Kansas State University Football Stadium Expansion

Solution

Longfellow Foundations was contracted to perform the concrete piles on the project. Longfellow partnered with Thrasher to complete the shotcrete portion of the project. Longfellow utilized auger cast drilling equipment they had on-site to install the grouted soil nails. This required cutting into the side of a 30-foot-tall hill to allow construction directly next to the current football stadium seating area. Working together, Longfellow Foundations installed grouted soil nails while Thrasher encased the grouted soil nails with #5 rebar to build an 8" x8" grid matt rebar cage, strip drains and 144 cubic yards of shotcrete for a total wall area of 3,300 square feet. The shotcrete wall was installed in multiple five-foot lifts. Through constant coordination between Thrasher, Longfellow and other contractors, the project was completed on time in a total of three weeks. 

Multiple solutions can be used to provide soil retention/shoring. Thrasher provides grouted soil nails and mechanical helical soil nails, and shotcrete for earth retention and shoring. Both systems can be installed in tight access areas, and neither causes vibration that could be harmful to existing structures. 

Soil nailing is a method of earth retention that relies on reinforcing strands or members installed within a soil mass to create an internally stable gravity wall/retaining system. Soil nail walls were first used in North America for temporary excavation support in the late 1960s and continued to gain recognition and acceptance during the 1970s and 1980s for higher-profile projects, including highway applications. 

Soil nail walls are typically more economical than other earth retention walls. 

Mechanical Soil nails are typically shorter than grouted soil nails for similar wall heights, so there will be reduced right-of-way (ROW) requirements.

There is less impact on adjacent structures since soil nails are not installed with vibratory energy like soldier piles or sheet-piles.

Overhead clearance requirements are less than driven soldier pile or sheet-pile wall construction. Soil nail walls can therefore be installed easily below bridges or other structures.

There is no need to embed structural elements below the proposed ground surface elevation on the soil nail wall's low side. Soldier pile and sheet-pile walls require minimum embedment depths for wall stability.

Soil nail wall construction is typically quicker than other earth retention methods.

Soil nail walls can be constructed in remote areas with smaller equipment.

Soil nail walls have performed well during seismic loading events due to the overall system flexibility.

A helical soil nail typically consists of square shaft lead and extension sections with small diameter (6 to 8 inches) helix plates spaced evenly along the entire shaft length. The helical soil nail is installed by application of torque, similar to installing a helical tieback. The helical soil nail is a passive bearing element, which relies on the movement of the soil mass and active earth pressures to mobilize the soil shear strength along with the nail. Mechanical soil nails like helical piers are installed using a torque monitoring system. Install torque is a direct correlation of capacity, which eliminates the need to proof-test helical soil nails.  

A grouted soil nail typically consists of auguring or drilling a predetermined size diameter and depth hole to meet the design criteria. The holes are drilled at a minimum 15-degree downward angle to hold the grout concrete. Once the hole is drilled, grout is pumped into the hole, followed by a reinforcing rod accompanied by centralizers. The grouted soil nail-like its helical soil nail counterpart, is also a passive element. Neither grouted nor mechanical soil nails need to be post-tensioned, but grouted soil nails are often proof tested to ensure design requirements have been met.   

Soil nail walls are constructed from the top down where the excavation proceeds. The construction sequence for a typical soil nail wall includes:

• Initial vertical excavation of about 3 to 5 feet deep depending upon design parameters and soil conditions.
• Installation of the first row of soil nails to the required inclination angle, torque and embedment length
• Placement of drainage medium (if required)
• Placement of wall reinforcement and bearing plates
• Placement of shotcrete to the required design wall thickness

After shotcrete has cured, repeat sequence for successive rows of soil nails. Continue the process to the final design depth (wall height).