Abstract

It is well established that compaction negatively affects agronomic productivity, that air permeability is a sensitive measure of the degree of soil compaction and therefore a good indicator of soil productivity impairment from compaction. Cohesive soils in urban settings are often heavily compacted by the common engineering practice to compact sub-grades of urban construction sites to 95% or more of the optimum density obtained in standardized Proctor tests. The objective of this study was to determine to what extent reducing compaction effort would increase the air permeability of Proctor test specimens.  Quantifying this relationship would permit more appropriate Proctor test specifications for the design of urban green spaces on cohesive soils.  We designed a portable transient flow apparatus for rapidly measuring air permeability and used it to measure air permeability on Proctor test specimens of three cohesive sub-grade soil materials covering a range of USDA textures (loam, silt loam and silty clay) and Proctor compaction characteristics.  We compacted test specimens at their Proctor optimum water content using efforts ranging from 100 to 25% (the lowest practicable value) of that used in the standardized Proctor test.  Results confirmed that compaction severely reduces air permeability of the test specimens and indicated that the common practice of compaction to 95% or more of the optimum Proctor density is probably not appropriate for construction of urban green spaces. Reducing compaction effort from 100 to 25% of the standardized Proctor test value increased air permeability 30, 89 and 42 times respectively for the loam, silt loam and silty clay test specimens.  More extensive studies are needed to correlate measured air permeability of Proctor test specimens to agronomic productivity of urban green spaces.

Key words: Urban soils, air permeameter, modified Proctor test.