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ATTERBERG LIMITSLiquid limit test:
A soil is place in the grooving tool which consists of a brass
cup and a hard rubber base. A groove is cut at the center of the
soil pat using a standard grooving tool. The cup is then repeatedly
drooped from a height of 10 mm until a groove closure of 12.7 mm.
The soil is then removed and its moisture content is determined.
The soil is said to be at its liquid limit when exactly 25 drops
are required to close the groove for a distance of 12.7 mm ( one
half of an inch)
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Plastic limit test:
A soil sample is rolled into threads until it becomes thinner
and eventually breaks at 3 mm. it is defined as the moisture
content in percent at which the soil crumbles when rolled into the
threads of 3.0 mm. If it is wet, it breaks at a smaller diameter;
if it is dry it breaks at a larger diameter.Shrinkage limit
test:
It is performed in the laboratory with a porcelain dish
approximately 45 mm in diameter and about 12.7 mm high.
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The dish is completely filled with wet soil. The mass and volume
of the wet soil is then recorded.
The dish is then oven dried, then the mass and volume of the
oven dried soil is also recorded.
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Significance of soil consistency
A knowledge of soil consistency or the relative ease with with
the soil can be deformed is important in identifying the soil
type.Soil consistency also describes us how the soil might perform
when used as a construction material.
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Consolidation Test
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What is Consolidation?When a saturated clay is loaded
externally, the water is squeezed out of the clay over a long time
(due to low permeability of the clay).
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When a compressive load is applied to soil mass, a decrease in
its volume takes place, the decease in volume of soil mass under
stress is known as compression.
The property of soil mass pertaining to its tendency to decrease
in volume under pressure is known as compressibility.
In a saturated soil mass having its void filled with
incompressible water, decrease in volume or compression can take
place when water is expelled out of the voids.
Such a compression resulting from a long time static load and
the consequent escape of pore water is termed as consolidation.
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1D LAB CONSOLIDATION TESTWe want to find the following
parameters to estimate the amount and rate of consolidation
settlementCompression indices:Coefficient of consolidation:
Equipment
The ring containing the soil sample can be fixed to the
container by a collar (fixed ring cell, Figure b) or is
unrestrained (floating ring cell, Figure c).
*Cc, Cr , and
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1D LAB CONSOLIDATION TESTA disk of soil is enclosed in a stiff
metal ring and placed between two porous stones in a cylindrical
container filled with water.A metal load platen mounted on top of
the upper porous stone transmits the applied vertical stress
(vertical total stress) to the soil sample. Both the metal platen
and the upper porous stone can move vertically inside the ring as
the soil settles under the applied vertical stress.
*
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1D LAB CONSOLIDATION TESTIncremental loads, including unloading
sequences, are applied to the platen, and the settlement of the
soil at various fixed times under each load increment is measured
by a displacement gage. Each load increment is allowed to remain on
the soil until the change in settlement is negligible and the
excess porewater pressure developed under the current load
increment has dissipated. For many soils, this usually occurs
within 24 hours, but longer monitoring times may be required for
exceptional soil types, for example, montmorillonite.
Each load increment is doubled. The ratio of the load increment
to the previous load is called the load increment ratio (LIR);
conventionally, To determine Cr, the soil sample is unloaded using
a load decrement ratioload decrement divided by current loadof
2.
*
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1D LAB CONSOLIDATION TESTWhat data are collected?Initial height
of the soil, Ho, which is fixed by the height of the ring.Current
height of the soil at various time intervals under each load
(timesettlement data).Water content at the beginning and at the end
of the test, and the dry weight of the soil at the end of the
test.
What plots are prepared?Time (log scale) versus height (to find
Cv using Casagrande and Fadum (1940) method, and C)Square root of
time versus height (to find Cv using Taylors method)Vertical
effective stress (log scale) versus void ratio at the end of each
increment ( to find Cc, Cr and zc
*
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e log v plotloadingv increases &e decreasesunloadingv
decreases &e increases (swelling)- from the above data
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Compression and recompression indicesCc ~ compression indexCr ~
recompression index (or swelling index)