Research Article  Open Access
Investigation of the LoadSharing Theory of the RC Pipes Rehabilitated with Slip Liners
Abstract
Sliplining is a preferred rehabilitation approach in the departments of transportation in China. Although the method is the most common rehabilitation technique, few research studies have been conducted on the mechanical behavior of a rehabilitated reinforced concrete pipe (RCP). A series of experiments were conducted on RCPs rehabilitated with a corrugated steel pipe (CSP), a steel pipe, a highdensity polyethylene (HDPE) pipe, and a shape steel bracket. The RCP rehabilitated with the CSP showed an increase in both the loadcarrying capacity (3.46 times greater than the RCP) and the stiffness (5.35 times greater than the RCP). The RCP rehabilitated with the steel pipe, HDPE pipe, and steel bracket exhibited an increase in the loadcarrying capacity (1.23, 1.50, and 1.31 times greater than the RCP, respectively), and the stiffness of these three pipes was not markedly changed. The sliplined pipe acts as a “pipe within a pipe” system. A “loadsharing” theory was proposed in this study and provides estimates of the loadcarrying capacity of the sliplined pipes.
1. Introduction
Sliplining, which involves inserting a liner into an existing culvert and grouting the space between them [1], is a preferred technique by the engineers [2], not only does it reduce reconstruction costs but also will not cause traffic disruption. A variety of pipe materials can be used as the liners such as highdensity polyethylene (HDPE) pipes and corrugated steel pipes (CSPs) [3, 4].
This rehabilitation technique is commonly used in China, but few researches have been performed when an reinforced concrete (RC) culvert is used. Zhao and Daigle [5] used a castiron pipe rehabilitated with a liner to conduct a twopoint loading experiment and found that the existing pipe, grout, and liner acted independently; an approach was proposed to estimate the service life of a sliplined pipe. McAlpine [6] used a rehabilitated concrete sewer to investigate a sliplined pipe and found that a composite model could be used to estimate the effect of the enhancement. Other researches are devoted to explore the mechanical properties of the rehabilitated pipes. SnapTite [7] considered that the compressive strength of the grout was not important, but Smith et al. [8] considered that a higher compressive strength of the grout resulted in a higher loadcarrying capacity of the rehabilitated pipe. Garcia and Moore [9] found that the steel culverts rehabilitated with sprayon liners responded like semirigid structures after rehabilitation. Moore and García [10] found that full interaction and partial interaction both occurred between the corrugated steel pipelines and spray on cementitious. Simpson et al. [11] found that the stiffness of the corrugated steel culverts rehabilitated with grouted HDPE pipe was larger than the prerehabilitated pipe and the negative arching of the soil had increased. Simpson et al. [12] found that the existing pipe carried most of the load, the grout and RC pipe (RCP) were bonded, and the ultimate loadcarrying capacity of the pipes depended on the bearing capacity of the unpaved ground surface. Tetreault et al. [13] concluded that the level of corrosion had no impact on the structural behavior and that paving the invert improved the structural performance. However, there are no guidelines for designing a sliplined RC culvert and the loadsharing theory of these culverts, so there is an urgent need to investigate the performance and loadsharing theory of the rehabilitated RC culvert.
In the current research, an RCP was used as a culvert and four liners were used to investigate the performance of the culvert before and after rehabilitation. A series of experiments was conducted on RCPs rehabilitated with a grouted steel pipe, CSP, HDPE, and shape steel bracket liners. The objectives of this research were to determine (1) the loadcarrying capacity of the five specimens (including an RCP, an RCP rehabilitated with grouted steel pipe, an RCP rehabilitated with grouted CSP, an RCP rehabilitated with grouted HDPE pipe, and an RCP rehabilitated with shape steel bracket, respectively), (2) the loaddeformation curves of five specimens, (3) failure characters and the cracks distribution of the specimens, and (4) the loadsharing theory of the sliplined pipes.
