Applying a spray-on waterproofing membrane is gaining acceptance on a growing number of tunnel projects.
The art of the spray-on membrane technique that creates a composite structure with the membrane sandwiched between the primary support and the final lining.
Neat fix in Lebanon
Water ingress through cracks in the existing concrete lining of the 30-year old Chekka highway tunnel north of Beirut forced Lebanon's Council of Development and Reconstruction to refurbish the parallel 360m and 390m long three-lane tubes. Design was based on casting a new 25cm thick reinforced in-situ concrete lining with a PVC waterproofing membrane behind.
In spring 2006 contractor Est. Antoine Makhlouf of Beirut tendered according to the project design and was awarded the contract. But the challenges of meeting the specifications soon became apparent. "What we needed was a solution that would allow us to retain the existing lining, which was intact, and that would provide the required waterproofing properties without encroaching excessively on the internal dimensions of the tunnels," General Manager Roger Makhlouf explained when he met TunnelTalk at the 2007 BAUMA exhibition in Munich.
The alternative was based on first repairing the existing lining then spraying the prepared surface with a 3mm coat of the Masterseal waterproofing membrane. A finish of tiles on the walls would then be applied to the final 4cm thick inner lining of steel fibre reinforced shotcrete to assist future cleaning. While the technique's limited track record was a concern, the advantages were difficult to refute.
1. No need to demolish the existing lining;
2. No need for concrete footings to support a new 25cm cast-in-place concrete lining that would interfere with the existing invert drainage channels;
3. Only minimal 5cm on the radius reduction of the interior tunnel profile as opposed to the 30-35cm profile reduction for the original design without removal of the original lining;
4. Simple repair should leaks appear in the new lining; and most significantly
5. A tremendous time saving from the expected eight months for the original refurbishment design to just six weeks for the alternative.
This last advantage was decisive considering the importance of the country's main six-lane north-south traffic route and the need to minimise the period of diversion and reduced contra-flow capacity through one tunnel while work progressed in the parallel tube.
The alternative was approved and Est. Antoine Makhlouf was given notice to proceed in August 2006.
Precise application
Central to accurate application of the Masterseal membrane is precision, consistency and high capacity spraying. This can only be achieved mechanically and in the case of the Chekka Tunnel, Est. Antoine Makhlouf rented a Meyco Logica Potenza computer controlled robot to apply the material to a uniform 3mm coating. Developed by Meyco (also a division of BASF) for precise spraying of materials including concrete, gunite, passive fire protection mortar, and sprayable waterproofing membranes, the electric-hydraulic Logica Potenza robots provide degrees of freedom from manual to fully automatic.
For automatic operation, the geometry of the surface is first scanned with a laser sensor. An on-board computer then calculates the kinematics of the operation and controls the boom and the spraying nozzle through the cycle and according to preset parameters of distance, speed and angle of the spraying jet to the surface. The maximum membrane-spraying capacity achieved to the required average 3mm thickness was about 1,800m2/shift or about 180m2/hr.
After sealing the cracks with epoxy and polyurethane resin grout injection and inserting a dilatation feature into the construction joints to relieve deformation cracking stresses, the concrete surface was cleaned and abraded using high-pressure water jetting to prepare for application of the Masterseal membrane.
Where in the past, the membrane product was delivered as a liquid material ready for spraying, the latest development of the concept by the UGC is to deliver the product as a powder and apply it using a rotor shotcreting machine and compressed air to feed the dry product through a 32mm diameter hose and introducing water to the stream at the nozzle. Control of the air supply, water and product was a manual operation while distance, speed and manipulation of both the boom and the spaying nozzle was controlled fully automatically according to parameters programmed into the robot’s control computer. The 3mm thick layer of Masterseal membrane was applied in one pass in sections of 4m long across the entire 13m diameter arch.
The contractor's personnel were trained to operate the Logica Potenza robot and achieved a production average of 1,500m2/10h shift or about 150m2/h. The membrane wass applied to each tube in just six working days. The coating then required about 48 hours to achieve the membrane's mechanical properties including the 1-1.2 MPa tensile bonding strength to the substrate surface.
Once both tunnels were completed to installation of the Masterseal membrane, the 4cm thick finish of steel-fibre reinforced shotcrete was applied as a single follow-on operation, again using the Meyco Logica Potenza robot on rent from BASF UGC. Working in full automatic mode, the crew achieved production rates of about 60-70linear meters of tunnel in every 10h shift to complete each tunnel in about seven to eight days.
