Road, rail and subway tunnels by the Reichtag, Berlin

Road, rail and subway tunnels by the Reichtag, Berlin, Germany
Ole Peter Jensen
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ABSTRACT: After reunion of West and East Germany, there has been a great effort to re-connect and upgrade the infrastructure between the two former German States. The project consists of three tunnels, constructed as cut & cover tunnels. The eastern tunnel is a two tracks U-Bahn (Metro) ending in an underground
station close to the new parliament for Germany (Reichstag). The central tunnel contains tracks for the ICErailway, starting with eight tracks at the north end and narrowing down to four tracks at the south end of the
project. The western tunnel is a four-lane highway tunnel. All tunnels were constructed in one single construction pit.
1 GENERAL DESCRIPTON
In 1995 the contractor Spie Batignolles GmbH
together with COWI as consultant won the project
“Projektlos 2 - Spreebogen” for a contract sum of
0.2 billion US$.
COWI was responsible for the detailed design
and planning of the construction pit and the tunnel
structures. Furthermore, for the planning of the diversion and the following re-establishing of the river
Spree. Other temporary works were included in the
design.
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Reichstag (Projektlos - 2). The tunnels then continues in bored tunnels (Projektlos - 3) under the Tiergarten to the new station at Potsdamer Platz.
2 PROJEKTLOS 2 - SPREEBOGEN
The project consists of three tunnels, constructed as
cut and cover tunnels. The eastern tunnel is two
tracks U-Bahn (Metro) ending in an underground
station close to the new German parliament ("Reichstag").
The central tunnel contains tracks for the ICErailway, starting with the possibility for eight tracks
at the north end and narrowing down to four tracks
at the south end of the project.
The western tunnel is a highway tunnel with two
bores, each with two traffic lanes.
Special construction methods were implemented
for construction of the tunnels, as groundwater lowering was not permitted.
The tunnels were therefore constructed in a special construction pit made watertight in order to
minimise the water volumes to be pumped.
3 CONSTRUCTION PIT
The Projektlos 2 is a part of a concept to
strengthen the infrastructure of the “Central Area” in
Berlin with the Lehrte Bahnhof (Projektlos -1) at the
north end of the development zone continued in cut
& cover tunnels under River Spree to the front of the
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The underground in Berlin consists mainly of
sand. In order to avoid lowering of the water table
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under the entire city, an agency was established to
co-ordinate and balance the water volumes that were
pumped out with the water volumes re-injected into
the ground.
The construction pit was divided into four sections surrounded by 1.20 meters thick diaphragm
walls. The diaphragm walls were designed with one
layer of ground anchors at the top of the wall and a
free standing height of up to 20 metres. After construction of the diaphragm walls and installation of
ground anchors the pit was excavated wet.
The underground in Berlin consists mainly of highly
permeable sand layers. Lowering ground water in
order to construct the cut and cover tunnels would
have impact on the water levels over a wide area and
therefore major ground water lowering schemes
were not permitted
Due to the great number of construction sites
working at the same time in Berlin, the city had established a water management company which took
care of the water pumped up and the re-injection of
water to keep a balance of the volumes and levels in
the city.
Excavation of the different parts of the construction pit was done wet excavation. The ground water
level was close to the surface and by excavating a
hole close to the diaphragm wall a basin was created. A small cut and suction dredger was lifted into
the excavated basin from where the dredger starts to
dredge the closed part of the construction pit by
pumping the materials to barges in the river.
At all time during the dredging work, the water
level inside the pit was kept higher than the ground
water level outside the pit.
The maximum allowable volume of water intruding the construction pit, when it was finalized and
empty for water, was 1.5 l/sec per 1,000 m².
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On completing the excavation, vertical tension
piles in the form of H-piles were installed, underwater concrete was placed and the pit was finally dewatered.
The construction pit was 500 meters long and
varied in width from 120 meters to 60 meters. The
pit was divided into four parts, where part E (95 x
120 m) was placed in the river, hereafter followed in
land part H1 (112 x 90 m), H2 (253 x 80 m) and last
part I (40 x 60 m).
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The diaphragm walls were constructed in a traditional way with guide beams and bentonite slurry to
keep the overpressure.
