Discussion of “Evaluating Subgrade Compaction for Different Soils Using Nondestructive Lightweight Deflectometer”

The subgrade, as shown in Fig. 1.1 of the "Typical Section for Rigid or Flexible Pavement Structures" in the AASHTO Guide for Design of Pavement Structures (AASHTO 1993), represents the top of the selected material or prepared subgrade on which the subbase course and overlying layers of the pavement structure rest.The authors' work is focused explicitly on this prepared subgrade, often the desired subgrade for accommodating a pavement structure because its material composition and compaction characteristics can be adequately controlled versus those of the original ground or roadbed soil.Satisfactory compaction of the subgrade layer is not just a requirement but a crucial factor in the overall performance of the pavement structure to accommodate the intended traffic loads.The authors identified a critical and urgent need for faster and less environmentally impactful techniques to evaluate the desirable uniformity of subgrade compaction.This need is pressing, given the limitations of the traditional methods, which use the slow sand replacement method (SRM) and the nuclear density gauge (NDG), which pose risks to the environment and human health.The authors' study introduces a novel solution to this problem by presenting a new method for rapidly detecting and evaluating the subgrade compaction and deformation modulus.This innovative method utilized the nondestructive lightweight deflectometer (LWD).It was tested on subgrade consisting of four soil types-silt (ML), well-graded gravel (GW), lean clay (CL), and poorly graded sand (SP)-along roads under construction in Gansu Province, China.LWD and SRM tests were conducted, with the authors detailing the test locations in Fig. 4 of the original paper and the LWD and SRM test methods in Fig. 5 of the original paper.The authors conducted a comprehensive series of tests (308 combined) using the LWD and SRM methods separately on the mentioned soil types.This thorough approach resulted in correlation analyses between the degree of compaction and dynamic deformation modulus (E vd ) for the combined results and results of the individual soil types.Based on the results presented, a few comments are considered appropriate for the readership, to which the authors may wish to provide their response.These are as follows.One of the significant outcomes of the analyses reported by the authors is the observed variability of 32.1% coefficient of variation (COV) of the E vd values of the combined data versus those of the individual soil types, which were reported to be much smaller.This finding was judged to result from the characteristic difference in the fine-grained soils, which, when compacted to the range -2% to 2% of optimum moisture content, resulted in the cementation of the materials.This range of moisture content is also known