G.2.2
This section includes requirements for subsurface investigation locations, timing, alternatives, investigation depth and vertical separation requirements, and subsurface reports.
G.2.2.1 Subsurface Investigation Locations
Subsurface investigations shall be performed at the site of the infiltration BMP, unless demonstrated to be infeasible. In such case, the subsurface investigation shall be performed as close as possible, but no more than 50 feet away, to obtain relevant subsurface information. Subsurface investigations can be conducted at the same location as the infiltration tests (G.3 Determining the Measured (Initial) Ksat).
G.2.2.2 Subsurface Investigation Timing
Subsurface investigations should be performed in the wet season (November through March) if possible, when soils may contain a higher water content and groundwater levels are typically higher. Refer to G.2.3 Simple Subsurface Investigation for wet season and dry season requirements for the different types of subsurface investigations.
G.2.2.3 Alternatives to Subsurface Investigation
In some cases, available data and the licensed professional’s interpretation of subsurface material characteristics can be used to demonstrate that infiltration is infeasible on a site and precludes the need for all of the subsurface investigation or infiltration testing. Examples of these instances include, but are not limited to:
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Groundwater monitoring data that meets the requirements of the groundwater monitoring section (G.5 Groundwater Monitoring), at the site of the proposed BMP showing groundwater elevations not meeting the vertical separation requirements (G.2.2.4 Investigation Depth and Vertical Separation Requirements).
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Identification by the licensed professional of hydraulically‑restrictive materials beneath the proposed BMP and within the vertical separation requirements (G.2.2.4 Investigation Depth and Vertical Separation Requirements).
To support these instances, the licensed professional must submit a stamped and signed letter that provides rationale and specific data supporting their professional judgment for each area deemed infeasible for infiltration.
G.2.2.4 Investigation Depth and Vertical Separation Requirements
Investigation depth is measured below the bottom of the proposed infiltration BMP. The bottom of the infiltration BMP is defined as the deepest portion of the proposed BMP where infiltrating water is expected to move into the underlying soil.
The vertical separation requirements depend upon the type of subsurface investigation required and the seasonal timing of the geotechnical exploration conducted to evaluate clearance and are typically 1 foot less than the minimum investigation depths summarized in G.2.3 Simple Subsurface Investigation. If groundwater or a hydraulically‑restrictive material is encountered within the vertical separation depth, then no further investigation is required.
Examples of materials that may be interpreted as hydraulically‑restrictive include:
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Glacially consolidated soils that have greater than 50 percent fines
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Glacially unconsolidated soils that have greater than 70 percent fines
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Bedrock
G.2.2.5 Subsurface Report
Projects that are required to perform subsurface investigations per Vol II–5.3 BMP Selection shall prepare a report documenting results of the subsurface investigations described in G.2.3 Simple Subsurface Investigation and infiltration tests described in G.3 Determining the Measured (Initial) Ksat. Refer to report submittal requirements in Volume II, Chapter 1.