Test: Properties of Soil - 1

The liquid limit of soil is the percentage of water content at which the soil passes from the plastic state to the liquid state or the Liquid limit indicates how much water the soil can hold without getting into the “liquid” state. The compressibility of soil is indicated by liquid limit, i.e. the compressibility of the soil generally increases with an increase in the liquid limit.

The formula for liquidity index is given as follows where w is the natural moisture content, w_p is the plastic limit, I_p is the liquid limit. So we can calculate the liquid limit by substituting the values.

⇒ I_p = 30%
Also Ip = w_l - wp
⇒ W_l = 30 + 25 = 55%

• The coefficient of curvature should lie between 1 and 3 for well-graded soil.
• The coefficient of gradation is also known as the coefficient of curvature and it is given by 
  
• The slope of the gradation curve of the soil gives the coefficient of curvature which represents the shape of the particle size distribution curve.

at unaltered water condition i.e. at undrained condition. Remoulded shear strength is less than undisturbed shear strength.

Soil has three phase representation as follows,

The uniformity of a soil is expressed qualitatively by a term known as uniformity coefficient, C_u, given by,

The larger the numerical value of C_u, the more is the range of particles.

The consistency index indicates the consistency (firmness) of a soil. It shows the nearness of the water content of the soil to its plastic limit. A soil with a consistency index of zero is at the liquid limit. It is extremely soft and has negligible shear strength. On the other hand, a soil at a water content equal to the plastic limit has a consistency index of 100%, indicating that the soil is relatively firm.

When the soil exists below Water, it is in a submerged condition. When a volume  V of soil is sub merged in water, then according to Archimede’s principle it displaces an equal volume of water. Thus the net mass of soil when submerged is reduced.

• The uniformity coefficient (Cu) is defined as the ratio of D60 to D10.
  
• A value of Cu greater than 4 to 6 classifies the soil as well graded.
• When Cu is less than 4, it is classified as poorly graded or uniformly graded soil
• Uniformly graded soil has identical particles with Cu value approximately equal to 1.
• A uniformity coefficient value of 2 or 3 classifies the soil as poorly graded. Beach sand comes under this category.
• Higher value of Cu indicates that the soil mass consists of soil particles with different size ranges.

The formula for water content is written as follows, where w = water content, G = Specific Gravity of soil grains, S = Degree of Saturation and e = Void ratio. 
Gw = Se

We can calculate the value of 'w' by substituting the values of G, S and e.

• 1 cm² of specimen of soil is taken and moistened if necessary and is rolled on a smooth glass plate to about 3mm diameter (plastic limit test).
• If crumbling does not occur, fold the thread, knead and roll again until the moisture of the soil reduces by drying during manipulation to plastic limit, which is indicated by crumbling that occurs during rolling.
• Characteristics of thread approaching plastic limit offers means of identification of soil.
• Plastic clays can be easily rolled and its plasticity can be easily found but if soil contains much silt, it can’t be rolled or becomes difficult to roll.

Water content,

Total weight,

Weight of solids in soil A,

Weight of solids in soil B

In mixed soil,
W_w = 500 + 333.3 = 833.3 gm
W_s = 500 + 666.7 = 1166.7 gm

We know that

For dry soil, w = 0;
For saturated soil, w = w_s
and for Wet soil 0 < w < w_s
Thus, dry density < wet density < saturated density.
Now

and

Thus submerged density < dry density

1. Constant head permeability test:

The coefficient of permeability for coarse-grained soil (Gravel and sand) is determined by means of the Constant-head permeability test. The degree of saturation of soil should be 100%.
By Darcy’s Law,

Q = k i A

Q=khA/L

k=QL/Ah

where, q = Discharge collected in time ‘t’, L = Distance between manometer taping points, A = Cross-sectional area of the sample, H = Difference in manometer levels i.e the head loss.

2. Consolidation Test: 

(i) This is used to determine the rate and magnitude of soil consolidation when the soil is restrained laterally and loaded axially. This test is conducted by an Oedometer. As disturbing specimens have a certain pre-consolidation pressure, so for conducting the consolidation test undisturbed soil specimen is required.

(ii) Oedometer is used to find out the void ratio at the end of various stress levels and hence curve is plotted. This curve is useful in calculating the settlement of the soil layer.

(iii) Consolidation test is generally performed on clay soil.

3. Pycnometer test is used to determine the specific gravity of cohesion-less soils and water content.

4. Hydrometer test:

• Measuring the particle size distribution(Grain Size Analysis) of fine-grained soils like clay and silt is best performed using the soil hydrometer test
• The hydrometer analysis of soil, based on Stokes’ law, calculates the size of soil particles from the speed at which they settle out of suspension from a liquid.

and 


⇒ D₃₀/D₁₀ = 2

Liquid limit:

It is the water content corresponding to the arbitrary limit between liquid and plastic state of consistency of soil.

It is defined as the minimum water content at which the soil is still in the liquid state, but has infinitesimal resistance against flow which can be measured by any standardised procedure.

With reference to the standard liquid limit device, it is defined as the minimum water content at which a part of soil cut by a groove of standard dimensions, will flow together for a distance of 12 mm under an impact of 25 blows in the device.
Other Important points:

Plastic limit: It is defined as the minimum water content at which a soil will just begin to crumble when rolled into a thread approximately 3 mm in diameter.

Shrinkage limit: It is defined as the maximum water content at which a reduction in water content will not cause a decrease in the volume of a soil mass. It is the lowest water content at which a soil can still be completely saturated.

Oven dry method is the most accurate and simplest method for water content determination.

In this method complete drying of soil sample occur and water content in sample is calculated accurately by a maintained temperature in the oven ( 105° C to 110° C) for 24 hours.

Note:

For highly organic soils a low temperature of about 60° C is preferable.

If Gypsum is present, the temperature should not be more than 80° C but for a long time.

Radiation method is based on energy loss during radio active isotropes material (E.g. cobalt-60) emmites on one end and received by detector on other end. This is an approximate method.

In calcium carbide method, Acetylene gas exerts pressure on the pressure gauge which indicate the water content present in soil sample. This is not an accurate method since no control over chemical reactions.

Sand bath method is field method and there is no control over heat given to soil sample. Therefore, it is not suitable for organic soil and soil having higher gypsum content. 

Pycnometer test is used to determine the specific gravity of cohesion less soils and water content.

Dry density or in-situ unit weight is determined by using the following methods:

1. Sand Replacement

2. Core-Cutter

3. Water Displacement

Other methods to determine the water content are:

1. Oven Drying Method

2. Calcium Carbide/Rapid Moisture

3. Sand Bath Method

4. Radiation method

5. Torsion Balance Moisture Meter

The grain size distribution of fine soil is determined using sedimentation analysis. Sedimentation analysis is performed using two methods:

1) Pipette method

2) Hydrometer method.

A hydrometer is a device used to measure the specific gravity of liquids.

The following three corrections are necessary for :

1. Meniscus correction: Since the hydrometer readings increase downward on the stem, the meniscus correction (Cm) is always positive.

2. Temperature correction: If the temperature at the time of the test is more than that of calibration of the hydrometer, the observed reading will be less and the correction (Ct) would be positive and vice versa.

3. Deflocculating agent correction: The addition of the deflocculating agent increases the density of the suspension and thus a correction (Cd) is applied which is always negative.

R = R_h' + c_m ± C_t - C_d
R_h' = Observed hydrometer reading
C_m = Meniscus correction
C_t - Temperature correction
C_d = Dispersive agent correction

Degree of saturation represents the portion of volume of voids which is filled with water i.e.