Abstract: The propagation of deep cracks directly affects the long-term service performance of expansive soil channel slope with the deep excavation. To study the influence of different degree of crack expansion on the strength of expansive soil, the membrane simulation method was used to prepare the expansive soil shear samples with different proportion of crack plane, and the direct shear tests under different overlying load were carried out to test the peak shear strength of the samples with different degree of crack expansion. Analysis shows that under the test conditions, the peak shear strength exhibits a nonlinear three-stage attenuation law of rapid attenuation, slow attenuation, and rapid attenuation with the crack expansion. Through the normalized processing, the attenuation ratio curve of the peak shear strength under the two overlying loads is obtained. It is analyzed that the overlying load has little effect on the attenuation ratio of the peak strength of the expansive clod, and the variation range is within 10%. The threshold of crack area proportion in three stages of nonlinear attenuation can be determined by multi-stage fitting of attenuation curve. When the crack propagation is at the threshold between rapid attenuation (stage I) and slow attenuation (stage II), the attenuation ratio of peak strength is (42.9%~47.8%), and when the crack propagation is at the threshold between slow attenuation (stage II) and fast attenuation (stage III), the attenuation ratio of peak strength is (50.3%~55.1%).The research results provide theoretical support for the design, calculation, operation and maintenance of deep excavation channel slope in expansive soil area.

Key words: Expansive Soils, Crack Propagation, Peak Shear Strength, Attenuation Ratio