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Start of Construction of Two Interesting Retaining Walls
April 2024
In the past few years, we have designed a whole slew of retaining walls – many of which are deserving of blog posts. However, RTGA is just starting the construction consultation phase for two different and unusual retaining wall systems in Reno, Nevada for Madole Construction.
The first is a set of eight temporary retaining walls for two bridges being widened on Interstate 395 in Northern Reno. The retaining walls are being constructed in the median between the northbound and southbound lanes of US 395 at Panther Valley and the North Virginia Street Interchange, just north and south of the bridges to be widened. As seen in the figure below, each wall will initially be a soil nail wall which will retain the soil (drawn gray in the diagram) under the existing freeway lanes.
In phase 1, they are conventional soil nail walls with shotcrete facing, up to 30 feet high. Shotcrete is a form of concrete where concrete is projected by compressed air (shot) at high velocities onto a surface. The bridges are skewed at 17 to 41 degrees, further complicating the soil nail layout.
After the soil in the middle of the freeway is excavated as shown above on the right, it will be replaced as fill to support the new MSE abutment. As the fill is placed, we will be adding plate anchors in the new filled zone facing inward (note red starter anchors pre-placed in the shotcrete wall above) so that it will be supported when the outer freeway lanes are removed on the left. Then the outer lanes (to the left) are removed and excavated in Phase 2, including the Phase 1 soil nails. The final retaining wall configuration is shown below, with the anchors supported by the same type of hollow-bar micropiles as used for the soil nails, except placed in backfill in horizontal layers. The shotcrete face is to be left in place between the right-facing and the left-facing excavation.
The picture to the right shows the start of the first 5 feet of excavation for the southern of the two bridges, where Madole Construction is installing test nails to verify soil bond strength. The soil looks much stronger than was indicated in the boring logs, so we may be able to shorten the nails some.
The second project is another temporary wall we call a “spile wall” to support a deep excavation for a parking garage one block from the University of Nevada, Reno campus. “Spiles” typically refer to rock anchors which knit support a tunnel as it is being excavated by nailing the rock on the ceiling in place before the material below is removed. The spiles are closely-placed in a line around the excavation face in an arch or fan.
We (and no one else we are aware of) call our planned design a “spile wall” because the front row of nails (typically at 1-1/2 foot spacing on center) basically knits together the soil in front so that there is no discontinuity, and soil arches between the nails so it cannot fall out. The plan section below is a 20-foot tall temporary excavation one block from the University of Nevada Reno campus, which will start construction in August. The property is adjacent to a railroad right of way, and shoring or excavation which would extend over the right of way line was not feasible. The cross section below shows the wall is supported by two rows of micropiles in an A-frame configuration, which are embedded in the soil. The nails act as a couple, tied by a small concrete cap beam, and the soil as a shear element, make the wall stand up. This is the tallest spile wall we have designed for temporary support in Reno in hard clay deposits, most of our previous spile walls have been in competent silty sands in excavations around Lake Tahoe.
The contractor confirms that this approach is cost effective compared to other shoring methods (including soil nail walls, which have the disadvantage that the mostly-horizontal soil nails typically would extend across property lines, which may not be permitted).
The last two photos are a temporary spile wall we designed (and Madole Construction built) on Moana Lane in Reno, which we observed during construction this winter. The wall allowed construction of a 1-story-below-ground parking level within 8 feet of the property line for a five-story garage. The entire wall at the toe was about three feet wide from the property line, and the wall fully supported a 12-foot-high excavation. The first photo shows the micropiles before installation of the cap beam, and the second shows the completed excavated face. The garage footing now takes up the entire width of the excavation. Because the micropiles were closely spaced, and the soil is cohesive, no shotcrete or other facing was used. The micropiles are generally a few inches behind the excavated face, sometimes the grout around the micropiles can be observed as a light gray streak, or a bar may be accidentally excavated. The general contractor reported that OSHA came out and reviewed the wall and our plans, and let the work continue.
