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7/31/2019 Soft Eye Case Study
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7/31/2019 Soft Eye Case Study
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7/31/2019 Soft Eye Case Study
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Fig. 6. Reinforcing cage for diaphragm
wall with localised GFRP section.
Typically, GFRPs will display a higher
tensile strength than steel, but when failure
occurs the result will be dramatic, resulting
in longitudinal failure of the polymer
matrix as the glass fibers within the bar fail.
This is known as the brooming effect.
The use of GFRP in tunnel construction
will facilitate TBMs to bore through the
concrete structures without any more
resistance then would be encountered in
hard materials of the same compressive
strength such as rock and un-reinforced
mass concrete.
Fig. 9. Adjacent cutter discs working
against hard material (RC) with GFRP
reinforcing.
Note the localised fragmentation of the
GFRP rebar and crack propagation within
the matrix of the concrete.
Bangkok MRTA Thailand
Fig. 7. Elevation & cross section of GFRP
Locally reinforced section of cage for soft
eye. (GFRP shows white against steel).
Concrete reinforced with GFRP willotherwise meet all the mechanical
requirements of high tensile reinforcing
bars in RC structures with minimal
changes to the reinforcement design for
this application.Red denotes GFRP Blk. denotes steel rebar.
Fig. 10. Comparative stress strain curve of
GFRP and high tensile reinforcing bars.Sample cage design complementary of
Coforce International consultants.
Strain measured in percentage elongation
Stressmeasuredink
ilonewtons
0
400
200
100
300
Steel reinforcing bar
Aslan GFRP
fy = 225 kN
fu = 300 kN
Steel reinforcing bar
Aslan GFRP Bar
CSA = 490 mm^2
CSA = 490 mm^2
fu = 343 kN
fy = N/A
UTS = 610 kN/mm^2
Yield @1% = 460 kN m^2
UTS = 700 kN mm^2
Yield = N/A
0.01% 0.02% 0.03%
The considerations before specifying a soft
eye opening for a TBM in a concrete
structure are essentially the same stress
parameters as would be considered when
selecting which TBM would be suitable tocut through any naturally formed hard
material.
This phenomenon is due to the high tensile
low-yield mechanical properties displayedby Aslan GFRP significantly reducing
transversal resistance compared to RCs
using traditional steel bars.
Fig. 8. Exposed section of GFRP from a
soft eye displaying typical brooming.
7/31/2019 Soft Eye Case Study
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Concrete is typically an easier material to
bore through compared to most rock mass
that would likely be encountered during an
excavation due to its homogenous nature
and predetermined uniaxial compressive
strength by design.
Before the introduction of soft eye
openings, the construction method to
overcome the impediment of RC shafts
would be to access the shaft with hydraulic
breaking/cutting equipment to break out
the concrete and steel before the TBM
could pass through the structure.
Fig. 11. Hand breakout at exit shaft.
Fig 11 shows a hydraulic breaker beinglowered into an exit shaft to breakout a
TBM from traditional steel reinforced
concrete.
Personal safety of those involved in such
an undertaking is greatly improved by
utilizing the soft eye technique, as no
person is required to access the shaft priorto placing the TBM in the launching shaft
or ahead of a breakout.
The contractor also benefits from
significant cost and time saving by the
construction of soft eyes. Placing GFRP
bars would be carried out in the same
manner as fixing traditional steel bars and
due to the lightweight of GFRPs ( that of
steel) the task would only be made easier.
Typically, a hand breakout would addseveral days to the construction program
depending on the depth of the shaft,
accessibility, safety considerations,
availability of manpower and resources
required to carry out the works etc.
Given these unique advantages of soft eye
openings more and more tunnelling
contractors are adopting this technique and
the future of soft eye openings looks
positive for some time to come.