3. Getting Started with Soil Compaction and CBR Testing
In this post, we’ll break down what soil compaction is, why it matters, how it’s tested in the lab (as per AS1289.5.1.1-2003), and how it’s verified in the field.
🔍 What is Soil Compaction?
· Compaction is the process of densifying soil by removing air from the voids between soil particles using mechanical force.
· The goal is to improve the load-bearing capacity, shear strength and stability of soil while reducing frost damage, settlement, swelling, compressibility and permeability.
· Measured quantitatively in terms of dry unit weight (Yd ) in KN/m3.
· Difference between compaction and consolidation:
Compaction | Consolidation |
1. Instantaneous phenomenon. 2. Soil always partially saturated 3. Densification due to reduction in the volume of air voids at a given water content. 4. Specific compaction techniques are used. | 1. Time dependent phenomenon. 2. Soil is completely saturated. 3. Volume reduction is due to expulsion of pore water from voids. 4. Consolidation occurs on account of a static load placed on the soil. |
Why it matters?
· Large air voids if left, may lead to compaction under working loads causing settlement of the structure during service or may get filled with water which reduces the shear strength.
· Increase in water content is also accompanied by swelling and loss of shear strength with time.
Types of Compactions?
· Vibration
· Impact
· Kneading
· Pressure
These different types of effort are found in the two principle types of compaction force: static and vibratory.
· Static force is simply the deadweight of the machine, applying downward force on the soil surface, compressing the soil particles. The only way to change the effective compaction force is by adding or subtracting the weight of the machine. Static compaction is confined to upper soil layers and is limited to any appreciable depth. Kneading and pressure are two examples of static compaction.
· Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to the machine’s static weight. The vibrating mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers). The compactors deliver a rapid sequence of blows (impacts) to the surface, thereby affecting the top layers as well as deeper layers. Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible. Based on the materials being compacted, a certain amount of force must be used to overcome the cohesive nature of particular particles.
These illustrations show the results of improper compaction and how proper compaction can ensure a longer structural life, eliminating future foundation problems.
Soil Types of Condition?
Every soil type behaves differently with respect to maximum density and optimum moisture. Therefore, each soil type has its own unique requirements and controls both in the field and for testing purposes. Soils found in nature are almost always a combination of soil types.
There are three basic soil groups:
- Cohesive
- Granular/Cohesionless
- Organic (this soil is not suitable for compaction and will not be discussed here)
1. Cohesive soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be moulded, but become very hard when dry. Proper water content, evenly distributed, is critical for proper compaction. Cohesive soils usually require a force such as impact or pressure and cannot be compacted by vibration. Silt has a noticeably lower cohesion than clay. However, silt is still heavily reliant on water content. Static pressure causes compaction in clayey soil.
2. Sand and gravel when subjected to vibratory loading may lead to large settlement. Liquefaction may occur in loose-sand. Thus, they are compacted generally by vibration. These soils obtain maximum density in either a fully dry or saturated state. Testing curves are relatively flat so density can be obtained regardless of water content.
Effect of Moisture:
The response of soil to moisture is very important, as the soil must carry the load year-round. Rain, for example, may transform soil into a plastic state or even into a liquid. In this state, soil has very little or no load-bearing ability.
Moisture Vs Soil Density:
Moisture content of the soil is vital to proper compaction. Moisture acts as a lubricant within soil, sliding the particles together. Too little moisture means inadequate compaction— the particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens the load-bearing ability. The highest density for most soils are at a certain water content for a given compaction effort. The drier the soil, the more resistant it is to compaction. In a water-saturated state the voids between particles are partially filled with water, creating an apparent cohesion that binds them together. This cohesion increases as the particle size decreases (as in clay-type soils).
📘 Lab Compaction Test: Proctor Test – AS1289.5.1.1-2018
This standard helps engineers determine the Optimum Moisture Content (OMC) and degree of Maximum Dry Unit Weight or density of a soil sample compacted. These values guide the required field compaction effort.
For a specific amount of compaction energy applied on the soil, there is one moisture content termed as Proctor Optimum Moisture content (OMC) at which a particular soil attains maximum dry density.
Maximum dry unit obtained is a function of compactive effort and method of compaction for a particular type of a soil.
Compactive effort is a measure of mechanical energy applied to soil mass
- Typical values of Yd max = 16-20kN/m3
- Typical values of OMC = 10-20%
Before starting compaction of soil in field we must know the compaction characteristic of soil.
