Soil Testing Definition & Soil Investigation Studies

Chemical Analysis

California Bearing Ratio (CBR) test is a penetration/load-bearing test meant for determining the mechanical strength of road subgrades & basecourses. This is an empirical test & gives a measure of the resistance of the ‘Subgrade’ (compacted layer consisting of naturally occurring local soil that occurs just beneath the pavement crust) to deformation under the application of load from vehicles plying on roads, especially highways. This test mainly used for evaluating the load- bearing capacity and also designing of newly laid roads, flexible pavements, unimproved airstrips, and for soils already under paved areas. The CBR test can be either carried out in-situ or at laboratories. Historically, this test was introduced by the California Department of Transportation during 1930’s. The measurement of CBR involves application of a pressure using a plunger (of standard area) to penetrate a soil sample at a specified rate which has been maintained under controlled density & moisture conditions. CBR is calculated as the ratio of pressure (or test load) required to penetrate a soil sample to that required for the corresponding penetration of a standard material, which is generally crushed California limestone which has a value of 100. The values from CBR tests are used for estimating the quantity or thickness of road pavement; stronger the subgrade higher is the CBR value, less is the thickness of the road pavement required & vice versa.
The CBR values of a soil sample depend significantly on the moisture content as it influences the compaction of soil, and this variation needs to be measured by varying the moisture content in a controlled manner. Moisture content can vary naturally in soil because of variations in rainfall, capillary action, seasonal movement of the water table, seepage etc., & these variations can be simulated by appropriate laboratory test methods. Standard test methods of measurement include BS 1377, ASTM D1883-05, ASTM D4429, & AASHTO T193.

CBR, laboratory, soaked single point

The soaked, single point laboratory method for determination of CBR is used for a soil specimen which has been maintained at a specified density and moisture content. This measurement is useful for determining the suitability of a soil sample in terms of meeting the specifications of CBR parameters before its actual use on-site or in the field.

CBR, laboratory, soaked three point

In this test, the soil specimen is first prepared in moulds to three different & specified density and moisture contents, in order to assess the soil’s CBR performance at the different levels of compaction and moisture.

In-situ Density of Soil

Density of soil can be measured either in the laboratory after sample collection, or on-field. However, the latter method is preferred to the former because it gives the density value of the soil at a particular depth in the field. Since soil is a non-homogeneous & complex substance comprising of multi phases, determination of in- situ or in-place density is quite method specific. Hence, comparison of density values can be done only if the test method is specified. In general, there are quite a number of such method types, like Sand Cone/Sand replacement method, Core-cutter method, Neutron radiation method, and Penetrometer test method. Each of these methods differs from the other, not only in respect of their suitability for a particular soil type but also in terms of the time & cost of conducting a test. Of all these methods, the Sand Cone method is the most preferred one because of its simplicity & low cost involved in its operation. The density values are reported after calculation of the soil moisture in order to give the dry density value.

Core Cutter Method of Density Measurement

This is a conventional method for measurement of density and is applicable for soft or fine grained soils. This method involves digging out a volume of soil by insertion of a cylindrical steel structure (of a fixed & known volume) & then weighing the combined mass. The density is calculated by taking the ratio of the mass of soil excavated to that of its volume. The moisture of the soil is determined by oven-drying method by taking soil specimens from both ends of the soil in the steel cylinder.

Density, in-situ, by sand cone

This in-situ determination of density of soil is known by the name of Sand Cone or Sand Replacement method, and is applicable mainly for coarse or stony soils. This method involves the determination of the moisture content and wet density/ bulk density of soil by excavation of a known mass of soil in the form of a standard hole using a combination of a compaction mold & a sand cone apparatus. These steps are followed by the determination of the volume of the test hole made by replacement of the excavated soil with a standard sand of a known density and mass. Prior to all these steps, standardization of the volume of the cone & estimation of bulk density of the sand are done. Standard test method followed is ASTM D1556

Importance of Density measurement of Soil

Density measurement gives a measure of the engineering usefulness of soil, as it provides information about the load-bearing capacity of soil, amount of compaction required for construction of any foundation, and degree of settlement of the soil under the application of a particular type of load. Such information or data is vital in the construction of footpaths, embankments, underground structures, and even high-rise buildings.

