Tunnel Lining Research Articles

Outcomes of one-stage and two-stage aneurysm repair in arteriovenous fistulae

ObjectiveMaintenance of long-term arteriovenous access is important in long-term care for patients with end-stage renal disease. Arteriovenous access is associated in the long term with the development of fistula aneurysms (FAs). This study aims to evaluate the outcomes of staged FA treatment in dialysis access arteriovenous fistulae (AVF). MethodsA retrospective review of all patients over a 12-year period with primary autogenous AVF was undertaken at a single center. Patients undergoing elective open aneurysm repair were identified and were categorized into three groups: single FA repair (single, control group) and staged and unstaged repair of two FAs (staged and unstaged). A staged repair was a procedure in which the initial intent was to treat both aneurysms in the AVF and in which the most symptomatic aneurysm was treated first. When the incision from the first surgery had healed, the second symptomatic aneurysm in the AVF was treated. An unstaged repair was a procedure in which the initial intent was to repair both symptomatic aneurysms simultaneously. All patients had a fistulogram before the FA repair. Thirty-day outcomes, cannulation failure, line placement, reintervention, and functional dialysis (continuous hemodialysis for 3 consecutive months) were examined. ResultsFive hundred twenty-seven patients presented with FA that met requirements for open intervention; 44% underwent single FA repair, whereas the remaining 34% and 22% underwent staged and unstaged repair of two FAs, respectively. The majority of patients were diabetic and Hispanic. Ninety-one percent of the patients required percutaneous interventions of the outflow tract (37%) and the central veins (54%). Thirty-day major adverse cardiovascular events were equivalent across all modalities. Thirty-day morbidity and early thrombosis (<18 days) were significantly higher in the unstaged group (4.3%) compared with the two other groups (1.3% and 2.1%, single and staged, respectively), which led to an increased need for a short-term tunneled catheter (8.9%) compared with the two other groups (3.4% and 4.4%, single and staged, respectively), Unstaged repair resulted in an increased incidence of secondary procedures (5.0%) compared with the two other groups (2.6% and 3.1%, single and staged, respectively). Functional dialysis at 5 years was equivalent in the single and staged groups but was significantly decreased in the unstaged group. ConclusionsOpen interventions are successful therapeutic modalities for FAs, but unstaged rather than staged repair of two concurrent FAs results in a higher early thrombosis, an increased secondary intervention rate, and a need for a short-term tunneled central line. Staged and single FA repairs have equivalent results. In the setting of two symptomatic FAs, staged repair is recommended.

Performance of the Flexible and Rigid Lining under Earthquake Impact and Weakness of the New Austrian Tunneling Method (NATM) Principles, a Specific Case Study of the Bolu Tunnel

The Bolu tunnel is located between the Istanbul-Ankara Highway and its construction took approximately 13 years. Since the beginning of the tunnel excavation, serious deformations and stability problems have been encountered. The basis of the problems encountered during the tunnel excavation is the fact that the geological units through which the tunnel passes are very weak and fault lines cut the tunnel location. The fault lines pass through secondary faults in the seismically active North Anatolian Fault. At these fault crossings, deformations occurred continuously so that revisions had to be made in the support systems. The 12 November 1999 Düzce earthquake occurred on the Düzce fault and a surface rupture 40 km long was caused. The rupture terminated 1.5 km west of the tunnel; it did not reach the tunnel. Throughout the earthquake, instances of collapse occurred at the areas excavated on the fault line at the entrance of Elmalık and where the deformations exceeded 1.0 m; this section of the tunnel had to be abandoned. After these problems, a new improved design for tunnel excavation was developed. These new support systems, which are Bernold lining and bench pilot tunnel systems, contain allowance for rigid lining that is the opposite of the new Austrian tunneling method (NATM) principles. Within the scope of this study, the causes of the collapse in the tunnel are investigated and the effect of the Düzce earthquake on the tunnel is discussed. Also, the applicability of the NATM in tunnels excavated on weak zones is also discussed.

