28 Grabosky and Gucunski: Upward Root Growth Pressure in Compacted Sand box between 1:1 and 1:2 depth to width ratios (in example, Cheng et al. 2004 and Chiroux et al. 2005). Westergaard (1926) defined the force required per unit soil deflection as the modulus of sub- grade reaction with regard to pavement deflections in a layered design as one aspect of soil layer evaluation and behavior. Still used today, the physical characteristics of soil, including texture, density, and moisture content, influence the soil behavior under load (National Cooperative Highway Research Program 1993). There have been a number of simplified models used to describe load transfer in pavement and soil systems. For example, in U.S. pavement design practice in the 1940s, a live load transmitted freely to a subgrade was commonly assumed to radiate down- ward as a truncated cone with line elements of 45 degrees, or a 1:1 slope (Hewes 1942). Viewed in the reverse direction, a root of 1 unit diameter, placed 6 units below the pavement, would spread over 13 units at the pavement. Given the likely differ- ences in lifting soil upward against a pavement surface, rather than displacing soil outward or downward, the 1:1 slope can serve as guidance rather than a given dimension. It is common to roughly estimate the distribution of vertical stress through soil depth using a 2:1 slope method across the distance of a rectilin- ear object as a convenient first approximation (Holtz and Kovacs 1981). However, all of those models were developed assuming purely elastic deformation, and thus might have limited applica- bility to the root growth problems where the plastic soil defor- mation is significantly present. More complex curvilinear distri- butions can be approximated using other methods with limiting assumptions regarding the soil in place (Liingaard et al. 2004). Roots cannot be assumed to impose loads in the same geo- metric pattern, speed of load increase, or over the same time period as traditional loading pattern in pavement design. A root segment can be modeled as a rectilinear object for a first approximation. The testing system was thus designed to col- lect data over a 72 cm wide area for a 2.54 cm root at 15 cm maximum depth, or roughly twice the expected testing surface. This paper describes a root growth laboratory simulation method to induce displacement within a sand system to mea- sure the force distribution on a pavement surface. The meth- od could test strategies in use to protect pavement from root growth with a standardized protocol for comparative evalu- ation and design guidance. A simple study will describe sand displacement in response to increasing input pressures. The current study includes examples of load profiles captured at the surface as it changes with sand layer depth as an exam- ple of the simulation method’s use. In this case, it was used to describe load profiles with changing moisture and den- sity when the sand was displaced from increasing root size. MATERIALS AND METHODS The Simulation Method A liquid-filled rubber gasket (Giant brand bicycle inner tube with a 0.9 mm wall thickness) was housed in a milled alumi- num holding block fabricated in the Rutgers Cook Campus Ma- chine shop (New Brunswick, NJ, U.S.) (Figure 1). The rubber gasket was held in place by the upper-milled aluminum hous- ing with a 2.54 cm × 27.94 cm centered opening, to provide an initial inflation surface of 70.97 cm2 . This apparatus, which will be referred to as the root, was designed to be filled with ©2011 International Society of Arboriculture pressurized water. Volume changes with an in-line pressure rated graduated cylinder. Increasing pressure inputs within com- pacted sand profiles, were used to simulate upward radial root growth. Pressure to the root was generated by compressed air through a manifold (SoilMoisture Manifold system 0700CG23, Santa Barbara, CA, U.S.). As the water was pressurized, it was forced into the root placed within a compacted sand profile. Figure 1. Schematic details of an inflatable rubber diaphragm to provide a laboratory simulation of a tree root. The image was adapted from AutoCad fabrication details (Cook agricultural engi- neering shop). Photo inset shows the “root” inflated at low pres- sure for demonstration purposes. The root was placed into a custom-fabricated box (Rutgers Cook Campus Machine shop) of 37.15 cm × 80.33 cm × 2.54 cm (7580 cm3 ). Exterior threaded columns were used to attach and clamp a metal plate box top into place, with braces to prevent lift- ing and flexing of the box top. The box top served as the simulation of a pavement wearing surface layer (Figure 2). The box wall edg- es were milled to accept additional nesting wall sections of equal depth (lifts). To adjust the distance between the root (placed on the base) and the top of the box, lifts added 2.54 cm depth increments. To map the load intercepted by the bottom surface of the box top resulting from sand displacement, a Novel Pliance Pe- dar pad (Novel Electronics, MN, U.S.) with a 16 × 16 grid of load cells was developed in a split pad layout to provide two pads containing load cell grids of 8 × 16 cells over an area of 24.8 cm × 72 cm. The pad fit into a milled recess in the box top to support the pad in a stable, consistent position. The pad was in direct contact with the sand once the box top was in place. Data were taken at 38 Hz from load cells with a maximum reading of 60 Ncm-2 (1 Ncm-2 = 10 kPa) and processed through a Pliance signal conditioner (Novel Pliance Hw pad with Plaince- m 8.3 standard sensor programming). Output from the entire load cell pad was labeled as a data frame (38 whole frames per second) and used to analyze the load distribution across the pad. Manifold-to-pad offsets as a calibration were developed to iden- tify the influence of the initial recess of the rubber gasket in the root apparatus housing and its resistance to stretching (Figure 3).
January 2011
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