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Geotechnical News • March 2014
39
GROUNDWATER
Dewatering a large excavation with well points
Consequences of a poor assessment of split spoon samples
Robert P. Chapuis
During my career first as a consultant,
and then as a professor, I have had the
privilege to be an expert in several
cases. This is the third legacy case
that I have presented in Geotechni-
cal News. Over the past few years,
I have asked for authorization to
publish scientific issues on selected
old (over 20 years) but still interest-
ing cases. Having no authorization
to publish is unfortunate for profes-
sional knowledge. The owners and
their current legal counsels have given
authorization, but with the request that
all names and legal issues be kept con-
fidential. In addition, no photographs
can be published, which would enable
identification of a site or person. The
actual year cannot be given.
Context
This case is about a large excavation
which was planned to be dewatered
using a wellpoint system, in order to
ensure stable working conditions at its
base. Wellpoints are small diameter
shallow wells of about 2 inch (51 mm)
diameter, with short screens, 50 to 90
cm long. They are driven vertically
and jetted, using high–pressure water,
into pervious or semi–pervious soils.
The hole around the wellpoint is filled
with filter sand to catch all seepage
in successive layers, and convey all
water to the screen. Wellpoints are
closely spaced (0.6 to 3 m) along the
edge of an excavation. Each group
of 50 to 80 wellpoints is connected
to a header main and pumped by a
high–efficiency vacuum dewatering
pump. In practice, due to head losses
in the ground, screen, pipes, connec-
tions, and flow rate limitations within
the vacuum pump, the drawdown
achieved by a row of wellpoints is
limited to 5 m.
Consider an excavation that must
reach a depth of 14 m below the water
table. A first series of wellpoints
would be driven from the bottom of
a first excavation levelled just above
the water table. After having exca-
vated the first 5 m of soil and having
lowered the water table by 5 m (to
have a dry bottom), a second series
would be installed 5 m deeper. Finally,
a third series would be installed at a
depth of 10 m to carry on the dewater-
ing and complete the excavation down
to 14 m, before building a reinforced
concrete raft or shallow foundations. A
wellpoint system means that the exca-
vation sides are made of soil sloping
at an angle usually between 1.2H/1V
and 2H/1V. Therefore, the excava-
tion occupies a large surface, which is
acceptable only in open fields, not in
already heavily constructed sites.
Till investigation
The site of this case history is far
from any developed areas. The natural
ground was roughly horizontal and
covered with a moist organic layer
0.4 to 0.9 m thick. The investigation
included several boreholes, which
used diamond drilling and a flush–
joint casing, with lateral or upwards
clean water jets as washing fluid for
the cored sediments. A split spoon was
used to take soil samples at intervals
of 1.5 m (5 feet). Split spoons are
known to provide class–4 soil samples
in any geotechnical classification, but
not intact samples as sometimes found
in geological papers. The contactor
had to excavate 4.5 m deep in a very
thick grey till layer (silty sand and
gravel, no plasticity) with a few thin
and yellowish sand–and–gravel layers.
The call–for–tender documents
included the full geotechnical report,
with borehole logs, sample grain size
distributions and a cross–section of the
area to be excavated, obtained using
interpolation between boreholes. The
contractor was requested to provide a
stable excavation with a dry and solid
bottom. The geotechnical report issued
warnings for the need to control the
surface water, and to control seepage
in the sand–and–gravel layers, which
could be achieved, for example, with a
wellpoint system.
Field work and observations
The contractor removed and put aside
the organic top layer, and then drained
the site surface with peripheral ditches
and sump pumps. Next, high–pres-
sure jetting was used to install a few
hundred wellpoints along the edge of
the planned excavation. The vacuum
dewatering system then started pump-
ing. During the first few hours, the
flowrate decreased from about 200 L/
min (40 gpm) to only a few L/min, and
stayed low over the following weeks.
The excavation easily progressed in
the till, which contained only a few
boulders. The excavated slopes were
stable and only little water was seep-
ing from the till upper portion, which