Environmental Engineering Reference
In-Depth Information
as you continue to remove sediment from the hole. Once the desired depth has been
reached, commonly about 1-1.5 m below the water table, it is time to assemble and
install the well.
Record the total depth of the hole by marking the auger rod at the point where it
is even with land surface when the auger is at the bottom of the hole. Remove the
auger from the hole and measure the distance from the mark to the bottom of the
auger. Add a distance, commonly 0.6-1 m, for the extent of the well casing that will
be above the ground. This is often called the “stickup.” The sum of these distances
will be the total length of the monitoring well. Assemble the well screen by gluing a
cap to the bottom of the well screen and a coupling to the top of the screen
(Fig.
3.42c
). If available, it is desirable to use a well cap that either is cone shaped
or that has the same outer diameter as the well screen to reduce resistance when
pushing the assembly into the loose sediments below the water table. The well
screen should be sized to be long enough that the water table is usually within the
screened interval of the well. The slot size (the width of the openings in the screen)
should be selected so that most of the sediment cannot pass through the well screen.
Well screens often have an interval at the bottom of the screen that does not have
any slots. This is called the sump, or the volume below the screen where fine
sediments that pass through the screen can accumulate without blocking the well-
screen openings. Be sure to record the presence of a sump and indicate the length of
the sump. This information will be important in determining the precise screened
interval of the well. The existence of a sump becomes particularly important if the
water table is below the bottom of the screened interval. Measurements of depth to
water will indicate an erroneous water level equivalent to the elevation of the
bottom of the well screen because water will be trapped in the sump. Drilling
small holes in the bottom of the sump prior to well installation may allow trapped
water to drain from the sump if the well goes dry.
Cut the PVC casing so that the total well length is the distance of the hole depth
plus the desired stickup length. If the hole is relatively deep, you may need to attach
another PVC coupling and another length of well casing to reach the desired total
assembly length. By now, the sediment in the auger hole may have settled and
solidified and it may be necessary to remove several additional buckets full of
recently slumped sediment from the hole. Keep removing sediment from the hole
until the auger has reached the bottom of the hole and the sediment is once again
poorly consolidated. At this point it is important to move rather quickly, especially
in sediments that readily slump and solidify, such as medium to fine sand. As soon
as the last bucket of sediment is pulled out of the hole, immediately shove the
completed well casing and screen into the hole and push it down until it stops. You
may need to pound lightly on the top of the well casing with the sledge hammer to
drive the well to the intended depth. It is prudent to place a board or drive cap on the
well casing to prevent damage to the top of the well casing. While pounding lightly,
grab the well casing and push downward, essentially vibrating the well downward
through the loose sediment. In most cases, you will be able to reach or get very near
the desired well depth. Once the well is in place, it is a simple matter of filling the
annular space between the edge of the augered hole and the well casing with