Air Volume Change Measurement in Unsaturated Soil Using a Digital Pressure-Volume Controller

Bankole A. Adams, Dvoralai Wulfsohn, and Delwyn G. Fredlund

Summary of the above paper by Karl Snelling, GDS Instruments.

ABSTRACT

This paper reports on the results of an evaluation of a GDS advanced digital pressure-volume controller as an air volume change indicator.  Effects of temperature, confined volume, and pre-compression the air its pressure-volume characteristics and the controller response are studied.  The GDS air pressure controller was also used to measure air volume changes of unsaturated soil specimens undergoing isotropic consolidation and axial loading in constant water content tests in a triaxial cell.

MAIN CONTENT

The paper starts with a scientific, yet simple to follow description of air volume change measurement - an excellent introduction into unsaturated soil testing.  A detailed, impartial description of the GDS digital air pressure-volume controller is then followed by a detailed study on the controllers pressure-volume response.

Results from 2 main areas of experiment are shown:

1) Response of the device from a known volume change

In these experiments, the GDS air pressure-volume controller was set to a constant target pressure, whilst a known volume change was introduced into the controller.  The total air volume, air volume rate and temperature were all varied independently.  As a result of experiments conducted and subsequent recommendations from this paper, the GDS air pressure-volume controller was "tweaked" for control of low air pressures with a reduction of the dead band (the band of pressure within which the controller accepts that it has achieved the set target pressure). 

2) Air volume change due to deformation of a soil specimen

Constant water tests were performed on 140x70mm cylindrical unsaturated clay loams.  The constant water test involved shearing with the pore-air drained, but the pore water undrained, such that the gravimetric water content remains constant.  The GDS air pressure-volume controller thus measures the air volume changes that occur in the specimen as it undergoes stress changes.  This arrangement, termed the axis-translation technique, allows pore air to drain into the artificial atmosphere maintained by the GDS controller, while the pore water drains to ambient atmospheric conditions.

CONCLUSIONS

The paper concludes with the following key points:

"By taking adequate precautions such as maintaining essentially isothermal conditions, preventing leakage at connecting points, pre-compressing the confined air space, and minimizing the total volume of the air confined space, the digital pressure-volume controller was found to be an excellent device for measuring air volume changes".