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Geotechnical News • March 2016
GEOTECHNICAL INSTRUMENTATION NEWS
½-inch of lateral building or excava-
tion support movement, say during a
cantilever excavation phase, are you
more concerned if no movement is
reported or if 5/8” of movement is
recorded? The reporting of “zero”
movement
may
be more indicative
of a problem and result in raising a
bigger flag to reassess the monitoring
system stability and or data process-
ing algorithm and suggest something
is not working properly or according
to expectations. Alternatively, 5/8” of
reported movement while potentially
alarming to one not familiar with the
design analysis,
may
support that the
structural engineer and monitoring
team deserve praise for their deforma-
tion analysis and movement reporting,
just 1/8” off from their estimate. Who
is best suited to evaluate these pos-
sibilities?
The evolution of roles,
sub-specialization
This discussion offers that a PLS may
be as suitably trained to administer
these programs as the PE. In the past
before recent Codes and Specifica-
tions, the PE may have performed land
surveying directly. This person likely
played a prominent role in the design
and construction inspection, and
performed optical monitoring from
the job site where physically aware
of ongoing construction progress and
activities, weather trends and other
external factors which affect the
adequacy of their recorded data. High
or sudden vibrations or rapid tempera-
ture swings resulting in poor survey
traverse closure and thereby increased
error were marked with an asterisk
as they were recorded or reported, as
the evaluation was made concurrent
with data processing by those famil-
iar and trained in recognizing these
occurrences. Potential to lose such
observations occurs more frequently
in automated data processing software
algorithms and or those in which third
party monitoring consultants perform
their tasks independently from other
trades.
Leading into the 1990s and to the
present day on many smaller projects,
the PLS generally provided the instal-
lation and as-built location of monitor-
ing “points” plus periodic readings
of delta x, y, z for interpretation by
“others”. The qualifications of the
“others” varied widely, from owner, to
owner’s representative in the form of
the general contractor or construction
manager, to an architect or engineer
likely specializing or sub-specializing
in a different discipline.
• How qualified are those people to
understand ground movements,
building response and/or to rec-
ognize typical red flags indicating
potential errant readings or system
flaws, or true signs of movement
versus scatter, or no reported
movement despite large seasonal
thermal variations?
• How intimate were these people to
the anticipated ground or building
response?
• How much did or does the risk
of underestimating or under-
recording or under-recognizing the
amount of deformation movement
matter, meaning what are the
inherent project risks?
• Are such things addressed in the
majority of boiler plate or recycled
project specifications?
Technology and methodology has
morphed into current practice, and
the efficiency of increased monitoring
frequencies has supported automa-
tion in hopes of achieving greater data
quality. As movement trends were
further defined by multiple readings
per day or hour, the less frequent
manual survey by PLS became less
cost efficient comparatively. There
seems to be a cross-over point at a fre-
quency of about two to three readings
per week at least in New York City,
where monitoring systems generally
become automated and the work scope
shifts from PLS to PE (unless a PLS
administers the automated system). A
PLS two-person crew, at $1,800 per
day with equipment and office support
performed three times per week results
in costs of about $5,400 per week or
$23,000 per month. Over the course of
several months, automation becomes
preferable and cost efficient while
realizing numerous other advantages
over manual survey.
Affordable redundancy by
Professional Land Surveyor
I advocate using a PLS to provide
monitoring point as-built and thus
licensed coordinates during the base-
line monitoring period, and periodi-
cally throughout the work as a sanity
check of an automated system. In
monitoring projects of 4 to 6 months
or longer bridging a seasonal change,
a building is likely to respond by
deforming through its maximum nor-
mal atmospheric drift or range as well,
irrespective of adjacent construction
activity. As introduced above, should
automated readings suggests either
zero movement or 5/8” of movement
whatever the case may be, a re-survey
of prisms by the same PLS and means
and methods
may
be appropriate to
verify the automated readings, or to
flag that a more detailed review of
one or both systems is warranted.
Recognize it is plausible that seasonal
thermal variation effects increase,
decrease or cancel out construction
induced movements over any par-
ticular time period, though it unlikely
movement trends would align with
environmental factors if that was the
case, hence the need for good baseline
data over a range of thermal condi-
tions and frequent readings. These
may be considered redundant read-
ings, so cost implications factor into
whether or when they are performed.
Collaboration among the
morphing evolution of roles
into subspecialties
Further sub-specialization of tasks
and consultants (not only in survey
or geotechnical disciplines but others
as well) puts a higher level of ethical
and technical responsibility on the part
of the PE designing, specifying and
or signing off on these programs or