The diaphragm walls have a thickness of 1.20
meters and a length varying from 25 to 30 meters.
The walls were divided into a general panel width of
7 meters.
The diaphragm walls are temporary walls that do
not have any structural function after the constructions of the tunnels are finalized.
The walls are designed for several load cases depending on different construction phases. When the
construction sequence for the different parts of the
construction pit were agreed, the amount of reinforcement in each wall panel were optimized
Along the inner side of the diaphragm wall was
excavated a trench for allowing installation of
ground anchors in the top of the wall. The holes for
the ground anchors are drilled through a preinstalled pipe in the diaphragm wall, fixed to the reinforcement cage. There were placed two anchors in
each reinforcement cage and with two cages in each
diaphragm panel give four anchors per wall panel.
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For construction of the three tunnels under the river
Spree it was necessary temporally to divert the river,
as it can be seen on the Ariel photo, picture 2. The
river diversion made it possible to construct a construction pit for the part of the three tunnels, which
finally will be placed below the river.
A new channel was excavated and bonds were
constructed out in the old channel to allow for construction of diaphragm walls forming the construction pit.
Measures were taken to protect the construction
pit for impact from the river barges.
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The fender structure consists of steel H-profiles both
as vertical piles and as horizontal beams between the
piles. The horizontal beams were also pre-stressed
by cables that would absorb the energy from a barge
impact during large deflection of the fender structure. The fender system was designed for a barge
impact force of 4 MN over a width of 2 meters.
The construction pit in the river was demolished
after the tunnels were constructed and back filled
with sand before the river was re-established.
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The ground anchors are installed inclined and
staggered with an angle of 20 deg and 35 deg from
horizontal. The anchors consist of 8 lines diameter
0.6 inch and with an average length of 20 to 30 meters and are stressed to a maximum of 1,200 kN per
anchor. The anchors are in general stressed twice
depending on the construction sequence of the pit,
excavation, empty pit for water etc.
The construction pit passes close to the corner of the
Swiss embassy. The Swiss embassy is an old, heavy,
three story building that is protected by a preservation order. Settlements of the building, which could
cause cracking or damage, were therefore of great
importance.
At this particular location the standard plane diaphragm panel was replaced with T-shape panels to
increase the stiffness. The tops of the T-shape panels
were connected to a 3 x 3 meter horizontal prestressed concrete beam. At the ends of the beam
were placed 18 numbers of ground anchors with a
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length of 50 to 70 meters and were reaching to the
opposite site of the building.
There was a program monitoring the behavior of
the building and there were no critical settlements at
any time during the construction period.
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The U-bahn vertical alignment was rising toward the
station placed in pit "H2". The part of the pit, which
contains the U-bahn, was separated with a longitudinal diaphragm wall. This was doing to avoid excessive excavation and re-filling before construction of
the U-bahn tunnel could commence.
To seal off from intruding ground water, a jetgrout layer was installed between the diaphragm
walls and deep under the U-bahn tunnel. The other
section of "H2" was constructed in the usually way,
with wet excavation and underwater concrete with
tension piles. This arrangement can be seen on picture 2.
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By using a stiff wall and by placing the grouted
part of the ground anchors opposite the building, the
soil volume under the building was pre-stressed,
thereby reducing the settlements.
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It was necessary to use temporary steel bracing
between the diaphragm walls in the pit for the Ubahn tunnel due to the water pressure from the other
part of "H2 ".
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The bottom of the construction pit was sealed off
with a concrete slab cast under water, anchored with
tension piles.
When the dredging in an enclosed part of the construction pit was finalized the bottom was cleaned
and prepared for installation of tension piles.
The tension piles consist of H-profiles, HEB 220,
with shear brackets welded on the toe of the piles
and a steel plate welded on the top of the piles forming the head. At the inside corners in the H-profile
were installed two injection pipes running down to
the pile toe. The piles had a length of 19.8 meters.
The piles were vibrated down into the bottom of
the pit from a floating barge. The piles were placed
in a pattern of 3 x 3 meters. After the piles were installed the toe of the piles were grouted by injection
of mortar through the pre-installed injection pipes.