General Manager Roger Makhlouf
According to Makhlouf, the whole composite liner system for both tunnels, comprising a total 750 linear meter of tunnel and 18,500m2 of arch area, was completed in six weeks, compared to eight months predicted to install a PVC membrane and a 25cm reinforced in-situ concrete liner.
When TunnelTalk asked about working with the Masterseal product Makhlouf said that there was very little rebound during application and only when wind through the short self-ventilating tunnel tubes was blowing particularly hard. "Water on the surface was not an issue because all weeping cracks had been sealed with chemical grouts beforehand so application was to a dry surface throughout. Humidity did affect the application but only in the time taken for the material to cure."
The flat, prepared surface of the original lining was said to have removed any issues of bridging or shadowing that could occur over a more rough or uneven surface and the Potenza robot ensured accurate coverage of every square inch. The steel-fibre reinforced shotcrete final lining was applied directly against the membrane without concerns of piercing or undermining its integrity.
"There was little difference between the estimated cost of the original design and the value engineering alternative", said Makhlouf, "and we did have to invest in developing a convincing proposal for the client and the project designer. But the entire process has been a most satisfying achievement especially in furthering a new industry technique and in cutting so much time and disruption from the anticipated construction programme. Time now will pass judgement on the durability and performance of the finished product."
For the Faroe Islands in the North Atlantic, the spray-on waterproofing system was used as part of a cost-effective design for the archipelago's second undersea road tunnel. The tunnel creates a fixed link for the 5,000 inhabitants of the Islands' second largest city on Bordøy with the main island Eysturøy. The first undersea tunnel, opened in 2002, and the new Nordøy Tunnel, completed in 2006 connect about 80% of the 60,000 population via fixed links.
The single tube two-lane bi-directional tunnel is 6.2km long x 64m2 in section and passes a maximum 150m below sea level with a minimum 40m rock cover. Following the Norwegian design standards for low cost road tunnels, the Nordøy Tunnel was excavated by drill+blast and is supported with rock bolts and shotcrete. A toll equal to the cost of a ferry crossing will pay for construction of the tunnel but with a design for 50,000 cars/day and only a current load of 2,000 cars/day, limiting cost was a central criterion for the owners.
Control of water ingress was a specific cost concern.
The Nordøy Tunnel with its cost effective sprayed on waterproofing membrane
Probe drilling to trigger programmes of pre-excavation cementitious grouting was carried out in order to limit water inflows during excavation to the allowed maximum of 300lt/km/min. Selective application of waterproofing would reduce long term pumping costs from the drained facility.
The Nordøy Tunnel was built for approximately $US49 million or about $US8 million/km.
Waterproofing criteria
Experience of the Island's first undersea tunnel demonstrated that seepage had diminished by a significant 20% over four years as calcification build-up seals leaking joints. Drips or small single water ingress points in the new tunnel's walls would enter the drainage system and were therefore acceptable but a completely dry surface was required were waterproofing was installed in areas were seepage exceeded a specified six drips/minute from a single dripping point.
Originally, and similar to the first undersea tunnel, waterproofing in the Nordøy Tunnel was based on the Nordic norm of installing 50mm sheets or 'blankets' of polyethylene (PE) material, fixed with dowels and finished with a 60mm thick layer of shotcrete to create a drainage shield to channel water towards the invert drains. PE however is flammable and to comply with modern design standards, the owner researched alternatives that would reduce the amount of PE in the tunnel by at least 50%.
"Several methods were considered including PVC membrane," said Sigurd Leifsson Lamhauge project manager of the state roads authority during his presentation at the 2007 RETC conference in Toronto in June. "Of the options, the sprayed-on membrane system offered significant financial benefits compared to the PVC membrane and cast concrete lining. It is a non-flammable material; can be applied over the heads and face plates of the rockbolts; is faster to install; is not as vulnerable to damage during installation; and reduces the thickness of the final lining which in turn reduces the excavated section of the tunnel. Another advantage is that the bond to the substrate and to the final layer of finishing shotcrete leaves no gap between the rock support and the waterproofing. The interfaces between the waterproofing membrane and the primary and final shotcrete are impermeable. This allowed us to apply the system in isolated areas to create an 'umbrella' type of situation with cracks and joints in the host rock allowing water to migrate around the barrier and into the drained part of the profile without taking the waterproofing down to the drainage channels and avoiding water pressure build-up behind the membrane. Also, should a leak might appear in the composite structure, that is exactly where the leak is occurring. It can be seen and readily repaired."
