ABSTRACT: The present work attempts to simulate and analyze the mechanical behavior of block-in-matrix (BIM) colluvial materials using virtual specimens. This study carried out a series of numerical simulation as virtual mechanical tests in order to look into the important factors that affect the mechanical properties of BIM colluvium. It was found that (1) the strength of BIM colluvium increases with increasing block proportion, (2) rock-block orientation affects the anisotropy of mechanical properties, raising confining pressure reduces the anisotropy, and (3) the rock-block aspect ratio has a minor influence on the mechanical behavior/properties. The effect of specimen size on the mechanical behavior/properties of BIM colluvium was also evaluated. It was shown that the variation of mechanical behavior tends to converge as the specimen size approaches to a representative elemental volume (REV). Due to the inevitable difficulties in obtaining colluvium specimens close to the REV for laboratory tests, it may be possible to adopt virtual mechanical tests to complement insufficient data of laboratory tests, provided an appropriate calibration can be made.
Colluvial materials are often complex mixtures of geomaterials of various sizes, shapes, fabrics, and porosities depending on the parent strata and the slope-failure mechanism. Very often, colluvium from previously failed rock slopes is composed of both hard rock blocks and soft clay matrix, and may be considered as a complex composite geo-material which can be treated as a "blocks-in-matrix (BIM) material . Unlike the common laboratory tests for intact rock or soil, conventional laboratory testing of BIM colluvium specimens (that may contain various sizes of rock blocks) is likely impractical since the specimen size is usually much smaller than the representative elemental volume (REV) of the in-situ BIM geo-material. Medley (1994)[1, 2] treated the composition of mélange as an example of a "block-in-matrix rock", or "BIMROCK". Medley considered that in a BIMROCK, the mechanical properties of the "blocks" in the BIMROCK have to be at least twice as great as those of the "matrix" in order that failure surfaces be forced to negotiate around blocks. Lindquist (1994)  performed a series of triaxial tests using artificial mélange BIMROCK specimens, and concluded that both the strength and stiffness of mélange BIMROCK varied with volumetric block proportion (termed "block proportion" hereafter). He also observed that as the block proportion increased, the failure surface tended to develop along block/matrix contacts interface and the failure surfaces appeared tortuous ("winding" or "zigzag" nature). Although both are BIM materials, BIM colluvium and mélange BIMROCK differ in the nature of their formation. However, in both forms of mixtures the mechanical properties of block and matrix may differ by several orders (e.g., 100 or 1000 times different). This study aims to simulate and analyze the mechanical behavior of BIM colluvial materials through a series of numerical simulations or "virtual mechanical tests".
2. DESCRIPTION OF COLLUVIUM AT THE STUDY SITE
The study site is within a huge landslide area (about 2.3 Km2) named Li-Shan located in the west wing of the central mountain chain of Taiwan with an average elevation near 1900 m.