2. Laboratory Tests
Five specimens were used in this experiment to investigate the loadsharing mechanism of the sliplined RCPs, including (1) an RCP, (2) an RCP rehabilitated with a CSP (RGC hereafter), (3) an RCP rehabilitated with a steel pipe (RGS hereafter), (4) an RCP rehabilitated with a HDPE pipe (RGH hereafter), and (5) an RCP rehabilitated with a shape steel bracket (RGB hereafter). The RCPs had an internal diameter of 1200 mm and a wall thickness of 120 mm. Doublelayer coldstretched steel bars (φ_{6}, HRB400) were arranged in the RCPs at a spacing of 50 mm. The strength grade of the RCPs is C60 with a compressive strength of 59.73 ± 2.77 MPa and an elastic modulus of 36 GPa, the strength grade of the steel bars is HRB400 Coldstretched steel bar with a tensile strength of 575 ± 9 MPa and an elastic modulus of 210 GPa. The pitch of the CSP was 200 mm with a depth of 50 mm with an intact wall thickness of 3 mm. The steel pipe had an internal diameter of 1100 mm and a wall thickness of 10 mm. The specification of the shape steel bracket is ∟70 × 5 with a leg length of 70 mm and thickness of 5 mm. The designations of the steel pipe, CSP, and shape steel bracket are Q235, which has a minimum yield strength of 235 MPa and a minimum tensile strength of 370 MPa and an elastic modulus of 210 GPa. The HDPE pipe (DN/ID 1000, SN 8) had an internal diameter of 1000 mm, a ring stiffness of 8.2 kN/m^{2}, a tensile strength of 16 MPa, and a modulus of 800 MPa. All pipes were 1000 mm long. The spacing of the shape steel bracket is 200 mm, and short shape steel was used to connect the shape steel bracket longitudinally.
Two types of grout were used, including (1) C40 concrete for RGC and RGS and (2) highperformance grouting material for the RGH. The C40 concrete has a compressive strength of 49.93 ± 4 MPa and an elastic modulus of 32.5 GPa, and the highperformance grouting material has a compressive strength of 87.7 ± 3 MPa and an elastic modulus of 38 GPa. No grout was used for the RGB, and the shape steel bracket is directly contacted with the RCP. The grout rings of RGC has a minimum thickness of 50 mm (from the crest of the CSP to the inner of the RCP), that of RGS had a thickness of 50 mm, and that of the RGH had a thickness of 20 mm. The RGB has no grout.
A twopoint loading experiment was used in this study, and the load was applied to the specimens using a 2500 kN hydraulic actuator, which was attached to a reaction frame over the pipes. A distributing girder and two base plates were used to ensure that the concentrated load could not cause a deterioration of the specimens or a stress concentration. The specimens were loaded to the ultimate state with a loading rate of 15 kN/min, and the loading was paused at various stages to observe the experimental phenomena. The schematic of loading frame is shown in Figure 1, and the specimens are shown in Figure 2.
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Four string potentiometers with an accuracy of 0.1 mm were used to measure the vertical and horizontal diameter changes. Two string potentiometers were installed inside the rehabilitated pipe at the crown and invert, and the other two were installed outside of the rehabilitated pipe at the springlines. Because the RCP, grout, and liners are in close contact with each other at the crown and invert, the diameter changes of the three pipe materials should be equal; in addition, the base plates were installed outside the pipe, making it impossible to install the string potentiometers outside of the pipe. Therefore, the string potentiometers were installed inside the rehabilitated pipe. However, if the RCP, grout, and liners were to separate from each other at the springlines, the diameter change of the RCP would be larger than that of the other components; considering the most unfavorable situation; the string potentiometers were, therefore, installed outside of the rehabilitated pipe to monitor the diameter change of the RCPs.
3. Experimental Results
3.1. Loads versus Diameter Changes
Figure 3 shows the results of the applied loads versus the diameter changes for the unrehabilitated pipe (RCP) and the rehabilitated pipe (RGC, RGS, RGH, and RGB). The vertical and horizontal diameter changes for each pipe are of similar magnitude but have opposite directions. It can be seen from Figure 3 that the liners increased the loadcarrying capacity of the RCP.
