Roadway Tunnel Research Articles

A Fast Method for Estimating the Emission Factors of Air Pollutants from In-Use Vehicles Fleet

The real-world emission factors of gaseous and particulate air pollutants emitted from in-use vehicles, can be rapidly estimated using monitoring data of their concentration profiles from inside roadway tunnels using a mobile laboratory equipped with fast monitoring instruments. The concentrations of CO2 and particle-bound polycyclic aromatic hydrocarbons (PM-PAHs) and NOx, were observed to increase linearly with traveling distance inside two successive roadway tunnels: the Hongjimun Tunnel and the Jeongneung Tunnel on the Naebu Express Way in Seoul, Korea, except for a small region of decrease. In the decreasing regions, within a few hundred meters of the entrance and before the exit, outside background air with low concentrations of air pollutants was thought to have intruded. From the slopes of the linear regression between distance and concentrations, a fleet-averaged (light-, medium-, and heavy-duty vehicles with 54%, 36%, and 10%, respectively) emission factor of CO2, PM-PAHs, and NOx at an average speed of ~60 km h−1 could be calculated as 197 ± 38 g km−1, 4.2 ± 0.8 × 10−4 g km−1, and 0.530 ± 0.230 g km−1, respectively, which are within the ranges of values reported in the literature. For each tunnel, the emission factors of CO2, PM-PAHs, and NOx estimated on days with higher-than-normal fractions of heavy-duty vehicles, were higher than those on other days. From these results, the new fast method proposed in this study is considered useful for estimating real-world emission factors of air pollutants by using a mobile laboratory as a complementary tool to traditional tunnel studies. This method can be used to rapidly make emission maps at roadway tunnels in mega-cities like Seoul, Korea, for urban air-quality management.

Critical velocity for preventing thermal backlayering flow in tunnel fire using longitudinal ventilation system: Effect of floor-fire separation distance

To design a reliable longitudinal ventilation system in a roadway tunnel, it is crucial to know the optimum value of the forced airflow velocity which must be applied to minimize hazards to users trapped in the building during a fire. This optimal value known as the critical longitudinal ventilation velocity, is a crucial criterion for tunnel safety when designing ventilation systems. Analysis of the influence of the fire development location from the tunnel floor on the critical velocity has rarely been studied, although this situation really exists due to the difference in vehicle sizes in practice. CFD simulations were performed in this paper to study this scenario and to estimate the critical velocity for a fire developing in different locations relative to the tunnel floor. The results indicate that as the elevation of the fire location increases from the tunnel floor, the critical velocity decreases. A new model involving the separation distance of the fire location on the tunnel floor and relating the critical velocity to the fire heat release rate is proposed. Furthermore, under critical conditions of ventilation velocity, the results show that the elevation of the fire source location from the floor has a significant effect on the maximum smoke temperature rise below the tunnel ceiling.

Vulnerability of Drop Ceilings in Roadway Tunnels to Fire-Induced Damage

Roadway tunnels often include a reinforced concrete drop ceiling that is hung from the liner to create a plenum that facilitates ventilation and houses utilities. Drop ceiling panels are lightweight compared with the much thicker tunnel liner and can experience significant damage from a fire on the roadway below. This paper examines the flexural response of drop ceiling panels in two representative tunnels to standard fire curves as well as several realistic fires due to vehicular accidents. Standard fire demands as per the Rijkswaterstaat and ASTM E1529 fire curves are uniformly applied to the ceiling panels, and heat exposure contours for typical vehicle fires with heat release rates of 30, 100, and 200 MW are generated from the software CFAST. The finite element analysis software SAFIR is used to evaluate the thermo-mechanical behavior of the ceiling panels when subjected to various thermal demands from the fire below. The analysis results indicate that drop ceiling panels are highly vulnerable to fire-induced damage and potential collapse both during a fire’s active heating phase (from simultaneous loss of capacity and restraint of thermal expansion) and during the subsequent cooling period (from tension that develops when the permanently deformed panel thermally retracts). The potential for fire-induced damage or collapse of the drop ceiling panels can be mitigated by reducing the fire hazard, removing the drop ceiling, or enhancing the fire resistance of the panels via the application of passive protection or structural hardening.

