Dynamic response of a double-deck circular tunnel embedded in a full-space

A three-dimensional dynamic model for calculating the ground-borne vibrations generated by harmonic loads applied on the interior floor of a double-deck circular tunnel is developed. The response of the system is obtained coupling the interior floor subsystem and the tunnel-soil subsystem in the wavenumber-frequency domain. The interior floor is modeled as a thin plate of infinite length in the train circulation direction and the tunnel-soil system is described using the Pipe in Pipe model. Some numerical instabilities of the resulting expressions are overcome by using analytic approximations. The results show that the dynamic behavior of the interior floor clearly influences the magnitude of the coupling loads acting on the tunnel structure. The soil response to a harmonic load acting on the double-deck tunnel is compared to the one obtained for the case of a simple tunnel finding significant differences between them for the whole range of frequencies studied. The proposed model extends the prediction of train-induced vibrations using computationally efficient models to this type of tunnel structure.     

Floor dust erosion during early stages of coal dust explosion development

An ignition of methane and air can generate enough air flow to raise mixtures of combustible coal and rock dust. The expanding high temperature combustion products ignite the suspended dust mixture and will continue to propagate following the available combustible fuel supply. If the concentration of the dispersed rock dust is sufficient, the flame will stop propagating. Large-scale explosion tests were conducted within the National Institute for Occupational Safety and Health (NIOSH) Lake Lynn Experimental Mine (LLEM) to measure the dynamic pressure history and the post-explosion dust scour depth. The aim of this effort is to provide quantitative data on depth of dust removal during the early stages of explosion development and its relationship to the depth of floor dust collected for assessing the incombustible content most likely to participate in the combustion process. This experimental work on dust removal on is not only important for coal mine safety but also for industrial dust explosions.     

The Underground Atlas Project

The systematic recording of underground structures, their location, use as well as other characteristics is something that is still missing from the global engineering community. The paper presents the development of the Underground Atlas Project which aims at gathering and indexing the major underground sites worldwide, becoming the focal point of information regarding subsurface structures. The main features of the project are found in the development of the electronic database to store and depict the spatial information of the site, as well as in the crowdsourcing concept that was selected for the data and information submission. To foster the submission process, a new taxonomy and categorization of underground space uses has been proposed, while a respective web service and mobile app was developed by the design team. The users can either browse through the records stored or, more importantly, can submit new content either by using their browser or by taking advantage of the geolocation capabilities of their mobile device. Finally, the Atlas will provide the opportunity for the meta-analysis of the data by using and benefiting from the accumulated knowledge and collective experience of the engineering community.     

Analysis of roof and pillar failure associated with weak floor at a limestone mine

A limestone mine in Ohio has had instability problems that have led to massive roof falls extending to the surface. This study focuses on the role that weak, moisture-sensitive floor has in the instability issues.Previous NIOSH research related to this subject did not include analysis for weak floor or weak bands and recommended that when such issues arise they should be investigated further using a more advanced analysis. Therefore, to further investigate the observed instability occurring on a large scale at the Ohio mine, FLAC3 D numerical models were employed to demonstrate the effect that a weak floor has on roof and pillar stability. This case study will provide important information to limestone mine operators regarding the impact of weak floor causing the potential for roof collapse, pillar failure, and subsequent subsidence of the ground surface.     

Hydrogen permeability in subsurface

Clean hydrogen is a promising option for reducing carbon dioxide emissions, but it has not yet been used as an energy carrier at the scale required for meeting the net-zero target by 2050. Hydrogen molecules are smaller than nitrogen and methane molecules. Hydrogen, nitrogen, and methane have densities of 0.09 g/L, 1.25 g/L, and 0.71 g/L, respectively, at the standard temperature and pressure. Our knowledge of the geological formations is based on responses to the larger and heavier gases; it is unclear whether we can apply this knowledge to store hydrogen at the required scale.We investigate the single-phase flow of hydrogen in the subsurface and compare it with the single-phase flows of nitrogen and methane. The comparison with nitrogen is helpful because it is used under laboratory conditions. The comparison with methane is also beneficial because engineers understand its behavior under in-situ conditions. We use the Knudsen number (Kn) to determine the flow behaviors under laminar conditions within two domains. The first is a permeable medium representing a conventional gas reservoir, and the second is caprock. Our study shows that the existing knowledge of the first domain's permeability applies to hydrogen flow; however, it is unrealistic for the second domain. The single-phase permeability of the caprock obtained by nitrogen in the laboratory underestimates hydrogen permeability at low pressures (<10 MPa), and the deviation is a non-linear function of pressure. Our study also shows that hydrogen permeability is always larger than methane permeability in the caprock. The difference between the two, controlled by the reservoir pressure, reached 70% in the caprock. The presented results have applications if hydrogen storage in gas reservoirs becomes a reality.     

