Train Inspection Pit Lifting Using Underpinning & Polyurethane Grouting, Washington, DC
58 sections of pre-cast commuter rail inspection pits were outside of installation tolerance at a new construction project in Washington, DC, requiring underpinning and polyurethane grouting. Each of the sections weight approximately 40,000 pounds, and were more than 12 inches thick, with two mats of embedded rebar. The sections were up to 1.75 inches out of tolerance, which was straight line slope with no more than 1/8″ in elevation across each joint. The deviation from tolerance was both side-to-side and end-to-end (each section was approximately 11 feet long and 5 feet wide).
Ability to meet the specified tolerance for position was most important, along with budget and time. Due to a very tight site, the work had to be completed before other phases of the project could commence. Additionally, all traffic to the site had to cross multiple 24/7 active Amtrak lines.
Slabjacking was originally requested to adjust the precast sections. However, during the repair design phase, it was determined that slabjacking with grout would not be able to achieve the level of tolerance required, particularly given the high loads and significant twisting on the short axis. Polyurethane foam injection alone was also ruled out from this perspective.
A repair using a combination mechanical underpinning using push (resistance) piers and high density polyurethane injection foam jacking was determined to be the most effective method for positioning each of the sections within tolerance. The chosen materials were wall-mounted resistance piers and high density polyurethane.
Each section was lifted primarily with resistance piers, and then void filling was completed using high density polyurethane grouting, which was also used for fine tuning of elevation. Frequent sampling of polyurethane was done to ensure integrity of material, and instantaneous load testing was achieved by transferring the entire load of each section to the underlying high density polyurethane from the piers.
In order to reduce expenses for the customer, the underpinning pier brackets were reused, as the repair was designed for the high density polyurethane to be the final load-bearing material.
Constant monitoring by a survey crew enabled each piece to be adjusted to tolerance and documented for as-builts and monitored for any resettlement.
All sections of the inspection pit were adjusted to achieve the owner’s tolerance using underpinning and foam jacking. A total of 116 ECP Model resistance piers were installed, along with 14,000 pounds of high density polyurethane foam. 14,000 pounds of high density polyurethane is equal to about 125 cubic yards of material. Due to the extensive compaction of adjacent and underlying soils (n>60), adequate lift was achieved from the resistance piers with a standard depth of around 10 feet.
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