1. Task definition

The federal highway B 10 Karlsruhe-Stuttgart connects the metropolitan areas of Karlsruhe and Pforzheim. Among others, it crosses the district of Karlsruhe - Grötzingen.

A traffic analysis in 1990 showed that the through road of Grötzingen experiences a traffic load of 28,000 vehicles/24 h, with heavy goods traffic between 9 and 12%.

Due to the traffic situation, several inner-city variants were investigated and a tunnel in a low-lying position in the town of Grötzingen directly south of the Karlsruhe - Pforzheim railroad line was approved for planning.

In the course of the altogether 1,738 m long construction measure, the 910 m long road tunnel, which is to be constructed in the groundwater using the cut-and-cover method, as well as the adjoining groundwater sumps with a length of approx. 58 m in the west and 140 m in the east are located.

Taking into account the constraint points:

• connection of the existing 4-lane extension of the B 10 i n the west
• existing retaining wall next to the B 10 in the Augustenberg area
• tracks of the federal railway line Karlsruhe - Pforzheim
• connection to the existing B 10 in the east

the following route was taken from west to east.

The tunnel initially runs in a left-hand curve with a radius of R = 800 m, which changes to a radius of R = 500 m. The tunnel is then connected to the existing B 10 line in the area of Augustenberg. After that, the axis is determined by the roughly parallel Karlsruhe - Pforzheim railroad line, which has a minimum distance of 5.45 m from the outer wall of the tunnel to the track axis. In the area of the ventilation centers, the distance is reduced to 3.75 m. The radius R = 2,100 m in the left-hand curve is followed by a straight line until the end of the tunnel.

The gradient is determined by the constraints of crossing under the Kirchstrasse railroad underpass, sufficient cover of the ventilation centers located on the tunnel and maintenance of the Oberaustraße municipal sewer, so that the trough position coming from the west initially has a gradient of 6% to the low point under Kirchstrasse. Towards the east, the gradient rises continuously at 0.75% over a length of 729 m before it is brought to the level of the existing B 10 with a gradient of 5% in the eastern portal area. The rounding radii of the trough are 1,500 m in the west and 2,500 m in the east.

The standard cross-section in the tunnel is a 12t rectangular cross-section. The total carriageway width is 7.50m plus two emergency walkways, each 1.0m wide, which are separated from the carriageway by a 15cm high curb, resulting in a clear width of 9.50m. The clear height of the driving space is 4.50 m plus the space required for the operational equipment such as lighting of 0.40 m, resulting in a clear ceiling height of 4.91m in the tunnel axis above the roadway.

The standard cross-section in the trough/ramp area is identical to that of the tunnel cross-section in terms of carriageway width and width of the emergency walkways.

The breakdown bay cross-section was widened by 2.50 m on both sides so that the clear width of the structure is 14.5 m here. In the area of the catch basin, the cross-section was widened by 2.50 m on one side.

2. Structural design

2.1 Geological conditions

To investigate the geological and hydrological conditions, 26 boreholes, 10 of which were developed as gauges, were sunk along the route.

Based on the results of the boreholes, the ground is divided into three layers:

• Below the ground surface, there is fill soil and a cohesive surface layer of sandy silts and clays of varying thickness.
• The middle layer contains talc of varying thickness, interspersed with stones and partly with more or less silt.
• The lowest layer consists of the bedrock of the Buntsandstein, which consists of fine-grained clayey sandstones with partly stronger fissuring.

The tunnel floor lies up to a maximum of approx. 12.5 m below the ground surface and binds almost along its entire length into the bedrock, which consists of uniform to partially bedded sandstone with intercalated clay layers of low thickness. The excavation floor lies mainly in the area of the compact rock.

In the area of the tunnel portals and the adjoining groundwater sumps, the excavation base lies partly in the area of the cohesive overburden or in the area of the talus. In this area, therefore, a soil replacement in a thickness of about 1 m was carried out for the foundation of the structure.

Over its entire length, the tunnel cuts into a contiguous groundwater table, which is between 3.0 m and 5.0 m below ground level.

