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.
This project exemplifies many unique features of the Stresswall Product. The project developer for the South Coast Medical Center Hospital expansion project, Sheldon Pollock in Santa Monica, requested that we provide a wall that would eliminate shoring that would have been required for any other wall option to support the existing Hospital embankment.
We proposed the “Terrace Wall” counterfort wall option which could be installed w/o shoring. A new building was to be constructed in front of and at a lower elevation than the existing five story structure. To do this a portion of the sloping embankment in front of the existing center would have to be removed w/o disturbing the existing building (maximum allowed settlement would have to be less than 0.5 inch!
To accomplish this the contractor cut deep slots into the existing slope between the existing deep caisson foundation for the existing structure. Since the site was is a stones throw from the ocean there is always a risk of morning rains etc. so the contractor was required to provide a site cover to eliminate any potential erosion should storms occur during wall construction. The slope face was covered every night and over weekends.
Following the slot cutting the contractor placed the open face counterforts in the slots and backfilled the portion of the counterforts w/in the slot cuts with slurry. Subsequently the .5 meter wide wall panels were placed in the Terrace Wall reverse batter voids.
Due the the spacing and orientation of the wall panels there is approximately half of the wall face area that could be planted. By selecting maintenance free, continuously blooming plants with an emitter irrigation system the concrete members quickly disappeared as the planting matured. As can be seen from the shots the result is a constantly changing pallet of color instead of a stark industrial look as would be the case for a conventional wall. In addition, the wall reduces the effect highway and other noise sources which is an advantage for patients in the new structure.
At the request of the ASCE we presented the Project for their annual meeting due to the unique features and advantages of the counterfort wall system.
The precast wall was an alternate in the Woodward-Clyde bid package and was selected by Granite Construction over the MSE wall alternate. An integral wave deflector was monolithically cast with the top wall panels instead of the field cast cap that would shown for the MSE wall.
Prior to the wall installation a trench was cut into the slope and a cip cut off wall was constructed behind rip rap. The precast wall components were then installed. Since sand was used for wall fill filter fabric/geotextiles were placed over the fill side of the precast elements.
The wall has been subjected to many extreme load conditions compared to a typical seawall/erosion control structure. Locals claim that the tallest wave ever recorded in North America occurred at Pacifica. There are shots of these giant waves at local establishments. Some smaller waves are shown crashing into the wall in the shots.
Also the wall was impacted by the San Francisco earthquake which hit shortly after the wall was completed. The existing MSE wall north of the fishing pier has not fared so well. We understand that due to the corrosion of the metal soil reinforcement and movement fill has been lost from behind the wall. To address this issue the city has apparently refilled voids with concrete fill. There have been no issues with the Stresswall Structure.
To lengthen the radius of a curve as well as to smooth out the road alignment a cut was required for Wiley Canyon. To accomplish this without shoring the Stresswall system was selected. Although the wall face is at a 1:12 batter (essentially vertical) random planting areas within the wall face for internal landscaping options were made possible with the large random planting “pockets” in the wall face. Plants included both large flowering bushes as well as vines and ivy.
The counterforts were placed in slots cut into the existing embankment so that no shoring was required as would have been the case for other wall options. Wall cut was utilized for wall fill.
Noveco Inc. Client
Installation of Stresswall into Existing Slope Scope Of Work
Single tier seawall structure placed along WashDOT SR31 at the Canadian Border.
The counterforts were placed in slots cut into the existing roadway embankment. Due to the constant border traffic it was necessary to construct the wall as one lane was kept open. Other wall options would have required shoring.
By utilizing the “slot cut” installation method temporary shoring was eliminated that would have been requires for other wall types. Therefore the overall wall cost and construction time was greatly reduced compared to other wall options.
This essentially vertical wall supports high E 80 loads (night traffic) and LRT commuter rail traffic during the day. By using reverse batter counterforts a planting space is provided between the wall tiers. Internal wall landscaping is unique to the Stresswall System and softens and aesthetically blends the wall face into the environment while eliminating the possibility of graffiti on the wall surface and associated removal costs.
This ASCE award winning wall is the result of a value engineering proposal submitted to SANDAG (formerly MTDB) by the contractor. The original plan design was a typical MSE wall that required very specific wall fill. All of the wall components are precast/prestressed elements and were made locally. The essentially vertical wall with internal planting offset between the counterfort tiers are possible due to the unique revers batter counterfort configuration. Morrison Knudsen was the engineer for the wall.
Brad Lewis (with KTU&A), the landscape architect, selected indigenous plants that alternatively bloomed continuously so that the wall face always has a variable color pallet. Since emitters are used for irrigation water use is at a minimum.
One of the factors that added VECP was the reduced cost of fill for the counterfort wall compared to the MSE wall. By utilizing construction debris from a landfill substantial savings were realized. In addition, the erection rate for the counterfort wall was substanially faster than would have been the case for an MSE wall.
The MSE wall also required internal cip foundation supports for the catenary poles for rail power lines. Due to the structural capacity of the precast counerforts special counterforts were designed to support both the top tier wall panels and to be utilized as foundation support members for the catenary poles.
This wall, as well as other Stresswalls supplied to SANDAG, are designed for locomotive E 80 traffic. During the day the rails are used by LRT lines whereas at night freight lines utilize the line.
This project is the result of a VECP proposed to the Bureau of Reclamation by the contractor. The wall supports the access road to the Theodore Roosevelt Dam Power Station.
The original design was a typical massive cip wall. Field testing determined that ground anchors would be required to both pre-compress the insitu soil as well as enhance wall stability. Therefore the wall has a unique foundation configuration. By including a cip grade beam with vertically oriented mini piles over which the counterforts were placed provided adequate foundation bearing capacity for the wall. In addition, following placement of the base counterforts, ground anchors (tie backs) where inserted through the cip grade beam that was cast over the tails of the counterforts.
Following the installation of the base tiers and anchors the subsequent upper tier components were placed. Site excavated cut material was used for wall fill.
The Stresswall concept was originally developed by Morrison Knudsen engineering under contract for the Colorado Department of Transportation. CDOT, in the early 70’s, was planning for the extension of I 70 west from Idaho Springs to Glenwood Springs Colorado. Due to the short construction season at elevations over 7,000 feet CDOT felt that a precast retaining wall option would be the optimum solution. The result was a combination of precast L-shaped counterforts with precast parabolic panels placed between adjacent counterforts.
Due to the shape of the 1 meter high wall panels the panel reinforcement was at a minimum since the resolved earth loads on the panels resulted in a compressive load being transferred to the counterforts. Therefore no tension cracks would develop or propagate on the panels as would have been the case for any other shaped panels. We now use rectangular section prestressed panels in combination with post tensioned counterforts to maximize the load capacity of the counterfort /panels assemblies. Accordingly all of the potential load points of the counterfort/panel assembly has an induced compressive load so no tension cracks can develop.