A MODIFICATION OF THE PINHOLE DISPERSION TEST AS A METHOD OF MEASURING SEDIMENT ERODIBILITY

The pinhole dispersion test has been used for thirty years by geotechnical engineers to identify dispersive soils. The goal of this study was to determine if the test could be suitably modified to measure the erodibility of sediment samples in the laboratory in a relatively simple, inexpensive way. The ASTM method involves packing a 3.8-cm-long sediment sample into a 3.8-cm-diameter metal cylinder, attaching it to a base plate, creating a horizontal 1-mm hole through the sample, attaching an end plate over screens and a layer of gravel, and running water through the hole for 5-minute periods at increments of head from 5.08 to 101.6 cm (2 to 40 in). Water discharge exiting the sample at any given head indicates the degree to which the hole diameter, and thus cross-sectional area, has been increased by erosion. Experimentation showed that the end plate for the testing device is not necessary and can limit discharge rate; a large washer is sufficient to hold the screens and gravel in place. Sample length can also be increased to 7.6 cm. The test can be partially automated by allowing the discharge to flow into a tipping rain gauge with a digital data logger. Testing on a range of stream bank sediment samples from the southeastern Piedmont and Coastal Plain indicates that intact specimens carved to fit the cylinder exhibit wide variability due to fabric inhomogeneities. Disturbed samples lightly packed into the test cylinder using a single push of a piston erode too quickly to be distinctive from one another. The best results are obtained by uniformly pounding layers of slightly moist, homogenized sediment into the cylinder. Still, multiple runs of the same sample do not always produce consistent results, and pre-test moisture content affects compaction and erodibility. At heads of 63.5 to 101.6 cm (25 to 40 in), the pinhole through erodible, non-dispersive samples tends to enlarge enough to reach the flow capacity of the testing device or rain gauge, thus data points for many samples converge at high head. We conclude that the modified pinhole dispersion test, while simple to perform, produces results that are only qualitatively useful as a test of sediment erodibility.