soft eye case study

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.


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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.

soft eye case study

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