Since the construction of a new high-end golf course and residential lots, extensive grading operations may have contributed to excess watershed in the local timber-lined waterway. During rain events, the water had begun to scour the toes of the 50+ foot tall slopes behind many of the new residential lots. The winter thaw and spring rains combined with the toe scouring has caused slope instability and the loss of the majority of this family’s backyard, a small timber retaining wall and distress to their two-story wood deck.

A new retaining wall was to be constructed to save what remains of the backyard and to reduce the risk of a global slope stability failure. Steel H-Piles were driven up to 40 feet deep on 6± foot centers below the old retaining wall location. The new 108 foot long and 2 to 15 foot tall wall was to be constructed utilizing horizontal timber reinforcing installed between the driven H-Piles. A total of 19 helical tiebacks were to be installed into the bank of the active landslide and were to be attach to the driven H-Pile via a bracket.

The soil deposit capable of providing sufficient holding capacity was only about 8 feet thick. Due to the spacing of the H-Piles and the shape of the semi-circle wall, the helical tiebacks were installed at alternating down angles of 5 and 10 degrees from horizontal in order to insure that the helices would be adequately spaced and the lead sections would stay within the stiff glacial till deposit.

Due to the slope being an active landslide, access in front of the new retaining wall did not allow even a small excavator. Anchored Walls designed a slide unit that a RS-12k Pengo drive motor could be mounted to. This unit would also allow the drive motor to be operated off to the side by a small excavator. After the tiebacks were installed, a custom bracket was design to connect the tiebacks to the H-Piles while accurately maintaining the installation down angle

The lead sections consisting of ether a triple 10-12-14, a triple 10-12-14 combined with a double 14-14 extension and a triple 10-12-14 combined with two double 14-14 extensions were needed to obtain the design capacities, which ranged between 34 and 50 kips. The helical tiebacks were installed to approximately 45 to 55 feet beyond the face of the new retaining wall. Since the retaining wall was within 30 feet of the existing house, all of the tiebacks extended beneath the house, approximately 10 to 15 feet below basement level.

Anchored Walls fought near record high rainfall events, high temperatures, limited working space and overlapping contractor schedules to complete this project ahead of schedule.

Anchored Walls and Grip-Tite Manufacturing would like to give special thanks to TK Concrete Contractors and Simon Welding for making this job a huge success.

Come stop by our booth at the Iowa State Fair. We are easy to find, just inside the North doors at Varied Industries Building and to the right. Our knowledgeable representatives will be able to answer any of your questions.

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