Field Density Test: Sand Replacement and Core Cutter Methods, Degree of Compaction and MoRTH Limits

What is a field density test in road construction?

A field density test is a check, done on the finished layer itself, of how tightly the roller has packed the soil. Density here means weight per volume: so many kilograms of soil in one cubic metre. The more tightly the grains are pressed together, the more soil fits into that cubic metre and the less room is left for water and air.

The test needs two things from the same spot: the volume of a small piece of the layer, and the weight of soil that came out of it. Weight divided by volume gives the wet density. A sample of the soil goes into an oven to find its water content, and taking the water out of the calculation gives the dry density, the figure every specification uses.

Dry density on its own means little, because every soil has its own ceiling. A gravelly soil may reach a dry density a clay never can. So the site figure is compared with the maximum dry density, written MDD, that the lab got from the same soil in the heavy compaction test of IS 2720 (Part 8): a 4.9 kg rammer dropped 450 mm, 25 blows on each of five layers in a 1,000 cm³ mould. The site figure as a percentage of the lab figure is the degree of compaction, also called relative compaction. That percentage is what the engineer signs off.

Where this sits in the job: the road construction layers are built one at a time, and each one is tested before the next goes on top.

How is the sand replacement method done?

The sand replacement method works out the volume of a hole by filling it with dry sand whose density is already known. It is the method MoRTH names for compaction control, and IS 2720 (Part 28) sets each step.

1. Calibrate the sand. The sand must be clean, uniformly graded natural sand that passes a 1.00 mm sieve and stays on a 600 micron sieve, oven dried and then left for about seven days so its moisture settles with the air. The lab finds how much sand fills the cone at the bottom of the pouring cylinder, and how much fills a container of known volume. The second figure gives the sand’s bulk density. The standard asks for this check every working day, because humidity changes how the sand packs.
2. Prepare the spot. A flat patch about 450 mm square is scraped level, and a metal tray with a round hole in the middle is laid on it.
3. Dig the hole. Soil is dug out through the tray’s hole, down to the depth of the layer being tested and no deeper than 150 mm. Every crumb is collected and weighed. Levering the digging tool against the side makes the hole bigger and gives a low reading.
4. Find the water content. A sample of the dug soil is oven dried as in IS 2720 (Part 2).
5. Fill the hole with sand. The pouring cylinder, filled to the same starting weight used in calibration, is placed over the hole and its shutter opened. The sand runs into the hole and the cone. Nobody vibrates the cylinder or the ground while it runs. The cylinder is weighed again when the sand stops moving.
6. Work out the volume. The sand that left the cylinder, less the sand that sits in the cone, is the sand in the hole. Divided by the sand’s bulk density, it gives the hole’s volume.
7. Repeat. The standard asks for at least three holes, averaged, because density changes from point to point across a layer.

The standard has two sizes of kit, and the layer thickness decides which one goes to site.

The standard groups soils by sieve. A soil is fine grained if at least 90% passes a 2 mm sieve, medium grained if at least 90% passes 20 mm, and coarse grained if at least 90% passes 40 mm.

It also gives one warning worth knowing on a gravel or sand layer. Where the material has little stickiness, especially when wet, the sides of the hole can slump in while you dig. The hole is then measured smaller than the soil it held, and the test reports a density higher than the truth.

How is the core cutter method done?

The core cutter method measures the volume directly: a steel tube of known size is driven into the layer, dug out full of soil and weighed. IS 2720 (Part 29) sets the kit and the steps.

• The cutter is a one-piece steel tube, with no welded joint, 130 mm long and 100 mm inside diameter, with a 3 mm wall bevelled at one end. That makes a volume of about 1,021 cm³, but the standard asks for the real cutter to be measured to the nearest 0.25 mm and its own volume used.
• The dolly is a 25 mm high steel collar that sits on top of the cutter, and the rammer is a steel staff with a 140 mm foot, about 900 mm long and about 9 kg.

A patch about 30 cm square is levelled. With the dolly on top, the cutter is rammed straight down until only about 15 mm of the dolly shows above the ground, without rocking it. The cutter is then dug out with some soil left hanging below it, both ends are trimmed flat with a straight edge, and the cutter and soil are weighed together. Take away the cutter’s own weight and divide by its volume, and you have the wet density. A sample from the core gives the water content. As with sand replacement, at least three cores are averaged.

The standard limits where this works. It is for fine grained soil, at least 90% passing a 4.75 mm sieve, free from stones. It says the core cutter is less accurate than sand replacement, and does not recommend it unless speed is essential or the soil is already well compacted. On stony soil the cutting edge is damaged and the core breaks up.

Sand replacement or core cutter: which method for which soil?

The soil decides. A clay or silty subgrade that cuts cleanly can be checked quickly with a core cutter. Anything with gravel or stone in it, which includes granular sub-base (GSB) and wet mix macadam (WMM), needs sand replacement, because a cutter cannot be driven through stone.

A third option turns up on large highway jobs: the nuclear moisture and density gauge, which reads density through the layer without digging. MoRTH allows it only where the engineer permits and the gauge is calibrated to give the same results as IS 2720 (Part 28), and it doubles the number of tests needed when a non-destructive method is used.

How is the degree of compaction calculated?

The degree of compaction is the field dry density divided by the lab maximum dry density, times 100. Three small formulas get you there:

1. Wet density = weight of soil from the hole or core ÷ volume of the hole or core.
2. Dry density = wet density × 100 ÷ (100 + water content in %).
3. Degree of compaction (%) = dry density ÷ maximum dry density × 100.