In rainy years, the groundwater can rise to ground level, so that a fluctuation range of up to 4 m must be assumed. The direction of groundwater flow is perpendicular to the tunnel structure.

For the cohesive gravels, the water permeability was about 10-3 to 10-4 m/s, so that a water accumulation of 1.9 to 9.5 l/s per 10 m could be expected.

For the red sandstone area, the water inflow rates in the compact rock were estimated to be low, although larger inflow rates had to be expected from large fissures that were not cut during the exploration.

In the course of the construction work, it turned out that the red sandstone was largely strongly fissured down to below the excavation bottom and thus generally water-bearing.

2.2 Structure design

From a ventilation point of view, the Grötzingen tunnel required three fan control centers, which were arranged above the structure for structural reasons. Fan control center l also includes the operations center, which houses all the equipment needed to operate the tunnel.

In the tunnel area, technical equipment such as ventilation, drainage, escape facilities. Warning and lighting equipment, etc., several special structures and civil engineering details were necessary. These include:

• hoist at both ends of the tunnel
• a reservoir with a capacity of around 72 m3 at the tunnel low point
• inspection shafts for maintenance and cleaning of the groundwater communication system
• a double-sided holding bay below the fan control center and a shortened breakdown bay on the south side above the reservoir, approximately in the middle of the tunnel
• an integrated emergency entrance in each fan control center, so that three escape stairs are available, in which the entrances to the control centers are arranged at the same time
• one above-ground chimney per control center, through which fresh air is drawn in emergency and installation niches
• niches in the tunnel ceiling for the ventilation systems.

Since the tunnel was constructed using the open cut method, the cross-section was designed as a rectangular section.

The dimensions of the walls, slabs and floor were mainly determined by the uplift safety, the earth pressure and the backfill. The buoyancy safety was verified for a groundwater level up to the top of the ground.

The average thickness of the invert slab in the standard cross-section and in ventilation centers l and III is 1.25 m, while the invert thickness had to be increased to 1.55 m in the area of the double-sided breakdown bay of ventilation center II.

The exterior walls in the tunnel and upper floors were designed with a thickness of 0.8 m to 1.0 m. The slab between the tunnel and upper floors had to be increased to an average thickness of 1.55 m. The slab between the tunnel and upper floors was adjusted to the height of the external formwork in the area of the control centers and amounts to 1.04 m.

Within the individual construction docks, the tunnel was constructed in 10 m long blocks. Each block was made as an impermeable concrete structure of B 35. The tunnel frame of the standard cross-section was concreted monolithically in a cast to prevent splitting and shrinkage cracks.

For the curing of the concrete, a 10 m long curing carriage with a heat-insulating skin was added analogous to the block length.

The joints were designed as space joints with an internal elastomeric waterstop with steel straps and post-injection capability. In addition, an exterior elastomer joint tape was installed. On the inside of the tunnel, the space joint was executed with a joint closure strip.

To avoid different settlements, the tunnel blocks were doweled to each other with 1.0 m long dowels.

For the west tunnel portal, a steel structure with five 3-joint frames mounted on the trough wall and a trapezoidal sheet metal cover as a gallery was chosen, which was planted with climbing plants.

In the east portal, the frame construction of the structure was retained.

The walls of the trough sections as well as the walls of the tunnel entrance areas were clad over a length of 22 m with a highly absorbent brick masonry for noise protection reasons, which was inserted into the concrete structure in a notch.

In the groundwater sumps, a waterproofing layer was applied to the concrete in accordance with ZTV-BEL B2. The pavement structure over the waterproofing is a total of 12 cm asphalt concrete including protective layer.

In the tunnel, the pavement structure was built up with 40 cm frost protection layer, 14 cm bituminous base course, 8 cm binder course and 4 cm surface course.

2.3 Drainage

The wastewater generated in the tunnel during firefighting and cleaning operations, as well as rainwater brought in by vehicles, will be collected in a hollow shelf channel and fed to the longitudinal tunnel drainage system via inspection shafts with wet siphoning approx. every 50 m. The water will be collected in the low point of the tunnel at km 0 + 847.25 in a reservoir with a collection volume of 85 m3 .

