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Energy tunnel provides an innovative and efficient approach to harvesting geothermal energy. Some investigations on a single energy tunnel have been reported in the literature. This study investigated the behaviour of twin energy tunnels in two configurations (i.e., parallel and perpendicularly crossing) by using a thermo-hydro-mechanical coupled numerical model developed and validated in previous work. The responses of ground and a tunnel (denoted by ET1) to three thermo- activation modes were determined: cooling ET1 itself alone ([ET1.sub.active]), cooling a nearby tunnel alone ([ET2.sub.active]), cooling both tunnels simultaneously ([(ET1 + ET2).sub.active]). The computed results reveal that the interaction between two tunnels becomes negligible when the clear distance reaches four times the tunnel diameter. When the clear distance is smaller, the interaction could impact the tunnel and soil displacements. In [ET1.sub.active] and [(ET1 + ET2).sub.active], the vertical responses (i.e., ground settlement, ET1 settlement and vertical convergence) in both tunnel configurations can be close to or even exceed the allowable values in tunnel management guidelines, depending on ground and tunnel conditions. The horizontal responses of ET1 (i.e., horizontal displacement and convergence) are always insignificant. The thermo-activation of ET2 alone has minor effects on ET1, except a strong impact on ET1 settlement in the case of crossing tunnels. Key words: energy tunnel, thermo-mechanical soil behaviour, geothermal energy, 1. Introduction Energy tunnel, which provides an innovative approach to harvesting shallow geothermal energy, is attracting increasing attention (Barla et al. 2016; Laloui 2019; Peltier et al. 2019; Insana and [...]

This paper concerns the problem of designing standard artificial tunnels for rock boulder protection. The proposed approach consists in uncoupling the problem of the dynamic response of the granular dissipative soil cushion placed on the top of artificial tunnels from the dynamic response of the reinforced concrete structure underneath. An already conceived elasto-viscoplastic constitutive model capable of simulating the penetration process of the boulder within the soil stratum and reproducing the force acting on the boulder is briefly described. Its simplified one-dimensional formulation is outlined. A modified version of the model, taking into account large displacements occurring when either impacts on loose granular soils or high energetic content impacts on dense sand strata are considered, is also introduced. To validate the approach, some in situ test results are numerically simulated. A simplified numerical approach is proposed for obtaining the evolution with time of both impact force and boulder penetration, bypassing the use of the rheological model. This goal is achieved by introducing some abaci obtained numerically with reference to an ideal dense sand soil stratum of reference. DOI: 10.1061/(ASCE)GT.1943-5606.0000363 CE Database subject headings: Sand, Soil type; Tunnels; Viscoplasticity; Numerical models; Granular media. Author keywords: Impacts; Sands; Artificial tunnels; Viscoplasticity; Numerical modeling.

This paper presents the results of a comprehensive research study to determine the potential causes for an inordinate distress developed on a shotcrete liner material of a tunnel located near Dallas, TX. This tunnel was originally founded on a limestone material. Distress locations were identified where possible delamination of shotcrete layer and moisture leaks were either suspected or noticed. As a part of the research, rock cores and white powderlike substance behind the liner were collected around the distressed locations, and these cores and powder material were subjected to chemical, mineralogical, and engineering tests to understand the potential causes of this distress. Mineralogical tests, in particular, X-ray powder diffraction analysis on a powder material and gel-like substances collected on the liner, showed the presence of anhydrite, gypsum, and ettringite traces. High amounts of sulfate measurements in chemical and energy dispersive X-ray spectroscopy studies also showed that both gypsum and ettringite formations were possible in and around the limestone material. Upon hydration, mineral expansion of ettringite and anhydrite led to heaving and subsequent cracking of the adjacent shotcrete layer. Engineering characterization tests including unconfined compression strength (UCS), indirect tensile strength (ITS), and triaxial tests on rock cores embedded with a powder type sulfate material revealed that low strength cores were obtained near high distress zones and high strength cores were collected at low distress zones. The UCS values ranged from 6.2 (high distress) to 13.8 MPa (low distress) whereas the ITS values of the cores varied from 0.5 to 1.1 MPa for the same distress locations. This indicates the potential loss of strength of these rock materials from the presence of gypsum material in them. Possible methods to mitigate this heaving problem behind the liner are also discussed. DOI: 10.1061/(ASCE)MT.1943-5533.0000087 CE Database subject headings: Sulfates; Tunnels; Shotcrete; Gypsum; Limestone; Rocks; Differential settlement. Author keywords: Sulfate heaving; Tunnels; Shotcrete; Ettringite; Gypsum; Limestone; Rock testing.

