Field Density Test Methods: Comprehensive Techniques for Assessing In-Situ Soil Compaction

What Is Field Density Test?

Various types of field density tests are being practiced in different parts of the world for the evaluation of in-situ soil compaction and knowing the relative degree of Compaction. FDT, Full name is Field Density Test, is a Quality Control test carried out at the site for knowing the increased Compaction or Density or achieved at a site on the soil layer.

The FDT is carried out by Laboratory persons of Contractor under the supervision of the QC team by the Consultant after completion of the required number of passes by compactor/roller determined in test fills at the site.

Compaction of the soil means pressing the soil particles close to each other by mechanical procedures. Air present at the void spaces are expelled in the soil mass as a result of Compaction, and so, the density is increased.

The Compaction and its testing by field density test (FDT) are very important and repetitive steps during earthwork phases of construction at a project. The Compaction of the soil usually increases the shear strength of this soil, and therefore the stability and bearing capacity. It’s also helpful in reducing the compressibility and permeability of the soil mass.

Whether you are constructing a multistory plaza, building, highways / pavements, or even in hydropower projects or Dam, you have to know the basic knowledge of field density tests.

Whether you are a quality site engineer or control engineer from the Contractor or from the supervision team, there are a lot of factors that must be known for ensuring the quality and sustainability of the structure.

Different Types of Field Density Tests

As Per Below types of Field Density Tests:

1. Sand Cone Method or Sand Replacement Method
2. Water Replacement Method of Field Density Test
3. Core Cutter method
4. Heavy oil Method
5. Rubber Balloon Method
6. Nuclear Moisture Density Meter

#1. Sand Replacement Method or Sand Cone Method-

The sand replacement test method is utilized to determine the in situ dry density of soil. The procedures, materials, equipment, and specifications of the test are based on the Indian Standard. This test is of significant importance, and it’s been widely utilized in various construction project sites.

The field density of natural soil is necessary for the estimation of soil bearing capacity with the aim of evaluation of pressures on underlying strata for computation of stability and settlement analysis of natural slope.

The sand replacement test method is also to determine the in-place density of compacted soil so as to compare it with the designated compaction degree. Therefore it specifies how much the Compaction of the soil is close to the designated compaction degree.

Apparatus

1. Sand – pouring cylinder
2. Excavating tools, and Soil cutting such as scrapper tool bent spoon
3. Calibrating container, 100mm diameter, and 150mm height
4. Plane surface: Perspex Plate or Glass or Other Plane Surface, 450mm square, 9mm thick or larger
5. A metal container to collect excavated soil
6. Weighing balance accurate to 1 gram
7. Metal tray, 40 mm deep and 300mm square with a hole of 100mm in diameter at the center
8. Weighing balance accurate to 1 gram
9. Moisture content cans
10. Oven

Calibrations of Instrument.

1. Measure the internal dimensions of this calibrating container and calculate its volume.
2. Fill the sand-pouring cylinder with sand, in about dia 10mm of its top. Dia the weight of the filled cylinder (M1).
3. Place the sand-pouring cylinder vertically onto the calibrating container. Open the shutter to allow the sand to run out of the cylinder. When there isn’t any further movement of the sand in the cylinder, close the shutter.
4. Lift the pouring cylinder from the calibrating container and weigh it into the nearest gram (M2).
5. Place the sand pouring cylinder on the glass plate. Open the shutter and allow the sand to run from the cylinder until no further movement of the sand will be noticed (sand fills the Cone of the cylinder), then close the shutter and remove the sand-pouring cylinder carefully.
6. Take the sand onto the glass plate and determine its weight (M3)
7. Repeat step 3 to step 6 two more times, and record mean weight (mean M3 and M2)
8. Determine the Filed dry Density of sand, as shown in the below Table.