Chloride content

The impact of chloride content in soil on its use for construction purposes is primarily related to its chemical tendency to cause corrosion, mainly under cyclic environmental conditions. Concrete structures can suffer deterioration of embedded reinforcement bar caused by ingress of chloride ions present in soil. Although chloride is a potential threat to concrete, the degree of deterioration increases with the concentration of moisture, permeability of the concrete, and the effect of cyclic freezing environment. Measurement of chloride in soil or concrete is usually done by wet chemical testing methods.

Electrical Resistivity

Electrical resistivity of a particular type of soil is a quantitative measure of the electrical resistance offered by it. This parameter is also known as the ‘Specific Resistance’ & it is measured as the electrical resistance between two opposite faces of a cube of material (like soil) with the dimension of the faces being 1 meter. It is generally reported as Ohm-meter (Ω-m) or as Ω-cm (especially in USA). Knowledge of soil’s electrical resistivity can be useful in the following areas:
(1) Sub-surface geophysical surveys conducted to identify locations of ores, depths of bedrocks, certain geological phenomena, and study of contamination of groundwater.
(2) Data from 'Soil Electrical Resistivity' studies are used in gauging the potential of the soil, which are intended to be used as backfills, in effecting corrosion of metal structures in contact with it, like underground pipelines & hence also aid in the development of protective designs.
(3) Design of economical extensive electrical grounding systems which are used in electrical substations, lightning conductors, high-voltage direct current transmission systems etc. An important reason for earthing electrical systems is to establish a common reference potential for the power supply system. This is achieved by means of a suitable low resistance connection to earth, and for this knowledge of electrical resistivity of soil and its variation with depth is essential.

Electrical resistivity of soil depends on a number of factors like:

  • Type of Soil or earth, like clay, loam, sandstone etc.
  • Soil stratification or different types of layers of soil.
  • Moisture content. Generally resistivity decreases with increasing moisture content.
  • Temperature. Generally resistivity increases with decrease in temperature.
  • Concentration of salts and metal ions.
  • Topography of the soil surface.

Type of Soil Range of resistivity values (Ω-m)
Clay, Fills, ashes, cinders, brine wastes 8 to 70
Shale, slates, sandstone etc. 10 to 100
Peat, loam & mud 5 to 250
Sand 200 to 3000
Gravel, sand, stones with little clay/loam 590 to 4580
Moraine gravel 40 to 10,000
Ridge gravel 3,000 to 30,000
Solid granite 10,000 to 50,000

There are basically two methods of test for measurement of electrical resistivity of soil. One is the Wenner 4-point method & the other is the Schlumberger method. The former method is the more common & preferred one than the latter, because of its better efficiency in terms of the ratio of received voltage per unit of transmitted current. The Schlumberger method is preferred in case where the resistivity is to be measured at different depths, as required in certain geological surveying.

Linear Shrinkage Test/Characteristics of Soil

The moisture content of soil has a significant impact upon a number of its important engineering properties, & is also susceptible to change with variations in environmental conditions. Hence, the study of the behavior of a particular soil under variable conditions of moisture becomes all the more important. Linear Shrinkage test is mainly used for characterization of ‘Expansive Soils’, and the information obtained from this test is used for interpretation or prediction of the ‘Cracking’ characteristics of soil resulting from shrinkage. Such characteristics are crucial in estimating the shrink/swell behavior of the Soil & also in the proper designing of buildings & other construction structures. The linear shrinkage value is considered a more reliable indicator of the ‘Plastic behavior’ of soil than the ‘Plasticity Index’ for those soil specimens with very low plasticity (i.e. ≤ 6%). Hence this test should be conducted along-with other tests which form part of the ‘Liquid Limit’ tests, in order to verify the plasticity index test result.