A probabilistic analysis of a shallow circular tunnel driven by a pressurized shield in a frictional and/or cohesive soil is presented. Both the ultimate limit state (ULS) and serviceability limit state (SLS) are considered in the analysis. Two deterministic models based on numerical simulations are used. The first one computes the tunnel collapse pressure and the second one calculates the maximal settlement due to the applied face pressure. The response surface methodology is utilized for the assessment of the Hasofer-Lind reliability index for both limit states. Only the soil shear strength parameters are considered as random variables while studying the ULS. However, for the SLS, both the shear strength parameters and Young's modulus of the soil are considered as random variables. For ULS, the assumption of uncorrelated variables was found conservative in comparison to the one of negatively correlated parameters. For both ULS and SLS, the assumption of nonnormal distribution for the random variables has almost no effect on the reliability index for the practical range of values of the applied pressure. Finally, it was found that the system reliability depends on both limit states. Notice however that the contribution of ULS to the system reliability was not significant. Thus, SLS can be used alone for the assessment of the tunnel reliability. DOI: 10.1061/(ASCE)GT.1943-5606.0000060 CE Database subject headings: Shields, tunneling; Settlement; Serviceability; Ultimate loads; Limit states; System reliability.

Underground tunnels are considered to be a vital infrastructure component in most cities around the world. Careful planning is always necessary to ensure minimum impact on nearby surface and subsurface structures. This study describes the experimental investigation carried out to examine the effect of existing piles installed in cohesive soil and extended to bedrock on the circumferential stresses developing in a newly constructed tunnel supported by a flexible lining system. A small scale testing facility was designed and built to simulate the process of tunnel excavation and lining installation in the close vicinity of preinstalled model piles. Lining stresses were measured for different separation distances between the lining and the existing piles Consistent decrease in the lining load was observed when the piles are located within a distance of one tunnel diameter from the tunnel. The results presented in this study indicated that measuring the lining response near existing pile foundations may be used to evaluate the extent of the interaction between the lining and the surrounding piles. DOI: 10.1061/(ASCE)GT.1943-5606.0000004 CE Database subject headings: Tunneling; Piles; Soil-structure interaction; Cohesive soils.

Incidents may arise in road tunnels, in particular at the tunnel entrance and exit. Investigation conducted in the road tunnels in the provinces of Guizhou and Yunnan in China showed that the concrete pavement surface skid resistance decreased rapidly inside the tunnels and was significantly different from that outside the tunnels. Pavement surface skid resistance is one of the most important factors that affect travel safety, in particular wet weather skidding accidents. Countermeasures are necessary to enhance the skid resistance on concrete pavements in tunnels. The writers made an effort to investigate possible pavement surfaces to provide long- lasting skid resistance, including porous concrete surface and open graded friction course (OGFC) surface. Porous concrete was paved directly on the regular concrete pavement and OGFC was paved on the existing surface of concrete pavement to provide sufficient skid resistance. Accelerated abrasion devices have been developed to evaluate the skid resistances of the porous concrete and OGFC, respectively. Regular asphalt materials will bum easily and emit toxic gas in fire, which may reduce the survival rate for tunnel users involved in fire accidents. Fire-retardant asphalt has been developed to reduce the tendency of asphalt materials to burn and reduce emission. CE Database subject headings: Tunnels; Skid resistance; Concrete pavements; Porous materials; Fire resistance.

