High Density Polyethylene Pipe Research Articles

Experimental study on gasketed bell-and-spigot joint behaviour of lined-corrugated HDPE pipe subjected to normal fault

Although the structural response of pipelines has been studied in relation to different geohazards, few studies have focused on the behaviour of flexible pipeline joints. In this paper, the response of a bell-and-spigot joint in a 600 mm dia. lined-corrugated high-density polyethylene (HDPE) pipe was investigated under differential ground movements imposed using a facility that simulates a normal fault. Two experiments were undertaken in this facility. In the first experiment, the kinematic responses of the pipe joint (i.e. axial, shear displacements and rotational angles) were measured using particle image velocimetry and string potentiometers. Strains were also monitored using optical fibres. In the second experiment, the pipe was sealed and leakage of the joint was captured through monitoring the internal vacuum pressure of the pipe. The results show that axial shortening, rotational angle and shear displacement of the pipe joint increased with increasing fault offsets. The joint began to leak when axial shortening, rotational angle and shear displacement of the pipe joint were 0·65 mm, 0·44° and 3·40 mm, respectively, and the joint clearly lost its functionality when those values reached 0·85 mm, 0·58° and 4·32 mm.

Liquid temperature effect on the hydraulic shock wave velocity in polyethylene pipes

Introduction. Providing people with high quality drinking water has always come first. However, its transportation through pipeline systems was often associated with some problems, such as the temperature of the water and the environment, as well as the possibility of water hammer on certain pipe sections. This was especially true for systems that use polyethylene pipes. Temperature is a key factor affecting the flexibility properties of polyethylene pipes, and it affects not only the design, but also the investment in the development of water supply networks. The purpose of these studies was to study the effect of water and ambient temperature on the density, properties of the pipe material and the speed of propagation of a hydraulic shock wave in polyethylene pipes.Materials and Methods. In the experiments performed, the method of field research was used, when tests are carried out on specialized equipment on samples specially made for the pursued purposes. Here, samples of high-density polyethylene pipes were used, which were subjected to tensile tests on a tensile testing machine, and each experiment was carried out three times.In the course of the experiments, the samples were exposed to certain temperature regimes (both external and internal), while the influence of the hydrodynamic pressure of the liquid in the pipe was also investigated, as a result of the change in time of the liquid velocity in its sections. To do this, the samples were supplied with liquid under a certain pressure in order to find out the influence on the pipes of an effect known as water hammer.Results. In the course of the research, it was found that the value of the elastic modulus of high-density polyethylene PE100 decreases with increasing water temperature, and the decrease at a temperature of 60° C reaches 60.21% compared to its value at a water temperature of +4° C. Based on the results of experiments to determine the effect of the elastic modulus of polyethylene with increasing temperature, an exponential equation was derived to calculate the value of the polyethylene coefficient as a function of time E = 1.312e-0,01t with the correlation coefficient R2 = 0.988 ; and based on the results of the studies carried out to calculate the value of the propagation velocity of a hydraulic shock wave, an exponential equation was derived as a function of time C = 275.9e-0,01t with the coefficient correlation R2 = 0.987 .Discussion and Conclusions. In the course of the research, it was found that such a phenomenon as water hammer has a harmful effect on the pipe walls, which, if possible, should be avoided even at the design stage of the water supply network. During the experiments, it was found that with an increase in temperature, the values of the elastic modulus of polyethylene decreased with a simultaneous decrease in the values of the propagation velocity of the hydraulic shock wave.

Time reversal of waves in hydraulics: experimental and theoretical proof with applications

Time reversal of waves is an intriguing wave property that underpins a breadth of applications in physics and engineering. Waves contain information about their sources and the media through which they propagate. Thus, time reversal of measured wave signals has the potential of localizing and characterizing wave sources and of inferring the properties of the medium. Herein, we experimentally demonstrate the time reversibility of acoustic waves propagating in water-filled viscoelastic high-density polyethylene (HDPE) pipes. Evidently, the two mechanisms that restrict time reversal are the stability of wave paths to perturbations and damping. Perturbations, however, are found to grow slowly in time (∼t 1/2) and are not critical for the time reversal of waves. To evaluate the effect of damping, we perform an order of magnitude analysis on the non-reversible terms of the coupled waveguide momentum equations and derive a dimensionless time reversal parameter TR showing that damping develops linearly with time (i.e. TR ∼ t). Subsequently, we apply the TR parameter to our experiments and relevant experimental proofs from the literature to find that the time reversal of waves only holds for TR ∼ 1 or less; hence providing a criterion to estimate the range over which time reversal-based wave techniques and methodologies are valid. Finally, we discuss the various existing applications of time reversal in hydro-environmental research and engineering and anticipate that the presented work will stimulate further development.

