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Geotechnical News • March 2014
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GEOTECHNICAL INSTRUMENTATION NEWS
others, but hard data makes for a more
compelling case.
From 2007 to 2012, more than 8,000
laser-distometer readings were col-
lected at the East Side Access Project
between 310 tunnel and cavern defor-
mation point pairs. Readings in the
tunnels were collected immediately
behind the tunnel boring machines
between targets installed on virgin
rock, and readings in the caverns were
recorded immediately following exca-
vation by drill and blast.
The data collected were analyzed to
validate the suitability of the laser-dis-
tometer as a replacement for the tape
extensometer. To evaluate the suit-
ability, all erroneous data (anchor bolt
damaged or destroyed) was removed
from the dataset, and all point pairs
indicating trending data (deformation
is occurring) were removed. This left
a “stable” dataset of more than 6,000
readings between 245 deformation
point pairs (Figure 4). The intent of
reducing the complete dataset to a
“stable” dataset is to analyze the laser-
distometer as a measurement tool,
rather than analyze the actual defor-
mation that may have occurred.
Results of the program and
conclusions
The scatter in the stable dataset plotted
as a histogram is an excellent approxi-
mation of the normal distribution.
Therefore, by calculating the mean
and standard deviation of the stable
dataset, it follows that 93.1% of read-
ings fall within the three millimeter
manufacturer specification of instru-
ment precision.
In reality, random errors are intro-
duced into the measurement process
by the operator. Assuming that random
errors by the operator introduce an
additional one-millimeter of error into
the readings, it follows that 96.3% of
readings fall within the instrument
specifications, and if two millimeters
of random error are introduced, then
99.8% of readings fall within the
instrument specifications.
The most significant random error
to be mindful of is “pointing” error,
whereby the laser beam is reflected
from different points on the second
target during successive readings.
Other random errors may be intro-
duced into the readings when the
laser-distometer is not snugly fitted
into the cradle, or when excessive dust
within the tunnel causes refraction of
the laser beam.
The laser-distometer has met the
requirements for deformation monitor-
ing at the East Side Access Project,
and has proven to be a viable alterna-
tive to the tape extensometer. Under
ideal conditions, it is possible that the
tape extensometer may yield defor-
mation readings of higher precision,
however this comes at a consider-
ably higher cost, both in material and
labor. Where project-specified limits
of movement allow for the substitu-
tion of a laser-distometer for the tape
extensometer, the option should be
seriously considered.
Chris Fagan
Geocomp Engineering, 139 Fulton
Street, Suite 917, New York, NY 10038.
Tel: 646-831-8941.
email: cfagan@geocomp.com.
Charlie Daugherty
Parsons Brinckerhoff, One Penn
Plaza, New York, NY 10119.
Tel: 212-465-5451,
email: daugherty@pbworld.com.
Commercial sources:
Leica Geosystems – laser-distance
meters
Leica Disto A3 (used for monitoring
at East Side Access described in this
article)
Trimble – laser-distance meter
Tape Extensometer
Slope Indicator -
Geokon -
ITMSoil -
RST -
Figure 4. Laser- distometer dataset.