Effective road pavement design for expansive soils in Ipswich

Abstract

[Abstract]: Ipswich City is located in south-east Queensland and is underlain by predominantly impermeable subsoils which cause periodic subsurface water-logging and typically classified as expansive, CH and MH soils. Council has battled with effective pavement design and construction due to poor subgrade behaviour. Unbound pavements are historically the dominant design used in Ipswich and are typically based on the AUSTROADS and Department of Transport and Main Roads manuals and experience. The subgrade of any foundation plays an important role in load bearing and support of traffic and pavement construction over expansive clays requires a suitable working platform to enable machinery to operate. Studies show the use of safe bearing capacity for subgrade assessment does not suit CH soils. Subgrade treatment needs to be a mandatory consideration coupled with using the lowest CBR readings to provide the maximum pavement thickness. The most effective method of subgrade treatment currently appears to be geosynthetics placed on the subgrade. The latest Austroads and Department of Transport and Main Roads manuals align with current world best practice for pavement design and it is recommended the latest versions continue to be used for new pavements. The Ipswich Planning Scheme Policy should refer directly to these documents and be specific for Ipswich soils, i.e. the greater depth pavement should be chosen based on a low CBR value and realistic design ESA's number. On expansive soils, a flexible surface should be constructed on an impermeable membrane or layer since flexible bases experience fatigue which can be easily maintained whereas rigid bases can crack rapidly and to the detriment of the entire foundation. Identification of the cause of pavement failure is necessary to determine the appropriate rehabilitation method for a failed pavement. Council should consider sustainable rehabilitation methods that maintain a flexible pavement with asphalt surfacing.