交通运输工程学报

Cover and Contents of Vol.23, No.4, 2023

Cover Contents

Review on surface protection technologies of loess slope

YAN Chang-gen, LIANG Zhe-rui, JIA Zhuo-long, LAN Heng-xing, SHI SHI Yu-ling, YANG Wan-li

Abstract: To address the serious water and soil erosion on exposed loess slopes, some significant research achievements on the surface protection technologies of loess slope were concluded. The development process of slope engineering protection technologies was outlined. The research directions and shortcomings in slope vegetation protection technologies were discussed. Slope protection performances and effects, such as shear strength and water stability of solidified loess, were analyzed. The future trend of surface protection technologies of loess slope was prospected. Research results show that slope engineering protection technologies include masonry protection, skeleton structure protection, geocell flexible protection, etc. The development process can be described as the gradual refinement of masonry operations, including the emergence of new structures, materials, and technologies on subdivided grids. Slope vegetation protection technologies generally conducts research and application from natural slope protection effect of vegetation, vegetation measures, and plant selection and matching. But it neglects the strong water sensitivity and barren soil fertility of loess, which leads to poor subsequent growth of slope vegetation. Therefore, slope protection cannot rely solely on vegetation. Slope protection performance and effect of loess are different under different curing material treatment conditions. Especially, biopolymers and fiber can play a positive synergistic role in maintaining long-term stability and ecology. Biopolymer-fiber-reinforced loess thus shows excellent anti-erosion effect and soil-water retention ability, which has great application potential and prospect. It is clear that the integrated ecological slope protection technology integrating engineering structure, vegetation, and solidified loess is a foreseeable trend in slope protection engineering. In addition, five areas are taken as the key research directions, including anti-scouring technology of solidified loess, restoration technology of ecological material, safe ecological monitoring system of slope protection, aging evaluation method of slope protection, and landscape design of plant matching.More>

2023, 23(4): 1-22. doi: 10.19818/j.cnki.1671-1637.2023.04.001

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Review on digital twin technology for highway construction and maintenance equipment

HUI Ji-zhuang, ZHANG Ze-yu, YE Min, GU Hai-rong, ZHANG Hao-bo, DUAN Yu

Abstract: To promote the development and application of digital twin technology for highway construction and maintenance equipment, starting from the three levels of components, systems, and equipment, the concept and composition of digital twin highway construction and maintenance equipment were discussed, and the digital twin hierarchical architecture of mechanism-data heterogeneous fusion was proposed. The current status and latest progress of multi-energy coupling of the hydraulic torque converter and rolling bearing fault diagnosis and life prediction of core components were investigated. The fusion mechanism-data heterogeneous digital twin architecture for highway construction and maintenance equipment at the component level was summarized. Based on the equipment power flow transfer and big data analysis, the digital twin technology at the system level was grouped into a digital co-motion system and a backend data management system. The power matching of the transmission system, the degradation of the hydraulic system performance, and the working condition sensing of the data management system were elaborated. The current technical applications and deficiencies were analyzed. Around the basic concept of digital twin equipment, the connotation and characteristics of digital twin technology for equipment-level highway construction and maintenance equipment were explained. The typical practical scenarios of digital twin technology for current highway construction and maintenance equipment in the division of working conditions, efficiency optimization, and quality control of operations were introduced. The challenges and key technologies faced by the digital twin highway construction and maintenance equipment were discussed and prospected. Research results show that the relevant research on highway construction and maintenance equipment currently focuses on the simulation analysis and experimental test verification under typical working conditions. There are problems of insufficient research on the mechanism and algorithm of multi-coupled fields under complex working conditions and simple technical working conditions. Furthermore, the problems of insufficient research on the coupling of external parameters and internal multi-physical fields, as well as the major differences between the experimental results and actual equipment use are present. The future direction should start from three aspects, namely the establishment of intelligent monitoring and remote control of mixed equipment group states, research based on different scenarios under the state analysis and data processing algorithm decision optimization and guidance, and building of effective "human-machine-ring" interaction mechanism under complex environments, so as to implement the digital twin technology for highway construction and maintenance equipment.More>

2023, 23(4): 23-44. doi: 10.19818/j.cnki.1671-1637.2023.04.002

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Theory and analysis method of lower-bound dynamic shakedown for design of flexible pavement structure

