Overcoming Subgrade Challenges with Innovative Design Solutions at Essendon FC

Geogrids have been used to stabilise soft roadway subgrades for more than 30 years, but the design is often based on an empirical, rather than a rigorous technical approach. Southern Geosynthetics has introduced new design software that allows simple, effective designs to be generated, increasing options for engineers.

At the new Essendon FC base, “The Hanger” at Tullamarine, designer Aurecon was faced with a real dilemma. The very soft reactive clay subgrade had repeatedly failed proof-rolling. Traditional options of digout and replacement with good quality fill, or in-situ cement-stabilisation were not possible due to the presence of a wide shallow service trench along the middle of the roadway alignment. With very little cover over services no further excavation was possible.

Savings of up to 30% of pavement thickness can be achieved with geogrid reinforcement.

Assuming a design CBR of 1%, and design traffic of 100,000 ESA, the design program solution called for placement of DUX CG30, a 30kN/ m biaxial geogrid combined with a non-woven geotextile at subgrade together with a 280mm “stabiliser” layer of crushed rock; this would then provide a working platform with CBR=11% and allow compaction of a second 180mm thick crushed rock pavement layer. The total pavement thickness of 460mm represents a saving of some 200mm, or 30% over traditional design.

Designer Aurecon opted for reduced thickness of 180mm of cement-treated first lift, followed by 200mm of Class 2 FCR and a wearing course of 40mm AC. Upon setting after a few days the cement increases the strength and stiffness of the first stabiliser layer and provides an improved base for compaction of the second lift. But even on initial placement, before the cement “went off ”, the grid and 180mm layer virtually eliminated rutting under construction truck traffic.

One of the concerns of the contractor was how to achieve satisfactory compaction of pavement layers. Standard road construction practice requires satisfactory “proof-rolling” of the sub grade, with no visible movement or cracking of the subgrade. Any soft, “unsuitable” areas are dug out and replaced with engineered fill to achieve a typical CBR of 15%. However, when building on very soft soils, construction practice must be modified. Roadbase must be placed more carefully, dump trucks should avoid directly trafficking the grid wherever possible. Materials should be spread with light, low groundpressure, such as the tracked bobcat such used here.