Underground medieval water distribution network in a Spanish town

The city of Alcudia de Crespins, in the centre of the Valencia province (east of Spain), has an exceptional water distribution system that in the past served fresh water to many houses in the town. This system is formed by more than one km of tunnels and underground cisterns, and dates probably in the late medieval times, while it has been in use and suffering modifications until 1955. This paper presents the structure and characteristics of such exceptional system, and explains the functioning parameters of the infrastructure.     

A framework for the future of urban underground engineering

A special and holistic approach is needed that captures aggregate attributes and emergent behaviors of the complex system of infrastructure systems in a region. Effective management of the impacts of future population growth, urbanization, and risks arising from continued evolution of our natural, physical and human/societal systems will require a systematic exploration and characterization of the urban subsurface, including much improved understanding and assessment of geologic risks. With recent cost escalations for underground construction projects, incentives are needed for the underground construction industry to develop and implement innovations in methods and technology, and smart integrated planning is needed to reduce costs both during construction and with life-cycle engineered design and operation of our subsurface facilities.The needed framework requires investigation of potential metrics that reflect the performance of aggregate functions of an urban environment so that we can holistically study system performance response under “normal” and “stressed” operation. Such a metric can support a cross-disciplinary exploration of urban resilience, and build knowledge as we develop and test theory and models that explore resilience of complex socio-technical systems. Econometrics with spatial and temporal granularity will help to understand the integrated functionality of our cities and to establish appropriate policies that will drive continuous improvement in the quality of urban life while providing natural, human, and physical urban environmental resilience. The underground in urban regions can become an important component of managing the increasing complexity of our physical systems, and can also make more significant contributions to improving the robustness and resilience of our future cities.     

Research on Combined Construction Technology for Cross-Subway Tunnels in Underground Spaces

Given the increasingly notable segmentation of underground space by existing subway tunnels, it is difficult to effectively and adequately develop and utilize underground space in busy parts of a city. This study presents a combined construction technology that has been developed for use in underground spaces; it includes a deformation buffer layer, a special grouting technique, jump excavation by compartment, back-pressure portal frame technology, a reinforcement technique, and the technology of a steel portioning drum or plate. These technologies have been successfully used in practical engineering. The combined construction technology presented in this paper provides a new method of solving key technical problems in underground spaces in effectively used cross-subway tunnels. As this technology has achieved significant economic and social benefits, it has valuable future applications.     

Rib stability:A way forward for safe coal extraction in India

Ensuring rib stability during pillar extraction is of prime importance in bord and pillar(BP) method of underground coal mining with caving. Rib stability has been assessed here by way of assessing factor of safety(FOS), a ratio of the strength of rib to stress on it. Earlier formulations for rib stability when applied to case studies gave very low FOS value suggesting significant ground control problems, which were contrary to the field observations. This has necessitated the need to revisit the concept of rib stability. The stress coming on the rib is estimated with the use of numerical modeling technique using the FLAC~(3D) software. The methodology of assessing rib-stability with the help of suggested rib-strength formulation has been validated at eight Indian coal mines. The outcome of this study finds relevance and importance in ensuring underground coal liquidation with improved safety and conservation.     

Dynamic response of underground gas storage salt cavern under seismic loads

A dynamic elastoplastic damage constitutive model is proposed based on the failure characteristic of rock salt under seismic loads. The coding of the proposed model is achieved by the embedded FISH (short for FLACish) language of FLAC^3D (Fast Lagrangian Analysis of Continua). Numerical models of bedded salt cavern gas storage facilities in China are developed by using FLAC^3D, and the proposed constitutive model is used in the simulations. The effects of seismic input angle, seismic acceleration, seismic moment, types of seismic waves, and gas pressure on the dynamic response, stress, displacement, plastic zone, and safety factor (SF) of rock masses that surround salt cavern gas storage facilities are studied. Results show that the seismic wave perpendicular to the surface poses the greatest risk to the safety of the cavern. With an increase in seismic acceleration, the cavern’s SF decreases and that of the lower structure of the cavern decreases more than that of the upper section. Plastic zones propagate from the cavern’s internal surface to the pillar, and then to the pillar and floor along the right and left corners of the cavern bottom. Higher internal gas pressure improves cavern safety. The acceleration and duration of seismic waves are critical factors in ensuring the safety of the cavern. The SF of the cavern’s lower structure is more sensitive to changes in seismic parameters than that of the other locations, which makes the cavern bottom more likely to be destroyed during an earthquake. Therefore, the lower structure should be the study target in the seismic design for a salt cavern gas storage facility. Results have been used in the seismic design of salt cavern gas storage facilities in China.