An Experimental based approach to estimate the overall modulus/CBR of geogrid/geocomposite-reinforced subgrades and use of the Austroads design chart

Abstract

It is essential to improve/stabilise weak subgrades in road construction to facilitate construction machinery, reduce the required gravel layer thicknesses (base/subbase), and enhance pavements' performance. Gravel replacement, rock-blanketing, and chemical stabilisation (mixing additives like cement/lime with subgrade soil) are traditional stabilisation methods of weak subgrades. Since these methods are expensive, time-consuming, and environmentally unfriendly, geosynthetics (geogrid/geocomposites) to stabilise (reinforce) weak subgrade has become a popular option as it is cost-effective, less time-consuming, and environmentally friendly. The lack of local research and investigations to estimate the modulus (or CBR) of the geogrid/geocomposite reinforced subgrade has hindered capturing the full benefit of geogrid/geocomposite in Australian pavement design. Therefore, it is essential to have a laboratory method to estimate the modulus (CBR) of geogrid/geocomposite-reinforced subgrades to be used in the chart based granular pavement design. In this research, geogrid/geocomposite-reinforced subgrades were modelled in a 1m x 1m x 1.2 m steel box and then monotonic plate load tests were conducted to estimate the strain-modulus (EV2). A series of model tests were conducted to investigate the effects of geogrid/geocomposite types and the granular cover thickness on the strain-modulus of the improved subgrade. A simple relationship between the strain-modulus and the CBR was developed based on the plate load test results of a know subgrade (CBR = 2.5%) and assuming linear elastic deformation of the pavement structure under 550 kPa plate pressure ( 200 mm diameter plate). Finally, a sample calculation is given to demonstrate the use of the Austroads granular chart to design a pavement using the improved strain-modulus and quantify the benefit of using geogrid/geocomposites