Aiming at the construction safety risk control problem of network construction project in underground space, five network methods including adjacent extension and construction, connection between existed structures extension, vertical direction extension,enlargingexistingunderground extension and multidirectional extension were proposed. The risk characteristics of network construction project were studied and analyzed. In planning and design stage, the safety planning and design methods and pre-redundancy design were established. In construction stage, pre-planning and design methods of quality evaluation,verification and feedback were constructed. The new planning and design technology could guarantee the safety in the whole process of network construction project. Based on the hierarchical control of the interference level, the safety pre-control measures for the four elements in construction projects including the extended structure, the existing structure, the ground and the surrounding environment were put forward. The new construction safety monitoring index system was given so that the safety risk prevention and control could be fully implemented in the construction process. The researched results provided theoretical guidance and technical support for the planning and design and safe construction of underground space network construction project.

To explicate the size of model boundary required for the analysis of ground settlement caused by circular tunnel excavation, the influences of the sizes of bottom boundary and horizontal boundary of the model on the ground settlement caused by a circular tunnel excavation were studied by the theoretical analysis and the numerical simulation. The Bobet, Park and Verruijt methods were adopted in the process of the theoretical analysis. The elastic and elastoplastic constitutive models were adopted in the process of the numerical simulation. The results of research showed that the mapping point of the circular tunnel vault on the ground surface gradually changed from settlement to uplift as the size of the model bottom boundary increased. When the model bottom was a half-space infinite body, the mapping point of the circular tunnel vault on the ground surface bulged infinitely. This phenomenon was obviously contrary to engineering reality. When there was an underlying hard layer at the model bottom boundary, if the elastic modulus of the underlying hard layer was 5 times that of the overlying soil, the underlying hard layer could be used as the bottom boundary condition of the model. The size of the model horizontal boundary was closely related to the size of the model bottom boundary and the tunnel radius. The size of the model horizontal boundary increased approximately linearly with the increased of the size of the model bottom boundary and the tunnel radius.

This paper discussed the auxiliary technique of TBM tunnelling through cyclic treatment of heating-Liquid nitrogen(LN₂)cooling on granite. The granites were subjected to the cyclic treatment of 200 ℃-LN₂ cooling, 300 ℃-LN₂ cooling and the 400 ℃-LN₂ cooling, and the numbers of cyclic treatment were 1, 3, 6, 12, 20 times, and all the treated specimens were subjected to uniaxial compression tests. Microstructure and the micromechanical properties of granites treated with cyclic treatment of 400 ℃-LN₂ cooling were studied by microscope and nanoindentation technology. The results showed that, under the same times of cyclic treatment, the higher the heating temperature was, the greater the deterioration degree of granites was. The change of macroscopic mechanical properties with the increase of the cyclic treatment times could be divided into three stages: rapid change stage(0-6 times), slow change stage(6-12 times)and nearly constant stage(after 12 times). The higher heating temperature, and the more times of cyclic treatment, the failure mode of granite under uniaxial compression would be more complex. The degradation induced by cyclic treatment of 400 ℃-LN₂ cooling was mainly due to the development of microcracks rather than the deterioration of mechanical properties of crystals themselves.

In order to improve the reliability of prediction of large deformation level of layered soft rock tunnels under complex geological conditions, a particle swarm optimization(PSO)based support vector machine(SVM)was proposed. The large deformation prediction method of tunnel solved the problems of complex calculation of multiple evaluation index weights and diverse boundary values in large deformation prediction. In order to fully consider the influence of layered soft rock surrounding rock strength, surrounding rock structure type, in-situ stress and groundwater on the large deformation of the tunnel, the 6 sub-indices of value: uniaxial compressive strength, rock inclination angle, initial in-situ stress state, burial depth, rock mass correction quality correction index ［BQ］ and groundwater development, were used to predict the large deformation level. This method constructed a large deformation prediction model based on in-situ stress inversion and on-site large deformation monitoring information according to the classification standard of large deformation grades, and used particle swarm optimization algorithm to adjust the penalty parameter C and the kernel function parameter Gamma to improve the model's performance accuracy. The research results showed that the use of particle swarm optimization-support vector machine(PSO-SVM)algorithm could avoid errors caused by traditional prediction methods such as geological comprehensive judgment method and strength-stress ratio method due to a single index and subjective reasons, and the prediction accuracy was high; this method used the large deformation information of tunnels that had occurred, a PSO-SVM large deformation prediction model conforming to the actual law of the target tunnel site was constructed; the PSO-SVM model had an accuracy of 86.36% in the prediction of the sample test set, which was better than the SVM and GS-SVM models. Taking the Baima Tunnel, a typical layered soft rock tunnel of the Jiu-mian Expressway as the research object, the proposed PSO-SVM model was used to conduct large-scale research. The deformation prediction was compared with the field measurement, and it was found that the prediction accuracy reached 80%, which verified the feasibility of the method.