Category Linear Shrinkage (%m/m) Expansive Rating
Low 0 – 12 Non-Critical
Medium 12 – 17 Marginal
High 17 – 22 Critical
Very High >22 Very Critical

The standard definition of this parameter is: The Linear Shrinkage of a Soil corresponding to the moisture content equivalent to the liquid limit, is the percentage decrease in the length of a bar of the Soil dried in an oven. This test calculates the Soil’s one-dimensional shrinkage. This test is performed on dispersive (or disturbed) Soils for which dispersion percentage is more than 50. The Soil sample used for the test consists of particles less than or equal to 425 μm by passing through calibrated sieves. Standard test methods followed are IS 2720 (Part 20), ASTM D-427, & BS 1377.

Classification of Soil

The categorization of Soil into different types or classes is done systematically on the basis of its intended use & the associated properties. Thus, three main broad type of categorization is generally made:
(1) Engineering Classification, which takes into account the engineering properties of Soil like Shear Strength, Compressibility, Plasticity, Liquid Limit, Particle Size Distribution etc.
(2) Agricultural Classification, which takes into account the agricultural properties of Soil.
(3) Geological Classification, which takes into account the geological processes of Soil formation like weathering & formation of different types of minerals & ores.
On the basis of these properties, Soil can be divided into three types based on the Particle Size (or Grain Size) & inter-particle forces of attraction. The three types are: Coarse-grained or Granular or Cohesion-less Soil, Fine-grained or Cohesive Soil, & Organic Soil. Out of these three Soil types, the last one, i.e. Organic is not considered for construction purposes because of the extreme difficulty posed by it to the process of ‘Compaction’. These three types of Soil comprise of varying proportions of different Soil particles, viz. Gravel, Sand, Silt, & Clay. All these Soil particles vary in particle size & the resulting inter-particle forces of attraction.

Soil Particle Type Permeability Foundation Support Pavement Subgrade Expansive Compaction Difficulty
Gravel Very High Excellent Excellent No Very Easy
Sand Medium Good Good No Easy
Silt None Moderate Poor Difficult Very Difficult
Organic Low Very Poor Not Acceptable Some Very Difficult

Grain-size Distribution Curve/Grading Curve

The sizes of the Soil particles (or the particle size distribution) are determined by ‘Sieve Analysis’. This test is a mechanical process which involves passing a known quantity of Soil through a series of calibrated sieves placed one above the other in order of decreasing aperture size or opening from top to bottom. The size of a particular Soil particle type is determined by the size of the particular sieve through which it passes. After this test, a ‘Grain-size Distribution Curve’ is plotted between the percentage of Soil particles retained on a particular sieve & the corresponding sieve size (expressed as a logarithm). The shape of this curve is an important characteristic of a particular type of Soil.

Particle size distribution by hydrometer analysis

This test method involves the determination of percentile quantity of particles (normally silt and clay sized particles) smaller than can be sieved, usually 63µm in diameter. The larger particles of the Soil specimen are first removed by sieving. Then the Soil particles passing the sieve are made up into a suspension by addition of water. Then as the particles settled, the reduction in density is measured over time. The ‘Hydrometer Analysis’ method is based on Stokes’ Law which describes the motion of a small spherical particle through viscous fluids, by relating the force required to move the sphere through the fluid with its viscosity & the velocity & radius of the sphere. The diameter of the Soil particles can then be calculated based on the speed at which they settle, measured indirectly by the fall in density of the suspension.
The analysis result of particle size based on this method is useful in aiding classification of the soil, and hence its performance as a fill material.

Consistency of Soil

Consistency of Soil refers to its mechanical response to the application of stress. This also refers to the strength & resistance to penetration of the Soil in its in-place condition. This property can also be called the ‘Rupture Resistance’ of the Soil. This property has a definite relationship with the deformability & firmness of a fine-grained Soil, which in turn depend on the inter-particle Cohesive & Adhesive forces. Consistency of Soil depends largely on the minerals comprising it & also on the water content.

