Pipe Roof Method Research Articles

Numerical Simulation Study on Frost Heave during the Freezing Phase of Shallow-Buried and Undercut Tunnel Using the Freeze-Sealing Pipe Roof Method

This article begins with the engineering geological conditions and freezing design scheme of the Gongbei Tunnel’s underground excavation section, then applies the mathematical model theory of the horizontal freezing-tunnel-formation freezing temperature field and frost heave displacement field, and builds a coupled two-dimensional finite element calculation model. The development law of the frozen soil curtain and the variation law of frost heave displacement during the freezing phase were studied by comparing on-site observation data. According to the findings of this study, the construction of the artificial frozen curtain is mostly based on two types of freezing tubes that freeze the soil between jacked pipes and seal the water. At 90 days, the thickness of the frozen soil curtain ranges from 2.32 m to 2.58 m, guaranteeing that its strength fulfills water-sealing safety criteria. The distribution and variation of frost heave displacement are highly related to engineering geological circumstances, the freezing scheme, the frozen soil curtain development process, and the pipe curtain structure. The maximum vertical frost heave displacement value at any time is located at the centerline, which is 155.67 mm at 90 d. The numerical simulation findings are acceptable and can potentially be utilized for predicting frost heave in subsequent projects. More research is required to effectively represent complicated working conditions and to develop more exact large-scale numerical models for tunnel excavation, support structure building, and other situations.

Field Temperature Data Analysis of the Water Resistance Performance of Freeze-Sealing Pipe Roof Used in Tunnel through Water-Rich Strata

A new presupporting technology named freeze-sealing pipe roof (FSPR) method was adopted in the construction of Gongbei tunnel (Zhuhai, P.R. China), which was a critical part of Hong Kong-Zhuhai-Macau Bridge (HZMB). The method combined the pipe-roofing with artificial ground freezing (AGF). The water resistance performance of FSPR surrounded by water-rich soils was studied based on the field temperature data. The time-dependent temperature changes of the frozen soil between two adjacent pipes were presented, and the thicknesses of the frozen soil in different freeze period were calculated. Besides, the remote temperature monitoring system used in this field research was presented systematically. The thermal effects of the Hurricane Nicole on the frozen soil wall were also observed. The results showed that the closure of frozen soil wall can be classified into two different modes. The frozen soil between the pipes before excavation had met the design requirements before excavation, and the FSPR maintained satisfied water resistance performance.

Field test and numerical simulation of ground settlement in super shallow buried station excavation supported by transverse pipe roof

The pipe roof is an effective means to control the surface settlement during the construction of the super-shallow buried subway station. The transverse pipe roof method can not only reduce the construction work surface, but also improve the construction efficiency by multi-stage flow construction. Based on the transfer station project of the Olympic Sports Center of Jinan Rail Transit Line 3, the ground settlement law caused by the excavation of ultra-shallow buried subway station under the action of transverse pipe roof is analyzed through field measurement. The mechanism of pipe roof controlling ground settlement and the influence of steel pipe spacing and steel pipe length on the bearing mechanism of pipe roof are analyzed by numerical simulation. The following conclusions are drawn. 1) The surface settlement curve caused by the excavation of the small pilot tunnel in the lateral direction of the station is a wavy U-shaped, and the surface settlement curve becomes a smooth U-shaped after the soil is broken. In the longitudinal direction of the station, the surface settlement is jagged, and the surface settlement above the pipe roof is less than the surface settlement between the pipe roofs. 2) After soil excavation, pipe roof can bear the load of surrounding rock and restrain the settlement of soil above, and soil arch effect is formed between pipes to restrain the relaxation of soil between pipes. With the increase of the spacing of the steel pipe, the height of the soil arch increases. After exceeding a certain spacing, the micro-soil arching effect disappears and the surface settlement increases. 3) The length and stiffness of pipe roof will affect the bearing capacity of pipe roof and there is an upper limit. When it exceeds the upper limit, increasing the length or stiffness of pipe roof has little effect on the control effect of ground settlement. The research can provide reference for shallow buried excavation station and other projects.

