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Using Pneumatics to Perform Laboratory Hydraulic Conductivity Tests on Gravel with Underdamped Responses

TitleUsing Pneumatics to Perform Laboratory Hydraulic Conductivity Tests on Gravel with Underdamped Responses
Publication TypeConference Paper
Year of Publication2011
AuthorsJudge AI, Ostendorf DW, Degroot DJ
Conference Name2011 American Geophysical Union Fall Meeting
Date Published12/2011
PublisherAmerican Geophysical Union
Conference LocationSan Francisco, CA
Other NumbersAbstract #H53F-1473
Keywords1828 Hydrology, 1838 Hydrology, 5114 Physical properties of rocks, Groundwater hydraulics, Infiltration, Permeability and porosity
Abstract

A permeameter has been designed and built to perform laboratory hydraulic conductivity tests on various kinds of gravel samples with hydraulic conductivity values ranging from 0.1 to 1 m/s. The tests are commenced by applying 200 Pa of pneumatic pressure to the free surface of the water column in a riser connected above a cylinder that holds large gravel specimens. This setup forms a permeameter specially designed for these tests which is placed in a barrel filled with water, which acts as a reservoir. The applied pressure depresses the free surface in the riser 2 cm until it is instantly released by opening a ball valve. The water then flows through the base of the cylinder and the specimen like a falling head test, but the water level oscillates about the static value. The water pressure and the applied air pressure in the riser are measured with vented pressure transducers at 100 Hz. The change in diameter lowers the damping frequency of the fluctuations of the water level in the riser, which allows for underdamped responses to be observed for all tests. The results of tests without this diameter change would otherwise be a series of critically damped responses with only one or two oscillations that dampen within seconds and cannot be evaluated with equations for the falling head test. The underdamped responses oscillate about the static value at about 1 Hz and are very sensitive to the hydraulic conductivity of all the soils tested. These fluctuations are also very sensitive to the inertia and friction in the permeameter that are calculated considering the geometry of the permeameter and verified experimentally. Several gravel specimens of various shapes and sizes are tested that show distinct differences in water level fluctuations. The friction of the system is determined by calibrating the model with the results of tests performed where the cylinder had no soil in it. The calculation of the inertia in the response of the water column for the typical testing setup was also verified by performing tests without soil. The friction coefficient of the cylinder base below the specimen where the water enters and exits throughout the test has a minor loss which is determined by analyzing these results. The hydraulic conductivity is then calculated by calculating the friction of the system and subtracting the friction loss from the frictional component of the damping frequency calibrated to the measured data for each test. This allows for a very precise and accurate calculation of the hydraulic conductivity of the soil tested because the closed form analytical model developed and used considers the underdamped responses which fit to the measured data unique to every test more easily than any other method. The average error in predicting the head values for preliminary results is 1 mm, or about 4% of the initial displacement for all tests.