(a) The following
equipment is required for performing a pit-bailing test (see Figure 8 in
Appendix A):
1. A back-hoe;
2. Wooden or metal stakes, string and a
hanging level;
3. A steel measuring
tape;
4. A pump
(optional);
5. A stop-watch;
and
6. A perforated pipe, with a
three inch diameter or greater.
(b) The following procedure shall be used for
preparation of the test pit:
1. Step One:
Excavate a test pit extending into but not below the soil horizon or layer to
be tested. The bottom of the pit should be a minimum of 1.5 feet below the
observed water level and a minimum of six feet below the proposed level of
infiltration. The bottom of the pit should be relatively flat and level. The
shape of the pit within the depth interval tested should be approximately
square or round. A rectangular or elliptical pit may be used provided that,
within the depth interval tested, the length of the long dimension is no more
than twice the length of the short dimension. The excavation made for a soil
profile pit as prescribed in
7:9A-5.2 may be used provided that
all the above requirements are met.
2. Step Two: Allow the water level to rise in
the pit for a minimum of two hours and until the sides have stabilized. If
large volumes of soil have slumped into the pit, this soil must be removed
before proceeding with the test. If the sides of the pit continue to slump and
cannot be stabilized, the test shall be abandoned. If water is observed seeping
into the pit from soil horizons above the zone of saturation in which the test
is being conducted, adequate means shall be taken to intercept and divert this
water away from the test pit, otherwise the pit-bailing test shall not be used.
If, during the excavation of the pit, the water level in the pit rises suddenly
after a hydraulically restrictive horizon is penetrated, and continues to rise
above the bottom of the hydraulically restrictive horizon, the pit-bailing test
shall not be used.
(c)
The following procedure shall be used for performance of the pit-bailing test
and the calculation of test results:
1. Step
One: Establish a fixed reference point for depth to water level measurements
which will not be disturbed during removal of water from the pit or which can
be temporarily removed and later re-positioned in exactly the same place. One
way to establish a removable reference level mark is as follows:
i. Drive stakes firmly into the ground on
opposite sides of the test pit, several feet beyond the edge, where they will
not be disturbed.
ii. Next, stretch
a string with hanging level from stake to stake, over the pit, and adjust the
string to make it level.
iii.
Finally, secure the string to the stakes and mark or notch the positions on the
stakes where the string is attached so that the string may be removed
temporarily and later repositioned exactly in its place.
2. Step Two: Measure the distance from the
reference level to the bottom of the pit and to the observed water
level.
3. Step Three: Lower the
water in the pit by at least one foot, by pumping or bailing. If the back-hoe
bucket is used to remove water from the pit, it may be necessary to remove the
reference level marker prior to bailing and re-position it in its original
position prior to beginning step four.
4. Step Four: Choose a time interval, based
upon the observed rate of water level rise. At the end of each time interval,
measure and record the information indicated in (c)4 i through iii below and
repeat these measurements until the water level in the pit has risen a total of
one foot or more.
i. Time, in minutes (the
time interval, in minutes, between measurements should be chosen to allow the
water level to rise by several inches);
ii. Depth of water level below the reference
string at the end of each time interval, to the nearest eighth of an inch or
one-hundredth of a foot; and
iii.
Area of water surface, in square feet. Measure appropriate dimensions of the
water surface, depending on the shape of the pit, to permit calculation of the
area of the water surface at the time of each water level depth measurement.
Entering a soil pit excavated below the water table can be extremely dangerous
and should be avoided unless the pit is relatively shallow and the sides of the
pit have been stepped and sloped as prescribed in
7:9A-5.2(e)3 to
eliminate the likelihood of sudden and severe cave-in of the pit. The distance
between two opposite edges of the water surface can be measured accurately,
without entering the pit, as follows. Place a board on the ground,
perpendicular to the side of the pit and extending out over the edge. Using a
plumb-bob, position this board so that its end is directly over the edge of the
water surface in the pit, below. Position a second board, in the same manner,
on the opposite side of the pit. Measure the distance between the ends of the
boards to determine the length of the water surface below.
5. Step Five: Determine whether an adequately
consistent set of data has been obtained in accordance with (e)5i and ii below.
i. Calculate the permeability for each time
interval using the following equation:
K[a] = (h[rise]/t) x [A[av.]/2.27 (H[LESS THAN]2[GREATER
THAN]-h[LESS THAN]2[GREATER THAN])] x 60 min/hr
where:
K[a] = permeability, in inches per hour;
h[rise] = difference in depth to water level at the beginning
and end of the time interval, in inches;
t = length of time interval, minutes;
A[av.] = average of water surface area at the beginning of
time interval (end of previous time interval) and at the end of the time
interval, in square feet;
H = difference between depth to assumed static water level
and actual or assumed depth to impermeable stratum, in feet (Depth to
impermeable stratum, if unknown, is assumed to be one and one-half times the
depth of the pit.); and
h = difference between average depth of water levels at
beginning and end of time interval and actual or assumed depth to the
impermeable stratum, in feet.
ii. If the calculated values of K[a] for
successive time intervals show either an increasing or a decreasing trend,
repeat Steps Three and Four until consecutive values of K[a] are approximately
equal.
6. Step Six:
Remove as much water as possible from the pit. Continue excavating the pit
until an impermeable stratum is encountered or as deep as possible considering
the limitations of the excavating equipment used and the nature of the soil
conditions encountered. Where no impermeable stratum is encountered, the
impermeable stratum shall be assumed to be at the bottom of the excavation. Due
to the potential safety hazards posed by the excavation of a large test pit
such as that required for this test, adequate safety measures shall be taken,
including the posting of warning signs and installation of a fence to prohibit
access to the pit by the public during periods when the pit is left
unattended.
7. Step Seven: Record
the depth to the static water level from the same reference level used in Step
One, (c)1 above. This step may be conducted either 24 hours after completion of
Step Six at (c)6 above or of Step Two at (b)2 above.
8. Step Eight: Recalculate the permeability,
K, using the following formula:
K = (h[rise]/t) x [A[av]/2.27 (H[LESS THAN]2[GREATER
THAN]-h[LESS THAN]3[GREATER THAN])] x 60 min/hr
where:
K = permeability, inches per hour;
The values of h[rise], t, and A[av]. are the values recorded
for these parameters in the last time interval of Step Four of this
subsection:
H = difference between depth to actual corrected static water
level and actual or assumed depth to impermeable stratum, recorded in Steps Six
and Seven, in feet; and
h = difference between the average depth of water levels at
the beginning and end of the last time interval recorded in Step Four and the
actual or assumed depth to impermeable stratum recorded in Step Six, in
feet.
