Prestressed composite concrete pipe (PCCP) has been widely used in water-transmission line and has been proven with many advantages over pure concrete or steel pipes, such as high performance with relatively low cost for materials as well as simplified installation and construction process. Recent efforts have been made to enable the PCCP structure suitable for pipe jacking method so as to replace the conventional cut and cover method. In this way, the construction time, disturbance to nearby structures, and the cost can be greatly reduced. In this paper, we present the full-scale experimental and numerical studies of PCCP and the evaluation of fracture and delamination behaviour of the structure when it is used with pipe jacking construction method subjected to various jacking forces and ground conditions.
Prestressed concrete cylinder pipe (PCCP) has been widely used in water-transmission line and has proven to have many advantages such as high performance price ratio and convenient installation process. PCCP also provides high bearing capacity, long durability, and resistance to corrosion and leakage. The total length of PCCP lines installed in North America is estimated to be 35,150 km [
The cut and cover method is mostly used in PCCP construction. Recently, PCCP structure has been attempted to be used with pipe-jacking construction method, for example, the water-intake project at Langshan Mountain, Nantong city, China. In that project, the pipe has an internal diameter of 2200 mm and the total jacking length was 340 m. Compared with steel pipe or other jacking pipes, PCCP used as a jacking pipe structure has many advantages, namely, (a) lower cost that PCCP has a much lower steel consumption than steel pipe; (b) faster construction that the spigot and socket structure as well as the flexible docking simplify the construction process; (c) better corrosion resistance since the steel cylinder and wire are protected inside the concrete; (d) improved leakage protection due to the fact that the steel cylinder and rubber gasket installed at the pipe joint under pressure will ensure good resistance to leakage.
The few projects using PCCP for pipe-jacking only use small diameter cross section and for short distance jacking. And the PCCP have been proven to be safe and feasible to be used in pipe jacking method. However, the question rises as to whether the structure remains safe and reliable when the pipe diameter is increased to 3600 mm and for long jacking distance. In this paper, we aim to study the mechanical behavior of the PCCP under such condition and possible failure mechanism subject to axial and lateral compressive loading conditions.
The present mapper will model the PCCP failure by employing materials models suitable for concrete and steel. The interface failure between layers of the structure will also be taken into account. The numerical methods for the modelling of concrete component/structure failure as well as concrete material include the conventional finite element method and other novel methods for capturing fracture propagation such as meshless methods [
The pipe tested in the experiment is shown in Figures
Cross section of the pipe.
Longitudinal cross section of the pipe.
In order to test the bearing capacity of the pipe and verify the numerical model, a three-edge bearing external load test subjected to ASTM C497 was performed firstly. Then several other tests involving two pipes were conducted with different relative initial angles between the two pipes and different acting oil cylinders in the axial direction. Three types of angle were applied in these tests: no deflection, small deflection (0.3°), and large deflection (0.5°).
In each of the axial loading tests, all of the acting cylinders including the lateral cylinders and axial cylinders were controlled by one loading system, to keep the deflection of two pipes constant during the whole loading process.
To better understand the mechanical characteristics of the pipes and to predict the possible failure modes, we built a 3-dimensional finite element model using commercial software ABAQUS to help us analyze this experiment. The mesh is shown in Figure
3D FE model of the experiment.
The loading process and boundary condition in our numerical model were applied as similar as possible to that of experiment. In the three-edge bearing test, the pressure at the top of pipe exerted by steel beam above was applied by surface traction, and the batten bracing the pipe was modeled by elastic foundation. In the axial loading tests, the axial forces and lateral forces were applied by concentrated force and pressure, respectively, and the guide rail was simulated by solid model.
In our finite element analysis, to avoid hourglass modes brought by reduced integration, volumetric locking brought by quadratic fully integration and shear locking brought by linear integration, we finally choose 8-node linear solid elements in incompatible modes (C3D8I) to model the concrete, wood cushion, and other solid materials. Prestressing steel was modeled using 2-node linear 3D truss element (T3D2), and the steel cylinder was modeled using the 4-node doubly curved shell element in reduced integration (S4R).
The interfaces between two pipes were considered as follows: the interface between the middle cushion and rear pipe was completely tied together, and the interface between the front pipe and middle cushion was modeled by surface-to-surface contact in the ABAQUS. There were some other interfaces using the same method to deal with as listed in Table
The interfaces using surface-to-surface contact.
The location of interface | Sliding formulation |
---|---|
The front pipe and front cushion | Small sliding |
The front pipe and middle cushion | Finite sliding |
Two pipes and their guide rail | Small sliding |
As to the boundary conditions, the base surfaces of two guide rails were fixed in the vertical direction. The rear surface of rear cushion was fixed in the axial direction.
The uniaxial stress (
Concrete uniaxial strain-stress relationship subjected to tension and compression.
A summary of the mechanism properties of the material used in the FE model is listed in Table
Material parameters.
