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34
Geotechnical News • March 2013
www.geotechnicalnews.com
GROUNDWATER
Field investigations
The owners wanted a new building
for their recently bought piece of land.
The land looked nice, had mature
trees, and a good value, being very
close to an urban area. The owners
retained the services of a geotechnical
company for the investigations, and
the services of an engineering com-
pany for the technical specifications,
drawings, call–for–tenders documents,
and quality control of the construction,
including as-built drawings.
A sketch of the piece of land is shown
in Fig. 1, with two accesses from
the streets. The owners and their
consultants agreed to perform a field
investigation with five boreholes, one
in the middle of the planned edifice,
and four at the corners. The geotechni-
cal company did the investigations,
established the stratigraphy, col-
lected soil samples, and installed two
monitoring wells in BH-1 and BH-5.
According to the geotechnical report,
the soil was a thick deposit of gravely
sand, with some silt and cobbles. The
standard penetration tests indicated
that the deposit was dense in its upper
2 m and very dense below. The pres-
ence of some construction debris, in
the upper 2 m, led the geotechnical
company, in its report, to suspect
some backfill. The water table was at
a depth close to 5.5 m. Taking into
account the soil type and the water
table depth, no groundwater problems
were anticipated during the planned
excavation down to a depth of about
2.5 m. The boreholes were stopped at
a depth of about 9 m without reaching
the bedrock.
Excavation and discoveries
When the excavation proceeded,
there was a big surprise. Below a
silty gravely sand about 1.5 m thick,
a very old dump was found under
about three quarters of the piece of
land, with all types of rubbish down
to a depth of about 5 to 5.5 m (Fig. 2).
The contractor threatened the owners
for unanticipated soil conditions, and
for extra costs (excavating 3 m deeper
than anticipated, having a broader than
expected excavation, transporting the
rubbish to the dump, buying accept-
able backfill material, backfilling the
big pit in layers and compacting them
to be able to support the new building,
etc.).
It was discovered that many years
ago, the previous land owners used
their lot to extract soils and sell them
to contractors. An old pit was deep-
ened until it reached the water table
and the limits of the lot, except in the
access zone. Then, the owners realized
they could make money by letting
people and companies bring rubbish
to the pit. The pit was filled in, and
the rubbish somewhat settled due to
decomposition and a few fires. Finally,
the surface was covered with five
feet (1.5 m) of backfill, more or less
compacted, the plan being to make the
piece of land look more natural and
get more value. The lot was sold and
bought many times between the old
events and the recent acquisition. This
gave enough time for trees to mature
and make the land look natural.
Clearly, the geotechnical investiga-
tion should have discovered the large
dump. Being asked more money by
the contractor, the owners were furious
against the geotechnical company for
having misled them. For the owners, it
meant that the geotechnical company
had not properly done its work, and
that the cost of the project was notably
increased. Keep in mind, however,
that this project was done a long time
ago, when the environmental rules
Figure 1. Sketch of the project with location of boreholes.