Investigation on Physical and Mechanical Properties of High Density Polyethylene (PE100) Using Novel Catalyst

Novel catalyst called super active catalyst for the production of high density polyethylene (HDPE) pipes grade 100 was prepared and localized for the first time in Iran. The purpose of this paper is to evaluate the performance of the new catalyst and compare it with Finix-112 catalyst which is a commercial catalyst for the production of HEPE grade 100. Extensive experiments were performed on the physical and mechanical properties of the product using both catalysts including melt flow index, particle size distribution, volatility, density, bulk density, scanning electron microscope-energy dispersive x-ray analysis, charpy impact strength. Comparison of the results showed that the hydrogen responsibility performance of the product with super active catalyst is 19%, charpy impact is 16.7% and pressure condition in first reactor about 28% was better than product with Finix-112 catalyst. Home-made super active catalyst can be a good alternative to imported finix-112 catalyst and save a considerable amount of foreign currency.

Experimental and numerical study of shallow‐buried high‐density polyethylene pipeline under collapse touchdown impact

AbstractThe debris from structures produced during the blasting demolition of engineered buildings (structures) may fall near buried pipelines, which threaten the integrity of the buried gas pipeline. In this paper, the dynamic response of nearby shallow‐buried high‐density polyethylene (HDPE) gas pipelines under collapse impact loads was investigated. The stress‐strain curve and material parameters of the HDPE pipe were obtained through uniaxial tensile tests. The strain distribution of buried pipelines under collapse loading was obtained. Subsequently, a 3D nonlinear finite element (FE) model of a buried HDPE pipeline impacted by a collapsed body was established. Finally, the effects of the impact velocity, touchdown angle, touchdown mode, offset distance, and pavement structure on the mechanical behavior of buried HDPE pipelines were analyzed in detail. The results show that, as the touchdown velocity and angle of the collapsed body increase, the von Mises stress of the pipeline is significantly affected. The pipe section close to the center of the impact zone tends to be damaged due to the von Mises stress reaching the maximum tensile strength. The maximum von Mises stress of the pipeline that is impacted by the edge of the collapsed body is the smallest, but the buried pipeline undergoes the largest displacement. The rigidity of the overlying pavement structure has a significant impact on the reliability of the buried HDPE gas pipeline. The stress concentration areas of buried pipelines mainly appear on the top, bottom, and the arches of the pipeline.

Experimental and numerical investigations on damage assessment of high-density polyethylene pipe subjected to blast loads

Buried high-density polyethylene (HDPE) pipelines have been extremely vulnerable to terrorist attacks and third-party damage in recent years, so it is important to study the vibration response and safety criterion of buried HDPE pipelines under the action of surface blast loads. In this paper, based on the full-scale field tests, the damage characteristics of HDPE pipes under the action of surface blast loads are studied. Next, a 3D numerical calculation model was established to analyze the response characteristics of the buried pipes subjected to surface blast loads and its reliability was verified using the field monitoring data. Then, the safety of the pipes under different blast load conditions was studied in combination with numerical simulation software, and different damage models of the pipe were established based on damage theory. The results show that the peak particle velocity (PPV) of both the pipe and the surface increases as the distance from the explosive decreases, and the vibration velocity of all measurement points is dominated by the vertical direction (Y). Meanwhile, as the surface explosive is triggered, the pipe is prone to overall bending deformation in the axial direction, resulting in the destruction of the pipe. Finally, based on the comprehensive numerical calculations, numerical simulation results were obtained for different damage levels of the pipes under surface blast loading. What’s more, the damage prediction model of the pipes is also proposed, and the critical curves corresponding to different damage levels are derived.

