HYDRAULIC CONDUCTIVITY OF GRAVEL SAMPLES USING A MODIFIED PERMEAMETER

This presentation describes a new permeameter that was developed to perform laboratory hydraulic conductivity tests on gravels with hydraulic conductivity values ranging from 0.1 to 2 m/s. A small diameter riser is connected to a large diameter cylinder, which holds the coarse-grained specimen saturated in a water bath. The release of pneumatic pressure applied to the free surface in the riser induces an underdamped oscillatory response of the water level in the riser, similar to an underdamped in situ slug test response in monitoring wells. A closed form model was derived to analyze the measured oscillatory hydraulic head data to calibrate the minor losses in the permeameter and the hydraulic conductivity of the specimen by performing tests with and without a specimen. The average model error of calibrated pressure head values in the riser for the tests considered is on the order of 5% of the initial displacement of about 2 cm. The hydraulic conductivity values presented are an average of replicate tests, which are within 10% of all tests performed on that specimen.

Two gravels with permeability values ranging from 0.5 to 1.5 m/s and noticeably different particle shapes were each arranged to specimens of the same six gradations from nine grain size ranges and tested. The hydraulic conductivity was higher for specimens that were larger in diameter, more uniform, and had a higher porosity, as expected. The hydraulic conductivity was then estimated using the Kozeny-Carman equation that considers the specific surface area of each specimen. This equation considers the porosity and specific surface by considering the grain size distribution, angularity, tortuosity, and sphericity of these specimens if the data are available. Marbles were also tested because the specific surface was easily calculated without the new methods proposed here to determine the average shape and tortuosity of the specimens. This equation estimated hydraulic conductivity values within 5% of the measured value for the reference marbles and within 20% of the measured value of the gravels on average, where the average shape and tortuosity were determined by new methods. Using the Kozeny-Carman equation with the described methods gives an excellent hydraulic conductivity prediction, especially considering that this is an order of magnitude estimate.