Study on the whole process application of advanced grouting pipe shed support under urban complex stratum conditions

The stability control of surrounding rock is always one of the important problems in tunnel construction. As a representative of tunnel pre-construction technology, advance grouting pipe roof support method is widely used in underground space engineering under various complex geological conditions, especially in strata with many fractured rock masses and rich groundwater content. Due to the complexity of the application of advance grouting pipe roof method, few studies have been conducted on the whole process of pipe roof application in actual projects. Taking the shield departure project of Guangzhou Metro Line 13 as the background, this paper uses the modified BQ method to grade the surrounding rock environment of the tunnel from the perspective of the stability evaluation of the surrounding rock of the tunnel. The result shows that the revised surrounding rock is grade v. further combining with the geological exploration report, it is found that the rock mass joints and fissures are large and may further develop under the change of the surrounding rock mass stress environment, and the formation water content is large. The surrounding rock is prone to large deformation under the action of groundwater, and even lead to surface collapse in serious cases. Therefore, it is necessary to adopt the advance grouting pipe roof support method. Subsequently, the influence of different pipe roof design parameters such as pipe roof diameter, number of pipe roofs, reinforcement thickness and layout range on its support effect is analyzed by using finite element software, and appropriate parameter values are determined and applied to the background project. Finally, considering the measurement error existing in actual monitoring, a dynamic safety evaluation method of tunnel structure based on reliability theory is proposed, and the background project is analyzed and evaluated by this method. The analysis shows that the tunnel structure with advance grouting pipe roof support meets the safety requirements, confirms the effectiveness of the design parameters, and can provide reference for tunnel construction under the same geological conditions.

An Analytical Solution to Steady-State Temperature Field in the FSPR Method Considering Different Soil Freezing Points

Taking the freeze-sealing pipe roof method (FSPR) adopted in the Gongbei Tunnel project as the background, this study develops a simplified calculation model by considering different soil freezing points, tube layout, and site conditions. The analytical solution of the linear single row tubes is then used to formulate the analytical solution of the freezing temperature field of two kinds of linear single row tubes, with equal spacing in the image plane. This is achieved through conformal mapping and the variable separation method. Finally, the analytical solution to the steady-state temperature field of FSPR in the object plane is obtained. The numerical solutions of common freezing parameters in freezing engineering are analyzed to evaluate the accuracy of the analytical solution, and the influence of parameter differences on the freezing temperature field are also discussed, to provide a theoretical reference for popularization and application of similar construction methods.

Mathematical model of temperature field and its analytical solution for Freeze-Sealing Pipe-Roof method induced by radial offset of adjacent jacking pipes

In order to achieve the purpose of sealing water between adjacent jacked pipes in construction of long-distance and large-section tunnel, an innovative method called Freeze-Sealing Pipe Roof (FSPR) was applied to the Gongbei tunnel, which is a key part of Hong Kong-Zhuhai-Macau Bridge in China. Since the pre-constructed jacking pipes provides great convenience for flexible placement of freezing tubes inside, it is crucial to study the temperature field of different freezing states. This paper first established a new mathematical model considering radial offset of jacking pipe according to the practical engineering, and then the analytical solution to steady-state temperature field was also derived. Moreover, temperature distribution of four typical cross-sections were analyzed to evaluate the freezing effect on different paths. Results show that the analytical results are in good agreement with the numerical results, and the quasi-double-circle freezing model can be well applied to the practical problem of radial offset. Soil between steel pipes will be lowered to a sufficiently low temperature and form a fairly reliable water-sealing curtain. By designing the radial offset between freezing tubes in solid and hollow pipes, the freezing effect can be better ensured. The conclusions have important guiding significance for design and construction of FSPR method in future projects.

