WLS-030 Slope Stability Report Requirements for Septic Systems

To provide requirements for the contents of a slope stability report to justify installation of a septic system on slopes over 30 percent or in potentially unstable slopes.

Slope stability reports shall be prepared by either a civil engineer or a geotechnical engineer registered in the state of California.

Slope Stability Reports shall include the following applicable items:

1. Cover Sheet
   1. Name of Project
   2. Site Address(es)
   3. Assessor’s Parcel Number(s)
   4. Preparer’s name and contact information
   5. Seal and signature of licensed civil engineer or geotechnical engineer
   6. Date Prepared
2. General Project Information
   1. Project description and scope of evaluation
   2. Description of proposed onsite wastewater treatment system (OWTS) type
3. Site Reconnaissance
   1. Description of slope geometry and topography including topographic maps and aerial imagery for steep slopes, ravines, or previous landslides
   2. Description of soils and geologic maps identifying weak, clay-rich, or loose alluvial soils
   3. Identification of observed groundwater levels or mottling during site evaluations and proximity to seeps, springs, or streams. Including wet areas that may indicate perched water tables
   4. Evidence of instability (example, sloughing, cracking, creep, landslide scars, bending trees)
   5. Proposed onsite wastewater treatment system (OWTS) location relative to slopes
   6. Photographs of the site slopes
4. Geologic Evaluation Analysis
   1. Methodologies used for evaluation (example, visual observation, slope stability analysis methods, software)
   2. Discussion of the presence of groundwater and seasonal variations thereof
   3. Impacts of anticipated effluent loading
   4. Factor of Safety analysis for the site under saturated conditions.
5. Downslope Hazard / Risk Assessment
   1. Identify the types of hazards (landslide, effluent breakout, etc.)
   2. Identify potential sources of harm (existing structures, roadways, human or animal exposure, streams, rivers, and natural resources)
   3. Analyze the probability of each hazard
   4. Analyze the severity of potential harm
   5. Identify and implement control measures to eliminate or reduce the risks
6. Mitigations and Recommendations for installation of proposed onsite wastewater treatment system (OWTS)
   1. Siting and layout
      Examples may include but are not limited to:
      • Place septic tanks and leach fields away from steep or unstable slopes
      • Maintain regulatory setbacks from slope crests, toes, and drainage channels
      • Avoid placing systems on fill or loose alluvium
   2. Drainage control.
      Examples may include but are not limited to:
      • Divert surface water away from septic areas
      • Install perimeter swales or interceptor drains to reduce slope saturation
      • Avoid leakage from system components; monitor and maintain pipes and tanks
   3. Slope reinforcement (as applicable).
      Examples may include but are not limited to:
      • Consider terracing or benches to reduce slope angle in critical areas
      • Retaining walls or soil nails may be used to fill or structures are needed
      • Ensure septic system footprint is not compromised by slope reinforcement
   4. Soil improvement (as applicable).
      Examples may include but are not limited to:
      • Use engineered fill where appropriate, with proper compaction
      • Avoid excessive disturbance of native soils during construction
   5. System design adaptations (as applicable).
      Examples may include but are not limited to:
      • Pressure-dosed or drip distribution can reduce infiltration on slopes
      • Lined trenches may prevent percolation into marginal soils
      • Design for long-term monitoring and maintenance to prevent saturation or slope weakening
7. Conclusion
   1. Summary of stability issues addressed
   2. Professional statement confirming site suitability