The loadcarrying capacities and stiffness of the specimens were used to compare the results. When the specimens were cracking, the RGC had approximately 3.70 times the loadcarrying capacity of the RCP (370 kN versus 100 kN) and 1.86 times the vertical diameter change of the RCP (1.7 mm versus 0.915 mm); the RGS had approximately 1.5 times the loadcarrying capacity of the RCP (150.3 kN versus 100 kN) and 1.0 times the vertical diameter change of the RCP (0.9 mm versus 0.915 mm); the RGH had approximately 1.16 times the loadcarrying capacity of the RCP (116.1 kN versus 100 kN) and 1.0 times the vertical diameter change of the RCP (0.9 mm versus 0.915 mm); the RGB had approximately 1.05 times the loadcarrying capacity of the RCP (105.5 kN versus 100 kN) and 2.44 times the vertical diameter change of the RCP (2.2 mm versus 0.915 mm). When the specimens were yielding, the RGC had approximately 3.62 times the loadcarrying capacity of the RCP (901 kN versus 249 kN), 1.68 times the vertical diameter change of the RCP (16.5 mm versus 9.835 mm), and 1.70 times the horizontal diameter change of the RCP (13.3 mm versus 7.84 mm); the RGS had approximately 1.07 times the loadcarrying capacity of the RCP (266.74 kN versus 249 kN), 0.34 times the vertical diameter change of the RCP (3.34 mm versus 9.835 mm), and 0.26 times the horizontal diameter change of the RCP (2.04 mm versus 7.84 mm); the RGH had approximately 1.2 times the loadcarrying capacity of the RCP (298.1 kN versus 249 kN), 0.62 times the vertical diameter change of the RCP (6.1 mm versus 9.835 mm), and 0.42 times the horizontal diameter change of the RCP (3.3 mm versus 7.84 mm); the RGB had approximately 0.91 times the loadcarrying capacity of the RCP (225.9 kN versus 249 kN), 0.99 times the vertical diameter change of the RCP (9.4 mm versus 9.835 mm), and 0.96 times the horizontal diameter change of the RCP (7.8 mm versus 7.84 mm). When the specimens reached their ultimate state, the RGC had approximately 3.46 times the loadcarrying capacity of the RCP (968 kN versus 280 kN), 0.65 times the vertical diameter change of the RCP (22.9 mm versus 35.425 mm), and 0.52 times the horizontal diameter change of the RCP (19.6 mm versus 37.61 mm); the RGS had approximately 1.23 times the loadcarrying capacity of the RCP (344 kN versus 280 kN), 1.0 times the vertical diameter change of the RCP (35.35 mm versus 35.425 mm), and 1.16 times the horizontal diameter change of the RCP (43.75 mm versus 37.61 mm); the RGH had approximately 1.51 times the loadcarrying capacity of the RCP (422 kN versus 280 kN), 1.46 times the vertical diameter change of the RCP (51.7 mm versus 35.425 mm), and 1.28 times the horizontal diameter change of the RCP (48.1 mm versus 37.61 mm); the RGB had approximately 1.31 times the loadcarrying capacity of the RCP (366 kN versus 280 kN), 1.27 times the vertical diameter change of the RCP (45.1 mm versus 35.425 mm), and 1.02 times the horizontal diameter change of the RCP (38.3 mm versus 37.61 mm).
The stiffness was also different for these specimens as shown in Figure 3. The RCP had an initial stiffness of 109.3 kN/mm and a secant stiffness of 7.9 kN/mm at the ultimate state; the RGC had a much higher initial stiffness than the RCP (217.6 kN/mm versus 109.3 kN/mm) and a much higher secant stiffness (42.3 kN/mm versus7.9 kN/mm); The RGS had an initial stiffness of 167 kN/mm and a secant stiffness of 9.73 kN/mm; the RGH had an initial stiffness of 129 kN/mm and a secant stiffness of 8.16 kN/mm; The RGB had an initial stiffness of 48 kN/mm and a secant stiffness of 8.12 kN/mm. From the loadcarrying capacity, diameter change, and stiffness enhancement in the twopoint loading tests, it can be inferred that the CSP improved the loadcarrying capacity of the RCP because of a significant increase in its stiffness and capacity but reduced the ductility of the rehabilitated pipe (reduction in the ultimate diameter change); the steel pipe, HDPE pipe, and shape steel bracket improved the loadcarrying capacity of the RCP slightly, but the stiffness of the RCP has not been changed much by these liners.
3.2. Cracks Distribution
The crack distribution at ultimate state is shown in Figure 4. The crack distribution of the RCP reflects the ductility of the specimens, i.e., if the cracks are distributed over a wide range with equal spacing and the maximum width of the crack is small, it indicates that the specimens have good ductility. The cracks of RCP distributed over a wide range and the maximum width is 5 mm; the cracks of RGC distributed over a smaller range than that of RCP and the maximum width is 18 mm, and the steel bar is broken; the cracks of RGS distributed a small range and the maximum width is 17 mm, the steel bar is broken; the cracks of RGH distributed over a wide range and the maximum width is 8 mm; the cracks of RGB distributed over a smaller range than that of RCP and the maximum width is 3 mm. The distribution of the cracks illustrated that the RCP, RGH, and RGB have better ductility than that of RGC and RGS.
