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Geotechnical News • September 2013
31
GEOTECHNICAL INSTRUMENTATION NEWS
As the Scottish poet Robert (Robbie)
Burns wrote, “The best laid schemes
o’ mice an’ men gang aft a-gley [often
go awry]”!
These examples have been derived
from the experience of working with
supervisors, associates and other col-
leagues in an organization which was
closed around 1999.
These cases are being written with
their contributions in mind from which
the writer has benefited. Some are
no longer with us. Others are retired.
The writer has therefore requested his
name to be withheld from publication.
Lightning and destruction of
electronics
A chimney, about 100 m high, had
been planned for construction on a
very dense till deposit about 9 m thick
over relatively flat bedrock. Tests on
soil samples in the laboratory had
been carried out to determine the usual
parameters and also the response to
cyclic loading related to wind effects.
The expected behaviour of the founda-
tions suggested no long term con-
solidation settlement of the structure
would occur and the calculated lateral
forces would not result in a cumulative
tilt. This project provided an oppor-
tunity to observe and document the
data which could be compared with
the assumptions used in the design and
a proposal for instrumentation was
approved.
During construction, within the area to
be covered by the foundation, pressure
pads were installed on the surface of
the soil. An anemometer was located
at the top of the chimney. Initial read-
ings indicated that the installations
were functional.
Soon after, there was a storm with
severe lightning. The electronics
installed for data collection were
zapped and destroyed. No repair was
possible and none was attempted.
Lessons learned
In retrospect, the disruption by light-
ning was a likely occurrence against
which the protection provided at that
time, four decades ago, was not effec-
tive. The increase in use of electronics
in many applications has probably
resulted in improvements in shielding
for preventing damage by lightning.
Specialists in this field should be
consulted.
A cofferdam on the foreshore of
a lake
A docking area was to be constructed
on the foreshore of a lake where the
bedrock surface was visible in shallow
water at the shoreline and the overbur-
den was about 2 m thick. The bedrock
surface sloped gently away from the
shoreline and soundings had reported
negligible overburden. Bedrock was
described as a shaley limestone.
The depth of water to be provided for
the equipment for docking was about
4 m. To facilitate excavation of the
rock for the for the docking area, a
cofferdam was constructed to enclose
a rectangular area extending 150 m
along the shore and about 100 m into
the water where the depth was about
5 m.
First, a rockfill embankment was built
on the three sides of the perimeter of
the area, extending into the water with
material from excavations in bedrock
for foundations for other structures at
the site. The impervious till material
from the overburden was then dumped
on the inner slope of the rockfill and
spread with a bulldozer to a top width
of about 5 m and a freeboard about 1
m above the lake water level.
It was the practice to observe the abut-
ments and downstream areas of dams,
during the first flooding of a reservoir,
for evidence of seepage or any unusual
conditions while the water level is
rising. The monitoring is continued for
some time after the maximum operat-
ing level is reached. Lowering of the
water level in an area enclosed by a
cofferdam creates a comparable situa-
tion but the project had arranged only
for the checks on the water levels dur-
ing the pumping. The opportunity to
detect, by inspection of the cofferdam,
any location where a leak may have
occurred was missed.
It was reported that the pumping for
dewatering had started and progressed
very well on the first day when the
submerged inner soil slope was partly
visible and appeared intact. Pump-
ing continued, but on the second day
the water surface had started to rise.
Additional pumping did not produce
any decrease in the water level.
The geotechnical engineering depart-
ment was called in to investigate and
find a solution. It was early winter.
A diver was sent down to inspect the
areas near the toe of the impervious
fill for any unusual signs of leak-
age. He described observing possible
movement of material from crevices in
the rock surface where characteristic
ridges caused by piping were noted
on the rock surface near the toe of
the impervious fill. In one location,
he was able to insert a piece of wood
about 50 mm thick which was secured
as a marker by covering with small
boulders.
It is likely that if observations had
been made during the initial pumping,
the locations of the piping would have
been noticed, and time would have
been saved.
A bedrock grouting program was
initiated with priority where the diver
had noted major crevices near the toe
of the fill. Check grouting was carried
out in the remaining sections.
Standpipe piezometers were installed
in the impervious fill at several loca-
tions for checking the water levels
during the resumption of pumping.
After some weeks of grouting opera-
tions, pumping was resumed. At
this time the condition of the inside
slope during the drawdown of the
water level in the enclosed area was
frequently checked. The area was
dewatered, and the inner slope of the
impervious zone was intact. The water
levels in the standpipes were gener-
ally below lake level except for one
case where the water in the standpipe
was higher than lake level. The small
ridge-like features where piping had