True Triaxial Piping Test Apparatus for Evaluation of Piping Potential in Earth Structures

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.