Dongming Zhang

Characterization of Model Uncertainty for Cantilever Deflections in Undrained Clay

AbstractThis work presents a critical evaluation of the model factor for the mobilized strength design (MSD) method for cantilever deflection in undrained soft to medium-stiff clay. The model factor is characterized in two parts. A correction factor η is first defined as the ratio between the wall-top deflection computed from the FEM and its corresponding value computed from the MSD. The hardening soil with a small-strain model is used in FEM. The statistics of η are evaluated using 82 numerical simulations. Because η is not random, and therefore cannot be modeled directly as a random variable, it is decomposed as a product of a systematic part (f) and a random part (η∗). The random part is modeled as a lognormal random variable with a mean of 1.01 and a coefficient of variation (COV) of 0.18. The model factor for FEM (eFEM) is next defined as the ratio between the measured wall-top deflection and the corresponding FEM result. The statistics of eFEM are evaluated using 45 field cases. The ratio eFEM is ra...

Deformational responses of operated shield tunnel to extreme surcharge: a case study

Abstract A field case of operated shield tunnel disrupted by unexpected extreme surcharge in Shanghai is reported in this paper. The deformational responses of segmental lining, in terms of convergences, settlements and joint open width, are highlighted. The surcharge caused by unexpected dumped soils with a maximum height of 7 m is six times larger than the design value of surcharge. Three hundred and sixty segmental lining rings of the operated metro shield tunnel below this surcharge are severely affected. The measured horizontal convergence to outer-diameter ratio (ΔD/Dout) ranges from 16.6 to 35.7‰, which is far beyond the averaged value for normal condition, i.e. 5.1‰. The trend of tunnel deformation (e.g. horizontal convergence and settlement) varying with surcharge level is found to be significantly non-linear. Correspondingly, the longitudinal joints are also severely disrupted and have large open widths. The rubber packer for joint near-tunnel springline loses its function of waterproof due to the release of 6 mm pre-compressive deformation. The majority of steel bolts of joint near-tunnel crown have passed yield state, while some are fractured. However, the circumferential joints are found to be in a sound workability.

Quantitative Geotechnical Risk Management for Tunneling Projects in China

To date, the tunneling in China is experiencing an age of fast development for decades. The potential risks behind the huge amount of construction and operation works in China was first formally realized and managed after 2002. The transition of risk assessment from a qualitative manner to a quantitative manner is on the way from the research gradually to the practice. This paper tries to share some experiences in the quantitative risk management for tunneling in China by introducing novel techniques and associated practical applications. The fuzzy fault tree analysis is used for hazard identification, the conditional Markov chain for probability analysis of soil spatial uncertainty, the quantitative vulnerability analysis for consequence evaluation and the field data based statistics for environmental impact risk analysis. All these novel methods have been validated successfully by applying into real cases shown in the paper. The dynamic feature of risk management is appreciated due to the different stages and scenarios of a tunnel project. The real-time monitoring technique developed using the LEDs and MEMS coupled with WSN could visualize the risk to the worker on site timely. The resilience analysis model to incorporate the high-impact low-chance risk for tunnel lining structure is introduced in the end of paper, which could assist the engineers to make the decision on performance recovery strategies once the tunnel goes through a significant disruption

Model Test Study of Soil Variation Impact on Shield Tunnel Segment Structure

In recent 10 years, several accident cases and theories showed that soil distribution around the shield tunnel has direct or indirect impact on the failure models of tunnel segment structure. In this paper, a 1:38.75 ration physical model test is developed and carried out to clarify the impact of the soil variation on the mechanical behaviors of the tunnel segment structure. Some conclusions are put forward that soil variation does have an influence on the structure, but with different grades in different places of the segment structure. The value of displacement, bending moment of the segment structure located in the variation area increase dramatically, and the extremum points move to side of the varied soil in the structure. However, the structure located in the unvaried region remains almost the same status as that in the green field.