15 August 2019, Volume 59 Issue 8

    

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MICROBIAL GEOTECHNOLOGY
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  Oedometer test of calcareous sands solidified using the MICP mixing method

  GUO Hongxian, LI Dongrun, MA Ruinan, CHENG Xiaohui

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 593-600. https://doi.org/10.16511/j.cnki.qhdxxb.2019.21.018

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  Microbially induced carbonate precipitation (MICP) was used to treat calcareous sand to improve the compressive properties of the sand. Oedometer tests showed how the compressive properties of the calcareous sand differed from quartz sand. The tests evaluated the effects of the particle size gradation, the relative density and the reaction solution concentration on the compressive properties of the carbonate sands. The tests showed that small amounts of bacteria solution and reaction solution mixed with the calcareous sand greatly improved the compressive properties of the sand after solidification with the compression index reduced by about 0.10 on average and the e-lgp curve having 2 or 3 distinct straight segments. The particle size gradation and the relative density both affect the reinforced sample compressibility with high-concentration reaction solutions further reducing the calcareous sand compressibility.
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  Self-healing of concrete cracks by microorganisms loaded in low-alkali cementitious material

  XU Jing, WANG Xianzhi

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 601-606. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.016

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  The primary issue for self-healing of concrete cracks by microbially induced mineralization is protection of the bacteria and the key to protecting the bacteria is the carrier material selection. This paper used calcium sulphoaluminate cement with 20% silica fume as a low-alkali binder type carrier material. An effective loading method was developed by regulating the binder pH and setting time through evaluations of the bacterial spore activity in this carrier. The concrete crack self-healing was successful up to a maximum crack width of about 322 μm that could be healed after 28 days. After the self-healing, the concrete compressive strength recovered up to 84% of the original strength with the water tightness completely restored.
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  Effect of mineralized bacteria type on concrete crack self-healing capacity

  ZHANG Jiaguang, XU Shunshun, FENG Tao, ZHAO Lin, LI Zhu

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 607-613. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.009

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  Microbially induced carbonate precipitation (MICP) can provide self-healing of cracks in concrete to prolong the service life of concrete structures. However, the survival rate of the bacteria incorporated into the concrete directly affects the self-healing capability. This study used concrete with a high porosity expanded perlite as the bacteria carrier which possessed excellent crack self-diagnosis and self-healing properties. The study analyzed the effects of different types of mineralized bacteria on the concrete crack self-healing and the microstructures of the crystals inside the concrete cracks. The results show that a pure culture of Bacillus cohnii and a microbial consortia of aerobic, facultative anaerobic and anaerobic bacteria both exhibited excellent healing capacities. The crack healing rates of the concrete after curing for 28 days were 73.3% for Bacillus cohnii, 83.3% for the aerobic, 63.3% for the facultative anaerobic and 41.5% for the anaerobic bacteria. The microstructure analyses show that the crystals in the concrete cracks formed by different types of mineralized bacteria were various forms of calcium carbonate crystals. The results of the present study will be useful for further studies of the crack-healing properties of concrete based on MICP.
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  Experimental study of microbial clogging of concrete cracks in sand

  LI Meng, YOU Shiquan, KANG Shaobin, CHENG Xiaohui, GUO Hongxian

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 614-618. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.021

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  Studies on microbial clogging techniques have shown that the provision of sufficient suitable nutrients for the microorganisms in geotechnical soils can produce effective microbial clogging in relatively short periods of time. This study applied various nutrients to two sand columns each with a cracked concrete slab. The tests recorded the ambient temperature and humidity, the pH, electrical conductivity, redox potential, OD₆₀₀ and the number of different microorganisms. Blocking occurred after various nutrients had been supplied for 25 days. The flow per unit time was significantly reduced with one test slab with potato exact dropping from an average of 85 mL/h to 0.5 mL/h and the other test slab with potato exact and Fe dropping from an average of 70 mL/h to 0.3 mL/h. The tests show that microbial metabolism plays an important role in concrete crack sealing in sand.
HYDRAULIC ENGINEERING
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  Analyses on finite earth pressure and slope safety factors

  JIE Yuxin

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 619-627. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.018

