Stiffness of coarse unbound granular material used in road construction

•Compared stiffness of open-graded and dense-graded coarse unbound granular material.•Characterization of stiffness using in-situ test standards for materials.•Composite stiffness measured with static Plate Loading Test.•Measuring the influence of the thickness of the interlocking layer. The implementation of mechanistic-empirical design (ME-design) and pavement prediction tools for Norwegian national roads introduced a need for characterization of standardized pavement materials. The current Norwegian pavement design system for public roads is empirical and does not differentiate on stiffness for subbase materials of crushed rock (CR) with different grading or maximum particle size. Hence, laboratory and in-situ testing were initiated to characterize the stiffness of coarse unbound granular materials (UGMs) used in road construction. This research paper addresses the in-situ characterization with static plate loading test (PLT). Three sections, each with different subbase materials, were constructed and tested in parallel: Section 1 consists of 60 cm open-graded CR 22/125 (lower sieve size/upper sieve size); Section 2 consist of 60 cm dense-graded CR 0/125 mm; and Section 3 comprises 10 cm dense-graded CR 0/32 mm + 50 cm open-graded CR 22/125 mm. The natural subgrade was composed of silty clay overlaid with two layers of improved subgrade of gravel and sand to level out local variations in stiffness. The PLTs were conducted according to the Norwegian test standard R211 using a 300-mm plate. In addition, PLTs were conducted according to the German standard DIN 18134 using 600-mm and 762-mm plates. The results show an average of 12–15 % higher stiffness for the dense-graded aggregate CR 0/125 mm in Section 2 than for the open-graded aggregate CR 22/125 mm in Section 1. Section 3 was designed to evaluate the influence of the thickness of a dense-graded interlocking layer on the open-graded CR 22/125 mm. The stiffness in Section 3 was on average 19–34 % higher than that in Section 1, showing a significant influence related to the thickness of the interlocking layer.