Lot NL 35 of the ‘Norra Laenken’ project in Stockholm forms part of the northern link of the City’s inner ring road. The project comprises the construction of approximately 2.6km of tunnels in hard rock with features specific to that of tunnel construction in Sweden. These include large cross sections, pre-injection grouting of the rock and tunnel advance lengths up to 6m during drill and blast excavation works. In addition, a new concept for the tunnel inner lining structure has been utilised on this project which comprises the use of precast elements, a suspended shell structure and a shotcrete lining. The primary function of the inner lining is to protect the traf_ c against ice, dripping water, fire and explosion.
INTRODUCTION
A dramatically increasing amount of traffic in the city of Stockholm in the mid-20th century led to a plan for a ring road around Stockholm’s inner city, to relieve the centre from most traffic. The first section of this ring road, called ‘Essingeleden’, was completed in the late 1960s and runs through the area’s western suburbs.
Further enhancements were opened for traffic only half a century later with a first section of the northern link – Norra Länken – followed by the southern link – Södra Länken.
The implementation of the remaining part of Norra Länken, scheduled for completion in 2015, will connect two major motorways coming from the north as well as two important ports in the northeast of the city with the already existing parts of the ring road.
As most of the 5km long ‘Norra Laenken’ will lead through areas with existing buildings, as well as the world’s first urban national park, the majority of Norra Länken will be underground. The excavation of in total about 11km of tunnel has mainly already been carried out in 10 major construction lots.
Figure 1 illustrates an overview of the entire awarded ‘Norra Länken’ project. From the entire 10 main construction lots the Hochtief Solutions was together with its JV partner Oden awarded with parallel main tunnels as well as two approximately 200m long connecting tunnels. Exemplarily a typical tunnel cross section is illustrated in figure 2, overleaf.
A new concept for the tunnel inner lining structure has been utilised on this project which comprises the use of precast elements, a suspended shell structure and also a sprayed concrete lining. The primary function of the inner lining is to protect the traffic against ice, dripping water, fire and the foce of an explosion.
EXCAVATION AND SUPPORT WORKS DURING DRILL AND BLAST TUNNELLING
The resulting total tunnel length of approximately 2,600m has been excavated by using conventional drilling and blasting methods. The cross section of the tunnels varied between approximately 80 to 340m2, with an average of 160m2, while the stroke (advance) length remained constant at about 6m during excavation.
Steel fibre reinforced shotcrete and rock bolts have been used in various thicknesses (0 to150mm) and lengths (4 to 6m) depending on the rock conditions to support the rock around the excavated tunnels.
As usual for hard rock conditions in Scandinavia the Q-Value classification system from Grimstad/Barton has been used to determine the appropriate support measures (i.e., shotcrete thickness, bolt length, spacing).
the construction contracts NL 35 and NL 51 in the eastern project area. The lot NL 35 has a total length of about 1,100m and consists of two The tunnel heading works were in the regular tunnel cross sections executed with full face heading. The advance lengths of up to 6m lead to very high advance rates for conventional tunnelling techniques. On the other hand this leads to irregularities during blasting such as over excavation profiles and enormous overbreak especially in case of unfavourable dipping of the rock joints.
The tunnels are located within the Stockholm granite and granite gneiss which is characterised by vertical dipping joints/ fissures and horizontal layering. Nevertheless, due to the suspended inner lining system, the resulting overbreak does not have to be compensated and backfilled with shotcrete.
In order to achieve the demands concerning the low groundwater inflow (underpassing of an urban national park) into the tunnels (about 4 litres/min per 100m of tunnel length), overlapping grout curtains were positioned around the tunnels prior excavation. Each curtain had a length of about 25m and an overlapping length of 7m.
The actual injection has been carried out by using microfine cementitious grout and a maximum pressure of 50 bar. Lugeon tests were carried out in controlled drillings through the temporary face to confirm the success of the grouting works.
Design and works preparation of the inner lining system
The primary function of the inner lining system is to protect the traffic against ice, dripping water and fire. Additionally it provides the facing of the tunnels traffic space. The tunnel lining system consists of precast concrete wall elements that are placed on both sides of the road and the suspended shell structure installed in the tunnel crown between these walls.
The shell is formed by a combination of a tunnel sealing membrane, suspended with a minimum distance of 500mm to the rock, and a 100mm thick shotcrete lining that is applied in a second step towards this membrane.
The main advantage of the suspended lining system is that in long frost periods in winter time no blasting development due to pressure can be formed because a stressless ice layer can be developed between the rock surface and the lining. A minimum distance of 500mm was considered within the design and construction.
