Soil can be regarded as the structural basis of any type of construction works as the majority of the structures are erected on the soil itself. In this regard, soil can be considered as the foundation of all types of surface construction.
Soil should have an acceptable quality to ensure adequate structural strength and safety. Thus, the first step in any type of construction work particularly road construction is the testing of soil. Several soil tests for road construction have to be carried out before the commencement of the construction work to understand the nature and properties of soil. Also, for any type of road construction, it is necessary to determine the profile of the soil. Soil tests are necessary for the planning and designing of the proposed structure being constructed. Various type of soil tests needs to be carried out whether in lab or in-situ.
Purposes of Soil Tests for Road Construction
a. To analyze the quality of the soil.
b. To determine the moisture content present in the soil.
c. To determine the compaction characteristic of various soil.
d. To find out the strength of the subgrade.
e. To determine the nature of the soil.
Soil Tests for Road Construction
The soil tests that must be carried out for road construction are briefly explained as follows:
1. Moisture Content Test
Moisture Content of soil is the amount of water it contains and is expressed as the percentage of dry mass. The test for determining the moisture content of the soil is vital for assessing the natural existing state of the soil. It is also necessary for understanding the bearing capacity as well as the probable settlement of the soil. It is carried out in the laboratory itself.
1. Non-corrodible airtight container
2. Weighing Balance of adequate sensitivity
4. Oven with the temperature maintained between 105 degree Celsius to 110 degree Celsius
The procedure for laboratory determination of moisture content can be listed as follows:
i. The air-tight container along with its lid must be first weighed.
ii. The specimen sample is then taken in the container and the container along with its lid and the sample is weighed.
iii. Then, the container is left in the oven. The specimen is dried to a constant weight at a temperature ranging from 105 degree Celsius to 110 degree Celsius for a period of about 16 to 24 hours.
iv. The container along with the dried sample is finally weighed.
v. The moisture content is then calculated using the following formula:
W= [(W2-W3)/(W3-W1)] * 100
W1= Weight of the container with a lid in grams
W2= Weight of the container with lid and wet sample in grams
W3= Weight of the container with lid and dry sample in grams
2. Specific Gravity Test
The specific gravity of soil may be defined as the ratio of the weight of soil in the air of a given volume at a standard temperature to the weight in air of an equal volume of distilled water at the same stated temperature.
Specific gravity test is necessary to get an idea of various soil properties such as void ratio, degree of saturation etc. It is also the type of test that is carried out in the lab itself.
1. Wash bottle with distilled water
2. Balance with a weighing accuracy of 1gm
3. Density bottle of 50ml with the stopper having a capillary tube
The laboratory procedure for determining the specific gravity of soil can be
listed as follows:
1. The density bottle must be cleaned and dried properly. First of all, it has to be washed with distilled water and drained. Then, it must be again washed with alcohol followed by ether.
2. The empty bottle with stopper is then weighed.
3. About 10 to 20gms of the soil sample is taken from the oven and allowed to cool in the desiccator. It is then filled in the empty bottle and weighed.
4. About 10ml of distilled water is added to the bottle and the soil sample is allowed to soak for about two hours.
5. Now, distilled water is filled in the bottle and it is kept under constant temperature water bath.
6. The bottle is taken out and again weighed.
7. Now, the bottle is emptied and filled with only distilled water and weighed.
8. The process is repeated three to four times to take average readings.
9. The specific gravity of soil is then calculated using the following formula:
Specific Gravity = (W2-W1)/[(W2-W1) – (W3-W4)]
W1= weight of density bottle in gram
W2= weight of density bottle with dry soil
W3= weight of density bottle with soil sample and water
W4= weight of density bottle with water
3. Particle Size Distribution Test
The particle size distribution of soil may be defined as the proportion of the dry mass of soil distributed over specified particle-size ranges i.e. gradation of soil.
The determination of particle size distribution is necessary for checking the suitability of the soil for the use in road construction. The purpose of this test is to determine the percentages of different grain sizes contained in the soil. Normally sieve analysis is used for determining the particle distribution for larger particles and hydrometer test is used for the finer ones.
2. Set of sieves
3. Cleaning brush
4. Sieve shaker
The procedure for determining the particle size distribution of soil can be listed as follows:
1. First of all, the sieves and the pan to be used at the bottom must be weighed.
2. The soil sample is then weighed.
3. The sieves are then arranged in the respective order and the soil sample
is carefully poured through the arrangement.