From the hole to the pass mark: one sand replacement test

The numbers above are a worked example in round figures, not readings from a real job. That subgrade passes, just: MoRTH asks for at least 97% on a subgrade.

Now a core cutter example on a different soil. The cutter holds 1,021 cm³, and the soil in it weighs 2,050 g, so the wet density is about 2.01 g/cm³. The water content is 15%, which brings the dry density to about 1.75 g/cm³. If the lab MDD for that soil is 1.82 g/cm³, the degree of compaction is about 95.9%.

The second result is a common one on site: good enough for the body of an embankment, short for the top 500 mm that forms the subgrade. Two habits keep the arithmetic honest. Use the MDD from the same borrow area as the soil you tested, because a different soil has a different ceiling. And take the water content from the soil that came out of the hole, not from a sample taken before rolling, since the layer dries in the sun while it is being worked.

How much compaction does MoRTH ask for each layer?

MoRTH asks for 95% of the lab maximum in an embankment, 97% in the subgrade and 98% in the granular layers above it. All of these are measured against the MDD from the heavy compaction test, IS 2720 (Part 8).

The same section puts two more limits around the test. The soil must be compacted within 1% above and 2% below its optimum moisture content, the water level at which it packs tightest in the lab; expansive clays are the exception and go on the wet side. And each compacted layer can be no thicker than 250 mm under a vibratory roller, or 200 mm under a static roller of 80 to 100 kN, roughly 8 to 10 tonnes.

Reaching 97 to 98% on a 250 mm layer is a job for a roller with real drum weight and vibration. You can compare road rollers and compactors side by side, with their drum and vibration figures, before you hire or buy.

Cement treated layers follow their own rules, set out in the guide to CTSB and CTB. The aggregate and soil limits for every layer are listed in the guide to road construction materials.

How many field density tests are needed, and where?

For earthwork, MoRTH Clause 903.2.2 asks for at least one set of ten density measurements on each layer for every 3,000 m² of compacted area. On the subgrade and earthen shoulders the area shrinks to every 2,000 m². The test method named is IS 2720 (Part 28), sand replacement.

Three rules sit behind that frequency:

• Test spots are chosen by random sampling, not by the supervisor picking the best looking patch.
• If a nuclear gauge or another non-destructive method is used, the number of tests doubles. Where results scatter widely, the set gets bigger.
• The layer is accepted on the set, not on single holes. The average of the set must clear the specified density plus an allowance tied to the standard deviation of the results, the measure of how much they scatter. One good hole cannot carry a set of poor ones.

For the granular layers, Table 900-3 sets one density test per 1,000 m² of GSB and one set of three tests per 1,000 m² of WMM, judged on the same lines. The next layer goes on only after the one below has been tested and accepted.

What does a failed field density test tell you?

A failed test almost always points back to one of four things on site, and the roller is only one of them.

• Wrong moisture. Too dry and the grains will not slide into place; too wet and water fills the gaps the roller is trying to close. Check the water content against the 1% above to 2% below window before blaming the machine. How moisture changes compaction is explained in the guide to soil compaction.
• A layer that is too thick. Over 250 mm loose under a vibratory roller and the bottom of the layer stays soft while the top passes. The test hole reaches into that soft part.
• The wrong drum for the soil. A smooth drum suits sand and gravel. Sticky clay compacts better under the padded drum of a sheepsfoot roller.
• Too few passes, or a different soil. If the borrow area changed, the MDD the test is being compared with may no longer fit the soil in the layer.

MoRTH’s answer to a soft area is more rolling first. If the density still is not reached, the soft material is dug out, replaced with approved material and compacted with light machines such as a small vibratory roller, a walk-behind double drum roller or a plate compactor. A low result on a CBR-designed road also means the soil is weaker than the design assumed, as the guide to the CBR test in road construction explains.

Which machines decide whether the test passes?

The single drum vibratory roller, the soil compactor, does most of the work on embankment and subgrade, after a motor grader spreads each layer and a water tanker sets the moisture. MoRTH Clause 305.3.6 calls for a self-propelled single drum vibratory or pad foot roller of 80 to 100 kN static weight, roughly 8 to 10 tonnes,, or a heavy pneumatic tyre roller, and asks the contractor to prove the roller on a compaction trial first. Earthmoving machines driving over the layer do not count as compaction.

On Indian highway jobs that work usually falls to soil compactors such as the L&T 1190D, the Dynapac CA305D and the HAMM HC 119i. How drum vibration reaches deep into a layer is covered in the guide to the vibratory roller, and the soil compactor guide explains how to size one for your job. When you are ready to plan the fleet, compare soil compactors and motor graders and connect with a dealer.

The test steps on this page follow IS 2720 (Part 28): 1974, Dry Density of Soils In-place by the Sand Replacement Method, together with IS 2720 (Part 29): 1975 for the core cutter and IS 2720 (Part 8): 1983 for the lab MDD. The compaction requirements and test frequencies follow the MoRTH Specifications for Road and Bridge Works (5th revision), Table 300-2 and Clauses 305, 401, 406 and 903. Every other road topic sits under the road construction guide.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. Test and compaction requirements quoted here follow the standards named above and are not a substitute for the contract specification or the instructions of the engineer in charge.