Dynamic Stress Concentration and Particle Velocity Response of a Composite Lining Tunnel with Imperfect Interfaces Subjected to Blasting P Waves

Dynamic Stress Concentration and Particle Velocity Response of a Composite Lining Tunnel with Imperfect Interfaces Subjected to Blasting P Waves

Treatment measures for the creep–slip phase of landslides induced by twin tunnels

Tunnel excavation in geologically challenging areas can lead to slope instability and consequent landslides, which pose a serious threat to the integrity of the tunnel lining. To address this issue, a study was conducted to investigate the stability of the tunnel–landslide system using the finite-difference method (FDM) based on the Hanshankou tunnel of the Shaoguan–Xinfeng expressway in Guangdong province, China. A plane-strain model was used to examine the effect of various treatment measures on the system, and the strength reduction (incremental) method was employed to determine the range of initial slope safety factors applicable to different treatment measures, with the maximum shear strain increment serving as the criterion. The ‘anti-slip pile + anchor frame beam’ reinforcement system was found to be the most effective solution, reducing daily deformation by 60.1% after implementation. The results indicated that the pile–anchor reinforcement system can significantly mitigate landslide creep, and the corresponding treatment measures can be categorised based on the critical slope safety factor under different conditions.

Analytical Study of Acceptable Cover Thickness and Temperature Limits of Tunnel Lining Concrete for Fire Resistance

Analytical Study of Acceptable Cover Thickness and Temperature Limits of Tunnel Lining Concrete for Fire Resistance

Risk Assessment Method and Application for Tunnel Lining Demolition Construction

The maintenance, reconstruction, and expansion of tunnels often require the removal of existing tunnel linings. However, there is a paucity of research on risk assessments during tunnel lining demolitions. In order to address this gap, we developed a risk assessment model for tunnel lining demolitions using a fuzzy hierarchy comprehensive evaluation and expert surveys. This model draws on on-site construction experience. A fuzzy hierarchy comprehensive evaluation assesses risk probability, while expert questionnaires evaluate risk loss. Our study applied this model to a tunnel project in Qinghai, determining that the overall risk level falls within category “V3”, indicating acceptable risk. Nonetheless, ongoing vigilance and monitoring measures are necessary throughout the construction process. These findings contribute to a deeper understanding of tunnel lining demolition risk assessments for improving project management and safety.

Numerical Simulation Analysis of the Influence of Entrance Wind Speed on the Wind Speed Distribution of Coal Mine Tunnel Sections

In order to further accurately obtain the wind speed of coal mine tunnels and achieve intelligent ventilation, taking the Wuhushan coal mine tunnel as the research object, the COMSOL Multiphysics numerical simulation software was used to simulate and analyze the influence of inlet wind speed on the wind speed distribution on the tunnel section, and the wind speed distribution law of the semi circular arch tunnel section under different inlet wind speed conditions was obtained. The research results indicate that the wind speed contour is basically parallel to the tunnel wall, and the wind speed gradient near the tunnel wall is large, while the wind speed gradient in the middle of the tunnel is small. The thickness of the boundary layer decreases with increasing wind speed. The ratio of the maximum wind speed to the average wind speed of the tunnel section is approximately 1.2125. The distance between the average wind speed line of the semi circular arch tunnel and the roof is 10.76%~11.02% of the tunnel height, and the error between the simulation results and theoretical calculation results is within 4%. The research results provide strong support for precise measurement of the average wind speed at fixed points in coal mine underground tunnels.

Automatic detection of delamination on tunnel lining surfaces from laser 3D point cloud data by 3D features and a support vector machine

AbstractA completely automatic algorithm for accurately detecting delamination on tunnel concrete lining surfaces using laser 3D point cloud data is first proposed to facilitate tunnel lining inspection. A mobile mapping system (MMS), which mounts laser sensors and a positioning system, is utilized to measure the geometries of the surfaces at high speed. The algorithm consists of two steps: extraction of the 3D shapes of anomalies and discrimination of delamination from appendages by a support vector machine (SVM). The article focusses on the second step. On tunnel linings, there are many conspicuous appendages such as cables, lights, signs, and water guides which mask the features of delamination. In this study, straightness, a novel 3D feature, is introduced to realize accurate discrimination. An automatic algorithm based on the SVM is developed and validated using real tunnel data, showing an accurate delamination map.

