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Home > Technology > Develop a New Type of Water Stop Structure Specialized for the near Sea Tunnel

Develop a New Type of Water Stop Structure Specialized for the near Sea Tunnel

Introduction:

Water leakage, as one of the most important detriments, is troubling tunnel construction and operation, and people even argued “ten tunnel nine leak”. Furthermore, the tunnel seams are the weak links of the tunnel waterproof. And as a matter of experience, about half of the leakage occurred in the seam during operation. Tunnel seams leakage brings a series of problems to the safety of tunnels in use. For instance, Damao tunnel, achieving completion and open to traffic in 1995, appeared serious leaking or even gushing water which posed a great threat to safe driving. The Zhongliang Mount tunnel on the Chengdu-Chongqing Expressway, which was built and put into use in the 1980s, is also troubled by the leakage, resulting in frequent traffic accidents inside the tunnel. Over 60 traffic accidents occurred within only a month, in spite of the continuous renovation which was still ineffective. Liuyang River Tunnel in operation appeared leakage of water in construction joints, deformation joints and other places in less than one year, which affected the high-speed rail line. In April 2013, Liuyang River tunnel leaking caused more than 20 trains to delay. In view of the hazards of water seepage and leakage, it is necessary to carry out in-depth study on its waterproofing problem.
Throughout the tunnel design and engineering applications, the main task of joints waterproof is to prevent, however, the real effect is poor. And the water leakage problem brought by the defects of the water leakage method, materials and so on is extremely serious. The waterproof material used in the tunnel joints generally includes steel edge rubber water stops, external tape-type water stop, pre buried grouting tube, water swelling water stop strip, joint sealant and so on.Generally the water stop strip is adopted together with other waterproof measures to block the leaking area. The traditional water stop is designed only with considerations of waterproof, neglecting the functions of drainage. When the tunnel is in the rich water area, the geological water is more inclined to enter the deformation seam. In order to reduce the leakage, a certain function of drainage can be developed by the structural improvement of the water stop.
Most material used for water stop strip in tunnel engineering is high-molecular polymer. There is a corrosive problem as the materials(EPDM) are long-term soaking in the marine. With further corrosion, the material strength, extensibility and other physical and mechanical properties will gradually decline until failure. Therefore, it is necessary to research and improve its corrosion resistance.
In summary, the traditional water stop has been unable to meet the specific requirements of tunnels. It is necessary to develop a new type of water stop structure specialized for the near sea tunnel.

1.Analysis of Waterproof Failure Mechanism

It is common that water stops used in tunnel engineering are permeable due to the stressed deformation. There are two specific reasons, one is tensile failure, the other is that two wings of water stops pulling from the concrete.

(1)Judgment standards of tensile failure
Two factors should be considered. One is that differential settlement and axial tension generate a pull force to water stop itself. The other is water pressure. When the pulling force is greater than the tensile strength, the water stop cracked. There are two results of differential settlement of the water stop including shear and tension. Whether invalid or not, it can be judged in the following two ways.
①Stretching differential settlement and seams and water pressure converted into equivalent tensile stress. The tensile stress of the water stop can be calculated by (1) and (2)
σ=Eε (1)
σ is pull stress produced by the deformation of water stop. E stands for elastic modulus. εis tensile strain.
The tension produced by the water pressure of the water stop is calculated by the formula proposed by the Hehai University through the classical film theory.
T=0.204Pb/槡ε (2)
T is the tensile stress of unit length of the water stop. P is water pressure. B is the width of the deformation caused by opened deformed seams. The parameters T, P, b are shown in Figure 1.

Figure 1 Stress of Water Stop Under Water Pressure

Judgment standard of tensile failure
σ<RL (3)
RL is tensile strength of water stop.
②Shear failure judged by (4), (5)
τ≤τf (4)
τ=Gγ (5)
τ is shear stress. τf is the shear strength of the water stop, take 4.44 MPa according to the research of Hao Jutao et al.

(2)Judgment standards of pulling from the concrete
Water stops with two wings poured in concrete, fixed by the adhesion. The adhesive stress of rubber water stop is 0.4 ~ 0.6 MPa. When the concrete and the water stop has a good contact, the cross-section is without blisters, bubbles, and the pouring quality is good, then take 0.5 MPa. To determine whether the water stop will be pulled from the concrete, the stress between the rubber water and the concrete should be considered.
When the tensile stress from circle and root is greater than the stress between the water stop and concrete, the water stop is pulled from the concrete, that is σw ≤0.5. (6)
σw means tensile stress produced by water pressure.
From (1) ~ (6), the safety of the water stop is mainly related to the displacement on both sides of the joint and water pressure, and the displacement on both sides of the joint is related to the tunnel structure and environment of engineering geology. From (2), it can be seen that to lower the water pressure under certain seam displacement can reduce the tensile stress of the water stop and then improve the safety of the water stop. Based on controlled drainage principle, you can reduce the nearby water pressure by setting the drainage channel in the seams. Therefore, the water stop can be improved to use structure of drainable water stop. When the drainable water stop can be discharged, with the increase in displacement, water pressure is decreasing [9] , then the stress produced by the water pressure is getting smaller and smaller. Under the condition of tensile stress caused by joint deformation, the total tensile stress is getting smaller and smaller, and the tensile safety factor is getting bigger and bigger. From (6) we can see that the anti-pull safety factor is growing. From (4), we can see that the shear safety of the water stop is unchanged.