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The cracks at the ends of the specimens can be used to illustrate the combination of RCP, grout, and liners (Figure 5). Circumferential cracks appeared on the interface between grout and RCP, it showed that there is a circumferential slip between them, and they were not completely bonded together (Figures 5(b)–5(d)). The RGB, on the other hand, only contains RCP and liner, no bonding existed between them at all, slip occurred on the contact surface, and the shape steel bracket was buckling (Figures 5(e) and 5(f)). This phenomenon is particularly clear in liners and grouts, liners, and grouts separated with each other especially at the springlines. Figure 5 shows that RCP, grout, and liners are likely to act independently, rather than a composite system.
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4. LoadSharing Mechanism
If RCP, grout, and liner act as a pipe within a pipe system, the vertical deflections of RCP, grout, and liner should be equal. Deflection of the pipe subjected to the concentrated load F is shown in Figure 6. The subscripts 1, 2, and 3 represent the liner, grout, and RCP, respectively, and the vertical deflections of each pipe under twopoint loading are expressed as [14]where is the vertical decrease in the diameter of each pipe, is the concentrated load shared by each pipe, is the mean radius of each pipe, is the modulus of elasticity, and is the moment of inertia of the wall’s cross section per unit length of the pipe.
Therefore, the vertical deflections are equal (Δ_{1} = Δ_{2} = Δ_{3} = Δ) and the following equation is obtained:
The concentrated load shared by each pipe should satisfywhere is the concentrated load carried by the sliplined pipe, is the concentrated load carried by the liner, is the concentrated load carried by the grout, and is the concentrated load carried by the RCP.
If a stiffness factor , as , is defined, equation (2) can be changed into the following equation:where is the stiffness factor of the liner, is the stiffness factor of the grout, and is the stiffness factor of the RCP.
Equation (4) indicates that the concentrated load is shared among the pipe materials and that the distribution of the load depends on the stiffness factor . It should be noted that concrete has different shortterm and longterm stiffness and the longterm stiffness decreases over time [5], and the term is not applicable. In assessing the load sharing and stresses, the value of the longterm stiffness taken at the expected design life should be used for the expected loads and the value of the shortterm stiffness should be used for loading conditions of a transient nature. The stiffness of a concrete component can be calculated using the Code for Design of Concrete Structures [15]. For this experiment, a shortterm stiffness was used to calculate the loadcarrying capacity and the expression is as follows:where is the shortterm stiffness of the RCP, is the elastic modulus of the steel bar, is the area of the tensile steel bar, is the effective thickness of the RC pipe, is the nonuniform coefficient of the strains, is the ratio of the elastic modulus of the steel bar to the elastic modulus of the concrete, is the reinforcement percentage of the tensile steel bar, is the ratio of flange area to web area, should be ZERO for a rectangular section, is the characteristic value of the concrete tensile strength, is the effective reinforcement percentage, and is the tensile stress of the steel bar at the crack section.
At this point, is changed into the following expression:
If equations (7) and (4) were substituted into (3), the loadcarrying capacity of the sliplined pipe can be calculated by the following formula:where is the load carried by the RCP, , and are the pipe stiffness of the liner and grout, respectively, and , , and are the mean radii of the liner, grout, and RCP, respectively.
When the loadcarrying capacity of the RCP is known, equation (8) can be used to calculate the rupture load of the rehabilitated pipe. The external load (concentrated load F in this experiment) was borne by the RCP directly; once the RCP was damaged after rehabilitation, the load shared by the grout and liner suddenly increased, resulting in damage. As a result, the loadcarrying capacity of the sliplined pipe indirectly depended on the RCP. Therefore, the loadcarrying capacity of the sliplined pipe should be calculated based on the RCP (equation (8)).