Association of Prophylactic Antibiotics With Early Infectious Complications in Children With Cancer Undergoing Central Venous Access Device Placement.

To evaluate the impact of prophylactic antibiotics on early infectious complications after central venous access device (VAD) placement in children with cancer. Despite the frequency of VAD procedures in children, the effectiveness of prophylactic antibiotics for reducing infectious complications is unknown. This was a retrospective cohort study of children with cancer undergoing central VAD placement identified in the Pediatric Health Information System database between 2017-2021. The primary outcome was the rate of early infectious complications (composite surgical site infections, central line-associated bloodstream infections, and bacteremia). Multivariable logistic regression was used to evaluate factors associated with early infection, and heterogeneity of treatment effect of prophylactic antibiotics was compared across subgroups. 9,216 patients were included (6,058 ports and 3,158 tunneled lines). Prophylactic antibiotics were associated with lower early infectious complications overall (1.3% vs. 2.4%; OR 0.55 [95% C.I. 0.39-0.79], P<0.001), an effect demonstrated for tunneled lines (OR 0.59, 95% C.I.: 0.41-0.84) but not ports (OR 3.01, 95% C.I.: 0.66-13.78). On multivariate analysis, prophylactic antibiotics (OR 0.67, 95% C.I.: 0.45-0.97) and solid tumors (OR 0.38, 95% C.I.: 0.22-0.64) were associated with reduced odds of early infections, while tunneled lines (OR 20.78, 95% C.I.: 9.83-43.93) and acute myelogenous leukemia (OR 2.37, 95% C.I.: 1.58-3.57) had increased odds. Prophylactic antibiotics are associated with reduced early infectious complications after central VAD placement overall. Despite recommendations from multiple national organizations against prophylactic antibiotics, these findings suggest a benefit in children with malignancy undergoing tunneled line placement.

Fiber Bragg grating displacement sensor based on synchronous deformation sensing for real-time monitoring of a tunnel lining.

A fiber Bragg grating (FBG) displacement sensor based on synchronous sensing is developed for real-time monitoring of a tunnel lining. The sensing principle and mechanical structure of the proposed sensor are analyzed and simulated, and its sensitization effectiveness and temperature compensation are verified. Equivalent model tests show that the sensor has a good linear sensitivity of 19.48pm/mm and an excellent precision of 5.13×10-2 m m in the displacement range of 0-25mm, which is basically consistent with the simulation results. The key traffic parameters of the train were successfully obtained by real-time monitoring of the tunnel lining in a field trial, which shows the superior capability of micro-displacement measurement of the sensor. Furthermore, good stability and excellent creep resistance have also been demonstrated. Our results provide theoretical guidance for the fabrication and package of the FBG displacement sensor, which is valuable for structure health monitoring (SHM) in civil engineering applications.

Analytical solution for seepage pressure of multilayered sand seabed considering interaction between wave-current action and shield tunnel

Existing theoretical studies of the seepage field around subsea shield tunnels under wave action take less account of the tunnel lining permeability, especially the effect of wave and current action on tunnel. Moreover, the solutions generally treat the sand as homogeneous and isotropic, ignoring the influence of multilayered seabed. In this paper, the analytical solutions are presented for the seepage pressure and liquefaction evaluation of sand around tunnel in multilayered seabed under wave and current action. The pore pressure response of pure seabed is obtained by the transmission and reflection matrix method. The mirror image method is introduced to establish governing equation of excess pore pressure reflected by tunnel, and the analytical solution of equation is obtained by Fourier series expansion under the condition of continuous seepage between sand and lining. Based on the superposition principle, the seepage pressure and liquefaction evaluation of sand around tunnel in multilayered seabed are obtained. Then the analytical solutions are compared with numerical and experimental results, and good agreement is obtained. In addition, the influencing factors of wave parameters (wavelength, period, current), seabed parameters (permeability, shear modulus, saturation) and tunnel parameters (lining thickness, lining permeability) are analyzed.