Failure Mechanism Analysis and Support Technology for Roadway Tunnel in Fault Fracture Zone: A Case Study

This paper introduces a case study on the failure mechanism and support design of a roadway tunnel in the fault fracture zone of the 106 mining area in the Yuandian no.2 coal mine. Based on the on-site geological conditions (in-situ stress test, borehole television imaging, and lithological analysis), the failure mechanism of the roadway tunnel in the fault fracture zone was studied. The test results showed that the high tectonic stress, fractured rock, and poor lithology are the primary reasons for the roadway instability. According to the support principles of grouting reinforcement, pre-reinforced support, and rational support range, a new type of combined support technology was proposed, including advanced grouting, grouting bolts, and grouting anchor cables. A 100 m roadway section was selected for field testing using the new support scheme, and detailed deformation monitoring was performed. Monitoring results showed that the roadway deformation under the new support was significantly reduced. During the roadway excavation process, no roof collapse phenomenon occurred, and the safety of roadway excavation was ensured. This successful case provides an important reference for similar roadway projects in the fault fracture zone.

Reusing Jet Grouting Waste as Filler for Road Asphalt Mixtures of Base Layers.

Secondary raw materials consist of production waste or material resulting from recycling processes, currently in large quantities, which can be injected back into the economic system as new raw materials. This study proposes jet grouting waste (JGW) as filler for hot and cold asphalt mixtures applied as base layers of road pavements and investigates the physical and mechanical properties. JGW is derived from soil consolidation performed during underground roadway tunnel construction. The research compares three asphalt mixtures: (a) hot mixture containing limestone aggregate-filler (HMA), (b) HMA containing JGW (HMAJ), (c) cold recycled asphalt mixture containing JGW (CRAJ). Leaching tests of JGW and reclaimed asphalt pavement (RAP) were conducted; the best configurations of the three mixtures were determined by using the volumetric method through gyratory compaction. Three mastics with filler-to-binder ratios reflecting those of the asphalt mixtures were investigated through delta ring and ball test and frequency sweep test at 0.05% stress by using a dynamic shear rheometer. The morphology of each mixture was further investigated by scanning electron microscopy. The results showed that CRAJ with 28 days of curing time reached the indirect tensile strength (ITS) of HMA (0.73 MPa) within 14 days and, among all studied mixtures, returned the lowest cumulative strain, which was on average 30% lower than that of HMA and HMAJ. The results of this study have shown that the cold alternative mixture, CRAJ, promotes the reuse of two types of waste, RAP and JGW, as it fully meets the reference Italian Technical Standard and ensures good mixture performance in addition to conserving natural resources.

Assessment of air quality in car cabin in and around Paris from on-board measurements and comparison with 2007 data

In this paper, we report results on commuter's exposure to particulate matter (PM). Particle number concentrations (PNC) were recorded simultaneously inside and outside the cabin of a vehicle during 45 field trips in and around Paris (France) between 2016 and 2017. For a fixed ventilation setting, the impacts of external parameters were investigated to assess commuter's exposure for a wide range of traffic conditions, including the presence of tunnels and ring road, various times during the day, the specific driving routes, different road types and vehicle speeds. Overall, our results indicate that roadway tunnels represent the worse situation for commuters resulting in the highest concentrations outside and inside the car cabin, followed by the ring road around Paris. Comparisons with a previous study undertaken in Paris and its inner and outer suburbs in 2007 reveal drastic reduction of Particle Number Concentrations (PNC) in the car cabin over this period of 10 years. More stringent emission regulations, changes in the vehicle fleet and development of low emission zones explain these promising results. Similar trends found in the recent literature for different countries corroborate the present findings.

Comparison of vehicle emissions by EMFAC-HK model and tunnel measurement in Hong Kong

On-road vehicular emissions were measured in a Hong Kong roadway tunnel and compared to those from a mobile emission model (EMFAC-HK) for the 2003 and 2015 fleets to assess emission changes and effectiveness of emission controls. EMFAC-HK results compared well with tunnel measurement for the 2015 fleet with differences ≤50% for most pollutants. Both measurement and modeled estimates show that non-methane hydrocarbon (NMHC) and PM2.5 decreased by over 40% from 2003 to 2015. Tunnel measurements also show that nitrogen dioxide (NO2) and liquefied petroleum gas (LPG) -related NMHC emissions increased due to expanded installation of diesel oxidation catalysts and LPG fueling. Motorcycles and LPG-fueled public light buses (3.7% of the fleet) contributed disproportionally high fractions of carbon monoxide (CO; 27%) and NMHC (49%) and deserve additional emission controls. Fuel economy improvements did not result in expected carbon dioxide (CO2) emission reductions, indicating that more aggressive CO2 reductions, particularly from heavy-duty vehicles, are needed.

