Column Settling Test

The column settling test was conducted in general accordance with USACE Engineering Manual 1110-2-5027. This laboratory test (conducted over a period of 15 days) simulates the settling

characteristics of a soil slurry mixture that would be used in a coastal restoration project. The column is 80 inches high with 13 sampling ports. Various tests were performed to determine the sedimentation rate, engineering and material properties of the slurry, and the amount of suspended sediment in the water column at a specified time during the test. The different components of the column settling test are described below.

Figure 4.7- Column Settling Test: set up (left) and schematic (right) Settling Column

Sampling Ports Mixing Container

Pump Tubing

27  4.4.1 Sample Preparation

The homogenized sample was used to create the slurry for the column settling test. The homogenized sample was placed in a mixing container and water was added until desired concentration was achieved, which ranged between 95 grams per liter (g/L) and 105 g/L. The desired salinity for a particular column settling test was created by using tap water, water from the site, salt or any combination of the three. An industrial paint mixer is used for 15 minutes or until the sample is completed mixed. The mixed slurry was left to settle for about five minutes to allow any coarse material to fall to the bottom. Once the slurry had achieved the desired concentration, the slurry was carefully transferred to pumping container, only scooping off the top to avoid the coarse material that has settled to the bottom.

The salinity and concentration were adjusted until desired levels were reached for a particular test. The slurry solids concentration was measured by determining its total dissolved solids (TDS). A slurry sample was collected and oven dried to measure the dissolved solids in the slurry. The TDS was calculated using the following formula:

1000 ∗

(4.4)

where TDS is the total dissolved solids or solids particle concentration, Wp is weight of dry particles, Gs

is the specific gravity, Ww is the weight of water.

A salinity meter was used to determine the salinity of the slurry. The salinity meter could measure salinities up to 10 parts per thousand (ppt). When the salinity was greater than 10 ppt, a manual method was used where a sample of water was oven dried to weigh the amount of salt in the water. The equation below determined the salinity of the slurry: where S is the salt content, Ws is the weight of the oven dried

salt, and Wpw is the weight of the salt water.

∗ 100

(4.5) where S is the salt content, Ws is the weight of the oven dried salt, and Wpw is the weight of the salt water.

After the slurry was prepared to the desired solids concentration and salinity, it was pumped into the settling column using an electric water pump. The column was filled to a height of 79.5 inches within one minute using the pump. The sample was continuously mixed during the pumping process to prevent cavitation and to ensure that a homogenous slurry was pumped into the column. Figure 4.8 shows the process of transporting the slurry from the mixing container to the settling column.

4.4.2 Sampling Protocol

At specified time intervals, soil samples from the column were collected to perform additional testing. The sampling protocol used for each test was based on the USACE Engineering Manual 1110-2- 5027. All sampling was performed using a 60 mL syringe, stopper, and needle, to extract samples from the center of the column. The sampling apparatus was rinsed between samples to prevent cross

contamination. Immediately after the slurry was pumped into the column, samples were taken from the even numbered ports to confirm the initial solids concentration of the slurry. After that, samples were taken from the six ports above the solid-water interface for the remainder of the test. During each

sampling period, 120 milliliter (mL) sample was extracted from the column and placed in labeled bottles. The sampling was performed after 1 hour, 2 hour, 4 hour, 6 hour, 12 hour, 1 day, 2 days, 3 days, 4 days, 7 days, 11 days, and 15 days. During each sampling time, the water height, slurry height, and temperature were recorded before the samples were extracted from the column. A photograph of the entire column was also taken at each sampling time, which is included in Appendix B of this thesis.

Figure 4.8- Mixing container (left) and pumping slurry into the column (right)

Figure 4.9- Sampling Protocol: Solid water interface (left) and sample extraction using needle (right)

29  4.4.3 Total Suspended Solids (TSS) and Turbidity

Total suspended solids (TSS) and turbidity analyses were run to determine the amount of suspended solids in the water column. These tests were performed on the samples collected from the settling column ports during the course of the experiment (15 days). The TSS analysis (Figure 4.10) was performed in general accordance with Environmental Protection Agency (EPA) Environmental Sciences Section (ESS) Method 340.2. The test included pouring a sample through a 0.47 micron pore size filter and weighing the dried solids on the filter. A vacuum pump was used to extract the water through the filter and distilled water was used to rinse the sample. TSS was calculated using the following equation:

∗ 10 (4.6)

where TSS is the total suspended solids in grams per liter (g/L), Wp is the weight of dried particles in

grams and Vt is the volume of the sample in liters.

The turbidity test was performed as an additional check to measure the amount of solids present in suspension. This test includes placing the sample in a vial and in a turbidimeter apparatus. The vial must be cleaned with a microfiber cloth and inverted twice to ensure no bubbles disturb the sample reading. The value displayed on the machine was recorded in NTUs. Figure 4.10 shows the apparatus used to determine the TSS and turbidity. The TSS and turbidity test results for each sample is included in Appendix B of this report.

5 Results and Discussion