Portal: Structural

A downtown Vancouver excavation and shoring project involving underpinning of two adjacent structures took on a strange twist when it was discovered that the three-level, early 1920s building to the north of the excava-tion had a precariously attached brick wall founded on a rubble footing, which was required to be underpinned to construct the new building to the south. The struc-tural engineer, acting for the owner of the

The Municipality of Maple Ridge in British Columbia, Canada, commissioned a spe-cialty contractor to build a 9 m (29.5 ft)diameter culvert, incorporated in a mechanically stabilized earth (MSE) wall, to provide a street crossing for a stream. The culvert had been partly sunk into the stream bed to adapt to the site conditions. The MSE walls were a maxi-mum 8 m (26.3 ft) high above the ground and 70 m (230 ft) long. Figure 1 shows part of the North Face of the MSE wall.
The fill used in the construction of the MSE wall contained a fraction of fine dredged river sand. After construction, fine particles of the sand dried out and started to migrate through the galvanized metal screen that comprised part of the MSE wall. This resulted in voids at the surface of the MSE walls. The voids were a concern as they constituted a potential cause for future settlement of the sidewalks and asphalt pavement constructed between the MSE walls. The specialty contractor decided to remedy this situation by arresting the migrating fill before it came to its natural equilibrium, by refilling the surface voids and applying a shotcrete lining.
REPAIR SPECIFICATION
The design engineer recommended filling the voids with shotcrete and stabilizing the surface of the MSE walls with a permanent shotcrete lin-ing. The specifications called for the following procedures:

Nicholson Construction Company was awarded the con- tract to build a reinforced shotcrete soil nail retaining wall along State Route 9900 in Blair County, Penn-sylvania (U.S.). The 330-ft-(100-m)-long, 35-foot-(10.7-m)-high permanent retaining wall was cut into the hillside to provide an access route to the new Blair County Convention Center (see Figure 1). The general contractor for this project was HRI, Inc. and the owner was PennDOT.

Soil nailing has become a popular method to shore exca-vations and to build retaining walls due to its versatility and cost effectiveness. Generally broken into two cat-egories, temporary and permanent ground retention systems, soil nailing has evolved into many variations. A range of dif-ferent means and methods have emerged, giving the construc-tion industry an incredibly adaptable shoring system. The im-portance of a well-seasoned soil-nail team, however, cannot be understated for the success of any project.
Temporary soil nailing is the system that utilizes the nail for a limited time during construction process for ground support. Once the permanent structure is in place, the nails are essentially aban-doned. By contrast, permanent soil nails remain in service for the life of the wall. The shotcrete application required during the shor-ing process can also be temporary or permanent, but not necessar-ily the same as the nail. Hence, a temporary facing may be con-structed with permanent nails or vice versa.
Typically, in basement construction, soil nailing is used as a temporary shoring system and is installed concurrently with the excavation for the basement walls. After the basement has been excavated and the soil nail system is in place, either a mat slab or footings are poured and a permanent cast-in-place con-crete or shotcrete wall is installed. If a shotcrete wall is cho-sen, it can be installed with many different finishes to fit the use of the structure. Typically, shotcrete basement walls are fin-ished with wood or rubber floats to create a sand finish. In tem-porary soil nailed walls, the structural floors and walls are in-stalled; the soil nailing retention system is no longer needed to support the walls. In some cases, the wall is installed in a top down manner concurrent with the soil nail anchor installation, in both temporary and permanent shoring systems.
In the past, the highway departments often utilized the per-manent soil nail wall system with shotcrete lagging, covered with a cast-in-place or precast facing to give the completed wall a cast-in-place look. Now, however, permanent soil nailed walls are being used extensively along West Coast highways and freeways.
Although materials, equipment, and methods vary greatly, soil nail retaining wall construction involves variations of the following very basic steps:

With more than 7,000 cubic yards (5,350 m3) of shotcrete to be used for slope reinforcement in the Mesa­Payson Highway expansion in Arizona, it may be the biggest shotcrete application in the country. The project consists of shotcreting thirteen freshly-cut mountainsides in the Kitty Joe Canyon of the Matazel Mountain Range on Highway 87 between the cities of Mesa and Payson. The total cost of the 17-mile (27 km) road expansion is $55 million. “The entire project will take more than seven months to complete,” said Mickey Garner, president of Shotcrete Specialties, the sub-contractor. “And we’d be done if it hadn’t been for a freak snow last spring,” she continued. The area had about 3 feet (1 m) of snow just as the company was starting the project.