TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418
(From "Foundation Engineering, " G. A. Leonards, ed., 1962. McGraw-Hill
Book Company. Used with permission of McGraw-Hill Book Company.)
Figure 2-3. Use of wellpoints where submergence is small.
c. Jet-eductor wellpoint systems. Another type of
supplementary method of dewatering if needed. Well-
points are more suitable than deep wells where the
dewatering system is the jet-eductor wellpoint system
(fig. 2-6), which consists of an eductor installed in a
submergence available for the well screens is small
small diameter well or a wellpoint screen attached to a
(fig. 2-3) and close spacing is required to intercept
jet-eductor installed at the end of double riser pipes, a
seepage.
b. Vacuum wellpoint systems. Silts and sandy silts
pressure pipe to supply the jet-eductor and another
pipe for the discharge from the eductor pump. Eductor
(D10
0.05 millimetre) with a low coefficient of per-
meability (k = 0.1 x 10-4 to 10 x 10-4 centimetres
wellpoints may also be pumped with a pressure pipe
per second) cannot be drained successfully by gravity
within a larger return pipe. This type of system has
methods, but such soils can often be stabilized by a
the advantage over a conventional wellpoint system of
vacuum wellpoint system. A vacuum wellpoint system
being able to lower the water table as much as 100 feet
is essentially a conventional well system in which a
from the top of the excavation. Jet-eductor wellpoints
partial vacuum is maintained in the sand filter around
are installed in the same manner as conventional well-
the wellpoint and riser pipe (fig 2-5). This vacuum will
points, generally with a filter as required by the foun-
increase the hydraulic gradient producing flow to the
dation soils. The two riser pipes are connected to sep-
wellpoints and will improve drainage and stabilization
arate headers, one to supply water under pressure to
of the surrounding soil. For a wellpoint system, the net
the eductors and the other for return of flow from the
vacuum at the wellpoint and in the filter is the vacuum
wellpoints and eductors (fig. 2-6). Jet-eductor well-
in the header pipe minus the lift or length of the riser
point systems are most advantageously used to dewa-
pipe. Therefore, relatively little vacuum effect can be
ter deep excavations where the volume of water to be
obtained with a wellpoint system if the lift is more
pumped is relatively small because of the low permea-
than about 15 feet. If there is much air loss, it may be
bility of the aquifer.
necessary to provide additional vacuum pumps to en-
2-5. Deep-well systems.
sure maintaining the maximum vacuum in the filter
column. The required capacity of the water pump is, of
a. Deep wells can be used to dewater pervious sand
course, small.
or rock formations or to relieve artesian pressure be-
(From "Soils Mechanics in Engineering Practice, "by K. Terzaghi and R. B. Peck, 1948,
Wiley & Sons, Inc. Used with permission of Wiley & Sons, Inc.)
Figure 2-4. Drainage of an open deep cut by means of a multistage wellpoint system.
2-4
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