Experimental study on the influence of cracks on tunnel vibration under subway train load

The impact of train vibration on the tunnel lining is a popular research topic. Under long-term train operation, the tunnel lining may incur damage, mainly longitudinal cracks, which will further increase the vibration caused by running trains, impacting the lining and the surrounding soil and eventually endangering the safety of train operation. To understand the influence of cracks on tunnel vibration under train load, we performed a model test of a shield tunnel with and without typical cracks based on a line of Hangzhou metro prototype to characterize its dynamic responses under train vibration loads. To simulate the tunnel lining material, the tunnel segment was lined up with polymer cast nylon with low creep rate and high strength. An inverted shaker simulated the train vibration load, and an accelerometer was used to measure the dynamic response of the tunnel structure and the surrounding soil. The results show that the dynamic response of the cracked section is significantly larger than that of the intact section, and due to the propagation characteristics of vibration, the expansion effect inside the tunnel is significantly greater than that of the surrounding soil. When the frequency spectrum results of the tunnel structure and the surrounding soil are compared, it is found that the vibration level of the surrounding soil due to cracks is mainly concentrated at around 100 Hz, while that of the tunnel structure at around 315 Hz, indicating that the cracks have a targeted effect on the expansion of the dynamic response. Meanwhile, the excessive vibration of intact segments is –2 or –1 dB, and the existing vibration reduction measures can meet the damping requirements. However, the excessive vibration caused by typical cracks can go up to 12 dB. Therefore, corresponding vibration reduction measures should be taken according to specifications. Finally, this paper proposes some vibration reduction suggestions and crack repair measures.

Effect of test parameters on the punching-shear strength of precast tunnel segments reinforced with GFRP bars

While assessing the load cases is crucial for designing the precast concrete tunnel lining (PCTL) segments, none of the tunnel-specific standards or codes explicitly consider punching-shear loads. This study aimed at determining the structural performance of full-scale precast concrete tunnel segments reinforced with glass fiber-reinforced polymer (GFRP) bars. The segments were constructed with a rhomboidal shape measuring 2100 × 1500 × 250 mm and subjected to point loading on their extrados surfaces until failure. Such loading simulates the geotechnical exposure conditions surrounding tunnels, such as rock expansion. The effects of concrete strength and GFRP flexural reinforcement ratio on the punching behavior were analyzed and are discussed herein. The load–deflection response, failure mode, punching strength, cracking behavior, reinforcement and concrete strain, deformability, and energy absorption were all evaluated in the experiments. The results reveal that increasing both the flexural reinforcement ratio and concrete strength enhanced the punching response and decreased crack width. In addition, using HSC in tunnel segments yielded higher pre-cracking stiffness, punching-shear capacity, and energy absorption. A theoretical investigation involving a comparison between predictions of the FRP-reinforced concrete slab design provisions and experimental results was performed to assess the applicability of the current punching equations for PCTL segments. The equations in the FRP-reinforced concrete codes predicted the punching capacities conservatively, while the researchers’ proposed equations overestimated. Moreover, CAN/CSA S806-12 showed the most accurate prediction of the punching capacity even if the concrete compressive strength exceeded 60 MPa.

Evaluation of segmental lining response during shield tunnel construction based on field measurements and 3D FEM simulation

Lining safety can be threatened more severely at the construction stage than at the service stage, because the loads at the construction stage are larger. This study presents an in-situ experiment for measuring the loads acting on the lining and the internal forces of the lining to investigate the lining response during both the construction and service stages. A three-dimensional (3D) finite element method (FEM) model that could simulate the main relevant components of the shield tunnel construction process was employed to gain further insight into the mechanisms of the lining response. The influences of the key segment position and assembly sequence on the lining behavior were examined through a comprehensive parametric analysis. The results showed that during the assembly stage, the insertion of a key segment resulted in the most significant increase in the axial force, followed by the assembly of an adjacent segment. The lining responses were significantly influenced by the key segment, particularly during the construction stage, whereas the assembly sequence had a limited effect on the lining responses. During the construction stage, the closer the key segment to the tunnel invert, the greater was the axial force and the smaller was the bending moment of the lining. During the construction stage, the closer the key segment to the tunnel invert, the smaller was the axial force of the tunnel lining and the greater was the bending moment. The circumferential compressive stress reached its maximum value at the joint interfaces. Finally, the jack thrust force was the greatest at the tunnel invert. The key segment should not be placed at this location.

