Drainage northern part of Zug, main canal to the lake & circulago

Go Bau AG exercises the function of the overall project manager and the chief building director in the mandate for BG engineers + consultants AG for the design and implementation of both projects for the realization. Marc Reinhard is the direct contact for both representatives of clients (urban drainage departement of Zug and Wasserwerke Zug AG). He leads the design team at BG engineers and consultants AG and manages the regular planning and building sessions in both projects.

Description of the project:

The City of Zug is located on the shores of the Lake of Zug, 20 miles south of the City of Zurich, Switzerland. With an average growth of about 1.2 % per year, the City’s population increased from 22’300 people in 1995 to 30’000 in 2017. In the mid 90’s, during heavy rainfalls, sewage water was still discharged at 27 locations around the city into the Lake of Zug due to the limited capacity of the combined wastewater sewer and the treatment plant. In 1995, the City Council decided to switch to a separated wastewater and surface water system to minimize the discharge of untreated wastewater into the Lake. Up to date, more than 2/3 of the sewer system has been switched to the new system and now, the main discharge line for the surface water of the northern part of the City is being built (blue line, situation plan in Figure 1).

The main discharge line comprises a total of 3 microtunnels consisting of reinforced concrete pipes of 4 m (13.1 ft) length with an inner diameter of 2’000 mm (78.7 in) and an outer diameter of 2’680 mm (105.5 in). Drive 1, as indicated in Figure 1, has a total length of 357.5 m (1173 ft), drive 3 of 589 m (1932 ft) and drive 5 of 882 m (2894 ft). Since it is a gravity system, the line is being built with a gradient towards the lake. Since drive 5 will be built by mining through the intersection shaft at drive 3, the first portion of drive 5 will be mined downwards to the intersection shaft, where a vertical curve brings it to an ascending slope to account for the changing flow direction (see Figure 2). The water outlet at the end of drive 1 (which is just on the shore of the lake) will be built separately.

Figure 1: Situation plan.
Black arrows indicate jacking direction. Blue: surface water main discharge line with flow direction.
Green: Circulago line

circulago WWZ

Apart from the City’s project to separate wastewater and surface water, the local energy company WWZ (Wasserwerke Zug) is realizing the “Circulago” project. In 2011, the City’s voters agreed to the popular petition “2000 Watt für Zug”, which demands the City’s public organizations to force the use of renewable energy, to reduce the maximum energy consumption to 2000 Watts and the carbon-dioxide emission to one ton per inhabitant per year on a long-term schedule. The “Circulago” project intends to use the lake’s water to supply green energy for heating and cooling purposes as a consequence of the new energy strategy.

A water intake 400 m (1312 ft) offshore will convey lake water to an underground heat exchange plant on the shore of the lake, where the lake’s water temperature will be transferred via a separate line to the power stations in the city. Heat pumps then produce heating / cooling energy and distribute it via a separate network to the consumers. The main lines between heat exchange plant and power stations will be built in a 2-pass procedure, with the first pass being two reinforced concrete microtunnels of 1600 mm (63 in) inner diameter and 1960 mm (77 in) outer diameter with pipe lengths of 3 m (9.8 ft) for drive 2 and 4 m (13.1 ft) for drive 4. The situation plan in Figure 1 gives an overview (green lines).

Drive 2 starts parallel to drive 1, crosses over drive 1 at the bottleneck between the piled abutments of a railway overpass and then runs parallel again to the receiving pit on the lake shore. The total length of drive 2 is 356.6 m (1170 ft) and the distance between drive 2 and drive 1, which is drilled first, goes down to 84 cm (2 ¾ ft). Drive 4 with a total length of 590 m (1936 ft) will be drilled after drive 3 and runs on the left side parallel to drive 3 in a minimum distance of 1.5 m (5 ft).


Simultaneous execution of the two projects

Many factors had to come together to simultaneously execute both projects, the first one being that the designer of the surface water discharge line was also assigned with the preliminary design studies for the Circulago project. However, it only became feasible due to a major change of the surface water discharge line’s alignment that took place after the contractor had already been assigned, which brought it to close proximity to the Circulago line. The alignment change was required due to a third party’s refusal of the formerly promised permits.

Thanks to the simultaneous execution, a lot of resources of any kind can be saved. For example, only one drive shaft is required for drives 1 to 4, only one microtunnel installation is required, overall construction time is reduced and importantly, it’s also easier to get the required permits for one bigger project than for two separate projects taking place within a short time frame.

Geology in the project perimeter is governed by alluvial soils. The top soil layer mainly consists of artificial fill (silty sand, some coarse material, gravel and construction rubble), under which a layer of rubble from the river Lorze with a thickness of up to 4 m (13 ft) can be found. Below that, younger and older lake and delta deposits are found, mainly consisting of sand with layers of silt and some organic material. The tunnels are between 3 and 8 m (10 – 26 ft) below ground and therefore mostly in the layer of older lake and delta deposits. Groundwater level is between 1 and 4 m (3 – 13 ft) above the tunnel crown.

The project is in the heart of a City close to a lake. Drive 1 and 2 cross a railway overpass, which is founded on piled abutments. Remaining piles from former buildings are expected. Also, any open excavation in the area requires water tight excavation supports, either strutted or tied back. Therefore, plenty of remaining strand anchors are known in the area and the alignment has to account for them.