2.Structural Optimization
2.1 Traditional Water Stop

(1)Back Stick Water Stop
The structure of traditional back stick water stop is shown in Figure 2.


Figure 2. Traditional Back Stick Water Stop (Unit:mm)

For the tunnel in rich water area, the main reason causing the failure of traditional back stick water stop is: it is only blocking instead of draining. Water leakage surrounded by back stick water stop through the waterproof layer is ready to burst out. If the back stick water stop itself is defective or has a construction quality problem, water leakage is unavoidable. If giving this groundwater a flowing channel, then the chance of leakage along the seam will be reduced.

(2)Medium Buried Water Stop
The traditional medium buried water stop is shown in Figure 3. Such a water stop only can block water not drain, with limited water resistance. If the water reaching the medium buried water stop is more with a certain water pressure, the water easily leak around the water stop.


Figure 3. Traditional Medium Buried Water Stop (Unit:mm)

Due to high water pressure and certain acidity of seawater, water stops should have a strong pressure capacity and strong resistance against chemical corrosion.

2.2 Optimization of New Type of Water Stop

(1)Back Stick Drainable Water Stop

The new structure of the back stick drainable water stop can be shown in Figure 4. The channel in the middle of the bottom of the water stop is reserved, where the water leakage goes through. Then the water flows into drainage channel. And finally the water through the corrugated drain pipe enters the longitudinal drain pipe. Compared with the traditional water stop, the new one mainly has the advantages in two aspects: Firstly, increase the thickness of middle part and geometric dimension to improve its ability to carry out water pressure. Secondly, endow water stop with drainage.

Figure 4 Back Stick Drainable Water Stop(Unit:mm)

(2)Medium Buried Water Stop


Figure 5 Medium Buried Drainable Water Stop (Unit:mm)

The new structure of the medium buried drainable water stop can be shown in Figure 4. Leakage water flows through the geotextile into the channel, and then goes through the drainage pipe into the longitudinal drainage pipe. Compared to the traditional water stop, the new type makes great progress: endowing water stop with drainage that reducing the chance of leakage is through draining first and then blocking. The Ω shape in the middle of the cross section makes it easier to position the water stop on the template than the conventional water stop.

3. Analysis on Structural Mechanics

The tunnel structure affords a certain amount of water pressure, therefore water stop need to have a strong pressure capacity. In the following, the traditional water stop is analyzed mechanically.
Material is made of EPDM, and its elastic modulus range is 1 ~ 5 MPa. This test is taking the concealed-digging section of Gongbei Tunnel, a coastal tunnel engineering as an example. In view of the safety reserve, water pressure would take 0.5 MPa 2 times bigger than the design value, and take the limit valve 30 mm [10] of tensile deformation and shear deformation, so as to check the mechanical properties of the water stop.

(1)Traditional Back Stick Water Stop
Semi-perimeter of grouting hole is 38mm. Semi-ring becomes a band through stretching with the length of 62 mm; what is called geometric deformation is that the water stop is elongated by 38 mm(from 62mm to 24mm). It shows that when the expansion joints opened 38 mm, the water stop has not produced tensile stress. When the maximum tensile length of the tunnel deformation seam is 30 mm, the back-sealed water stop does not produce tensile stress. Therefore, the mechanical properties of the back-sealed water-stop can meet the requirements.

