Progressing experimental methods for the hydrological characterization of structural soil substrates

Rising demand for functionality of green infrastructure in urban environments led to the development of combined systems for stormwater retention and infiltration together with urban trees. Frequently, a special type of substrate is used based on coarse gravel or cobbles (ca. 5 – 20 cm) as structure element ensuring load bearing capacity as well as stable pore volume and combine it with a fine growing substrate with a certain storage capacity for water and nutrients. These systems got different names in different parts of the world. They are called structural soils in the US and Singapore, Stockholm systems in Northern and parts of Central Europe, as well as sponge city substrates for urban trees in Austria. Despite progress in technical knowledge about Dos and Don´ts in the installation of structural soils and their stormwater retention functionality, there are no standard lab methods for their hydrological characterization by now.

The main goal was to develop a lab method to determine the retention capacities at different matric potential states and the respective hydraulic conductivity of structural soil substrates. A major challenge therein is to handle the dimensions of the cobbles in lab conditions. For hydrological characterization, the multi-step-outflow method and the evaporation method were combined. The adopted changeable lab setup allows to determine the saturated hydraulic conductivity as well as the total pore volume at the beginning. Afterwards a ceramic pressure plate is used to perform the multistep-outflow method by applying certain negative pressures with a suction pump. In a third step the setup is changed to an evaporation method, which is used to determine the volumetric soil water content at more negative matric potentials.

The first results provide a promising basis for further developments. For example, the available water capacity of structural soil substrates can be narrowed down to around 5 percent by volume, while the air capacity is around 21 percent by volume. This study represents a first step for developing appropriate methodology for a practicable hydrological characterization of structural soils. For the future, the experiment is intended to be extended by observations of wetting front characteristic and to be applied in standard procedures by a wide range of geotechnical or soil science laboratories.