Evaluation of trench backfills at highway cross-drains

A highway cross-drain is an important element in a highway drainage system. This structure, however, has been related to the ride quality deterioration caused by pavement surface dips above this structure. In-situ probing techniques, such as the cone penetration test (CPT), miniature cone penetration test (Mini-cone) and dynamic cone penetrometer (DCP), were used to diagnose the causes of the dip problem. The field evaluations indicated that when a pavement surface dip occurred, the trench backfill underneath was softer than the adjacent subgrade soil. Because of its simplicity and potential as a quality control tool during the backfill process, the DCP test was given special attention. The field case histories and full-scale laboratory tests confirmed that DCP results can provide a reasonable estimation of the stiffness (modulus) of different trench backfills and subgrade soils. A possible way to prevent a pavement dip from occurring at a cross drain is to make a backfill stiffer than adjacent subgrade soils. The compaction results from the full-scale laboratory tests indicated that sand is not always successfully used for highway cross-drains due to its poor gradation and difficulty in compaction. Other materials, such as crushed limestone and recycled asphalt pavement (RAP), are good alternates because of their good workability and high stiffness when compacted.