Burying a plastic septic tank requires meticulous planning and execution to ensure its stability, longevity, and proper function. The key is to provide a stable foundation, prevent tank flotation, and backfill correctly to support the tank's structure.
Proper installation is paramount for any septic system, especially with plastic tanks which are lighter and require specific handling to prevent issues like shifting or floating.
1. Site Selection and Preparation
The first critical step in burying a plastic septic tank is choosing the ideal location.
- Sub-Soil Drainage: Begin by selecting a site with good sub-soil drainage. This is crucial to ensure that any potential leakage or overflow from the system is managed naturally without saturating the surrounding soil or contaminating groundwater. Poor drainage can lead to system failure and health hazards.
- Accessibility: Ensure the site allows for future access for maintenance, pumping, and inspections. Consider the path for heavy equipment during installation and future servicing.
- Regulations: Always check local building codes and health department regulations regarding setbacks from property lines, wells, foundations, and water bodies.
2. Excavation Dimensions
Once the site is chosen, prepare the pit for the tank.
- Pit Size: Excavate a pit that is sufficiently larger than the tank itself. You must allow clearance around the tank to properly place and compact backfill around the lower half of the tank. This extra space (typically 12-24 inches on all sides) is vital for proper compaction and ensuring the tank is uniformly supported.
- Depth: The depth should accommodate the tank's height, the required bedding material, the inlet/outlet pipe slopes, and the final cover depth (typically 6-12 inches of soil cover).
3. Bedding Preparation
The foundation for your plastic septic tank is critical for its structural integrity.
- Material: Provide well-compacted or undisturbed bedding of a sand/gravel mixture or clean, granular soil. This provides a stable, uniform base that will not settle unevenly over time, which could stress the tank.
- Thickness: The bedding material should be at least 6 inches minimum in rock terrain to protect the tank from sharp points and provide adequate cushioning. In other soil types, a similar depth is generally recommended for optimal support.
- Shaping: Shape the bedding to suit the underside of the tank. This ensures that the tank's entire bottom surface is supported evenly, preventing point loading or stress on specific areas that could lead to cracks or structural failure. Use a screed or level to achieve a perfectly flat and compact surface.
4. Tank Placement and Water Filling
Placing the tank correctly and preventing flotation are key considerations for plastic tanks.
- Lowering the Tank: Carefully lower the plastic septic tank into the excavated pit, ensuring it is centered and level. Use appropriate lifting equipment.
- Preventing Flotation: Plastic tanks are lightweight and can float, especially in areas with high water tables or during heavy rains. To counteract this:
- Fill with Water: As you begin backfilling, simultaneously fill the septic tank with water. This internal water weight counteracts the buoyancy from external groundwater or backfill pressure, preventing the tank from shifting or floating out of position. Maintain the water level slightly above the level of the backfill.
- Anchoring (Optional but Recommended): In areas prone to high water tables, consider using anti-flotation devices like concrete deadmen anchors or hold-down straps. These are typically strapped over the tank and anchored into the earth on either side, providing additional security against buoyancy.
5. Backfilling and Compaction
Proper backfilling is essential for the tank's long-term stability.
- Material: Use clean, granular fill material (sand, gravel, or excavated soil free of large rocks or debris). Avoid using clay or organic material, which can settle unevenly.
- Layering: Backfill in layers, typically 6-12 inches at a time.
- Compaction: Properly place and compact backfill around the lower half of the tank first. This ensures the tank's base and sides are fully supported. Use hand tampers or light mechanical compactors, being careful not to strike the tank directly. Compact each layer thoroughly to prevent future settlement, which could leave voids or put uneven pressure on the tank.
- Even Distribution: Ensure backfill is evenly distributed around the tank to prevent unbalanced pressure. Continue filling and compacting in layers up to the desired grade. Remember to keep the tank filled with water during this entire process.
6. Final Grading and Landscaping
The final steps involve finishing the burial site.
- Inspection Ports: Ensure that access risers and inspection ports are extended to the final grade level for easy future access.
- Final Grade: Create a slight mound over the tank area to shed surface water away from the septic system. This prevents water from pooling directly over the tank and potentially entering the system or causing saturation.
- Landscaping: Plant shallow-rooted grasses or ground cover over the septic area. Avoid planting trees or shrubs with deep root systems, as roots can damage the tank or drainfield. Do not drive vehicles over the tank area.
Summary Table: Key Septic Tank Burial Steps
| Step | Key Action Points |
|---|---|
| 1. Site Selection | Select a site with good sub-soil drainage. Ensure accessibility and compliance with local regulations. |
| 2. Excavation | Dig a pit larger than the tank (allowing clearance for backfill). Ensure adequate depth for tank, bedding, pipes, and cover. |
| 3. Bedding Preparation | Provide well-compacted or undisturbed bedding of sand/gravel mixture or clean, granular soil. Bedding should be 6″ minimum in rock terrain and shaped to suit underside of tank. |
| 4. Tank Placement | Carefully lower the tank. Fill the tank with water during backfilling to prevent flotation. Consider anchoring in high water table areas. |
| 5. Backfilling | Use clean, granular material. Backfill in layers, properly placing and compacting around the lower half of the tank. Continue compacting evenly up to final grade. |
| 6. Final Grading | Extend access risers to grade. Create a slight mound over the area to shed water. Landscape with shallow-rooted plants. |
By following these detailed steps and incorporating the essential considerations for plastic tanks, you can ensure a successful and durable septic tank installation. For further technical specifications, always refer to the tank manufacturer's instructions and consult with local permitting authorities or a certified septic system professional.
