Effect of Removing the Amelioration Period on Design and Construction of Lime Stabilised Subgrades in Local Government

Abstract

Lime stabilisation of subgrades to improve the California Bearing Ratio (CBR) in local government roads is not a new concept and has been widely used and accepted for many decades in Queensland. The process of spreading and mixing lime into non-compliant subgrade materials is simple and effective where permanent CBR improvements can be increased by a factor of well over 10.

The most common construction standard in Queensland is to mix the lime into the subgrade material over a two day period to facilitate amelioration and breakdown of heavy clay particles. The spreading and mixing requirement in TMR’s construction specification, (MRTS07A Insitu Stabilised Subgrades using Quicklime or Hydrated Lime) is based on research that optimised the amelioration period. This research found maximum improvements to unconfined compressive strength (UCS) measurements of lime stabilised subgrade materials when the material was stabilised with a 14 hour amelioration period. Local government’s adaptation of TMR’s specification that requires lime to be mixed over a minimum two day period (to allow for the 14 hour amelioration period) is considered conservative in local government applications. This is due to the significant difference in the way subgrades are designed at state and local government levels.

Two of TMR’s primary objectives in specifying a two day mixing process is to ensure adequate treatment of the material so that their strength requirement to achieve a target UCS of 1.5MPa is obtained, as well as ensuring sufficient particle breakdown, particularly in regions where highly reactive clays exist. In contrast to TMR’s compressive strength requirement, most local government roads where low CBR subgrades exist, the designer regularly imposes a CBR improvement requirement for the subgrade material to exceed 5%.

The inconsistency that exists within the local government industry is the design and construction practices of stabilised subgrades (eg. a new housing subdivision or rehabilitation of an existing road). Whilst the design often has a requirement for the subgrade CBR to achieve a minimum CBR (say 5%), the corresponding construction specification which commonly follows TMR’s MRTS07A stipulates that the lime must be mixed over a two day period (driven by the amelioration period and desire to achieve a target UCS of 1.5MPa). This construction practice is significantly conservative for local government to the point where it is unnecessary and costly.

This paper explores and compares single day and multiple day mixing processes with an emphasis on the effect of strength gain measured by UCS and CBR and how these correlate to the intended design outcomes. It will be shown that a single day mixing process with no amelioration period of the lime-soil mixture is sufficient in achieving the majority of local government design assumptions.