QIAN Jian-gu, DAI Yu-chen

Abstract: To study the mechanical response and service performance of flexible pavement subgrades under long-term traffic loads, the current research state, existing issues, and frontier of the shakedown theorem in the design process of flexible pavement subgrades were reviewed. The basic principles of classical upper- and lower-bound dynamic shakedown theorem and their application and development in transportation geotechnics were introduced. The critical criterion and numerical analysis method of lower-bound shakedown were discussed in detail. Based on the case studied by the dynamic finite elements within artificial boundaries, the dynamic responses of pavement-subgrade system under the traffic moving loads were revealed. The effects of cross-anisotropic materials and wheel-pavement friction on the dynamic shakedown of the road structure were discussed. Research results show that the dynamic responses of the road structure under the traffic load are significant for the shakedown limit. The lower-bound shakedown limit level reduces with the growing traffic moving speed and reaches the minimum value when the moving speed approaches around the Rayleigh wave velocity of the structure system. The material mechanical properties of subgrade, the degree of anisotropy, as well as the wheel-pavement friction coefficient may also produce considerable effects on the lower-bound dynamic shakedown limit of the flexible road structure. The lower-bound dynamic shakedown limit of the road structure system increases at first and then decreases with the increase in the elastic modulus ratio of the upper and lower layers of the structure. The optimal modulus ratio corresponding to the maximum shakedown limit indicates that the critical position of the shakedown limit changes from the lower subgrade to the upper pavement. When the horizontal friction is considered, the increase in the wheel-pavement friction coefficient will obviously reduce the dynamic shakedown limit of the structure and weaken the influence of moving load speed on the dynamic response of the road structure.More>

2023, 23(4): 45-59. doi: 10.19818/j.cnki.1671-1637.2023.04.003

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Characteristics of subgrade temperature field of gravel road in high altitude permafrost region

BAO Wei-xing, LIU Ya-lun, MAO Xue-song, LI Wei, QIN Chuan, GUO Qiang, CHEN Rui

Abstract: In order to study the effect of highway construction on the thermal state of permafrost layer in high altitude area, the investigation on the road diseases in the permafrost regions along the Xinjiang-Xizang Highway was conducted, a temperature monitored cross-section of the ground and meteorological monitoring sites were built in the area with an altitude of 5 400 m, and the air temperature, ground temperature and radiation intensity were monitored. According to the monitoring result, the heat flux at the upper limit of the frozen soil was calculated, and the changing characteristics of ground temperature in permafrost layer were analyzed. Based on the heat conduction theory and heat diffusion theory, a ground temperature-depth theoretical prediction model of the permafrost under natural subgrade and normal subgrade was proposed. Research results show that the road diseases in the permafrost region are mainly caused by the large amount of heat absorption of asphalt pavement. Although the active and passive protection measures such as hot rods and thermal insulation layers have some positive effects, they cannot change the rapid degradation of the permafrost. The largest temperature difference between the natural foundation and the subgrade center in the study area is up to 19.66 ℃ in a day, and the temperature difference between the left and right shoulders is up to 4.94 ℃ in a day. The temperature of the deep permafrost under the natural foundation maintains at about -6.0 ℃, and the temperature of the deep permafrost in the lower part of the subgrade center maintains at about -5.6 ℃. The temperature of the lower part of the subgrade is more drastic than that of the natural foundation, and the temperature of the subgrade of the isothermal layer is higher. The radiation intensity in the study area increases significantly at 10:00-18:00 in a day, the peak radiation intensity is between March and June in a year, and the shallow ground temperature is mainly affected by the annual cycle variation of radiation intensity. The annual heat fluxes of the permafrost layer in the lower part of the natural foundation, subgrade center, shady slope shoulder, and sunny slope shoulder are -4 001, -14 649, -4 487 and 58 303 kJ·m^-2, respectively. The heat dissipation rate of the subgrade center is greater than that of the natural foundation, and a large amount of heat is absorbed at the shoulder of the sunny slope road. The isotherm of the natural foundation appears at a depth of 9.79 m, while the isotherm of the subgrade center appears at a depth of 9.61 m, indicating the isotherm of the subgrade center is shallower. The compaction of the subgrade soil makes the temperature change of the shallow part of the permafrost more obvious, which has a positive effect on the heat dissipation of the lower permafrost in the short term. Under the sunny-shady slopes effect, the increase of the permafrost temperature in the lower part of the slope reduces the thermal stability of the subgrade and results in uneven settlement.More>

2023, 23(4): 60-74. doi: 10.19818/j.cnki.1671-1637.2023.04.004

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Influences of water level rise on dynamic responses and long-term settlement of high-speed railway subgrade

HU Jing, TANG Yue, ZHANG Jia-kang, JIANG Hong-guang, BIAN Xue-cheng, DENG Tao

Abstract: Based on the Biot theory, a 2.5-dimensional finite element analysis model of the track-subgrade-multilayered saturated soil foundation coupling system was established, and a calculation method for the cumulative settlement of the subgrade considering the actual train cyclic load was proposed. The influences of water level rise, train speed, and axle load on subgrade dynamic response and long-term settlement were discussed. Research results show that the amplification effect of water level rise on the soil vibration intensity is not limited to the depth of water level change, but will lead to the increase in the vibration of the entire subgrade and foundation section. This full-section vibration amplification effect increases with train speed. When the water level rises to the inside of the subgrade, significant excess pore pressure will generate inside the subgrade, and the maximum value can reach 27.52 kPa, resulting in a large drop in the effective stress. Then, the stress path of the soil element in the subgrade will approach the failure line. When the water level only rises within the foundation, the cumulative deformation of the subgrade under the train cyclic load is small, and the railway settlement mainly comes from the foundation. When the water level rises to the inside of the subgrade, the cumulative deformation of the subgrade develops rapidly with the loading cycles, which is 19.54 mm after one million times of loading and exceeds the allowable value largely, indicating that the subgrade waterproofing plays a key role in the long-term cumulative settlement control of the railway line. The train speed and axle load affect the cumulative deformation of the subgrade and foundation, and the increase significantly with the axle load of the train. The increase in the axle load has a stronger influence on the cumulative deformation of the subgrade than the foundation. Thus, the effect of axle load on the cumulative deformation should be well considered in the design.More>

