For this project both the wall foundation and the wall face utilized precast double tee segments that were post tensioned together in the field. In addition, to eliminate shoring that would have been required for conventional wall options, the double tee footing extended in front of the wall. To increase sliding resistance a cip grade beam was linked to the double tee foundation segment following wall erection. Due to the post tensioning the wall tee stems are always in compression thereby eliminating the possibility of tension cracks forming on the fill side of the wall which can occur with typical cip walls.
The Mid-Coast Transit Corridor Transit Project is currently utilizing All the Walls’ Stresswall precast post-tensioned/prestressed counterfort wall system for the Midcoast Corridor Transit Project. The MCTC project extends the light rail from Old Town north to UCSD and La Jolla. It also adds a second heavy rail track through the entire Midcoast corridor.
The rail corridor runs through Rose Creek Canyon with 4 tracks: 2 light and 2 heavy E 80 rail. Due to the additional live tracks, there is a need for many different retaining walls along the alignment. There are both cut and fill wall applications since the track grade is also being raised in places to be above the 100-year flood elevation.
All the Walls originally approached Mid Coast Transit Constructors, the general contractor for the project, starting in 2015 proposing the Stresswall design as an alternative, cost effective option that met or exceeded all of the E 80 and LRT load criteria. MCTC realized the cost effectiveness and time savings available with the system so they successfully value engineered Stresswall for many of the walls.
Walls 508 and 563 were originally designed as CIDH soldier pile walls with two-stage tiebacks. Conventional walls of this type would be very expensive and time consuming to construct, as well as the associated inherent risks involved with drilling in difficult areas.
After months of extensive design review meetings with SANDAG, NCTD, WSP, HDR, TY Linn and other design firms, the Stresswall option was approved for construction. The acceptance of the Stresswall system on this project saved millions of dollars in construction costs that were credited back to SANDAG and MCTC and also saved months on the critical path construction schedule. Construction of these walls has gone very well and all parties are pleased with the design and final product.
Wall 482 ( 4,500 sq. ft.) was built over a weekend with two crews working around the clock while the track was shutdown. This site was excavated and the precast components installed and backfilled in 47 straight hours. The original approved plan design was an MSE wall backfilled with Cell Crete, which would not have been cost effective.
Walls 600 and 624 were both originally designed as MSE walls, however North County Transit District (NCTD) no longer allows MSE walls to be built to support their track embankments, so Stresswall was selected as the most viable alternative. The change to Stresswall eliminated the shoring that would have otherwise been required for MSE walls, resulting in substantial excavation savings while providing with a total precast/prestressed product.
This project used All the Walls’ patented proprietary retaining wall system. It was also awarded PCI’s 2019 Best Non Highway Bridge Project.
The result of a VECP submittal by Faltiron Construction the Stresswall option was selected due to speed of erection and cost savings due to elimination of shoring that would have been required for any other wall option at that location. Typical shots show the “slot cut” excavation method wherein the existing emabankment was substantially left in place instead of removal which would have been required for other wall types. By cutting 4″ wide slots into the embankment at 15′ centers the precast counterforts could be placed w/o total wall excavation or any site shoring.
E 80 & LRT Track Support WallWall Scope Of Work
POST TENSIONED & DOUBLE TEE E-80 WALLS
This precast system utilizes large double tee members that are typically used for long span bridges to form post-tensioned retaining walls. Double tee elements are also used for the wall foundation elements, resulting in an efficient and expeditious wall installation.
After placement of the DOT footing elements on a granular leveling foundation course, the vertical DOT double tee wall panels are set over vertical sleeved post tensioning bars secured to the base tees. Following the subsequent post tensioning of the face and base tee segments, the structurally competent wall can be backfilled. Since the DOT wall tees are typically 10 to 12 feet wide with heights of over 50 feet, wall installation square footage per day is significantly more than is the case for any other wall option. In addition, because the wall elements are post tensioned together, the concrete elements are always in compression, so no tension cracks can develop, which is typically the case for other conventional wall options.