TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418
Cockfield formation as encountered during installa-
(2) Control of water. The Contractor shall control
tion of the system.
all surface water, any seepage into the excavation, and
(b) The wellpoints shall have a minimum screen
hydrostatic pressure in the semipervious stratum be-
or slotted length of 30 inches and shall be of the "bot-
neath the excavation, regardless of source. Any open-
tom suction" type. The wellpoint screens shall be of 30-
ing in the wood lagging, through which seepage is car-
mesh cloth or have #25 slots. The wellpoints shall be of
rying any soil, shall be promptly caulked. Any water
a high-capacity type with a diameter of approximately
seeping, falling, or running into the excavation as it is
2 inches. The jet-eductor pumps and pressure and re-
dug shall be promptly pumped out. The entire periph-
turn riser pipes shall have a capacity and be operated
ery of the excavation area shall be suitably diked, and
at a pressure which will produce a minimum flow of 2
the dike maintained, to prevent any surface water
gallons per minute with a static lift of 25 feet. If the
from running into the excavation. The Contractor
Contractor uses jet-eductor pumps with a yield capac-
shall be fully responsible for disposal of all water from
ity greater than 3 gallons per minute, it will be neces-
the excavation and from the wellpoint and relief well
sary for him to redesign the pressure and return head-
systems in an approved manner at no additional cost
er lines. The pressure pumps and recirculation tank
to the Government.
shall be designed by the Contractor. The pumps shall
(3) Wellpoint and pressure relief system data. The
have adequate head and pumping characteristics to
Contractor shall submit for approval by the Contract-
match the number and capacity of the jet-eductors be-
ing Officer, within 10 calendar days after receipt of
ing used. Both the pressure and return header lines
Notice to Proceed, complete information regarding
shall be provided with pressure gages at the pumps
motor generator, pumps, wellpoints, jet-eductors, well
and across the excavation from the pumps. Overflow
screens, and any other equipment that he or she pro-
from the recirculation tank shall be arranged so that
poses to utilize in dewatering and relieving hydrostatic
the flow from the wellpoint system can ye readily
pressure, and in maintaining the excavation in a dewa-
measured.
tered and hydrostatically relieved condition. The data
(c) The filter sand to be placed around the well-
to be submitted shall include, but not necessarily be
points and up to within 10 feet of the ground surface
limited to, the following:
shall be a clean, washed sand (A) having a gradation
(a) Characteristics of pumps and motor gener-
falling within the following range:
ator, and types and pertinent features of well screens,
Filter Sand A
U.S. Standard
wellpoints, and jet-eductor pumps.
Percent Passing
Sieve Size
(b) Plans for operating, maintaining, and moni-
78-100
10
toring the wellpoint and relief well systems.
16
35-82
20
15-58
(4) Jet-eductor wellpoint system.
3-32
30
(a) The soils at the site consist essentially of fill,
40
0-13
50
0-3
clays and silts, and some silty sand and clay sands with
occasionally pea gravel above the Cockfield formation.
(d) The wellpoints shall be installed by a combi-
The jet-eductor wellpoint system has been designed to
nation of driving and jetting (with a hole puncher) a
lower the groundwater table and intercept most of the
12-inch "sanding" casing to approximately 1 foot be-
seepage that will flow toward the excavation. Because
low the top of the Cockfield formation, rinsing the
of the impervious Cockfield formation at about the
"sanding" casing until there is no more sediment in the
bottom of the excavation, it is not possible to lower the
casing, centering the wellpoint and lowering it to the
groundwater table and intercept most of the seepage
bottom of the hole, placing Filter Sand A in the casing
that will flow toward the excavation as it is dug. Be-
around the wellpoint in a heavy continuous stream up
cause of the impervious Cockfield formation at about
to within 10 feet of the ground surface, and then fill-
ing the remainder of the hole with a thick bentonitic
the bottom of the excavation, it is not possible to lower
grout or by pouring in a mixture of dry sand contain-
the groundwater table completely down to the top of
this stratum or to intercept all seepage above it. By
ing 10 percent granular bentonite, up to the ground
proper installation, the jet-eductor wellpoint system
surface. Before the working day is over, all wellpoints
installed that day shall be pumped with a small centri-
should intercept most of the seepage and stabilize the
soil sufficiently to install the wood sheeting subse-
fugal pump, capable of producing a 25-foot vacuum,
quently specified without any significant sloughing of
until the effluent becomes clear. Jet water used for in-
the soil behind the sheeting prior to placement of the
stalling the wellpoints shall be clear polished so that
pea gravel backpack. The fact that some minor seepage
return of jet water up through the sanding casing is
may bypass the wellpoints should be anticipated by
achieved before the casing has been driven to a depth
the Contractor. The tips for the wellpoints shall be in-
of more than about 10 feet from the surface. It may be
stalled approximately 12 inches below the top of the
necessary or expeditious to prebore the holes for the
G-5
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