The loadcarrying capacity of the RCP (280 kN) was already obtained by this twopoint loading experiment, taken as the loadcarrying capacity of the prerehabilitated pipe (F_{3}); B_{s} could be calculated by equation (5), where E_{s} should be 2.1 × 10^{5} MPa, A_{s} should be 650.9 mm^{2}, h_{0} should be 110 mm, ψ should be 0.707, α_{E} should be 5.83, ρ should be 0.0054, f_{tk} should be 2.85 MPa, ρ_{te} should be 0.011, and σ_{sq} should be 575 MPa; the mean radii of the RCP (r_{3}) should be 660 mm, B_{s} should be 1376053836.208 MPa mm^{3}, and φ_{3} should be 4.786 MPa. The moment of inertia of the grout could be calculated with the help of drawing software, such as AUTOCAD, since their cross sections are irregular. The stiffness factors φ_{1} of the liners can usually be querying in the parameters supplied by the manufacturer. Stiffness factors are summarized in Table 1.

The calculation results are shown in Table 2. The percent error for RGC and RGS (8% and 13.4%) is considerably higher than that for RGH and RGB (0.6% and 0.5%), and this may due to the thickness of the sliplined pipe wall and the buckling of the steel pipe. The RGC has a thickest wall than the other pipes, this will cause the load to diffuse in the pipe wall tremendously, so that the range of the loads act on the liners will be larger than that act on the RCP, and this indirectly improves the loadcarrying capacity of the liners. Since the theoretical analysis did not consider the diffusion of the loads and assumed the ranges of the loads act on the RCP, grout, and liner are same, this will make the calculated value is lower than the tested value. The RGS, on the other hand, takes the steel pipe as the liner; the steel pipe is easy to buckle under pressure, and this will make the actual loadcarrying capacity lower than that when it yields, while the theoretical analysis assumed the steel pipe could yield which make the calculated value higher than the tested value. Overall, it can be seen from Table 2, the maximum difference between the theoretical and experimental results is less than 13.4%, the minimum difference between them is 0.5%; the calculated results are so close to the experimental results. This proves that the above theory is very reasonable.
 
Note: F_{c} is the calculated loadcarrying capacity, as F_{3} × Σφ_{i}/φ_{3}; F_{t} is the tested loadcarrying capacity. 
In the theoretical derivation, the shortterm stiffness is used to consider the nonlinear effects of the RCP, and elastic stiffness is used for the grout and liners. The liners, on the other hand, have formed plastic hinge at the crown, invert, and springlines, when the sliplined pipes reached the ultimate state. Once the plastic hinge formed, the liners should be regarded as destructive [16]; the maximum stiffness that the liners can provide is the elastic stiffness, which should be used in loadsharing calculation. Foamed cement banking, cement mortar, and fine aggregate concrete are always used as grout; once these materials cracked, they cannot carry any loads, but only when they work alone. In a sliplined system, the radial pressures applied by the RCP and liners will constrain the radial deformation of the grout. Moreover, friction exists on the contact surface of RCP, grout, and liners, though it will not prevent slipping, it will reduce the tensile stress of grout. The friction is always opposite to the tensile stresses caused by the bending moments, and this is similar to an imaginary force that resists the tensile stress of grout at the crack, which allows the grout to act like an elastic body and to exert an imaginary pull. If the elasticity hypothesis is wrong, the calculated results in Table 2 cannot be as good as the experimental results, which indirectly shows that the elasticity hypothesis is correct.
The results also indicate that the plasticity approach and composite behavior method used by Smith et al. [8] are likely not appropriate for rehabilitating RCPs given the behavior demonstrated by these specimens.
5. Conclusions
This paper presented a loadsharing theory to estimate the loadcarrying capacity of the sliplined pipes. The CSP, steel pipe, HDPE pipe, and shape steel bracket were used as liners to rehabilitated RCPs. Including an RCP, there are 5 pipes were tested in twopoint loading experiments. The current investigation was undertaken to examine the effect of different liners on the performance and loadcarrying capacity of rehabilitated RCPs. The following key conclusions can be drawn from this work:(1)The liners increased both the loadcarrying capacity and the stiffness of the RCP. Among them, the CSP increased the loadcarrying capacity of the RCP most (3.46 times greater than RCP), followed by HDPE pipe, shape steel bracket, and steel pipe (1.51, 1.31, and 1.23 times greater than RCP, respectively). Although the CSP increased the loadcarrying capacity of the RCP, it reduced the ductility of the RCP, whereas HDPE pipe, shape steel bracket, and steel pipe did not.(2)Cracks distributions of the specimens imply that the RCP, grout, and liners act independently, and a loadsharing theory was proposed based on compatibility of vertical deformation. The loadsharing capacity of the RCP, grout, and liners depends on the stiffness factors (φ) of the pipes. When the liners were used to rehabilitate the RCPs, the liners should not be treated as new pipes for the structural analysis, and the loadcarrying capacity of the RCP and grout should also be considered. Once the RCP is damaged after rehabilitation, the loads distributed by the grout and liner will suddenly increase so that they cannot withstand a large load and damage will occur.(3)A “loadsharing” theory was proposed in this study, and equation (8) provides estimates of the loadcarrying capacity of the sliplined pipes (the maximum difference between the theoretical and experimental results is less than 13.4%, and the minimum difference between them is 0.5%). Since different liners were used to compare the calculated results and experimental results, this theory is highly reliable.