Batched-image detection model and deployment method for tunnel lining defects using line-scan cameras based on experiments study

High-performance line-scan cameras are highly accurate and efficient for tunnel detection, however, the exponential growth of image data creates new challenges for real-time image processing. The inference speed of existing tunnel detection algorithms is insufficient for massive amounts of image data, and traditional data structures limit the parallelism of the detection system, thereby reducing the speed of image processing. In this study, a deep learning (DL) network is developed for real-time tunnel defect detection based on the you only look once (YOLO) family. The network includes innovative defect feature-extraction modules, optimization of the network architecture, and improvement of the decoupling heads to increase defect detection accuracy and efficiency. Furthermore, we developed critical technologies for real-time detection. First, our method employs a deployment method for DL networks based on TensorRT, which enables our network to be applied directly to C++ image acquisition programs away from the traditional DL frameworks. The proposed line-scan data structure enables the raw images to be fed into the neural network in batches, which, combined with the simultaneous technique of image acquisition, detection, and storage, significantly increases the efficiency of real-time detection. The paper demonstrates the advantages of the proposed model and hardware system through an experiment study. The test results indicate that the mean average precision and F1-score of our network were 86.07% and 84.53%, respectively, while it had 21.42 M parameters. Combined with the proposed storage strategy, a recall rate of more than 90% was achieved for defect detection. Moreover, batch image processing method has a significant advantage in terms of inference speed, with 243.9 FPS in PyTorch, 381.46 FPS in TensorRT, and 305.3 FPS in real-time detection, which is five times faster than existing defect-detection models. This study significantly increases the efficiency of tunnel lining detection, which has positive implications for the visual detection of other infrastructures.

Study on Temperature Distribution Law of Tunnel Portal Section in Cold Region Considering Fluid–Structure Interaction

The design of tunnels in cold regions contributes greatly to the feasibility and sustainability of highways. Based on the heat transfer mechanism of the tunnel surrounding rock–lining–air, this paper uses FEPG software to carry out secondary excavation and development, then the air heat convection calculation model is established by using a three-dimensional extension of the characteristic-based operator-splitting (CBOS) finite-element method and the explicit characteristic–Galerkin method. By coupling with the heat conduction model of the tunnel lining and surrounding rock, the heat conduction-thermal convection fluid–structure interaction finite-element calculation model of tunnels in cold regions is established. Relying on the Qinghai Hekashan tunnel project, the temperature field of the tunnel portal section is calculated and studied by employing the fluid–structure interaction finite-element model and then compared with the field monitoring results. It is found that the calculated values are basically consistent with the measured values over time, which proves the reliability of the model. The calculation results are threefold: (1) The temperature of the air, lining, and surrounding rock in the tunnel changes sinusoidally with the ambient temperature. (2) The temperature of each layer gradually lags behind, and the temperature variation amplitude of the extreme value of the layer temperature gradually decreases with the increase in the radial distance of the lining. (3) In the vicinity of the tunnel entrance, the lining temperature of each layer remains unchanged, and the temperature gradually decreases or increases with the increase in the depth. The model can be used to study and analyze the temperature field distribution law of the lining and surrounding rock under different boundary conditions, and then provide a calculation model with both research and practical value for the study of the temperature distribution law of tunnels in cold regions in the future.

Full-scale testing of precast tunnel lining segments under thrust jack loading: Design limits and ultimate response