Freezing Effect of Enhancing Tubes in a Freeze-Sealing Pipe Roof Method Based on the Unsteady-State Conjugate Heat Transfer Model

The freeze-sealing pipe roof (FSPR) method was applied as an innovative construction technology to the Gongbei Tunnel of the Hong Kong–Zhuhai–Macau Bridge. A freezing scheme involving master freezing tubes, enhancing freezing tubes, and limiting freezing tubes is the key component of the freezing effect of the FSPR method during the construction process under various working conditions. This is related to whether the thickness and temperature of the frozen soil meet the design requirements under various complex working conditions, and it is also related to frost heave control and energy saving. Based on the unsteady-state conjugate heat transfer model, different freezing schemes of enhancing freezing tubes—that is, the shape, layout, operating duration, and heat preservation—were simulated to analyze the freezing effect, which can be measured by the thickness of frozen soil around the steel pipes and the average temperature of the frozen soil curtain. The results show that the greater the contact area between the enhancing tube and the inner wall of the steel pipe, the better the freezing effect, and that the semicircle enhancing freezing tube scheme is superior to the other three shapes of freezing tubes. The arrangement of enhancing freezing tubes far away from the excavation surface, without heat preservation measures, has a better freezing effect due to the function of the hollow pipe as a freezing pipe. Moreover, the enhancing freezing tube can be operated intermittently to control frost heave. Our research simulated the temperature fields of different media—such as steel pipes, frozen soil, and air—providing a design basis for similar projects, such as the combination of the pipe-roofing method and artificial freezing method.

Unsteady-State Conjugate Heat Transfer Model of Freeze-Sealing Pipe Roof Method and Sensitivity Analysis of Influencing Factors of Freezing Effect

The freeze-sealing pipe roof (FSPR) method has been applied to the Gongbei tunnel of the Hongkong–Zhuhai–Macau Bridge, which is the first application of this method in the world. The purpose of the ground-freezing method is soil waterproofing. Temperature is an important indicator for measuring the freezing effect; however, the FSPR method involves unsteady-state conjugate heat transfer between frozen soil, steel pipes, concrete, air, and other media. This paper proposes an unsteady-state conjugate heat transfer model and establishes a global solution algorithm of a strong coupling governing equation based on the virtual density method. Then, a calculation based on COMSOL software is realized and validated. The sensitivity of different factors such as initial formation temperature, different soil layers, and brine temperature on the freezing effect was studied by simulating the FSPR model. It is concluded that the brine temperature had a greater impact on the freezing effect, followed by the soil layer, whereas the formation temperature had the least impact. For muddy silty clay, if the brine temperature is −20 °C, it takes 44 days to meet the design requirements of 2 m. If the brine temperature is −30 °C, 27 days is enough. When the formation temperature is 20 °C, it takes 20 days for medium gravel sand to reach the thickness of the freezing curtain, and 32 days for muddy silty clay. Compared to other soil layers, the freezing effect of the medium gravel sand is relatively better. This research has a certain impetus to similar multimedia freezing heat transfer issues.

Frozen curtain characteristics during excavation of submerged shallow tunnel using Freeze-Sealing Pipe-Roof method

The Freeze-Sealing Pipe-Roof (FSPR) method, which has been applied for the first time in the Gongbei Tunnel of the Hong Kong-Zhuhai-Macao Bridge, is a new approach of tunnel pre-support that allows flexible adjustment of freeze tube arrangement and can be adapted to different environmental conditions. When the FSPR method is used to construct shallow burial submerged tunnels, the frozen wall to hold back groundwater during excavation will be weakened by air and water flows inside and outside the tunnel, and its waterproof performance needs to be further investigated. In this paper, a two-dimensional numerical model of the temperature field considering excavation and moving water boundary is established based on the preliminary design scheme and in-situ conditions and is used to analyze the variation in frozen curtain properties with various active freezing times during excavation. The results show that excavation has a weakening effect on both sides of the frozen wall, with a greater effect on the inner side, and a positive temperature appears in the local area inside the jacked pipe. The concrete fill in the jacked pipe obviously improves the freezing efficiency, and the tunnel excavation after 60 days of active freezing in the interval filling mode can ensure that the frozen soil thickness at the thinnest segment exceeds 2 m, i.e., the design requirement. In practice, the active freezing time can be extended appropriately to reduce the influence of river water flow above the tunnel. The study serves as a technical reference for the design and implementation of similar projects.