Calibration for Dry Density of Sand – Table 

Procedure of Sand Replacement Method

1. Expose an area of about 450mm square on the top layer of the soil mass. Trim the surface down to a level surface using a scraper tool.
2. Place the metal tray on the leveled surface.
3. Excavate the soil through the central hole of the tray, using the hole in the tray as a pattern. The depth of the excavated hole must be about 150mm.
4. Collect all the excavated soil in a metal container and determine the mass of the soil (M).
5. Remove the metal tray in the excavated hole.
6. Fill the sand pouring cylinder within 10mm of its top. Determine its mass (M₁).
7. Place the cylinder directly over the excavated hole. Allow the sand to run out of the cylinder by opening the shutter. Close the shutter when the hole is completely filled, and no further movement of sand is observed.
8. Remove the cylinder from the filled hole. Determine the mass of the cylinder (M₄).
9. Take a representative sample of the excavated soil. Determine its water content.
10. Determine the dry density of soil, as shown in Below Table.

Observations and Calculations for Dry Density Test of Soil – Table 

#2. Water Replacement Method-

The water displacement test method is utilized to determine the dry density of soil. It’s suitable for evaluating the in-place dry density of cohesive soil because of the nature of the specimen required.

The soil sample ought to be trimmed to obtain a regular shape and then coated to make it impervious to water. After that, the total volume of this waxed specimen is found by determining the volume of water displaced from the specimen.

Apparatus of Water Replacement Method:

1. Water displacement apparatus
2. Weighing balance, accuracy 1g.
3. Paraffin wax (density of paraffin (Pp)= 0.91 g/ml).
4. Cutting knife
5. Brush
6. Water content container
7. Heater
8. Oven
9. Measuring jar

The Procedure of Water Replacement Method of Field Density Test

1. Take the soil specimen.
2. Trim it to a regular shape. Avoid re-entrant corners. Weigh the specimen.
3. Take a paraffin wax and melt it onto a heater. Apply a coating of melted paraffin wax into the specimen with a brush. When it’s hardened, apply another coat. Take the mass of this waxed specimen (Mt).
4. Fill the water displacement apparatus with water. After the flow occurs, close the valve.
5. Place a measuring jar below the overflow tube of the apparatus. Open valve.
6. Immerse the waxed specimen slowly into the water from the apparatus. Water overflows. Collect the overflowed water from the jar. Determine the volume of the water collected (Vt).
7. Take out the waxed specimen from the apparatus. Dry it from outside.
8. Remove the paraffin wax by peeling off it.
9. Cut the specimen into two pieces. Take a representative sample for the water content determination.

Calculations Sheet of Water Replacement Method:

#3. Core Cutter Method:

By using core cutter method, bulk density of soil can be quickly calculated and by determining the moisture content of the soil the dry density of the fill can be calculated and hence the voids percentage. A high percentage of voids indicates poor compaction of soil.

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#4. Rubber Balloon Method:

Relevant Code: ASTM Designation: D 2167-66

Apparatus Required

1. Balloon Density Meter
2. Soil augur or trowel
3. Moisture tight container
4. Weight balance
5. Rubber bulb pump
6. Density plate with fasteners
7. Rubber balloons
8. Drying Oven

The Procedure of the Rubber Balloon Method

1. Position the density plate on a flat surface and set the volume measuring (A) apparatus in the recessed hole in the density plate
2. Hold the apparatus down firmly in position, open the control valve, pump the balloon down with the rubber bulb until the water level in the graduated cylinder has reached its lowest position, and record this volume, which is the initial reading.
3. Pump the balloon back into the cylinder by inverting the rubber bulb, and close the control valve.
4. Place the density plate in a level position on the material to be tested, dig a test hole about4 in diameter, and 4 in. Deep, retain all the material which is removed from the hole, then measure and record the mass of the excavated soil.
5. Set the apparatus in the recess in the density plate, hold it down firmly, open the control valve, pump the balloon into the hole, and record the lowest point reached by the water in-cylinder, which is the final reading.
6. Invert the pressure-vacuum bulb and pump the balloon back into the cylinder.
7. Subtract the initial reading from the final reading and obtain the volume of the hole in cubic feet.
8. Mix the material thoroughly and secure a representative sample of not less than 100 gm for moisture determination.
9. Make density calculations based on the volume of the test hole and dry or wet weight (as required) of materials removed.

