Tunnelling is an insidious form of sub-surface erosion, resulting in considerable damage even before surface manifestations are evident. Tunnel erosion is caused by the movement of excess water through a dispersive (usually sodic) subsoil.
Sheet erosion is often a precursor to the onset of tunnelling.
Compacted bare areas generate runoff which flows directly into the subsoil via surface cracks, rabbit burrows, or old root holes. Once concentrated in the subsoil the runoff causes the sodic clays to disperse and form a suspension or slurry. Provided there is sufficient gradient, the slurry is able to flow beneath the soil surface. If the subsoil is exposed through erosion or construction work, the slurry is able to rapidly flow onto the surface. Once formed, tunnels continue to enlarge during subsequent wet periods. Eventually tunnels reach a point where the roof collapses resulting in potholes and formation of erosion gullies.
- Tunnel erosion appears as a series of tunnels that form beneath the soil surface
- It is both a chemical and physical erosion process
- Associated with changes in catchment hydrology or uneven saturation of clay subsoils
- Usually associated with sodic soils derived from Triassic sandstone, Permian mudstones and re-deposition of these sediments in Quaternary deposits.
Prevention and Control
Tunnel erosion is particularly difficult and expensive to control and not always successful. A combination of mechanical, chemical and vegetative measures are usually required to control or prevent tunnel erosion.
Mechanical
Mechanical methods include the construction of banks, gully checks and terraces to direct runoff away from the tunnelling and deep ripping, chisel ploughing and other types of cultivation surrounding and above the tunnel erosion to promote more even infiltration.
- Contour banks and drains to divert surface water from susceptible areas
- Deep ripping, chisel ploughing and cultivation of all areas adjacent and above the eroded area
- Ripping and excavating of all tunnels to a depth of at least 15 cm below the tunnel base
- Ripping should be conducted in multiple directions finishing with ripping along the contour
- Repacking excavated tunnels with stable clays.
Chemical
Tunnel erosion is a chemical process, requiring chemical amelioration. Subsoils should be treated with high applications of gypsum (>10 t/ha) and / or lime to displace sodium ions with calcium ions within clays and provide an electrolyte balance.
- Apply gypsum / lime to displace Na+ with Ca++ and produce electrolyte effect
- Selection of either Lime or Gypsum depends on soil pH, electrolyte concentration and time required to stabilise soils
- Gypsum (Ca2S04) fast acting, readily dissolvable, slightly acidic, forms protective electrolyte quickly, more expensive, no deposits in Tasmania
- Lime (CaCO3) slow acting, dissolvability depends on grade, strongly alkaline, cheaper, readily available in Tasmania.
Vegetation
With all earth works in tunnel affected areas, re-establishing vegetation is essential to not only bind soils and provide a protective cover, but also to maximise uniform water use. In many cases topsoil may have to be brought onto the site to produce a level finish and establish a rapid vegetation cover. Resurfacing of the reclaimed area may also be required to ensure runoff is evenly distributed across the site.
- Essential to bind soils, protect surface soils and maximise uniform water use
- Non dispersive topsoil may have to be brought in
- Resurfacing area to spread runoff
- Revegetate with grass within reclaimed areas
- Establish trees above and around reclaimed area to maximise water uptake.
Guidelines and a Technical Reference Manual have been prepared to assist landholders, planners and engineers to identify, understand and manage dispersive soils.
Dispersive Soils and their Management - Guidelines for Landholders, Planners and Engineers (540Kb)
Dispersive Soils and their Management - Technical Reference Manual (3Mb)