An elasto-plastic stress-strain theory for cohensionless soil with curved yield surfaces is used for prediction of soil and pore pressure behavior in undrained triaxial tests on saturated sand specimens. Prediction of pore pressures is performed using the condition that no volume change occurs in undrained tests for any increment in load. The soil stress-strain relations are calculated from the effective stress-paths deduced from the applied deviator stress, the cell pressure, and the predicted pore pressure. Effects of back pressure and cavitation of pore water can be calculated from the theory. Predictions of stress-strain relations, pore pressure variation, expansion after cavitation, effective stress-paths, and critical confining pressures compare favorably with experimental results for loose and dense Sacramento River Sand. The importance of employing curved failure surfaces for realistic prediction of undrained behavior is emphasized, and it is shown that most soils exhibit curved failure surfaces.