Taking Na-bentonite as filling medium and cement mortar as fissure wall

the filled fractures are made. By means of normal compression and lateral restraint

the constitutive relations of filled fracture with expansive medium and under the conditions of different fracture widths and different water contents

are discussed. A large number of stress-strain data are obtained. It is shown that the fractures filled with expansive medium can be divided into three stages: rheological phase

compression phase and the elastic stage in the pre-peak. At the beginning of the experiment

the filled fracture is in the rheological state; that is to say

the stress nearly remains unchanged

but the strain continues to increase. With the finite deformation and the normal stress increase

the filled fracture overall intensity increase

and demonstrate elastic characteristics gradually. Based on the results

the mechanical model of inverted Saint-Venant body is put forward; pre-peak constitutive equation of filled fracture with expansive medium is established; and its mechanism is analyzed.