Calculation of Rubber Balloon Method

where:

V1= Volumeasure initial reading, cubic centimeters

V2= Volumeasure final reading, cubic centimeters

W = weight of wet soil taken from test hole, grams

V = volume of a hole, cubic centimeters

DW = Wet density, g/cc

D = Dry density, g/cc

w = Moisture content of the sample expressed as a percentage of the weight of the dry soil.

1. Calculate the moisture content, w (expressed as a percentage of the weight of the dry soil), of the soil as follows:
2. w = weight of dry soil x 100%
3. Calculate the volume, wet and dry density of the soil as follows
4. V = V2 – V1

DW = W / V

D = (DW x 100) / (100 + w)

#5. Nuclear Moisture Density Meter

Apparatus Required

1. Portable Nuclear Moisture-Density Gauge
2. Transport case (Type “A” Package)
3. Charger
4. Reference Standard Block
5. Transport Documents (Bill of Lading)
6. Leveling Plate/Drill Rod Guide
7. Drill Rod w/extraction tool
8. Hammer (4 lbs.) used for Driving the Pin
9. Safety Glasses
10. Square-Point Shovel
11. No. 4 sieve
12. Set Balance Scales
13. Drying Apparatus
14. Miscellaneous Tools like Mixing Pans and Spoons

The Procedure of Nuclear Moisture Density Meter

There are two different procedures to determine percent density and percentage moisture utilizing the portable nuclear density gauge. The methods are direct transmission and backscatter. The direct transmission method requires punching a hole to the surface of the material being tested and lowering the source rod to the desired depth of this test.

This method is used to test soil and aggregate materials. Please note that if testing soils, the backscatter position won’t be utilized as a way of acceptance for density. From the backscatter procedure, the source rod is lowered to the first notch below the safe position placing the source and detectors in the same horizontal plane.

No hole is necessary for the probe because it’s flush with the bottom of the gauge. This method is used to test aggregate (sub-base and base course) and asphalt materials.The Roller Pattern is performed first. The purpose is to determine the number of passes to be made by the roller at various combinations of static or vibratory rolls to achieve maximum density for this depth of the material using this roller.

The data collected from the gauge, when properly plotted, will provide a graphical comparison of the number of roller passes essential to produce a properly compacted product. Once completed, this data is used to establish a Control Strip(s).

The Control Strip determines the target values for density, which can define the acceptance criteria for the material placed and compacted using the previously determined roller pattern. The values determined from the control strip won’t change until a new roller pattern is required. The Control Strip provides an accurate way of evaluating materials that are relatively uniform and exhibit smooth surfaces.

Roller Pattern

The Roller Pattern is constructed on this same material being placed and, once established, will be utilized for the remainder of the project. The Roller Pattern is 75 ft in length plus some additional area to accommodate the lateral positioning of the roller. The width and depth of the material depend on the design of this project.

Listed Below are the Steps Used to Construct a Roller Pattern:

#1. Establish an area at least 10 ft from any structure, and 33 ft from other radioactive sources (another gauge) to take standard counts. This area may be asphalt, concrete, or a well-compacted soil using a minimum density of 100 lb/ft3. Don’t set the gauge on truck beds, tailgates, tabletops, etc..

After taking a standard count. Turn the gauge on and let it warm up. At this time, standard counts can be taken and recorded.

Note: A standard count is going to be taken each day of usage. If count fails, refer to the gauge Manual of Operation and Instruction guide for further instructions.

#2. To prepare a Roller Pattern, place the material onto a section of roadway approx. 75 ft in length for the typical application width (an area of at least 100 yards), and in the proper loose depth before any rolling is started. The Compaction is to be completed uniformly and in the same manner for the remainder of the job.

The moisture content of aggregates must be kept as near optimum as possible throughout the rolling operation. Water has to be added when needed to maintain optimum moisture. To speed up operations, select the 15-second mode on the readout panel and record the moisture and density readings. When testing the control strip and test section, select the 60-second mode for acceptance.

#3. Make two (2) passes (1 pass is counted each time the roller crosses the test site) using the roller over the entire surface of the Roller Pattern. Make sure the previous passes have been completed over the whole surface before the next pass is started. When testing asphalt materials, take a nuclear test for density only, using the Backscatter Method.