Frost heave and thawing settlement of the ground after using a freeze-sealing pipe-roof method in the construction of the Gongbei Tunnel

The port section of the Gongbei Tunnel of the Hong Kong Zhuhai Macao Bridge is constructed by the freeze-sealing pipe-roof method, a new construction method that combines the pipe-roof and artificial ground freezing techniques. If the frost heave and thaw settlement of the ground are not effectively predicted and controlled during tunnel construction, then the surrounding environment of the tunnel will be adversely affected. In this study, thermophysical and mechanical parameters such as frost heave ratio and thawing settlement coefficient are obtained by the laboratory test of frozen-thawed soil. Then, the frost heave and thawing settlement laws of the ground are simulated by the full-coupled analysis method of transient temperature and displacement according to the secondary development technology of the ABAQUS finite element software considering the orthotropic deformation characteristics of frozen soil during the construction of the Gongbei Tunnel. The numerical simulation results show that the maximum vertical frost heave displacement of the stratum is 164.47 mm and the maximum horizontal frost heave displacement of the stratum is 53.80 mm, while the maximum vertical thawing settlement displacement of the stratum is −69.45 mm and the maximum horizontal thawing settlement displacement the stratum is −43.89 mm. Moreover, the error of the maximum vertical frost heave displacement between numerical simulation and field measurement is only 5.89 mm, which shows that the numerical simulation method proposed in this paper has high prediction accuracy.

Model Test and Numerical Simulation Analysis on Freezing Effect of Different Freezers in Freeze-Sealing Pipe-Roof Method

The Gongbei Tunnel Project is a key phased project of the Zhuhai Connection Line of the Hong Kong–Zhuhai–Macao Bridge, and the freeze-sealing pipe-roof (SFPR) method was first applied successfully. During this period, in order to obtain the best arrangement of freezers, three configurations, that is, solid pipe with circular freezer (SCF), empty pipe with double circular freezer (EDCF), and empty pipe with special-shaped freezer (ESF), were designed to analyze their freezing effects. The model test results show that the freezing temperature fields excited by different arrangements of freezers are obviously different, and the freezing effect of ESF is the best. The numerical simulation results indicate that the temperature cloud diagram of SCF at 42 h is butterfly-shaped after opening the limit pipes, and the temperature cloud diagrams of ESF and EDCF are the same as the model test results. However, the processing method of ESF is relatively complex, which adopts arc angle steel welding requiring a lot of reverse welding and high welding quality. Eventually, EDCF is recommended to ensure the quality of frozen walls due to its lower cost and better freezing effect. This research plays a basic role in the implementation of the freeze-sealing pipe-roof method.

Numerical Study on Water Sealing Effect of Freeze-Sealing Pipe-Roof Method Applied in Underwater Shallow-Buried Tunnel

The freezing-sealing pipe-roof method is a new presupporting technique, which fully combines the advantages of pipe-roof method and artificial ground-freezing method, and can adapt to the construction needs of underground projects in complex and sensitive strata. After the Gongbei Tunnel of Hong Kong–Zhuhai–Macao Bridge, this method will be applied for the first time in an underwater shallow-buried railroad tunnel, and there are still many urgent problems to be solved. In this article, based on the field situation and the preliminary design scheme, a convective heat transfer model under complex boundary conditions was first established. Then, the development of frozen wall thickness during the active freezing period was solved by numerical simulation for three different pipe filling modes, and the cloud map of temperature distribution in the whole section is analyzed. After that, the moving state of river water was characterized by different heat transfer coefficients, and the weakening effect of flow velocity on the top freezing wall was studied. Finally, six critical water sealing paths were selected, and the temperature differences of the frozen curtain were calculated. The results show that the mode with interval concrete filling can form a reliable frozen curtain within the scheduled time, whereas the nonfilling mode cannot achieve the water sealing requirement. River water has a large effect on the temperature at the boundary of jacking pipe and almost no effect on the center of the jacked pipe. It takes approximately 15 days from the frozen soil covering the pipe wall to reach the designed thickness, and the freezing effect of empty pipe lags approximately 28 days compared with that of solid pipe, which requires targeted enhancement measures in field projects.