Model properties for efficient synthesis of nonblocking modular supervisors

Supervisory control theory provides means to synthesize supervisors for systems with discrete-event behavior from models of the uncontrolled plant and of the control requirements. The applicability of supervisory control theory often fails due to a lack of scalability of the algorithms. This paper proposes a format for the requirements and a method to ensure that the crucial properties of controllability and nonblockingness directly hold, thus avoiding the most computationally expensive parts of synthesis. The method consists of creating a control problem dependency graph and verifying whether it is acyclic. Vertices of the graph are modular plant components, and edges are derived from the requirements. In case of a cyclic graph, potential blocking issues can be localized, so that the original control problem can be reduced to only synthesizing supervisors for smaller partial control problems. The strength of the method is illustrated on two case studies: a production line and a roadway tunnel.

Comparison Thermal Insulation Effects of Different Thermal Insulation Materials for a Roadway Tunnel at Southeast Edge of the Qinghai-Tibet Plateau

In seasonal frost regions, tunnels are affected by frost damage after excavation. At entrance and exit section of tunnels, thermal insulation layer (TIL) is widely used to mitigate this problem. In this study, a moisture-temperature coupled mathematical model was established. Based on field observed air temperature at entrance section of Altun mountain tunnel, the thermal regime of surrounding rocks was investigated. Moreover, a comprehensive evaluation method was established to evaluate the thermal insulation effects of seven thermal insulation materials. The results showed that, with increase in thickness of TIL, maximum frozen depths of surrounding rocks decreased almost linearly. At the vault (H1), arch hance (H2), foot (H3), and inverted arch (H4) of the tunnel, the thermal insulation effect was different. When the thickness of TIL reached to 5 cm, there would be no seasonal freezing process at the four points. Seven types of TIL were considered for the thermal insulation of the tunnel. The optimal thickness of them was 4.2, 11.0, 5.5, 6.3, 10.2, 12.8 and 8.5 cm. To ensure the surrounding rocks do not experience freezing process, rigid polyurethane foam would be the best choice. Considering the cost, dry process aluminum silicate fiber would be a better one.

Numerical Study on Thermal Insulation of a Roadway Tunnel at Southeast Edge of the Qinghai-Tibet Plateau

Tunnels in seasonal frost regions are heavily affected by damage related to icing problem and cyclic freeze-thaw of surrounding rock. To mitigate frost related damage, thermal insulation is widely used at entrance and exit section of tunnels. In thermal insulation design, length of the insulated section and thickness of the thermal insulation layer (TIL) are two important parameters. In this study, a coupled mathematical model of heat and moisture transfer was built for tunnels in cold regions. Then, field observed air temperatures at entrance section of a roadway tunnel at Altun Mountain, southeast edge of the Qinghai-Tibet Plateau, were collected and analysed. Based on the mathematical model and field observed inside air temperatures, seasonal freeze-thaw of surrounding rocks of the tunnel entrance was numerically investigated with consideration of the thicknesses and thermal conductivities of insulation layer. The results showed that, within the tunnel, the air has its mean annual temperature increase and the its amplitude decrease with increase in the distance from the entrance. The significant variations mainly occurred within the section 375 m from the entrance. Without the TIL, the maximum seasonal freeze depth of surrounding rocks at the entrance can reach to 1.5 m. To ensure that the surrounding rocks do not experience freezing, the minimum thickness of the TIL with a thermal conductivity of 0.037 W/(m·°C) should be 7 cm. While when the thickness of the TIL is 5 cm, its thermal conductivity should be not greater than 0.014 W/(m·°C).

