Environmental Engineering Reference
In-Depth Information
Spacing of wellpoints (ft)
Poles
Ground water
lowering
(ft)
1
1
1
60
50
Gravel
O
1
2
Very fine
gravel
O
40
2
5
10
Gravel
2
2
3
4
5
30
25
1
3
Coarse
sand
O
20
3
4
15
2
10
4
5
3
4
5
15
Very fine
gravel
1
9
8
7
6
5
5
6
Medium
sand
O
10
7
8
10
9
15
4
10
Coarse
sand
15
3
Fine
sand
15
2
Medium
sand
Fine
sand
O
1
FIGURE 8.43
Wellpoint spacing for stratified sands and gravels. (From Mansur, C.I. and Kaufman, R.I.,
Foundation
Engineering,
G.A. Leonards, 1962 and Griffin Wellpoint Corp.)
wellpoints. There must be a sufficient depth of pervious materials below the level to which
the water table is to be lowered to permit adequate submergence of the well screen and
pump. Deep wells are particularly effective in highly stratified formations containing free-
draining gravels. They are used also to dewater tunnels and to stabilize deep-seated slide
masses, but they are costly and continuous pumping is required to maintain stability.
Method
The wells, containing submersible or turbine pumps, are usually installed outside the
work area at spacings of 20 to 200 ft (6-60 m). The well diameter can vary from 6 to 18 in.
(15-45 cm), and the screen is commonly of lengths from 20 to 75 ft (6-22.5 m). Often used
in combination with wellpoints as shown in
Figure 8.46,
the deep wells are installed
around the perimeter of the excavation and the wellpoints within the excavation.
Electro-osmosis
Application
Electro-osmosis is used to increase the strength of silts when they are encountered as thick
deposits in open excavations, slopes, or tunnels. It has been used infrequently in the
