1791
Technical Committee 204 /
Comité technique 204
investment profits, the possibility of implementing construction
projects on the gradual basis, the possibility of using the
facilities after the project completion. is the most feasible
method for Korean companies to promote their businesses in the
construction market.
2.3
Status of construction mehods
A 2-lane traffic tunnel was to be constructed under an
expressway where many vehicles pass through 2 conventional
trenchless methods(UPRS, PRS) and PSTM method were used
in the initial design, in which are applied to the structures such
as underpass box #3, #5, underpass #4, Hadong IC ramp,
respectively, in Fig. 2.
Figure 2 Location map of construction methods
The former is Upgrade Pipe Roof Structures, which is
interconnected to steel pipes to propel the pipe through
guidance rail in direct type. Pipe Roof Structures method means
that it is interconnected to steel rods in steel pipes to be filled
with reinforced concrete inside the pipes by strengthening
lateral stiffness. The latter is Pressurizing Support Tunneling
method, in which applies the pressure larger than the magnitude
of the relaxed earth pressure around excavated area to the
ground so that radial deformation occurred during excavation
can be recovered.
In addition, temporary structural system for a trenchless
method was introduced. This structural system is a type of
hybrid member (STS method, Steel Tube Slab method)
consisting of mortar-filled steel tubes laterally connected by
headed reinforcements without disruption of the surface.
However, before the excavation was started, it is included a
very thin overburden thickness of 0.9 m. The road construction
under highway is the area to fulfill the several important criteria
of implementing construction projects in an urban area(Kim, D.,
2012).
On geotechnical condition, weathered ground is more than
30m thick under the upper highway. Fig. 3 shows the shape of
the load-deformation curve in tunnel, in which IE indicates the
support pressure of support installed at D in conventional tunnel
methods and its corresponding radial deformation at final stage
is OI, while as support pressure increases to I’E’, resulting in
decrease of radial deformation to OI’ (Daemen, 1977).
Figure 3 Load- deformation curve
The tunnel construction, in which steel pipes are installed to
surround the tunnels, then soil nailing is placed at face to retain
the ground as temporary retaining system, follows excavation.
H-beams are installed and cement milk is injected into the pipes
to support the ground, and finally shotcrete is sprayed, and the
concrete inner linings are casted (Kim, D., 2012).
3 RESULTS OF MONITORING
To minimize the inconvenience at existing equipment operation,
many trenchless methods have been developed and used much.
Existing trenchless methods insert stiff structure (rectangular
type pipes or circular type pipes) to lower part under the ground
in order to protect existing roads or structures before excavation
of tunnels. These stiff structures are assembled each other in the
lateral direction. And then, the lower part of this stiff structure
will be excavated. These kinds of existing method are
developed from the concept which huge inserted steel pipe-
concrete structures control the displacement by supporting the
overburden load. To minimize the displacement, and to
increase the stiffness of existing structures, the structures are
becoming larger and larger to be many obstacles. These
obstacles are concerned with excavating and grouting the
foundation, excavation delay on hard foundation, waste of work
costs which is caused by excessive section, and long period of
construction (H
an
, 2012).
It should be noted that another tunnel was constructed by a
conventional trenchless method right next to the tunnel by
PSTM. As shown in Table 4 and Figure 4, a couple of faults
were created and significant ground settlement up to 30 cm
occurred in the area where the conventional method was used,
due to pipe jacking and excavation under construction (Kim, D.,
2012).
Table 4 Comparison of ground displacement(settlement)
Method
Under excavation
(Max, mm).)
After excavation
(mm)
Remarks
24.0
1.0
Measured
PSTM
2.15
0.88
Calculated
Other
49.7
35.0
Measured
As the time elapses in Figure 4, the actual ground
displacement state of tunnel was more than predicted in design.
