Determination of interface shear strength is crucial in the design of many geotechnical structures. To study the effect of interface roughness, particle geometry (size and shape), and sand gradation on the interface friction angle, direct interface shear tests were performed for 10 sands with varying particle sizes, shapes, and gradations and four steel surfaces with different levels of rusting. When sheared along the same interface, the interface friction angle was greater for sands with smaller particle sizes and more angular or elongated particle shapes. For a given sand, the interface friction angle increases with increasing surface roughness. For sands with uniform particle size, a unique relationship was found between the normalized surface roughness and the ratio of the sand–steel critical-state interface friction angle to the internal critical-state friction angle of the sand. Given the same surface roughness and mean particle size, smaller critical-state interface friction angles were mobilized for graded sands than for sands with uniform particle sizes.

The Dynamic Cone Penetrometer (DCP) is a device that is used in the construction industry for the assessment of in situ soil compaction quality. Over the past few decades, numerous correlations have been developed between the DCP test results and soil strength and stiffness parameters. This paper proposes a comprehensive set of criteria and recommendations for quality control (QC) of compacted subgrade that take into account the inherent statistical variability of DCP test results. For the development of the QC criteria, a new statistical methodology is used to extract representative test values from the raw field DCP test data. In order to use the proposed QC criteria, soils are first classified into two broad categories (fine-grained and coarse-grained) depending on their fabric and response to compaction efforts. Test results indicate that (i) for fine-grained soils, the DCP test values have good correlation with the plasticity index (PI), which is indicative of the type and amount of clay content of the soil and (ii) for coarse-grained soils, the DCP test values have good correlation with the optimum water content of the soil, which is directly related to its in situ density. DCP blow count correlation equations are presented for both soil categories. Recommendations for field DCP testing and data analysis are also provided to highlight the significance of the statistical distribution of the DCP test results in QC testing of compacted subgrade.