In early July 2010 during a routine inspection Missouri Department of Transportation (MoDOT) engineers discovered the ground had settled causing pavement settlement on a highly travelled section of I-470 in southeast Kansas City.
On July 8, MODOT closed the westbound lane because of minor settlement and pavement cracking. Eventually, the entire westbound highway was closed when multiple lanes of pavement collapsed. Traffic was rerouted north on both I-435 and U.S. 71 highway.
Initial speculation was that two months of constant rains prior to this event may have caused the embankment to shift resulting in the collapse.
This section of pavement is part of the larger I-470/ I-435/ U.S. 71 Three Trails Crossing interchange. The collapsed portion of the highway consists of five lanes and handles 60,000 drivers per day. MoDOT accelerated the design and construction process to reduce extensive delays and detours.
On July 22, just 14 days after the highway was closed, MoDOT awarded a $6.7 million contract to Pyramid Contractors Inc. of Olathe, Kan. ,to repair the damaged highway with a contract wrap-up date of Sept. 30.
According to Perry Allen, District Construction Engineer for MoDOT, “To expedite the repairs MoDOT also included a financial incentive of $40,000 per day up to $1 million for early completion.”
HNTB Corp of Kansas City, Mo., provided the design for the repairs on a design-build format which allowed Pyramid to start work immediately. Perry stated the only consideration given to the bridge design to expedite construction was to utilize as many standard features as possible.
This made the work extremely routine allowing Pyramid to utilize existing form forming material and practices. This allowed Pyramid to focus on streamlining existing processes then trying to determine the quickest way to execute construction on “new” unique design details.
According to Perry, “The contract called for the removal of fill material, old pavement and walls in the collapse area and build a 222 ft. (67.7 m) bridge extension. After the existing pavement was removed, Pyramid excavated the existing roadbed to the bottom of the bridge cap elevation. They used the existing fill as a falsework to form the bridge deck caps. This saved them the time of excavating first and then constructing traditional falsework to support the caps.”
Hayes Drilling Inc. of Olathe, Kan., provided the shaft drilling for the new bridge.
According to Perry, Hayes had two rigs, a Soilmec R-625 and SR-40 hydraulic drill, on site and operated them 24 hours per day. Perry said the new bridge extension would have seven piers at original end bent, three piers each at the two new intermediate bents, totaling 13 piers. The deepest pier would be was one of the middle-bents and measured 71 ft. (21.6 m) in length.
“Pyramid elected to pour the deck in two pours, rather than one. This allowed the first pour to cure permitting Pyramid to pour and complete the approach slab and pavement before the bridge,” said Perry. “Pyramid also chose not to slip form the barrier walls on the bridges allowing the bridge deck pour within 24 hours.”
The summer weather also played a role in assisting the construction process. During this period there were 16 days where the high temperature was 90 degrees and three days where the temperature was 100 degrees or above.
“The hot weather had a positive effect on the concrete popurs,” said Perry. “The need for speed in construction was assisted by early strengths generated by concrete pours made during hot conditions. If the ambient temperature was around the 70 degree level then concrete strength generation would have taken and additional one to three days for this mix design.
All of the bridge concrete was a MoDOT B-2 mix. This mix consisted of a higher cement content than other MoDOT mix designs and develops strength rapidly, especially with higher ambient temperatures. We were getting 4,000-plus PSI installed as early as 36 hours in some pours.
Subsequent investigations by MoDOT eventually determined that water infiltration as well as high water levels on streams during storm events caused a draw down effect on the embankment, Perry said. “Sub surface investigation prior to the original construction found no evidence of a water table within the existing embankment. Post-failure subsurface exploration discovered a water table present within the elevations of the fill where one did not exist before possibly caused by the above average precipitation over the last two years. Otherwise, there was no other significant change in the area that would affect the drainage condition.”
The highway was reopened to traffic on Aug. 27, 2010. In the end 20,000 man hours were expended to complete the project ahead of scheduled. Pyramid received $800,000 for early completion.
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