- Green technology in the United States has been on the rise over the past few decades in the United States. However, certain green technologies have been developed in the absence of design standards. Ecoroofs, which are vegetated soil masses placed on the top of a building’s roof structure, present several engineering concerns. Several important engineering concerns include overstressing of the roof structure due to ponding or excessive saturated soil dead loads during storm events, sliding of the ecoroof material causing overtopping or complete failure of the parapet walls during a seismic event, and the increase of inertial load at the top of the building. Standardization of ecoroof soil and the development of design codes for ecoroof systems in the United States would minimize the possibility of structural failure. Accordingly, this study presents the quantification of the geotechnical index properties and their variability, static stress-strain and volumetric response, and dynamic soil response and properties for ecoroof soil.
A field exploration program, which included gathering undisturbed Shelby tube samples of ecoroof soil, was conducted in Portland, Oregon. The field exploration
provided the basis for the geotechnical characterization of ecoroof soil. Sample statistics of the geotechnical index properties showed significant variability. Sieve testing showed that field samples of ecoroof soils tended to cluster; therefore, three target gradations were selected to serve as representative ecoroof gradations, which varied in the amount of fines present (i.e. material passing No. 200 sieve), median grain sizes, and uniformity of the gradation. Drained static simple shear tests were conducted on specimens reconstituted from each of the target gradations. The tests were performed to investigate the effect of relative density, applied vertical stress, and gradation on static shear strength, each of which contribute to variations in ecoroof strength. To understand the effect of organic matter on the strength parameters, three undisturbed specimens were sheared and the test results indicated that the organic content has significant influence on the volumetric and stress-strain response of ecoroof soil. Preliminary dynamic soil properties, such as the modulus reduction and damping characteristics, and the liquefaction susceptibility of ecoroof soil was determined from a series of undrained cyclic simple shear tests on reconstituted and undisturbed tube specimens. Many reconstituted ecoroof soil specimens exhibited liquefaction under undrained cyclic strain controlled conditions, and variations in liquefaction susceptibility, were attributed to the type of soil gradation. Cyclic simple shear tests on undisturbed tube specimen specimens indicated that the organic content influences the dynamic response of ecoroof soil. Accordingly, this study provides a baseline with which practicing engineers can begin to assess the loading associated with ecoroof soil, and may be used to guide future studies and code development.