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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.
An undisturbed soil sample has a plastic limit of 25%, a natural moisture content of 40% and liquidity index of 50%, Its liquid limit in % will be
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%
A well graded soil has a coefficient of curvature between
The ratio of the undisturbed shear strength to the remoulded shear strength in cohesive soil under undrained conditions is
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,
Which of the following is a measure of particle size range?
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.
The principle involved in the relation
γ_{sub.} = γ_{sat}  γ_{w} is
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.
Given for a soil sample:
Degree of saturation = 90%
Specific gravity of soil grains = 2.70
Void ratio = 0.30
The water content of the sample is
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.
For distinguishing clays from silts in the field, a moist soil is rolled into a thread of 3 mm diameter. This test will indicate the
Which one of the following is the water content of the mixed soil made from 1 kg of soil (say A) with water content of 100% and 1 kg of soil (say B) with water content of 50%?
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
Which one of the following gives the correct decreasing order of the densities of a soil sample?
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
Match ListI with Listll and select the correct answer using the codes given below in the options:
1. Constant head permeability test:
The coefficient of permeability for coarsegrained soil (Gravel and sand) is determined by means of the Constanthead 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 = Crosssectional 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 preconsolidation 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 cohesionless soils and water content.
4. Hydrometer test:
For a given soil sample,
If C_{c} = 1.0 and C_{u} = 4.0, then the value of D_{30}/D_{10 }would be
and
⇒ D_{30}/D_{10} = 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.
Which of the following methods is most accurate for the determination of the water content of soil?
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. cobalt60) 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 insitu unit weight is determined by using the following methods:
1. Sand Replacement
2. CoreCutter
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
Valid range for S, the degree of saturation of soil in percentage is
Degree of saturation represents the portion of volume of voids which is filled with water i.e.
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