Rehabilitation of field tunnel erosion using techniques developed for construction with dispersive soils
M. A. Hardie A D , W. E. Cotching B and P. R. Zund CA Tasmanian Institute of Agricultural Research, University of Tasmania, PB 98, Hobart, Tas. 7006, Australia.
B Tasmanian Institute of Agricultural Research, University of Tasmania, PO Box 3523 Burnie, Tas. 7320, Australia.
C NRM South – Southern Tasmanian Natural Resource Management Committee, 13 St John Avenue, Newtown, Tas. 7008, Australia.
D Corresponding author. Email: Marcus.Hardie@utas.edu.au
Australian Journal of Soil Research 45(4) 280-287 https://doi.org/10.1071/SR06154
Submitted: 3 November 2006 Accepted: 28 May 2007 Published: 28 June 2007
Abstract
Past repairs of field tunnel erosion using mechanical treatments (deep ripping, contour furrowing, contour ripping, etc.) and reestablishment of perennial vegetation have often failed, resulting in further tunnelling. Techniques to prevent ‘piping’ (tunnel erosion) in earth dams constructed using sodic clays have not been used in the repair of field tunnel erosion; however, these techniques have the potential to reduce recurrent failure. Installation of an optical fibre cable in a Grey Sodosol and Grey Dermosol at a site near Dunalley, Tasmania, in November 2001, resulted in the formation of a 380-m-long tunnel erosion system. Detailed site investigations and soil analysis in 2004 indicated that the tunnel erosion resulted from the consequences of inadequate compaction during the installation of the optical fibre cable, and secondary processes such as capture of surface and subsurface flow, dispersion of sodic clays, and translocation of dispersed clay platelets through the poorly compacted fill. Repair works consisted of excavating the entire length of the tunnel system, chemical amelioration with gypsum, compaction of repacked fill to reduce internal porosity, and installation of sand blocks to capture and remove water moving along the reinstalled cable. Inspections conducted 2 years after completion of the repair works indicated the site to be stable; however, it is acknowledged that this is too short an interval to adequately assess the long-term stability of repair works and techniques employed at the site. Due to the costs associated with the application of dam construction techniques, their use for the repair of field tunnel erosion is likely to only be justified where continued erosion poses a risk of damage to high value infrastructure.
Additional keywords: sodicity, dispersion, piping, optic fibre cable, gypsum.
Acknowledgments
The authors wish to acknowledge the partnership and opportunity provided by Shane Beresford and Telstra who provided an opportunity and support to try undemonstrated approaches to repairing tunnel erosion. We wish to acknowledge the concern for detail and dedication of Robert Godrich and his team who did the earthworks, and Richard Doyle and Shaun Lisson for input on the manuscript. The work reported in this paper was conducted whilst the primary and secondary authors were employed by the Conservation Branch, Department of Primary Industries and Water, Tasmania (DPIW).
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