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Geotechnical News • March 2016
GEO-INTEREST
• The merits of the mini-trial method
of ADR for a practical technical
matter under a presiding Judge
with relevant experience, was
demonstrated.
“Fireman to the rescue” type
assignment
This summary focuses primarily on
lessons learned by the participating
Geo-Consultant. It illustrates pitfalls
which can be encountered through
limited involvement in a potential
dispute situation. A more detailed
technically related account is given by
Rowe and Seychuk (1995).
A young geo-engineering consultant
received an “SOS” call from a Munici-
pal Consulting Engineer stating that
construction of a sewer was experi-
encing wet ground conditions during
trench excavation and that assistance
was urgently required. Without hesita-
tion, or any previous involvement in
the subject project, the keen young
engineer proceeded to the site. The
engineer’s expeditious participation,
in a satisfactory design resolution of
the problem, unfortunately became a
“Horror Story” as discussed later.
Upon his arrival on site he noted that
the base of the trench excavation in
silty to sandy soil was in a “quagmire”
condition with only dewatering (sump
pumping) in use for groundwater
control. Furthermore, examination of
available records indicated that the
invert of the sewer had been lowered
below the depth of available geo-
technical information. The “rescue”
engineer requested additional bor-
ings and piezometers to depth. The
Constructor negated this requirement
on the grounds of time constraints
and instead excavated a test pit which
could not be taken to the necessary
depth because of the high groundwater
“soupy” conditions.
After a proper wellpoint system was
agreed upon and employed, the dis-
turbed soil in the trench problem area
was able to be removed and replaced
with lean concrete to invert level.
Dewatering to below invert level was
maintained throughout the remainder
of trench excavation operations, along
the sewer route, but the Construc-
tor objected (on the basis of cost
and workability issues) to the use of
well-graded granular material for the
trench bedding and insisted on “clear
stone”. The geotechnical engineer did
not agree with this on the grounds that
the surrounding sandy silt subsoil fines
could migrate into the “clear” stone
and cause settlement of the pipe. The
Prime Consultant came up with a com-
promise solution with the use of gravel
but with a filter fabric “wraparound”
to prevent soil fines migration into the
stone around the pipe and detrimen-
tal impact on basal ground support.
Trench excavation and pipe installa-
tion continued in that manner (with
“prior” wellpoint dewatering) along
the route. The geotechnical engineer
monitored construction operations for
a short while in the problem area, and
the Municipal’s engineer then took
over all site monitoring and compli-
ance responsibility.
About a year after completion of con-
struction, several “sink-holes” devel-
oped beneath the roadway surface at
locations where the Geo-Consultant
had not been involved. The site Devel-
oper initiated a lawsuit against all par-
ties involved – including the “rescue”
engineers consulting organization,
whereupon the Constructor and Prime
Consulting Engineer “combined” their
defence forces. In its “lone” defence,
the “rescue” engineering Firm carried
out extensive field and laboratory test-
ing that conclusively showed that the
filter fabric was effective in prevent-
ing soil fines migration into the clear
stone at the failure locations where the
natural subsoil was coarser grained
than at several other nearby locations
tested where the surrounding soil was
finer grained and where no failures
occurred.
So at the trial, the basic issues in the
dispute “boiled” down to:
a. Whether the failures were the result
of inadequate design and selection
of the filter cloth;
b. Or whether they were related
to movement of the subgrade
soil through tears, or open gaps
between the geotextile sheets (con-
struction related).
The “rescue” Geo-Consultant argued
alternative (b) while the Construc-
tor/Prime Consultant took position
(a). Notwithstanding the compelling
presentations by the Geo-Consultant’s
Team, the Judge concluded that:
• The geotextile permitted migration
of the natural soil through it and
should not have been used;
• There was no evidence of inade-
quate overlapping, or the presence
of gaps or tears in the geotextile.
• The “rescue” Geo-Consultant gave
opinions based on inadequate
information and did not stress the
importance of, borehole investiga-
tions, but relied on a shallow test
pit which did not go down to at
least sewer pipe invert level;
• If the Geo-Consultant was pressed
to proceed without adequate
subsurface information, he should
have either refused to do so, or
written a qualification report stat-
ing that his opinion is provided on
insufficient information, together
with a clear warning of the risks
involved; and
• The Geo-Consultant did not give
adequate instructions to the Prime
Consultant, or the Contractor, on
good practice procedures for geo-
textile installation prior to leaving
the site.
Based on his findings, the Judge ruled
that there was no evidence that either
the Contractor or the Prime Consultant
was negligent. The Geo-Consultant
was therefore solely liable for all the
costs in conjunction with the damages
incurred, including associated Legal
and Expert witness costs.
