One steel grid and five thin-walled concrete-filled steel tubes (CTST) used as the supports of tunnel were tested in site for investigating the mechanical behavior. The mechanical influences of thickness, node form, and concrete on CTST were gained and compared with the impacts on steel grid. It is indicated that high antideformation capacity of CTST improved the stability of surrounding rock in short time. The cementitious grouted sleeve connection exhibited superior flexibility when CTST was erected and built. Although the deformation of rock and soil in the tunnel was increasing, good compression resistance was observed by CTST with the new connection type. It was also seen that vault, tube foot, and connections were with larger absolute strain values. The finite element analysis (FEA) was carried out using ABAQUS program. The results were validated by comparison with experimental results. The FE model could be referred by similar projects.
Although steel grid and steel arch are common used structures for tunnel support, they are not good enough for large deformation control of soft rock. As a new structure, concrete-filled steel tubes (CFST) with high bearing capacity and good plasticity have a broad application prospect [
Compared with ordinary CFST, less steel and welding work are needed for CTST because of the thin wall. Previous research mainly focused on the experimental behaviour of CTST in the laboratory [
In this study, tests have been done in a cable tunnel in Jinan while an FE analysis by ABAQUS program is conducted to do further research on the application of CTST in tunnel.
The cable tunnel is located in Shunhua Road in Jinan. As the tunnel in strong weathered rock, it is 145 m long. The thickness of overlaying soil of the tunnel is about 6.7~8 m. The maximum excavation height is 4.6 m while the largest excavation span is 6.75 m. For presupport, advanced small pipes with 1.5 m longitudinal distance are supplemented. Then, C25 early-strength concrete is sprayed for primary support. Subsequently, steel grid or CTST is used to support the tunnel. At last, C30 waterproof concrete with seepage resistance
In the study, the steel tube is segmented for installation and transport. One end of segment is sealed while the other is opened. The segment is sealed by precast head pale once the concrete vibrating finished. Finally, the segments are spliced in site. According to the results of geological exploration, the counter-arch for the support is not set owing to the small effect of soil arch. The size and segmentation of CTST support are shown in Figure
Size and segmentation of CTST support.
5 CTST specimens and 1 steel grid specimen are tested. The details of the test specimens are shown in Figure
Parameters of CTST specimens.
Specimens | ST-1 | ST-2 | ST-3 | ST-4 | ST-5 |
---|---|---|---|---|---|
|
159 | 159 | 159 | 159 | 159 |
|
5.5 | 4.5 | 3.5 | 5.5 | 5.5 |
Concrete | Filled in | Filled in | Filled in | Not filled in | Filled in |
Connection | Cementitious grouted sleeve | Cementitious grouted sleeve | Cementitious grouted sleeve | Cementitious grouted sleeve | Ordinary sleeve |
Note:
Steel grid section.
For the butt joint in the tunnel, a new cementitious grouted sleeve connection is designed and shown in Figure
Details of cementitious grouted sleeve connection.
According to “metallic materials at ambient temperature tensile test method” (GB/T228-2002), steel tube Q235B is used in the test. The yield strength of the steel tube is 255.3 MPa while the tensile strength is 330.7 MPa. The elastic modulus is 2.06 × 105 MPa and Poisson's ratio is 0.30. As what is shown in Figure
Manufacturing procedure of steel tube in factory.
The mix proportion of cement, sand, and gravel used in the concrete is 1 : 1.51 : 2.45. The water cement ratio of the cement is 0.41. The expansion agent ratio is 11%. The water reducing agent ratio is 0.5%. According to the relevant Chinese standards, compression tests are carried out, as shown in Figure
Compressive strength of concrete at different ages.
Age | 3 d | 7 d | 28 d |
|
|||
Compressive strength (MPa) | 16.3 | 24.8 | 45.5 |
Material test of concrete.
Vault and convergency displacement are monitored in the test. The settlement value is tested with a level, and the convergency values are tested with a convergence device.
It is shown in Figure
The location of resistance strain gauge and fiber.
As what is shown in Figures
The location of pressure gauge.
Pressure measurement in situ.
The colligation of the pressure monitor
Read the pressure monitor in situ
The settlement-time curves in the vaults and convergency value-time curves in both sides are shown in Figures
Settlement-time curves in the vaults.
Convergency value-time curves in both sides.
Figures
Load-settlement curves in the vaults.
Load-convergency value curves in both sides.
It is shown in Figure
It is indicated in Figure
In this study, tangential strains of 5 CTST supports are mainly monitored.
