Lin ZHANG^{1}^{#}^{+}, Cunli CHEN^{2}, Tonglu LI^{1}, yan LI^{1}

^{1}Chang'an University, China, ^{2}Xi'an University of Technology, China

Constant moisture content is an important aspect of geotechnical engineering tests. In order to investigate the stress-strain，suction change and critical state characteristics of Q_{3} undisturbed loess from Lanzhou, China under constant water content, the GDS unsaturated triaxial instrument with controllable drainage conditions was used, and the pore-water pressure, axial stress and strain of the undisturbed loess with different initial suction during the isotropic compression and triaxial shear tests were measured. The analysis of the test results shows that: the greater the initial suction of the isotropic compression test, the smaller the compressibility and the greater the degree of suction reduction; the greater the isotropic stress, the greater the degree of saturation increase; the stress-strain curve of triaxial shear test is hardening type, the deviator stress has little effect on the decrease of suction, and mainly affects the increase of saturation; the critical state lines of undisturbed loess with different initial suction in q-p plane are parallel straight lines, and the critical state line of deviator stress and effective average stress can be expressed uniquely by the critical state line of saturated soil; with the increase of the initial suction，the critical state lines of unsaturated soil in e~lgp' plane moves up parallel，and the relation between their intercept and the initial suction is hyperbolic, the slope of the critical state line of saturated soil in e~lgp' plane is obviously smaller, nevertheless the relationship between the ratio of critical void ratio of unsaturated soil (under different initial suction) to saturated soil and failure saturation (or gas saturation) can be expressed by a normalized exponential function.