Milatz, MariusMariusMilatzGrabe, JürgenJürgenGrabe2022-05-032022-05-032014-076th International Conference on Unsaturated Soils (UNSAT 2014)http://hdl.handle.net/11420/12420This contribution deals with laboratory tests on unsaturated sand specimens for the investigation of the relationship between matric suction and stress-strain behaviour. The main question concerns the effects of matric suction on soil stiffness and strength, as well as a change in matric suction due to pore water pressures and saturation changes on cyclic loading. For this purpose triaxial tests and simple shear tests with either monotonous or cyclic loading are performed on unsaturated sand specimens. The laboratory tests can either be performed as controlled suction tests with drained boundary conditions or as constant water content tests. Similar to the conditions encountered in nature, matric suction is applied in the form of negative pore water pressures in the controlled suction tests, while the pore air pressure remains atmospheric. The "-special"- challenge when investigating sands in unsaturated conditions is the low level of matric suction: Both in nature and in laboratory tests, in these soils the effects of suction are limited to the range 0-10 kPa. For this reason, in the laboratory tests discussed in this paper, the control of low suction values is obtained by negative pore water pressures applied at the bottom of the specimens through a porous disc, using a computer-controlled vacuum regulator with a high accuracy of 0.05 kPa. This technique was also applied to the determination of the Soil-Water Characteristic Curve (SWCC) for drying and wetting of the soil and is used in combination with a triaxial and simple shear device. The measurement of suction on loading is obtained by tensiometers placed at specimen mid height in the triaxial test and in the loading topcap of the simple shear device: This allows for the control of applied suction and the measurement of change in matric suction during monotonous or cyclic loading. © 2014 Taylor & Francis Group.enConference PaperOther