(2)Traditional Buried Water Stop
①Stress produced by the deformation of water stop
The thickness of the water stop is 10 mm; its diameter of inner circle is 20 mm; its perimeter of inner circle is 62.8mm. After the circle is stretched, the middle becomes a seam of which the length is half of the circle, that is, 31.4 mm. So the distance on both sides of the root is 31.4 mm + 20 mm = 51.4 mm, the water stop is elongated by 11.4 mm, that is, the geometric deformation. This shows that when the expansion joints opened 11.4mm, the water stop has not produced tensile stress.
When the water stop continues to stretch 18.6 mm, a tensile strain is produced ε = 18.6 / 51.4 = 0.36. From σ = Eε, tensile stress of unit length of the water stop is T = Etε × 1. t is the thickness of the ring after stretching; So T=E×0.02×0.36=0.0072E. The thickness of the root is 10 mm, so the tensile stress isσ=T/0.01.
Elastic modulus of EPDM takes 4 MPa, so the stress isT=0.0072 m×4 MPa=28.8kN•m-1m, tensile stress of circle root is σ=28.8kN•m-1/0.01 m=2880kPa.
②Stress from the water pressure
In equation (2), water pressure P produced by 50 m deep water is 500 kPa, b=0.070 m. The tensile stress produced by the water pressure is shown in equation (7)
T=0.204Pb/槡ε=7.14/槡ε (7)
Stress-strain curve of EPDM rubber water stop can be obtained from equation (7) shown in Figure 6. Based on the results of the tensile test, let ε = 15%, T = 18.4kN • m-1 can be obtained. The tensile stress of circle” σ = 920kPa”, tensile stress of circle root” σ = 1840kPa”


Figure 6. Stress-Strain Curve of EPDM Rubber Water Stop

The stress superposition caused by deformation and water pressure can be obtained: T= 47.2kN•m-1, σ=2360kPa(tensile stress of circle), σ=4720kPa(tensile stress of circle root)
③Checking on tensile failure
The tensile stress of the root of the EPDM water stop is 4.72 MPa with 50 m water pressure: tensile strength of EPDM is 12 MPa; so the tensile safety factor is 12/4.72=2.54 and the stress level is 39.4%. Many studies [7] think polymers (including elastomers and plastics) are subjected to tensile stresses with a service life of more than 100 years when the stress level does not exceed 20%. When the stress level is between 20% and 30%, the water stop can still be safe to use with a service life less than 100 years. With 30% to 50%, the water stop can’t be used for a long time; If more than 50%, the water stop should not be used because of the short life. It can be seen that the water stop can’t be used for a long time when it is displaced in the deformation seam by 30 mm with water pressure of 50 m. As a result, thicker circles and wings should be designed and created.
④Checking on water stop due to shear failure
The shear stress of the water stop is mainly related to the shear displacement. The shear stressτ=2.55 MPa is calculated from equation (5). Therefore, the shear safety factor is τf/τ=4.44 MPa/2.55 MPa=1.74. The water stop meets the safety requirements and will not fail due to shearing.
⑤Checking on pulling from the concrete
Based on the foregoing calculations, stress on the circle and root isT=47.2kN•m-1with 50 m water pressure. The adhesion that the rubber pours into the concrete is 0.5 MPa, while the adhesion poured by steel is 0.5 MPa. The rubber width of both upper and lower sides of each wing is 0.155 m, and the steel width is0.195 m. Therefore, the adhesion is 0.5 MPa×0.155 m+0.36 MPa×0.195 m=148kN•m-1. Safety factor of pulling from the concrete is 148 kN• m-1/T = 3.14. It can be seen that the water stop will not be pulled out when the concrete pours well under the conditions of the contact surface between water stop and concrete without water blisters, bubbles and others. However, the safety factor will be lower than the above data if the pouring quality is not ideal, in that case, you need to widen or stiffen the wings, or try to reduce the external water pressure.
The new type of water stop structure can’t simply apply the formula (1) ~ (6) for mechanical analysis, however, the limit water pressure can be tested by a water pressure test.

4. Research on Corrosion Resistance

Seawater generally has a certain pH degree. Materials using in tunnel near sea are mostly EPDM. Corrosion would appear after long-term soaking. Especially in acidity, the strength and the extension will be gradually reduced. Therefore, the material requires a good chemical resistance, which is to ensure its durability as an important indicator. The study found that barium sulfate and graphite can improve the acid resistance of rubber [11-13]. The optimum dosage of EPDM can be further determined by experiment.

Filler Mass fraction/% Remarks
Barium sulfate 5,15,30,50  
Graphite 5,15,30,50  
Combination packing
(Barium sulfate + Graphite)
10,30,60 Mass ratio 1:1

The procedure is as follows:
①Resistant tests on hydrochloric acid. Put sample with EPDM in glass bottle featured with high temperature and pressure which contains hydrochloric acid (10% mass fraction). Place the glass bottle in the water bath at 100 ℃ for 48h. And then take it out and test the tensile strength, tear elongation and strength.
②Resistant tests on sulfuric acid. Put sample with EPDM in glass bottle featured with high temperature and pressure which contains sulfuric acid (50% mass fraction). Place the glass bottle In the water bath at 100 ℃ for 48h. And then take it out and test the tensile strength, tear elongation and strength.
The testing device is shown in Figure 7, and the testing results in Figure 8-10.