A Frost Heaving Prediction Approach for Ground Uplift Simulation Due to Freeze-Sealing Pipe Roof Method

Freeze-sealing pipe roof method is applied in the Gongbei tunnel, which causes the ground surface uplift induced by frost heave. A frost heaving prediction approach based on the coefficient of cold expansion is proposed to simulate the ground deformation of the Gongbei tunnel. The coefficient of cold expansion in the model and the frost heaving rate from the frost heave test under the hydration condition can achieve a good correspondence making the calculation result closer to the actual engineering. The ground surface uplift along the lateral and longitudinal direction are respectively analyzed and compared with the field measured data to validate the model. The results show that a good agreement between the frost heaving prediction model and the field measured data verifies the rationality and applicability of the proposed model. The maximum uplift of the Gongbei tunnel appears at the center of the model, gradually decreasing along with the lateral and longitudinal directions. The curve in the lateral direction presents a normal distribution due to the influence of the constraint of two sides, while the one along the lateral direction shapes like a parabola with the opening downward due to the temperature field distribution. The model provides a reference for frost heaving engineering calculation.

Analysis of Ultimate Bearing Capacity and Parameters of Steel Support Cutting Pipe Roofing Structure

A new pipe-roof construction method, the steel support cutting pipe method (SSCP), was proposed to improve the construction security and accuracy of pipe jacking as well as underground space usage. The pipe-roof method is one of the underground excavation methods which push multiple steel pipes into the soil, then connect the steel pipes horizontally to form a whole. The proposed structure’s failure mode and force characteristics were determined through theoretical analysis, and then its ultimate bearing capacity and influencing parameters were analyzed through laboratory experiments and numerical simulation. The research results show that the structure’s bearing capacity depends on the steel pipe’s buckling load; the structure’s failure mode is a result of the steel pipe’s buckling. The ultimate bearing capacity of the pipe-roof structure first increases and then decreases with the increase of the steel pipe chord height ratio. The ultimate bearing capacity reaches the maximum when the ratio is 0.33. In addition, the structure’s ultimate bearing capacity is positively related to the steel pipe wall thickness and the pipe section’s length. This can be obtained from the relationship curve showing that the steel pipe wall thickness should be selected according to the engineering requirements and that the pipe section’s length is preferably 2.3 times the diameter of the steel pipe in the construction design.

Application of Under-pinning Pipe Jacking on Tunnel Auxiliary Construction

When a new structure is constructed in an overcrowded underground space, it is necessary to consider how to reduce the impact of the new structure on the surrounding existing structures and the surrounding ground, and one of the methods is the under-pinning method using pipe jacking. The under-pinning method is a general term for the method to reinforce the lower part of an existing structure with piles and pipes in advance to reduce the impact of new construction on the existing structure and surrounding ground. In this study, various numerical analyses were carried out assuming that the under-pinning method using pipe jacking was applied to the case where a new structure was to be installed underneath the existing structure. As a result of comparison with the under-pinning method, it was clarified that the under-pinning method with pipe jacking suppressed the influence on the existing structure and the surrounding ground by supporting the overburden pressure above the pipe over a wide area. In addition, it was found that the combination of the under-pinning method with the pipe-roof method, in which pipes are placed on top of the new structure, is more effective to suppress the deformation of the surrounding ground around the new structure than the under-pinning method alone.

Novel pipe-roof method for a super shallow buried and large-span metro underground station

To address the inadequacies of traditional pipe-roof methods, the steel support cutting pipe method (SSCP)—a novel pipe-roof method that improves construction security and underground space usage—is proposed. To further explore the applications of SSCP, its design scheme ought to be optimized. The failure mode and mechanical behaviors of the SSCP were investigated through laboratory experiments. Subsequently, a series of finite element models (FEMs) was established to study the deformation characteristics. Further, the parameters of the steel support of the proposed structure were optimized using fuzzy mathematics. The results indicated the ultimate bearing capacity to be 366.8 kN, and the specimen began to yield when the external load reached 70% of the ultimate value. The longitudinal spacing of the steel supports, transverse steel support size, and vertical steel support size had significant effect on the vertical deformation of the steel support and the ground settlement. Finally, the optimal combination of steel supports for the SSCP structure was obtained.

