This paper presents a series of unconsolidated-undrained (UU) triaxial compression tests for investigating the behavior and failure mechanism of geotextile-reinforced clay and the effects of sandwiching nonwoven geotextile in a thin layer of sand (sandwich technique) on improving the shear strength of reinforced clay. Test variables include confining pressures, the number of geotextile layers, and thicknesses of the sand layers. The mobilized tensile strain of reinforcements, estimated according to the residual tensile strain by using a digital image-processing technique, was used to directly quantify the effects of soil-geotextile interaction on the shear-strength improvement. The test results showed that the shear strength of reinforced clay increased as the number of geotextile layers was increased. Failure patterns were changed from classical Rankine-type failures for unreinforced soil specimens to bulging (barrel-shaped) failures between adjacent geotextile layers. The effectiveness of reinforcing clay by applying nonwoven geotextile can be attributed to an increase in the apparent cohesion of the reinforced clay specimen. Regarding the sandwich technique, the test results revealed that layers of sand encapsulating the reinforcement can effectively provide an improved soil-geotextile interaction, thereby enhancing the shear behavior of reinforced clay. The shear strength increased as the thickness of the sand layer was increased. An optimal value of sand-layer thickness for maximum shear-strength improvement was not observed at large confining pressures. An appreciable shear-strength improvement was still observed when the sand-layer thickness was increased from 15 to 20 mm at sigma(3) >100 kPa. The sandwich technique contributes to shear-strength improvement by increasing the friction angle of reinforced specimens. The mobilized tensile strain and force of the geotextile increased as the number of geotextile layers, thicknesses of the sand layers, and confining pressure were increased. The mobilized tensile strain and force were strongly correlated to the strength difference between reinforced and unreinforced soil. This experimental finding demonstrated that mobilized tensile strain and force directly contribute to the shear-strength improvement of reinforced clay. (C) 2015 American Society of Civil Engineers.