Different types of field density test are being practiced in different parts of the world for evaluation of in-situ soil compaction and knowing the relative degree of compaction.

FDT which is acronym of Field Density Test is a Quality Control test carried out at site for knowing the increased density or compaction achieved at site on the soil layer.

FDT is a part of in-situ soil testing which is a broad subject of geotechnical engineering.

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


Compaction of the soil means pressing the soil particles close to each other by mechanical methods. Air present in the void spaces are expelled from the soil mass as a result of compaction and therefore the density is increased.

The compaction and its testing by field density test are very important and repetitive steps during earthwork phases of construction in a project.

The compaction of the soil generally increases the shear strength of the soil, and hence the stability and bearing capacity. It is also useful in reducing the compressibility and permeability of the soil mass.

Whether you are constructing a building, multistory plazas, highways / pavements or even in Dam or hydropower projects you have to know the basic knowledge of fdt

What is Soil Compaction
What is Soil Compaction

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

Types of FDT Tests

The following are the most common types of field density test:-

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

The compaction or increase in density is achieved at site with the help of Tampers, different types of roller like steel drum roller, pneumatic-tyred roller, sheep foot rollers or different types of vibratory compactors.

In some advance scenarios where above methods won’t work; the engineer can demand the use of compaction piles, precompaction method, compaction by explosives, compaction by pounding or terra probe method.

Methods of Field Density Test
Methods of Field Density Test

Discussing all of the above methods is actually out of the scope of this post. In today’s article my main focus would be on the different types of fdt methods enlisted above.

I would be answering questions like what method  is suitable for particular type of soil. Which test is more economical and easy to perform? Which fdt test method give more accurate result?

So let’s dive in.

In one of my previous article I have already explained the field density test calculations that are performed after carrying out the tests. In that post I have discussed about the sand replacement method and core cutter method.

These two methods are actually the most common of all. I have worked for many mega projects in all of them we are either using sand replacement or core cutter.

From these two methods,  sand replacement is even more common even to such an extent that whenever you talk in the field about FDT you are assumed to refer sand replacement method.


Have you ever seen someone performing FDT on the concrete?

I have not seen but I actually have performed this test myself. What?

Yeah! If you haven’t work on the Dam or Hydropower project which has mass concrete structures you might not know about roller compacted concrete.

Roller compacted concrete is actually a hard slump low strength concrete that is poured in layers and compacted by vibratory rollers. This is low permeable concrete that is usually used around the banks of water reservoirs.

In this concrete the compaction is also tested by fdt usually sand replacement method.

What is FDD test?

FDD is another acronym that is relevant to FDT. FDD test full form is Field Dry Density Test so one someone says FDD test he or she is actually referring to as FDT or field density test. So, FDD in civil engineering is a test carried out in the field to determine the dry density of the soil achieved at site.

Now that you know about the FDD and FDT acronym. Do you know what is MDD or MWD? I’m just asking this because these two are also relevant to this subject. Well, MDD is for Maximum dry Density while MWD is maximum wet density.

MDD is the density that is achieved in the laboratory at the optimum moisture content (OMC). It is the maximum density you can achieve for given conditions. So, quality control officers use this as a threshold value to determine if the pavement or earthwork is of good quality or not.

Frequency of FDD Test

Well as per the IS code 2720 Part-28 here are some of the recommendations for frequency of tests and permissible limits.

Frequency of test

  • For embankment                 –  1 test  per 1000 m2 area
  • For granular sub base         –  1 test per 500 m2 area

Permissible limits for different reaches

For permissible limit here are recommended values:

  • for embankment           –              minimum 95%
  • For subgrade                 –              minimum 97%
  • For granular sub base –              minimum 98%

FDT Test Procedure

The procedure followed for field density test is generally as follows:-

  1. The surface of the soil is leveled and brush is used to clean the surface from fines.
  2. The apparatus of field density is used to get the sample of soil for bulk weight using physical balance. In core cutter it is obtained from the soil in the core while in sand replacement, the soil on the flat tray is weighed.
  3. After knowing the weight of the soil, the next step is to determine the volume of the hole or that of the soil. In sand replacement or rubber balloon method it is measured indirectly while in core cutter method you can calculate the volume of core.
  4. The weight is divided by volume to determine the in-situ bulk density of the sample.
  5. A representative sample of the soil is taken for determination of moisture content in the filed or at site by speedy moisture test.
  6. After knowing the moisture content the dry density of the soil is determined using following formula.
    1. Dry density = (1 + bulk density / moisture content)
  7. Now you know the field dry density or in-situ dry density and that is afterwards compared with the maximum dry density (MDD) obtained already from the Proctor test or modified proctor test.
  8. Water replacement method for field density

Comparison of Different Methods

The following points include the draw backs, benefits, pros and cons of different methods of FDT and they will help you in judging the efficacy and usability of the method in your case at site.  

  1. The core cutter method is actually well suited for soft soils like clay soils or other cohesive soils that are placed as fills. This method cannot be used for coarse grained soil as the core cutter would not penetrate through them due to high resistance at the tip of the instrument.
  2. In comparison to core cutter method, sand replacement method or sand cone method is known to be better as it can be used in different types of soils and the results obtained are also much more appropriate. The main difference between sand replacement method and core cutter method is the procedure. In core cutter method you use a core and volume is used as that of the core. While for sand replacement the volume is obtained indirectly by using a calibrated sand.
  3. However Sand replacement method the calculation is lengthy as it involves many steps and you need more area of the reach to test. It also tend to be less accurate than nuclear density gauge, which is very costly but is more accurate.
  4. The rubber – balloon method although can get large samples, direct readings are obtained but it is little awkward method that is slow and can be abandoned easily in case of balloon breakage.
  5. Nuclear density test is the fastest of all the methods and is easiest to redo when needed. You can get more tests for statistical reliability.
  6. However, in case of nuclear density test you won’t be able to get any sample and it involves radiation which may be harmful and damaging to the operator of the instrument if suitable precautions are not taken in to account.
  7. Nuclear density test cannot be used if rocks are in the path and it can lead to ambiguous results if miscalibrated.

FDT Meaning in Construction

FDT is an acronym for Field Density Test. FDT is a in-situ density test of soil that is used in various types of construction works like highways, DAM construction, Embankment filling, and backfilling in order to access the quality of density achieved at site. FDT like many other tests on soil in civil engineering is a quality control tests. Other tests like CBR, Proctor Test, Moisture Test, Vane Shear Strength Test etc. are also carried out to access different parameters of soil.

The density we obtain as a result is called in situ dry density of the soil. So, if anyone asks you about which of the methods is used to find in situ density of soil, the answer would be simply – the sand cone method.

Experts call this as in-situ density because it is measured at the field with actual depth of the soil. You are measuring the weight of the excavate material and then divide it with the in-situ volume of the hole. Generally when you determine the density, it is the bulk density i.e. the density with actual moisture condition. So, after obtaining the moisture content we determine the dry density.


Final Words…..?

Actually different projects have different requirements so selecting the type of test is a very important step that can be very tricky if you don’t know the different types of tests or their pros and cons.

So I have tried to cover all the tests that are practiced as in-situ dry density test and while I am writing the article I have searched a lot but was not able to find some article so I thought it would be very helpful for ones like me.

So while I am publishing the article I would be preparing an infographic that would surely help you in remembering the different methods so wait for that and stay tuned with

So If this article has helped you please don’t forget to share it with your friends.

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Written by saadiqbal

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