Load-strain curves in the outer edge of the CTST supports.
C1
C2
S3
Load-strain curves on the center axis of the CTST supports.
A1
A2
Load-strain curves in the inner edge of the CTST supports.
S1
S2
B1
Load-strain curves at the connections of the CTST supports.
D1
D2
D3
D4
Strain distribution curves in the outer edge of the CTST supports.
Strain distribution curves in the inner edge of the CTST supports.
It is obvious that the tensile strain was gradually transformed into compressive strain along the direction of vault to the straight wall in the outer edge. All the straight walls are in compression. The maximum tensile strain of outer edge is located in the vaults while the maximum compressive strain is generated near the lower connection. The inner edges of the CTST supports are all in compression with the maximum strain in the vaults. But the maximum compressive strain of ST-5 is located near the lower connection. The compressive strain values of vault, foot, and area near the lower connection are relatively larger.
The finite element analysis (FEA) of ST-1~ST-3 by ABAQUS program is conducted. Four-noded shell element with reduced integration (S4R) is used for steel tube. Von-Mises kinematic hardening rule is adopted for steel material. The elasticity modulus of steel takes 206 GPa. Poisson's ratio takes 0.30.
The three-dimensional 8-noded solid element with reduced integration (C3D8R) is used to mesh the concrete. The elasticity modulus takes
Considering the influences of surrounding rocks at the scope of 5 times the width, the spacing of support (0.5 m) was taken as the width of surrounding rock. Considering that the mid-separate wall was established between CTST supports in the experiment which could affect the force situation, a vertical constraint was added at both sides in the outer edges of the concrete lining. Only 1/2 model is needed when calculation according to the structure symmetry, as shown in Figure
Mesh of steel tube model.
The whole mesh of tunnel
Lining and support mesh
In this test, the displacement values were taken as the load to apply to the supports, and the FEA results were compared with the testing results.
Based on the FEM results, the maximum strain distribution curves of the inner and outer edge of ST-1~ST-3 are shown in Figures
Comparison of the maximal strain between FEA and test results.
Location | Specimen | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
ST-1 | ST-2 | ST-3 | ||||||||
|
|
|
|
|
|
|
|
|
||
The outer edge | Vault | 179.7 | 185.3 | 3.0% | 124.2 | 134.8 | 7.9% | 131.3 | 143.4 | 8.4% |
Largest corner | −97 | −108 | 10.2% | −66.3 | −72.2 | 8.2% | −103.7 | −103.4 | 3.0% | |
Top of the straight wall | −175 | −185.8 | 5.8% | −145 | −133 | 9.0% | −194.2 | −191.2 | 1.6% | |
Foot | −122.9 | −133.7 | 8.1% | −92.9 | −98.9 | 6.1% | −125.3 | −135.4 | 7.5% | |
|
||||||||||
The inner edge | Vault | −240.8 | −252.7 | 4.7% | −192 | −191.3 | 4.0% | −224.3 | −247.3 | 9.3% |
Largest corner | −188.1 | −215.3 | 2.6% | −128.5 | −147.9 | 1.3% | −175.8 | −185.4 | 5.2% | |
Top of the straight wall | −156.2 | −147.4 | 6.0% | −109.4 | −100.3 | 9.1% | −149.9 | −144.7 | 3.6% | |
Foot | −213.4 | −230.3 | 7.3% | −164.5 | −171.4 | 4.0% | −217.5 | −233.7 | 6.9% |
Note:
Strain distribution curves in the outer edge of the CTST supports.
Strain distribution curves in the inner edge of the CTST supports.
From Table
In line with the results of the experimental study and FEA analysis, the results could be summarized as follows: With better compression resistance, CTST benefits to resist against the deformation of the tunnel. The deformation of tunnels with CTST can reach the steady state in a short time. All of the CTST supports perform elastic during the experiment. The higher steel ratio and concrete inside, the larger stiffness of CTST obtained. The cementitious grouted sleeve connection with superior flexibility exhibits better resistance to deformation. The tensile strain is gradually transformed into compressive strain along the direction of vault to the foot in the outer edge. The inner edges of the CTST supports are all in compression. The absolute strain values of vault, connections, and tube foot are larger. The mechanical behaviors of CTST supports in the tunnel are analyzed with ABAQUS software package. The analysis results agree well with the test results. The maximum error is 10.2%. The model and the parameters chosen are relatively reasonable and could be used for the structural analysis of the similar supporting projects.
The authors declare that they have no conflict of interests.