Unsaturated Soil Testing in the Laboratory

GDS provides a number of solutions for the testing of unsaturated soil. Each is based around the requirement to either define, or explore areas around the Stress Dependent Soil Water Characteristic Curve (SDSWCC) by varying the matric suction of the soil (the difference between the pore air pressure and the pore water pressure). The matric suction is the principal variable in defining the state of stress in an unsaturated soil.

GDS provides the ability to perform unsaturated testing in triaxial, consolidation, direct or simple shear, resonant column, hollow cylinder and true triaxial apparatus. All of these apparatus use the axis translation method, which is direct control of matric suction, using precise control of the pore water and pore air pressures through the use of a high air entry porous disk (maximum achievable suction equal to the high air entry value of the disc, chosen typically as either 500kPa or 1500kPa).  The axis translation method was a technique developed on the principal of elevating pore air pressures which, then allowed matric suction to be controlled using positive pore water pressures, preventing cavitation in the water drainage system. Based on the axis-translation principle, the matric suction can be controlled over a range far greater than the cavitation limit for water under negative pressure.

The soil–water characteristic curve (SWCC) is the relationship between suction and water content or degree of saturation for an unsaturated soil. It is now generally accepted that unsaturated soil behaviour is governed by two independent stress state variables, i.e. net stress and matric suction (Fredlund and Morgenstern, 1977). Therefore, it is necessary to consider the influence of net stress on the SWCC. However, the SWCC of a soil is conventionally measured by means of a pressure plate extractor (for example), in which no external stress is applied and volume change of the soil specimen is assumed to be zero. Hence, unsaturated testing within triaxial / consolidation equipment etc described below to determine the Stress dependant Soil Water Characteristic Curve (SDSWCC) is nowadays considered essential and the next logical step in unsaturated soil assessment.

Unsaturated Triaxial Testing (SDSWCC) 

GDS has been supplying unsaturated triaxial testing systems into Universities for more than 20 years. Working alongside HKUST in China (Charles Ng) to develop what we call the HKUST unsat method (method B in our range), and developing our own low range differential pressure transducer and suction probe, enabled us to stay at the forefront of unsaturated triaxial testing technology.

The axis translation technique is applied to triaxial testing with the addition of an unsaturated testing pedestal, which has a high air entry disc bonded in to it. This allows pore pressures to be applied and controlled at the base, along with higher air pressures applied to the top-cap of the sample. This is the principal in all of the axis translation direct control/measurement of matric suction systems. The challenge for the triaxial test is in the critical measurement of sample volume change. At GDS we give the customers the choice of 4 methods (methods A, B, C and D) which are all explained in the datasheet which can be found by following the above link.

All GDS triaxial systems can be upgraded into an unsaturated testing system, along with many systems from other manufacturers. Part of the GDS speciality is upgrading systems from other manufacturers where those manufacturers do not provide these advanced options.

Unsaturated Shear Testing (SDSWCC)

Compared to the triaxial test, the direct shear test is simpler to perform and requires shorter test durations due to the smaller drainage paths. A high air-entry ceramic disk is installed in the lower part of the direct shear box, air pressure is controlled using a GDS software controlled air pressure regulator, and pore water pressure is controlled using a GDS pressure/volume controller. Sample volume is measured directly from the shear travel and sample height (due to the fact that direct shear samples are constrained, sample volume measurement is relatively simple).

Unsaturated Consolidation Testing (SDSWCC)

Whether using the Rowe and Barden type system or a CRS type system, the sample is contained within a circular, fixed diameter ring equipped with a high air-entry ceramic disc at its base. Vertical stress is applied through a loading frame (CRS) or using the upper chamber pressure (Rowe and Barden), and axial force is measured with a load cell or directly from the pressure in the upper chamber. Because the radial deformation is zero for the K0 condition, the total volume change of the specimen is measured from the vertical displacement of the soil specimen. Using either of these apparatus, the volume change can be accurately measured and the stress-dependent soil–water characteristics curve (SDSWCC) can be determined. Note: Both the Rowe and Barden and CRS type apparatus, even if first supplied for fully saturated testing only, can be upgraded to an unsaturated version at any time.

Unsaturated Resonant Column Testing

The resonant column’s ability to measure soil stiffness at small strains with enough strain variability to measure the upper portion of the modulus degradation curve within soils, makes it an extremely attractive apparatus for testing unsaturated soils. A high air entry porous stone is fitted into the base pedestal, and pore air and water pressures are applied in the same way as a triaxial test. Note: A GDS resonant column apparatus can be upgraded to become an unsaturated RCA at any time.

Unsaturated Hollow Cylinder Testing (SDSWCC)

All GDS hollow cylinder apparatus can be upgraded to perform unsaturated hollow cylinder testing. A unique high air entry porous stone is fitted into the base pedestal, which consists of a number of circular bonds in the porous disks. The pore air and water pressures are applied in the same way as a triaxial test.

Unsaturated True Triaxial Testing (SDSWCC)

The GDS true triaxial apparatus can be upgraded to perform unsaturated tests. High air entry porous stones are fitted into the base actuator plate, and pore air and water pressures are applied in the same way they are in a triaxial test.