Measurement of Structural Performance of Fusion Weld with Change of Welding Parameters in High-Density Polyethylene

Abstract In the last few decades, the use of high-density polyethylene (HDPE) is continuously increasing such as HDPE pipes. These pipes are frequently used in the construction of long-buried infrastructures including water mains, sewers, and gas pipelines. These pipes are available in standards lengths and can be customized as per desired lengths using the fusion welding technique. In this article, the effect of changing the welding parameters such as temperature, heat, and soaking time on the performance of the fusion welding joint in HDPE is presented. The structural performance is measured by comparing the tensile strength of the unwelded and welded structure. Furthermore, the structural performance of extruded HDPE pipes and injection-molded HDPE fittings, such as elbows, tee-joints, and close end cap connections, are also discussed. The findings of this research are very useful in understanding and improving the structural performance of HDPE fusion welds with the help of comparative studies based on tensile strength on the welded joint in HDPE.

Experimental investigations of the critical values of J-integral and crack tip opening displacement (CTOD) of high-density poly-ethylene PE100 elbow

The present study aims to characterize the cracking behaviour of high-density poly-ethylene (HDPE) pipe elbow. Experimental studies were performed to determine the mechanical behaviour of HDPE and study the anisotropy of the material. Tension tests were performed on compact tension (CT) specimens to identify the toughness value, expressed by the critical value of the J-integral JIc. These tests were carried out on specimens cut from a PE100 plate with a different orientation angle with respect to the extrusion direction. The testing results have shown that HDPE can be considered as an isotropic material. The methods used to calculate the JIc value resulted in different value. Due to these uncertainties, a study of a failure criteria using crack tip opening displacement (CTOD) was proposed and conducted. The combination of the J-integral and “CTOD” method gave the possibility to determine for the critical value of the J-integral as a value of the fracture toughness of the HDPE. The obtained results suggest that the value of 8.4 kJ/m2 can be considered as the JIc fracture toughness required to initiate the crack for the HDPE.

Impact of operational temperature changes and freeze\u2013thaw cycles on the hydraulic conductivity of borehole heat exchangers

A large share of the primary energy is consumed to provide space heating. Geothermal energy offers a regenerative alternative. For reasons of efficiency and environmental protection, it is important to ensure the system integrity of a borehole heat exchanger (BHE). Previous investigations have focused on the individual components of the BHE or on the grout and pipe systems’ integrity. This study focused on the analysis of the hydraulic system integrity of the complete subsoil–grout–pipe system as well as possible thermally induced changes. For this purpose, a pilot-scale experiment was built to test a 1-m section of a typical BHE under in situ pressure, hydraulic and temperature conditions. During the tests the hydraulic system permeability of the soil and the BHE was measured continuously and separately from each other. In addition, the temperature monitoring array was installed in a 50-cm cross-sectional area. Significant temperature-related fluctuations in the sealing performance could be observed. Hydraulic conductivity limits required by VDI 4640-2 (Thermal use of the underground—ground source heat pump systems, 2019) were exceeded without frost action. The succeeding application of freeze–thaw cycles further enhances the system permeability. The study shows that the thermally induced effects on the system integrity of the BHE are larger and more significant than the subsequent frost-induced effects. The hydrophobic character of the high-density polyethylene (PE-HD) pipes as well as its high coefficient of thermal expansion seem to be the main points of weakness in the system. Optimization research should focus on the interface connection between grout and pipe, whereby hydrophilic pipe materials such as stainless steel or aluminum should also be considered as well as manipulation of the pipe surface properties of PE-HD.

Degradation of sunlight exposure on the high-density polyethylene (HDPE) pipes for transportation of natural gases

High-density polyethylene pipes are prevailing in transportation of combustible natural gases, so their reliability deserves special attention. Amongst the diverse ageing effects that are usually taken into account, including internal pressure, chemicals, temperature and vibration etc., the effect of sunlight exposure has been seldom concerned because the natural weathering test takes a really long time. Though the Xenon arc lamp is always used to simulate the ultraviolet environment, the natural weathering test is still the optimum to evaluate the performances degradation of materials to simulate the actual service conditions. In this paper, such a natural weathering test with sunlight exposure time up to 24 months was performed on the PE100 class high-density polyethylene pipes. The mechanical and the chemical properties before and after the test were measured to evaluate the extent and trend of degradation, and particularly the Fourier transform infrared microscope was utilized to quantitatively investigate the oxidation degradation mechanism in the microscopic level.

Simulation Analysis of Limit Operating Specifications for Onshore Spoolable Reinforced Thermoplastic Pipes.

