Pluviation of the Samples

Several methods are available for manipulating the density of sediments including tamping, vibration and pluviation. The method for manipulating the density of sandy sediments adopted in our laboratory is pluviation, as is defined by the ASTM method (D 4253-83). Pluviation, by definition, is a process which consists of raining cohesionless soil particles onto a sample holder, by using only the force of gravity and using appropriate sieve meshes to act as diffusers. [77] This method consists of the following steps:

1. A hopper or container suspended above the sample is filled with the sediment of interest.

2. The bottom of the container is allowed to fall out using an opening mechanism. According to ASTM standards, this opening system should have a diameter within the range of 5 mm to 70 mm. [77]

3. The sediment freely falls until encountering diffusers. The sand hits sieve meshes, which act as diffusers. There are a total of six meshes which are rotated 45 hori-

Figure 5.2: Grain size distribution of sample B1-6 derived from performing a sieve-shaking analysis of the sediment.

than the bottom five sieves, which should all be uniform in size.

4. The sand exits the bottom-most sieve and continues to fall until reaching the sample holder. The distance between the bottom-most sieve and the top of the sample holder is denoted as the ”Height of Drop” (HD).

5. After the sand has fallen, the sediment is allowed to settle for 30 minutes. 6. The sand is then leveled off to create a perfectly flat surface.

The density of the sediment within the sample holder after pluviation is directly related to HD. However, beyond a certain critical value of HD, the height increase fails to impart additional kinetic energy onto the falling particles, and the densification of the material will not increase. [77] The pluviation method is often preferred by the geotechnical community for creating varying densities of sand samples for several reasons, including: uniform spreading of particles throughout the sample holder, accurate creation of densities, and repeatability of density profiles by using the same value of HD across runs. [77]

A custom pluviation device was developed and characterized by a former lab mem- ber for the purpose of manipulating sand density. [78] This device was used in sample preparation for this set of laboratory experiments. For this experiment, the first stage of roughness preparation consisted of pluviating the sample at a constant value of HD of 50 cm. In this way, all samples had approximately the same density prior to gener- ating roughness patterns, mitigating the potential for varying sample density to affect the measured BRDF under laboratory conditions. Repeatedly performing 15 pluviations with sample B1-6 at this constant drop height revealed that the density was constant to within ∼ 1.5%. After performing this pluviation routine, we mechanically created surface roughness for the samples using meshes. We discuss the development of this process in the next subsection.

5.2.4 Creation of Roughness Profiles

Over the course of the summer field experiments described above, we encountered both wavelike sediment roughness and randomly distributed roughness. The goal of this labo- ratory experiment was to assess the differences in directional scattering properties of these two different types of surface roughness. In this experiment, we used grid-like mesh struc- tures as well as wave-like meshes to create these roughness profiles. For each of these mesh types, we used meshes with different grating spacings to compare directional scattering effects for sand samples with varying spatial frequencies. An example of these meshes is shown in Figure 5.3. This image shows two different meshes with grating spacings of approximately 25 mm. Note that one of the meshes exhibits a grid-like pattern that when pressed into a sediment sample would create a roughness patten in which the distribution

Figure 5.3: Examples of the meshes that will be used to generate macroscopic surface roughness for the samples used in this laboratory experiment. The grid-like mesh is shown on the left, and the wavelike mesh is shown on the right.

of facet normal vectors has no preference for azimuthal direction in terms of its slopes normal angles; in other words, the azimuth component of the slope normal angles has a uniform probability distribution function. The other mesh has only vertical gratings and would produce a more wavelike roughness structure when pressed into a sediment sample. During the testing phases of this experiment, the wave-like mesh was pressed into a sam- ple and an image was captured of the surface under diffuse lighting conditions, shown in Figure 5.4. The photometric effect of wavelike roughness is immediately apparent despite the fact that there is no directed illumination onto the sand surface.

In order to examine the role of roughness in the BRDF of sediments, we created three different roughness patterns of varying spatial frequencies for both the wavelike roughness mesh and the grid-like roughness mesh. The grating spacing was 25 mm for the coarsest grid, and 10 mm for the least coarse grid. For the third roughness profile, the sample was perfectly smooth and no grating was used. This sample was meant to be the control case for the effects of roughness. The grids used were approximately 8 inches in diameter. This diameter is large enough to ensure that only the roughness pattern will be within the field-of-view of a 5 degree fore-optic at extreme zenith angles of 65 degrees.

For our series of experiments, we used a constant pluviation drop height value of 50 centimeters. After each drop, the cone of sand on top of the sample holder was leveled off to create a uniformly smooth surface. When creating a unique roughness case, we pressed a sieve mesh into the sample to a depth of approximately 15 mm and then removed the mesh vertically from the sample. We made the assumption that this approach creates a sample that is approximately constant in density between runs, with the macroscopic surface roughness being the only free variable. We acknowledge that this is an approximation due

Figure 5.4: An example of the wavelike roughness profile produced on a test sample of sand using the wavelike mesh shown in Figure 5.3.

to the local disturbances that are created by pressing the mesh into the sand.

We wanted to characterize the effect of wavelike roughness on the directional reflectance of a medium, and also compare this effect to similar frequencies of roughness that are not distributed with azimuthal preference. For this reason, we used four different meshes in this study. These meshes included both grid-like and wave-like meshes in which the grating spacing was 10 millimeters and 25 millimeters. Examples of the prepared sediments resulting from using these four different meshes on the sample B1-6 are shown below in Figure 5.5.