Types of Soil Tests for Road Construction

Types of soil tests for road construction project requires the site investigation to be carried out to understand the soil profile. For road construction works, the properties of soil at subgrade level are required.

The common soil test for road construction includes classification of soil, particle size distribution, moisture content determination, specific gravity, liquid limit and plastic limit tests. Moisture content, particle size and specific gravity tests on soils are used for the calculation of soil properties such as degree of saturation.

The soil tests can be laboratory tests or in-situ tests. The laboratory tests should be carried out on every sample taken for determination of particle size and moisture content.

Types of Soil Tests for Road Construction

Following are the various types of soil tests for pavement construction:

In-situ Moisture Content

The moisture content of soil test is carried out in laboratory. It is expressed as percentage of water in soil to its dry mass. The moisture content in a soil signifies the various properties of soil such as compaction, permeability, particle size etc.

Specific gravity of soil

Specific gravity of soil is the ratio of the weight of soil in 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. This test is also carried out in laboratory.

Particle Size Distribution (By wet sieving & pipette method)

This test determines the particle size distribution of soil from the coarse sand size down to fine clay size. The data from particle size distribution test is used to determine suitability of soil for road construction, air field etc. This test can also be used to predict soil water movement although permeability tests are more generally used.

Compaction test – Proctor test

This soil compaction test also called as Proctor test is used for the determination of the mass of dry soil per cubic metre when the soil is compacted over a range of moisture contents, giving the maximum dry density at optimum moisture content. Thus this test provides the compaction characteristics of different soils with change in moisture content. This is achieved by densification of soil by reducing the air voids.

The degree of is measured in terms of its dry density of soil. The dry density is maximum at the optimum water content.

California Bearing Ratio (CBR) Test

California Bearing Ratio test is conducted in laboratory. This tests provides the load penetration resistance of soil. CBR value is obtained by measuring the relationship between force and penetration when a cylindrical plunger is made to penetrate the soil at a standard rate.

The CBR test is used for the evaluation of subgrade strength of roads and pavements. The CBR value obtained by this test is used with the empirical curves to determine the thickness of pavement and its component layers. This is the most widely used method for the design of flexible pavement.

Even though provision of subsoil drains reduces the effect of water on subgrade, fully soaked CBR tests shall be considered to be appropriate for road construction projects.

Following points should be taken care of while soil testing for road construction:

• Sampling and Testing: Sampling of soil for tests in laboratory or in-situ is to be carefully done by experienced engineer. The requirement for the various mass / volume of soil at different points of a road project shall be followed as per the specification and standard codes.
• Test Data Logging: Logging of all the soil sample and test data shall be done by trained staff who has the knowledge of soil properties and tests results.
• Testing Frequency: The testing frequency of soil shall be as per input from Engineer. The decision on the testing frequency is usually taken on the basis of results obtained from the previous tests.

Soil Tests Required for Shallow and Raft Foundations

Soil Tests for Shallow and Raft Foundations

Soil tests required to determine safe bearing capacity of shallow foundations and raft foundations are discussed here. These tests are as per IS 6403 – 1981.

Apart from ascertaining the highest level ever reached by the groundwater table and tests for classification of soil as per IS 1498 – 1970 based on grain size analysis as per IS 2720 (Part –IV)– 1985, index properties of soil as per IS 2720 (Part-V) – 1985, the following tests are required to determine safe bearing capacity based on shear strength consideration:

1. Standard penetration test as per IS 2131 – 1991 for coarse grained / fine grained cohesionless soils with semi-pervious clayey soils (i.e.  soils with clay upto 30%).

2. Direct shear test (controlled strain) as per IS 2720 (Part – 13) – 1986. Consolidated undrained tests for cohesive and for  soils and consolidated drained tests for cohesion less soils. The results may be compared with standard penetration test / static cone penetration test results.

