The pinhole test provides one method of identifying the dispersive characteristics of clay soils that are to be or have been used in earth construction. The piping failures of a number of homogeneous earth dams, erosion along channel or canal banks, and rainfall erosion of earthen structures have been attributed to the colloidal erosion along cracks or other flow channels formed in masses of dispersive clay (2).
This test method models the action of water flowing along a crack in an earth embankment. Other indirect tests, such as the double hydrometer test (Test Method D 4221
Method A and Method C of the pinhole test require the evaluation of cloudiness of effluent, final size of the pinhole, and computation of flow rates through the pinhole in order to classify the dispersive characteristics of the soil. Method B requires only the evaluation of the cloudiness of effluent and final size of the pinhole to classify the dispersive characteristics of the soil. The computation of flow rates through the pinhole in Method A serves primarily as a guide to the proper equipment and specimen performance under sequential pressures applied during the test. All methods produce similar results and any method can be used to identify dispersive clays.
The use of Method A or Method C results in the accumulation of data relative to sequential flow rates through the pinhole and consequent enlargement or erosion of the hole. The pinhole erosion test was developed for the purpose of identifying dispersive soils and is not intended to be a geometrically scaled model of a prototype structure. Since the theory of similitude was not used in the design of the pinhole test, quantitative data are not obtained. The quantity of flow through the pinhole, amount of soil erosion, or the rate of soil erosion should not be extrapolated to actual field conditions (3). However, such data may be useful in performing qualitative evaluations of the consequences of such erosion in terms of dam failure, loss of life and property. They also may be used in considering the cost effectiveness of defensive design measures necessary to minimize the effects of failure due to dispersive clays. For example, the amount of colloidal erosion that will occur in a soil classed as ND2 (very slightly dispersive) will be very small for a relatively long period of time. Such erosion may not be significant in evaluating the cost-benefit relationships in projects where public safety is not involved or where normal maintenance procedures will handle the problem. In such cases, classifying the soil as ND (nondispersive) using Method B of the pinhole test should be adequate.
Pinhole tests that result in classifying soil as slightly dispersive (ND3 by Method A or Method C or SD by Method B) indicate high uncertainty about the existence of significant problems to be considered in the design or stability of a structure. In such cases, it is advisable to resample and test a number of other soils from the same area to generate an adequate statistical sample for problem evaluation. The original slightly dispersive sample may come from an area on the edge of a more highly dispersive soil.
In a few physiographic areas or geoclimatic conditions, or both, neither the pinhole ......
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