SS9901-CS050321AECOM
Turner Collie & Braden
17300 Dallas Parkway, Suite 1010, Dallas, Texas 75248
~ 972 7353000 F972735.3001 wwwlcb.aecom¢om
March 21,2005
Mr. Kenneth M. Griffin, P.E.
Director of Engineering & Public Works
City of Coppell
PO Box 9478
Coppell, TX 75019
Subject: DeForest and Sandy Lake Wastewater Lift Stations
Dear Mr. Griffin:
Attached is a copy of the trip report prepared by Rob Osterman of our office to document his
observations during his visit to the DeForest Lift Station site last Friday, March 18, to assess the
condition of the excavation and shoring system. Please let us know if we can be of any further
assistance in this matter.
Sincerely,
Erika K. Cooper, P.E.
Project Engineer
Attachments Coppell Lift Station Trip Report
Figures 1 through 5
c: Rick O'Dell, City of Coppell
File
Page 1 of 2
Coppell DeForest Lift Station
Trip Report
18Mar05
Ken Griffin, City of Coppell, contacted Turner Collie & Braden Inc. (TCB) regarding the excavation
for the DeForest Lift Station for the City of Coppell. Mr. Griffin indicated that the excavation was
failing and requested TCB make a site visit, by qualified personnel, to observe and ascertain
conditions of the failure. This discussion indicated that the shoring was failing and the excavation
was falling into the hole.
TCB sent Rob Osterman, P.E. to make the initial site visit. On arriving at the site, it was obvious
that the ground on the north was falling off as shown in Figure I and the ground on the west side
was sinking as shown in Figures 2 and 3. The sheeting had moved in places due to the dirt
movement and the ring beams had also moved. Rick O'Dell (City of Coppell construction
inspector) arrived at the site and noted that the sinking had been a continuous occurrence since
earlier in the week and it was getting worse. The following observations were made.
Method of excavation presents opportunities for ,qround instability
The hole has been excavated substantially below the in-place sheeting as seen in Figure 4. This
exposes significant open face excavation prior to Canfer installing additional sheeting. The earth
pressure from the surrounding dirt then appears to cause the dirt to flow under the sheeting and
into the hole. This can be seen on the west side where several sink holes have appeared. The
dirt on the north wall appears to be sloughing off and forcing dirt under the sheeting.
Rinq beams were not insta{led in accordance with Canfer's excavation and shoring submittal
The submittal drawing indicates that the ring beams are to be connected by one of two methods.
First the ring beams are to have %" splice plates with a 5/16" fillet weld all around at the end of
each beam. The two beams are then to be connected by 4-%" bolts. Secondly, the ring beams
can be connected with a butt weld all around. The shoring as installed only has plates welded
with a single weld to the ring beams to make them continuous. Figure 4 shows these plates are
located only on the interior flange of the beams. Further, the beams apparently were not properly
tied to the supporting structure. The ring beams were broken loose in several locations and the
splices were either broken or severely damaged. It is unknown whether the ring beams are
without voids between the soldier piles as required in the submittal, but no shims were observed.
Minimal dewatering observed
Only one well point on the northwest corner of the bracing is installed as shown in Figure 5. It
was approximately five feet outside of the bracing. The geotechnical report noted that
groundwater was found at 13 to 14.5 feet and noted that this may vary during periods of heavy
precipitation. The geotechnical report also noted that measures should be taken to control
groundwater prior to excavating below a depth of ten feet. The report indicates that the site
should be dewatered to a depth of two feet below the excavation depth prior to excavation. The
use of one well appears questionable to dewater the entire site. The geotechnical report also
indicates that filters be used to reduce the loss of soil fines which would result in settlement. The
removal of soil fines could cause settlement as noted during this observation. The actual design
of the dewatering system is noted as the contractor's responsibility in the geotechnical report.
Construction Equipment
Machinery owned by MasTec was observed on the northeast side of the excavation. The dirt that
sloughing off is on the north and west sides of the excavation. The west side of the excavation is
a considerable distance away from the equipment. The contract documents and Canfer's
excavation and shoring submittal make note of the potential for construction equipment in the
vicinity of the excavation and the contractor's responsibility to account for the associated loads. It
was reported by Mr. O'Dell that the sinking did not appear to stop when the MasTec machinery
was moved further north. Mr. O'Dell reported that Canfer had indicated they would not fix the
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shoring excavation problem until the MasTec machinery was removed. Discussion with Canfer's
foreman indicated that pipe would be installed on Sunday and the machinery would be removed
by early next week.
We recommend the contractor provide a solution immediately and install temporary measures to
stabilize the excavation.
P:\CoppelI\DF-SL~LS Const\Coppell Lift Station.doc
F~gure 1
Figure 2
Figure 3
Figure 4
Figure 5