An exact analysis for three-dimensional dynamic interaction of monochromatic seismic plane waves with two lined circular parallel tunnels within a boundless fluid-saturated porous elastic medium is presented. The novel features of Biot dynamic theory of poroelasticity along with the appropriate wave field expansions, the pertinent boundary conditions, and the translational addition theorems for cylindrical wave functions are employed to obtain a closed-form solution in the form of infinite series. The analytical results are illustrated with numerical examples in which two identical tunnels, lined with concrete and embedded within water- saturated soils of distinct frame properties (i.e., soft or stiff soils), are insonified by plane fast compressional or shear waves at end-on incidence. The basic dynamic field quantities such as the hoop and axial stress amplitudes are evaluated and discussed for representative values of the parameters characterizing the system. The effects of formation material type, angle of incidence, incident wave frequency, and the proximity of the two tunnels on the liner stresses are examined. Particular attention is paid to the influence of bonding and drainage conditions at the liner/soil interface on the dynamic stress concentrations. Limiting cases are considered and good agreement with the solutions available in the literature is obtained. CE Database subject headings: Stress; Tunnels; Saturated soils; Three-dimensional models.

Critical for the seismic safety of immersed tunnels is the magnitude of deformations developing in the segment joints, as a result of the combined longitudinal and lateral vibrations. Analysis and design against such vibrations is the main focus of this paper, with reference to a proposed 70 m-deep immersed tunnel in a highly seismic region, in Greece. The multisegment tunnel is modeled as a beam connected to the ground through properly calibrated interaction springs, dashpots, and sliders. Actual records of significant directivity-affected ground motions, downscaled to 0.24 g peak acceleration, form the basis of the basement excitation. Free-field acceleration time histories are computed from these records through one-dimensional wave propagation equivalent-linear and nonlinear analyses of parametrically different soil profiles along the tunnel; they are then applied as excitation at the support of the springs, with a suitable time lag to conservatively approximate wave passage effects. The joints between the tunnel segments are modeled realistically with special nonlinear hyperelastic elements, while their longitudinal prestressing due to the great (7 bar) water pressure is also considered. Nonlinear dynamic transient analysis of the tunnel is performed without ignoring the inertia of the thick-walled tunnel, and the influence of segment length and joint properties is investigated parametrically. It is shown that despite ground excitation with acceleration levels exceeding 0.50 g and velocity of about 80 cm/s at the base of the tunnel, net tension and excessive compression between the segments can be avoided with a suitable design of joint gaskets and a selection of relatively small segment lengths. Although this research was prompted by the needs of a specific project, the dynamic analysis methods, the proposed design concepts, and many of the conclusions of the study are sufficiently general and may apply in other immersed tunneling projects. CE Database subject headings: Tunnels; Earthquakes; Dynamic analysis; Soil-structure interaction; Vibration; Nonlinear response; Seismic effects.

Experiences show that the design of overlay mortars is a complicated matter as far as long-term durability and dimensional stability are concerned. In many cases, applied mortars crack or delaminate a few years after application. The same is true for passive fire protection mortars (PFP mortars). The mortar must remain crack free and must not delaminate during the planned service life, that means the fire protection requirements must be combined with the durability requirements. In this paper, an approach based on a numerical model is used to forecast the long-term behavior of PFP mortars. By means of this model, the time-dependent humidity, temperature, stress, and strain distributions, as well as the crack formation can be calculated in a realistic way. The minimal adhesive strength between overlay and substrate which is necessary to prevent delamination during the planned service life has been calculated. Material properties and climatic boundary conditions have been taken into account. The results allow the tailored optimization of PFP mortars. DOI: 10.1061/(ASCE)0899-1561(2007)19:6(484) CE Database subject headings: Concrete durability; Fire protection; Mortars; Numerical models; Tunnel construction.