Aiming at the problem of insufficient bearing capacity of existing pile foundation in silty clay stratum, the effect of improving bearing capacity of existing pile foundation by side grouting was studied systematically through grouting simulation experiment. The reinforcement mechanism was mainly through the compaction effect of grouting material on the soil around the pile, increasing the friction resistance of the pile-soil interface, restricting the settlement of the pile body, and improving the bearing capacity of the pile foundation. The results showed that the grouting water cement ratio, grouting pressure, grouting hole layout position and grouting amount were the four main controlling factors for the reinforcement effect of pile foundation. The combined action of slurry water-cement ratio and grouting pressure directly affected the compaction effect of slurry on silty clay stratum by controlling the shape of slurry veins. According to the different compaction effects, the combination of different water cement ratio and grouting pressure was divided into three reinforcement design areas: strong, medium and weak. Among them, the compacted soil with a slurry water-cement ratio of 0.8 and a grouting pressure of 2 MPa improved the physical and mechanical strength to the greatest extent. The increase of the bearing capacity of pile foundation was positively correlated with the amount of grouting, and inversely proportional to the distance between the formation position of grouting vein and the pile body. According to the different distance between the formation position of grouting vein and the pile body, the soil reinforced by grouting was divided into the strong action area and the weak action area. In addition, the improvement of lateral friction resistance of pile side grouting was mainly generated in the middle and lower section of pile body. In practical engineering, the reinforcement of this section reduced the consumption of manpower and material resources.

Attention should be paid to the dynamic response of railway track in seasonally frozen environment under moving tram loads in order to ensure the safety of tram during operation period. The black clay in Zhangjiakou was selected as the typical frozen soil sample, and the hydraulic and mechanical properties of the sample were tested under the environment of temperature variation. The long lined track of the tram was assumed as an infinite homogeneous beam resting on a Pasternak foundation, and the analytical solution for long lined tracks subjected to moving traffic loads was obtained based on the beam-elastic foundation theory. Parametric analyses were performed to investigate the influence of the freeze-thaw behaviors of soil on the dynamic response of railway subgrade. Results showed that the dynamic response of subgrade was affected by the freeze-thaw behaviors of soil, and the dynamic stiffness of foundation would decrease with the increase of temperature, and thus contributed to the amplification of dynamic responses of the track.

In order to study the effect of upper step length on tunnel deformation in tunnel step construction, the stability of surrounding rock and supporting structure was analyzed by using three dimensional finite element model, taking the step length and circular excavation length of a tunnel in Gansu province as parameters. The results showed that the length of the upper step had a significant impact on the plastic area range, surrounding rock stress, arch subsidence and surrounding convergence; the maximum surface settlement gradually increases. Besides 25 m from the tunnel center line, the surface settlement growth rate slows down and stabilizes; the length of the upper step is 1~1.5 times, about 15 m, and the circulating excavation meter was 1.5 m. During the tunnel construction, the monitoring results of surrounding rock displacement and surface settlement met the construction safety requirements, and verify the safety and accuracy of the analysis of the model data analysis. The research results can provide some reference for the tunnel constructed by step method.

