To guarantee the reliability of the its water supply, the Munich City Works began replacing its exiting supply pipes with a deep pressure water pipe system in bored tunnels in the early 1990’s.
German contractor, Bilfinger Berger has been continuously engaged in the construction of a 30km long, four tunnel system that draws on the vast water resources found in the Mangrall Area, 35km south of the city (Figure 1). To date, three of the four tunnels, totalling 11.8km, have been completed, namely the 5.4km Mangfall tunnel, the 4km Muehthal tunnel, and the 2.4km Hachinger tunnel.
All of the tunnels have a primary pre-cast concrete segmental lining with an i.d. of 2.9m and a 2.2m i.d. steel pipe inner lining, with the void between the pipe and the segments filled with mortar (Figure 2).
The completed tunnels have already been commissioned and put into service using an existing aqueduct to by-pass the unfinished length.
The Munich City Works officially let the approximate US$50M Bill of Quantities type contract for this final and longest section, namely the 17.4km long Hofoldinger Tunnel, to Bilfinger Berger in the summer of 2002.
Tunnelling began in September 2002 and as of February 2004, some 40% of the tunnel had been driven, keeping the contractor well on target to complete in the 50 month tunnelling window. The entire project programme allows for a 65 month construction period, to ensure the new supply system is up and running by the end of 2007.
Geology
The Hofoldinger Tunnel is situated entirely within the Quaternary gravel of the so-called Munich gravel field and is entirely above the groundwater table.
The alignment runs through very mixed ground. In the same horizon very different material can be encountered, from stable hard rock to loose unstable ground. The grounds encountered are loose gravel with sand and silt, boulders and hard conglomerates, which are intermixed in the various formations.
Synsedimentary erosion and dislocated deposits provide for additional disturbances and obliterate earlier boundaries between the layers. In some areas, the coarse-grained series are interrupted by interglacial weathered clayey-silty layers and top soils which have a lateral size of several hundreds of meters.
Tunnelling operations are all being carried out from a central start shaft. With the exception of the ventilation, which has to be continuously moved, the entire supply and mucking activities to and from the drive will be carried out through this shaft.
Currently the 9.43km long southeast leg is being driven at a gradient of 0.15% in the direction of the finish shaft at “Grub” where it will connect to the Mangfall Tunnel. Upon completion, the machine will be transported back to the start shaft and set off on the 8.04km long, 0.23% decline northwest drive to the reception shaft at “Gleissental” where a connection to the Hachinger Gallery Tunnel will be made.
Tunnelling
The very unpredictable distribution of hard and soft ground has set Bilfinger Berger a very exacting tunnelling task. Due to the inhomogeneous nature of the soil, the technology used for the drives has to have a wide range of capabilities for the excavation of rock and soft ground. The driving machine has to be able to:
The decision was made to employ an adjustable shield machine, which can be used (through a change of the driving unit) as a hooded shield with roadheader technology or as a full face TBM with mechanical support of the face. The machine being used comprises available Lovat technology and a newly designed hooded shield from Herrenknecht.
It was possible to utilise the Lovat components used during the previous Mangfall, Muehlthal and Hachinger tunnel drives without modification. With the exception of the cutterhead, the entire driving unit with its middle shield, propulsion jacks, erector unit, shield tail and backup system could be reused. A hooded shield, more suitable for the mixed ground conditions, replaced the cutterhead.
The shield is made up of the hood and machine shields 1 and 2. The hood is connected to Shield 1 by a steerable articulated joint. The hood’s variable face angle can be adjusted by five inbuilt forepoling plates positioned at the front that allows the geometric cutting shape to be matched to the slope angle of the face material when the tunnel face is unstable. In addition, these forepoling plates are equipped with hydraulic breasting plates for partial face support. The base of the hood is funnel shaped enabling feed onto the chain conveyor.
Shield 1 contains the control platform, the excavating tool and its hydraulic power pack. The excavating tool, which also serves as the loading facility for the chain conveyor, is mounted on a sled that allows the front edge of the shield cutter to be reached even when the forepoling plates are extended.
Shield 2 serves as an adaptor to the Lovat system and is the connection to Shield l. This has been developed as a passive joint and is equipped with a device to control the roll of the machine.
There are two means of excavating the face using the open mode shield hood – a rotating (360°) mounted excavation arm with backhoe shovel and hydraulic hammer or, alternatively, a roadheader with an axial cutting head can be installed. For any forward ground treatment, both types of tools can be supplemented with a drilling attachment.
In anticipation of the difficult mixed ground conditions, the drive began using the backhoe excavator. However after a few hundred meters, it was replaced by the roadheader, as the backhoe was prone to problems and the ground conditions looked to be better suited to the roadheader. After the unit was replaced, the performance increased so much that it has not yet been necessary to refit it with a full face cutting head, as was originally planned.