Since there is escape of pore water during box shear, partial drainage vitiates the consolidated undrained test. Hence this test is not exact for semi-pervious soils such as clayey sands / silts (i.e. with clay more than 15% but less than 30%). For such soils, triaxial tests are required if shear strength is critical criterion.

3. Static cone penetration test as per IS 4968 (Part -3) – 1976 for foundations on non-stiff clayey soils such as fine grained soils (i.e. more than 50% passing through 75 micron sieve). In fine and medium coarse sands such tests are done for correlation with standard penetration test and to indicate soil profiles at intermediate points.

4. Unconfined compressive strength test as per IS 2720 (Part-10) – 1973 for highly cohesive clays except soft / sensitive clays.

5. Vane shear tests for impervious clayey soils except stiff or fissured clays.

6. Triaxial shear tests for predominantly cohesive soils. If shear strength is likely to be critical.

Soil Tests for Shallow Foundations

Tests required to determine allowable bearing pressure for shallow foundations on settlement consideration:

1. Standard penetration test as stated above.

2. Consolidation test as per IS 2720 (Part-15) if the settlement of clayey layer /layers calculated on the basis of liquid limit and in-situ void ratio indicates that settlement may be critical. Consolidation test is not required if the superimposed load on foundation soil is likely to be less than pre-consolidation pressure (assessed from liquidity index and sensitivity or from un-confined compressive strength and plasticity index).

3. Plate load tests as per IS 1888 – 1982 for cohesionless soils and  soils where neither standard penetration test or consolidation test is appropriate such as for fissured clay / rock, clay with boulders etc..

Soil Tests Required for Raft Foundations

(As per Para 3 of IS 2950 (Part-1) – 1981.

Apart from other tests for shallow foundations, the following soil tests are required especially for raft foundations:

1. Static cone penetration test as per IS 4968 (Part-3) – 1976 for cohesionless soils to determine modulus of elasticity as per IS 1888 – 1982.

2. Standard penetration test as per IS 2131 – 1981 for cohesionless soils and  soils to determine modulus of sub-grade reaction.

3. Unconfined compressive strength test as per 2720 (Part -10) – 1973 for saturated but no pre-consolidated cohesive soil to determine modulus of sub-grade reaction.

4. As specified in IS 2950 (Part -1) – 1981¸ plate load test as per IS 1888 – 1982 where tests at Sl. No. – 1 to 3 above are not appropriate such as for fissured clays / clay boulders.

5. In case of deep basements in pervious soils, permeability is determined from pumping test. This is required to analyze stability of deep excavation and to design appropriate dewatering system.

Types of Soil Tests for Building Construction

Types of Soil tests for building construction works depend on properties of soil. Design of foundation is based on soil test report of construction site.

Soil tests for construction of buildings or any structure is the first step in construction planning to understand the suitability of soil for proposed construction work.

Soil which is responsible for allowing the stresses coming from the structure should be well tested to give excellent performance. If soil shouldn’t tested correctly then the whole building or structure is damaged or collapsed or leaned like leaning tower of Pisa. So, soil inspection or testing is the first step to proceed any construction.

Types of Soil Tests for Building Construction

Various tests on soil are conducted to decide the quality of soil for building construction. Some tests are conducted in laboratory and some are in the field. Here we will discuss about the importance of various soil tests for building construction. The tests on soil are as follows.

• Moisture content test
• Atterberg limits tests
• Specific gravity of soil
• Dry density of soil
• Compaction test (Proctor’s test)

Moisture Content Test on Soil

Moisture content or water content in soil is an important parameter for building construction. It is determined by several methods and they are

• Oven drying method
• Calcium carbide method
• Torsion balance method
• Pycnometer method
• Sand bath method
• Radiation method
• Alcohol method

Of all the above oven drying method is most common and accurate method. In this method the soil sample is taken and weighed and put it in oven and dried at 110o + 5oC. After 24 hours soil is taken out and weighed. The difference between the two weights is noted as weight of water or moisture content in the soil.

