Underground Construction - June 2018 - 37

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Whatever the reason, obtaining current field
measurements is important and can be incorporated
into most pipe and manhole inspection programs
that utilize NASSCO's Pipeline Assessment
and Certification Program (PACP) and Manhole
Assessment and Certification Program (MACP) for
data collection.
Of course, there is always the use of a record
drawing, assuming it exists, can be located, and is
legible. Even if it is legible, however, measurements
like pipe inverts, pipe diameters, and rim elevations
do change over time, and in some cases that time
is upwards of 100 years! In other cases, when a
municipality's first GIS was initially developed, some
digitization may have been performed and data
pulled from those record drawings outdated due to
repairs or was entered incorrectly in the first place.
Additionally, post-construction settlement caused
by poor compaction or developing voids in the soil
surrounding the pipe, over time, may alter original
elevations shown on record drawings or road
resurfacing and road reconstruction projects may
raise surface elevations.
For example, a New England community installed
a sewer line under a local highway in the early
1800s. The initial pavement deteriorated due to
snow, ice, and continuous vehicular traffic on the
highway. The community then repaved the highway
three times in the last 20 years, each time raising
the road elevation, thus requiring that sewer frames
and covers be raised to the new elevation. In
this case the manhole inverts are now deeper by
several inches.
So, if you are a collection system owner updating
or developing an accurate hydraulic model of your
system, using GIS-based drawings for trenchless
sewer rehabilitation, or assigning criticality to
assets for prioritizing efforts, consider making the
PACP and MACP fields that require these various
"measurements" mandatory for your operators
during CCTV work or routine opening of manholes
to obtain current data in addition to the original
purpose of the site visit.
There are several inexpensive tools on the market
that can speed up the process for obtaining these
measurements while providing great accuracy. The
PACP standard recommends that a survey rod be
used to measure depths (rim to invert, rim to grade,
grade to invert, etc.), while a measuring wheel be
used to determine the length between manholes (or
other access points to the sewer). A measuring

obtaining accurate field
measurements is critical.

tape is too flexible and can introduce errors in
measurements, therefore is not recommended.
For measuring accurate inverts and pipe diameters,
a PIPE-MIC®II can be used and easily attached to
the bottom of a survey rod, using an extension arm
and vertical plumb rod for accurate measurements.
As mentioned previously, these practices are
probably the most critical and beneficial when
creating a hydraulic model. Hydraulic models are
intricate and sensitive, using calculated slopes
between manholes to calculate flow capacity. Many
municipalities use hydraulic models to understand
and predict the capacity of their systems. The first
step in developing a hydraulic model is building
a physical model network which includes pipes,
manholes, catch basins, outfalls, and any pump
stations. The data associated with these types of
assets (depths, sizes, lengths, etc.) must be current
and accurate so that the results can be used
confidently to make key decisions on needed sewer
system improvements such as storage, upsizing, or
parallel sewers.
As a best practice, obtaining current field
measurements for pipes, manholes, and other
structures as part of a routine PACP/MACP
assessment program will provide the necessary
accuracy for proper management and maintenance
of sewer collection systems in lieu of record
drawings. Accurate measurements will also improve
the use and validity of a hydraulic model and
contribute to accurate contract documents and
bid quantities which can ultimately result in fewer
change orders.
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Underground Construction - June 2018

Table of Contents for the Digital Edition of Underground Construction - June 2018