Development of real-world emission factors for on-road vehicles from motorway tunnel measurements

Abstract In India, there is uncertainty in vehicular emission estimation due to the lack of reliable emission factors (EFs) that represent the real-world driving conditions. In this study, real-world EFs of particulate (PM2.5, PM10, PM2.5-10, PNCs, and BC) and gaseous (CO, CO2, VOCs, and NO2) pollutants from on-road vehicles were developed from the roadway tunnel measurements. Air pollutants measurement campaign was carried out at the Eastern Freeway tunnel (South-bound) in Greater Mumbai in April 2019. Particulate and gaseous samplers were deployed at the entry and exit of the tunnel to measure the targeted air pollutants covering both peak and off-peak traffic hours. The average traffic in the tunnel was ~2450 h−1, dominated by Light-Duty Vehicles (LDVs) (~98%) with an average (±SD) vehicular speed of 59 (±5) km h−1. Almost ~93% of vehicles plying inside the tunnel were with ≤10 years of age and 87% of the fleet predominantly complied with Bharat Stage-IV (BSIV) emission standards. The measured average (±SD) fleet EFs for PM2.5, PM10, PM2.5-10, BC, NO2 and VOCs were 44.2 (±26.9), 118.1 (±28.5), 69.7 (±20.1), 147.0 (±31.5) and 92.9 (±20.7) mg veh−1 km−1, respectively. For CO and CO2, it was 1.6 (±0.3) and 132.4 (±25.0) g veh−1 km−1, and 2.1 (±0.5) ( × 1013 particles veh−1 km−1) for particle number concentrations. The coarse particle EF was 1.6 folds higher than the fine particle EF, signifying the importance of non-exhaust vehicular emission sources. Compared to laboratory-based EFs, the measured real-world EFs were 1.8, 4.8, and 2.2 folds higher for PM2.5, CO, and NO2, respectively. This is likely due to the presence of aged and poorly maintained vehicles in the real-world which were not taken into consideration by the lab-based studies. Capsule We developed hourly EFs of particulate and gaseous pollutants for on-road real-world vehicular fleet (having predominantly LDVs) from air pollution measurements and traffic characterization inside a motorway tunnel in Mumbai, India. Exhaust as well as non-exhaust EFs are developed and compared with the laboratory-based re-constructed EFs (which were 2–5 fold lower than real-world EFs).

Study on the Freeze‐Thaw Process of the Lining Structures of a Tunnel on Qinghai‐Tibet Plateau with the Consideration of Lining Frost Damage

In seasonally frozen ground, there are many frost problems in highway road tunnel after its excavation due to the heat exchange between the cold air and lining structure inside the tunnel. To mitigate these frost‐related damages, thermal insulation layer is widely used at entrance and exit sections of the tunnel. In this study, a coupled mathematical model of heat, moisture, and stress was built for tunnels in seasonally permafrost regions. Then, based on the field‐observed air temperature inside a roadway tunnel at Altun Mountain on the Qinghai‐Tibet Plateau (QTP), seasonal freeze‐thaw process of the surrounding rocks (SR) and lining structures were numerically investigated with the consideration of insulation methods: without insulation (WTIL) and laying the insulation layer on the inner surface of the second lining structure (STIL). Combined with the principle of Miner damage accumulation, the stress regimes of the lining structures of tunnel were investigated in WTIL and STIL. The results show that there was a significantly thermal disturbance of the SR after the tunnel excavation. In the 5th year of the operation period, the maximum seasonal freeze depth (MSFD) of the SR can reach 1.6 m at the vault of the arch and that at the inverted arch was only 1.0 m due to the pavement inside the tunnel. Then, both the absolute maximum value of the maximum principal stress (MAPS) and minimum principal stress (MIPS) in cold season were bigger than those in warm season comparing the value of the stress filed of the lining structure. In the same way, both the MAPS and MIPS of the lining structure in WTIL are bigger than those in STIL in numerical simulation. The positions of the maximum tensile stress of the primary lining structure in STIL and WTIL were inverted arch. For the lining structures, the greater tensile stress was generally harmful. Thus, the inverted arch of the tunnel should be laid on the insulation layer.