Specific Gravity Test on Soil

Specific gravity of soil is the ratio of the unit weight of soil solids to that of the water. It is determined by many methods and they are.

• Density bottle method
• Pycnometer method
• Gas jar method
• Shrinkage limit method
• Measuring flask method

Density bottle method and Pycnometer method are simple and common methods. In Pycnometer method, Pycnometer is weighed in 4 different cases that is empty weight (M1), empty + dry soil (M2), empty + water + dry soil (M3) and Pycnometer filled with water (M4) at room temperature. From these 4 masses specific gravity is determined by below formula.

Dry Density Test on Soil

The weight of soil particles in a given volume of sample is termed as dry density of soil. Dry density of soil depends upon void ratio and specific gravity of soil. Based on values of dry density soil is classified into dense, medium dense and loose categories.

Dry density of soil is calculated by core cutter method, sand replacement method and water-displacement method.

Core Cutter Method for Soil Dry Density Testing

In this methods a cylindrical core cutter of standard dimensions is used to cut the soil in the ground and lift the cutter up with soil sample. The taken out sample is weighed and noted. Finally water content for that sample is determined and dry density is calculated from the below relation.

Sand Replacement Method for Soil Dry Density Testing

In this method also, a hole is created in the ground by excavating soil whose dry density is to be find. The hole is filled with uniform sand of known dry density. So by dividing the mass of sand poured into the hole with dry density of sand gives the volume of hole. So we can calculate the soil dry density from above formula.

Atterberg Limits Test on Soil

To measure the critical water content of a fine grained soil, Atterberg provided 3 limits which exhibits the properties of fine grained soil at different conditions. The limits are liquid limit, plastic limit and shrinkage limit. These limits are calculated by individual tests as follows.

Liquid Limit Test on Soil

In this test, Casagrande’s liquid limit device is used which consist a cup with moving up and down mechanism. The cup is filled with soil sample and groove is created in the middle of cup with proper tool. When the cup is moved up and down with the help of handle the groove becomes closed at some point.

Note down the number of blows required to close the groove. After that water content of soil is determined. Repeat this procedure 3 times and draw a graph between log N and water content of soil. Water content corresponding to N=25 is the liquid limit of soil.

Plastic Limit Test on Soil

Take the soil sample and add some water to make it plastic enough to shape into small ball. Leave it for some time and after that put that ball in the glass plate and rolled it into threads of 3mm diameter.

If the threads do not break when we roll it to below 3mm diameter, then water content is more than the plastic limit. In that case reduce water content and repeat the same procedure until crumbling occurs at 3mm diameter. Finally find out the water content of resultant soil which value is nothing but plastic limit.

Shrinkage Limit Test on Soil

In case of shrinkage limit, the water content in the soil is just sufficient to fill the voids of soil. That is degree of saturation is of 100%. So, there is no change in volume of soil if we reduce the shrinkage limit. It is determined by the below formula for the given soil sample.

Where M1 = initial mass

V1= initial volume

M2= dry mass

V2= volume after drying

Pw = density of water.

Proctor’s Compaction Test on Soil

Proctor’s test is conducted to determine compaction characteristics of soil. Compaction of soil is nothing but reducing air voids in the soil by densification. The degree of Compaction is measured in terms of dry density of soil.

In Proctor’s Compaction Test, given soil sample sieved through 20mm and 4.75 mm sieves. Percentage passing 4.75mm and percentage retained on 4.75mm are mixed with certain proportions.

Add water to it and leave it in air tight container for 20hrs. Mix the soil and divide it into 6 – 8 parts. Position the mold and pour one part of soil into the mold as 3layers with 25 blows of ramming for each layer.

Remove the base plate and Weight the soil along with mold. Remove the soil from mold and take the small portion of soil sample at different layers and conduct water content test. from the values find out the dry density of soil and water content and draw a graph between them and note down the maximum dry density and optimum water content of the compacted soil sample at highest point on the curve.