Faced with extended dry periods, many Australian farmers are considering repairing failing or leaking dams to boost their water supply for when the rain returns. However, dam repairs are typically complex and costly projects that should only begin when the root causes of failure and suitable restoration plans are clearly understood.
Repairs are generally only recommended for larger, well-constructed dams. Before any maintenance starts, it is crucial to gather a comprehensive history of the dam—understanding who built it, what equipment was used, and how the dam has performed over time.
Typical Reasons for Dam Failure
Common factors contributing to dam failures include:
- Inferior construction methods
- Use of unsuitable materials
- Insufficient compaction during construction
- Soil contamination
While neglect or poor maintenance can worsen issues, they are rarely the primary cause of structural failure.
Diagnosing Dam Issues
Pinpointing the exact reason for a dam’s failure can be challenging. It is essential to determine whether water is leaking from the floor or the wall, and whether the problem is localised or widespread. Ideally, inspections should be conducted over several seasons—winter, spring, and summer—to fully understand the dam’s condition.
Inspection Guidelines for Landholders
Landholders should assess dam condition by:
- Inspecting the dam embankment for seepage, settling, cracking, or tunnelling
- Observing the waterline for unusual surface movement or whirlpools
- Checking downstream for soft areas or exceptionally lush vegetation, which may indicate seepage
- Testing soil firmness in suspicious areas with a steel probe or crowbar
- Looking for unusual signs such as exceptionally clear or bright yellow water, which could indicate unsuitable soils or tunnelling
- Regularly monitoring and photographing the dam to record changes in water level or visible issues
- Inspecting after significant rainfall to identify emerging problems
- Examining any breaches in the dam wall for unsuitable materials like sand, gravel, or topsoil
The Value of Soil Testing
Soil testing is essential for any repair job. Samples should be taken from the failure site, surrounding areas, and any potential borrow pits. Landholders can often use established guidelines to conduct basic assessments, including soil texture, aggregate stability, and shrinkage.
Methods of Repair
The best repair method depends on the location, cause, and extent of the failure. Options may include:
- Reworking existing dam materials
- Sourcing suitable soils from nearby
- Adding lime, gypsum, bentonite, or specialised sealing products
Expert advice is recommended before using manufactured products, as locally sourced materials often yield better results.
Typical Dam Repair Processes
Dam repairs usually involve:
- Clearing the site for safe machinery access
- Preparing the foundation to ensure strong bonding with repair materials
- Placing repair materials in layers no thicker than 100 mm, and moistening them as needed
- Compacting each layer thoroughly with a padfoot roller
- Ensuring the total thickness of repair materials exceeds 300 mm
- Shaping the final slopes to a maximum gradient of 3:1 (horizontal:vertical)
Safety Considerations
Major safety risks associated with dam repairs include unstable soils, steep slopes, heavy machinery, and dust exposure. All tasks should be handled by trained and experienced personnel, with strict safety procedures and reliable emergency communication in place.
Conclusion
By following these recommendations, and understanding the complexities involved, landholders can significantly increase the likelihood of successful, long-lasting dam repairs.