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|Allan Block retaining walls have been used in more settings than anyone dreamed
possible. Applications continue to come along adding to the system's list of successes. New
latest success story comes all the way from "Down Under" in Adelaide, South
Australia, where a well-known highway, the Southern expressway, was extended. This extension passes
through the rolling landscape of the suburbs of Adelaide. To best utilize the space of the growing
countryside, a single retaining wall was built over a two-mile span with heights up to approximately
17 ft (5.2 m).
The Southern Expressway itself is a unique highway. Between the hours of 10:00 AM
to 3:00 PM, the traffic runs in only one direction, towards the town of Adelaide. During the
peak afternoon hours, the direction of traffic flows away from the town to facilitate the traffic
returning to residential areas. The construction plans for the expressway called for a
two-mile long retaining wall along the north side. This wall was needed to not only retain the parking
lot and soil behind it, but also act as a sound barrier. To achieve the desired decrease in sound
decibels, the height of the wall needed to be high enough to block the sight line of the neighboring
structures. This meant that the wall needed to be as high as 17 ft (5.2 m).
The team at Adbri Masonry proposed the Allan Block system as the best
retaining wall product to meet all the requirements and specifications of the project. Allan Block
Corporation assisted Adbri Masonry's engineering department in offering immediate and comprehensive
support, as well as a design and any revisions they required. An Allan Block representative was also
called upon to conduct an on-site-training program for the construction crew, who had no prior
experience with the Allan Block product.
Adbri Masonry's Engineering Department worked with Allan Block and the team at
Maccaferri to make the design suitable for the site. The AB Pitched Stones were chosen because
they would use less reinforcement than a more-vertical type of block. Due to the setback of
the 12-degree system, the retaining wall required more blocks to achieve the desired height for the
sound barrier, than a six-degree system. Using the AB Pitched Classic in the sections designed as
sound barriers, helped to meet this challenge while still keeping costs reasonable.
Each section of the wall was analyzed for internal and external stability. Some sections of the retaining wall had a slope above as large as a 2:1. In addition to the slope, a surcharge from the parking lot located at the top of the slope and a point load created by the pedestrian bridges going across the highway added more weight to the wall. This weight added more pressure to sections of wall, which meant longer grid lengths were needed in those sections. The wall was further designed to withstand small amounts of seismic force due to the seismic coefficient of 0.1, which is considered minimal for seismic activity.
Due to the multiple loads affecting the wall, two types of geogrid
reinforcement were used. The two grid types used in
the design were Miragrid 3XT and Miragrid 5XT. In
some sections, the base of the wall required
stronger reinforcement than the top. The existing
site soil is a clay material with a friction angle
of 30 degrees. However, during excavation, various
soil conditions were encountered such as hind marsh
clays, silty sands and stiff clays. This variety of
soil conditions required the site's Geotechnical
Engineer to evaluate the wall to be sure that its
design could accommodate the surrounding
environment. The wall was now designed and
engineered to have grid lengths ranging from 5 to
17 feet (1.5 to 5.2 m) in length. With the design accomplished, it
was time to begin construction.
As always on a construction site, time constraints were extremely tight. Adbri Masonry needed to act quickly. They
manufactured approximately 110,000 Allan Block units in 19 days. This was no easy task, especially
since the blocks are tumbled differently that the methods most commonly used in the United States.
With two miles of retaining wall, the owner wanted to have a product that would be timeless in
appearance. The feeling was the straight split face or a three-way split face would soon be out dated
on a wall that was projected to have a 75-100 year life cycle. With this in mind, Adbri Masonry took
the AB Stones and Classic to another level, giving them a carved look by pitching the block. Noticeable
in the photos, the pitched face gives the block a natural stone look.
Once this process was complete, the blocks were transported to Adelaide. Proper grading of
the site was done to direct water away from the wall and help manage surface drainage to prevent erosion. The
foundation was then prepared by laying the base course, and installing the drainpipe. The wall was then built on grade,
providing yet another unique aspect to this project. This simplified installation process minimized time and costs to
the client. The retaining wall was built at an amazing rate of 2,153 ft²(200 m2) per day by a group of 12 people, with no prior
experience. The Southern Expressway of Adelaide South Australia was completed flawlessly, giving us another great
success story for Allan Block Retaining Walls.