Crews from Structural Systems Inc., Honolulu, recently completed the installation of tieback anchors at two locations along the Hamakua Coast of Hawaii. In Honomu, a failing earthen slope was stabilized with 29 anchors ranging from 50 feet to 60 feet deep. Farther up the road at Laupahoehoe, an additional 64 anchors to depths as great as 65 feet were installed to stabilize a failing rock wall. All anchors were temporarily cased their entire length, and grouted during extraction of the casing to construct the required bonded lengths and provide the specified capacities. One hundred percent of the anchors were tested and tensioned. Some anchors were as much as 24 feet below the finished top-of-the-wall elevation.
Structural Systems mounted a custom-engineered-and-fabricated
platform to an excavator to install the four rows of anchors stabilizing the
two retaining walls. It used a RDS550 rotary drill from TEI Rock Drills. Wireless
controls allow the crew the flexibility to operate the drill at the point of
installation or remotely. Casing is installed at the same time as the
down-the-hole pneumatic hammer drills the borehole to the specified depth and
diameter. Drill cuttings are blown out the top of the casing. Drilling and
casing at the same time ensures a consistent and clean borehole. Casing while
drilling assures the design team that the grout will be properly placed, and
constructed to the specified diameter and cross-sectional area.
A demanding performance testing sequence was completed,
whereby each anchor was both incrementally loaded and unloaded in stages to 25
percent, 50 percent, 75 percent and 100 percent of the maximum test loads, with
intervening unloading to 0 percent between the loading sequences. Upon
completion of testing, each anchor was tensioned/stressed to its specified
lock-off load, and secured with nuts and washers in accordance with the project
Access conditions were extremely difficult at this site. After
all the anchors were tensioned and tested, the retainment system will
receive another layer of reinforced gunite.
Earthquake Rockfall Repair Project
February 21, 2012