Spoolable reinforced plastic line pipes (RTPs), exhibiting a series of advantages such as good flexibility, few joints, long single length, light weight, easy installation, etc., have been widely used in the onshore oil and gas industry such as oil and gas gathering and transportation, high pressure alcohol injection, water injection, sewage treatment, and other fields. However, due to the lack of clear standard specificationof the limit operating properties for RTPs, three typical failure modes, i.e., tensile, flexure, and torsion, frequently occur in terrain changes, construction operation, and subsequent application, which seriously affects the promotion and use of RTPs. In this paper, the stress distribution of a non-bonded polyester fiber reinforced high-density polyethylene (HDPE) pipe (DN 150, PN 2.5 MPa) was systematically studied by the finite element method (FEM), and then the limit operating values under the axial tensile, coiled bending, and torsion load were determined. The corresponding experiments were conducted to validate the reliability and accuracy of the FEM model. The FEM results showed that the critical strain for axial tensile was 3%, the minimum respooling bend radius was 1016.286 mm, and the limit torsion angle of this RTP was 58.77°, which are very close to the experimental results. These limit values will be useful to establish normative guidelines for field construction and failure prevention of onshore RTP.

Scale precipitation on HDPE pipe by degassing of CO2 dissolved in water

Abstract In Algeria, high-density polyethylene (HDPE) is widely used in drinking water pipes. This study is focused on the precipitation of calcium carbonate, a major constituent of scale, from calcocarbonically pure (CCP) water in HDPE pipe. Studying scaling in natural conditions is very difficult because it occurs over many years. For this, accelerated scaling is caused by the degassing CO2 dissolved in water. The kinetic study has shown that the germination time and the critical pH decrease with the hardness (30, 40 and 50 °f) and temperature (30, 40 and 50 °C) of water. On the other hand, scaling process efficiency (η) and the supersaturation coefficient (Ωcal) of CaCO3 increase with these parameters. The CaCO3 precipitation occurs both in solution and on walls of HDPE. By the weighing method, it is shown that the deposit mass increases with hardness and temperature. Calcium carbonate precipitates much more in homogeneous phase than in heterogeneous one. The study also showed that heterogeneous nucleation on HDPE is much less important than on PA, PVC, chrome and Inox. These measurements are supported by the characterization of X-ray diffraction deposits and by scanning electron microscopy, which recognizes that the precipitate obtained consists mainly of calcite.

An Accurate and Efficient Fitness-For-Service Assessment Method of Pipes with Defects under Surface Load

With continued urbanization in China, the construction of urban gas pipelines is increasing, and the safety of gas pipelines are also increasingly affected by urban development and the increased scope of buildings and roads. Pipes with defects are more likely to fail under the surface loads. In this study, uniaxial tensile tests of high-density polyethylene (HDPE) pipes were carried out to obtain the real material parameters of pipe. A pipeline-soil interaction finite element model of HDPE pipeline with defects under surface load was established. The failure mechanism of the urban gas pipeline was studied and the influence of parameters such as internal pressure, defect position, defect depth on the mechanical behavior, and failure of pipelines were analyzed. A failure criterion for HDPE pipes with defects under surface load was proposed based on the limit-state curves obtained under different working conditions. Furthermore, an accurate and efficient fitness-for-service assessment procedure of pipes with defects under surface load was proposed. The results showed that maximum Mises stress of the pipeline gradually increased with increasing surface load and the position of maximum stress changed from the top and bottom of the pipe to the defect position and both sides of the pipe. Finally, when Mises stress of the HDPE pipe exceeds the yield limit, failure will occur. Internal pressure, defect location, and defect depth were found to influence the failure process and critical surface load of the pipeline. Safety evaluation curves of the gas pipeline with defects under surface load were obtained by calculating the critical failure load of the pipeline under various working conditions. Finally, a nonlinear fitting method was used to derive a formula for calculating the critical surface load under different defect parameters. The proposed method provides a useful reference for urban gas pipeline safety management.