The above test on aggregates and asphalt materials should be made at three randomly selected points within the area to be tested. Choose points with good surface conditions, and try to spread the 3 tests over most of the 75-ft section, making sure to not place the gauge closer than 18 inches into an unsupported edge.

Make sure you mark the exact location where the gauge is placed. (If using spray paint to mark the locations, don’t spray the gauge with paint.) The gauge, when in use, will always be positioned parallel with the roadway, with the origin end toward the direction of this paver.

Obtain Total and Average for Both Moisture and Density.

All further tests for your Roller Pattern have to be reached in the same 3 locations, together with the gauge source rod pointing at the same direction as the first test. The plot, the average dry density versus the number of pliers, moves on the graph.

#4. Make additional passes using the roller over the whole surface of the Roller Pattern, and again obtain and record the 3 readings for density and moisture at the same location as the previous set of readings.

Calculate the average from the readings.

Continue the testing and rolling of this section until the Roller Pattern reaches its maximum density before decreasing or the curve levels off. To be certain, this is a sufficient degree of Compaction, make one additional roll over the whole surface and test again.

Point Note: The number of passes that are indicated does not necessarily need to be put at two (2) every time. It might be found that in some instances, one pass could be sufficient between readings and, in other instances, 3 or 4 passes would be required.

An accurate count of this required passes should be maintained and might vary depending upon sub-grade conditions, roller efficiency, types of materials, and moisture content. Notes on determining Maximum Attainable Density with Roller Pattern/Control Strip Technique

The Control Strip shall be rolled until maximum dry Density for granular materials or maximum density for asphalt materials is obtained. Materials compacted to maximum density provide a solid platform on which to construct pavement. Materials at maximum density increase pavement load carrying capacity and pavement life.

In the interest of good construction procedures and practice, the inspector must use these guidelines to the best of his / her ability. In brief, the change in density in a typical Roller Pattern, for example, on Aggregate Base Material, Type I, Size 21B, will look as shown below:

It could be seen in the above that continued rolling after 10 passes resulted in diminishing returns. This is typical for many Roller Patterns. Based on an analysis of the type, the following is recommended as a guideline for granular materials. In the event that the increase in dry density for a Roller Pattern on granular material is less than 1 lb/feet3, one additional pass shall be required.

For asphalt base, the same guidelines as for granular materials must be utilized, with the exception that after the increase becomes less than 0.5 lb/ft3 per pass, one additional pass will be required. If the density doesn’t increase by 1 lb/ft3 with the additional pass, rolling ought to be discontinued. Occasionally, there’ll be instances where a decrease in density rather than a small increase will occur.

This usually occurs for two reasons: a false break, in which the density levels off well before maximum density is achieved, and over rolling. In this case, consideration ought to be given to the number of passes already made and the materials involved, making certain that the break occurring in the Roller Pattern curve isn’t greater than 1.5 lb/ft3.

After the break is greater than the above value, re-compact the material into the maximum dry density based on the peak of this roller pattern. A new roller pattern should to be established whenever there are multiple lifts of material, or there is a change in the following:

Source of material 

1. Compaction equipment
2. The visual change in subsurface conditions
3. Test section readings are significantly above the target values by more than 8 lb/feet3
4. Another Control Strip will be established.
5. Gradation or types of material
6. Nuclear Density Gauge

Control Strip

To prepare a Control Strip, an additional 300 feet. of roadway is required extending by the Roller Pattern area (same spreader box width in the same designed depth). This area is to be rolled the same number of passes from the Roller Pattern.

In order to determine the maximum dry Density of the Control Strip, 10 readings for moisture and density ought to be performed and recorded over the entire 300 feet. Section. Calculate and enter the data. The Target Values of 98% and 95% of the average dry density can now be determined.

The dry density determined in the average of the Control Strip must compare within 3 lb/ft3 of the roller pattern’s maximum dry density. This applies to both asphalt and aggregate materials.

Point Note 1: When testing Asphalt Concrete, the gauge ought to be programmed into the asphalt mode.

Point Note 2: When testing aggregates, a verification test will be performed in the completion of the control strip using the direct transmission method or other methods approved by the engineer.