Ground Response to Pipe-Roof Tunneling of a Large Underpass in Shanghai Soft Soil

Pipe roof method is an effective way for urban underpassing scheme. Due to the successive jacking of multiple pipes, the characteristics of ground response are distinctly different from single pipe or twin pipes. In this paper, the gradual ground loss by successive pipe-jacking and the causes of settlement are analyzed. The conclusions drawn from this paper show that: (1) The maximum incremental was right above the pipe axis which could be controlled within 1mm. (2) The shield effect of the former pipe on the latter one resulted in a 50% reduction of maximum incremental settlement. (3) The distribution of cumulative settlement curve was reflected by superimposed effect of staged ground settlements. (4) The results of settlement monitoring illustrated that the field measured settlements were basically tally with those of the simulated values, the goodness of fit was more than 85%.

Investigation on the Performance of Pipe Roof Method Adjacent to the Underground Construction

In this paper, the behaviour of the pipe roof method with different pipe roofs disposition schemes including the pipe roof gate system and L-shaped system is discussed in terms of ground settlement reduction in the condition of underground space construction adjacent to the existing structure by using numerical simulation method. Considering the construction sequences of parallel underground tunnels with rectangular cross sections, the appropriate parameters and ground response characteristics of the corresponding pipe roof methods are evaluated by the ground surface displacement. The results indicated that the construction impacts from new buildings can be reduced by the pipe roof method. However, the theoretical performance of pipe roofs is highly dependent on the spacing between two structures. Generally, the L-shaped system is more effective in case of the distance (D) that is less than or equal to 1/2 width of box culvert (R) under the condition of a new structure constructed nearby the existing building. When two structures are constructed simultaneously, the excavation and the construction of structures influence each other reciprocally. The conclusions could provide a reference for the pipe roof method with the application scenario of adjacent building protection.

Freezing Temperature Field of FSPR under Different Pipe Configurations: A Case Study in Gongbei Tunnel, China

As a new tunnel presupport construction technology, the freeze‐sealing pipe roof method (FSPR) has been successfully applied to the Gongbei Tunnel Project for the first time in China. To overcome the practical difficulties of this new method in the freezing construction process, such as welding difficulties of the profiled freezing tube, refrigerant leakage, and long construction period, based on the principle of an equal cooling capacity, an improved design of freezing tube was proposed. By designing three different pipe configurations and using scaled model tests and numerical simulation, the feasibility of the improvement and the difference in the freezing temperature field were studied. The research results show that the improved design with double circular freezing tubes exhibited a lower temperature and a higher cooling rate during the test, and the time required to meet the freezing design requirements was significantly shortened. Finally, a composite structure of “pipe roof and frozen soil curtain” with a better carrying capacity and water sealing performance was formed. It was recommended to strengthen the temperature monitoring at the pipe wall in the preliminary stage of the freezing construction to ensure the water sealing effect between the pipes. In the later stage, attention should be paid to monitoring the thickness of the frozen curtain and reducing excessive frost heave through technical means such as cooling capacity control. The improvement and configurations proposed in this research could effectively replace profiled freezing tubes in the original project, better adapt to horizontal freezing projects of similar curved tunnels, achieve the freezing goal faster, and provide a reference for the promotion of the FSPR.

Analytical solution to steady-state temperature field of Freeze-Sealing Pipe Roof applied to Gongbei tunnel considering operation of limiting tubes

In order to solve the problem of steady-state temperature field with complex boundary conditions and provide a more complete theoretical guidance for Freeze-Sealing Pipe Roof (FSPR) method, which is a new pre-supporting system for tunnel construction. This paper proposed a FSPR freezing model based on the simplification of Gongbei tunnel considering the operation of limiting tubes, and the analytical solution of steady-state temperature field was first investigated using the superposition and boundary separation methods of the Laplace equation. Then we verified the accuracy of the analytical solution by comparing the analytical results with the numerical results in different characteristic cross-sections. Finally, we introduced the on-site monitoring scheme of Gongbei tunnel and analyzed the field measurements. The results show that the analytical solution agrees well with the numerical simulation and field measurement. The maximum errors between calculated values and field temperature values are within 2 °C, which demonstrates the applicability of the analytical results in practical engineering. Field test also shows that the limiting tubes can effectively control the local development of frozen soil without affecting the overall water sealing effect of FSPR method.