Pressure Effects on the Lifetime of Gas High Density Polyethylene Pipes

Abstract The purpose of this study was to propose low gas pressure effects on lifetime of natural gas high-density polyethylene (HDPE) pipes by thermal-oxidative aging. The new method to assess the lifetime of HDPE natural gas pipes is based on gas pressure testing. An approach to monitor oxidative induction time (OIT) has been used to predict lifetime. Natural gas HDPE pipes were used to evaluate the effects of low gas pressures on oxidative induction time. In order to emphasize the pressure effects, relatively low temperatures at 45, 55, 65, and 75 °C were utilized for the exposure. The low-pressure conditions were created using air at levels of 0, 0.1, 0.2, 0.3, and 0.4 MPa. The property of high density polyethylene pipes was effectively monitored using the low pressure OIT test. The results show that the aging reaction rate of high density polyethylene pipes increased exponentially with temperature and gas pressure according to the Arrhenius equation. Analytical models were developed to predict the aging reaction rate and lifetime of natural gas HDPE pipes.

Crack layer model for semi-elliptical surface cracks in HDPE pipes and application in buried pipes with complicated loading conditions

The crack layer (CL) model can be used to theoretically simulate unique discontinuous slow crack growth (SCG) and the consequent lifetime of pipe-grade high-density polyethylene (HDPE). However, its application has been limited to several specimen configurations and 1-dimensional (1D) crack growth in pipes. In this study, a 2-dimensional (2D) CL growth model was formulated for a semi-elliptical surface crack in an HDPE pipe, for the first time. The 2D semi-elliptical discontinuous SCG kinetics of the inner surface of the pipe were successfully simulated. In addition, the proposed model was applied to a buried HDPE pipe under the complicated loading condition, e.g., internal pressure, ground pressure induced by structures and traffic loads, as well as the point load due to stones contacting the pipe surface. As the point load exerted by such hard objects within the soil medium can reduce the structural integrity of the buried pipes, this effect is a valid consideration. Various levels of these loading conditions were applied for semi-elliptical CL growth simulations, with different stone sizes in contact with the buried pipe. The results indicate that the diameter of the stone influences semi-elliptical crack propagation behavior and significantly reduces failure time even under the same external loading condition. In addition, it was observed that the lifetime reduction rate depends on the ratio of the internal pressure to ground pressure. This study broadens the applicability of the 2D CL model for buried HDPE pipes to more practical situations and quantifies the effect of stone-embedded soils on SCG kinetics and its lifetime.

Structure Evolution and Hoop-Reinforcing Mechanism of Bionic-Inspired Off-Axial Glass Fiber-Reinforced High-Density Polyethylene Pipes Fabricated via Rotating Co-extrusion

Triple-layer rotating co-extrusion is employed to fabricate hoop-enhanced polyethylene composite pipes with bionic off-axial glass fiber (GF)-reinforced high-density polyethylene (HDPE) middle layer. One kind of super-“hybrid shish-kebab”-like structures formed by GFs and interfacial lamellae tend to spirally align with a fiber deviation angle of 55.7° as the rotation rate reaches 20 rpm. Encouragingly, the hoop strength and hydraulic damage resistance are improved by 25.5 and 24.1%, respectively, even with a low fiber aspect ratio, which can be attributed to the enhanced hoop loading capacity and the energy dissipation effect of crack bifurcation contributed by deviated hybrid structures. Innovatively, the relationship between processing field and structure evolution has been systematically investigated by the simulation of melt flow track, shear rate, and flow rate, indicating that fiber deviation is closely related to the shear rate. This work develops a feasible strategy with both academic value and industrial application prospect for the hoop enhancement of fiber-reinforced HDPE pressure pipes and improves its adaptability to service requirements as well.

Hydraulic Transients in Viscoelastic Pipeline System with Sudden Cross-Section Changes

It is well known that the water hammer phenomenon can lead to pipeline system failures. For this reason, there is an increased need for simulation of hydraulic transients. High-density polyethylene (HDPE) pipes are commonly used in various pressurised pipeline systems. Most studies have only focused on water hammer events in a single pipe. However, typical fluid distribution networks are composed of serially connected pipes with various inner diameters. The present paper aims to investigate the influence of sudden cross-section changes in an HDPE pipeline system on pressure oscillations during the water hammer phenomenon. Numerical and experimental studies have been conducted. In order to include the viscoelastic behaviour of the HDPE pipe wall, the generalised Kelvin–Voigt model was introduced into the continuity equation. Transient equations were numerically solved using the explicit MacCormack method. A numerical model that involves assigning two values of flow velocity to the connection node was used. The aim of the conducted experiments was to record pressure changes downstream of the pipeline system during valve-induced water hammer. In order to validate the numerical model, the simulation results were compared with experimental data. A satisfactory compliance between the results of the numerical calculations and laboratory data was obtained.

