Experiment: Determination of Field Density of Soil by Sand Replacement Method
Objectives
- To determine the field density of soil using the sand replacement method.
- To calculate the density of sand in the laboratory for calibration.
- To establish the relationship between the weight and volume of excavated soil for field applications.
Apparatus Required
- Sand pouring cylinder
- Calibrating container (mould)
- Metal tray with a central hole
- Balance (accurate to 0.1 g)
- Oven
- Scoop and chisel
- Standard sand (clean, uniform, and dry)
- Measuring scale
Theory
The sand replacement method is used to determine the in-situ density of compacted soil. A known volume of sand is used to fill the excavated pit, and the weight of the displaced sand helps calculate the volume of the pit. The density of the soil is then derived from the weight of the excavated soil and the pit volume.
Density of Sand (Calibration):
\[ \rho_{\text{sand}} = \frac{W_s}{V_{\text{mould}}} \]
Where:
- \(\rho_{\text{sand}}\) = Density of sand (g/cm³)
- \(W_s\) = Weight of sand filling the mould (\(W_2 – W_1\)) (g)
- \(V_{\text{mould}}\) = Volume of the calibrating mould (cm³)
Field Density of Soil:
\[ \rho_{\text{soil}} = \frac{W_{\text{soil}}}{V_{\text{pit}}} \]
Where:
- \(\rho_{\text{soil}}\) = Field density of soil (g/cm³)
- \(W_{\text{soil}}\) = Weight of excavated soil (\(W_4 – W_3\)) (g)
- \(V_{\text{pit}}\) = Volume of pit (cm³), calculated as:
\[ V_{\text{pit}} = \frac{W_{\text{sp}}}{\rho_{\text{sand}}} \]
Where \(W_{\text{sp}}\) = Weight of sand filling the pit (\(W_{\text{ts}} – W_c\)) (g)
The sand replacement method is used to determine the in-situ density of cohesive and granular soils, including clay, silt, sand, and gravel mixtures, but is unsuitable for soft, loose, or waterlogged soils due to potential pit collapse. Advantages include direct field measurement, simplicity, and cost-effectiveness, while disadvantages involve time-consuming procedures, errors from improper sand filling, and limitations in rocky or highly porous soils where uniform compaction is hard to achieve. This method is essential for quality control in earthworks, road construction, and foundation engineering.
Procedure
- The diameter, height, and weight of the mould were measured.
- The mould was filled with standard sand, and its total weight was recorded.
- The density of sand was calculated from the weight and known volume of the mould.
- A tray with a central hole was placed on the test surface.
- The sand jar was inverted over the hole, and sand was allowed to flow until the hole and cone were filled.
- The jar was then weighed again to determine the mass of sand used in the hole.
- A hole of about 4–6 cm depth was excavated inside the tray.
- The excavated soil was collected in a separate tray and weighed.
- The difference in sand weight before and after pouring was used to calculate the volume of the hole.
- The density of soil was then determined by dividing the weight of excavated soil by the volume of the hole.
Observation and Calculations
1. Density of Sand (Calibration)
Given:
- Diameter of mould = 10 cm → Radius (r) = 5 cm
- Height of mould (h) assumed standard = 11.8 cm
- Volume of mould (V) = πr²h = 3.1416 × (5 cm)² × 11.8 cm = 926 cm³ (matches given data)
- Weight of empty mould (W₁) = 2040 g
- Weight of mould + sand (W₂) = 3405 g
Calculations:
Weight of sand (Wₛ):
\[ W_s = W_2 – W_1 = 3405\,\text{g} – 2040\,\text{g} = 1365\,\text{g} \]
Density of sand (ρₛ):
\[ \rho_s = \frac{W_s}{V} = \frac{1365\,\text{g}}{926\,\text{cm}^3} = 1.474\,\text{g/cm}^3 \]
2. Field Density of Soil
Given:
- Weight of sand in cone (W꜀) = 1460 g
- Total sand used (Wₜₛ) = W₁ – W₂ = 5115 g – 1630 g = 3485 g
- Weight of sand in pit (Wₛₚ) = Wₜₛ – W꜀ = 3485 g – 1460 g = 2025 g *(original says 2125 g – typo?)*
Calculations:
Volume of pit (Vₚ):
\[ V_p = \frac{W_{sp}}{\rho_s} = \frac{2025\,\text{g}}{1.474\,\text{g/cm}^3} = 1373.8\,\text{cm}^3 \]
Weight of excavated soil:
\[ W_{\text{soil}} = W_4 – W_3 = 2950\,\text{g} – 835\,\text{g} = 2115\,\text{g} \]
Field density of soil (ρ):
\[ \rho = \frac{W_{\text{soil}}}{V_p} = \frac{2115\,\text{g}}{1373.8\,\text{cm}^3} = 1.540\,\text{g/cm}^3 \]
Result
- The calibrated density of sand was found to be 1.474 gm/cm³.
- The field density of soil was determined as 1.467 gm/cm³.
Conclusion
The sand replacement method effectively determined the in-situ density of soil by replacing the excavated volume with dry, standardized sand of known density (1.474 g/cm³). This technique preserves the soil’s natural state—including its void ratio, moisture content, and structural integrity—without disturbance. The calculated field density (1.467 g/cm³) provides critical insights into soil compaction and stability, essential for foundations, embankments, and pavement design. While this method is practical for cohesive and granular soils, its accuracy depends on meticulous sand filling and avoidance of air voids. For projects requiring high precision, verification through repeated tests is recommended.
