Forensic Audit Report-WPCA Phase IV, Part B. Contract ... - Trumbull

Forensic Consulting Services Report
Town of Trumbull, Connecticut
Page 20
must recognize in the practically sense the technical concerns that could develop as a result of less
than adequate bedding of the pipe.
In regards to the “filter fabric envelope”, in lieu of fully encasing the pipe bedding material (all sides
with an overlap typically on the top), it appears only a single carpet-type layer of fabric was placed
above the bedding material just prior to the installation of the gravel fill and backfilling.
Unfortunately, although this may produce a filtering effect for fines descending from above, the
migration of fines entering the voided areas of the stone bedding from the exposed sides and bottom
may still potentially promote post-construction settlement of the trenches. This may be especially
true in areas of high groundwater where either the migration of groundwater within the trench is
constant and/or a seasonal variation in elevation expedites a transfer or flushing of fines.
Unfortunately, we were unable to acquire any supporting documentation and/or acknowledgement as
to why or when this “filter fabric” modification was actually made, as well as who initiated and
approved the modification for the Town. The response to our inquiries from Town representatives
indicated that this modification was initiated “early on in the field” during prior sewer contracts thus
being carried over to Contract 3 as a Town accepted practice. It is clear however that the original
design elements for this particular item of work was indeed part of the Contract No. 3 Design
Documents as originally bid and that despite a substantial reduction of work and material costs based
on the volume of pipe installed there were no subsequent credits requested and/or received by the
Town for allowing such a substitution by the contractor.
“Trench Drain” Details
A standard detail entitled “Trench Drain” is delineated on Design Sheet D-3. The composition of
this element of work as detailed appears to resemble what is commonly referred to as a “trench dam”
or “cut-off wall”. This type of installation typically is constructed as a means of controlling
groundwater by preventing or otherwise inhibiting large volumes of groundwater from migrating
downstream along a newly constructed trench. Although the existing detail is somewhat vague, the
installation requires a concrete trench dam to be constructed, keying it into both sides and bottom of
the excavated trench a minimum of 6-inches. It is important to note the intended design thickness of
the concrete dam and the frequency of installation along a trench is not provided.
The detail also specifies the installation of a perforated PVC pipe (4 linear feet in length) to be
installed “with holes down” parallel, but below the sewer main along with a solid PVC pipe
extending perpendicular from the outside limits of the trench “to outlet”. It appears the intended use
of this PVC pipe is to serve as a sub-drain, redirecting any collected groundwater within the
upstream side of the trench dam towards some type of unspecified “outlet”. It is important to note,
there is no pipe diameters specified for this installation nor are there any specifics to better describe
the intended destination and/or method of the “outlet” discharge.
In regards to payment, although there is no specific reference to the method of measurement and
payment for this particular item of work, it appears to be all inclusive (concrete dam and PVC subdrain
combined). As such, Pay Item No. 33 entitled “Trench Dam” with an established Unit Price of