2023, 23(4): 75-91. doi: 10.19818/j.cnki.1671-1637.2023.04.005

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Bearing capability of collapsible loess subgrade through cement-fly ash treatment

DENG You-sheng, LI Long, SUN Ya-ni, YAO Zhi-gang, MENG Li-qing

Abstract: To investigate the reinforcement mechanism of cement-fly ash post-grouting on pile-net composite subgrade in collapsible loess areas, the static load tests in laboratory were carried out on the grouted cement-fly ash gravel (CFG) piles, the influence of post-grouting on the collapsibility coefficient of the soil samples around the piles was analyzed, and the changing rules of additional stress, pile side friction resistance and pile tip resistance in the depth direction of post-grouting pile-net composite subgrade under vertical static load were studied. Based on the Boltzmann mathematical model and load transfer function, the reinforcement mechanism of pile side friction resistance and pile tip resistance was investigated, and their calculation formulas after grouting were given. The influence mechanisms of elastic modulus of pile, post-grouting depth, pile-net replacement rate, and cushion layer thickness on the bearing capacity of pile-net composite subgrade were discussed by the numerical simulation method. Research results indicate that under the same static load, the collapsibility coefficient of the cement-fly ash post-grouting soil around the pile is less than that of the natural soil sample and less than 0.015. After post-grouting, the vertical additional stress of the pile top in the pile-net composite subgrade gradually decreases under the static load, the vertical additional stress of the soil between the piles decreases first and then increases, and the pile side friction resistance increases by about 1.54 times compared with the un-grouting pile. With the increase in post-grouting depth, the maximum stress in the depth direction of pile body increases first and then decreases, and the maximum stress is obtained at the depth equal to pile length. When the pile-net replacement rate is doubled, the stress and settlement decrease in the depth direction, among which the peak stress decreases by 24% and settlement decreases by 26%. With the increase in cushion layer thickness in the pile-net composite subgrade, the stress in the depth direction of the subgrade gradually increases. Therefore, the cement-fly ash treatment of collapsible loess subgrade can weaken the collapsibility of subgrade soil and improve the bearing capacity. In the construction process, the effects of elastic modulus of pile, post-grouting depth, pile-net replacement rate and cushion thickness on the bearing capacity of the subgrade should be considered.More>

2023, 23(4): 92-103. doi: 10.19818/j.cnki.1671-1637.2023.04.006

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Long-term stability analysis of loess cutting shallow slope based on wet-dry cycle test

SHI Yu-ling, CHANG Zhou, AN Ning, YAN Chang-gen, LAN Heng-xing, YANG Wang-li

Abstract: To evaluate the shallow soil strength deterioration effect of loess slope under wet-dry cycle, the laboratory direct shear tests were carried out under different wet-dry cycle paths on Q₃ undisturbed loess from Dingxi, Gansu Province. The effects of cycling times, cycling amplitude and lower bound water content on the shear strength of the soil were analyzed. A strength degradation model considering three parameters of wet-dry cycle was established, and the long-term stabilities of loess cutting shallow slope under different wet-dry cycle paths were compared by the strength reduction method. Test results show that the cohesion of undisturbed loess first decreases and then tends to be stable with the increase of wet-dry cycles, which can be fitted by the hyperbolic function. The internal friction angle decreases linearly. After 10 cycles, the maximum deterioration degrees of cohesion and internal friction angle are 27.64% and 9.88%, respectively. Under the same wet-dry cycles, the degradation effects of the cycling amplitude on the cohesion and internal friction angle of undisturbed loess are greater than that of the lower bound water content. The long-term stability coefficient of loess cutting shallow slope follows an exponentially decreasing function during the wet-dry cycle. The maximum reduction of slope stability coefficient under different wet-dry cycle paths is 61.5%, and the reduction of stability coefficient accounts for 85% of the total reduction after 6 cycles. The stability of loess cutting shallow slope is affected by the cycling amplitude and lower bound water content in the wet-dry cycle, which shows that the slope stability increases first and then tends to be stable with the increase of lower bound water content. However, with the increase of cycling amplitude, the stability coefficient decreases linearly. In engineering practice, the water content varies with the depth in the slope, and the wet-dry cycle paths are different, so the layering effect of wet-dry cycle should be considered in long-term stability analysis of loess cutting slope.More>

2023, 23(4): 104-115. doi: 10.19818/j.cnki.1671-1637.2023.04.007

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Water-salt migration rules in chlorine saline soil under precipitation infiltration

