Reviewing whether moisture content is a necessary requirement in fill compaction control

Abstract

As a quality control under end-product specification, a minimum relative compaction (RC) (e.g., 95%) and range of moisture content (m.c.) (e.g., 3% of optimal m.c.) of the compacted fill are usually specified. This dissertation discusses whether the control on m.c. is a necessary requirement in fill compaction works. A brief review of many local, Mainland China and overseas specifications and guidance documents on fill compaction control was carried out. Some of the specifications do not require control on m.c. of the compacted fill. This includes one local specification published by the Architectural Services Department. A literature review is conducted to see the relationship between m.c. and common engineering properties of soils. A summary of the results of a recent related study (Chung and Chu 2023) is presented on the effects of m.c. on shear strength, compressibility, stiffness, CBR and air voids content of the compacted soils using four different types of fill materials (sandy GRAVEL, gravelly SAND, silty SAND and SILT/CLAY). The results of this study show that effects of m.c. is minimal for coarsely grained soils. This dissertation then focuses on the effect of m.c. on the collapse potential of the fill materials. A series of laboratory tests was conducted on a well graded silty SAND (completely decomposed granite) and a well graded silty CLAY (completely decomposed metamorphic sedimentary rock) compacted to 95%, 90%, 85%, 80% and 75% of RC at different m.c. using oedometer of different thickness and small and large shear boxes of different thickness. The results clearly show the relationship between the density of fill (RC) and m.c. on the collapse potential. In particular, for silty SAND compacted up to 85% of RC, m.c. is not critical in terms of severity of the collapse potential. For silty CLAY compacted up to 95% of RC, m.c. in the dry side of optimal is important in terms of severity of the collapse potential. A brief discussion is also given on the differences in measuring collapse potential using different apparatus.