Search

Tunneling in karstic geology confronts numerous challenges due to unpredictable occurrence of voids. The current approach of karstic void risk assessment is qualitative or semi-quantitative and lacks consideration of the spatial variability and distribution of voids. This often influences the pricing strategies, and design and construction activities on tunnel projects. This paper presents a geostatistical modeling-based methodology to develop a quantitative assessment of karstic void risk for a tunnel project in a karstic geological setting. The methodology is applied on an actual mixed-ground tunnel project situated in a karstic geological environment in Malaysia. The geology at the tunnel project site consists of sedimentary rock formations with limestone as the predominant rock type overlain by weak sedimentary residual soils. Pluri-Gaussian simulation (PGS) technique, a stochastic geostatistical-modeling algorithm, is applied to characterize the spatial distribution of voids in 3D along tunnel alignment. Simulations from PGS take into consideration the anisotropic distribution of voids on the tunnel project site. PGS utilizes void data from borehole investigations to model different void sizes (Vs) as categorical variables. The variability in multiple realizations from PGS technique is used to quantify the uncertainty in occurrence probabilities, number, and frequency of karstic voids. The proposed methodology demonstrates the ability to develop probabilistic estimates of occurrence frequency of different void sizes. Probabilistic assessments indicating 95% confidence interval (CI) on number of voids and respective occurrence probabilities are presented. The probabilistic assessment results are applied to estimate the grout quantity required for void treatment, while considering uncertainty in void occurrence. A minimum, mean, and maximum cumulative grout volume of about 2000 m3, 4000 m3, and 8000 m3 (for 95% CI), respectively, is estimated along the alignment.

Urban soft-ground tunneling projects involve significant risks related to the spatial variability and uncertainty in geotechnical parameters. However, standard practice typically does not i...

Ground volume loss induced by shield tunnel construction is the major factor leading to ground settlement and deformation. The general equations predicting surface settlement based on ground volume loss involve a settlement trough width coefficient (parameter i) which in previous models was set as a constant in both the transverse and longitudinal directions. In this work, the equations predicting surface settlement during the construction were modified by introducing the parameter j – the width coefficient in the longitudinal direction, assumed to be different from that in the transverse direction. A model shield machine was adopted to carry a laboratory test under 1 g to investigate surface settlement induced by earth-pressure-balance shield tunnel construction in unsaturated sandy soil. The surface settlement during the excavating observed in the test was compared with that predicted by general equations from previous studies and the modified. The results showed that surface settlement above shield machine obtained by the modified equation proposed here fits the test data better than those obtained by the general equations because of the introduced longitudinal width coefficient.

Recent extreme weather with heavy rainfall has brought new challenges to the operation of a karst tunnel as a large amount of groundwater flowing toward the tunnel in a short time exceeded the drainage capacity and caused high external water pressure. By evacuating the air in the inner space of the tunnel structure, an apparatus was developed to simulate the external water pressure on the tunnel structure. It was also employed to study the behavior of a highway tunnel structure with a large cross section considering cavities behind the liner. The results showed that the bending moment of the liner increased with the external water pressure, which helps to decrease the nonuniform distribution of the thrust force, and the maximum bending moment and eccentricity were generally located at the knees or side walls. Lower external water pressure contributed to reducing the eccentricity of the liner while higher external water pressure does not further reduce the eccentricity. Under external water pressure, fractures appeared first at the knee, then at the invert and finally at the vault. The air tightness tests under external water pressure also showed that the cavity decreased the bearing capacity of the liner and significantly aggravated the fractures in the liner nearby. The results help to estimate the bearing capacity of the liner under external water pressure, and the method of simulating external water pressure can also be used to study tunnels with different cross section geometries.

This paper examines the role of slurry TBM operational parameters, namely face, annulus and grout pressures, on ground deformation. The analysis includes both field data and computational modelling. Field data from the very successful Queens bored metro tunnels (6.9 m diameter) project in New York City is employed to illustrate phenomenologically how ground deformation was controlled via slurry TBM parameters. Computational modelling is then employed to support the experimental findings and to further investigate the relative influence of TBM parameters through parametric analysis. The results quantify the importance of the various pressures on controlling ground deformation.