Subgrade Lime Stabilisation
A Pragmatic Investigation into Pavement Design, Field Performance & Construction Quality Control
Trent McDonald
Pavement Management Services
John Tannock
Ipswich City Council
Abstract:
Lime stabilization of poor subgrade materials is by no means a new concept and has been implemented by road authorities for decades; however, uncertainty still exists surrounding the adopted design procedure and material characterisation in Queensland, Australia. Currently, two principal design procedures for lime stabilised subgrade layers are adopted from organisational and state government authorities in Queensland, namely Austroads and the Department of Transport and Main Road (DTMR) in Queensland. The disparity between the two procedures leads designers to adopt suitable lime contents and design parameters according to the two methods which produce considerably different results and begs the question - which method is more suitable and are we being too conservative?.
This paper peruses to scrutinise quantifiable data obtained from Falling Weight Deflectometer (FWD) testing on an existing, failing road, which has been identified as having unsuitable subgrade for pavement construction. The FWD testing was carried out at various stages to assess the natural and stabilised material including pre-construction, at subgrade level (natural), at subgrade level (stabilised), and also at 7, 14 and 28 days to observe strength over time. The FWD deflection data was back analysed on site before proceeding with the adopted pavement design as a Quality Control (QC) measure. The results were then compared to laboratory tests (CBR and UCS), other field tests (DCP), and weighed against the pavement design methodologies and material parameters to provide guidance on future stabilised pavement designs. Furthermore to the pavement design process, the construction process was also investigated to observe the effect of reducing the amelioration process from a two day stabilisation process to a single day process which has been investigated by industry to allow greater time savings during construction.
Introduction
Pavement Management Services (PMS) was commissioned in early 2019 to conduct a pavement design of Briggs Road in Ipswich, Queensland - which is a collector street, servicing an industrial and sports precinct in the region. The road was exhibiting structural distresses such as fatigue cracking, pavement failures, potholes and numerous heavy patches indicating ongoing issues, and was in need of reconstruction.
During the pavement design investigation, adverse subgrade conditions were determined through field and laboratory geotechnical testing which indicated an expensive subgrade and indications of low strength. Based on these findings, a subgrade treatment was considered consisting of insitu stabilisation using lime.
The pavement design was conducted using the mechanistic empirical design procedure detailed in the Austroads Guide Part 2 (Austroads, 2017), however, conjecture arose regarding the design methodology of the lime stabilised layer and assigning the material properties which lead to this investigation.