Concrib chose the geogrid reinforced retaining wall solution to support the embankment prior to the construction of a new road reserve at Varsity Heights in Reedy Creek.

An elevated, premium location at Reedy Creek on the southern Gold Coast, Varsity Heights offers sweeping views to the ocean while offering a tranquil bushland setting. The proposed construction of the main 20m wide road reserve with a 20 kPa load required a retaining wall up to 7m to support the embankment.

The original specification of a concrete crib wall presented a problem with a geotechnical investigation of the foundation profile revealing a 3.5m deep layer of soft clay, overlaying competent Argilite material. This in-situ foundation condition initially led to a foundation design incorporating 450 dia bored piles at 2m centres along the length of the front and rear of the wall footing.

However, this pile design proved to be relatively expensive and presented some constructability issues with the soft clay and presence of groundwater.

Seeking a different approach, Concrib engaged Geoinventions Consulting Services to offer an alternative solution. Geoinventions recommended a hybrid-crib/ geogrid reinforced soil wall design to be constructed on some foundation improvement.

This design combines the excellent drainage characteristics of a crib wall face with the remaining structure consisting of reinforced soil using geosynthetics. The foundation improvement was achieved by a geotextile wrapped rock mattress block beneath the crib section as well as a 750mm deep rock mattress reinforced at the base with geogrid beneath the reinforced soil zone.

Miragrid 8XT was used in the reinforced soil zone and the grid length was typically 0.8H, where H is the wall height. This innovative design addresses the issues of a high 20kPa surcharge, delivering constructability as well as a cost effective solution to the client.