Slurry Concrete Diaphragm Walls help improve key Infrastructures
The Santa Ana River is designed to improve the scour protection of the BNSF Railway Bridge foundations during large discharges of the Prado Dam, which is one mile upstream from the project site. The original BNSF Bridge was designed for scour depths of 14 feet but due to the Prado Dam being modified to handle larger discharge flows, deeper scour has the potential to occur. The existing railroad bridge includes three main tracks that carry over 80 freight and passenger trains daily, which intensifies the project coordination.
The designed protection for the bridge consists of slurry concrete diaphragm walls surrounding piers #2 to #5, with the same technique being used at the East & West abutments. The diaphragm walls for the abutments include challenging T Panels designed as cantilever walls due to the proximity to the existing bridge foundations. The railroad bridge must remain active during construction therefore work must be performed from under the bridge with overhead clearances of only 28-30 feet. This requires a specially designed low overhead hydrocutter to perform panel excavation to depths of approximately 100 feet. Based on these height restrictions, the reinforcing steel cages require as many as five splices during installation at each panel location.
For approximately 18 months The Santa Ana River was temporarily diverted so construction of the pier protection could begin, and construction of the diaphragm walls could commence.
MMB and the Corps worked collaboratively to design the individual diaphragm wall panels. Because some of the panels had to be installed beneath the existing bridge structure, a special low-overhead hydrocutter was used to excavate the ground to make room for the panels.
Deepest diaphragm wall panels measured approximately 95 feet long. While panel sizes varied slightly, they were generally between 9 feet and 21 feet wide and 4 feet thick. The diaphragm wall design included tiebacks at each abutment, but they were not installed underneath the footprint of the bridge as their installation would have interfered with the abutment foundations. In the places where tiebacks couldn’t be installed, reinforced-concrete T-shaped diaphragm wall panels were used. During the construction of the diaphragm walls, the below grade sections of the T panels were built, and the above grade sections were formed as CIP concrete walls. Diaphragm wall rebar was coupled with CIP wall rebar.
Due to extensive planning, preparation, and execution, the work areas were able to be quickly restored during any major weather events to allow work to continue. Daily weather and river conditions are always assessed and monitored consecutively using multiple tools including constant contact with the dam operators. The project team worked very hard to establish an Emergency Action Plan with all stakeholders to reduce risk for all parties and always ensured the safety for personnel.
One technical challenge is the excavation of the diaphragm wall in the existing riverbed. The subsurface conditions include layers of sand, gravels, cobbles, and boulders. Additionally, the integrity of all diaphragm wall panels was tested using Crosshole Sonic Logging (CSL) for acceptance. Excellent QC measures were applied on all work phases from excavation over support fluid preparation, rebar and tremie concrete placement to ensure the highest standards are being achieved.
This project was a joint venture between Malcolm Drilling, Malcolm International and Bauer Foundations.