Keneng Zhang

Numerical simulation on the ground deformation by Pipe-roof Pre-construction Method

Its one of the key problems that ground deformation and influeced to the near buidings couseed by the city underground space construction. The surface deformation and vault subsidence regularity are analysed for station within the turkmen excavation to ensure the technology successful application to underground subway station in Shenyang Metro Line 2, based on FLAC3D simulation. Results show that, (1) the surface deformation and vault subsidence could be less by using Pipe-roof Pre-construction Method in the full-section turkmen excavation, and in the whole Pipe-roof Pre-construction Method construction process of surface subsidence caused by the proportion is less too, which could meet the requirements of controlling the surface subsidence target. (2) with the great influence of horizontal loads, the underground structure in advance could partially offset the ground deformation caused by vertical loads. the surface deformation and vault subsidence tend to be stable, and the sedimentation curve rebounds a little. Pipe-roof Pre-construction Method is much better than depth-span ratio underground structure in controlling the ground deformation. (3) surface deformation and vault settlement of shallow buried underground arch shell structure were basically equal, which built by Pipe-roof Pre-construction method, and there was a phenomenon that the surface deformation was bigger than vault settlement during station within the turkmen excavation. (4) The supporting structure beforehand built by Pipe-roof Pre-construction method could almost completely replace the supporting function of the earthwork excavated to the surrounding rock. The Pipe-roof Pre-construction method deserves further research because it has no bad influence to near environment, has obviously social and environmental benefits, and can be used to build traversing urban traffic hub of the underground engineering.

On the decay of strength in Guilin red clay with cracks

In order to research the effect of cracks in red clay on shear strength through dry-wet cycle test, the experimenters used imaging software and a mathematical model to determine fractal dimension and crack ratio of surface cracks in red clay in Guilin, China. After each dry-wet cycle, direct shear tests were carried out on the sample, and such variables as matrix suction on the crack propagation process of red clay were analyzed. The mechanics model was established and obtained the critical condition of soil cracks. The results show that with the increase in the number of dry-wet cycles the shear strength of the samples would decrease. But the rule of shear strength of sample 3 is slightly different from samples 1 and 2. The shear strength of red clay has a good correlation with fractal dimension and crack ratio, which could be an identification index of the strength of red clay.

Analysis of the ground deformation to large cross- section tunnel by Pipe-roof Pre-construction Method

Urban underground engineering construction may have bad influence on the environment and even cause disasters. Pipe-roof Pre-construction Method(PPM) had been used in building a metro station of line No.2 of Shenyang subway. Its very important to analyze and evaluate the deformation of the environment in order to ensure the safety of constructing procedure. So, based on the detailed study of geological conditions and PPM, Flac3D software combined with field monitoring data in stations earthwork phase is used. And then, research on the surface and vault settlement, arch feet headroom convergence and tunnel structure deformation law is presented. The results show that: (1) numerical simulation is close to the monitoring data, which ensures the large cross-section single arch tunnel construction safety, and provides guidance for the first successful application and research of this method in China mainland area. (2) The surface deformation and vault subsidence could be less by using PPM to build large sections tunnel, and in the whole PPM construction process of surface subsidence caused by the proportion is less too, which could meet the requirements of controlling the surface subsidence target. (3) It deserves further research and extension for the PPM to build underground space, because it has obviously social and environmental benefits, and can be used to build traversing urban traffic hub of the underground engineering.

Numerical Simulation for the Excavation in Tunnel Construction

Tunnel excavation is often done in underground engineering such as civil tunnel construction and mine excavation. The numerical simulation can output colorful results to interpret the tunnel excavation effect more easier. Among all the numerical simulation methods, fast lagrangian explicit finite difference code of continua (FLAC3D) is widely used to solve practical problems, especially in field of elasto-plastic characteristic and construction procedure. So in the present paper, 3 steps of excavations in a tunnel is modeled by FLAC3D to present the stress and displacement distribution, and give guidance for the real practice.

Numerical Modeling for the Stress Displacement Distribution of Tunnel after Excavation

The tunnel excavation is often existed in civil engineering, in which the minimum roof thickness of tunnel is an important index in tunnel construction and design. Some work have been done on this field, by analytical analysis method and numerical simulation method based on linear criterion. In order to add some conclusions to the roof thickness prediction by nonlinear criterions, in the present paper, the calculation model for excavation of tunnel is built by numerical method, the nonlinear criterion is applied to describe the failure situation of the surrounding rock mass; then, according to the reduction method for thickness of roof, the thickness of the roof after excavation in tunnel is analyzed with the nonlinear criterion, to give some guidance for the real practice.

Effect of Soil Conditions on the Strength of Cemented Soil with Deep Bidirectional Mixing

Soil condition is a major impact factor on the strength of cemented soil. In order to study the relationship between the soil conditions and the strength of cemented soil in deep mixing column under the circumstance of thick soft clay in the Nansha area, the uniaxial and unconfined compressive strength tests were carried out, from which stress-strain curves of the cemented soil and strength-depth curves of column body were obtained under three different soil conditions. Analyzing these curves, conclusions can be made that the strength of different parts of cement soil from deep mixing pile varies greatly in the Nansha area. Among the samples, the cement mixed with silty clay has the highest strength, while the lowest one is the cement mixed with mud. Besides the above results, compressive strength values(fcu)of cement soil is determined, which can be of great significance to calculate the bearing capacity of deep mixing piles.

Numerical Simulation of Shield Tunnel Passing Through Underground Structure

Shield tunneling is often adopted in underground engineering such as civil tunnel construction and mine excavation. The program FLAC3D is used to simulate the process of the tunnel excavation through underground structure in Shenyang in this paper. The analysis results show that,(1) the soil below the end wall suffers great displacement, when the shield approaches the end wall of underground framework from different directions, so the soil under the end wall needs to be reinforced. (2) Increasing pressure and volume of grouting can’t significantly reduce the amount of surface subsidence when the drilling of the shield acrosses through the independent foundation. (3) The influences of shielding to the construction are limited because of the constraint function to the surrounding rock above the tunnel by the great entire rigidity of under-ground framework. The results of numerical simulation exactly matches the monitoring data when the stiffness of under-ground frame structure is considered, and it can provide guidance for engineering practice.

Experimental Study on Vertical Bearing Behavior of Composite Foundation with Tapered Rigid Pile

In order to study the vertical bearing behavior of composite foundation with tapered rigid pile, three static load model tests of composite foundation with 5 different tapered rigid piles were performed as well as a static model test of composite foundation with 5 cylindrical rigid piles under the same geological conditions. The test results indicated that the average pile-soil stress ratio of composite foundation with tapered pile is lower than that of cylindrical rigid piles. Tapered pile was beneficial to take full advantage of the bearing capacity of natural foundation and could effectively relief pile-tip stress concentration in composite foundation. The axial forces decay gradually along pile depth under load. However, the axial force attenuation grads of tapered pile are larger than that of cylindrical rigid piles. The axial force attenuation speed of the center pile, which increases along pile depth with the increase of the obliquity, is much faster than that of corner pile in composite foundation with tapered rigid pile. The skin frictions of the piles mostly focus on the range from pile top to L/3 of the pile length.