Atterberg Limits

Since the Consistency of a Soil varies with its water content, the gradual increase in water content causes the Soil to attain the following Consistency states (in the increasing order of water content): (Dry) Solid, Semi-solid, Plastic, & (Wet) Liquid. The water content at which the Consistency changes from one state to the other is called a Consistency Limit or Atterberg Limit. Atterberg Limits are important for characterizing or describing the Consistency of fine-grained Soils. There are three Atterberg or Consistency Limits & they are:
(1) Shrinkage Limit (Ws): This refers to the moisture content at the threshold/transition between the Semi-solid & the Solid (Dry) state. This is also the point at which no further reduction in volume occurs with further reduction in moisture content. (2) Plastic Limit (Wp): This refers to the moisture content at the threshold/transition between the Semi-solid (brittle or crumbly state) to the Plastic state.
(3) Liquid Limit (WL): This refers to the moisture content at the threshold/transition between the Plastic to the Liquid state. At the Liquid state, the Soil begins to behave like liquid or tends to ‘flow’.

Plasticity index

The ‘Plastic Index’ or ‘Plasticity Index’ is the difference between the Liquid Limit & the Plastic Limit, & it denotes the region where the Soil has a Plastic Consistency. The Consistency of most Soils in the field is generally Plastic or Semi-solid. This test method covers the determination of the Atterberg limits, the Liquid limit & the Plastic limit, and then ‘Plasticity Index’ is calculated. For determining the ‘Liquid limit’, the Soil's resistance to penetration by a standard cone is measured at varying moisture contents. The moisture content at which the penetration of the Soil specimen by the cone to a depth of 20mm takes place is taken as the value of Liquid Limit. For determination of the ‘Plastic Limit’, a Soil sample is slowly dried, using the heat of the hands until it can no longer be rolled into a 3 mm thread without cracking and shearing (or crumbling). At this point the moisture content is measured to get the Plastic Limit. Standard equipment used for the test is the Standard Casagrande Liquid Limit Device, & the Standard test method followed is ASTM D4318.
The determination of the Plasticity Index is useful in aiding classification of a Soil specimen, and also in assessing or pre-determining its performance as a fill material, where particular emphasis is placed on swelling and shrinking characteristics of Soil.

Soil Compaction, Consolidation & Settlementt

Compaction of Soil is the process of mechanically increasing the density of a Soil (or Densification) by application of static or moving loads (also called Compacting Force). Compaction results in rearrangement of the Soil particles with the end result of reduction in the Void ratio (ratio of the Void volume to the Solid volume). The degree of Compaction of a Soil is measured in terms of its ‘Dry Unit Weight’, which is defined as the density of the Soil when it is completely dried out. The Dry Unit Weight of a Soil correlates with the degree of packing of the Soil grains. There are four control factors affecting the extent of Compaction: (1) Compaction Effort/Energy (2) Type of Soil & Gradation (3) Moisture Content (4) Dry Unit Weight. The effect of increasing the density as a result of Compaction is the pronounced enhancement in the load-bearing capacity of the Soil. For each type of Soil, there exists an optimum moisture content for Compaction, i.e., the moisture content at which a given pressure will create the densest material. Soil Compaction is crucial in the construction of highway embankments, earth dams, reinforced earth walls, road bases, runways, & a number of other engineering structures, where either the condition of existing Soils need to be improved or loose Soils are used as fills.
Advantages of Compaction of Soil
(1) Increased Shear Strength of the Soil which enhances its load-bearing capacity.
(2) Reduced Compressibility (the tendency of a Soil to decrease in volume under load) of the Soil which prevents ‘Settlement’ under the application of working loads. Settlement is defined as the vertical subsidence of any structure or building as the Soil is compressed. Excessive Settlement of Soil can result in serious damage (or even failure) to a structure.
(3) Reduced Permeability of the Soil or hydraulic conductivity which will prevent water absorption, & hence will reduce the tendency of the Soil to expand or swell or shrink or even to liquefy.