An assessment of the structural performance of concrete tunnel linings under tire load is presented for the Lainzer Tunnel, a shallow tunnel recently constructed in Austria. Prior to the installation of the concrete lining at this tunnel, large-scale temperature experiments were conducted, revealing almost no reduction of the lining thickness in consequence of spalling for the employed lining concrete, reinforced by polypropylene fibers. In the absence of spalling, dehydration of concrete under high temperatures is the safety-determining process. Within the thermochemomechanical material model employed for the tunnel-safety assessment presented in this paper, dehydration is described by the decrease of the hydration extent, resulting in the reduction of both stiffness (thermal damage) and strength (thermal decohesion). The distribution of the hydration extent is determined by means of a thermochemical analysis. The obtained results serve as input for the subsequent chemomechanical analysis, providing insight into the deformation state of the ground-lining compound structure and the stress state in the lining. As a function of the stress state, a level of loading L is introduced, with L=0 for the unloaded material and L = 100% for stress states reaching the compressive strength of concrete. During fire exposure the size of L increases because of increased loading by thermal expansion, on the other hand, and because of thermal decohesion resulting in a reduction of the compressive strength, on the other hand. The difference between 100% and L; represents the remaining load-carrying capacity of the lining, giving access to the safety of the tunnel for the chosen fire-accident scenario. DOI: 10.1061/(ASCE)0733-9445(2006) 132:6(961) CE Database subject headings: Tunnels; Fires: Concrete; Loads; Linings: Safety.

The Tymfristos tunnel, a 1,365 m long road tunnel with an 11 m section diameter, was excavated through the highly tectonized, low strength Flysch formation of central Greece. About 1 year after the completion of the excavation in 1995, convergence of up to 20% followed by failure of the support system was recorded. A 501 m long segment, in which the failure occurred, was reexcavated based on a more detailed study of the ground conditions; however, this second phase was also followed by an up to 6% convergence. The detailed geodetic study of the three-dimensional kinematics of the support shell in relation time, ring closure delays, and support measures reveals an unusual time-history of convergence occasionally continuing even after the ring closure and a nonuniform distortion of the lining. Furthermore, uncontrolled deformation of certain 'weak' sections propagated bidirectionally to adjacent, previously stabilized sections producing a 'chain effect,' responsible for destabilization and resumed convergence of long (~ 90 m) segments. CE Database subject headings: Convergence: Deformation analysis; Tunnels: Failures: Greece.

According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report in 1982, radiation from radon contributes about 50% of all natural radiation sources to the public. The situation is worse in Hong Kong where large area is granitic containing larger amount of uranium when compared to the global mean. Radon level can therefore be on the high side during tunneling especially where poor ventilation is envisaged. This article includes realistic field study of radon in tunnel under construction. A maximum concentration of over 30,000 Bq/[m.sup.3] was recorded (1 Becquerels per cubic meter is equivalent to 0.027 pico Curies/L). Radon concentration in tunnel environment is found to be related to groundwater ingress, ventilation provision and geology. A review of the current statutory control in Hong Kong is done as well as its comparison with other countries' practices. In fact, the situation can be improved through education, adopting control legislation and more specifically, having a suggested radon management plan in every tunnel drilling project. CE Database subject headings: Radon: Tunnels: Drilling: Ventilation.

A numerical method for the analysis of tunnel-wall interference effects in forced-oscillation testing is presented. The method is based on the wall-pressure signature concept and utilizes the computed wall-pressure distributions. The induced wall pressure field is computed by using an unsteady three-dimensional full Navier--Stokes solver for a 70-deg pitching delta wing in a wind tunnel. To validate the code, the computed static results are shown to have good agreement with the experimental data of Wentz in lift and drag coefficients. On the other hand, the computed static and dynamic normal force coefficients are, in general, higher than those in a second set of data. However, the predicted hysteresis curves of normal force coefficient in large-amplitude-oscillation testing agree reasonably well with this second set of data. The computed unsteady pressures on the tunnel walls and floor are used to generate the interference flowfield that is analyzed for the upwash and blockage corrections. The corrected results are shown to agree well, in the slope and magnitude of hysteresis loops, with the computed results in free-air conditions. It is also shown that the hysteresis in upwash and blockage corrections increases with the reduced frequency.

