276 Grabosky et al.: Plant Available Moisture for use Under Pavement Figure 2. Predawn leaf water potential data transformed into a negative natural log and plotted for linear regression slope analy- sis for treatment comparison and assignment of high matric po- tential (assigned at 1500 kPa or -7.31 to determine plant available moisture endpoint). Regression line endpoints correspond with minimum and maximum value observations on the x axis for each treatment. DISCUSSION After compaction, there was adequate porosity for normal plant growth functions in all soil-aggregate systems tested, above a pro- posed benchmark minimum of 22% in urban soils (Craul 1992). The systems drained quickly with a high internal gravitational porosity, providing aeration (Grabosky et al. 2001) which also provided rapid infiltration ability. The rate of rapid drainage can be influenced by the stone-soil mixing ratio, choice of soil, choice of aggregate, and compaction level. Plant available moisture ca- pacity was observed to be a function of compaction intensity, soil type and aggregate type. In all testing cases, the designed soils provided a conservative estimate of plant available moisture from 7% to 14% by volume, within normal expectations of field soil conditions. The range of test results generally corresponded to the plant available moisture behavior of loamy sands or some sandy loams (Donahue et al. 1983, Craul 1992), demonstrating the bulk system plant available moisture in the designed stone-soil mix- tures are not limited to a contribution from the soil component alone. The test findings were consistent with observed plant re- sponse in field installations of pavements using stone-soil systems as base materials in comparison with control pavement sections or nonpaved plant installations (Grabosky et al. 2002; Smiley et al. 2006; Bühler et al. 2007; Grabosky and Bassuk 2008). The free drainage, infiltration and total porosity data suggest that moisture at field capacity is held weakly. The initial stor- age may reflect the type of soil in the system (Table 3), the type of aggregate used (Table 4) or the level of compaction. Grain surface roughness of aggregates has been demonstrated in fine measurement levels (surface pitting on the micron level) to influ- ence moisture release and movement behavior in clean gravel in small aggregate testing with the assumption that intra-granular and surface features drive water retention behavior (Tokunaga et al. 2003). Surfaces as rough as the manufactured Stalite mate- rial (visual surface pitting, on the millimeter scale), as well as internal porosity from the heat expansion need more focused attention before satisfactorily explaining the mode of aggregate surface roughness on the stone-soil system hydraulic properties. The soil component visually coats the aggregate in mixing, and thus is assumed to impact the behavior of the aggregate surface. Studies of smaller clean aggregates describe important consider- ations needed to fully understand and model low tension mois- ture drainage behavior in response to distance from a water table or less permeable layers (Tokunaga et al. 2002; Tokunaga et al. 2003), which could translate to plant available moisture storage consideration in these paved systems, suggesting an avenue for future testing. The study presented here differs in aggregate size, compaction level, sample size of observation, and presence of an agricultural soil in the stone matrix and was designed to de- scribe general behavior. However, the data from this study pro- vided evidence that at low tension, the depth of the material will provide a continuum of moisture content increasing with depth to the lower boundary of the designed material and the underly- ing soil. A middle elevation point of the designed material layer for scaling of bulk system behavior is a reasonable first approxi- mation for future testing and definition (Liu and Dane 1995). Data from the present studies suggest that the depth of a de- signed soil profile will impact moisture regimes, thus potentially root system architecture. Moisture content at high matric tension (between 1200 and 1500 kPa) was characterized by the large amount of aggregate in the system muting soil component mois- ture release differences at high tension and was found to be fairly consistent across studies which varied by soil and aggregate. The use or consideration of porous aggregates could be fac- tored in when assessing irrigation needs in normal operations, but should not be assumed to be the only criterion for determin- ing the overall performance of a mix design. Soil chemical as- pects such as aggregate and soil pH and cation exchange capacity ought to be further tested and considered over time. Aggregates must function over time as the load-bearing lattice of the sys- tem. As such, aggregate soundness/durability, whether defined from LA abrasion testing or sodium bath aging, must meet the engineering demands of the chosen wearing surface material, the expected load, acceptable settlements over time and pavement service life. Northern climates need to look at aggregate sound- ness with consideration to freeze-thaw cycles and potentially road salt as well as soundness over time under loading regimes. Hopefully, these studies will provide some insight into irri- gation management strategy and soil volume requirements for stone-soil mixtures. For stormwater management, the infiltration and permeability data of the systems may be helpful for scaling and planning water interception in sheet flows from pavement. It may also be useful in providing a holding reservoir of water for plant use in transpiration or deep infiltration below pavement. While most of the study data describes a dewatering event, ini- tial data in describing the systems dealt with innundation satura- tion in comparison with a possible total saturation. The differ- ences between de-watering characteristic and iterative recharge of moisture in this manner would likely be subject to measurable hysteresis at low tension in the aspects of the soil-stone inter- face but bulk behavior would likely mimic the large pore struc- tures in the stone matrix for rapid initial stormwater acceptance. It is apparent that the systems can be managed and used in design much like agricultural soils in terms of plant available moisture in the total volume of designed root zone. The materi- als behave similarly to existing field soils in water movement and in plant response, however more research is needed to fairly characterize details of lateral flow and provide predictable sys- ©2009 International Society of Arboriculture
September 2009
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