Geotechnical Analysis

Partial Size Distribution

A particle size distribution is needed to classify the type of soil found on the site. To achieve this the team performed a sieve analysis to determine the distribution of particle size based on their diameter. In the lab, the samples of soil collected from the site were dried, and processes leading to the end results were conducted using ASTM C136. The drying process began by separating our samples and removing any organic material from the sample. Once this was done the samples were put into the drying oven at about 200 degrees Fahrenheit. Leaving the samples in the oven overnight when gathering the samples, the next day they were completely dry to make sure that the weight of the sample was just due to the sediment. All the samples were then measured for their weight by subtracting the weight of the pan from the weight of the sample. Last thing done before shaking the samples the material was poured into the top of the towers with the sieves shown in Table 6‑1. By shaking the samples and allowing them to filter through different sized sieves, we were able to determine the distribution of particles based on their mass comparing it to the total mass that the sample starter with.

In the sieve analysis, the sieve numbered 4, 10, 20, 40, 60, 80, 100, and 200 were used. The gap size in each one of the sieves can be seen in the table below. As the number descends the size of mesh the particles travel through gets smaller resulting in smaller quantities of sediment.

By disrupting the soil and allowing the soil to go down through the different sized sieves the distribution of particles can be seen. For the site, four different soil samples were collected to perform sieve analyses. The results for each of the particle size distributions for each of the different samples, ordered by the sample number.

Soils that pass through the #200 sieve are considered silty, while anything in between the #200 and the #4 sieve being sand, and anything above the #4 sieve is considered gravel. A range of 4.4%-8.5% was found to be silty, 58.5%-78.7% was found to be sand, and 17.7%-40.1% was found to be gravel comparing each one of the tests. As seen in their graphics most of the soil was within the sand category ranging from the #200 sieve to the #4 sieve. After analyzing the data, it was determined through using Figure 6‑2 the soil type of all the soils was stone fragments, gravel and sand. Starting from the top you must determine the amount of material passing the No. 200 sieve. In the tests ran all the samples had less then 35% passing this point shown with the red box on the top right of the figure. Next looking at the third row the amount passing through the No. 10,40, and 200 is considered. There was more than 50% passing though the No. 10 sieve so it cannot be the first column. Finding this the soil is considered to be A-1b defining it as the stone fragments gravel and sand.

The losses through the sieve analysis due to the mass of soil being lost are shown below. These losses are through the shaking proses and the transfer of the samples from the initial drying pan to the tower. Some material may have stuck to the bottom of the drying pan showing that not all the material went into the tower. Also, when the sample is being shook, some soil may be dislodged from the tower even with the lid on the top.

After gathering this data, the Coefficient of Uniformity (Cu) and Coefficient of Curvature (Cc) could be determined by finding the particle size when 10% (D10), 30% (D30), 60% (D60) of the materials passed through the sieves. Using the equations below the coefficients can be defined for grater classification of the soil.

With this result, there are a few defining factors that can be concluded. When looking at this figure the Soil is first determined if it is mostly gravel or sand. This can be done through analyzing. The mass of soil not passing through the No. 10 sieve is considered gravel. The mass that is in between the No. 10 and No. 200 sieve is considered sandy. Using both the figure and Table 6‑1 the soil can be determined to be mostly sand. The next step is to follow the flow path that is applicable to the soil shown in the figure below. The soil that we gathered can be defined as poorly graded sand with clay & gravel (or silty clay & gravel).

First, the soil that is based on site currently can be used as the base material for the proposed expansion of the parking space. Second, there will be no need for a large amount of cut of materials on-site for a new shipment of proper soil type. This result will save on construction costs for cutting materials and removal of materials. Any larger pieces of material that need to be broken up can be used for fill material as well.

Atterberg Limits

The Atterberg Limit testing, regulated by ASTM D-4318 [9], is responsible for gathering the soil’s Liquid Limit and Plasticity Index. Following the steps set forth by the ASTM standard the team was able to gather these values for the soil samples. The team performed this test after the completion of the drying and sieving process stated in the previous section. Drying the samples is a very important part of the Atterberg limit testing because any additional moisture would result in inaccurate and skewed values. After the team is done drying and sieving the samples collected from the site in Tucson for the previous testing, the samples were massed and compiled into their particle sizes. After the samples were separated, the material that possessed the smallest particle size were collected and used in the ASTM tests. In the team’s case, the usage of the material that passed through the #200 sieve was prioritized as that was the recommended size in the ASTM manual.

The plasticity of the soil allows for deformation and change under stress without shearing or cracking, which can lead to degradation and unsafe conditions. This is tested and completed with ASTM D-4318. The plasticity was collected using a method of repeatedly “tapping” or impacting the soil at a ratio of water to the soil that would cause deformation after a prescribed number of impacts. At that prescribed number, the ratio of water to soil represents the plasticity of the soil.

The liquid limit of the soil is the amount of water our soil is able to withstand before it begins to flow or deform, and the testing situation holds the sample at the lowest shear strength as to test the material at the limit of the material. The Liquid limit testing was done to the directions of ASTM D-4318, where the same dried and sieved soil in the previous section is mixed with an amount of water so the soil could be rolled by hand into a long thin piece. When the testing material is rolled thin enough to be sheared apart by the force of rolling, it could be concluded that the particles could no longer be actively held together. At that point, the maximum amount of water, or the liquid limit, would be determined by the percentage of water by weight. When the log is sheared apart, the diameter of that log was recorded and if the diameter was within 3-5 millimeters, listed in the ASTM manual, the ratio of water to dirt would then be listed as the Liquid limit.

The results of the testing, seen above, are mostly similar thought the 4 samples done. Although with sample 2 the definition found for the soil type is different then the other samples. This difference can be concluded accurate due to the location of where the sample was taken. The sample was collected near the culvert where most of the water is flowing to from the site. A larger liquid limit can be found in this area do to the nature of the culvert found on site.

The liquid limit, as listed above, is the difference between the Plastic Limit of the soil that is measured in the Casagrande cup ASTM testing while the liquid limit is calculated in the rolling ASTM testing.