Challenge

A new 250-foot long gabion basket retaining wall was proposed along a wooded hillside. The retaining wall would support excavations made into the slope for the purposes of constructing and then maintaining a walking trail below. The tallest section of wall would consist of two tiers of stacked gabion baskets with each tier of three baskets being five feet high above grade (one foot buried). The proposed gabion baskets were three feet wide and either 1.5 feet or three feet tall, filled with six to ten-inch crushed stone. The preconstruction hillside conditions disallowed access to the project site with traditional track or truck-mounted drill rigs. Hand auger borings completed to depths of about nine feet at the base of the wall and within the proposed retained soils encountered medium stiff to very stiff clayey silt. With the sloped hillside conditions and the soils encountered, the taller sections of wall would require tiebacks or anchors to provide lateral stability.

Solution

The gabion wall design incorporated 73 helical tiebacks to support design working loads of 5, 10, 15 and 20 kips. The tiebacks consisted of the Model 150 (1.5-inch round corner square bar) with 10” single-helix, 8”-10” double-helix, and 10”-12” double-helix lead sections. The tiebacks were installed at a downward angle of six degrees from horizontal and at a spacing of five or ten feet. Standard extensions were used to advance the tiebacks to lengths of 17 to 37 feet, which corresponded to the greater of the design length or the length where the minimum required installation torque (1,000 to 4,000 ftlbs) was achieved. The tiebacks were advanced to torque-correlated ultimate capacities of at least twice the design working loads (FOS ≥ 2). Two verification tests and five proof tests were completed to maximum test loads of two times and 1.5 times the design working loads, respectively. Creep measurements were taken at the maximum test loads over ten minute hold periods. The maximum creep movement for each test was 0.007 inch or less, which was less than the specified maximum allowable creep of 0.04 inch. The helical tiebacks were connected to wide-flange beam and C-Channel walers within the gabion baskets. Crushed stone was then placed and compacted around the tiebacks and walers. Tieback installation and testing was completed in seven days.

Print (PDF)

Project Summary

Architect: Walker Macy

Structural Engineers: KPFF Consulting Engineers; SFA Design Group

Geotechnical Engineer: Geotechnical Resources, Inc.

General Contractor: Brant Construction

Certified Tieback Installer: TerraFirma Foundation Systems

Products Installed: (73) Foundation Supportworks® Model 150 Helical Tiebacks, Single 10”, Double 8”-10”, and Double 10”-12” Lead Sections, Installed Lengths of 17 to 37 feet, Design Working Loads of 5, 10, 15 and 20 kips