ZHANG Liu-jun, QIU You-qiang, ZHANG Fa-ru, LI Xiong-fei, LIU Jun-yong

Abstract: To explore the salt loss of chlorine salt soil caused by precipitation infiltration, the artificially configured coarse-grained soils and fine-grained soils with different salt (sodium chloride) contents were taken as the research objects, nearly 500 groups of test data were obtained under 14 conditions by the self-designed simulation test device for laboratory precipitation infiltration, and the effects of water infiltration times, soil particle size, and salt content on the water-salt migration characteristics of soil samples were compared and analyzed. The relationship between salt migration and water migration under the action of precipitation infiltration was established, the depth of infiltration effect was determined, and the distribution characteristics of water and salt in the soil column after precipitation infiltration were revealed. Research results show that for fine-grained saline soil, with the increase in precipitation infiltration times (1-4), the peak points of moisture content and salt content move significantly downward, and the salt will gradually accumulate to the middle and bottom of the soil column. As for coarse-grained saline soil, the moisture content is relatively uniform within the height range of the soil column after the second precipitation infiltration, and the continuous increase of precipitation infiltration times does not change this uniformity, while the salt will quickly accumulate to the bottom of the soil column with the moisture. The "salt along with water" relationship of chlorine saline soil is related to its easy dissolution in water, so the subgrade filled with chlorine saline soil should strengthen waterproof measures, especially coarse-grained soil filler. Although coarse-grained soil has better compactability than fine-grained soil, it is strong of collapsibility after soaking water, and the disease is even more serious. The increase in salt content within a certain range does not change the overall law of water-salt migration in fine-grained or coarse-grained soil but will reduce the migration rates of water and salt. Compared with the fine-grained soil with low salt content, the depths of peak points of water and salt of fine-grained soil with higher salt content will lag behind 5-10 cm. Under the same condition of precipitation infiltration, the relative increment in salt migration of fine-grained soil is different for different salt contents, but the absolute increment is basically the same, indicating that the salt carried by water in fine-grained soils with different salt contents is certain.More>

2023, 23(4): 116-127. doi: 10.19818/j.cnki.1671-1637.2023.04.008

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Soil-water characteristic curve of geopolymer stabilized aeolian sand with clay

CHEN Rui, CHEN Hai, BAO Wei-xing, LAI Hong-peng

Abstract: In order to study the water retention capacity of geopolymer stabilized aeolian sand with clay, the geopolymer was used to stabilize the aeolian sand with clay. Based on the pressure plate instrument method, the volume water contents under different matric suctions were measured, and the soil-water characteristic curves (SWCC) of stabilized aeolian sand were drawn and fitted. The effects of clay content, geopolymer content, curing time, and fiber length on the water retention capacity and fitting parameters of stabilized aeolian sand were studied. Research results show that with the increase in the clay content from 20% to 30%, the volume water content of aeolian sand sample under the same suction increases by about 3%, and the water retention capacity increases significantly. With the increase in the geopolymer content from 8% to 12%, the SWCCs of stabilized aeolian sand with 20% and 30% clay show an overall upward-moving trend, and the corresponding volume water contents with 10 kPa suction increase by 7.0% and 5.9%, and the corresponding volume water contents with 600 kPa suction increase by 4.3% and 4.2%, respectively. However, the extension of curing time has little effect on the improvement of the water retention capacity of stabilized aeolian sand, and the change range of volume water content is less than 1%. Basalt fiber with a length of 6 mm can barely improve the water retention capacity of stabilized aeolian sand. But increasing the fiber length to 12 mm slightly decreases the water retention capacity of stabilized aeolian sand, and the change ranges of both are less than 1%. The SWCC of stabilized aeolian sand with clay has two steep drop sections, which can be fitted by the multi-segment Van Genuchten model. The bimodal fitting parameters can reflect the distributions of large and small pores of the samples. Clay can only affect the parameters of large pores, while the geopolymer can affect the parameters of large and small pores. The scanning electron microscope and the mercury intrusion porosimetry test results show that the filling effects of geopolymer gel and clay reduce the porosity of soil samples. The overall pore volume in the stabilized aeolian sand varies slightly after the geopolymer is added. However, parts of the large pores change into small pores, the peak density of large pores decreases from 0.45 mL·g^-1 to 0.22 mL·g^-1, the peak density of micro pores increases from 0.02 mL·g^-1 to 0.05 mL·g^-1, and the average pore size decreases. The cementation of geopolymer leads to the formation of aggregates in the soil, which encapsulates some of the free water and prevents the water loss, improving the overall water retention capacity of stabilized aeolian sand.More>

2023, 23(4): 128-141. doi: 10.19818/j.cnki.1671-1637.2023.04.009

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Compression characteristics of filled jointed rock under dry-wet cycles