Blast Vulnerability of Drop Ceilings in Roadway Tunnels

Tunnels are critical links within modern transportation networks and are susceptible to accidental explosion from vehicle fuel and cargo. They also present a target for terrorism due to unscreened public access. Roadway tunnels constructed with circular, oval, or horseshoe cross sections in the United States within the last 100 years have typically consisted of a road surface, tunnel liner, and drop ceiling. The tunnel liner was the main structural component that resists any over-tunnel loads as well as preventing substrate intrusion and cave-ins. The drop ceiling is hung from the liner and creates a plenum above the roadway for ventilation of vehicle exhaust as well as throughput for electrical conduits, utilities, fire alarms, and fire suppression systems. The typical design of a tunnel drop ceiling only considers gravity loading and forces exerted by pressure within the ventilation plenum—severe impulsive uplift loading from a blast on the roadway below therefore presents significant risk of ceiling panel damage or collapse. This study evaluates the likelihood of significant damage and collapse of tunnel drop ceilings under blast loading. Six scenarios consisting of three trinitrotoluene (TNT) equivalent charge sizes in two representative tunnels were analyzed using computational fluid dynamics (CFD) to generate blast demands on the drop ceiling, followed by a blast vulnerability assessment of the drop ceiling panel elements. Dynamic single-degree-of-freedom (SDOF) analyses were verified against finite-element (FE) solutions and used to evaluate the damage for a wide range of blast demands. The results indicate that modestly sized explosive hazards can induce significant damage to the drop ceiling, ranging from permanent deformation up to widespread collapse. Hardening options include flexural enhancement with fiber-reinforced polymer (FRP) as well as retrofit of the hangers. As an alternative, removing the ceiling and reconfiguring the ventilation system with modern jet fans can be a more cost-effective option for blast mitigation depending on the tunnel owner’s operational and/or financial constraints.

Roadway tunnels: A critical review of air pollutant concentrations and vehicular emissions

Air quality is a widespread problem with the presence of pollutants in indoor and outdoor environments that generate significant consequences for the population, ecosystems and exposed materials. Vehicular traffic is one of the main sources of air pollutants and, therefore, needs to be studied and analysed in detail.This review reported the results of studies conducted on tunnels, in particular for the measurement of concentrations and the definition of emission factors. The characteristics of the tunnels, available ventilation systems, type of vehicular traffic, and geographical distribution, in addition to concentrations and emission factors, are discussed. Light-duty vehicles are the most frequent category in the case studies. Between the fuels used, gasoline is by far the most widespread.Pollutant concentrations concentrations can reach very high levels. For example, atmospheric particulates (with an aerodynamic diameter of 10 µm) and nitrogen dioxide have also reached levels of 1490 µg/m3 and 4982 µg/m3, respectively.

Research on Influence of Road Tunnels with Different Lanes on Surrounding Rock Characteristic Curve

Convergence confinement method is an important guidance method for tunnel construction and support design. Numerical simulation method was used to comparatively analyze the ground reaction curve and the plastic zone under different rock grade and roadway tunnel size. The results show that the change of tunnel size has different effects on the maximum deformation of the tunnel arch crown, the ground reaction curve and the plastic zone range. Finally, some suggestions were put forward for the construction and optimization of the large span arch tunnel support structure. The research results may provide some guidance for related engineering

Connecting Continents: Challenges of the Eurasia Tunnel in Istanbul

The Eurasia Tunnel, the first bored tunnel crossing of the Istanbul Strait (Bosphorus) in Istanbul, Turkey, is a site that presents many unique challenges, including very poor ground conditions, unusually high water pressures, and severe seismic demands. Although this historic waterway separating Europe from Asia has been spanned by three bridges and crossed by an immersed tube railway tunnel, the Eurasia Tunnel is the first bored tunnel across the strait, and the first roadway tunnel crossing.

Development and Application of Complete Equipment for High-speed Tunnel Boring and Bolting Machines

With the improvement of coal mining speed and mechanization level inChina, traditional tunnel boring methods can no longer meet the actualneeds. In order to solve the problems of low effciency, high labor intensity, slow tunnel boring speed, bad working environment and poor safetyin traditional tunnel boring, on the basis of analyzing the developmentand application of coal roadway tunnel boring equipment at home andabroad, complete equipment for high-speed tunnel boring and boltingmachines was developed by using the integrated technology of tunnelboring and bolting. The complete equipment for high-speed tunnel boring and bolting machines has the functions of tunnel boring and boltingsynchronization, once-tunneling, negative pressure dust removal, digitalguidance, independent cutting feed, digital cutting, safety monitoring anddata interaction, which has the advantages of safety in use, reliability andeffciency

