SW6191004_Response to Request For Additional Information_20200110_1/22/2020

SW6191004 Response to Request For Additional Information 1/23/2020

1. The drainage area table on Page 3 of the SWU-101 application incorrectly lists the
drainage area to SWM#2 as 42,209 ft2, whereas the calculations show it as 2.215
acres. Please revise the application form to correct this discrepancy.

Response: The application has been corrected to 2.215 Acres = 96,485 SF. The On-
site value was also corrected to 90,440 SF.

2. Please make the following revisions to the Supplement EZ form:
a. Complete the information for the seasonal high water table (Fields 22 & 23).

Response: SHWT has been added and the SHWT memo of findings has also been
included in this submittal.

b. Revise the bottom elevation to reflect the elevation of the subsoil at the bottom
of the stone layer (Field 29).

Response: Field 29 has been revised to the bottom of stone. Please note that due to
SHWT results and infiltration test elevations the SWM facilities have been revised.
SWM#1 was reduced to a 1.5 foot gravel depth in order to match the infiltration test
elevation of 240.83. SWM#2 was changed to a infiltration basin with no stone in
order to come closer to the infiltration test elevation. Further explanation of
nitration test elevations provided below in item #4.

c. Complete the information for the infiltration trenches (Fields 34-43).

Response: Fields 34-43 have been updated for SWM#1 since the stone component
was removed from SWM#2.

3. The boring location map shows no borings in the areas of the infiltration systems to
determine the seasonal high water table. Please explain how the elevation of the
seasonal high water table was determined in these areas.

Response: SHWT investigation was performed by Amber Coleman, LSS, PWS on
November 7, 2019 after the initial geotechnical investigation. The findings are
summarized in a memo dated December 6, 2019 which has been included with this
submittal.

SW6191004 Response to Request For Additional Information 1/23/2020

4. The infiltration tests should be completed at the final elevation of the subsoil. The
Supplement EZ form indicates that the infiltration tests were completed at the
existing grade of the site, which does not accurately reflect the infiltration rates of
the soil at the excavated elevations.

Response: SWM#1 was revised to raise the stone to the elevation of the infiltration
test. SWM#2 was converted to an infiltration basin to raise the infiltration point as
closely as possible to the test elevation. The test elevation was at 225.25 the basin
elevation is at 223.77 or roughly 1.5 feet below the test elevation. As the designer of
record (DOR) it is my opinion that this facility will still perform per the MDC due
to the results of the SHWT which revealed sandy-loams and loamy-sands with no
clay features within an elevation range of 219.16 ft to 224.5ft. The sand structures
are also consistent with the Geotech borings around the site and the NRCS web
soil survey results (Hydrologic Group A soils). The consistency amongst all
sources of information (infiltration test, boring logs, SHWT observations, and
NRCS web soil survey) along with a design factor of safety of 2.0 gives confidence
that the facility will perform as designed and meets the MDC.

5. Provide pdfs of all revisions, 2 hardcopies of revised plan sheets, and 1 hardcopy of
other documents. Pdfs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW_Project_Submittal