Behaviour of unsaturated soils for road pavement structure under cyclic loading

Abstract

The geotechnical engineering practice has not been advanced in South Africa. As no significant integration of unsaturated soil mechanics (USM) is yet to be covered in pavement design. Stiffness of subgrade soils that is determined by resilient modulus (𝑀𝑟), is an important component in the design of flexible pavement structures and railway embankments. Environmental effects, such as seasonal variations in moisture content considerably influence the subgrade properties that are failed to be considered in any realistic pavement and railway embankment design. The seasonal changes in moisture condition affects the stress state of subgrade due to changes in the matric suction and this is considered an important stress state variable in unsaturated soil mechanics. Therefore, particular attention was directed in this research towards evaluating the response of unsaturated pavement structure under cyclic loading.

Several factors are needed to be considered during pavement design, which are dependent on 𝑀𝑟 in order to provide an accurate assessment of the support provide by the subgrade. Roadbed with high plasticity index (PI) and swelling potential, are prevalent across Free State and Northern Cape and this possess an uncommon challenges to design Engineers. This challenge is majorly as a result of 𝑀𝑟 value of high plasticity index subgrades depends on the moisture content. Furthermore, the pavement structures are generally under unsaturated conditions, such that the Groundwater Table depth is below the depth of the subgrade in consideration for pavement design. The classic soil mechanics considers the pavement structures to be under saturated conditions. This means, that the soil mass under consideration consists of two phases, solids (soil particles) and water. This assumption is generally acceptable, as it makes it possible to develop simple analytical solutions that lay the foundation for geotechnical engineering. However, since subgrade soils for pavements exist largely in an unsaturated state. It is reasonably realistic to employ the fundamentals of unsaturated soil mechanics in order to explore the resilient performance and deformation characteristics of subgrades using 𝑀𝑟- suction correlation.

The evaluation of 𝑀𝑟 properties of unsaturated subgrade soils and evaluation effects of suction and swelling stress on 𝑀𝑟 of the studied subgrades were summerized in this research. Furthermore, this study developed mathematical predictive models. As well, reviewed the design of flexible pavement, as per AASHTO 1993 flexible pavement design guide by comparing designs made with both unsaturated 𝑀𝑟 design value and conventional laboratory 𝑀𝑟 design value. Prior to the design of flexible pavement exercise carried out in this study, some geotechnical tests were laboratory conducted using both the unsaturated soil mechanics and classic soil mechanics approach i.e. unsaturated CBR, shear stress, resilient modulus tests and filter paper test to measure suction.

The laboratory result of the tested soils reveal that the unsaturated CBR and 𝑀𝑟 values were 1.5 to 2.5 times higher than that of the conventional CBR and 𝑀𝑟 values. Among other results, are shear strength result that followed similar trend, on the contrary, the shear stress parameter i.e. ∅𝑏 of the tested soils were 1 to 1.5% lower than that of the classical soil mechanics.

The SWRC curves were evaluated through the entire range of volumetric water content using filter paper techniques. It is evident that 𝑀𝑟 depends on matric suction, which also varies with moisture content, thereby, a 𝑀𝑟 -matric suction relationship provides sound theoretical framework to account for moisture variation in unsaturated subgrade soils. Three different SWRC models (Seki, Van Genuchten, and Fredlund and Xing) were used for curve fitting, the SWRC revealed that Seki’s SWRC model best fitted the laboratory data with coefficient of determination, R2 values ranging from 0.95458 - 0.99986. Whereas, Van Genuchten R2 values were in the bracket of 0.85796 - 0.93317, and Fredlund and Xing R2 values were within the range of 0.89959 – 0.96142.

The SWRC curves evidenced that the subgrade soils with fine content (50%>P200) like FSS 1, 2 and NCS 2 and 3 recorded higher air entry values (AEV) within the range of 152 kPa – 250 kPa. Whereas, the subgrade soils with lower fine content yielded AEV between the range of 90kPa -120kPa. The means that soils with high fine content starts to desaturate at a very high AEV compared to soils with lower fine content, due to inability of the soils to maintain saturation.

The analysis of experimental data obtained from the prepared specimens at different moisture contents were used for multi-regression analysis using "NCSS11” software package. The predictive mathematical models were developed for unsaturated CBR. This model performed well against Ampadu’s (2007) model for prediction of unsaturated CBR.

In addition, all the predictive models developed in this study i.e. Models 7, 8, and extended Yang et.al model yielded more satisfactorily results than, Yang et.al (2005) model, Liang et. al (2008) model and when compared with the laboratory measured 𝑀𝑟. Thus, the predicted 𝑀𝑟 values using all these models were 1.2 to 1.5 times higher than laboratory measured 𝑀𝑟 values with R2 within the range of 0.91 – 0.96 on curve validation. The results showed that all the tested subgrade soils are highly depended on 𝑘1 parameter. Whereas, the effects of 𝑘2, and 𝑘3 are proportional to deviatoric and confining stresses relatively to 𝑀𝑟 values.

Lastly, AASHTO 1993 pavement design guide was used for the design exercise, on the samples prepared on the dry side of optimum. Subgrade 𝑀𝑟 reflect the range of stress states, commonly developed beneath the pavements that are subjected to moving wheel loads. According to the design exercise in this study, the predicted 𝑀𝑟 design value for the subgrade provided sufficient thickness that can support the entire pavement structure. Whereas, the measure resilient modulus design value, underdesigned the pavement thus, required higher asphalt thickness layer.