Blast Vulnerability Assessment of Road Tunnels with Reinforced Concrete Liners

Tunnels are a critical component of our transportation infrastructure, and unexpected damage to a tunnel can significantly and adversely impact the functionality of a transportation network. Tunnel systems are vulnerable to potential threats of intentional and accidental blast events because of their relatively unrestricted public access. These events can lead to spalling and breach of the tunnel liner which, depending on the surrounding media, can result in local damage and progressive collapse of the tunnel. Current approaches for evaluating blast-induced damage to a tunnel liner either require significant computational effort or oversimplification such that accurate spatial distributions of damage cannot be obtained. This study presents an effective approach to predict and map the damage to a reinforced concrete liner of a roadway tunnel from various explosive threat sizes and tunnel geometries. A literature review of existing studies is conducted, and potential scenarios of blast events are examined with varying charge position and size. Rectangular, horseshoe, and circular tunnel geometries, each with the same traffic throughput, are evaluated. An efficient analytical approach to determine the spatial distribution of blast-induced spall and breach damage is presented and shows good agreement with numerical models analyzed in LS-DYNA. The proposed approach is then used to examine the relationship between increasing blast hazard intensity and the extent of spall and breach damage. Inflection points in this relationship can be used to identify hazard levels at which a progressive collapse evaluation would be warranted.

Compositional multivariate statistical analysis of the hydrogeochemical processes in a fractured massif: La Línea tunnel project, Colombia

Hydrogeochemical and isotopic data were used to improve the understanding of 1) Groundwater flow in a complex fractured aquifer system, 2) Surface water – groundwater interactions, and 3) Groundwater evolution within the La Línea Massif in the Cordillera Central of the Colombian Andes, using two roadway tunnels drilled there as a natural laboratory. The La Línea Massif is a crystalline structure formed by two igneous-metamorphic fractured complexes (Cajamarca Group and Quebradagrande Group), plus andesitic porphyry rocks from the Cenozoic period. The tunnels traverse eight main fault zones, where higher infiltrations of groundwater into the tunnels were reported. Water samples were collected at the surface and within the tunnels in three different sampling rounds in 2010, 2014 and 2015. Major ion concentrations and δ18O and δ2H stable isotopic ratios were measured. This database is a unique opportunity to investigate water-rock interaction processes through time in a dewatering crystalline rock massif in such an important area in the Colombian Andes. Compositional data analysis (CoDa) techniques were implemented on the major ion data before a multivariate statistical analysis (MSA). The MSA included a compositional principal component analysis (PCA) and a compositional hierarchical cluster analysis (HCA), combined with more classical graphical methods for the classification of water chemistry data, such as Piper and Stiff diagrams. According to the compositional HCA and PCA, the obtained components and clusters are first related to the location of the samples (surface or tunnel samples) followed by the geochemical processes. The geochemical variability of the massif is associated with the weathering of silicates and pyrite oxidation that generates carbonate dissolution as a result of the drainage caused by the tunnels. A temporal evolution characterized by a decrease in pH, conductivity and Mg2+ concentrations was observed. The analysis of the stable isotopes, 2H and 18O, suggests that the sampled groundwater has a common origin: recharge waters with specific signatures for the east and west watersheds that exhibit alterations due to hydration because of the dissolution of plagioclases and rock-water interactions with materials exposed to hydrothermal fluids.

Improving the existing roadway tunnels capacity by adding new tunnels—a structural approach

The roadway tunnel is considered a good solution for the success of modern roadway networks. It can help to overcome possible traffic congestion and considerably reduce journey time. The continuous growth of traffic volumes leads to increase congestion and decrease safety. This leads to the need for extra tunnel space. The extra tunnel space can be achieved either by the widening of the existing tunnel or by adding a new one. The choice of the suitable method is dependent on many factors like tunnels alignment, site conditions, construction method, tunnel operation, risk assessment…etc. The current research investigates the second alternative through a specific case study as an example. The method comprises adding two new tunnels to an existing twin roadway tunnels. The investigated problem considers the new tunnels to be added vertically or horizontally. The influence of the new tunnel construction on the existing tunnels is investigated considering both the variation of relative position and spacing distance in a parametric study context. Several numerical models are employed to check the construction sequence and the tunnelling safety. These models are used to evaluate the induced stresses in surrounding ground for two different soil types, straining actions in tunnels’ liner and deformations of both ground and liner. The result demonstration shows how to find out the minimum practical and safe spacing distance between the driven new tunnels and the existing ones without the need for the relatively expensive soil strengthening techniques.