Geotechnical Analysis/Soils Reports

1.4.5 Geotechnical Analysis/Soils Reports

Geotechnical analysis/soils reports are required to document subsurface investigations, groundwater monitoring, characterization of infiltration receptor, and groundwater mounding and seepage analyses, per Volume II, Chapter 5. In addition, KCC 19.700.725 requires a geotechnical report whenever development is proposed in a geologically hazardous area or shoreline setback, or when DCD determines that additional soils and slope analysis is appropriate on a particular site.

The following report types may be combined or provided separately, depending on who prepares the reports and approval by DCD reviewers to combine reports. Where a licensed professional is required to prepare reports, the reports shall bear the signature and stamp of the licensed professional in the state of Washington. Where provided separately, the Drainage Report shall reference other reports by title, date, and name of company or licensed professional.

The minimum information to be included in each report is detailed below:

Geotechnical analysis – Provide the following:
1. Potential impact of stormwater BMPs on slopes 15% or greater or otherwise sensitive slopes, per Volume II, Chapter 5.
2. For BMPs setback less than 50 feet from steep slopes, information necessary to support the proposed setback.
3. Information required per KCC 19.700.725.
4. Results and conclusions.
5. Raw data and calculations, to be included in an appendix.

Subsurface investigations and infiltration testing – For projects required to perform subsurface investigations and infiltration testing per Volume II, Chapter 5, provide the following:
1. Simple Subsurface Investigation Report, Standard Subsurface Investigation Report, or Comprehensive Subsurface Investigation Report, as required per Volume II, Chapter 5.
2. Small Pilot Infiltration Test Report, Large Pilot Infiltration Test Report, and/or Deep Infiltration Test Report, as required per Volume II, Chapter 5.
3. For all subsurface investigation and infiltration testing reports, provide the following:
  1. Description of the methods used and the standards upon which the methods were based.
  2. Maps of investigation and testing locations.
  3. Discussion of soil and groundwater conditions found.
  4. Results and conclusions.
  5. Raw data and calculations, to be included in an appendix.
Groundwater monitoring – For projects required to perform groundwater monitoring per Volume II, Chapter 5, provide the following:
1. Description of methods used and the standards upon which the methods were based.
2. Map of monitoring locations relative to the project site.
3. Description of groundwater levels relative to the investigation depth and vertical separation requirements per G.2 Subsurface Investigation.
4. Results and conclusions.
5. Raw data and calculations, to be included in an appendix.
Characterization of infiltration receptor – For projects required to perform characterization of infiltration receptor per Volume II, Chapter 5, provide the following:
1. Depth to groundwater and to hydraulically restrictive material.
2. Seasonal variation of groundwater table based on well water levels and observed mottling of soils.
3. Existing groundwater flow direction and gradient.
4. Approximation of the lateral extent of infiltration receptor.
5. Volumetric water holding capacity of the infiltration receptor soils. The volumetric water holding capacity is the storage volume in the soil layer directly below the infiltration facility and above the seasonal high groundwater mark, or hydraulically restrictive material.
6. Horizontal hydraulic conductivity of the saturated zone to assess the aquifer’s ability to laterally transport the infiltrated water.
7. Results and conclusions.
8. Raw data and calculations, to be included in an appendix.
Groundwater mounding and seepage analysis – For projects required to perform groundwater mounding and seepage analysis per Volume II, Chapter 5, provide the following:
1. Description of data used.
2. Analysis procedures, including modeling tools and methods.
3. Potential for groundwater mounding or seepage as a result of proposed infiltration facilities.
4. Results and conclusions.
5. Raw data and calculations, to be included in an appendix.