Deep Underground Science and Engineering最新文献

Earthquakes triggered by dynamic disturbances have been confirmed by numerous observations and experiments. In the past several decades, earthquake triggering has attracted increasing attention of scholars in relation to exploring the mechanism of earthquake triggering, earthquake prediction, and the desire to use the mechanism of earthquake triggering to reduce, prevent, or trigger earthquakes. Natural earthquakes and large-scale explosions are the most common sources of dynamic disturbances that trigger earthquakes. In the past several decades, some models have been developed, including static, dynamic, quasi-static, and other models. Some reviews have been published, but explosion-triggered seismicity was not included. In recent years, some new results on earthquake triggering have emerged. Therefore, this paper presents a new review to reflect the new results and include the content of explosion-triggered earthquakes for the reference of scholars in this area. Instead of a complete review of the relevant literature, this paper primarily focuses on the main aspects of dynamic earthquake triggering on a tectonic scale and makes some suggestions on issues that need to be resolved in this area in the future.

The fractured surrounding rocks of roadways pose major challenges to safe mining. Grouting has often been used to reinforce the surrounding rocks to mitigate the safety risks associated with fractured rocks. The aim of this study is to develop highly efficient composite ultrafine cement (CUC) grouts to reinforce the roadway in fractured surrounding rocks. The materials used are ultrafine cement (UC), ultrafine fly ash (UF), ultrafine slag (US), and additives (superplasticizer [SUP], aluminate ultrafine expansion agent [AUA], gypsum, and retarder). The fluidity, bleeding, shrinkage, setting time, chemical composition, microstructure, degree of hydration, and mechanical property of grouting materials were evaluated in this study. Also, a suitable and effective CUC grout mixture was used to reinforce the roadway in the fractured surrounding rock. The results have shown that the addition of UF and US reduces the plastic viscosity of CUC, and the best fluidity can be obtained by adding 40% UF and 10% US. Since UC and UF particles are small, the pozzolanic effect of UF promotes the hydration reaction, which is conductive to the stability of CUC grouts. In addition, fine particles of UC, UF, and US can effectively fill the pores, while the volumetric expansion of AUA and gypsum decreases the pores and thus affects the microstructure of the solidified grout. The compressive test results have shown that the addition of specific amounts of UF and US can ameliorate the mechanical properties of CUC grouts. Finally, the CUC_22-8 grout was used to reinforce the No. 20322 belt roadway. The results of numerical simulation and field monitoring have indicated that grouting can efficaciously reinforce the surrounding rock of the roadway. In this research, high-performance CUC grouts were developed for surrounding rock reinforcement of underground engineering by utilizing UC and some additives.

Frictional stick–slip instability along pre-existing faults has been accepted as the main mechanism of earthquakes for about 60 years, since it is believed that fracture of intact rocks cannot reflect such features inherent in earthquakes as low shear stresses activating instability, low stress drop, repetitive dynamic instability, and connection with pre-existing faults. This paper demonstrates that all these features can be induced by a recently discovered shear rupture mechanism (fan-hinged), which creates dynamic ruptures in intact rocks under stress conditions corresponding to seismogenic depths. The key element of this mechanism is the fan-shaped structure of the head of extreme ruptures, which is formed as a result of an intense tensile cracking process, with the creation of inter-crack slabs that act as hinges between the shearing rupture faces. The preference of the fan mechanism over the stick–slip mechanism is clear due to the extraordinary properties of the fan structure, which include the ability to generate new faults in intact dry rocks even at shear stresses that are an order of magnitude lower than the frictional strength; to provide shear resistance close to zero and abnormally large energy release; to cause a low stress drop; to use a new physics of energy supply to the rupture tip, providing supersonic rupture velocity; and to provide a previously unknown interrelation between earthquakes and volcanoes. All these properties make the fan mechanism the most dangerous rupture mechanism at the seismogenic depths of the earth's crust, generating the vast majority of earthquakes. The detailed analysis of the fan mechanism is presented in the companion paper “New physics of supersonic ruptures” published in DUSE. Further study of this subject is a major challenge for deep underground science, earthquake and fracture mechanics, volcanoes, physics, and tribology.