CHAI Shao-bo, SONG Lang, LIU Huan, ABI Erdi, LIU Shuai

Abstract: In order to explore the effects of dry-wet cycles on the compressive mechanical strength and deformation and failure characteristics of filled jointed rock, the artificial jointed rock samples with various fillings were prepared and then pre-treated by 0 (drying state), 1, 5, 10, 15, and 20 dry-wet cycles, which resulted in certain cumulative damage. On this basis, static uniaxial compression tests and dynamic impact tests were carried out on filled jointed rock samples, and in the process of dynamic impact tests, the impact failure characteristics of filled jointed rock were observed with the help of a high-speed camera. Research results show that with the increase in the number of dry-wet cycles, the static and dynamic compressive strengths of the filled jointed rock continue to decrease, and the decrease rates gradually becomes smaller. After 20 dry-wet cycles, the total deterioration degree of the static and dynamic compressive strength of the rock samples is between 20% and 30%. The dynamic compressive strength reduction law of rock samples conforms to the exponential function distribution through fitting. With the increase in the number of dry-wet cycles, the static failure mode gradually develops from splitting failure to shearing failure. In the process of dynamic impact, the degree of fragmentation of filled jointed rock and the degree of smashing and spattering of the filled joint group are aggravated, verifying that the dry-wet cycles will seriously affect the deformation and failure characteristics and dynamic impact resistance of filled jointed rock. In addition, the stress wave transmission coefficient of the rock sample decreases continuously with the increase in the number of dry-wet cycles, and after 20 dry-wet cycles, the coefficient decreases by about 10%. It can be seen that the dry-wet cycle has a significant impact on the stress wave propagation ability.More>

2023, 23(4): 142-153. doi: 10.19818/j.cnki.1671-1637.2023.04.010

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Design of borehole in-situ shear test system

LIU Xin, LAN Heng-xing, YAN Chang-gen, DONG Zhong-hong, BAO Han, WU Yu-ming, ZHANG Fan-yu, ZHU Yan-bo, ZHANG Jun

Abstract: In order to solve the problems of shallow detection depth and over-reliance on empirical formulas in the traditional in-situ soil strength test, a borehole in-situ shear test system was proposed, and a prototype was developed. The whole test system consisted of an in-hole cutting subsystem, which could anchor and cut the soil ring at any position in the borehole, and an in-hole shear subsystem with a reducing wing, which could perform shear tests on the soil ring and record shear force and shear displacement through sensors. Based on the borehole shear test in the model box and the direct shear test in the laboratory, the reliability of the borehole in-situ shear test system was verified. Research results show that the shear plane of the borehole shear test in the model box is parallel to the direction of the shear force, and the stress-strain curve is found to be consistent with the expectation. The strain softening behavior occurs for loess in the borehole shear test in the model box and direct shear test in the laboratory, and the peak shear strength of the loess decreases with the increase in water content. The brittle failure is found in the direct shear test in the laboratory, while in the borehole shear test, the loess shows plastic failure. The peak strength is higher in the borehole shear test than in the direct shear test in the laboratory under the same void ratio, water content, and loading pressure. The above discrepancy is attributed to the fact that the stress distribution along the shear plane is not uniform in the direct shear test in the laboratory, while in the borehole in-situ shear test, the stress on the shear plane is stable and remains unchanged. Compared with the direct shear test in the laboratory, the borehole in-situ shear test system exhibits a higher test accuracy.More>

2023, 23(4): 154-164. doi: 10.19818/j.cnki.1671-1637.2023.04.011

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Calculation method of semi-ellipsoid loose load on tunnel in weak and fragmented stratum

HAN Xin, YE Fei, LIU Chang, HAN Xing-bo, JIA Yan-ling, WANG Dong-fang

Abstract: To effectively control the tunnel pressure from the surrounding rock and optimize the support structure of the tunnel, the loose load applied in the tunnel lining structure in the weak and fragmented stratum was studied. Based on an experiment in the literature about the loose deformation of the surrounding rock in sandy stratum, the semi-ellipsoid loose load model for the weak and fragmented stratum was put forward by using the semi-ellipsoid to fit the loose range of the stratum. The calculation equations of the loose load with the tunnel convergence deformation were developed. The distribution characteristics of the loose load and its changing with the tunnel convergence deformation were analyzed. For quickly calculating the loose load, a numerical calculation procedure by scatter and summation of the loose load was developed by dividing the semi-ellipsoid boundary of the loose zone into many short straight lines. To further improve the engineering practicability of the calculation model, the quadratic polynomial was used to fit the loose load, and the simplified calculation method of the loose load with the tunnel convergence deformation was obtained. Research results show that the proposed semi-ellipsoid model can accurately represent the loose load applied to the tunnel and its dynamically developing process with the tunnel convergence deformation. Due to the presence of frictional resistance on the slide surface, the vertical stress of the stratum in the loose range is less than the initial gravity stress, and the load in the loose range is transferred to the surrounding stratum by the frictional resistance on the slide surface, which obviously manifests a soil arching effect in the loose range. The increase in the tunnel convergence deformation causes an extended loose range of the surrounding stratum of the tunnel and weak constraints on the loose ellipsoid from the surrounding stratum, and the soil arching effect of stratum weakens. The internal friction angle of the stratum and the eccentricity of the ellipsoid have great influences on the calculation results of the loose load, so these two parameters should be determined according to the test of geological conditions. When the semi-ellipse boundary is discretized into three segments, the results of the numerical calculation by scatter and summation can have a good consistency with the analytical solution, and the error is about 3.9%.More>