· It gives the density that must be achieved in the field.
· Provides the moisture range that allows for minimum compactive effort to achieve required density.
· Provides data on the behaviour of the material in relation to various moisture content. (i.e how permeability etc. will be affected by m/c).
Procedure Summary:
1. Prepare the Specimen:
o Mix 2.5 kg of soil with pre-determined water contents:
§ G1: 4% & 16%
§ G2: 8% & 20%
§ G3: 12% & 24%
2. Compaction:
o Compact the soil in three layers inside a mould.
o Apply 25 blows per layer using a standard hammer.
o Use an extension collar for the final layer.
3. Weighing:
o Weigh the mould and soil to find wet density.
4. Moisture Content:
o Collect a sample to determine water content.
5. Plot and Analyse:
o Plot Dry Unit Weight vs Water Content.
o Identify the peak point = OMC & Max Dry Unit Weight.
🏗 Field Compaction: Specifications and Equipment
Relative Compaction
Once lab values are established, field compaction is evaluated by:
🧪 Field Inspection: How is Compaction Verified?
After compaction, engineers verify density with field tests: It is important to know and control the soil density during compaction. Following are common field tests to determine on the spot if compaction densities are being reached.
🚧 CBR Test – California Bearing Ratio (AS1289.6.1.1:2014)
The California Bearing Ratio (CBR) is used to measure the strength of subgrade soil for road design.
Purpose:
· Ensures the soil can support pavement loads
· Helps determine whether subgrade needs strengthening or if thicker pavement is needed
CBR Value Formula:
Laboratory CBR Procedure:
· Prepare soil sample (152 mm dia, 127 mm height)
· Compact in five layers using a hammer (2.5 kg or 4.5 kg)
· Penetrate a plunger at 1 mm/min; record load at every 0.25 mm up to 7.5 mm
· Calculate CBR value at 2.5 mm penetration
🛣 Types of Pavement Based on Subgrade Strength
1. Flexible Pavement:
· Bituminous layers, granular bases
· Transfers load across layers
2. Rigid Pavement:
· Reinforced concrete
· Stronger, more durable, and spreads loads over wider areas
Design Considerations:
· Option 1: Improve subgrade → thinner pavement
· Option 2: Keep subgrade as is → thicker pavement
💡 Final Thoughts
Understanding soil compaction and strength testing is essential for safe and long-lasting construction. Whether you’re building a road, preparing a building foundation, or simply curious about how soils support structures—compaction is where it all begins.
If you’re a beginner, start by observing a lab compaction test, review the compaction curve, and learn how field density is verified with simple tests we discussed above or the CBR.
Q1.
ReplyDelete1. Sand Cone Test: The Sand Cone Test is when a cone with sand is put on top of soil and sand is poured into a hole in the soil. The amount of sand used to fill hole helps to figure out the soil density. This test is good because it gives big sample size and is pretty accurate. But it takes many steps, needs big area, and takes time to set up. It can also have problems like air pockets under the plate or sand being weird and compacted. It's cheap
2. Baloon Densometer test: The Balloon Densometer is a test where balloon is blown up in a hole in soil, and the balloon's volume is measured. This helps to know the soil density. It’s good because it gives direct reading and can work with open-graded materials. It has big sample size, but it’s slow, balloon can break, and sometimes surface isn’t even, which can mess up the test. It costs moderate.
3. Shelby tube: The Shelby Tube is when a steel tube is pushed into the ground to get a soil sample and weigh it to find out its density. This test is useful for deep soil samples like under pipes. But it gives small sample size and sometimes the sample doesn’t stay intact. You can also mess up by pushing the tube too much and disturbing the soil, and rocks can block the sample. It’s cheap.
4. The Nuclear Gauge Test: This test uses radioactive isotopes to figure out the soil’s density and moisture by measuring radiation that passes through the soil. It’s fast, easy to repeat tests, and you can do more tests quickly. But it’s expensive and dangerous because of the radiation. Sometimes it gives wrong readings because of rocks or moisture, and the equipment costs a lot.
Q2.
