Laboratory Soil Tests

Sieve Analysis:

The most fundamental soil test of all, is the sieve analysis. It is used to determine what type of soil are we dealing with, in the first place. It is like asking a person his or her name (is the soil mainly gravel? sand? silt? clay? and in what proportions?). Through this test, soil particles are passed through successively smaller sieves and the portion retained in each sieve is recorded. With this, grain size distribution of the soil can be obtained and put on a graph. Very fine soils can not be separated with this, (but now at least we know the total fine particles portion, the portion that passed the smallest possible sieve) and those fine particles must be measured with a different method, called “hydrometer analysis”, which is discussed next.

Hydrometer test:

Fine grained soils such as silts and clays are put in water and by using the principle of different size particles settle in different amount of time, the particle size distribution of these fine grained soils can be determined, because larger particles settle faster in a liquid, based on Stoke’s Law. This test is for fine grained soils only. The coarse grained soils’ particle size distribution is determined by sieve analysis as described above.

Unconfined Compressive Test:

Unconfined test can be done on only self supporting soil, such as clay. Sand can not stand freely, so this test is not suitable for sands. Basically a cylindrical clay sample is obtained. After that a compressive load is applied on the cylinder and the rate and amount of loading vs. deformation and force at failure is measured. Because the test is rapid, only undrained shear strength can be measured with this test, where water does not have time to escape from clay pores.

As you may recall, undrained shear strength can be expressed as:

τ=c + sinØ

Here everything is in terms of total stress, not effective stress (so, without apostrophe sign), because water in clay does not have time to escape, and we can only measure in terms of total stress.

Soil testing 2

Consolidation Test (Oedometer Test):

This is the process we described in section “Consolidation Settlement” before, also see figure Soil Strength 23b in that section. This test is also called confined compression test or oedometer test. It is used to measure the void ratio vs. applied loads in clay soils. It consists of a short cylinder (disc) of soil, enclosed by a metal ring with porous plates are placed at the top and bottom of cylinder. Then the cylinder is loaded slowly, at such a rate that water in clay has enough time to escape, so that the clay is consolidated. Therefore this test can take days or weeks, as the permeability of clay is very low. The load is applied, consolidation takes place, and then at some point it is released. The clay recovers some of the original height but not all. At the stage, clay is in overconsolidated state, which means, it had greater stress on it in the past. Recovering some of the height means, it is swelling, and it is also in suction mode, it means, it sucks the water back. But only some of the original height can be recovered. Then it is loaded again. Until that load reaches the maximum previous load, the settlement is very slow. And then, when the load exceeds maximum past load, settlement is faster again. That portion of the curve is called the virgin curve. 

Direct Shear Test:

We had described this test under previous section, angle of internal friction. It is basically, enclosing a soil disc, in what is called a direct shear box. Here as the vertical load is applied from top, the two halves of the box is slid on each other. The normal force vs shear force is measured to measure shear strength and angle.

Triaxial Test:

A much more improved version of the unconfined compressive test and direct shear test mentioned above, the most complex laboratory test for soils is called triaxial test, where a cylindrical soil sample that is carefully transported from the field, is wrapped in rubber membrane and tested under different loading and water drainage conditions, to measure the soil’s most important property, it’s shear strength. Also see Soil Strength > Angle of Internal Friction, where we discussed other details on triaxial test.

Unlike the unconfined compression test, which can only measure undrained strength, triaxial test can measure both undrained and drained strength, by carefully controlling letter water out or not. The undrained shear strength is not a fundamental soil property. It depends on loading rate, how the testing is performed, initial stresses in the soil, boundary conditions of the soil and other things. So during testing, the engineer must choose the best way of testing to represent as close as possible to field conditions.

Triaxial tests are usually for clays, which has cohesion and can stand on its own, because it is difficult to obtain an undisturbed sand sample and put it in the test equipment in its natural state.

Triaxial tests can be of three types:

  • Consolidated & Drained (CD Test)
  • Consolidated & Undrained (CU Test)
  • Unconsolidated & Undrained (UU Test)

It all depends on what variables we need to use for our project and therefore what we must simulate in the lab.