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  This paper analyzes recent geotechnical engineering studies such as the finite earth pressures, slope safety factors and critical slip surfaces. However, current methods for calculating the finite earth pressure are not appropriate since they predict that the earth pressure decreases to zero as the soil mass width decreases, which violates the soil mechanics laws. The Rankine earth pressure theory can still be used to calculate the finite earth pressure. The slope safety factor is defined according to the strength reserve with no need to distinguish between the "anti-sliding force" and the "sliding force". The shear strength definition shows that the soil shear strength should be used to calculate the safety factor for a given soil slope slip surface. However, a slope cannot necessarily be adjusted to develop its "maximum anti-sliding ability". Furthermore, the critical slip surface corresponding to the minimum safety factor may not be the first surface to slide. Thus, this investigation helps discriminate the key concepts for understanding geotechnical mechanics.
  
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  CNN model for mining safety hazard data from a construction site

  LIN Peng, WEI Pengcheng, FAN Qixiang, CHEN Wenqi

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 628-634. https://doi.org/10.16511/j.cnki.qhdxxb.2019.26.008

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  A convolutional neural network model was used to mine typical safety hazard data from an infrastructure construction site for site intelligent safety control. Safety hazard data from a hydropower station during construction was analyzed to show the typical safety hazard data characteristics. The learning and mining used a convolution neural network (CNN) with the convolution layer, pooling layer, fully connected layer and training and testing processes defined here. The data mining and learning improves the safety flat-closed loop management safety for the construction site for intelligent safety management. The method then automatically identifies typical safety hazards for construction sites. The results provide guidance for automatic classification and analysis of safety hazards for a construction project.
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  Coordinated regulation of ball valves in pumped storage power stations for extreme conditions

  HUANG Wei, YANG Kailin, GUO Xinlei, MA Jiming, LI Jiazhen

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 635-644. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.013

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  For some pumped storage power stations, the maximum pressures at the spiral case inlet and the draft tube inlet cannot be limited to their allowable ranges by simply optimizing the guide vane closing law for extreme conditions. This paper presents a control strategy using coordinated regulation of the intake ball valve and the guide vane for such cases. A one-dimensional simulation is used to analyze the influence of the guide vane closing law, pump-turbine characteristics and ball valve closing law on the pump-turbine rotational speed and the pressures at the spiral case inlet and the draft tube inlet. A case study shows that this cooperative control mode has little effect on the first stage of the speed and the pressures at the ball valve inlet and the draft tube inlet, but significantly reduces the secondary peaks in the speed and ball valve inlet pressure and improves the second minimum draft tube inlet pressure. Moreover, this control mode also effectively prevents the units from entering the "S" shaped unstable area for small guide vane openings, so the system reduces the water hammer and pressure pulsations in pumped storage power stations.
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  Shape optimization of a trapezoidal canal structure for coupled temperature-water-soil conditions in cold regions

  WANG Yi, LIU Jincheng, LIU Quanhong, WANG Zhengzhong

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 645-654. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.010

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  The shape of a large trapezoidal canal in a cold region should be optimized to improve the water delivery and minimize frost damage. This study used a double objective analysis to optimize the hydraulics and the frost damage prevention for a canal using the hierarchical sequence method. A frost heave model was developed for a concrete lining canal based on thermo-hydro-mechanics frost heave theory for freezing soil to predict the canal deformation for various temperature, water, and soil conditions. The optimal hydraulic design was predicted first with the canal section parameters then used as the solution set space for the frost damage prevention optimization. The overall stiffness index of the lining was set as the optimization object with the allowable maximum normal displacement and stress in the lining set as the optimization constraints. The model was solved using the secondary hierarchical sequence method in COMSOL. The results show that this method can consider both the water delivery and the frost damage effect on the canal section. The practical case results indicate that the overall stiffness index of the lining is reduced by 30%-48% from that of the original design using this method to get an optimized canal section, which improves the adaptability to frost damage of the lining structure. This method can also be used for similar analyses involving hydraulics and frost damage effects.
CONSTRUCTION MANAGEMENT
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  Urban and industrial diffusion mechanisms in China's venture capital investments