2023, 23(4): 165-177. doi: 10.19818/j.cnki.1671-1637.2023.04.012

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Bipolar coordinate solving method of ground displacement caused by shallow tunnel excavation

LAI Hong-peng, YAO Yi, GAO Qiang, LIU Yu-yang

Abstract: Based on the bipolar coordinate system and Mohr-Coulomb criterion, the elastoplastic solution of ground displacement caused by the shallow tunnel excavation in vault direction was derived by considering the characteristic of dilatancy and combining the surrounding rock stress solution of Jeffery and Massinas semi-infinite spatial circular tunnel with the equilibrium equations. The new solution was further verified by the Peck formula, Park formula, Loganathan-Poulos formula, and measured data. The relationships between the ground deformation mechanism and existing solutions were revealed. The quantitative methods for determining the empirical parameters (ground loss rate and gap parameter) subjected to ground parameters and construction factors were provided. Research results show that fewer hypotheses are needed by the elastoplastic solution. An difference of less than 2% is presented in comparison with the Peck formula and Park formula, and an difference of 9.5% is shown in comparison with the Loganathan-Poulos formula. The ground deformation mechanism, Peck formula, and various modified elastic formulas are further elucidated from the perspective of elastoplastic analysis by the bipolar coordinate solving method. In other words, both the elastic and plastic deformations can be caused by the shallow tunnel excavation. When the ground deformation is calculated by the empirical formula methods, typified by the Peck formula, the values of ground loss rate in diverse regions and construction conditions correspond to the ground elastoplastic deformations under different ground parameters (cohesion, internal friction angle, Poisson's ratio, weight, and elastic modulus) and construction boundary conditions (buried depth, excavation radius, and support force). Various modified elastic solutions, typified by the Park formula and Loganathan-Poulos formula, can also be deemed as the modifications offsetting the discrepancy between the ideal elastic solution and the elastoplastic solution by the approximate means of the cross-section ovalization and sedimentation. Therefore, the field construction can be better guided by combining the elastoplastic solution with the existing formula.More>

2023, 23(4): 178-189. doi: 10.19818/j.cnki.1671-1637.2023.04.013

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Responses of loose sand ground and surface structure caused by tunnel construction

XU Jing-min, XU Cheng-hua, SHI Ye-hui, ZHANG Ding-wen, XU Tao, XU Wen, CHENG He-lan

Abstract: Four centrifuge tests were performed to simulate the construction of tunnels with the relative depths (burial depth-diameter ratio) of 1.3 and 2.0 in the sand ground, and the migration and deformation laws of soil layers and ground buildings were analyzed. The ground volume loss caused by tunnel construction was modelled by extracting the liquid from the model tunnel, and a two-storey aluminum frame structure model was designed. The movement data of the strata and structure caused by tunnel construction were measured by particle image velocimetry (PIV), and the horizontal and vertical displacements of the surface and building raft foundation, the movement and shear deformation of the deep ground, the shear deformation and classification of the frame structure, as well as the shear deformation modification factor of the building and relative shear stiffness were analyzed. Research results show that the surface settlement trough width increases from 3.4 m to 5.6 m when tunnel relative depth increases from 1.3 to 2.0, and the maximum settlement of ground building increases from 32.3 mm to 49.5 mm, but the deformation degree decreases. The deformation of ground frame structure due to tunnel construction mainly exhibits shear deformation, and the proportion of bending deformation is negligible. The contractive deformation of loose sand soil due to tunnel construction leads to the fact that the volume loss of surface ground is always greater than the tunnel, and a deeper tunnel indicates a greater difference. A large gap (27 mm) exists between the building raft foundation and the ground in the shallow tunnel test, whereas no gap forms in the deep tunnel test, thereby increasing the constraint of the building raft foundation on the horizontal movement of the surface soil. The building shear deformation modification factor decreases gradually with the increase in tunnel volume loss, and the change rate of shallow tunnel is greater. The data of building shear deformation modification factor-relative shear stiffness for the two relative tunnel depths are within the existing empirical envelopes, indicating that the shear deformation modification factor is also applicable to the deep tunnel.More>

2023, 23(4): 190-204. doi: 10.19818/j.cnki.1671-1637.2023.04.014

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Mechanical deformation characteristics of support system for boulder-cobble tunnel

ZAN Wen-bo, LAI Jin-xing, CAO Xiao-yong, FENG Zhi-hua, QIU Jun-ling, ZHANG Wen-jie