A 1.5 meters thick concrete slab was casted under
water and the head of the piles were embedded in
the concrete. The water in the pit was pumped out
after the concrete had reached its strength and the
pressure below the slab were then taken over as tension in the piles. The concrete slab was not reinforced the forces were carried by a compression arch
between the pile heads.
soil in between the piles were monitored in three
level for vertical and horizontal stresses and with rod
extensiometers the vertical movement of the soils.
Based on all the results from the pile tests the allowable shear stress to be used in the calculation of
the pile bearing capacity was fixed to τ = 150
kN/m².
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The U-Bahn tunnel is constructed as one cross section with two tubes and a constant overall width of
15 meters. The cross section separates into two independent tunnels close to the underground station
to make room for a central platform in the station
area.
The ICE-railway tunnel varies in width from 69
meters at the north end under river Spree to 24 meters at the south end. Under river Spree the tunnel is
tied to the vertical tension piles used for the underwater concrete in order to get enough safety against
uplift; those piles will work as permanent tension
anchors for the tunnel.
The highway tunnel (road B96) has a constant
width of 24 meters and the cross section is divided
into two tubes with two lanes in each.
An extensive full-scale field test of the piles was carried out before any of the piles were installed. A test
area was excavated down to a level just above
ground water level, so the test piles would be placed
in water. An area of 20 x 54 meters was prepared for
testing of five single piles with different lengths and
a group test of five piles. Two geotechnical boring
were carried out in the area together with SPT tests,
all to give a good knowledge of the ground conditions.
The necessary anchor lengths of the piles were
calculated to 18.20 meters and the necessary working load was calculated to be 1,000 kN. The single
pile test were carried out with pile lengths varying
from 13.76 to 17.30 meters and the test load of 700
kN, 1,200 kN and 2,000 kN were applied to the piles
in three steps with full load release between the
steps. Some of the single piles were even tested up
to 2,900 kN. The results from the tests were plotted
for each pile in two diagrams load versus time the
load was applied and deformation of pile head versus load. The distribution of the axial force, based on
strain, was measured by strain gauges placed along
the pile.
4 TUNNEL CONSTRUCTION
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The tunnel construction was traditional cut and
cover tunnel construction, with the tunnels divided
into 10 to 20 meters casting segments. The concrete
used was dense B-35 concrete also classified as watertight concrete. By casting in segments of 10 to 20
meters it was possible to control the crack development from the concrete temperature and keep the
crack width below the required 0.15 mm.
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A group of five piles were likewise tested and for
any group effect. Besides monitoring each pile the
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The construction sequence was simply to construct bottom slab, walls and last the roof slab. For
tunnels along the pit walls, highway and metro, the
pit walls were utilized as outer formwork. In this
case the pit walls were cleaned and straighten out
and a thin drain layer was placed before the tunnel
wall was casted.
In the final condition, when ground water rises
the drainage layer ensures that the tunnel wall is subject to the full water pressure. Special measures were
taken to ensure that the tunnel bottom and roof slabs
were supporting the pit wall when the ground anchors were released. The effects of this arrangement
is that in final condition the tunnel wall carries the
water pressure and the pit wall carries the soil pressure and transfers the load to the tunnel bottom and
roof slab.
The joint between the tunnel segments were
equipped with waterstops and shear keys which allows the segments to contract and expand, and that
accommodate small rotations.
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5 REMARKS
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All of the tunnels had sufficient safety against uplift
by their weight, except for the part of the main railway tunnel under the river where the available space
was limited.
The tension piles below the railway tunnel were
extended and were cast into the bottom slab of the
tunnel.
In the final condition when the river is back in
position the uplift from that part of the railway tunnel will be taken as permanent tension in the piles.
All other tension piles will not have any effect in the
final condition.
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The project was very complex with special construction pit and three tunnels side by side.
Especially the construction of the pit had several
obstructions. Bunkers from world war two, unexploded ammunition, cellars, pieces of tunnels etc.
which could not be located due to registrations and
drawings have been lost or burned during the war.
But, also the tunnel construction calls for an experience contractor to handle huge quantities of concrete, reinforcement and steel.
The contractor has in professional way and with
high standard finalized this complex project in time.