Standard Subsurface Investigation

G.2.4 Standard Subsurface Investigation

This section summarizes the minimum requirements of a Standard Subsurface Investigation. Refer to Table II-5.4 to determine the minimum subsurface investigation requirements for a project.

Table G.2. Standard Subsurface Investigation Elements.
Minimum Investigation Depth and Vertical Separation Requirements
Season	Minimum Investigation Depth (feet)^a	Minimum Vertical Separation (feet)^a
Season	Minimum Investigation Depth (feet)^a	Groundwater	Hydraulically- Restrictive Layer
Infiltration Basins
Wet Season (November–March)	6	5	5
Dry Season (April–October)	7	6	5
All Other Infiltration BMPs
Wet Season (November–March)	2	1	1
Dry Season (April–October)	4	3	1
Characterization for each soil and/or rock unit (strata with the same texture, color/mottling, density, and type)
Unified Soil Classification System (USCS) classification or textural class Material texture, color/mottling, density and type Relative moisture content Grain size distribution, including fines content determination Presence of stratification or layering Presence of groundwater Iron oxide staining or mottling that may provide an indication of high-water level Cation exchange capacity (refer to Volume V, Section 5.6 of the Ecology Manual)
Detailed logs for each investigation
Map showing the location of the test pits or borings Depth of investigations Investigation methods (hand augers, test pits, or drilled borings), material descriptions Depth to water (if present) Presence of stratification Existing boring or groundwater information
The report shall document how the information collected relates to the infiltration feasibility of the site based on the setbacks provided in Vol II–5.3.2 Determine Infiltration Feasibility and Appendix G. If more than 2,000 square feet of the site infiltration will occur within a single BMP, the Standard Subsurface Investigation report shall be prepared by a licensed professional in accordance with Volume II, Chapter 1.
Notes: The minimum investigation depth and vertical separation shall be measured from the bottom of the BMP. The bottom of the BMP is defined as the deepest portion of proposed BMP where infiltrating water is expected to move into the underlying soil. For Small PITs, sampling of distinct materials below the bottom of the BMP and within the vertical separation depth is required. Beyond this depth, samples should be collected every 2.5 feet.