Abstract: In response to the engineering and technical problems such as the extreme difficulty in excavation and support construction, and the frequent collapse disasters at the arch in the Nianggaicun Tunnel of Lhasa-Nyingchi Section on the Sichuan-Xizang Line, a combination of excavation and support system containing "three-bench complementary cyclic excavation+section steel arch+shotcrete+double-layer dense reinforcement mesh+multiple groups of feet-lock bolts (pipes)+grouting behind the lining" was proposed. Two typical sections of boulder-cobble tunnel were selected to conduct the test research on the stress and deformation of the support system. The load action characteristics of the surrounding rock, the mechanical properties of the support system, and the deformation laws inside and outside the tunnel were analyzed. The load-bearing action mechanism of the new support system was revealed for the boulder-cobble tunnel, and corresponding new prevention and control principles were summarized and proposed. Analysis results show that the surrounding rock pressure is mainly a loose collapse load at the arch and distributes unevenly around the tunnel. The average load-sharing ratios of initial support and secondary lining are 67.65% and 32.35%, respectively. The feet-lock bolts are either tensile or compressive, and the maximum tensile and compressive forces reduce by 45.9% and 20.0% after optimization, respectively. The force of the secondary lining is overall small with enough structural safety reserve. The vault subsidence is less than 15 mm, and the horizontal convergence is 8-9 mm in the tunnel body. At the tunnel portal section, the deformation is asymmetric and significantly affected by the shallow bias and rainfall conditions. The maximum settlement of the arch part reaches 52.4 mm, and the horizontal convergences of the upper and lower benches are 11.4 and 15.6 mm, respectively. Under the similar unfavorable conditions, the inverted arch and secondary lining should be constructed as early as possible to ensure the construction safety. The support system design for the boulder-cobble tunnel follows the prevention and control principles of "less disturbance, strong arch foot, anti-overbreak, dense reinforcement mesh, and frequent grouting". It can timely control the expansion of the loose zone at the arch and mobilize the self-bearing capacity of the deep surrounding rock, achieving the purpose of improving the mechanical performance of the support structure and effectively avoiding the occurrence of arch collapse disasters.More>

2023, 23(4): 205-217. doi: 10.19818/j.cnki.1671-1637.2023.04.015

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Improved transient electromagnetic radar method of void identification behind tunnel lining

GENG Qing-qiao, JIA Yuan-hua, YE Ying, WANG Deng-ke

Abstract: In view of the technical problems of detection behind the tunnel lining such as insignificant image features, and the low accuracies of void location and range identification, an improved transient electromagnetic radar (TER) method to detect the void behind the lining was proposed based on the transient electromagnetic theory and the detection requirements of tunnel lining structure. The detection depth was increased by improving the turn-off time of the transmitting system. The image resolution was improved by using the equivalent circuit of receiving coil and weak signal enhancement algorithm. On the basis of eliminating the strong interference signal and increasing the transmitting magnetic moment, the data noise was suppressed by smooth filtering and multi-period superimposed sampling. Based on the difference in the apparent resistivity, the TER map of the relevant medium of lining structure was established, and the indoor lining defect simulation test was carried out through the different combinations of various concrete components. The detection accuracy and feasibility of the improved TER method to detect the void behind the lining were preliminarily evaluated. A field application was carried out in Haobei Tunnel of Beijing Metro Line 6. By comparing the test results of TER images with the sampling results of drilling, the effectiveness of the improved TER method to detect the void behind the lining was further verified. Analysis results show that the improved TER method has higher imaging resolution and better defect identification effect. It can directly and clearly reflect the actual characteristics of the void defects behind the lining and effectively identify lining thickness, buried depth of steel bar, and specific locations and scopes of the voids behind and inside the lining. It can quantitatively describe 10-20 cm voids and locate and determine the magnitude of the voids less than 10 cm. Besides, it has a good reference for the identification result of the target body with a depth of more than 30 cm behind the lining. Meanwhile, the improved TER method is less disturbed by metal mediums and can accurately locate the void defects behind the lining with a void range greater than 30 cm in a wide range of continuous detection.More>

2023, 23(4): 218-232. doi: 10.19818/j.cnki.1671-1637.2023.04.016

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Force and deformation characteristics of subway station foundation pit under severe cold conditions

YANG Xiao-hua, CAI Shi-chun, HE Shao-hua, SHAO Ying, YAN Chang-gen

Abstract: Based on Hohhot Metro Line 1, a self-developed freeze-thaw cycle test device was used to conduct the laboratory test on the temperature distribution of foundation pit soil, the frost heaving amount of surface soil, and the stress and deformation characteristics of underground continuous wall, and the stress and deformation characteristics of the foundation pit under different wind speeds, water contents, and temperatures were analyzed by the numerical simulation. Analysis results show that the soil around the foundation pit exhibits bidirectional freezing characteristic during the cooling process from 5 ℃ to -30 ℃, and the maximum freezing depth near the underground continuous wall can reach 18.2 m (the bottom of the foundation pit downward 1.09 m). The maximum deformations of foundation pit soil and underground continuous wall increase with the increase in the freeze-thaw cycles and tends to be stable in six freeze-thaw cycles. The maximum surface uplift during the last freeze-thaw cycle can reach 3.85 times that during the first freeze-thaw cycle. The horizontal frost heaving force is approximately parabolic in distribution along the underground continuous wall. The maximum frost heaving force appears in the middle of the underground continuous wall and can reach 775.8 kPa at -30 ℃. Wind speed has a significant effect on the heat exchange of foundation pit soil. The horizontal earth pressure of the foundation pit is linearly correlated with the wind speed of 0-0.4 m·s^-1. The earth pressure fluctuates and increases with the wind speed of 0.4-2.5 m·s^-1. When the wind speed is greater than 2.5 m·s^-1, the earth pressure is basically stable. When the wind speed is 0-0.4 m·s^-1, the surface deformation is linearly correlated with the wind speed. When the wind speed is 0.4-2.5 m·s^-1, the deformation increases step by step. When the wind speed is greater than 2.5 m·s^-1, the deformation is basically stable. When the water content increases from 13.3% to 33.3%, the maximum horizontal earth pressure increases by 44.2%. Under different constant negative temperatures, lower ambient temperature indicates that the maximum horizontal frost heaving force is closer to the bottom of the foundation pit. At -30 ℃, the maximum horizontal frost heaving force can reach 0.95 MPa, and the maximum surface deformation can reach 56.6 mm.More>

