With Rembco’s Help, The University of Tennessee Renovates And Expands Strong Hall
Sophronia Strong Hall at the University of Tennessee dates back to 1925, when it (or as the locals would say, “she,”) opened as a female residence house. Currently, the University is renovating the structure into a large and modern science class and laboratory facility. The two-year, $114 million project marks the first big step in addressing the university’s need for general class and laboratory space.
The new nine-story, 268,000-square-foot building will be home to the anthropology and earth and planetary sciences departments and also provide critical instruction and lab space for the general biology and chemistry departments.
The renovation and expansion project will preserve 20,000 square feet of the original structure, including the building’s distinctive front arches. But since the new building will be much larger, there were issues with stabilization. So UT called Rembco.
As construction planning began, UT considered several approaches for stabilizing a deep excavation until the foundation and basement walls could be built and back-filled. One option for shoring the 26’ deep cut was pile and lagging. It’s not a great option, however, because a pile and lagging wall requires immense piles and tie-back anchors if it’s greater than about 10’ tall. That was too expensive.
Another option, laying back the cut, would avoid a shoring system altogether. On this site, however, an excavation with a 1:1 slope would require about 26’ linear feet for the lay back. That would extend the construction site boundary into an adjoining parking lot on one side and into the adjoining streets on two other sides. So laying back the slope was not an option.
The third option was a soil nail wall. A soil nail wall is less complicated and less expensive to build than a pile and lagging wall. And it requires much less space than a lay back because it can be built with a vertical or near vertical face.
Soil nailing is generally the most economical option for a cut wall–especially when it’s a 26-footer!
The University apparently came to the same conclusion, so officials contacted Rembco to provide the design and drawings for a temporary soil nail wall. When our proposal was approved, we began construction and testing of the 26’ wall.
This isn’t our first experience with similar construction at the University. In fact, Rembco first provided a similar shoring system for UT more than 30 years ago. Here’s how we do it.
The Soil Nail Wall Demystified
Starting at grade level, the site is excavated 4’ to 6’. Next, we begin at one end of the vertical cut face and drill a near horizontal hole to a prescribed depth in the face of the cut. As the drill rod is withdrawn, the hole is pumped full of slurry grout. Then, a threaded steel bar with centralizers is inserted into the grout filled hole. While the cemented bar is left to set up, it’s time for the next hole.
The drill rig is moved a pre-determined distance along the face of the cut and the process is repeated. We do this along the entire length of the wall – constructing a horizontal line of equally spaced soil nails. After installing the last nail at this elevation or “lift”, reinforcing wire is pinned all along the vertical face. The wire covered face is then sprayed with a 4” blanket of shotcrete. The threaded bar tips protrude through the shotcrete allowing metal plates to be installed and tightened until they are snug against the shotcrete facing. That’s a soil nail wall.
For a taller wall, we build onto the bottom of the section we just built. The section is begun by excavating another 4’ to 6’ from the jobsite. Then the drilling, grouting and shotcreting process begins again. With this approach, an entire vertical or near vertical wall is built from the top down, in 4’ to 6’-lifts, until the lowest level of the excavation is reached.
As with many of our projects, Rembco’s portion of Sophie’s face-lift was done in a limited space and with lots of other construction activity. But we finished on time and under budget.