemical effects of groundwater seepage must be managed, ideally at the design stage. Otherwise, sintering can result in significant long-term costs for maintenance of an underground asset. “It is not so easy to understand all of the chemical processes,” said Wegmüller. “It is helpful to involve an expert.”

“It is not so easy to understand all of the chemical processes,” says Wegmüller, “It is helpful to involve an expert.”