Material | Young’s modulus (GPa) | Yield stress (MPa) | Density (kg/m3) |
---|---|---|---|
Concrete | 35.5 | 32.4 (compressive)/2.64 (tensile) | 2500 |
Steel cylinder | 206 | 300 | 7800 |
Prestressing wire | 205 | 1570 | 7800 |
Configurational steel bar | 190 | 500 | 7800 |
Wood cushion | 0.4 | — | — |
Guide rail | 200 | — | — |
Iron band | 1000 | — | — |
The numerical model is set up corresponding to the experiments. Some local output parameters which cannot be measured accurately such as strain and stress can therefore be obtained from the FE modelling and can be used for the reference of design and construction.
The prestressing of the pipe is applied using equivalent temperature descent method [
Initial stress of prestressing wire.
Initial radial stress of concrete.
The three-edge bearing test, which aimed at determining the strength of pipe withstanding the vertical crushing loads, also established the load-strain curve of many parts of the pipe and verified our numerical models. The maximum vertical loading amount was 2600 kN in our model. Figure
Circumferential strain of inner wall.
Figure
Damage distribution of concrete in three-edge bearing test.
A remarkable phenomenon during the test was the delamination of exterior concrete and intermediate concrete core. To analyze this phenomenon, the radial stress and circumferential shear stress were illustrated in Figure
Stress at the interface of exterior concrete and intermediate concrete core.
Radial stress
Circumferential shear stress
In this section the axial loading test was analyzed in four aspects: the concrete, the interface between external cover and intermediate concrete core, the prestressing wire, and the influence of wood cushion. These numerical analyses were verified by monitor data.
Figure
Axial strain of concrete at 90°, section 1.
The axial stress contour plot of tests 1, 2, and 5 was shown in Figures
Radial stress of the front pipe in test 1.
Radial stress of the front pipe in test 2.
Radial stress of the front pipe in test 5.
For the axial tensile stress, the initial value at the inner wall of the spigot of the pipe is 2.10 MPa, which is close to the limit tensile strength of the concrete. And the tensile stress decreases when loaded without angle whereas increases to 2.35 MPa when loaded with angle. The deflection angle does not have impact on the maximum tensile stress (see Figure
The maximum stress of concrete under different angles (test 7~10).
For the axial compress stress, in tests 1 and 2, the maximum axial compress stresses occur at the intermediate concrete core near the socket. In test 5 the maximum compress stress is located at the concrete external surface near the spigot at the left side, the same side where the center of axial eccentric force is located. Other tests where pipes were loaded with certain angle have similar axial stress distribution like test 5.
Figure
The circumferential stress contour of test 5 is depicted in Figure
Circumferential stress of the front pipe in test 5.
Distortion of the front pipe in test 5 (deformation scale factor: 100).
This section presents the analyses on the surface between external concrete cover and intermediate concrete core. This interface is important because that is also where the prestressing wire is located. Once the interface delaminates, the prestressing wire might be damaged due to corrosion. Zarghamee et al. [
Radial strain on the surface at 270°, cross section 1.
Figure
Radial strain at the surface of the front pipe.
Other tests on deflection angle show similar strain results as in test 5. The maximum radial strain grows significantly with the deflection angle. And the highest value of maximum radial strain is approximately 247
The ultimate tensile strain of the concrete material used in the experiment is 76.5
Figure
Stress increment of prestressing wire in test 5.
From the results it can be seen that the damage of prestressing wire should not occur during substantial pipe-jacking process, since the load conditions in experiment are way more critical than the actual jacking conditions.
Only few literatures have been devoted to the study of the influence of cushion on the mechanical characteristics of the jacking pipes. Wang [
Schematic diagram of curved pipe jacking.
A new FE model is established in which the elastic modulus of wood cushion is set as 200 MPa, half as large as before. Other parameters and conditions are the same as the FE model simulating test 5.
After Young’s modulus of the wood cushion has been changed, the stress distribution of concrete is almost the same as the previous modelling results. The maximum axial compressive stress reaches 23.2 MPa, about 10% less than the previous result. The maximum axial tensile stress is 2.35 MPa, remaining constant compared to the previous result. The maximum radial strain at the surface between external concrete cover and intermediate concrete core drops to 128
In this paper, a study on the mechanical performance of PCCP used as a pipe-jacking structured by both experimental tests and FE modelling is presented. A three-dimensional FE model is established for the fracture and delamination analysis of PCCP in a three-edge bearing test and several axial loading tests. The results are verified by experimental data. A number of structural problems are highlighted and suggestions are given accordingly. Firstly, the initial axial tensile stress at the inner wall of the spigot of the pipe is high and may cause circumferential cracks during pipe-jacking, which has been reproduced in full scale experiments. The axial eccentric force increases the tensile stress and results in the local structure damage. Secondly, the deflection angle in pipe-jacking project for this pipe should not exceed 0.5° during the piping process. Thirdly, the prestressing wire exhibits low stress growth and low prestress loss during the loading. And it will not be damaged due to pipe-jacking process. Finally, softer wood cushion will improve the structure safety of the pipe. Further study is needed to determine the relationship between material property of cushion and the mechanical characteristics of pipe, providing suggestions on the design of cushion.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge the supports from the NSFC Program (51379147), the National Basic Research Program of China (973 Program: 2011CB013800), the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1029), and Shanghai Chenguang Program (12CG20).