Prolonged drought period causing cracks to form as bank dries.
The large number of ring tanks on the Downs has resulted in the large number of failures. Percentage of failures probably hasn’t changed.
Vegetation causing excessive drying and cracking in ring tank walls.
Poor construction i.e. soil types, compaction and moisture content.
Inadequate maintenance i.e. keeping top of ring tank grass and weed free and controlling erosion and gulleys.
Grass planted on the inside batter provides good erosion control and in the event of a leak, can slow the expansion of the breach. However, it can also cause significant problems.
Grass roots, especially African Star Grass, have been observed up to 9m into the bank. The roots of this grass have been observed to tunnel into banks in search of moisture in dry periods whereas native grasses tend to die off. The tunnels created by this grass can be large enough to create a potential leakage point.
Trees and large weed roots can cause a similar problem.
One attendee said that he has no vegetation at all growing on the majority of his ring tanks because he believes it contributed to the failure he experienced several years ago. Another is seriously considering the same approach on a storage that has given him a problem recently, but he is concerned about the control of erosion on the inside batter without the grass buffer.
Ashley Theuerkauf gave a comprehensive presentation on the dams he has repaired and what similarities he has seen within the failed banks. He also presented data from a Dynamic Cone Penetrometer that was able to compare the differences in compaction between repaired and older sections of dam walls. Repair costs can be incredibly high, especially if quality material has to be sourced from a considerable distance. Costs in the order of $5 - $6/m3 are common.
Pockets of dry soil were observed in 100% of all failed dams.
Roots were present in 50% of all failed dams.
Soil moisture content within the failure zone was highly variable.
There was a high variation in the distances between the water entry and exit points through the bank.
The blacker soils had seemingly formed hard individual blocks (like a stack of bricks) from 50mm cubes to 150mm cubes with significant cracks between each block.
Pipeline failures or poor installation practices caused some dam failures.
Geofabric – A Semi-permeable membrane placed vertically within the bank by means of an excavated trench. It could possibly prevent water tunnelling through the bank and rapid increase in flows (but not proven). High cost and difficult installation.
In dams constructed with dispersive soils, the application of Gypsum on the top and inside batter can improve soil structure and reduce the risk of tunnelling and erosion. Also placing a layer of topsoil on the inside batter could reduce the problems encountered with dispersive soil.
Physical barrier to reduce erosion created by waves i.e. something similar to swimming pool lane ropes.
Identify a grass or other plant that could prevent erosion and not have a severe impact on bank moisture content.
Keep outside batter and top of dam free of vegetation.
Keep top of dam cultivated.
Inside batters ranging from 5:1 to 6:1 could reduce erosion risk. Difficult to do in existing storages.
The concept involving wetting the top of embankment by either head ditch or trickle tape could cause internal slumping in dry walls and uneven moisture content due to variability in compaction.
Recommendations of Meeting:
Investigate the possibility of using EM surveys to identify areas of risk within the bank, eg dry, low compaction or excessively wet areas.
CDIL is funding Robert Baigent from WA to survey two storages on the Downs with Ground Penetrating Radar and Gamma-Ray Spectrometry. Andrew Smart from Precision Ag in Moree will survey with EM and GPS technology. The surveyed banks will be test drilled to ground-truth any significant differences in readings. If it is found that one or more of these machines can identify suspect areas it may be necessary to take readings as dams are being filled to (possibly) identify leaks before they penetrate right through a bank.
The possibility of using Polyacrylamide (PAM) to seal the bottom of leaking dams and maybe help in reducing the risk of wall failure will be investigated.
We would like to thank Ashley Theuerkauf, Mark Wheeler and Tim Eckert (DRN & M) and Peter Watts (FSA Consulting) for their time and the information they provided us with at this workshop.
If anyone has an idea on how we could better manage this problem could you please contact your local CDIL representative or Ian Hayllor on 0428 795548.