  DUAN Liaoliao, SUN Weizeng, ZHENG Siqi

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 655-662. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.017

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  Venture capital plays an important role in advancing the development of high-tech industries for promoting industrial and economic growth. However, venture capital is not evenly distributed among various regions and industries in China, which inhibits the development of backward regions and industries. Micro-data related to China's venture capital investments are analyzed to construct an investment decision-making model to analyze the factors affecting venture capital investments. The results show that the industrial and urban experience of the venture capital, the maturity of the start-up companies, and the industrial and urban experience of the joint investors all significantly increase the investment probability. In addition, the venture capital experience and the start-up company maturity are found to strongly influence the venture capital's entry into new cities and new industries, while the joint investors' experience does not have much effect.
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  Factors and principles influencing control right allocation in public-private partnership projects

  WANG Shouqing, NIU Yunshi, WU Di, CHU Xiaoling

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 663-669. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.020

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  Reasonable allocation of control rights is a precondition for successful implementation of public-private partnerships (PPP) projects. However, the optimal allocation schemes often differ for different projects. Therefore, the factors influencing the control rights and allocation principles have important theoretical and practical meanings. This paper describes the project features reflected by different influencing factors as the importance of the three dimcnsions of various decisions in PPP projects based on the control rights, decision theory and failure mode and effects analysis (FMEA) processing. The relationships between these key factors and the decisions are verified using the factor analysis with this study identifying 15 possible factors that influence the control rights of PPP projects. Then, an OLS regression analysis and a Logistic regression analysis are used to explore how these factors impact the allocation of PPP project control rights. The results show that 12 of the 15 key factors have different effects on the control rights of governments and enterprises. Four control right allocation principles are identified that make full use of the professional advantages of enterprises, provide reasonable decentralization of the government power, ensure reasonable earnings for the enterprises and provide win-win cooperation between the government and the enterprises in PPP projects.
AEROSPACE ENGINEERING
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  Geometric design and characteristic analysis of herringbone face gears

  FENG Guangshuo, GU Yongpeng, LAN Xudong, ZHOU Ming

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 670-682. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.015

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  A herringbone face-gear drive was developed in this project. The design of the herringbone face-gear geometry used the entire tooth surface precise modeling method. A geometry shaper was modeled mathematically with a precise, three-dimensional mathematical model then developed for the herringbone face-gear pairs. The loaded tooth contact analysis (LTCA) method was used to analyze the characteristics of the herringbone face-gear drives, including the tooth contact area, the axial force on the pinion gear, the contact ratio, the transmission error, the effect of the inner and outer face widths, and the effect of the inner and outer tooth phases. A numerical study shows that the advantages of herringbone face-gear drives include no axial force on the pinion, simplified bearing support, larger transmission ratios, larger helix angles, higher bearing capacities, less vibration and noise, high-speed applications, high-load applications, lower sensitivity to the sense of rotation, and being applicable to both clockwise and counterclockwise rotations. Finally, this paper summarizes the design rules for herringbone face-gear drives.
  
MECHANICAL ENGINEERING
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  Suppression of imbalance vibrations in magnetically suspended spindles based on zero-bias current control

  RONG Hai, ZHOU Kai, MAO Feilong

  Journal of Tsinghua University(Science and Technology). 2019, 59(8): 683-688. https://doi.org/10.16511/j.cnki.qhdxxb.2018.25.059

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  Biased current control consumes large amounts of power which increases system temperatures and causes temperature drift in sensors which then affects the levitation accuracy. The rotor suspension is also affected by unbalanced forces, sensor runout and other effects which lead to vibrations. This paper presents a zero-bias current strategy based on a multiple frequency notch filter. The zero-bias current control algorithm reduces the power consumption, handles the nonlinearities in the zero-bias current strategy, and translates the nonlinear relationship between the current and the displacement into a linear relationship between the magnetic force and the displacement so that a linear control strategy can be used. The multiple frequency notch filter then suppresses the vibrations. Three controllers are analyzed in tests with no unbalanced control, only single frequency notch filter control and multiple frequency notch filter control to verify the system effectiveness. The method not only suppresses the vibrations, but also significantly reduces the power consumption.