Keywords: soft rocks, tunneling, finite element analysis, numerical analysis, underground excavation, geotechnical engineering.

Due to the rapid development, in recent years, there are more and more underground excavation constructions in the western Taiwan area. There are different difficulties during underground excavation constructions, and most of the countermeasures are still on the empirical side. This study first reviewed the past underground excavation constructions in order to assess the common difficulties during construction. With calibrating with the physical modeling and in situ monitoring results, the finite element software ABAQUS [1] was used to study the behavior, including typical failure mechanisms for soft rock tunneling in the western Taiwan area.

The western foothill of Taiwan Island is mainly covered by young sedimentary rock formations, such as the Pliocene Cholan formation and the Plio-Pleistocene Toukoshan formation. Those formations are categorized to weak rock due to their low strength. Besides, they are generally poorly cemented, with high porosity and low weathering resistance.

Through a series of preliminary tests, a special artificial soft rock [2,3] was adopted for both the physical and numerical modeling. The physical model comprises three parts, i.e., a chamber with confining and draining function to make the soft rock block, a mechanical driving system to drive or rotate the pipe into the block, and a monitoring system to simultaneously record the readings from gauges such as pressure meters, laser scanner or TDR (time domain reflectometry) probes. To simulate different types of tunneling, both open type and close type (with cutting head) steel pipes are used in the physical modeling.

As the influence of tunneling to adjacent rock mass is three-dimensional, the three-dimensional finite element software ABAQUS was applied to simulate physical modeling as well as a real tunneling case. Through previous study on the mechanical behavior of those soft rocks, the extended Drucker-Prager Model was adopted as the constitutive law, and was implemented in the numerical simulation.

The comparisons show that different models fairly consistently predict the mechanical behavior of the rock mass influenced by the tunneling. And the finite element software could be used to more accurately study the mechanical behavior of soft rock tunneling, including face stability, failure mechanism, plastic zone development, etc.

1

Hibbitt, Karlsson & Sorensen Inc., "ABAQUS Version 6.3 User's Manual", Volume 1-3, U.S., 1988.

2

D.Z. Gu, M.R. Jafari, G. Mostyn, "An artificial soft rock for physical modeling", Geotechnical Engineering of Hard Soil-Soft Rocks, Anagnostopoulos et al. (ed.), Balkema, Rotterdam, 517-524 (1993).

3

Y.C. Liu, "Physical and Numerical Modeling of Shield Excavation in Soft Rock", Master Thesis of NCHU, Taichung, Taiwan, 2003.

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