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When earth pressure balance tunnel boring machines (EPB-TBMs) tunnelling in abrasive sandy ground, soil additives are critical for promoting excavation surface stability, reducing residual soil permeability, decreasing screw conveyor torque, avoiding mud cake siltation and preventing excessive tool wear. However, the existing studies mainly focus on the first four aspects. The influence mechanism of soil additives on cutting tool wear is still unclear due to the lack of reliable test methods. In the present study, a newly developed WHU-SAT tester (Soil Abrasion Tester developed by Wuhan University) was used to evaluate the influence of conditioning additives (foam additives and bentonite slurry) on soil abrasivity. Scraper wear in additive-conditioned sand has been compared with that in water-conditioned sand to reveal the mechanism of foam additive and bentonite slurry. The soil additive parameters (solution concentration and injection ratio) were optimized to reduce scraper wear in abrasive sandy ground. The results indicate that the mitigation effect of the foam additive on scraper wear is related to the lubrication of the active agent and the cushioning of the air bubbles. The optimized foam solution concentration and foam injection ratio for the quartz sand samples are FSC = [5%, 10%] and FIR = [22.5%, 25.0%], respectively. The aggravation in scraper wear caused by bentonite slurry is related to the improvement of particle gradation and the increase of sand cohesion. The bentonite slurry parameters should be selected to keep the water content of the conditioned sand away from the peak water content to decrease scraper wear. [ABSTRACT FROM AUTHOR]

Two main methods are commonly used in tunnel excavation, drilling and blasting (DB) and tunnel boring machine (TBM) tunnelling method. To better understand the differences and predict rockbursts, the characteristics of microseismic (MS) events in two parallel tunnels excavated by DB and TBM methods were compared, including temporal distribution, spatial distribution, and intensity distribution. The results indicated that the characteristics of MS events induced by two excavation methods had some differences. In the DB excavated tunnel, the number of MS events decreased exponentially as the time to a blasting cycle increased. The MS events predominantly occurred within 15 min to a blasting cycle. On the other hand, in the TBM excavated tunnel, the MS events primarily occurred while the TBM was working. Similarly, when a tunnelling cycle was completed, there was an exponential decrease in the number of MS events, which mainly occurred within 15 min. The distribution of MS events in relation to the tunnelling face exhibited similarities for both excavation methods, presenting a normal distribution. The range of the TBM excavation (approximately 2.3 times of tunnel diameter) was generally larger compared to the DB excavation (approximately a time of tunnel height). In terms of energy levels, both the DB and TBM excavations predominantly produced low-level energy MS events (less than 30 kJ). For moderate and high-level energy events, the percentage of TBM excavation induced events was higher than that of DB excavation induced. In the TBM excavation, surrounding rocks maintain good integrity and high bearing capacity, and a large tangential stress gradient is near the tunnelling face. It is more likely to induce a large rupture scale of seismic sources and lead to a more prolonged and widespread active period. On the other hand, the fracturing zone induced by the DB excavation reduces bearing capacity of surrounding rocks, and strain energy within surrounding rocks is released to a certain extent. The tangential stress gradient is relatively small, so it is associated with more localized and short-lived seismic activity in the DB excavated tunnel. [ABSTRACT FROM AUTHOR]

The mechanical properties of rock, especially the uniaxial compressive strength (UCS), the elastic modulus (E), and the Brazilian tensile strength (BTS) are critically important for rock engineering. Since standard laboratory tests have some limitations, various indirect methods have been developed for estimating these mechanical properties, e.g., the indentation test. In the indentation test, an indenter is forced into the rock to obtain the load-displacement curve. Most estimation models of rock mechanical properties by indentation test are appropriate for fine-grained rock or soft rock, and there is little estimation model for coarse-grained rock and hard rock. Therefore, it is important to establish an estimation model appropriate for rock with large grain sizes and high strength. Since the specimen size may affect the test result, specimens with different sizes were tested first to study the specimen size effect. The result showed that both the specimen diameter and height affect the specimen failure pattern, the load-displacement curve, and the indentation indices. The proper specimen size is suggested using in the indentation test with a 7-mm diameter indenter. Then, 8 types of rock were prepared for indentation tests, uniaxial compression tests, and Brazilian tests. The UCS, BTS, and E all increase with indentation indices, e.g., the energy per unit (W) and the indentation modulus of the first peak (IM1). There are linear correlations between the UCS, E, BTS, and indentation indices (W and IM1) with high determination coefficients. [ABSTRACT FROM AUTHOR]

