Determining resilient modulus (MR) for subgrades is crucial in advancing Mechanistic-Empirical (M-E) pavement design. This study models the responses of a poorly graded sand and two silty sands through Suction-controlled Repeated Load Triaxial tests (RLT), factoring in moisture content and equivalent suction levels based on Soil Water Retention Curves (SWRC). Using a predictive model, the authors calculate the subgrade’s MR for a typical pavement cross-section in Sweden, incorporating layer parameters for critical seasons and climatic zones defined by the Swedish Transport Administration (STA). Results highlight the materials' sensitivity to moisture-suction and their stress dependency. Notably, the predicted MR for silty sands exceeded STA-recommended values across seasons and climatic zones. The poorly graded sand aligns well when the coefficient of earth pressure at rest k0 equals 1, except under wet conditions, in such case STA-recommended values are optimistic. Comparisons with existing data support the findings, particularly for the silty sands. In summary, this research sheds light on three subgrades and offers a reproducible method to expand the database of subgrade materials. Furthermore, if offer insights for enhancing M-E pavement design, considering different climatic conditions and materials.

This study investigates the influence of moisture and suction on the resilient modulus (MR) of subgrade soils. The research employs suction-controlled Repeated Load Triaxial (RLT) tests on three sandy materials with varying fines content. The soil water retention curves (SWRC) for the three materials were obtained and allowed expediting suction equilibrium outside the triaxial chamber by controlling the water loss and thus inferring the yielded suction with the SWRC parameters. The results show that MR increases with lower moisture content and higher suction. Two stress-based models and two moisture-based models are evaluated for predicting MR. The findings indicate that stress-suction models provide a good fit for the silty sands, but only the model where suction is an independent variable suits all tested materials. Additionally, a proposed suction-based model demonstrates promising results. Overall, the study highlights the importance of considering both moisture and suction for accurate MR characterisation of subgrade soils.

Rutting, a prevalent failure mode in flexible pavements, largely stems from subgrade issues. Despite this, there is a lack of standard protocols to evaluate subgrade rutting or permanent deformation (PD). This study attempted to characterise PD in subgrades, focusing on a poorly graded sand and two silty sands. Moisture contents above and below optimum levels were considered to account for seasonal variations. The research involved adapting a test to assess the PD by determining typical stresses on the subgrade. Moreover, given these soils’ unsaturated state and medium- to fine-grained nature, suction is an important factor. Suction-controlled multi-stage repeated load triaxial tests were conducted, and the results were fitted by a PD model modified to account for suction. The characterisation was compared with the subgrade strain criterion used in pavement design solutions. Results indicated discrepancies between the PD characterisation and strain criteria predictions, with the silty sands performing better than the poorly graded sand, consistent with the shakedown theory.

This study investigates the accumulation of permanent deformation (PD) in sandy subgrades using multi-stage (MS) suction-controlled repeated load triaxial (RLT) tests. Three sandy subgrade soils with varying fines content were tested under a range of moisture states to reflect seasonal field conditions. Two PD models were evaluated: the Rahman & Erlingsson model and a modified variant incorporating net mean stress and matric suction. The analysis revealed that suction/moisture conditions influenced PD accumulation primarily through their effect on resilient strain. The number of load cycles and stress levels significantly affected PD growth, with stress ratios (σd/σ3) above 1.5 leading to steady accumulation of plastic strain convergent to Shakedown Range B, across different soils. The enhanced model captured these dependencies using a generalizable term combining suction and stress state, offering comparable performance to the original model decoupling the influence of moisture state over one of the regression parameters and considering it in terms of its associated yielded suction. Overall, the results highlight the combined influence of stress path, moisture state, and material behaviour on PD, contributing to improved mechanistic-empirical pavement design.