Structural Soil Investigations at Cornell University

"Over the past several years, Cornells Urban Horticulture Institute, among others, has been developing alternative layered pavement systems to integrate pavement stability and horticultural requirements for street trees surrounded by pavement.  The most recent approach has focused on gap-graded skeletal soil materials (SSMs).  Initial testing formulations of SSMs were successful in establishing seedlings and demonstrating the efficacy of a hydrogel in preventing aggregate separation.  A second study demonstrated the potential for English oak to quickly establish in fully compacted materials with bearing strengths exceeding minimum criteria for pedestrian and parking sub-base materials.  Several observations were made from these studies.  Adding fine-grained material quickly impacts the formation of the stone skeleton and its strength.  Excessive soil can result in structural and horticultural failure.  A zone of overlap exists between horticultural and structural requirements.  The maximum amount of soil is likely to be dictated be engineering demands and the minimum soil by horticultural demands.  Larger paved installations were deemed necessary for observing plant response over time."