The California Bearing Ratio (CBR) test is used to check how strong the subgrade is for road design, making sure it can handle the load of the pavement and traffic. To do this, a soil sample (152 mm in diameter, 127 mm high) is prepared and compacted in five layers with a hammer. Then, a plunger is pushed into the subgrade at 1 mm per minute, and the load is recorded every 0.25 mm up to 7.5 mm. The CBR value is calculated using the formula CBR = P/Ps (100) where
P is the load on the penetrometer at 2.54 mm of penetration, and Ps is the load on the penetrometer at the same depth for standard crushed stone (6900 kN/m^2). This test helps decide if the subgrade needs to be strengthened or if thicker pavement is needed. There are two types of pavements: flexible (made of bitumen and granular layers) and rigid (made of reinforced concrete). If the subgrade is weak, you can improve it to use thinner pavement, or if the subgrade is strong, you can keep it as is and use thicker pavement.
1.
ReplyDelete1.1 Sand Cone Test: Measures in-situ soil density by filling a hole with calibrated sand. The procedure is as follows:
Excavate a small hole in the soil.
Collect and weigh excavated soil sample.
Use a sand cone apparatus to fill the hole with sand from a known volume container.
Weigh the sand used to fill the hole.
1.2 Nuclear Density Test: Quickly measure soil density and moisture using radiation detectors. It is performed in the following way:
Placing nuclear gauge on the soil surface.
Send radiation into soil and measure counts of neutrons (for moisture) and gamma rays (for density).
1.3 Drive Cylinder Method (Core Cutter): Determine field density by extracting a known volume of soil. The following procedure is carried out:
Drive a metal cylinder of known volume into the soil.
Extract the soil sample and weigh it.
The device internally converts counts to wet density directly from gamma counts and moisture content from neutron counts.
2.
The California Bearing Ratio (CBR) test evaluates the strength and load-bearing capacity of soil or base materials used in road and pavement construction. It involves compacting a soil sample in a mold, soaking it if required to simulate field conditions, and then penetrating it with a standard piston at a controlled rate. The test measures the force needed to push the piston into the soil at depths of 2.5 mm and 5 mm. The CBR value is calculated by comparing the measured penetration force to standard reference forces (1370 kg for 2.5 mm and 2050 kg for 5 mm), expressed as a percentage. A higher CBR indicates stronger soil.
Q1.
ReplyDeleteSand Cone Test:
In this method, a metal cone filled with sand is placed over a small hole dug in the soil. Sand is poured until the hole is filled, and the amount used tells us the soil’s density. It’s reliable and covers a large sample area, but it takes time, needs space, and requires careful setup. Errors can happen if sand traps air or compacts wrongly. It’s inexpensive.
Balloon Densometer Test:
Here, a balloon is expanded inside a hole, and the balloon’s volume is measured to find soil density. It gives a direct reading and works even in coarse soils. The sample size is decent, but the test is slow, balloons can burst, and uneven ground can cause mistakes. It costs a medium amount.
Shelby Tube Test:
This involves pushing a thin steel tube into the ground to collect an undisturbed soil sample. The sample is weighed to calculate density. It’s useful for deeper layers, like beneath pipelines. However, the sample is small, can be disturbed while pushing, and rocks may prevent proper sampling. It’s cheap.
Nuclear Gauge Test:
This test uses radioactive sources to measure soil density and moisture by tracking radiation through the soil. It’s quick, repeatable, and allows many tests in little time. The downsides are high cost, strict safety requirements, and possible errors from rock or moisture variations. It’s expensive.
Q2.
The California Bearing Ratio (CBR) test measures the strength of soil used as subgrade in road construction to check if it can handle traffic loads. A cylindrical soil sample (152 mm wide, 127 mm high) is compacted in layers, then a plunger is pressed into it at a steady rate of 1 mm per minute. Load readings are taken every 0.25 mm up to 7.5 mm.
The CBR value is calculated with:
𝐶
𝐵
𝑅
=
𝑃
𝑃
𝑠
×
100
CBR=
P
s
P
×100
where P is the test load at 2.54 mm penetration, and Ps is the standard load for crushed stone (6900 kN/m²).
The results guide pavement design:
Flexible pavements (bitumen + granular layers).
Rigid pavements (reinforced concrete).
If the soil is weak, it may need stabilization or a thicker pavement. If it’s strong, a thinner structure can be used.
The Sand Cone Test involves creating a small hole in the soil and then pouring sand from a cone into it. By measuring how much sand fills the hole, the soil’s density can be calculated. This method is accurate and works well for relatively large areas. However, it requires several steps, careful setup, and enough space. Potential errors can occur due to air trapped under the plate or improperly compacted sand. The test is inexpensive to perform.
