A Spectacular Concreting Job
At a railway tunnel in Istanbul, Putzmeister concrete pumps
and a concrete wet spraying machine are involved in producing
the huge tunnel elements for the underwater section, in lining
the tunnel with shotcrete and in backfilling tunnel formworks
on land. A report.
The metropolis of Istanbul, which boasts of a population
of 15 million and stands at the dividing point between Europe
and Asia, is currently seeing the construction of a spectacular
railway tunnel, designated the Marmaray Project. Putzmeister
concrete pumps and a concrete wet spraying machine are involved
in producing the huge tunnel elements for the underwater section,
in lining the tunnel with shotcrete and in backfilling tunnel
formworks on land.
Istanbul is home to around a fifth of the population of Turkey.
Traffic planners are focussing on modernising public transport
and developing the infrastructure in order to relieve the
burden on the city’s chronically congested bridges over
the Bosporus and the associated access roads. The Marmaray
Project, which was established in the 1980s, is an integral
component of this measure. In May 2004, tenders were finally
approved and the project was started. An important section
of the structure is a 1.4 km tunnel segment which lies on
the floor of the Marmara Sea at the southern end of the Bosporus.
This segment consists of enormous prefabricated concrete
elements which are lowered to a depth of up to 58 m, sealed
and interconnected. The tunnel connects the European suburb
of Halkali with Gebze on the Asian side of Istanbul.
Work is being conducted on both sides of the waterway to
establish a connection between the tunnel and the existing
rail and metro network. The construction measures include,
among others, 9.8 km of driven tunnel sections, 2.4 km of
open cut tunnel sections, three new underground stations and
37 stations located above ground, which are currently undergoing
a comprehensive modernisation programme. A 63 km existing
railway line is being expanded. The client is the Turkish
General Directorate for Rail, Port and Airport Construction
(DLH). The project is being undertaken by a Japanese/Turkish
consortium (contract section BC1) under the management of
the Taisei Corporation, together with Kumagai, Gama and Nurol.
The construction costs are estimated at $ 3.5 billion.
Efficient tunnel elements
Concreting of the eleven 135 m long, 15.3 m wide and 8.6 m
high tunnel segments is being conducted in a dry dock on the
Asian side of Istanbul in the suburb of Üsküdar.
This is where Oyak Beton is utilising several of its Putzmeister
truck-mounted concrete pumps (with boom sizes M 24-4, M 36-4
and M 47-5) to fill the base, wall and ceiling form-works
of the caissons from the edge of the dock. Approximately 5,000
cu m of concrete in varying strength classes (up to C40) is
required for each tunnel element. The upper segment is concreted
with a flooded basin.
Sturdy tugboats tow the finished and sealed, reinforced tunnel
segments from the dock to a floating platform in the Marmara
Sea in a catamaran construction. The 30,000 tonne reinforced
concrete constructions are carefully towed between the two
hulls. The catamaran is manoeuvred to the exact pre-calculated
position above the seabed, in which dredgers have excavated
a deep channel. The special ship is held in place by thick
steel coils, which are secured to 13 buoys anchored on the
seabed. The catamaran houses huge cranes with cable winches,
which are used to lower the tunnel sections into the artificial
ditches. Cameras and measuring instruments monitor the precise
alignment of the tunnel segment, which is achieved by filling
and emptying ballast tanks in a similar manner to a submarine.
The critical period is during the final phase where there
is a risk of the large rubber seals being damaged; this is
because these are required to produce a watertight connection
between the segments. The top of the tunnel element is subsequently
protected against damage with a 4.5 m thick sediment layer.
It has long been known that a dangerous current runs through
the passage between the Black Sea and Mediterranean Sea. Strictly
speaking, there are two directions in which the water flows
through the Dardanelles, the Marmara Sea and the Bosporus:
In the lower water layer, there is a strong current running
from south to north; in the upper layer, however, the water
flows in precisely the opposite direction. In order to be
able to position these tunnel segments accurately at the calculated
positions, despite this strong and often fluctuating current
speed, the Japanese experts use a specially developed system
for predicting current.
A significant challenge for the tunnel constructors is posed
by more than 10,000 earthquakes that are registered in the
region every year (most of which, however, are barely perceptible).
The reason for these earthquakes is the Anatolian Fold, which
meets at the two continental plates. It is located some 15
km from the tunnel. The safety measures include, for example,
special protection for the connection points of each individual
segment under water. And where the Marmaray tunnel merges
with the land tunnel, particularly powerful, flexible and
elastic rubber seals are used which will not break in the
event of an earthquake.
Versatile construction methods
While the 1.4 km tunnel section is being established from
caissons at the bottom of the Marmara Sea, entirely different
tunnelling methods are being utilised at other sections of
the 76.3 km Marmaray Project. Tunnelling in areas along the
banks and those areas susceptible to soft ground conditions,
for example, is conducted in a shield tunnelling process with
tubbing support. In other sections, the inner formwork is
concreted by stationary Putzmeister concrete pumps, which
are made up of both modern
BSA 1408 E units and older fully functional pumps. Other sections
are built using an open construction method, often involving
the use of PM truck-mounted concrete pumps. For shotcreting,
Putzmeister provides a compact SPM 400 wet spray manipulator.
The spray arm of these machines has a working range of 2 m
- 8.5 m and its spraying capacity is up to 20 cu m/h.
With an annual production capacity of 4.5 million cu m, Oyak
Beton is one of the largest ready-mixed concrete producers
in Turkey. Oyak Beton has its own concrete laboratory and
is represented throughout the country thanks to its multiple
mobile and stationary mixing plants. Besides numerous truck
mixers, the company also operates a fleet of around 70 Putzmeister
truck mounted concrete pumps in varying sizes. In addition
to concreting the tunnel segments, the company also delivers
and pumps concrete to other sections of the Marmaray Project
(contract section BC1 and CR1). Oyak Beton is also currently
involved in other major construction projects in Turkey.