While the experiments were undertaken under twopoint loading, the RCPs were buried in the soil in practical engineering. The surrounding soil would influence the vehicle loads distribution and make an interaction between the soil and RCP, and the stress state of the RCP will be very different from these experiments. Therefore, the surrounding soil should be considered in the future research.
Abbreviations
RCP:  Reinforced concrete pipe 
RGC:  Reinforced concrete pipe rehabilitated with a corrugated steel pipe 
RGS:  Reinforced concrete pipe rehabilitated with a steel pipe 
RGH:  Reinforced concrete pipe rehabilitated with a highdensity polyethylene pipe 
RGB:  Reinforced concrete pipe rehabilitated with a shape steel bracket 
Δ_{i}:  Vertical decrease in the diameter of each pipe 
Δ_{1}:  Vertical decrease in the diameter of the liner 
Δ_{2}:  Vertical decrease in the diameter of the grout 
Δ_{3}:  Vertical decrease in the diameter of the RCP 
F_{i}:  Concentrated load shared by each pipe 
F_{1}:  Concentrated load shared by the liner 
F_{2}:  Concentrated load shared by the grout 
F_{3}:  Concentrated load shared by the RCP 
r_{i}:  Mean radius of each pipe 
r_{1}:  Mean radii of the liner 
r_{2}:  Mean radii of the grout 
r_{3}:  Mean radii of the RCP 
E_{i}:  Modulus of elasticity of each pipe 
E_{1}:  Modulus of elasticity of the liner 
E_{2}:  Modulus of elasticity of the grout 
E_{3}:  Modulus of elasticity of the RCP 
I_{i}:  Moment of inertia of the wall's cross section per unit length of the pipe 
I_{1}:  Moment of inertia of the liner 
I_{2}:  Moment of inertia of the grout 
I_{3}:  Moment of inertia of the RCP 
F:  Concentrated load carried by the sliplined pipe 
φ:  Stiffness factor, as 
φ_{1}:  Stiffness factor of the liner, as 
φ_{2}:  Stiffness factor of the grout, as 
φ_{3}:  Stiffness factor of the RCP, as 
B_{s}:  Shortterm stiffness of the reinforced concrete, N·mm^{2} 
E_{s}:  Elastic modulus of the steel bar, MPa 
A_{s}:  Area of the tensile steel bar, mm^{2} 
h_{0}:  Effective thickness of the RCP, mm 
ψ:  Nonuniform coefficient of the strains 
α_{E}:  Ratio of the elastic modulus of the steel bar to the elastic modulus of the concrete 
ρ:  Reinforcement percentage of the tensile steel bar 
f_{tk}:  Characteristic value of the concrete tensile strength, MPa 
ρ_{te}:  Effective reinforcement percentage 
σ_{sq}:  Tensile stress of the steel bar at the crack section, Mpa 
ρ_{2}:  Density of the grout, kg/m^{3} 
f_{2}:  Compressive strength of the grout, MPa 
F_{c}:  Calculated loadcarrying capacity of the sliplined pipe 
F_{t}:  Tested loadcarrying capacity of the pipe 
1:  Liner 
2:  Grout 
3:  RCP. 
Data Availability
The (experimental results) data used to support the findings of this study are included in the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This paper is a result of a research project funded by the National Natural Science Fund (51278202), Science and Technology Support Program of Hunan Province (2015039), and Guangzhou Communication Investment Group Co., Ltd.
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Copyright
Copyright © 2019 Baijian Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.