The challenges and obstacles facing the 'Big Dig' project, and the use of advanced engineering techniques to overcome the complex circumstances are discussed. The project aims to construct an underground superhighway and a tunnel beneath Boston Harbor, in difficult subsurface geological conditions.

Estimates of ground deformations and liner stresses in a tunnel are usually obtained from empirical correlations or from past experience on similar tunnels. These correlations account for only a few of the significant factors, and extrapolation to other cases is questionable because similitude conditions are not generally fulfilled. In this paper, complete analytical solutions for a shallow tunnel in saturated ground are obtained. Two different drainage conditions have been considered: full drainage at the ground-liner interface, and no drainage. The solutions cover different construction processes and soil conditions: (1) dry ground; (2) saturated ground with and without air pressure; (3) with and without a gap between the ground and the liner; and (4) applicability for short term analysis (i.e., undrained excavation and liner installation) and for long term analysis. Since the ground and the liner are assumed to behave elastically, the solutions obtained are restricted to cases where ground deformations are small, such as stiff clays and rocks, or when the excavation method prevents large deformations of the ground.

This article brings out background information and evidence of a forensic investigation into a massive sinkhole accident in Georgia resulting in two fatalities. Data indicate that soil erosion coupled with a failure in the sewer tunnel built on top of fill soils are the causes of the accident.

A large number of centrifuge model tests for simulating the shield construction process in dry sand was carried out by using a newly developed 100 mm diameter miniature shield tunnel. Different construction processes were modeled by three series of tests. A 'buried tube test' was conducted to measure the lining stress under a centrifugal acceleration, a 'tail void test' simulated the process of tail void formation only, and a 'shield test' dealt with the complete process of shield construction. Lining stresses, transverse and longitudinal surface settlements, and earth pressures around the tunnel were simultaneously measured during advancement of the shield at a centrifugal acceleration of 25g. The lining stress at the crown elevation was well predicted by Terzaghi's loosening earth pressure. From the comparison with the recent field measurements and the comparative results of three series of tests, it was confirmed that the shield construction process was successfully simulated in a centrifuge. An experimental formula for estimating the surface settlement above the shield was deduced by a function of tail void thickness and cover-to-diameter ratio.

The south cut and cover of the Confignon tunnel in Geneva, Switzerland, lies on deposits of clay and clayey silt of high and varying compressibility. Without improvement of the foundation, settlements under the final loads of the cover are unacceptable for the project. The improvement of the foundation was achieved by a preload drain system. Monitoring settlements under the fill loads and heave at fill removal indicate: (1) maximum settlement values of 34 cm (340 mm); (2) reduction of the compressibility of the foundation to one-third of its original value in the preloading process; (3) values of the in-situ coefficient of consolidation 1.5-2 times larger than the design values derived from the oedometer test results; and (4) a computed settlement 1.5 times the monitored settlement at the location of the maximum settlement. Monitoring the settlements of the concrete structure under the cover loads indicates: (1) a present maximum settlement of the order of 8 cm (80 mm); and (2) a predicted maximum settlement over the 80-year life period of the project on the order of 12 cm (120 mm) with low differential settlements.

The Inland Feeder designed to increase the amount of state-produced water for Southern California comprises of the Arrowhead East and Arrowhead West Tunnels. The tunnels incorporate structural features like impermeable lining, unique seismic sections and extensive probing and grouting to account for earthquakes and a ground-water monitoring program with 100 monitoring sites. The use of thick-walled steel pipes in fault zones serves a means of protection for the tunnel-workers during earthquakes.

The construction of tunnels at the complex Fort Point Channel I-90 portion of Boston's Central Artery Project involves tunnel jacking, deep soil mixing and outsized immersed tube tunnels by the engineers and contractors. Contacts worth over a billion dollars have been awarded for the purpose. The design and construction of Fort Point Channel faces challenges regarding the use of new techniques and weak clay soils

A common misconception about technological development is that engineers go ahead willy-nilly with whatever is physically possible at a given time and expect the rest of the world to fall […]