Evaluate unsaturated soil behavior using constant water content triaxial tests

Abstract

The triaxial test has been extensively used to evaluate both saturated and unsaturated soil behaviors. The conventional triaxial test apparatus for saturated soils cannot be used to test unsaturated soils due to difficulties in soil volume and suction measurement. In 1961, a suction-controlled triaxial test apparatus was developed to investigate behavior of unsaturated soils. Since this development, the suction-controlled test has been widely used for unsaturated soil characterization. Most important concepts concerning unsaturated soil mechanics were developed based upon results from suction-controlled tests. However, the suction-controlled triaxial test on unsaturated soils, which is a drained test, is usually laborious, time-consuming, and costly, and may not be justifiable for routine engineering projects. The constant water content (undrained) test has been widely used to investigate saturated soil behaviors. However, for unsaturated soils, due to difficulties in direct, rapid, and reliable suction measurement, the constant water content test was rarely used for unsaturated soil behavior evaluation. In addition, accurate volume change measurement of unsaturated soils was a great challenge for researchers. Recently, the Modified State Surface Approach (MSSA) has been developed to calibrate unsaturated soil behaviors. According to MSSA, both results from suction-controlled and constant water content triaxial tests can be used for constitutive behavior calibration on unsaturated soils. In this study, a new triaxial test system was developed to investigate unsaturated soil behaviors through constant water content triaxial tests. To measure soil suction variation during testing, a new type of high-suction tensiometer was developed based on a commercial miniature pressure transducer. A 15 bar air-entry ceramic disc was used as the filter of the high-suction tensiometer. After saturation and calibration, this new type of high-suction tensiometer could be utilized for matric suction measurement on unsaturated soils with a maximum measurable suction up to 1100 kPa determined via a free evaporation test. To measure the volume change of unsaturated soils during triaxial testing, a photogrammetry-based method was developed by integrating photogrammetry, optical-ray tracing, and least-square estimation techniques. Through two validation tests on a stainless steel cylinder and a saturated sand specimen, the average point and total volume change measurement accuracy were determined to be approximately 0.065 mm and 0.05%, respectively. With this method, the conventional triaxial test apparatus for saturated soils can be used for triaxial testing on unsaturated soils without any modification. In addition to total volume change measurement, the newly developed photogrammetry-based method can also be used to investigate the deformation characteristics of soils during triaxial testing such as full-field deformation, volumetric strain non-uniformity, full-field strain distribution, and shear band evolution process. To evaluate the performance of the new triaxial testing system, a series of constant water content triaxial tests were carried out on unsaturated soils. New methods were proposed to characterize shear strength of the tested unsaturated soils. Also, an example was given to calibrate the constitutive behavior of an unsaturated soil based on results from the constant water content triaxial tests. Analysis results indicated that the proposed triaxial testing system is a cost effective and time efficient alternative to the suction-controlled triaxial testing system. In geotechnical and highway engineering, many projects involve unsaturated soils at shallow depths with low confining stresses (less than 100 kPa). To investigate the behavior of unsaturated soils at low confining stresses, the new triaxial testing system was simplified to a modified unconfined compression testing system. In this simplified system, negative air pressure (i.e., vacuum pressure) was used to provide the low confining stress for the triaxial tests. The high-suction tensiometers were used to monitor soil matrix suction variation during testing. A photogrammetric method was utilized for deformation measurements of unsaturated soils during triaxial testing. A series of undrained triaxial tests was also carried out to demonstrate the use of the modified unconfined compression testing system for unsaturated soil behavior evaluation under different confining stresses.