Geotechnics Lab Report: Soil Sieving

Introduction

Soil composition varies significantly in terms of particle shapes and sizes, depending on its location. This lab aims to illustrate the process of soil grading through sieving, utilizing the methods specified by BS 1377, which includes both dry and wet sieving. Wet sieving is suitable for cohesionless soils, while dry sieving is appropriate for soils with minimal silt and clay content. Excessive silt and clay in dry sieving can lead to inaccuracies due to particle clogging. Sieving is essential in civil engineering for classifying coarse-grained soils by their particle size, ensuring proper construction practices based on ground characteristics.

Method

The equipment used for this experiment comprises a range of sieves varying in size from 37.5 mm down to 63μm, allowing the creation of a particle size distribution curve. The procedure begins with obtaining a soil sample to ensure an even representation of larger particles. Visual inspection of the soil helps gain initial insights into its characteristics. The soil is then weighed, and the scale reads 1156.

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5g (±0.1g accuracy). This sample is added to the top sieve, securely fastened to a sieve shaker. The shaker is set at an amplitude of 2mm and a frequency of 50Hz, and the stopwatch is started for a 2-minute duration (deviating from the British standard of 10 minutes). After shaking, each sieve is removed, and the retained mass on each is measured. Smaller sieves are meticulously brushed to ensure accurate measurements, minimizing errors. The bottom pan captures particles smaller than 5 mm, which are then subjected to another 2-minute shaking using a set of smaller sieves ranging from 3.

35 mm to 63μm. After each shake, the contents of each sieve are weighed. Any discrepancies in mass can be attributed to human error and the scale's accuracy. Subsequently, a table is constructed, depicting particle size against the percentage of the total soil mass passing through each sieve. This table facilitates more precise soil grading than visual inspection alone.

Findings and Calculations

The obtained data was used to construct a graph illustrating the percentage of soil passing through each sieve against the particle size. To calculate the percentage retained, the mass retained in each sieve was divided by the initial soil mass and multiplied by 100. The percentage passing was determined by subtracting the retained percentage from the previous percentage (with the first sieve set at 100%). This graph allows for precise soil grading. It is evident from the graph that the majority of the soil consists of medium gravel, accounting for approximately 65% of the total soil mass.

Two coefficients are then calculated to further assess the soil's grading: the coefficient of uniformity (Cu) and the coefficient of curvature (Cz). The coefficient of uniformity (Cu) is determined using the formula Cu = D60 / D10, resulting in a value of 8.6. Since Cu is greater than 4, it indicates that the soil is well graded. The coefficient of curvature (Cz) is calculated using the formula Cz = (D30^2) / (D10 * D60), yielding a result of 2.88. This value also signifies that the soil is well graded, as it exceeds the threshold value of 1.

Results

The key results from this experiment are summarized in the table below, showcasing the particle sizes and their corresponding percentages of total soil mass:

These findings emphasize the distribution of particle sizes within the soil, crucial information for engineering and construction applications.

Conclusion

This lab successfully demonstrated the process of soil grading through sieving. By analyzing the percentage of soil passing through different sieves and calculating the coefficients of uniformity and curvature, we determined that the soil is well graded. Such knowledge is vital for civil engineering projects, ensuring that construction is tailored to the specific soil characteristics, thereby enhancing safety and stability.