In modern practice, precast segmental tunnel linings are typically installed via a tunnel boring machine (TBM), which advances by thrusting against the previously installed segmental ring. The forces applied through the thrust jack pads can induce significant bursting and spalling tensile stresses and strains in the segment, and improperly designed segments can suffer from cracking as a result. An experimental study has been conducted to evaluate the progression of damage from initial cracking to ultimate capacity for full-scale precast tunnel liner segments under thrust jack loading. The baseline segment design is composed of steel fiber reinforced concrete (SFRC), and the impact of supplemental conventional steel bar reinforcement and load application eccentricity were also investigated. Six full-scale tests were performed with a thrust jack load per pad up to 22.2 MN (which is ∼ 3.8 times the maximum expected installation thrust force). At the maximum expected thrust jack load during installation (5.78 MN per pad), the segments were virtually undamaged, and hairline cracking initiated between the load pads on only one test. At the TBM’s ultimate jacking capacity (9.55 MN per pad) surface cracking was observed between and under the load pads; however, the crack width remained below 0.2 mm for all specimens. The formation of cracking limit states was accurately predicted by pre-test linear and nonlinear finite element (FE) models. At overload conditions, the baseline SFRC-only segment exhibited a radial bursting failure. The inclusion of supplemental conventional reinforcement does not reduce the level of cracking damage or strain development below the TBM’s ultimate jacking capacity; however, at overload conditions, the supplemental reinforcement mitigates cracking and prevents a radial bursting failure at 20.3 MN per pad. A load eccentricity of 38 mm towards the extrados surface increased the transverse strain and the formation of transverse cracking at a lower load level.

Thermo-mechanical behaviour of microbial induced carbonate precipita-tion (MICP) sand for geothermal pavements

Introduction&#x0D; Ground source heat pump (GSHP) or shallow geothermal energy systems are gaining attention for helping combat global warming and the negative effects of urbanisation caused by human activities. In recent decades, energy geo-structure techniques have developed by using subsurface infrastructures to exchange heat with the ground. These techniques can provide space heating and cooling while still preserving the primary structural function. As a result, they have become a valuable part of geothermal energy systems. Researchers have developed an understanding of incorporating ground heat exchangers into foundation piles, retaining walls, and tunnel linings with modest additional cost [1-4]. Pavements are also structures in contact with the ground that have the potential to be used as energy geo-structures (i.e., geothermal pavements), yet, their use has not been extensively studied [5-8].&#x0D; Soil thermal conductivity is an important factor influencing the efficiency of geothermal pavements since heat transfer in soils occurs primarily by conduction [9]. Microbially induced calcium carbonate (CaCO3) precipitation (MICP) is an innovative technique for strengthening sandy soils by coating and binding soil grains with calcium carbonate crystals. The distribution and arrangement of the CaCO3 within the pore spaces play a crucial role in determining the resulting strength of the treated sand. In addition, these crystals can act as thermal bridges to enhance the soil's thermal conductivity [10]. Combining geothermal pavements with MICP sand is still nascent, and the limited number of studies that exist mainly focus on the associated thermal property changes [11]. However, since the principal function of pavements is transmitting (dynamic) loads to the subbase and the underlying soil, the thermal conductivity of the MICP-treated pavement may vary as a result of the applied mechanical loads (e.g., due to the partial or total loss of thermal bridges and/or particle rearrangement). This research thus investigates the changes in the thermal conductivity of MICP-treated sands as they are subjected to quasi-static triaxial compression. The experimental results collected can deepen our understanding of the thermo-mechanical behaviour of MICP-treated sands and provide practical insights for using MICP to reinforce the subbase or underlying soil of geothermal pavements.&#x0D; Methodology&#x0D; This research performed a series of quasi-static triaxial tests on MICP-treated Houston sand, fine-grained, high-purity silica sand. Sporosarcina pasteurii (strain designation DSM 33) was used for the MICP treatment of the soil specimen. To study the effect of CaCO3 content on the thermo-mechanical performance of MICP-treated sand, three cementation solution treatment cycles were applied, yielding theoretical CaCO3 contents of 0.6%, 1.6% and 2.7% by weight, respectively. Details on the MICP treatment of the samples can be found in [12]. Samples for triaxial testing were treated in cylindrical tubes of 50mm inner diameter and 100mm height. To investigate the influence of the CaCO3 content on the soil thermal conductivity, MICP-treated specimens were air-dried prior to triaxial testing Triaxial tests were subsequently conducted in dry conditions to isolate the effect of the MICP and avoid the influence of water content on soil thermal conductivity (λ). Furthermore, a new miniaturised transient sensor was embedded in the triaxial samples to monitor the λ changes during the sharing phase [12].&#x0D; Results&#x0D; An example of the evolution of the deviatoric stress with axial strain under 50kPa confining stress (σ3) in the triaxial cell is shown in Figure 1a. Compared to the untreated sand, MICP-treated samples lead to higher peak strength and stiffness. Importantly, λ changes during triaxial testing for different CaCO3 contents are summarised in Figure 1b. Results indicate that the increase in CaCO3 content can significantly improve λ, and that λ rapidly decreases post-peak strength due to dilation and CaCO3 bond breakage. Once the samples reach its ultimate state, λ remains unchanged.