The cone penetration test (CPT) contributes to the design and analysis of piles regarding geometry, installation effect, and pile capacity (shaft and toe resistance). MiniCone, as an alternative to CPT sounding, has been used to carry out field and laboratory investigations by physical modeling. More tests can be practically carried out through light equipment and small soil mass, involving fewer errors caused by boundary conditions. Furthermore, it can be used for in situ testing, such as quality control, assessment of ground improvement, and subgrade characterization. A database comprising MiniCone and CPT records in field and physical modeling is proposed with a variety of cone diameters. The case study records in the database have been obtained from 140 tests compiled from data from 26 sources. The sources include the results of 20 physical modelings and field data from six sites in 10 countries. The data comprise MiniCone and CPT cone tip resistance (), and sleeve friction (). The different cones are used in sandy, silty sand, and clayey soils via simple chambers (1 g), calibration chambers, and frustum confining vessels. In addition, correlations were found in penetration records in terms of physical modeling types, cone diameters, penetration rates, and soil densities. Moreover, and are related to capacities of pile toes and shafts using proper correlation coefficients less than unity, respectively. Correlations and dominant factors in geotechnical practice between MiniCone, CPT, and pile have been reviewed and discussed.

During shield tunneling in highly abrasive formations such as sand–pebble strata, nonuniform wear of shield cutters is inevitable due to the different cutting distances. Frequent downtimes and cutter replacements have become major obstacles to long-distance shield driving in sand–pebble strata. Based on the cutter wear characteristics in sand–pebble strata in Beijing, a design methodology for the cutterhead and cutters was established in this study to achieve uniform wear of all cutters by the principle of frictional wear. The applicability of the design method was verified through three-dimensional simulations using the engineering discrete element method. The results show that uniform wear of all cutters on the cutterhead could be achieved by installing different numbers of cutters on each trajectory radius and designing a curved spoke with a certain arch height according to the shield diameter. Under the uniform wear scheme, the cutter wear coefficient is greatly reduced, and the largest shield driving distance is increased by approximately 47% over the engineering scheme. The research results indicate that the problem of nonuniform cutter wear in shield excavation could be overcome, thereby providing guiding significance for theoretical innovation and construction of long-distance shield excavation in highly abrasive strata.

To understand the mechanical response pattern of the existing structure and ground due to the construction of metro tunnels underneath, the finite difference method is adopted to study the torsional deformation and stress variation of the existing structure and the effect of underground carriageway structures on the surface subsidence. The curves of the maximum differential subsidence, torsion angle, and distortion of the cross-section of the existing structure show two peaks in succession during traversing of two metro tunnels beneath it. The torsion angle of the existing structure changes when the two tunnels traverse beneath it in opposite directions. The first traversing of the shield tunnel mainly induces the magnitude variation in torsional deformation of the existing structure, but the second traversing of the subsurface tunnel may cause a dynamic change in the magnitude and form of torsional deformation in the existing structure. The shielding effect can reduce the surface subsidence caused by metro tunnel excavation to a certain extent, and the development trend of subsidence becomes slower as the excavation continues.

Biochar is a carbon sink material with the potential to improve water retention in various soils. However, for the long-term maintenance of green infrastructure, there is an additional need to regulate the water contents in the covers to maintain vegetation growth in semiarid conditions. In this study, biochar-amended soil was combined with subsurface drip irrigation, and the water preservation characteristics of this treatment were investigated through a series of one-dimensional soil column tests. To ascertain the best treatment method specific to semiarid climatic conditions, the test soil was amended with 0%, 1%, 3%, and 5% biochar. Automatic irrigation devices equipped with soil moisture sensors were used to control the subsurface water content with the aim of enhancing vegetation growth. Each soil column test lasted 150 h, during which the volumetric water contents and soil suction data were recorded. The experimental results reveal that the soil specimen amended with 3% biochar is the most water-saving regardless of the time cost. Soil with a higher biochar content (e.g., 5%) consumes a more significant amount of water due to the enhancement of the water-holding capacity. Based on the experimental results, it can be concluded that the appropriate ratio can be determined within 1%–3%, which can reduce not only the amount of irrigated/used water but also the time cost. Such technology can be explored for water content regulation in green infrastructure and the development of barriers for protecting the environment around deep underground waste containment.