ReplyDelete2. Balloon Densometer Test:
In the Balloon Densometer Test, a balloon is placed in a soil hole and inflated. Measuring the balloon’s volume allows calculation of the soil density. This method provides direct readings and works even for open-graded soils. It covers a fairly large sample, but the process is slow, the balloon may tear, and uneven surfaces can affect accuracy. The cost is moderate.
3. Shelby Tube Test:
The Shelby Tube Test uses a steel tube pushed into the ground to extract a soil sample, which is then weighed to determine its density. This method is suitable for collecting deeper soil samples, such as those beneath pipelines. Drawbacks include a small sample size, risk of disturbing the soil if pushed incorrectly, and potential obstruction by rocks. It is a low-cost method.
4. Nuclear Gauge Test:
This test measures soil density and moisture using radiation from a radioactive source. It is quick, allows repeated measurements, and can test multiple locations efficiently. However, it is expensive, involves radiation risks, and readings can be affected by rocks or moisture variations. Equipment costs are high.
California Bearing Ratio (CBR) Test:
The CBR test evaluates the strength of subgrade soil for road construction, helping determine if it can support pavement and traffic loads. A soil sample, 152 mm in diameter and 127 mm high, is prepared in five layers and compacted with a standard hammer. A plunger is then pressed into the sample at a rate of 1 mm per minute, and the force required is recorded at intervals of 0.25 mm up to 7.5 mm penetration. The CBR value is calculated using:
CBR (%) = P/PS * 100
where Ps is 6900 kN/m². This test helps engineers decide whether the subgrade needs reinforcement or if a thicker pavement layer is required. Roads with strong subgrade can use thinner pavements, while weaker subgrades may need improvement or thicker pavements. Pavements can be flexible (bitumen with granular layers) or rigid (reinforced concrete).
Q1.
ReplyDelete1. Sand Cone Test
The Sand Cone Test is performed by placing a sand-filled cone over a small hole dug in the ground. The sand flows into the hole, and the amount of sand needed to fill the space is measured to determine the soil’s in-place density.
This method is reliable and uses a large sample volume, making it fairly accurate. However, it is time-consuming, requires a flat working area, and involves several steps. Errors may occur if air gaps form under the base plate or if the sand compacts incorrectly. The equipment required is inexpensive.
2. Balloon Densometer Test
In the Balloon Densometer Test, a rubber balloon attached to the device is placed into an excavated hole. The balloon expands to fill the hole, and its volume is used to calculate the soil density.
This test provides direct readings and works well even for coarse or open-graded materials. Although it uses a reasonably large sample volume, the process is slow, the balloon can be punctured, and uneven surfaces may cause inaccurate results. The test has a moderate cost.
3. Shelby Tube Sampling
The Shelby Tube method involves pushing a thin-walled steel tube into the ground to collect an undisturbed soil sample. The sample is then weighed to determine its density.
This method is useful for obtaining deeper samples, such as those beneath foundations or pipelines. However, the sample size is small, the soil may become disturbed if the tube is pushed too far, and gravel or stones can prevent proper sampling. The method is relatively inexpensive.
4. Nuclear Gauge Test
The Nuclear Density Gauge uses radioactive sources to measure soil density and moisture by detecting how much radiation passes through the soil.
This test is very fast, easy to repeat, and allows many measurements in a short time. However, it is costly, requires strict safety procedures, and results can be affected by stones or uneven moisture. The equipment is expensive and requires trained operators.
Q2.
The California Bearing Ratio (CBR) test is used to evaluate the strength of subgrade soil for pavement design. A soil specimen (152 mm diameter, 127 mm height) is compacted in five layers and then tested by pushing a plunger into the soil at a rate of 1 mm/min. The load is recorded for each 0.25 mm of penetration up to 7.5 mm.
The CBR value is calculated using:
CBR= Ps/P×100
where P is the penetration load at 2.5 mm and Ps is the standard load for crushed stone (6900 kN/m²).
The CBR result helps determine whether the subgrade needs improvement or whether a thicker pavement layer is required.
Pavements can be either flexible (bituminous layers over granular layers) or rigid (reinforced concrete). Stronger subgrade soils allow thinner pavement sections, while weaker soils require strengthening or thicker pavement layers.