Test Sections

To complete a test section, 5 readings are required. Each test section for asphalt concrete will be a one-quarter mile in length for the full width of the roadway or one-half mile in length or half the width of the roadway. Each test section for aggregate base, sub-base, and select materials will be a one-half mile in length per application width.

The length of test sections for shoulders is going to be the same as the mainline. If possible, test alternating sides. Five (5) readings will be done on each test section for both moisture and density using the same method of test used on the Control Strip and Roller Pattern.

Rolling is continued until none of the 5 readings will be less than 95% of the Control Strip density, and the average of these 5 readings is equal to or greater than 98% of the Control Strip density. This doesn’t apply to aggregate shoulder material, which requires an average density of 95 ± 2 percentage points of their control density, with individual densities within 95 ± 5 percentage points of the control density.

No other test is going to be required unless specified by the engineer. When test section readings are significantly below or above the target values by more than 8 lb/ft3, another Control Strip will be established. When testing turn lanes, deceleration lanes, acceleration lanes, and crossovers, take 2 or 3 readings on each, whichever is needed, to complete the full test section.

Point Note: For sections of roadway less than 900 ft, the direct transmission method or other approved testing methods for density determinations might be used. If obvious signs of distress are observed while rolling, evaluate, and cease rolling, the area of distress. Such signs include cracking, shoving, etc.

Structural failures may cause the gauge to give an erroneous reading indicating more Compaction is needed if actually over-compaction is causing the failure. If this situation occurs, it should to be brought to the attention of the engineer.

FAQ: Field Density Test

What Is a Field Density Test (Fdt)?

A Field Density Test (FDT) is a Quality Control test performed on-site to determine the in-situ density of soil. It evaluates how well the soil has been compacted during construction by comparing the achieved density to the required specifications.

Why Is the Field Density Test Important in Construction?

The FDT is crucial for ensuring the stability and load-bearing capacity of soil used in construction projects. Proper soil compaction improves shear strength, reduces compressibility and permeability, and enhances the overall stability of structures.

What Are the Common Methods Used for Field Density Testing?

Common methods include:

1. Sand Cone Method: Uses sand to replace an excavated soil sample to determine density.
2. Water Replacement Method: Determines soil density by measuring the volume of water displaced by a coated soil specimen.
3. Core Cutter Method: Involves taking a core sample of soil to measure its density.
4. Rubber Balloon Method: Uses a rubber balloon to measure the volume of a test hole and thus determine soil density.
5. Nuclear Moisture Density Meter: Employs nuclear radiation to assess moisture content and soil density.

How Does the Sand Cone Method Work?

In the Sand Cone Method, a sand-pouring cylinder is used to fill an excavated hole. The amount of sand needed to fill the hole is measured, which allows for the calculation of soil density by comparing the volume of sand used to the volume of the hole.

What Is the Procedure for the Water Replacement Method?

The Water Replacement Method involves coating a soil specimen with paraffin wax, immersing it in water to measure displaced water volume, and then calculating soil density based on the volume of water displaced and the mass of the specimen.

How Does the Core Cutter Method Differ from Other Methods?

The Core Cutter Method involves extracting a cylindrical core of soil from the field. The core’s volume and mass are measured to determine bulk density, which is then used to calculate dry density by accounting for moisture content.

What Is the Rubber Balloon Method?

The Rubber Balloon Method uses a balloon to measure the volume of an excavated hole. By comparing the initial and final volumes of water in the balloon, the density of the soil can be calculated based on the weight of the excavated soil.

How Is the Nuclear Moisture Density Meter Used?

The Nuclear Moisture Density Meter uses nuclear radiation to measure the density and moisture content of soil. It can perform tests through direct transmission or backscatter methods, depending on the material being tested.

What Are the Advantages of Using a Nuclear Moisture Density Meter?

The Nuclear Moisture Density Meter provides rapid and accurate measurements of soil density and moisture content, making it ideal for large-scale projects where quick, on-site testing is needed.

How Do I Choose the Appropriate Field Density Test Method?

The choice of method depends on factors such as soil type, project requirements, and available equipment. For cohesive soils, the Water Replacement Method may be preferred, while the Sand Cone Method is commonly used for granular soils.