Consistency Test:

Through consistency test, the plastic and liquid limits of a soil can be determined, which are defined by water (moisture) contents. Plasticity applies to clay soils only.

There are four states of consistency, for fine grained soils, expressed in terms of moisture content of the soil. These go as follows: Solid, Semi Solid, Plastic and Liquid. As the soil takes in more water, it’s volume increases and it transitions between these states. 

Soil Testing 3

As can be seen in the graph,

The limit moisture content where soil transitions from solid to semi solid state is called shrinkage limit, SL.

The limit moisture content where soil transitions from semi solid to plastic state is called plastic limit, PL.

The limit moisture content where soil transitions from plastic to liquid state is called liquid limit, LL.

So for example, below the shrinkage limit, soil is considered as solid, or above the liquid limit, it is considered as liquid and so on…. These limits are also called Atterberg’s limits, after the scientist who defined them.

Plasticity Index, a very important property for a fine grained soil is expressed as:

Plasticity Index = Liquid Limit – Plastic Limit

in other words, PI = LL – PL

The larger the Plasticity Index, the more plastic is that soil, which means it has a larger ability to expand and contract. This is an unwanted situation below footings or slabs.

Plasticity is one of the two most important characteristics of fine grained soils. The other was already mentioned before, which is cohesion (which has such importance that fine grained soils are often called as cohesive soils). Both plasticity and cohesion are a result of fine grained soils holding adsorbed water films in between particles. Plasticity is a soil’s ability to deform without cracking or breaking apart. Sands and gravels are coarse grained soils, and they are non cohesive and they are not plastic.

Other than the consistency test, (we will not give details of the test here) another way to determine plasticity of the soil is just by rolling the soil over a flat surface by hand, but that can only give rough results.

Specific Gravity Test:

Through this test, the specific gravity of solid soil particles can be determined. Specific gravity was defined before, under the section Phase Relationships.

Compaction Test (Proctor Test):

This test is done to determine the maximum dry density and the corresponding water content of a soil for a given compactive effort. It is performed by filling a cylinder of a known volume with soil, and dropping a tamper of known weight for a given number of times from a known height. The amount of work done on the soil by this specific effort is known as the compactive effort. For each compactive effort, there is a different Optimum Moisture Content (OMC), to obtain a different maximum dry density which would correspond to a different curve in the graph below. By increasing the compactive effort, the maximum dry density that can be obtained increases, while the OMC decreases. Before performing compaction test, sieve analysis must be done. A typical graph of a compaction test for different compactive efforts, may look similar to the one below. The higher the curve, the more compactive effort, because it means we obtain a higher density for the same water content.

Soil Testing 4

Based on this graph, we can also see that, for the same compactive effort, (for one of the curves in the graph), when the soil was dry, as the moisture content increases, first this serves as a lubrication, and therefore enables better compaction, hence more dry density. But adding of water helping increase in dry density can only continue upto a point. This point is called optimum moisture content. After this point is exceeded, as we add more water, all it will cause is to increase the total volume of the soil, as it will be harder for soil to take in more water and the soil starts to swell. Increase in volume will mean decrease in dry density which is the downsloping part (right part) of each individual curve.

Permeability Tests:

Permeability is a measure of how easily water flows through soil. There are two types of tests for measuring the permeability. One is for coarse grained, highly permeable soils, and the other is for fine grained, low permeable soils.

Constant head permeability test is for coarse grained soils. It is a system where water is constantly poured into the system, and it goes through the soil. As the soil is coarse grained, it is highly permeable and lets water out from below fast and that is why we can (and need to) constantly pour water into the system. The rate of water flow is measured, which is required to keep a constant height of water above the soil. This rate of water flow is then related to the permeability of the soil by formula.

Falling head permeability test is for fine grained, low permeability soils. For those soils, it is not possible to constantly pour the water into the system, as in the test above, because the soil will not let the water pass through easily. Instead, only a small amount of water is poured first, and then the time that passes for it to slowly drain through the soil is measured. This time is then is related to the permeability of the soil by using a formula.

In the next post of this series, we will discuss “Geotechnical Reports”

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