A tall wall in a tight space

November 2009 » PROJECT CASE STUDY
A large-block retaining wall with soil nails stabilizes a steep slope and saves homes.
Lindsey Manthei
Soil Nail Launcher designed a solution using 58, 29-foot-long, grouted soil anchors, all installed in a single phase. Redi-Rock 41-inch anchor blocks attach to the soil anchor rods using an engineered bracket.

Peaceful Valley, nestled along the Spokane River, is one of the oldest neighborhoods in Spokane, Wash. Most of the houses in the 40-acre valley were built at the turn of the 20th century. When the Harwood family bought a home in Peaceful Valley about five years ago, they were concerned with the 25-foot-high retaining wall that formed the back property line of the 100-foot-deep lot. They had an engineering analysis conducted to put their minds at ease, and the river rock and grout wall passed the engineering tests.

Residential retaining wall, Spokane, Wash.

Soil Nail Launcher, Inc
Wilbert Precast

Product application
Soil nailing reinforces a tall, large-block Redi-Rock wall in a tight space.

However, record-setting precipitation during two winters and construction of a foundation for a garage above the wall caused the 100-year-old wall to fail “violently,” owner Gary Harwood said. The wall toppled in April 2008, causing damage to the house and the garage below the Harwood’s property, as well as threatening the foundations of the Harwood home and garage.

“We were just thankful that no one was hurt,” Harwood said.

Harwood discovered some interesting things about his retaining wall after it collapsed. He had known that the house had been built around the turn of the 20th century as a boarding house, but he had not known that, for the first 20 to 30 years, boarding house residents dumped their garbage behind the house and down the slope. He also had not known that when the original retaining wall was built in the early 1900s, the trash had been used as backfill and had decomposed over time, creating a cavity behind the wall that inhibited proper drainage. The trash even included an antique car frame.

To increase the usable backyard space, the previous owners of the property extended the retaining wall on top of the original wall using a small-block retaining wall product, Harwood said. This portion of the wall did not have proper reinforcement or footings and was backfilled with heavy soil. All of these factors likely contributed to the wall failure.

Neighbors and the local fire station had used Redi-Rock retaining walls, the Harwoods were familiar with the product and began to research it online. They liked the look of Redi-Rock and were impressed that many walls could be built without reinforcement and hoped that their wall could be a gravity wall as well.

The Harwoods hired an engineering firm that determined that reinforcement would be necessary for this wall because of the height, soil conditions, and load-bearing requirements. But, because access to the site was extremely limited and the terrain was so steep, finding an appropriate solution was not a simple task.

“In most cases, projects have the space and access to permit the excavation necessary for a geogrid-reinforced wall. In our case there was neither,” Harwood said.

The engineers recommended soil anchors for the project; however, Harwood said that soil anchors are not used prevalently in the Spokane area and it was difficult to find the expertise and installation skill the project required. The original soil anchor design called for three phases of drilling and block installation, making it labor- and time-intensive — and thus cost prohibitive. Searching for an alternative, Harwood found Soil Nail Launcher, Inc., online and was impressed with its experience and reputation for solving reinforcement issues such as this. Soil Nail Launcher was able to design a solution that involved 29-foot-long, grouted soil anchors that could be installed in one phase instead of three.

Soil anchors are soil reinforcement tools used to stabilize the soil behind a retaining wall when it isn’t feasible to excavate behind the wall and place geogrid. Soil Nail Launcher has developed a system that uses a declassified British military cannon to “launch” the anchors into the earth behind a wall.

According to the Soil Nail Launcher website, the cannon “can accelerate a 1.5-inch-diameter, 20-foot-long steel bar to 220 miles per hour. As these high-speed projectiles enter the earth, a shock wave is generated at the tip that causes the soil particles to ‘jump away.’ The bar enters the earth without significant abrasion. The soil particles then collapse onto the bar, providing surprisingly high pullout resistance — many times greater than for driven bars and rods.”

Grouted soil anchors, which were specified for this project, require installing the rods behind the proposed wall, then filling the holes with concrete, which provides adequate reinforcement for the wall.

“We went in with our machine after the first course of blocks was set and measured up the slope to determine the placement for the three rows of anchors. We were able to drill all of the anchors and test them within a week,” said Steward, a consultant for Soil Nail Launcher.

A total of 58 anchors were installed, ready to be attached to every block in the base, fourth, and eighth courses of blocks. The next week, Redi-Rock crews were onsite installing blocks and attaching the soil anchors to the blocks.

The soil anchor rods were attached to Redi-Rock 41-inch anchor blocks by an engineered bracket. The anchor block is manufactured with a void in the middle of the block that is specifically designed to integrate with soil anchor systems. Redi-Rock retaining wall blocks weigh about 1 ton each, harnessing the power of gravity to build taller retaining walls. The blocks interlock using a patented knob-and-groove design.

Construction of the wall was accomplished entirely from the front side of the wall, which was difficult considering Wilbert Precast’s crew had only about 10 feet of space to work with between the neighbor’s garage and the front of the new wall. Wilbert brought in 500 cubic yards of fill and, because of space constraints, had to place it using a telebelt extended over the neighbor’s roof.

The retaining walls were installed as quickly as possible in an effort to save the foundation of the new garage and to prevent further erosion. Because the new wall abuts an existing concrete wall on one side and an existing rock wall on the other side, Wilbert installed shotcrete to tie the walls together.

From installation of the new retaining wall to clean-up, the entire project was completed in about a month. The finish date was May 2009.

The total project cost is expected to be about $320,000, which includes repairs to the neighboring buildings and landscaping.

The 100-year-old retaining failed “violently” in April 2008.
Wall height, soil conditions, soil-bearing requirements, and limited space required use of soil anchors instead of geogrid for retaining wall reinforcement.
Soil nail and retaining wall installation and clean up took about one month.

Lindsey Manthei is communications specialist for Redi-Rock International. She can be reached at lindsey@redi-rock.com.

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