The water is collected at the tunnel's low point at km 0 + 847.25 in a reservoir with a collection volume of 85 m3 .

Drainage is provided via a suction pipe in the control shaft to the ground surface. Depending on the degree of contamination, the water is either disposed of via pump trucks or fed into the wastewater sewer in Kirchstraße.

Surface water generated in the trough sections and in the tunnel entrance area is transferred to the submersible pumps located in the portal areas to the foul water sewers.

2.4 Operating facilities, equipment

The equipment required for the tunnel's electrical supply and ventilation control system is housed in fan control center l, which also serves as the operations center. The ventilation control center l is located underground in the main area of the tunnel above the driving floor. Access is from the tunnel via the emergency stairwell and above ground via Kirchstraße.

Rooms for transformer, battery, 20-KV, NSP and ventilation control are provided in the operations control center.

The entire tunnel operation is automatically monitored and remotely controlled via a digital I&C system, with self-sufficient on-site controls installed for all operations.

Power will be supplied from EnBW's 20 KV medium-voltage grid. The transfer station including transformer is located in the fan center l. An additional spur feed is provided as a reserve.

In the event of a power supply failure, a battery-powered uninterruptible power supply (UPS) designed for a period of one hour is arranged for all safety systems, the fire alarm system, the information lights, the measurement and control systems and the night-time passage lighting.

The tunnel is illuminated with high-pressure sodium lamps in a single-row arrangement. For maintenance reasons, these are arranged off-center to the tunnel axis. The adaptation section is provided with counter-beam lighting, the inner section with mixed contrast lighting.

A semi-transverse ventilation system with spot fresh air supply is planned as the ventilation system.

For this ventilation system, three fan control centers will be distributed throughout the tunnel above the driving floor and each equipped with two axial fans. In the intermediate ceiling between the fan control center and the driving floor, each fan control center has a ceiling slot for the punctual supply of fresh air.

In the event of a fire in the tunnel, the direction of rotation of the axial fans is reversed in the nearest ventilation center, and the smoke is extracted via the ceiling slot and conducted outdoors via the exhaust shaft to the ventilation center.

A maximum of two fan stations were required for smoke extraction, so that fresh air can always be blown into the tunnel via the third. The fans are each designed for 25 m3/s of supply or exhaust air.

A total of six emergency telephones are installed in niches in the tunnel. Two dry fire extinguishers are located in each of the niches, as well as a hydrant connection for the fire department and a fire alarm system.

Fire is detected in the tunnel by an automatic line fire detection system. A dry line is installed in the tunnel for fire fighting. Filling of the hydrant line via the connection line in fan center l is automatically initiated by the fire alarm system.

3. Construction sequence

3.1 Excavation pit shoring

Anchored, watertight sheet pile shoring was provided as excavation pit shoring, which could be pulled out again after backfilling. A gravel pile wall with a cased borehole had to be constructed before the sheet pile base was backfilled, since coarse debris in the gravel and in the cohesive surface layers above could cause problems during backfilling. Furthermore, the base of the sheet pile wall had to be tightly integrated into the rock. To ensure the tightness of the sheet pile base, an additional foot injection was provided.

The loosening boreholes with auger drilling equipment ordered instead of the gravel pile walls for the installation of the sheet piles were technically feasible, but cohesive fine material got stuck on the auger flights and was conveyed upwards, so that the conveyed soil quantities of approx. 0.5 m3 per borehole could have led to subsidence in the immediate railroad area or in the area of the development.

Excavation into the red sandstone was only possible to a depth of approx. 10 cm. When the excavation pit was dug, the rock proved to be very brittle and cracked, so that the base support of the sheet pile walls broke off in places.

Due to the strong fissuring of the red sandstone, considerably more water penetrated the excavation pit. The planned sinking and seepage wells were largely ineffective, since the inflowing water mainly ran in the deep-lying fissures. As a result, the shoring concept had to be modified.