The steering of the shield is being aided by an active target display unit, supplied by VMT, which visually provides the machine operator with the planned actual axis as well as the three dimensional position of the TBM. It also recommends the type of segment ring to be installed. All measurements and machine technical data are transmitted online to the construction office.
Logistics
Transportation of the material is by rail vehicles, hauled by battery operated locomotives.
Muck is discharged into one of four trains, comprising a Manrider, Loco, five X 3.5m3 mine cars, grout car and three segment cars. The muck from each one metre advance fills the five mine cars. All muck cars are side tipped at the shaft by a hydraulic tipping station. Clayton locomotives are the prime movers except for special trains pulled by Belloli Battery Loco’s. The track gauge is 600mm with 18kg/m rails mounted on steel ties set every one meter to a concrete segment.
At the start of the work, excavated material was removed from the start shaft on a continuous vertical conveyor belt system.
Due to technical difficulties, the belt system was replaced by a conventional skip hoisting system. As a temporary solution mine cars were brought to the surface and tipped until the installation of a 15m³ skip. The skip can hold the same volume as a one metre advance given the c.a. 30% bulking factor. It is hoisted and emptied (bottom discharged) by an unmanned, program-controlled cable crane.
Lining
Both the northerly and southerly drives have an excavated diameter of 3.4m and are being lined with fibre reinforced segments creating an i.d. of 2.9m. The 6-part conical segments ring thickness is 180mm, 1m long and has a taper of ± 5mm.
The C45 concrete segments are produced to a design by C.V. Buchan and are reinforced with 40kg/m³ of steel fibre, supplied by Bekaert.
The connection between the segments is formed by forged steel 25mm bolts across the radial joint (two per segment) with each segment circle joint connected by two tapered wooden dowels. The steel fibre reinforced segments have proved very successful, given the thickness of only 180mm.
After the primary lining has been completed, the final lining of the tunnel will be carried out through the previously built intermediary working shaft. For this purpose, a steel lining pipe (DN 2200, wall thickness 17.5mm) will be brought into the tunnel and welded together. The steel pipe will be set into position by a centring device and secured against uplift.
Backfill of the annular space between the segments and the steel pipe will be carried out in accordance with a system protected by patent, developed during the previous projects (Mangfall, Muehlthal and Hachinger Tunnels). The mortar backfilling can be done continuously, along the entire length of the already installed pipe lining, without intermediary partitioning of the void (‘fluffing up’). In this way, installation of the steel pipe and the backfilling are carried out completely independent from each other. At an optimal rate of backfilling, this process allows the void to be filled without creating cavities or the need to proof grout the annulus void.
Finally the inner surface of the pipes will be sealed to protect the quality of the potable water with a drinking water neutral concrete mortar lining.
Shaft construction
There are seven working shafts, up to 30m deep, and a further three boreholes for ventilation, each with a diameter of 1.2m.
The working access shafts have a diameter of 14m and will be used in particular for lowering the steel pipes. The other shafts have a diameter of 8m, with the exception of the two start shafts at Hofoldinger Forst, that are elliptic in shape 22 X 18m and 22m deep.
As there was no water present, other than a little perched water, the construction of the shafts was relatively simple. This was done using the shotcrete method, sinking between one and one and a half meters per advance. The 250mm-500mm thick (depending on geological conditions) shotcrete lining, placed during the shaft sinking, was reinforced with two layers of mesh.
Progress and conclusions
Current progress is within programme, with rates of advance averaging at some 23m/day, and reaching an impressive 30m/day in good geological conditions.
The principles of the design of the machine selected by Bilfinger Berger has so far been confirmed by the successful performance.
During the boring of the Hofoldinger tunnel several notable improvements have been made to the design and construction methodology. The change of the partial face excavation from backhoe to roadheader has lead to a marked increase in production, whilst redesigning the shaft mucking system from vertical conveyor to a skip system has made muck removal considerably more efficient.
Also the redesign of the roadheader head, so as to reduce pick wear down to average pick consumption of one pick per 3m advance, has minimised idle time required for machine maintenance.
The modified hooded shield technology has thus far been able to handle every conceivable tunnel face scenario, whether it be with conglomerates. boulders, clay and silt, non-cohesive gravel or inflowing perched water. By being able to alter the attack of the shield to reflect the angle of internal friction of the soil at the face, this tried and tested technology has been adapted to provide an extremely effective solution.
Of course, there are some disadvantages to the excavation method. The driving performance fluctuates according to the existing ground conditions, so that the number of personnel and the flow of materials require careful coordination. Whilst tunnelling through the conglomerates, close attention has to be paid to the over-cut and steering of the machine, and finally, limitations have definitely been discovered when using this system below the groundwater level.
In conclusion when one uses any type of technology, one should not forget that success depends on the people working with and operating the technology. Without the expertise, motivation, and experience of the workers on-site, even the best technology quickly reaches its limitations.
Related Files
Figure 1 – Plan map of the 30km network of bored water tunnels
Figure 2 – The Hofoldinger’s simplified long and cross section