Figure 7 Testing Devise


Figure 8 Tensile Strength of Fillings with Different Components


Figure 9 Elongations of Fillings with Different Components


Figure 10 Tearing Strength of Fillings with Different Components

(1) With the increase of the barium sulfate filled in the water stop, the tensile strength after the acid resistance test increased first and then decreased. Particle size of barium sulfate is small, and is dispersed uniformly in EPDM. Barium sulfate is an inert filler. Stability In the acidic medium is better. With the amount increasing, under high temperature environment, its crosslinking with EPDM is getting better and better. Up to a certain amount, the value falls down.
With the increase of the sulfate, the tensile strength is gradually reduced, because the lamellar structure of graphite make acidic media impossible to intrude. The more graphite it added, the worse crosslinking it has.
With the increase of the combination packing, tensile strength is getting smaller and smaller and the combination of filler and EPDM is worse. When the test takes 15% barium sulfate (mass fraction, the same below) or 5% graphite or 10% combination packing, tensile strength is larger. When graphite takes 5%, tensile strength is largest. The tensile strength after hydrochloric acid and sulfuric acid resistance test meets requirements.

(2)With the increase of the barium sulfate filled in the water stop, the tear elongation after the acid resistance test increased first and then decreased. With the increase of the sulfate, tear elongation is gradually reduced. With the increase of the combination packing, tear elongation is getting smaller and smaller. When the test takes 30% barium sulfate or 5% graphite or 10% combination packing, tear elongation is larger. When graphite takes 5%, tear elongation is largest. On this occasion, cross-linking with rubber is the best. The tear elongation meets requirements after hydrochloric acid and sulfuric acid resistance test.

(3)Tear strength doesn’t appear to be much different from the above tests. With the increase of the sulfate, change of tear strength after acid resistance test in the beginning is not great, but later the strength is reduced. With the increase of the combination packing, tear strength gradually decreases. The tear strength meets requirements after hydrochloric acid and sulfuric acid resistance test. When the test takes 30% barium sulfate or 5% graphite or 10% combination packing, tear strength is greater. When graphite takes 15%, tear strength is the largest.

Because of the same amount of combination packing, the mechanical properties of the graphite-filled materials after acid resistance test are relatively good. Based on the above analysis, 5% graphite added in the EPDM can effectively improve the chemical resistance of the water stop.

5. Installation

Construction errors caused by the water stops under an unreasonable stress are the reasons why tunnel water leakage occurs in many cases, leading to the failure of water stops and water leakage of tunnel. The good construction quality is related to simple installation of the water stop. The causes of water leakage associated with the construction quality of the tunnel mainly include: Firstly, no proper fixing of the water stop results in displacement or curling. When pouring concrete on both sides, any collision could result in skewing, overlapping and even water leakage. Secondly, the concrete is lack of vibrations and pounding or the water stop is improperly handled, causing uncompleted compact of concrete and water stop. Problems are more likely to appear when the water stop is drainable.
For the installation of a new type of water stop, please note the following matters:
(1) The water stop should be centered and pulled by a wire and fixed on steel through the support of baffle, so as to ensure that the center hole of the water stop coincides with the seam. Besides, the water stop should be kept straight and not twisted.
(2) When pouring and vibrating concrete at the seams, put the water stop straight and keep it static. The deviation between the center line and the halfway line shall not be greater than 20 mm so as to make sure that the seam is waterproof.
(3) The installation slot should be pre-set in the lining of water stop. The upper part is secured to the concrete end of the template with steel bars, and the lower part is inserted into the reserved slot. Place the drain pipe into the reserved channel of the water stop. And then fix the water stop with waterproof concrete. Finally pour concrete. After the construction of concrete, there is no joint in the middle of each water stop.

6. Conclusion

(1) The traditional water stop can’t meet the requirements of high water pressure near sea and corrosion and other engineering, which is easy to cause water leakage. Therefore, we should design a new type of water stop with the drainage, good corrosion resistance.
(2) It is found that reducing water pressure to the water stop can prevent the water stop from pulling off and the wings separating from the concrete to improve the waterproof performance after analyzing the waterproof failure mechanism. Therefore, it is recommended that the traditional water stop is arranged in a drainage type that can reduce water pressure and then drain away excess water.
(3) (3)Physical mechanical tests are conducted under different acid conditions of EPDM water stop with different fillers according to the nearby sea environment. It is found that the resistance performance on chemical corrosion of new water stop by adding 5% mass fraction of graphite is the best.
(4) The new type of drainable water stop is improved in structure and material. In this paper, the material properties and mechanical properties are studied by laboratory test. The effect should be further verified in the actual project.

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