2023, 23(4): 233-247. doi: 10.19818/j.cnki.1671-1637.2023.04.017

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Design and application of modular step-by-step scale model test for earth pressure balance shield construction

HUANG Da-wei, XU Chang-jie, LUO Wen-jun, JIANG Hao, LIU Jia-xuan, LI Qing

Abstract: To improve the pertinence of experimental research on shield construction-related problems, a modular step-by-step scale model test method for the earth pressure balance shield construction was proposed, and the model test-related device was designed. To avoid the mutual influence of multiple key influencing factors, the key operation links for the shield construction such as the excavation, segment assembly, and synchronous grouting were simulected in a separate and sequential manner. To reduce the cost of experimental research on the cutterhead opening rate, a model shield machine with a geometric similarity ratio of 1∶10 and an adjustable cutterhead opening rate was developed. To ensure the similarity of the longitudinal and transverse stiffnesses of the model shield tunnel, a model shield tunnel with a geometric similarity ratio of 1∶10 and the longitudinal and transverse stiffnesses set on demand respectively was designed. The segment ring adopted the modified homogeneous ring model, and the segment rings were connected by compression spring bolts. To facilitate the test and save the test cost, the model shield tunnel was assembled externally and then put into the steel sleeve, so as to avoid the segment assembly in a narrow space, and the model shield machine did not need to process the segment assembly machine. To realize the precise control of grouting pressure and grouting amount, a constant pressure synchronous grouting device suitable for the indoor model test was designed. The experimental study on the influence of slag soil output control on the surface settlement during the tunneling construction was carried out by using the model shield machine. Analysis results show that the slag soil output per unit tunneling distance is directly related to the over-excavation control in the process of shield tunneling, thus affecting the surface settlement. When the unearthed rate is less than 1, the change in the unearthed rate has little effect on the surface settlement. When the unearthed rate is greater than 1, the increase in the unearthed rate will significantly increase the surface settlement. To maximize the scientific research value of the whole model test, it is suggested to continue to carry out the relevant model test of shield tunnel response research after the completion of shield tunnel construction.More>

2023, 23(4): 248-257. doi: 10.19818/j.cnki.1671-1637.2023.04.018

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Calibration method of asphalt pavement fatigue damage prediction model based on accelerated pavement test

HAN Zhen-qiang, HU Li-qun, SHA Ai-min

Abstract: To improve the reliability of fatigue damage prediction model for asphalt pavements in engineering applications, a calibration method of the model was proposed based on the small-scale-specimen test and full-scale accelerated pavement test (APT). The evolution characteristics of asphalt pavement fatigue damage were analyzed to establish a cumulative fatigue damage analysis method by the nonlinear incremental recursive (NIR) method, which was suitable for the shift and extrapolation of the fatigue damage prediction model from small-scale test to full-scale test. The fatigue damage prediction model for the full-scale asphalt mixture layer was established based on the fatigue life prediction model obtained by small-scale specimen test, and the transfer equation of the fatigue life prediction model was used as the tool to predict the fatigue damage under the full-scale APT condition. In order to determine the model transfer equation, the fatigue damage calibration equation based on the APT was proposed, and the calibration methods of the undetermined coefficients were derived. The APT was carried out by the heavy vehicle simulator on the full-scale asphalt pavement test sections with graded-crushed stone composite base. The fatigue damage prediction model of asphalt mixture layer was calibrated and validated by integrating the dynamic modulus test and the four-points-bending-fatigue test results of the core samples drilled from the APT sections. Research results show that the NIR method can consider the impacts of material nonlinearity, performance decay, and loading history on the fatigue damage accumulation of asphalt pavement structure, conforming to the actual evolution law of the pavement fatigue damage. The calibrated fatigue damage prediction model can be used to predict the cumulative fatigue damages of the asphalt mixture layers of test sections in different loading intervals, and 50% and 90% of the predicted results have an error of less than 3.1% and 20.0% compared to the measured results, respectively, indicating that the calibrated prediction model has certain reliability. Therefore, the proposed model calibration method can be taken as a reference for the establishment of fatigue damage prediction model of asphalt pavement based on the APT and provide more reliable performance prediction models for the decision-making on pavement design and maintenance.More>

2023, 23(4): 258-270. doi: 10.19818/j.cnki.1671-1637.2023.04.019

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2023 Vol. 23, No. 4

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