Soils
Selecting appropriate materials for filters and drains is a crucial component of successful embankment repairs and construction. A couple questions to ask yourself:
- Do I have materials suitable for construction?
- Have I sized the filters and drains properly?
On-site Materials
It is rare to find natural materials that can satisfactorily serve as filters. The filter and drain materials will likely need to be imported. Why are on-site materials unsuitable for filters?
- On-site materials often have variations in gradations and quantities.
- Natural deposits often have coarse particles that don't meet the filter requirements necessary to prevent segregation.
- Natural materials typically contain clay/silt particles, which decreases permeability dramatically. A soil with as little as 7% clay can be essentially impermeable.
Is processing on-site materials an option?
- The cost of bringing in new material is likely less than the cost of the additional effort required to test on-site materials and verify material properties during construction.
Where can on-site materials be used for cost savings?
- If the content of fines is adequate (<5%), there is potential for on-site materials to be used in the shell downstream of the filter.
- On-site materials may be considered for random fill used in the top couple feet of the dam within the freeboard zone.
Filter and Drain Materials
Modern practice and guidelines dictate the use of zoned embankments that include, at a minimum, engineered filters downstream of earth cores and around conduit penetrations and toe drains. Engineered filters include single stage and two stage filter configurations. Two stage configurations include a filter layer and drain that offer greater protection against concentrated flows and the potential for seepage that can occur at contaminations in the filter layer.
Sand Filter
- Provides a barrier between the core of the dam and the downstream shell. The sand filter captures flow traveling through the embankment and prevents the movement of soil particles.
Drain Rock
- Accommodates any concentrated seepage that is greater than the capacity of the filter layer.
Typical Filter and Drain Configurations
Note: The content on this page was originally created as part of DamOutletWorks.org (DOWL, 2018). It has subsequently been updated and reformatted as part of the Dam Safety Toolbox.
Revision ID: 4754
Revision Date: 12/02/2022