In the past major damages in existing road and railway tunnels in Sweden occurred due to ice pressure. This led to additional remedial works to be executed associated with additional costs.
The novel inner lining system was applied the first time, therefore innovative, economic and practical solutions amongst others for design details were required during the design and works preparation phase.
A close cooperation and early continuous communication between client, JV, designer and membrane supplier was a key element for the success of the project. In the calculations for the inner lining and precast concrete elements the following load cases were taken into account:
¦ Dead load of membrane and lining
¦ Creep and shrinkage
¦ Loads resulting from ice formation
¦ Temperature loads (+20 °C to -15 °C)
¦ Pressure and suction loads caused by traffic
¦ Falling stones
¦ Impact load on vertical walls caused by traffic
¦ Exceptional accidental loads
¦ Loss of a rock bolt
¦ Explosion
¦ Fire
A practical full scale load test for the lining system for proof of the load case explosion was developed and executed which demonstrated the omission of a collapse of the lining (‘falling down’) in case of explosion within tunnel.
ConstruCtion sequenCe for the inner lining
The construction sequence was broken into eight stages:
Foundation for ventilation fans
The foundations for the ventilation fans were planned in the contract as two single foundations for a pair of fans as in-situ concrete.
To optimise the installation process the foundations were re-designed as one large pre-cast block, which holds the pair of fans. The foundations are hoisted up to the ceiling with the help of chain blocks and fixed to an already levelled ceiling and installed rock bolts. The gap was subsequently contact grouted. The installation takes only half the time of conventionally cast foundations and the tunnel is less blocked because of it. Six pieces of acid resistant stainless-steel rock bolts M24 are used per foundation.
Concrete curbs and side-strip foundations
The outline for the side strips is formed with concrete curbs, which are acting as ‘kickers’ for the wall foot. The curbs are produced by a diesel driven curb former (commonly used in the US or in Americas for road construction or e.g. CFRF dam concrete blinding).
The curb shape is custom made to take into account the different slopes of the road in the tunnel and to be installed on the same filling layer and supporting the wall foot. An installation performance of 120m per shift has been achieved. Once the curb is installed the side strips can be completed. The side strip for the culvert is used as a walkway. To lead the forces from the walls into the rock, unreinforced beams are casted between rock and curb with a common distance of 3m at the wall joints. In between the beams, 0-4mm graded crushed rock is filled and compacted to form the walkway. Behind the culvert membrane the water is able to drain into the tunnel invert.
On the opposite ‘rock’-side, the relative small distance between curb and rock is filled with drainage concrete which allows for transferring loads and draining of water.
Drilling and installing rock bolts
A drill rig with a twin drill boom (c/c 1.20 for the main pattern of bolts) is used.
The double drill on the machine is mounted on an 8m long rail allowing longitudinal movement of rock bolts. The arm for the double drill moves in radial direction. The positioning is computer aided and the lining outline and bolt positions are programmed.
Once the rig is set up and the laser positioned it finds the hole positions with the help of the system. Furthermore the distance from the lining outline to the actual rock surface for each hole is recorded. The drill depths are then suited to bolt lengths staggering 200mm allowing for the pre-defined minimum drill depth of 600mm.
An installation list is produced which indicates the appropriate rock bolt length for each hole position. This method simplifies the installation process and furthermore avoids the cutting of bolts, which are double corrosion protected (hot-dip galvanised and epoxy coated). The system enables drilling within a tolerance of plus or minus 50mm.
Therefore the wall bolts need to be drilled ‘manually’ with surveyor assistance to achieve greater accuracy. The suspensed sections for the wall elements only allow for a tolerance of plus or minus 30mm.
The bolt installation is carried out from a truck mounted mobile platform with a size of 3m by 5m.
Cement grout is used (w/c ratio continuously checked by mud-balance method) to ensure additional corrosion protection. Spacers ensure that the bolt centres in the hole. Any epoxy type resin was not allowed due to a design life of 80 years.
The drilling performance for membrane bolts was 500 holes per working shift, and bolt installation performance was about 400 bolts per working shift.
Membrane installation
The 1.5mm strong LDPE membrane had to fulfill defined features from Trafikverket regarding fire resistance. Certain flashover time and smoke mass were defined.
The installation is carried out from a purpose made working platform, which enables workers to roll down the membrane and tighten it in radial and longitudinal direction to get a proper base. This is important for the sprayed concrete application.
The radial moving arm equipped with platform and membrane roll dispenser is sitting on a four-leg construction. The frame is driven on caterpillars and the legs can be adjusted in height to allow traffic to pass underneath.