4. The stack of the sieve is then placed in the mechanical shaker and switched
on for about 10 minutes.
5. The stack of sieves is then removed from the mechanical shaker and the
weight of soil retained on each of the sieves is recorded.
6. The results are then analyzed.
4. Proctor Test
Proctor test is a type of compaction test that is carried out to determine the compaction properties of soil. In actual, this test is used for determining the mass of dry soil per cubic metre when the soil is compacted over a range of moisture contents providing the maximum dry density at the optimum moisture content.
Proctor test includes establishing the relationship between the moisture content and dry density of soils compacted in a mould of a specific size with a 2.5kg rammer dropped from a height of 30cm.
1. Proctor mould having an internal diameter of 10.2cm and height of 11.6cm with a detachable collar and detachable base plate.
2. A mechanical rammer weighing 2.5kg.
3. Sample extruder
4. Sensitive balance
5. Straight edge
6. Graduated cylinder
7. Mixing tools such as the spoon, tray, trowel etc.
The procedure for conducting the Proctor compaction test can be listed as follows:
1. Approximately 5kg of the oven-dried sample is taken in the given pan. Water is then poured in it such that the representative sample is dampened four to six percentage points below the optimum moisture content.
2. The Proctor mould is then weighed without the base plate and collar. After that, the collar and base plate are fixed.
3. Place the sample soil in three layers and each layer is compacted by 25 blows from the 2.5kg hammer.
4. The collar is then removed, compacted soil is trimmed with the straight edge and the mould is weighed.
5. The sample is then removed and slightly cut to obtain the sample for moisture content.
6. Then the results are analyzed as follows:
Wet density gm/cc= weight of compacted soil/944
Dry density= wet density/(1+w)
W= moisture content of the soil
5. California Bearing Ratio Test
California Bearing Ratio test is a type of test that is carried out to check the strength of the subgrade of pavement. By comparing the results of this test with a set of standard curves, the thickness of the subsequent layers can also be determined.
It is a type of penetration test and extensively used in the design of flexible pavements. This test was developed by the California Division of Highways of the United States.
CBR is defined as the ratio of force per unit area that is required to penetrate a soil mass with a piston of 1.25mm/min corresponding to that required for the penetration of a standard material.
1. Steel Cutting Collar
2. Dial Gauges
4. IS Sieves
5. Penetration Plunger
6. Disc and loading machine
7. Cylindrical Mould and Rammers
The procedure for conducting the CBR test is as follows:
1. Firstly, an undisturbed sample specimen is prepared by attaching a cutting edge to the mould and pushing it gently in the mould to remove the soil on the outside.
2. The density of the sample is determined and is remoulded at Proctor’s maximum density or Optimum Moisture Content. The sample must be such that it must pass through 20mm IS sieve but must be retained on 4.75 IS sieve.
3. About 4.5kg to 5.5 kg of the sample is mixed with water. The extension collar and base plate are then fixed to the mould and the disc is placed over the base plate.
4. The soil mix is then compacted in several layers by a rammer. Generally, for light compaction, the soil is compacted in three layers by 55 blows from a 2.6kg rammer.
5. The collar is then removed and the soil is trimmed. The mould is then turned over and the base plate and disc are removed.
6. The mould is then weighed and bulk density, as well as dry density, is determined.
7. Then the mould along with the surcharge weight is placed on the penetration test machine.
8. The load is applied to the piston such that the rate of penetration is 1.25mm/min.
9. The load readings at penetrations of 0.5,1.0,2.0,2.5,3.0,4.0,5.0,7.5,10 and 12.5 mm are recorded.
10. The mould is then detached from the loading equipment and moisture content is determined.
Advantages of Soil Tests for Road Construction
a. It helps to finalize the requirement of soil for road construction.
b. It gives the knowledge about various properties of soil.
c. It helps to determine the moisture content present in the soil.
d. It helps to find out the strength of road or pavement subgrade.
e. It helps to analyze the quality of the soil.
f. Material requirement for road construction and the nature of road can be easily determined.
g. It helps to predict the thickness of the road pavement.
i. It helps to determine the load-bearing capacity of the road.
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