Cooling underground substations worldwide using heat pumps

The use of underground space in urban environments has increased at an accelerated rate over the past few years, particularly to satisfy transport infrastructure needs. Transport tunnels are being constructed at increasingly greater depths in the already congested subsurface. While transport tunnels have demonstrated significant potential as sustainable heat sources for heating under - and above-ground spaces, due to the large area in contact with the ground [4,5], cooling the substations that serve the tunnels, containing plants and machinery, is a task that poses a significant economic and environmental challenge. To tackle this, a new method that takes advantage of airflow in the tunnels and the thermal mass of the ground surrounding the tunnels has been recently introduced [2]. Similar to energy tunnels [1], these systems use water-filled high-density polyethylene (HDPE) pipes integrated into the tunnel space to exchange heat, however using the pipes to reject heat from the substations to the tunnels. This work adopts four cities, namely Sydney in Australia, Guangzhou in China, London in the UK, and Stuttgart in Germany, and investigates the suitability of this approach to cool substations for different meteorological and geological conditions around the world. Moreover, varying heat exchanger lengths and tunnel air temperatures are incorporated in the analysis, to assess the impact of these key parameters on system efficiency and performance.&#x0D; A detailed 3D finite element heat and mass transport model is utilised in this work, simulating the tunnel lining, surrounding soil, the airflow within the tunnel, the pipes attached to the tunnel linings (tunnel air side), and the circulating fluid within the pipes [2]. The location farfield temperature and material properties are shown in Table 1, for the soil conditions in the four locations and for the common materials (shaded in grey). A parametric analysis is undertaken to investigate the performance of these systems under varying conditions. For each of the locations, five different pipe leg length values are used: 50 m, 100 m, 150 m, 200 m, and 300 m and the air velocity is assumed as 0.5 m/s, typical of these tunnels. In addition, two different distributions for the temperature of the air entering the tunnel are used: the estimated tunnel air temperature () distributions based on available literature and measurements [3, 6, 7], as well as the surface air temperatures (), both shown in Figure 1-left. The latter is used to understand the impact of natural ventilation of tunnels on the system, which could inform the placement of the pipes with respect to the positions of ventilating shafts along the tunnel length.&#x0D; The results are shown in Figure 1-right, in terms of the cooling thermal power that a single pipe loop can provide (total pipe loop length being ), when operating constantly over 20 years – to replicate the real conditions in cooling substations. These values are obtained such that the temperature of the fluid does not reduce below 5 °C, such that the heat pump efficiencies remain high. The locations are shown based on the line colour and the air temperature distribution used for each location is represented by the line type (solid:, dashed:). The results show that the cooling provided can vary significantly based on the investigated parameters and suggest that the temperature of the air flowing in the tunnel is very influential to the system performance. The most favourable outcome is achieved for Stuttgart (green), which has the lowest tunnel air and farfield temperature, and the least favourable for Guangzhou (red), which has the higher air and farfield temperature. Comparing the two air temperature distributions used (solid vs dashed), using instead of , which is lower in all cases, increases the performance of the system. This is most obvious for the case of London, with an increase between 2.5 kW and 5 kW, likely due to the low value of the farfield temperature as opposed to the significantly high tunnel air temperatures (). The length of the pipes used in all cases increases the amount of energy, however that increase is logarithmic in nature and therefore after about 150 m to 200 m the investment might not be justified. Overall, this work shows that there is significant potential to utilise heat pump technologies to cool tunnel substations, especially in cooler climates such as central-northern Europe.

Potenential for thermal energy from Tunnels beneath Manchester and Crewe: a case study

potenential for thermal energy from Tunnels beneath Manchester and Crewe: a case study