A failing slope on a bluff on 22-acre Alcatraz Island (The Rock, a former prison in San Francisco Bay and now popular national monument) was recently repaired by Boulderscape, San Juan Capistrano, Calf., a design/build firm that specializes in shotcrete. “The bluff had eroded so much that the Warden’s House (a historic structure) was only 4 feet from the slope,” says Steve Jimenez, senior vice president, commercial sales, for Boulderscape. “Rocks were falling from the harsh weather and wind and threatening to undermine the building.”
For this application, Boulderscape used Quikrete Shotcrete MS and Shotcrete MS – Fiber Reinforced to successfully repair and stabilize the slope. The fiber-reinforced shotcrete was applied first as a structural layer, explains Jimenez, followed by a layer of mesh and then the architectural layer. In addition to facing dangerous application conditions, a condensed shotcrete schedule was required to avoid disruptions to regular tour operations and to match the habitat and migration patterns of local sea birds.
“Navigating a variety of unique factors made this a very fulfilling project for everyone,” says Jimenez. “We couldn’t start until the indigenous birds migrated from the island in November, and we had to complete our portion of the work before the birds returned in February, which meant everything had to be expedited starting with the delivery of material.”
Alcatraz is one of the most popular national monuments in the country, with annual visitors of more than 1.5 million. This prompts the National Park Service to want to minimize disrputions. During the day, when tourists were visiting, Boulderscape was required to cover everything up. “Originally they wanted us to work only between 8 in the morning and 5 in the afternoon,” says Jimenez. “But it’s too windy during the day to be spraying shotcrete. I finally convinced them that the only way was to work at night when the wind is a little calmer. We lit it up with four 4000-watt light towers on the bluff facing San Francisco.” So much for being inconspicuous.
In one weekend, 131 bulk bags (3000 pounds each) of Shotcrete MS – Fiber Reinforced and 126 bulk bags of Shotcrete MS were delivered to Alcatraz Island for the project. Quikrete delivered 16 truckloads of material to Pier 50 in San Francisco. The bulk bags were then shipped on two barges to Pier 33 on Alcatraz Island where they were offloaded at night by crane one-by-one before finally being transported to the jobsite on trailers pulled by four-wheel all-terrain vehicles.
Beyond the challenging schedule, the application process was full of danger. The nozzlemen had to be hoisted more than 60 feet in manlifts and with ropes to spray-apply the shotcrete before sculptors rappelled down the cliff to shape the material. The winds on Alcatraz are continuous and given that the work was in the winter, cold. But, according to Jimenez, “The end result was a structurally sound slope that matched the surrounding environment.” Boulderscape workers carved the face and used water tools to give it an eroded appearance. This was followed by stains to make the shotcrete match the surrounding bluffs and the old concrete.
Boulderscape’s part of the project was completed in under three weeks. “This was one of the most exciting projects we’ve ever done,” brags Jimenz.
Shotcrete MS – Fiber Reinforced is a single component silica-fume-enhanced repair and restoration material that achieves more than 9000 psi compressive strength in 28 days, and features very low rebound and permeability characteristics. This material meets the requirements of a shotcrete material: It can be applied through a wet or dry process to deliver high strength, high adhesion, low rebound, and low sag. For more information, visit www.quikrete.com.