RESEARCH INTO THE DEFORMATION AND STRENGTH CHARACTERISTICS OF PARTLY SATURATED SOIL IS DESCRIBED. EQUATIONS ARE DEVELOPED FOR THE APPLICATION OF THE PRINCIPLE OF EFFECTIVE STRESS TO THE STRENGTH RELATIONSHIPS. TESTS WERE CONDUCTED ON CYLINDRICAL SAMPLES BROUGHT TO FAILURE BY AXIAL COMPRESSION. THE FOLLOWING FOUR DIFFERENT CONDITIONS OF DRAINAGE WERE USED WITH COMPACTED SAMPLES: UNDRAINED TESTS, CONSTANT WATER CONTENT TESTS, CONSOLIDATED-UNDRAINED TESTS WITH FULL SATURATION, AND DRAINED TESTS WITH FULL SATURATION. APPARATUS USED FOR THE MEASUREMENT AND CONTROL OF PORE WATER AND PORE AIR PRESSURE IS DESCRIBED. TIME HAS AN IMPORTANT INFLUENCE ON THE OBSERVED VALUES OF PORE WATER AND PORE PRESSURE. SEVERAL INDEPENDENT FACTORS ARE INVOLVED: (1) CHANGE IN PORE PRESSURE AT CONSTANT WATER CONTENT AND CONSTANT ALL ROUND STRESS, AND (2) TIME LAG IN EQUALIZATION OF PORE PRESSURE THROUGHOUT THE SAMPLE DURING SHEAR. RESULTS OF TRIAXIAL TESTS AND A DETAILED SERIES OF CONSOLIDATED-UNDRAINED TESTS AND DRAINED TESTS ON SAMPLES OF THE SAME SIZE SHALE SATURATED BY A BACKPRESSURE ARE GRAPHICALLY PRESENTED. TEST RESULTS SHOW THAT IT IS NECESSARY TO USE A MODIFIED MATHEMATICAL EXPRESSION FOR EFFECTIVE STRESS WHEN THE DEGREE OF SATURATION DROPS APPRECIABLY BELOW 100%. THE USE OF THE MORE GENERAL EXPRESSION FOR EFFECTIVE STRESS TAKES ACCOUNT OF BOTH PORE WATER AND PORE AIR PRESSURE. THE X FACTOR VALUE OBTAINED FROM TESTS ON A SOIL DRAINED BY SUCTION AND FROM TESTS ON A COMPACTED SOIL INDICATE ITS DEPENDENCE ON DEGREE OF SATURATION, THOUGH NO SIMPLE GENERAL RELATIONSHIP IS APPARENT FOR COMPACTED SOILS. THE DIFFICULTIES ENCOUNTERED IN OBTAINING RELIABLE VALUES OF PORE WATER AND AIR PRESSURE, AND OF EFFECTIVE COHESION INTERCEPT AND EFFECTIVE STRESS, POINT TO THE CARE WHICH MUST BE USED IN DRAWING CONCLUSIONS FROM LABORATORY TESTS. TEST RESULTS MAY INDICATE COHESION INTERCEPTS OF CONSIDERABLE MAGNITUDE NOT ONLY IF INSUFFICIENTLY FINE POROUS DISCS ARE USED TO MEASURE PORE WATER PRESSURE IN PARTLY SATURATED SOIL, BUT ALSO IF TESTS WITH BASE MEASUREMENT OF PORE PRESSURE ARE RUN TOO RAPIDLY FOR PORE PRESSURE EQUALIZATION TO OCCUR. TEST RESULTS DO NOT SUPPORT THE VIEW THAT THE COHESION INTERCEPT IS IN GENERAL, ZERO. A VALUE OF 2.3 PSI IS CLEARLY INDICATED FOR THE COMPACTED CLAY SHALE WHEN SATURATED. OTHER TESTS INDICATE A VALUE OF ABOUT 1.4 PSI FOR THE BOULDER CLAY WITH 18% CLAY FRACTION. ONLY THE BOULDER CLAY WITH 4% CLAY FRACTION APPEARS TO HAVE A NEGLIGIBLE COHESION INTERCEPT, AT LEAST IN THE FULLY SATURATED STATE.