Usually the membrane is installed in 3.75m wide lanes (250mm welding overlap) starting at the rock side. The rolls are pre-confectioned with a certain length suited for the tunnel sections minimising the cutting waste. The installation performance was 900m2 per shift.
Fixation parts for wall elements
As Trafikverket needs to access any pipes or drains lying in the tunnel invert without influencing the lining construction, the walls are hung from the tunnel ceiling rather than based on foundations. Therefore suspension frames had to be developed that are able to take up the tolerances from the bolt installation.
For the vertical fixation gallowlike hangers with slotted holes and a turnable head plate with a slotted hole were developed. To fix the hangers to the wall the slotted holes allow for longitudinal tolerances. The turning plate to fix the hanger to the vertical bolt allows for combined latitudinal and longitudinal adjustments.
For horizontal fixation a plate with big bores and suitable washers was designed to adjust the walls.
A turnbuckle type adapter between fixing plate and horizontal bolt makes it possible to do adjustments in a latitudinal direction.
Wall installation
The wall elements are delivered to the site on racks on special trailers. To get a good performance for mounting the wall elements an installation system consisting of a 24t tunnel excavator equipped with a fully articulated vacuum plate has been developed. The challenge here was the rough surface which had to match the sprayed concrete surface of the ceiling.
Whereas the roughness is not a problem for a vacuum system with regard to holding the sucking force as long as the pump is running, it is more difficult to hold the vacuum if the pump fails for any reason while it is lifting an element. For this case a safety period of 20 minutes to either secure the area and/or mechanically fix the wall element was agreed upon.
Coordination of the wall producer as well as the vacuum plate manufacturer was necessary. The solution was a combination of a homogenous formed rough surface by means of a form inlay and special sealing rubber on the vacuum plates.
Before lifting and installing the walls they are equipped with an EPDM rubber to seal the element’s joints. The EPDM sealing fulfills the necessary fire resistance proven by Hochtief’s earlier research on EPDM sealed segmental lining (tubbings) for TBM tunnels carried out at the University of Brunswick. The walls are brought into rough position by one operator in the machine’s cabin. A second operator with a remote control for the vacuum plate carries out the final adjustment and placement. The wall is now fixed to the rock bolts via the fixation parts.
Reinforcement mesh installation
Before installing the reinforcement mesh the membrane needs to be prepared for the sprayed concrete with all built in parts. All necessary cable ducts are installed.
The dilatation joints every 22 to 30m need to be fire resistant and are prepared with a fire resistant cloth and a PE profile to get a controlled crack.
Now the mesh (Ø6mm at 150mm) is installed. The mesh for the elliptical sides is custom made and pre bent, the middle part is standard mesh.
The installation is carried out by a truck mounted crane and a working platform.
Additionally, the mesh is cut into pieces with a maximum weight of 30kg to ensure also manual installation. Most important here are the spacers, which are glued to the membrane and hooked in to the mesh. The result is a sturdy and stiff base necessary for a good result when applying the sprayed concrete.
Sprayed concrete application
A ‘Meyco Potenza Maxima’ shotcrete robot is used. The machine was developed for the Scandinavian market to reach for the long rounds.
It is equipped with a long arm with less articulation features, e.g., a robojet arm, and enables to spray areas of 15m length and 20m width without repositioning the machine. Additionally an automatic movement either in longitudinal direction or circumferential direction can be preset.
The shotcrete is applied in three layers. The first layer, about 10 to 20mm, stabilises the membrane. A lower pressure and flow of shotcrete is used here to reduce rebound as the membrane acts like a drum skin.
When the first layer has hardened (usually by the next day/ shift), the second layer is applied and filled behind the mesh. Now the concrete spraying works ‘as usual’, since the first layer gives a sturdy base.
The final layer (mesh to final outline) can be applied after four to five hours. The average spraying performance was 5m³ per hour. The final contour is checked by 3D laser scanning.
SUMMARY AND OUTLOOK
Typical techniques, equipment and materials from conventional tunnelling (NATM/SEM) and mechanised tunnelling have been used such as:
¦¦Rock bolts, shotcrete, grouting
¦¦Sprayed concrete units, rock drills, working platforms, excavator, vacuum system
A combination of two mechanised construction operations for future tunnel projects in hard rock conditions could be possible by application on a gantry system or after runner of a hard rock gripper TBM.
Also potential application of the developed lining system is under consideration of the client Trafikverket for future tunnel projects in Sweden.
The system at NL 35 was built within budget and planned time frame with a very satisfying quality product for the client – also because of our main modification to use precast walls on both sides.
