20130987 Ver 2_Application_20140923

B-4591 Coffer Dam Permit Modification/Permit Reissuance Request Narrative
There is a need for the installation of a sheet pile coffer dam on the B-4591 Island Creek Bridge Project
at the New Hanover and Pender County line on SR 1002 Holly Shelter Road/Island Creek Road. The
water level of Island Creek is relatively high and has been relatively high for the several weeks preceding
this writing. The water level of estimated approximate elevation 2.2 as of my visit to the site on 9/16/14
and another reported water level elevation of 2.7 on 9/18/14 would put the water surface of Island
Creek at approximately 1.5 feet high and 2 feet high respectively on the interior cap which is to be
poured concrete (elevation of bottom of proposed cap is 0.7). While the concrete would be poured in a
form, there would be a high likelihood that the stream waters would interact with the live concrete
thereby potentially raising the pH of the stream and potentially causing water quality problems.
Therefore, a sheet pile coffer dam is proposed to isolate the interior bent location and allow for the
pouring and curing of the interior concrete cap to be done in the dry. The proposed coffer dam would
be a rectangular shape and would enclose the interior bent area to separate the interior bent area from
the remainder of Island Creek. The coffer dam would have interior dimensions of more or less 12’ by 40’
and exterior dimensions of more or less 15’ by 43’. I say more or less because the sheet pile has sort of
a zig zag shape to it but the aforementioned 15’ by 43’ exterior dimension should encompass the area of
the impacts of the proposed sheet pile coffer dam.
For installation of the coffer dam, steel sheet pile sections would be brought in and driven into the
stream bed. The sheet pile sections have receiving pieces along the length of one side of the sheet pile
and insert pieces along the length of the other side of the sheet pile such that each sheet pile section fits
securely to the next sheet pile section. Also, the ST Wooten representative that I met with onsite on
September 16, 2014 to discuss the proposed sheet pile coffer dam proposal plans to obtain saw dust
and spread the saw dust in the described connection areas of the sheet pile sections. The saw dust gets
carried into any remaining voids at the connections of the sheet pile sections by the suction of the water
moving through the remaining voids. The saw dust then swells up from exposure to the water and
thereby provides a better seal to minimize water entry into the interior of the coffer dam. The quantity
of saw dust seems minimal to accomplish this as the ST Wooten representative was literally
demonstrating that he would take a handful(s) of saw dust and spread it by hand into each seam
between the sections of sheet pile. Once installed, the area encircled by the sheet pile would be
pumped dry. The pumped water would be routed into a special stilling basin (silt bag) for treatment.
The silt bag would be placed on a bed of 57 stone as per specifications. Wattles with PAM applied
would be located near the silt bag and on the down gradient side of the silt bag in order to further treat
the discharge from the silt bag. The discharge water would then flow over a grassy shoulder and grassy
fill section down to the silt fence near the toe of existing fill and would receive final treatment through a
wattle break in said silt fence and would then discharge into the wetlands beyond the wattle break. The
location of the silt bag and other above described treatments for the discharge is at approximate station
22+25 LT.
Upon dewatering the coffer dam area and maintaining it dry, the form for the pouring of the interior
concrete cap would be constructed on the already installed interior bent driven concrete piles. The
concrete cap would be poured and allowed to cure for several days prior to removing the form. Once
these tasks are accomplished the sheet pile coffer dam would be removed. All of this work from
beginning of installation of the coffer dam to completing the removal of the coffer dam would take
twelve days maximum but we would hope to have all accomplished within ten days, so we are
requesting twelve days in case we need the extra couple of days.
This proposal has been discussed with the NCDOT Hydraulics Unit and the NCDOT Hydraulics Unit is
supportive of this proposal. Discussions with the NCDOT Hydraulics Unit occurred during the week of
9/15/14.
In further support of this permit modification/permit reissuance (as the case may be) request, please
see attached Permit Drawing 2.1 of 9 dated September 22, 2014. This permit drawing shows the
proposed sheet pile coffer dam location by means of two lines with hatching in between them. The
drawing also shows the proposed temporary surface water impacts and associated temporary channel
impacts associated with the coffer dam proposal in red. This is the area of Island Creek that would be
impacted by the coffer dam itself within the stream as well as the area encircled by the coffer dam
which would be dewatered for approximately ten days with a twelve day maximum time period of
impact as described previously. This area of temporary surface water impacts is approximately 645
square feet (15’ by 43’) or 0.01 acres. The length of temporary stream impacts associated with this
same effort and as measured along the near centerline of the stream is 43 feet. Please see the revised
impact summary table (Rev. 2 of Permit Drawing 9 of 9 on 9/22/14) which reflects the aforementioned
temporary surface water impacts and temporary stream impacts as well. Also, the sheet pile has been
attempted to be shown to approximate scale line width on the permit drawing considering the fifteen
inch wide out to out of the zig zag sheet pile sections connected together. The sheet pile material itself
is probably only one to one and one half inch wide. However, as discussed the zig zag pattern
incorporates about a 15 inch width. Please note that 18 inches wide was used for the outer dimension
of the sheet pile rather than 15 inches for ease of calculation and drawing depiction. Also, when
considering the interior bent area shown in the plan view at Station 23+25, please consider that the
interior bent location shown in the plan view is represented by a line that scales out at two feet wide.
The cap, however, is three feet and eight inches wide and is thirty-one feet and ten inches long. So the
cap is actually about one foot wider to each side than the two foot wide line that represents the interior
bent location in the permit drawing attached. I say this so that you will have the preceding information
in case you are checking the measurement for the Contractor requested four foot distance from interior
edge of coffer dam to near edge of cap. Also, station 23+00 is noted on the permit drawing for your
reference; please look toward top of the page slightly left of center and below the north arrow to see
the denoted Station 23+00. Station 23+00 very closely coincides with the existing western end bent (of
the bridge that has just been removed). Also, I noted a new Site 7 (center top on drawing as well as new
notation on impact table) for the proposed coffer dam in keeping with the original submission of
denoting separate site numbers for the separate impact areas.
Stonewall Mathis
9/22/14