2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

G.2.5 Comprehensive Subsurface Investigation

Refer to Table II-5.4 to determine the minimum subsurface investigation requirements for a project. The comprehensive subsurface investigation report shall be prepared by a licensed professional. Refer to report submittal requirements in Volume II, Chapter 1.

Table G.3. Comprehensive Subsurface Investigation Elements.

Minimum Investigation Depth and Vertical Separation Requirements

Season

Minimum Investigation Depth (feet)a,b

Minimum Vertical Separation (feet)a

Groundwater

Hydraulically- Restrictive Layer

Infiltration Basins

Wet Season (November–March)

6

5

5

Dry Season (April–October)

10

8

5

Permeable Pavement

Wet Season (November–March)

2

1

1

Dry Season (April–October)

4

3

1

All Other Infiltration BMPs

Wet Season (November–March)

4

3

3

Dry Season (April–October)

10

8

3

Characterization for each soil and/or rock unit (strata with the same texture, color/mottling, density, and type)

Same as G.2.4 Standard Subsurface Investigation

Detailed logs for each investigation

Same as G.2.4 Standard Subsurface Investigation
Notes:

  1. 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.

  2. If the bottom of the BMP is not known, the minimum investigation depth shall be 16 feet below grade. Investigations that will also serve as groundwater monitoring wells shall not be less than 20 feet below the bottom of proposed BMP and the criteria for vertical separation to groundwater or hydraulically‑restrictive materials listed above shall apply.