First, in accordance with the official design, gravel pile walls were again constructed by cased boreholes, into which the sheet piles were vibrated. As far as technically possible, the sheet pile wall axis was shifted outward by 50 cm to ensure the rock support of the sheet pile wall base.

As a next step, in order to reduce water leakage from the fissures in the red sandstone, the sheet pile walls (sheet pile and bored pile walls) were moved to 30 cm below the excavation pit.

During excavation, however, it became apparent that the interconnected fissure water system was now forcing the water over the bottom into the excavation pit, so that the shoring system could be changed to a short-long system. Here, the reinforced piles were placed 30 cm below the excavation pit, while the unreinforced piles were embedded 1.0 m into the rock. This saved considerable additional costs with slightly higher water ingress.

The sinking and seepage wells were not continued. Despite the accumulating water of 60-80 1/s on average, which rose to over 100 1/s in some areas, the predicted total water withdrawal volume from wells and excavation pits was not exceeded.

For the discharge of the groundwater and rainwater accumulating in the excavation pit, only a second settling basin had to be constructed and additional gauges set in line with the construction progress.

In areas of directly adjacent buildings and directly next to the railroad line, an overcut, tie-back bored pile wall was provided, which was partially inclined up to 10:1 against the tunnel due to the proximity of the buildings and the railroad tracks. The embedment in the rock was 1.0 m.

The area below the sheet pile and bored pile wall in the red sandstone area was sloped vertically and secured with wire mesh, shotcrete and rock nails. The groundwater flowing in through fissures was diverted via hoses.

3.2 Dewatering

To limit the groundwater impoundment, the maximum shoring length was limited to 200 m.

According to the hydrogeological report, the inflow rate for a 200m construction dock was up to 1 42.5 l/s. Therefore, on the upstream side, drawdown wells were arranged at 10m intervals for groundwater communication, with alternating drawdown and observation wells. On the downstream side, to prevent the groundwater from sinking, seepage wells were arranged at 10m intervals.

The excess pumped water, which could not be returned to the groundwater via seepage wells, was fed to the Pfinz River via a pressure line.

The residual water in the excavation pit, which pressed through the shoring and out of the bottom, and the precipitation water were also fed to the Pfinz via settling basins and CO neutralization.

3.3 Construction sequence

In order to ensure a continuous construction progress, the construction docks with a maximum length of 210 m were divided into two individual docks each, so that the construction measures comprised a total of 20 individual docks.

Due to the construction of the Kirchstraße underpass, the construction work started at Block 22 (Kirchstraße area). From there, the tunnels and trough blocks were constructed to the west.

After completion of the western area, the tunnel was constructed in an easterly direction from Block 27. The special area of fan center l and reservoir was initially left out. Parallel to fan control centers II and III, the tunnel entrance area east dock 17 and 18 was constructed.

The last construction phase was the fan control center l Dock 11/12. At the same time, the blocks of the east groundwater basin (Dock 19 and 20) were constructed.

After the shoring was constructed over two individual docks, the excavation of the excavation pit, the shell of the tunnel, the groundwater communication system, the backfilling of the excavation pit and the removal of the shoring were completed. For this reason, a concrete transverse bulkhead was placed in each dock before the shoring was pulled, preventing groundwater inflow from the completed section to the next excavation.

Only after this transverse bulkhead had become effective could the shoring be pulled and the shoring for the next individual dock be started.

 

 

• Country: Germany
• Region: Baden-Württemberg
• Tunnel utilization: Traffic
• Type of utilization: Road tunnel
• Client: Regierungspräsidium Karlsruhe
• Consulting Engineer: Bung Beratende Ingenieure GmbH, Gackstatter und Partner, SHB Schindler Haerter AG
• Contractor: Wolff & Müller GmbH & Co. KG
• Main construction method: Open
• Type of excavation: cut-and-cover
• Lining: In-situ concrete
• No. of tubes: 1
• Tunnel total length: tunnel 9,100m, trough western section 57.55 m, trough eastern section 140.0 m
• Contract Volume: Roughwork approx. 53.5 mill. DM
• Construction start/end: 1997 till 1999
• Opening: 1999