Grain-Size Distribution Effects on the Mechanical Behavior of Granular Soil in Response to EPBS Tunneling

Abstract

A set of model tests using a miniature Earth Pressure Balanced Shield (EPBS) machine are carried out to examine the effects of grain-size distribution on the mechanical behavior of granular soil in response to EPBS tunneling. The combined effects of particle size (D50) and tunnel depth on the vertical movement and volume loss of ground are studied. A discussion of the implications of the soil arching effect and volumetric strain associated with D50 to the results is also provided. The results show that the deformation patterns of ground are highly dependent on the grain-size distribution of the soil, especially for relatively deep tunnels. The value of trough width parameter (k) for fine soil exhibits an obvious nonlinear increase with the depth. However, it is linear for the soil with a relative large grain size. The particle size has a significant influence on the variation of soil volume loss at the surface level, especially for relatively deep tunnels. Typically, when EPBS tunneling is undertaken in granular soil, a target value of the pressure in the chamber of the EPBS machine and grouting pressure in the tail are required to define to control the soil volume loss within an acceptable range, which is important to keep surface settlement in compliance with maximum specified values. The results in this study highlight the effects of the grain-size distribution of granular soil on the surface settlement trough and volume loss. Potentially, data from testing of model tunnels could provide a useful basis for making this assessment. According to the findings in this study, for fine granular soil, a tunneling engineer with a target volume loss at the surface level will have to target less volume loss at the tunnel level than the desired maximum value for the surface volume loss. In the granular soil with a relatively large grain size, the target values for volume loss at tunnel level can be greater than the desired maximum value for the surface volume loss. The results in this study confirm that the settlement trough in the fine granular soil is steeper than that in the granular soil with a relatively large grain size. This suggests that tunneling in granular soil with a relatively large grain size is likely to induce a wider settlement trough but probably causes less damage to buildings, whereas in fine granular soil, the settlement trough is likely to be narrower but may potentially cause greater building damage.