Fabrication of anti-scaling HDPE/fluorinated acrylate polymer/nano-silica composite for landfill leachate piping system

Clogging generally happens to the leachate piping system, which poses a risk to the environment. A low surface energy nanocomposite is prepared to mitigate the cloggings, by adding the fluorinated acrylate polymer and hydrophobically modified nano-silica into high-density polyethylene (HDPE) substrate. The best addition of the fluorinated acrylate polymer and the nano-silica is given as 15% and 5%, to produce the composite with a low surface energy of 29.4 mJ/m2. Through the characterization of contact angle (CA), electrochemical corrosion, scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), atomic force microscope (AFM) and thermogravimetry (TG), the composite shows low wettability, good corrosion resistance and thermal stability. The surface hydrophobic property of the composite remains unchanged after being immersed in an acidic (pH = 2) and an alkaline (pH = 12) solution, indicating that the prepared composite has strong adaptability to the extreme environments. In addition, the composite shows better anti-scaling performance than that of the commercial high-density polyethylene (HDPE) and polyvinyl chloride (PVC) pipe materials by application of a dispensing leachate immersion test. The results provide insights into engineering practice for the design and manufacture of pipe materials for leachate collection and transport.

Structural response of non-perforated and perforated corrugated high-density polyethylene pipes under variable loading

The structural response of small diameter (<300 mm) corrugated high-density polyethylene (HDPE) drainage pipes installed on agricultural lands, has not been thoroughly examined. Additionally, characteristics of imposed wheel loads from agricultural field machinery are not adequately represented in conventional structural design practises for buried pipes. This study investigates the response of single-wall corrugated HDPE pipes (100 mm diameter) under loading from 12 agricultural soil textures, two water table positions, and three agri-machinery wheel loads using a finite-element-based numerical model. Effects from two friction conditions (smooth and bonded) at the soil-pipe interface were assessed. Simulations were also done to determine the impact of variable perforation characteristics (shape, size, and configuration) on the stress distribution and deformation of the corrugated pipes. The results show that bonded friction conditions generally induce larger stresses in the pipe wall than smooth friction conditions, but the differences are not statistically significant across all soil and load types. The critical load case occurs for a loaded grain wagon on silt loam soils. The results also indicate that perforations in the pipe wall induce high stress concentrations at the edges of rectangular slots and circular holes, increasing the risk for failure by ductile yielding. Specifically, corrugated HDPE pipes perforated with 5.6 mm diameter holes produce stresses that exceed the yield strength at buried depths shallower than 0.9 m. In conclusion, perforation characteristics should be considered when assessing the structural adequacy of corrugated HDPE pipes for land drainage.

An efficient and sustainable approach for cooling underground substations

With rapid rates of urbanisation and significant improvements in construction technologies, the number of subsurface infrastructure projects has drastically increased in recent years. In addition to their primary functions, these structures have shown great potential as energy geo-structures, exchanging heat with the ground to heat and cool spaces. Given their large contact area with the ground, energy tunnels are proving to be a sustainable source of thermal energy for effectively heating under- and above-ground spaces. However, their efficiency in cooling-dominated conditions has not yet been adequately studied. This paper tackles one of the key challenges regarding transport tunnels: sustainable cooling of underground substations, by introducing an efficient and cost-effective cooling method. The method takes advantage of airflow in the tunnels and relatively stable ground temperatures and involves heat exchangers in the form of water-filled high-density polyethylene (HDPE) pipes being integrated into the tunnel space. The efficiency of the proposed system is numerically assessed by analysing the spatio-temporal variations of temperature in the ground, substation, tunnel air, tunnel structure and heat exchangers caused by continuous heat rejection from the substations. A detailed 3D finite element heat and mass transport model is used, and alternative placements of heat exchangers are investigated. Results show that heat exchangers placed on the tunnel lining, and hence exposed to the tunnel airflow, could efficiently supply a substation’s cooling demand, without significantly increasing the temperature of the tunnel air or the ground. The substantial economic benefits of this cooling system compared to a conventional cooling system is also demonstrated.