Relying on the open excavation subway foundation pit project of Aixihu in Nanchang, the excavation of the foundation pit was numerically simulated by ABAQUS software. The reliability and applicability of the numerical model were verified by comparing the numerical calculation and field monitoring results of the horizontal displacement of the diaphragm wall, supported axial force, surface settlement. The bending moment variation law of diaphragm wall during excavation was analyzed. The research showed that there was a significant difference in the bending moment between the earth facing side and the excavation side of the diaphragm wall, the excavation side was tensioned as a whole and the earth facing side was compressed as a whole. The maximum bending moment of the excavation side was 2.3 times than that of the earth facing side. In the original design scheme, the main reinforcement on both sides of the diaphragm wall was symmetrically reinforced, and there was a large optimization space in the design scheme; the effect of crack control was not good only by increasing the amount of reinforcement. Under the condition of constant excavation depth and internal support spacing, after the asymmetric reinforcement optimization of the existing design, the reinforcement amount on the earth facing side and excavation side was reduced by 38.3% and 11.4% respectively, and the load sharing characteristics of the structure were more reasonable, which could greatly reduce the use of reinforcement, so as to reduce the project investment and construction scale.

In order to study the stability of excavation face of Jinan Yellow River Tunnel with large diameter slurry balance shield passing through permeable sand layer, a three-dimensional calculation model was established according to the relevant parameters of typical nodes of the project. Combined with the fluid-soild coupling theory, the variation of water seepage, pore water pressure distribution, soil displacement and soil stress under different slurry pressure were analyzed. The results showed that the displacement of the excavation face changed suddenly in the process of the slurry pressure reducing. The excavation face was actively damaged, the displacement expanded upward, and the instability form conformed to the wedge model. Due to the effect of soil arch, the horizontal stress of the soil increased. In the process of slurry pressure decreasing, the height of soil arch area expanded to the surface. According to the analysis of the displacement and stress state of the soil on the excavation face, the lower limit of the slurry pressure to maintain the stability of the excavation face was 0.34 times of the water and soil pressure. It was consistent with the existing test results, which was larger than that without considering the fluid-solid coupling.

In order to reduce or avoid the disaster of rockburst during the construction of the long and deep tunnel TBM, the small pilot tunnel excavation technology to release the stress of the surrounding rock in the tunnel in advance was adopted, and the stress release effect through the microseismic monitoring equipment was monitored. The results showed that the excavation of the small pilot tunnel could effectively release the stress in the tunnel and reduce the intensity and frequency of rock bursts. On this basis, the effects of different single-shot doses and the cross-sectional dimensions of small pilot holes on stress release were studied. The results showed that the single-shot dose was basically proportional to the microseismic monitoring data, that was, within a certain range, the greater the single-shot dose was, the better the stress relief effect was; the larger the cross-section size was more conducive to stress relief. In actual engineering, the stress should be released as much as possible based on the actual situation and under the premise of ensuring safety.

Based on a large foundation pit excavation project directly above a shield tunnel in Hangzhou, the influence of gabled reinforcement and divisional and block excavation measures on tunnel deformation were analyzed from the perspective of measured data.To study the deformation characteristics of tunnel in the process of divisional and block excavation, the vertical deformation, horizontal deformation and convergence deformation of subway tunnel in each stage during foundation pit excavation were compared, combined with the deformation control value. The analysis results showed that the reinforcement of "door" triaxial mixing pile strengthened the anti-deformation ability of soil and effectively reduced the deformation influence of upper unloading on shield tunnel. The excavation method of divisional and block foundation pit made use of the time-space effect, which greatly reduced the uplift deformation of the tunnel in the process of foundation pit excavation and unloading. The influence of mixing pile construction on the convergence value of tunnel was much greater than that of foundation pit excavation. Light disturbance construction technology could be adopted for similar projects in the future.

In order to build a refined three dimensional mesoscopic concrete model, a lattice-based aggregate placement algorithm was proposed, which was universal to polyhedral aggregates composed of patches, and after parallel optimization through OpenMP, the aggregate placement efficiency was improved about 50%. The basic idea of the algorithm was to discretize the continuous model domain through a series of ordered lattices, traverse the set of points surrounded by the aggregate to be cast, and judge whether the aggregate was put successfully through the status indicators of these points,which effectively avoided the complicated geometric calculations when judging the intrusion between aggregates. Combined this algorithm with laser scanning technology, the constructed three-dimensional numerical model of concrete could faithfully reflect the characteristics of its internal aggregate form, gradation, and spatial distribution, and the aggregate volume content of the model was as high as 60%, which fully met the requirement of concrete in numerical research.