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Geotechnical News • June 2013
www.geotechnicalnews.com
GROUNDWATER
had been altered (in past centuries)
and given some secondary porosity by
frost action (fissures) and biological
action (roots and burrows).
Only one sand–and–gravel layer was
found which was clearly visible as a
sinuous yellow seam within a grey
till. The yellow ribbon-like layer was
in a horizontal plane about 2 m below
the top of the till layer. The ribbon
was 2–3 m wide and 30 cm thick
in its middle. The sand–and–gravel
thickness decreased toward the rib-
bon edges. When plotted on a map,
this unique sand–and–gravel seam
was located between the boreholes,
which could not have detected it. The
boreholes had detected a few thin
yellowish sand–and–gravel layers, but
only one was found in the excavation.
This single layer conveyed little water,
which was easily controlled.
The owner and the contractor thought
that a lot of time and money had
been spent on the wellpoint sys-
tem, whereas such a system was not
required for this excavation. At this
time, I was hired as an external expert,
to examine the data and to try to
uncover what happened.
I visited the site, took soil samples and
photographed the yellow ribbon-like
layer. This was a puzzling case. To
better document the case, I asked to
see the handwritten field reports by the
drilling inspector. The engineer had
a copy at the construction site. The
field reports were very detailed, and
provided very useful information.The
drilling reports described the casing
progression, as well as the washing
and sampling operations. All split-
spoon samples had a high recovery. At
all times, the upper portion of the sam-
ple, 7 to 12 cm high, was a yellowish
sand–and–gravel, whereas its lower
portion was a grey till. The upper and
lower portions of the sample were kept
and analyzed separately. Each sand–
and–gravel sample appeared as a thin
aquifer layer within an aquitard till.
Because the ground surface was nearly
horizontal, and samples were taken
at similar depths in each borehole,
all yellowish sand–and–gravel layers
were found at similar elevations in all
boreholes. The engineer thought that
there could be continuous thin aquifer
layers, as shown by interpolation dot-
ted lines in the cross–section of the
geotechnical report. The presence of
aquifer layers was also the reason why
the engineer believed that there was a
risk of significant water inflow into the
excavation, and a need to control the
water by using a wellpoint system.
All needed information was there. I
announced that it was time to explain
what had happened. Our small group
including employees of the owner,
engineer and contractor, returned to
the excavation. There, I took a grey till
sample of about one kilogram, and put
it in a plastic bag. Then I brought the
sample to the outlet of the wellpoint
system, where a little clear water was
discharging into a ditch connected to
surface water. I used the clean water
to wash the till in the plastic bag,
letting the fines escape from the bag.
To everyone’s surprise, the grey till
turned into a yellow sand–and–gravel.
Everybody suddenly realized what had
happened. What had been interpreted
as a thin aquifer layer was simply the
result of locally and partially washing
the till during the drilling operations,
before taking a split–spoon sample.
Conclusion
As a technical expert, and as usual, I
was not involved in the out–of–court
settlement. All parties had understood
the situation, and they did not need
me anymore for legal and financial
considerations. However, because the
case never went public, many profes-
sionals were kept unaware of this
problem and its simple cause from soil
washing. This negative aspect of the
confidentiality rules works against the
Professional Order’s and the Corpora-
tion’s mandate to protect the public,
and is unfortunate for the engineering
profession.
However, this case was useful for my
research. It revealed, for example, that
damage and erosion due to drilling
operations must be taken into account.
It also motivated further research
on hydraulic short-circuits along the
well casings (Chapuis and Sabourin
1989; Chesnaux et al. 2006) and their
influence during slug tests (Chapuis
2001) and pumping tests (Chapuis and
Chenaf 1998).
References
Chapuis, R.P. 2001. Extracting
piezometric level and hydraulic
conductivity from tests in driven
flush–joint casings. Geotechnical
Testing Journal, 24(2): 209–219.
Chapuis, RP, and Chenaf, D. 1998.
Detecting a hydraulic short–circuit
along a monitoring well with the
recovery curve of a pumping test
in a confined aquifer: method and
example. Canadian Geotechnical
Journal, 35(5): 790–800.
Chesnaux, R., Chapuis, RP, and
Molson, J.W. 2006. A new method
to characterize hydraulic short–cir-
cuits in defective borehole seals.
Ground Water, 44(5): 676–681.
Chapuis, R.P. and Sabourin, L. 1989.
Effects of installation of piezom-
eters and wells on ground water
characteristics and measurements.
Canadian Geotechnical Journal,
26(4): 604–613.