Both Svein Kristiansen of Førøyakonsortiet and Lamhauge for the owner stressed the importance of preparing the substrata surface for application of the spray-on membrane. "It was essential to have a smooth dry surface to achieve a continuous membrane," said Kristiansen. "Smooth blasting and scaling achieved a smooth excavation profile. Also aggregate in the primary lining shotcrete was limited to 4mm grain size. Aggregate of 8mm could be used if well graded and if the application was aimed particularly at achieving a smooth finish. Major irregularities in the substrate required an additional layer of smoothing shotcrete."
To achieve a dry surface, wet patches were sealed with a rapid curing coating and point leaks were controlled with drainage half pipes, polyurethane injection, or drainage holes and hoses to the drainage system. "Once cured, the condition of the membrane is irreversible," said Lamhauge. "Curing of the recommended 3mm thick membrane layer to a Shore A hardness and a bonding tensile strength of 1-1.2MPa took 10-14 days in the colder climate of the Faroes. Once cured the final 4cm of permanent steel-fibre shotcrete lining was applied directly to the membrane."
Similar to the Chekka Tunnel project in Lebanon, the Førøyakonsortiet crews were trained to use the Meyco Logica Potenza spraying robot. After scanning the profile, a 3m wide section of tunnel was covered with a precise 3mm coating of Masterseal membrane. Monitoring the membrane powder consumption and taking periodic measurements by rescanning the profile and comparing the difference between the first and second scan results ensured quality control of a 3mm layer to achieve a minimum 2mm thick membrane.
Of the 40,000m2 of crown profile that required waterproofing in the 6.2km long tunnel, 50% or 20,000m2 was sealed with the sprayed-on waterproofing membrane. Total water inflow being pumped from the now operating undersea tunnel is between 100-150Lt/min/km, "which is cost effective for us for the long term", said Lamhauge. "We are very pleased with the spray-on membrane system. It is cost effective; it is a chemical polymer which ensures long-term durability; it presents no fire-life safety risk to tunnel users; the original tunnel support is not hidden and so it too can be monitored continuously through the design life of the tunnel; and any cracks or leaks that might occur in the waterproofing system can be identified without delay and readily repaired."
The sprayable waterproofing membrane concept was first introduced by the UGC division of BASF about 10 years ago and was originally supplied as a water-based material with the consistency of a gloss paint. It is a chemical polymer product that is non-toxic, non-flammable, and environmentally benign. In its liquid form the material was used on several projects including the Collombey road tunnel in Switzerland, the Bergen rail tunnel rehabilitation project in New Jersey, USA; the Zapata and Lo Prado highways tunnels in Chile; and the Machadinho hydropower project in Brazil.
Masterseal345 was used to waterproof the complex junctions on the Prague Metro
While many of the advantages of the system were realised on these projects, there were issues including running of the liquid based material if applied too thickly and the sensitivity to ambient conditions such as humidity and temperature both during application and in storage. An independent assessment of the system by UK consulting engineers Mott MacDonald focused on the engineering criteria and guidelines for effective application of the system including guidelines for the minimum quality and finish of the primary lining or substrate against which the membrane is to be sprayed and the methods of applying the material under different ambient conditions. The University of Innsbruck also published a test report on the concept.
From these and early field experiences, UGC developed the next generation for release in 2004, which improved various material properties and converted the liquid-based supply to a powder supply - introducing water at the nozzle - a technique well developed and established by the dry shotcreting procress. In its dry form, Masterseal has been applied to waterproof cross passages and utility enlargements on the Prague metro extension and the current Budapest Metro Line 4 construction, as well as on the Lausanne Metro in Switzerland and the Chekka and Nordøy Tunnels in Lebanon and the Faroe Islands respectively.
On many projects, preparation of the substrate and quality of workmanship has failed to maximise the properties and advantages of the concept. For the San Diego LRT Trolley Tunnel in California, the contractor failed to achieve a suitable shotcrete substrate without applying an extra smoothing layer, which was not in the contract budget. An alternative waterproofing system was therefore applied.
On the Wolf Creek highway tunnel project in Colorado, a contractor with limited previous tunnelling experience was contracted to apply only the final lining. As such, the lining contractor had no control over the quality of the substrate of rockbolts and primary support shotcrete as installed by the project's Part 1 excavation contractor. Preparation of the surface for application of the membrane incurred significant time and cost overruns, claims for which eventually went to arbitration.
The ruling found in favour of the contractor, the court awarding extra payment for application of the sprayed-on waterproofing membrane. The ruling found no fault with the membrane concept itself and the tunnel (now in operation) remains waterproofed with the Masterseal sprayed-on membrane as part of its composite-shell lining.
