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An area of tunnel gully erosion induced by the excavation of a drain through a residential subdivision in Southern New South Wales was examined.
Twelve months after excavation, tunnels extend 7 m from the drain wall and threaten residential blocks only 5 m further away.
A survey of a major tunnel by excavation and probing indicated tunnel formation along soil cracks around large (50 to 100 cm diameter) structural units.
Soil analyses rated the soil as highly dispersible by all the routine tests used by the Soil Conservation Service (SCS) of New South Wales, namely, Emerson's Crumb Test, Ritchie's Dispersal Index, and Floyd's Sticky Point Test, and showed that this dispersibility could be attributed to a high exchangeable sodium percentage (ESP) (mean 10.5). In addition, the soil had a low volume expansion (<5 percent) and was relatively impermeable.
These analyses have been compared with published criteria to show that the soil could have been identified as highly susceptible to tunnel erosion prior to excavation. This would have permitted an improved channel design, with a more gently sloping batter and stressed the need for lime stabilization, topsoiling, and immediate revegetation to minimize the possibility of tunnel development.
clays, piping (erosion), soil stabilization, soil tests
Soil conservationist, Wagga Wagga Research Center, Soil Conservation Service of New South Wales, Sydney,