Abstract
Abstract Current methods available for testing the piping potential of soils in dams (pinhole test, hole erosion test, and slot erosion test) are limited to cohesive soils that maintain an open hole within the sample. These tests do not adequately simulate conditions within a zoned embankment, where zones of non-cohesive materials are present under relatively high confining stresses. A new apparatus, called true triaxial piping test apparatus or TTPTA, was developed for testing a wider variety of soils under a wider range of confining stresses, hydraulic gradients, and pore pressures than current tests allow. The TTPTA is capable of applying a range of confining stresses along three mutually perpendicular axes in a true triaxial test apparatus. Pore pressures are also controlled through regulated inlet and outlet pressures. The test determines the critical hydraulic gradient and, more importantly, the critical hydraulic velocity at which piping is initiated in non-cohesive soils. Detailed descriptions of the test apparatus and test method are presented, as are initial test results using TTPTA. Three sets of initial tests were conducted using uniform sand to (1) assess the repeatability of test results, (2) evaluate how the rate of change of inflow impacts the critical discharge rate at which piping is initiated, and (3) evaluate how the angle of seepage affects the critical velocity for piping initiation. These initial tests were conducted to evaluate the method and to help set test parameters for future testing. It is found that the TTPTA is capable of yielding fairly consistent results with 10 % scatter in repeat tests. The seepage angle tests demonstrate that the angle between seepage flow direction and the direction of gravity is an important factor to consider when evaluating piping potential. The rate of change in seepage also has a minor influence on test results, but a change in flow rate of 5 (mL/min)/min could produce reliable results. Based on the results, the hydraulic gradient is found to be a less reliable indicator of piping potential than the hydraulic velocity for non-cohesive soils. The TTPTA is capable of simulating conditions within small to medium sized embankments.
Highlights
Numerous earth structures, earth dams, commonly fail by piping
INITIAL TEST RESULTS Three sets of tests were run on a uniform fine SP sand
The purpose of these initial tests was to 1) to assess the repeatability of test results, 2) to evaluate how the rate of change of in-flow rate impacts the critical discharge rate at which piping is initiated, and 3) to evaluate how the angle of seepage affects the critical velocity for piping initiation
Summary
Earth dams, commonly fail by piping. approximately half of all dam failures are due to piping (Foster, Fell, and Spangle, 2000), with approximately 33% of all piping failures possibly attributed to backwards erosion piping (Richards and Reddy, 2007). Dispersive soils are highly prone to piping failure (Aitchison, et al, 1963) and these tests were developed to evaluate a soil’s piping potential in areas with dispersive soils Prior to these early laboratory tests, empirical methods were available to assess piping potential. Bligh’s method was later improved by the work of Lane (1934) These empirical methods have a significant shortcoming; they are based on seepage flow paths consistent with internal erosion and do not adequately address the potential for backwards erosion. Terzaghi’s theory does not consider the physico-chemical or other properties of soils that influence piping potential It was originally based on the case where seepage flow is vertically upward into a cofferdam, acting in direct opposition to the downward force of gravity. Does this theory apply to the case with seepage exiting at a downward angle on the downstream slope of an embankment dam? No standardized laboratory tests have been developed to assess piping potential in non-cohesive soils that could be used to evaluate piping potential in a way that would take all these other factors into account
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