Types of Soil Compaction

The method of Soil Compaction selected for a Soil type depends on the type of Soil, i.e., whether Coarse-grained or Fine-grained. Broadly, there are two methods of Soil Compaction depending on the type of Compaction force utilized. One is the ‘Static Force Soil Compaction’ in which the deadweight of the machine applies a downward force on the Soil surface. The force due to this form of Compaction is confined to the upper layers of the Soil only. Kneading & Pressure are two examples of Static Compaction. The other type is the ‘Vibratory Force Soil Compaction’ which uses an engine-driven mechanism of applying a downward force in addition to the machine’s static weight. Vibration & Impact are examples of Vibratory Compaction.

Difference between Compaction & Consolidation

Both Compaction & Consolidation are Soil densification methods but the mechanisms for the two are fundamentally different. Soil Compaction results in removal of air-filled porosity, with no outward fluid or water flow. Soil Consolidation results in removal of water-filled porosity achieved by outward flow of water. The outward flow of water is a function of Permeability of the Soil.

Proctor Compaction Test & Modified Proctor compaction test

The Proctor or Modified Proctor Test is used to determine the maximum density of a Soil required for a particular site. The Modified Proctor Test was developed during World War II by the US Army Corps of Engineering, in order to get a better insight into the Compaction required for airfields to support heavy aircrafts. The first step is the determination of the maximum density achievable, followed by the determination of relationship between Soil density (Dry) & moisture. Both the tests involve application of a standard Compaction effort to a number of samples of the Soil of increasing moisture content, and measuring the resulting dry density of each sample. The comparison between the two test methods is as follows:

Standard Proctor Test Modified Proctor Test
Mold Size: 1/30 cubic feet Mold Size: 1/30 cubic feet
Height of drop: 12 inches Height of drop: 18 inches
Weight of Hammer: 5.5 lb Weight of Hammer: 10 lb
3 layers of Soil 5 layers of Soil
25 Blows per layer 25 Blows per layer
Compaction Energy: 12,375 Compaction Energy: 56,250
Preferred for low Shearing Strength Soils Preferred for High Shearing Strength Soils
Std. Test Method Adopted: ASTM D698 & AASHTO T99 Std. Test Method Adopted: ASTM D1557 & AASHTO T180

Plate Bearing

The Plate Bearing (or Loading) Test is used to assess Shear failure in Soil, i.e. deformability assessment of soils & rocks. This test involves in-situ testing of Soil by placing different loads on the plate & then noting ‘Settlement’ at certain intervals. A graph is plotted between the applied pressure or load & the resultant displacement or deflection (mean Settlement) produced. During the test care should be taken to protect the testing equipment from sunlight, moisture & any other adverse weather conditions. The test is carried out by using a hydraulic device that transfers pressure in a stepwise manner through a circular rigid plate onto the surface of the earth or rock half-space, until the Displacement or Pressure criterion is satisfied. Typical diameters of the rigid plates used for testing on-site are 300, 420, 600, & 760 mm.
This test is normally used to measure or assess the short term Settlement of road sub-grades or building footings under their proposed design load. However, the main application of this test is the Compaction check of roads & airfield earthworks. The value of Settlement against load is then used to check whether the Soil meets the design load-settlement criteria or not. Standard Test Method followed is IS: 1888-1982 In addition to determining the values of Settlement, other Soil parameters can also be measured, or calculated from this test. These include Modulus of Sub-Grade Reaction & Permanent Deformation Characteristics of the Soil.

Sulphate content

Concrete in contact with Soil containing high levels of Sulfate (SO42-) can suffer attack & undergo deterioration. Testing of Sulfate levels in Soils are generally conducted to check whether special cements need to be incorporated into a concrete mix to prevent such detrimental attacks. The measurement of Sulfates in Soil is often made by the traditional Wet Chemistry method. The most common method employed is the ‘Gravimetric’ process which involves precipitating the Sulfate as Barium Sulfate using Barium Chloride solution. The calculation of the concentration of Sulfate is done taking into account the mass of the precipitate obtained & the mass of the original sample taken for analysis.