Abstract: The holistic analysis of all the pores within the range of experimental measurements is often performed when studying the fractal characteristic of soil pores under compression. However, this studies show that the fractal behavior of large pores has great difference with that of small pores, especially their response to compression. To illuminate the different fractal characteristics, the different responses of large and small pores under compression, and describe their variation laws, the pore-size distribution data of Wuhan clay with different dry density are obtained by mercury intrusion method. Afterward, these pore-size distribution data are analyzed and fitted to obtain the fractal dimensions based on fractal theory. The result shows that, there is a special critical pore-size in the pore volume and pore-size distribution curve. The distribution laws of pores larger and smaller than critical pore-size, as well as their responses to compression deformation, are significant different. The pores larger than the critical pore-size are defined as large pores and those smaller than the critical pore-size are defined as small pores. The distribution of small pores has stronger natural fractal characteristic, and does not vary with the dry density, which is referred to as intrinsic fractal characteristic. The distribution of large pores is far from the intrinsic fractal characteristic, which indicates that its fractal behavior is weaker than that of small pores. With the increasing of dry density, the fractal characteristic of large pores is gradually enhanced to continually approach the intrinsic fractal characteristic of small pores. Therefore, the intrinsic fractal characteristic of small pores can be used as a compacted reference indicator of clay, and the essence of compression process is that the large pores adjust to a stronger fractal distribution under compression and gradually approach the reference indicator.

Key words: mercury intrusion method, fractal theory, critical pore-size, intrinsic fractal characteristic