Using a single specimen and a single apparatust. Full suction double-cell extractor can measure full-suction drying and wetting stress-dependent soil-water characteristic curves (SDSWCCs) and stress-dependent permeability functions (SDPFs).

During a test, the Geo-Experts can apply various isotropic and anisotropic stress states to a soil specimen in triaxial space (i.e., (p – ua)-q-s space, where p is total mean stress; ua is pore-air pressure; q is deviatoric stress; and s is soil suction). Desired stress paths can be applied by controlling axial stress, cell pressure and soil suction independently. Due to the flexible controls of stress paths, measured SDSWCC and SDPF can thus be more closely represent actual field condition.

On the other hand, the Geo-Experts can apply and control soil suctions ranging from 0.1kPa to 300MPa using three different techniques, namely Axis-translation, Osmotic and Vapour equilibrium techniques. A full-suction SDSWCC can thus be obtained using a single specimen and a single apparatus. In addition, a SDPF with wide range of suctions can also be determined.

During SDSWCC and SDPF measurements, any total soil volume changes due to applied stresses and suctions can be measured by a double-cell extraction device and a high-accuracy DPT accurately. Axial displacement of the soil specimen can also be measured by a linear variable differential transformer (LVDT).

Key Features:
(1) Can measure full-suction drying and wetting stress-dependent soil-water characteristic curves (SDSWCCs) and stress-dependent permeability functions (SDPFs) using a single specimen and a single apparatus;
(2) Can apply stress states in triaxial space;
(3) Can control suctions ranging from 0.1kPa to 300MPa by three different control techniques;
(4) Can measure total soil volume changes using a double-cell extraction device and a high-accuracy differential pressure transducer (DPT);
(5) Can monitor applied vertical stress using high-precision internal pressure transducer

Working principle:
1、Controls of stress and suction
The Geo-Experts allows a soil specimen to be tested under (p – ua)-q-s space. Any isotropic and anisotropic stress states can be applied by controlling an axial force provided by a vertical loading system in combination with a cell pressure applied by pressurised air supply independently.
On the other hand, three different techniques can be used to apply and control suctions in the Geo-Experts. They are:
(1)  Axis-translation technique (ATT)
(2)  Osmotic technique (OMT)
(3)  Vapour equilibrium technique (VET) (for SDSWCC only)
2、Measurement of total soil volume change
The Geo-Experts equips a double-cell extraction device. An open-ended, bottle-shaped inner cell is mounted inside the outer cell. The inner cell is initially filled with water. A reference tube with a fixed water level is mounted on the external sidewall of the inner cell. Any changes of water level inside the inner cell due to soil volume change would result in pressure difference between the inner and the outer cells. The differential pressure is monitored by the high-accuracy DPT. Total volume change of the soil specimen during a test can then be determined through a calibration curve.
3、Measurements of SDSWCC and SDPF
The Geo-Experts can measure steady-state SDSWCC and SDPF in a single test. After applying a desired stress state to a soil specimen, soil suction is controlled by any of the three techniques. When steady-state is reached at each suction level, both equalised gravimetric and volumetric water content are determined and a SDSWCC is obtained. At the same suction level controlled by the ATT and OMT, the Geo-Experts can also measure SDPF based on Darcy’s law. The hydraulic head at the top and the bottom boundaries of the soil specimen can be controlled independently. A known hydraulic head gradient across the specimen is hence established. Outflow rate at the bottom boundary is measured by recording mass change of a solution vessel. At steady state, permeability is determined by dividing the steady-state outflow rate by the applied hydraulic head gradient.

Some measured SDSWCCs and SDPFs are shown in Figs 1 and 2, respectively, to illustrate the effects of stress state.

Figure 1. Measured SDSWCC (Ng & Chen 2008)

Figure 2. Measured SDPFs at 1D stress condition at (a) 0kPa, (b)40kPa and (c) 80kPa (Ng & Leung 2011)