Field density is an in-situ test that measures the density of natural or compacted fine and medium-grained soils. When used on compacted materials in the field, the field density refers to the degree of compaction on a layer by comparing the density achieved at the field with the maximum dry density (MDD) obtained in the laboratory, as described under the proctor test. The field density is expressed as a percentage of the MDD, known as the degree of compaction. Various methods are used to determine the field density including:

1. Core Cutter : The core cutter method involves driving a metal cylinder (core cutter) into the compacted soil to extract a sample. The mass of the sample is then weighed, and the volume of the core cutter is used to calculate the field density. This method is generally more suited for fine-grained soils and may not be ideal for coarse-grained soils or soils containing large aggregates.
2. Rubber Balloon Method : The rubber balloon method involves placing a rubber balloon in a hole dug in the compacted soil. Water is then pumped into the balloon to measure the volume of the hole. The apparatus is a bit delicate, therefore, not widely used.
3. Nuclear Density Gauge: This method uses a nuclear gauge to measure the density and moisture content of the soil. The gauge emits gamma rays into the soil and measures the scattering of these rays to determine density. The Nuclear Density Gauge method has the advantage of providing rapid and accurate results and is particularly useful for large-scale projects where time is of the essence. Additionally, it can measure both density and moisture content simultaneously. The downside of this method is that due to the use of radioactive materials, operators must undergo specialized training and adhere to safety protocols.
4. Sand Replacement Method: The sand replacement method involves digging a small hole in the compacted soil, weighing the excavated material, and then filling the hole with calibrated sand to determine its volume. It is the most widely used procedure for determining the in-place density of compacted soil and is the subject our discussion today.

Sand Replacement Procedure – According to BS 1377:9 1990

a) Sand in Cone

1. Fill the apparatus with sand to constant mass say 8kg or 16kg, depending on its capacity.
2. Place the apparatus in a tray, open the shutter, and allow a volume of sand equivalent to that of the excavated hole to run out then close the shutter.
3. Put base plate on a flat surface (e.g glass plate) and place the apparatus on it, open the shutter and allow the sand to run out freely without any vibration.
4. When there is no further run-out, close the shutter, collect the sand that has filled the cone and weigh it. Alternatively, weigh the remaining sand in the container and get the difference to determine the sand in cone.
5. Repeat step 1 to 4 at least three times and take the average value as the weight of sand in cone (m_a).

b) Sand Calibration (determination of bulk density)

1. Measure the volume of the calibration container (V_c).
2. Fill the apparatus with sand to constant mass, say 8kg or 16kg, and record the weight (m_b).
3. Fix cone to the sand apparatus, close shutter and place it over the calibration container.
4. Open the shutter and allow the sand to run out freely.
5. When no further run-out is experienced, close the shutter and weigh the sand remaining in the bottle. Repeat step 2 to 5 at least three times and record the average weight (m_c)
6. Calculate the bulk density of sand as follows:

c) Field Density (determination of the field compaction)

1. Fill sand pouring apparatus with known mass of sand according to its capacity, say 8kg or 16kg (m1).
2. Level the place to be tested and brush away any loose material.
3. Place base plate in position and dig a round hole about 150mm deep for a 150mm layer or 200mm deep for a 200mm layer.
4. Collect all soil dug out of the hole, determine its weight (M₀) and take a small portion for moisture content (w).

NOTE: If moisture content is to be determined later, put the soil in an airtitght container or platic bag.

5. Place the sand pouring apparatus over the hole (with the base plate) and allow the sand to run out freely without vibration.
6. When there is no further run-out, close the shutter and weigh the sand remaining in the bottle (m₂).

Calculations:

Presentation of Results and Interpretation

The presentation of the results for field density test is straightforward. The data entry form ideally captures the calculation of bulk density of the soil by determining the volume of the hole dug out at the field, by using the predetermined density of the sand. Upon determination of the bulk density of the soil, the dry density of the soil can easily be determined as is clear in the data entry forms attached below:

Significance of Field Density Test in Engineering Applications

The field density test of compacted soil is of paramount importance to engineers as it ensures that the material used in the construction of earthworks or pavement works has been compacted to the required density. The level of compaction on any fill or pavement layer has a direct correlation to the layer’s ability to carry loads, the minimizing of settlement and erosion resistance. Inadequate compaction can lead to excessive deformation, reduced load-bearing capacity, and potential structural failures, making the Field Density Test an essential quality control measure in construction projects.

If the field density is below the required specification, additional compaction is necessary. This may involve using heavier compaction equipment, increasing the number of passes, or adjusting the moisture content of the soil to achieve optimal compaction.

Video Version

Click on the video below to see how the field density is determined using the sand replacement method: