Field Density Test (FDT) under BS 1377: Part 9

The Field Density Test (FDT) under BS 1377: Part 9 involves determining the in-situ density of compacted soil. This test ensures that the soil's compaction meets design requirements. BS 1377: Part 9 outlines various field test methods for assessing soil properties.

Common Methods for FDT in BS 1377 Part 9:

1. Sand Replacement Method:

   • A known volume of soil is removed, and the void is filled with calibrated sand.
   • The weight of sand used gives the volume of the hole.
   • The in-situ density is calculated by dividing the weight of the excavated soil by the volume of the hole.

2. Core Cutter Method:

   • A cylindrical metal core cutter is driven into the soil to extract a sample.
   • The weight and volume of the soil in the cutter are used to calculate the in-situ density.
   • Typically used for cohesive soils.

3. Nuclear Density Gauge:

   • Measures density and moisture content using nuclear radiation.
   • This method is faster and non-intrusive but requires specialized equipment and training.

Key Aspects to Consider:

• Preparation: Ensure the soil surface is level and free of debris for accurate testing.
• Calibration: Equipment, especially for the sand replacement and nuclear gauge methods, must be calibrated properly.
• Moisture Content: It is essential to determine the moisture content of the soil, as it impacts compaction and density.
• Safety: Follow safety protocols, especially when using the nuclear gauge.

The Sand Replacement Method is a widely used field test under BS 1377: Part 9 to determine the in-situ density of soil. Here's a comprehensive, step-by-step explanation of the process:

---

Objective:

To determine the in-situ density of compacted soil for quality assurance of earthworks and embankments.

---

Equipment Required:

1. Sand Pouring Cylinder:
   • A calibrated cylindrical container with a cone-shaped pouring nozzle.
2. Calibrated Sand:
   • Dry, uniform sand with a known density (calibration done in a controlled environment).
3. Metal Tray:
   • A flat, circular tray with a central hole for creating the test excavation.
4. Weighing Balance:
   • Sensitive enough to measure weights accurately.
5. Moisture Content Equipment:
   • Oven, container, and balance to determine soil moisture content.
6. Tools for Excavation:
   • Small spade, scraper tool, or similar implements.
7. Measuring Tape:
   • For measuring dimensions of the hole (if necessary).

---

Pre-Test Preparation:

1. Site Selection:

   • Identify the area where the compaction or in-situ density needs to be tested.
   • Ensure the surface is level and free of debris.

2. Calibration of Sand:

   • Determine the bulk density of the sand by pouring it into a pre-calibrated container of known volume.
   • Record the mass of sand and calculate the density using:
     $$\text{Bulk Density of Sand} = \frac{\text{Mass of Sand}}{\text{Volume of Container}}$$

---

Testing Procedure:

1. Prepare the Test Area:

• Place the metal tray on the soil surface, ensuring it's flat.
• Use the central hole in the tray as a guide to cut a small, neat hole in the soil.
• Carefully excavate the soil up to the required depth (~10-15 cm for most cases), collecting and weighing the excavated soil.

2. Measure the Volume of the Hole:

• Place the sand pouring cylinder over the metal tray hole.
• Open the valve to allow sand to flow and fill the hole completely, including the cone below the cylinder.
• Close the valve once sand stops flowing.
• Weigh the remaining sand in the cylinder to calculate the amount of sand used.

Calculate Volume:

$$\text{Volume of Hole} = \frac{\text{Mass of Sand Used}}{\text{Bulk Density of Sand}}$$

3. Determine the Soil Mass and Moisture Content:

• Weigh the excavated soil to get the total mass.
• Take a representative sample of the excavated soil for moisture content determination using the oven-drying method.
• Calculate the dry mass of the soil using:
  $$\text{Dry Mass of Soil} = \frac{\text{Mass of Excavated Soil}}{1 + \frac{\text{Moisture Content (\%)}}{100}}$$

4. Calculate the In-Situ Density:

• The in-situ dry density is calculated as:
  $$\text{Dry Density} = \frac{\text{Dry Mass of Soil}}{\text{Volume of Hole}}$$

---

Post-Test Calculations:

1. Compare with Specification:

   • Compare the dry density obtained with the specified compaction requirement.
   • Typically, results are expressed as a percentage of the Maximum Dry Density (MDD) obtained from laboratory compaction tests (e.g., Proctor test).
   $$\text{Compaction (\%)} = \frac{\text{Field Dry Density}}{\text{MDD}} \times 100$$

---

Advantages:

• Accurate and reliable for small-scale in-situ density determination.
• Simple and cost-effective equipment.

---

Limitations:

• Time-consuming compared to modern methods like nuclear density gauges.
• Errors may arise from improper sand calibration or uneven soil surfaces.
• Unsuitable for very coarse or granular soils.

---

Tips for Accurate Results:

1. Ensure sand is dry and uniform.
2. Minimize disturbances around the test area.
3. Perform multiple tests for consistency.
4. Regularly calibrate the pouring cylinder and sand density.