In this blog, we will discuss how to perform the Field Density Test (FDT Test) of soil by the Sand Cone Method. This test is very useful and commonly adopted at construction sites. The sand cone method is one of the most common techniques used for FDT in highway construction. The dry density test is carried out in the field to check the compaction of the layers.
What is Field Density Test (FDT)?
The Field Density Test (FDT) is a soil testing method used in construction to determine the compactness of soil at a site. It ensures that the soil meets the required density for stability and load-bearing capacity, reducing future settlement and structural failures.
What is Sand Cone Method?
The Sand Cone Method is a widely used technique to determine the field density of soil by excavating a small hole, filling it with uniformly graded dry sand, and calculating the soil’s density based on the weight and volume of the sand used.
What is Field Density Test by Sand Cone Method?
The Field Density Test by Sand Cone Method is a process in which a hole is dug in the compacted soil, and the excavated soil is weighed. A calibrated sand cone apparatus is then used to fill the hole with sand, and the volume of the hole is determined. Using this data, the soil’s density is calculated and compared with design specifications to ensure proper compaction.
Importance of Field Density Test
The Field Density Test (FDT) is conducted to:
- Ensure proper compaction of soil layers in construction sites.
- Prevent future settlement and structural failures.
- Verify that the soil meets design specifications.
- Improve the load-bearing capacity of the soil.
Compaction in Construction
Compaction is crucial for preparing the sub-base grade and other layers of pavement. It enhances stability and reduces settlement in embankments.
The required compaction levels for different layers in highway construction are:
- Base layer – 98%
- Sub-base layer – 98%
- Type 1 layer – 95%
- ABC layer – 100%
Apparatus Required
To perform the field density test using the sand cone method, the following apparatus is needed:
- Sand cone apparatus with a metal base plate.
- Calibrated sand (dry and uniform in size).
- Weighing balance (accurate to 1g).
- Moisture content containers.
- Digging tools (such as a trowel and chisel).
Procedure of FDT Test
The steps to perform the FDT Test are as follows:
Step 1: Calibration of the Sand Cone
- Take the weight of the empty sand cone.
- Fill it with dry sand and weigh it again.
- Determine the bulk density of sand by using a standard container of known volume and measuring the weight of sand required to fill it.
Step 2: Site Preparation
- Select a flat area on the construction site for the test.
- Remove any loose material and level the surface.
- Place the metal base plate over the test spot.
Step 3: Excavation of Soil
- Dig a hole in the ground through the base plate, collecting the excavated soil.
- Measure the weight of the excavated soil.
- Collect a sample of soil to determine the water content using the oven-drying method (IS 2720 Part 2: 1973).
Step 4: Filling the Hole with Sand
- Place the sand cone filled with calibrated sand over the hole.
- Open the valve, allowing sand to flow into the hole until it is full.
- Close the valve and weigh the cone with the remaining sand.
Step 5: Calculation of Field Density
The volume of the hole is calculated as:
volume of the hole = Mass of sand in hole/Density of used sand
The wet density is calculated as:
wet Density= mass of wet excavated soil/volume of hole
The dry density (𝛾_d) is calculated as:
Dry density = wet density*100%/moisture content of soil+100
The degree of compaction is determined using:
Degree of compaction = dry density*100%/Maximum dry density
Step 6: Determination of Dry Density
To obtain the dry density (𝛾_d), use the formula:
𝛾d = 𝛾/1+w
Where:
- w = moisture content of the soil sample (in decimal form)
Example Calculation
- Weight of excavated soil (W_s) = 1500g
- Weight of sand used to fill the hole = 1800g
- Bulk density of calibrated sand = 1.6 g/cm³
- Volume of the hole (V) = 1800 / 1.6 = 1125 cm³
- Moisture content (w) = 8%
- Field density (𝛾) = 1500 / 1125 = 1.33 g/cm³
- Dry density (𝛾_d) = 1.33 / (1 + 0.08) = 1.23 g/cm³
Advantages of Sand Cone Method
- Accurate and reliable results when properly performed.
- Simple procedure requiring minimal training.
- Applicable for all soil types, including cohesive and granular soils.
Limitations of Sand Cone Method
- Time-consuming process compared to other methods like the nuclear density test.
- Difficulties in conducting the test in highly compacted or very loose soils.
- Errors due to improper sand calibration or moisture variations.
IS Code for Field Density Test
The Indian Standard (IS) Code that governs the field density test of soil using the sand cone method is IS 2720 (Part 28): 1974. This code provides guidelines for conducting the test accurately and obtaining reliable results.
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Conclusion🎯
The Field Density Test by Sand Cone Method is an essential procedure for ensuring proper compaction of soil in construction projects. By following IS 2720 (Part 28): 1974, engineers can determine the soil’s density accurately, ensuring the long-term stability of structures. Despite some limitations, it remains a preferred method due to its simplicity and reliability.