With the rapid development of urbanization and industrialization, electric power consumption continues to increase on a worldwide basis. Traditional overhead transmission lines cannot satisfy the requirements for modernization of power distribution systems. Cable tunnels with ultra-high-voltage and complex network have become the trend of urban power transmission lines for their advantages in flexibility, expandability, and convenient maintenance. Although cable tunnels have good adaptability in power transmission, their application is severely restricted by construction and inspection technologies. Especially for long-distance cross-river tunnels installed with 1000 kV equipment, there is still a lack of reliable solutions to deal with complex challenges encountered during the construction and inspection of ultra-high-voltage cable tunnels. In the present study, complexity features of the first underwater gas-insulated transmission line for 1000 kV power in the world, Sutong GIL Yangtze River Crossing Cable Tunnel, have been analyzed. Engineering challenges encountered during the construction and inspection of long-distance cable tunnel have been discussed. Key technologies including embankment deformation control, gas explosion prevention, synchronous transportation organization, dust-free equipment installation, and robot intelligent inspection have been developed. The application of these technologies has enabled the safe and efficient construction and inspection of a high-quality cross-river cable tunnel. [ABSTRACT FROM AUTHOR]

The estimation of soil abrasivity and cutting tool wear during soft ground tunneling is complicated and arduous work due to the lack of a generally accepted testing system. To date, geotechnical baseline reports (GBRs) and geotechnical data reports (GDRs) still cannot provide reliable soil abrasion indices for tool life prediction. This is considered a deficiency in the geotechnical investigation of shield-driven tunnel projects. In this study, the Multifunctional Shield Test System (MSTS) developed by the Key Laboratory of Safety for Geotechnical and Structural Engineering of Hubei Province at Wuhan University is presented. The MSTS was motivated by the need for a test platform with the ability to simulate undisturbed soil conditions and actual tool working environments to explore the mechanism of cutting tool wear, bentonite slurry penetration, and filter cake formation in various geomaterials with grain sizes including clay, silt, sand, and gravel (< 15 mm). Experimental studies on the effect of alloy hardness on cutting tool wear are conducted. The soil samples used in the tests are dense fine silty sand from the Sutong GIL Yangtze River Crossing Cable Tunnel. When the wear extent is plotted against the alloy hardness, an inverted S-shaped band bounded by the highest and lowest alloy hardnesses is formed. The sensitive hardness interval is HRC = [47, 52]. If the alloy hardness is higher than 52 HRC, the wear extent is relatively small. Since alloy hardness is negatively correlated with bending strength, using a cutting tool material with a hardness slightly higher than the upper limit of the sensitive hardness interval is optimal. Thus, an alloy with a hardness interval of HRC = [52, 63] is recommended for the dense fine silty sand ground at the Sutong GIL Yangtze River Crossing Cable Tunnel. [ABSTRACT FROM AUTHOR]

Rock strength is one of the most important parameters in geotechnical engineering. During excavation using a tunnel boring machine (TBM), a rapid and effective method to estimate rock strength is needed. The TBM muck produced during excavation contains blocky fragments which are suitable for a point load test (PLT). To use the PLT on blocky fragments to estimate rock strength, it is necessary to find the relation between the point load strength (PLS) of blocky fragments and uniaxial compressive strength (UCS) of surrounding rocks. In the present study, PLT was performed on both blocky fragments and core samples of limestone for the Yinsong Water Conveyance Project, China. The UCS of limestone was acquired. The PLS of blocky fragments and core samples was compared. The results show that the blocky fragments' PLS is lower than that of core samples; the declining ratio depends on the fractured degree of sampling sites; blocky fragments with width and thickness in a certain range are suitable for estimating rock strength; and when the size of the blocky fragment exceeds a certain range, it means the blocky fragment was not generated from the tunnel face. The limestone core samples simultaneously drilled from side wall have an average conversion factor from PLS to UCS of 24.61. The mean conversion factor from PLS of blocky fragments to UCS of limestone in the present study is 40.38. [ABSTRACT FROM AUTHOR]

To investigate the size effects on the cracking behavior of flaw-containing specimens under compressive loading, rectangular parallelepiped specimens containing a single centrally located flaw are numerically loaded using the bonded-particle model (BPM). Effects of the ratio of flaw size (length) to specimen size on cracking behavior are carefully studied. The numerical results show that increasing the ratio of flaw size to specimen size within a reasonable range (≤0.33) has no obvious effects on the first crack initiation stress, but decreases the uniaxial compressive strength. In specimens of a high flaw size to specimen size ratio, the newly generated cracks have longer extension length, which allows for easier crack identification as compared with that of the low ratio specimens. These phenomena are also observed in the physical test. Based on the present study, recommendations are given on choosing appropriate specimen widths in the BPM simulation. The failures of flaw-containing specimens as well as the outward bending effect of narrow specimens are also discussed. [ABSTRACT FROM AUTHOR]

Bonded-particle model (BPM) is widely used to model the cracking processes in rock/rock-like materials. However, the discrete particles in the BPM cannot produce a continuous displacement field, which is comparable with those in physical experiments and numerical method such as the finite element method. Displacement trend lines are introduced to analyze the displacement field type of the BPM in the present study. Three displacement field types (DF_I, DF_II and DF_III) are defined and used in the analysis of the cracking processes. The study shows that the type of displacement field may evolve during the cracking processes. Researchers are advised to examine the displacement field before and right after a particular crack has formed. An examination of the associated type of displacement field can help reveal the nature of the crack. [ABSTRACT FROM AUTHOR]