Environmental Engineering Reference
In-Depth Information
Q
w
r
w
R
dh
Aquiclude
(clayey soil)
H
w
H
h
Aquifer
(sands)
FIGURE 8.32
Flow from confined aquifer to a fully
penetrating artesian well from a circular
seepage source. (From Mansur, C.I. and
Kaufman, R.I.,
Foundation Engineering,
G.A. Leonards, 1962.)
h
w
D
Well
r
dr
Equations 8.13 and 8.14 are valid only when there is no head loss in the well, i.e., when the
head at the well
h
w
is equal to the water level in the well. Since some head is required to
force water through the filter and well screen, there will be some head loss. The above
equations are valid, therefore, provided that
h
w
is considered as the head at the periphery
of the well, and not the water level in the well. Relationships are available for estimating
head loss (see, e.g., Mansur and Kaufman, 1982, p. 314).
Combined Artesian-Gravity Flow
In the artesian case above, the water level remains in the impervious stratum. It is possi-
ble at high pumping rates to lower the water table to below the top of the aquifer or per-
vious stratum. Under these conditions, the flow pattern close to the well is similar to that
of a gravity well, whereas at distances farther from the well the flow is artesian.
Overlapping Cones of Depression
When several wells are close together, the cones of depression overlap, causing interfer-
ence, and the water table becomes depressed over a large area. At any point where the
cones overlap, the drawdowns are the sum of the drawdowns caused by the individual
wells. When wells are too closely spaced, flow to each is impaired and the drawdowns are
increased.
Slots
A line of wells, such as wellpoints, or a dewatering trench may be simulated by a slot.
Solutions may be found in Mansur and Kaufman (1962) for fully or partially penetrating
slots, from single- or two-line sources, in gravity, artesian gravity, and artesian conditions.
8.3.4
Practical Aspects of Groundwater
General
The practical aspects of groundwater problems can be grouped into three major cate-
gories: flow quantities, stability problems, and water quality.
Quantity of flow
is of concern for water supply and for excavations made for structures,
mines, and tunnels and through, beneath, and around dams.
Stability
in soil formations is related to operate pressures occurring in slopes, excavation
bottoms, and beneath embankments and pavements; in rock masses it is related to the soft-
ening or removal of fillings in fractures, and to the development of high pore- or cleft-
water pressures in slopes or beneath structures.