Until recently, it is believed that the rupture speed above the pressure wave is impossible since spontaneously propagating ruptures are driven by the energy released due to the rupture motion, which is transferred through the medium to the rupture tip region at the maximum speed equal to the pressure wave speed. However, the apparent violation of classic theories has been revealed by new experimental results demonstrating supersonic shear ruptures. This paper presents a detailed analysis of the recently discovered shear rupture mechanism (fan hinged), which suggests a new physics of energy supply to the tip of supersonic ruptures. The key element of this mechanism is the fan-shaped structure of the head of extreme ruptures, which is formed as a result of an intense tensile cracking process with the creation of intercrack slabs that act as hinges between the shearing rupture faces. The fan structure is featured with the following extraordinary properties: extremely low friction approaching zero; amplification of shear stresses above the material strength at low applied shear stresses; creation of a self-disbalancing stress state causing a spontaneous rupture growth; abnormally high energy release; generation of driving energy directly at the rupture tip which excludes the need to transfer energy through the medium. The fan mechanism operates in intact rocks at stress conditions corresponding to seismogenic depths and in pre-existing extremely smooth interfaces due to identical tensile cracking processes at these conditions. This is Paper 1 (of two companion papers) which discusses the fan theory and extreme ruptures in experiments on extremely smooth interfaces. Paper 2 entitled “Fan-hinged shear instead of frictional stick-slip as the main and most dangerous mechanism of natural, induced and volcanic earthquakes in the earth's crust” considers extreme ruptures in intact rocks. Further study of this subject is a major challenge for deep underground science, earthquake and fracture mechanics, physics, and tribology.

Post shut-in seismic events in enhanced geothermal systems (EGSs) occur predominantly at the outer rim of the co-injection seismic cloud. The concept of postinjection fracture and fault closure near the injection well has been proposed and validated as a mechanism for enhancing post shut-in pressure diffusion that promotes seismic hazard. This phenomenon is primarily attributed to the poro-elastic closure of fractures resulting from the reduction of wellbore pressure after injection termination. However, the thermal effects in EGSs, mainly including heat transfer and thermal stress, may not be trivial and their role in postinjection fault closure and pressure evolution needs to be explored. In this study, we performed numerical simulations to analyze the relative importance of poro-elasticity, heat transfer, and thermo-elasticity in promoting postinjection fault closure and pressure diffusion. The numerical model was first validated against analytical solutions in terms of fluid pressure diffusion and against heated flow-through experiments in terms of thermal processes. We then quantified and distinguished the contribution of each individual mechanism by comparing four different shut-in scenarios simulated under different coupled conditions. Our results highlight the importance of poro-elastic fault closure in promoting postinjection pressure buildup and seismicity, and suggest that heat transfer can further augment the fault closure-induced pressure increase and thus potentially intensify the postinjection seismic hazard, with minimal contribution from thermo-elasticity.

Nonlinear time-history analysis can be used to determine the liquefiable behaviors of the tunnel–sand–pile interaction (TSPI) model with the consideration of sand anisotropy. This study presents the nonlinear response of the TSPI model with the existence of liquefaction under seismic excitation. The analysis reveals that tunnel and pile behave as isotropic elements, while sand shows isotropic, orthotropic, and anisotropic characteristics. Three constitutive models including UBC3D-PLM (two yield surfaces associated with the hardening rule), NGI-ADP (yielding with associated plastic potential function), and a user-specified constitutive model are adopted to evaluate the isotropic, orthotropic, and anisotropic behaviors of sand. On this basis, two finite element-based codes (ETABS 18.1.1 and Plaxis 3D) are used to evaluate sand behaviors and responses. Responses of the tunnel, sand, pile, and excess pore pressure ratio are recorded in the interaction zone by varying the pile diameter, tunnel diameter, and tunnel–pile clearance. Compared with the orthotropic and isotropic conditions, lower variations of results are found in the anisotropic condition, except for the case of generation of excess pore pressure. In addition, the present reanalysis results are in agreement with previous analytical and case study results, which further indicates the effectiveness of the finite element-based numerical codes.