OBJECTIVE
To
determine the settlements due to primary consolidation of soil by conducting one
dimensional test.
The
test is conducted to determine the settlement due to primary consolidation. To
determine :
iii. Pressure-void ratio relationship.
From the above information it will be possible for us to
predict the time rate and extent of settlement of structures founded on
fine-grained soils. It is also helpful in analyzing the stress history of soil.
Since the settlement analysis of the foundation depends mainly on the values
determined by the test, this test is very important for foundation design.
a)
A ring of diameter = 60mm and height = 20mm
b)
Two porous plates or stones of silicon carbide, aluminum oxide or porous
metal.
c)
Guide ring.
d)
Outer ring.
e)
Water jacket with base.
f)
Pressure pad.
g)
Rubber basket.
PRINCIPAL
INVOLVED
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
and 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.
Then
the load is applied on the saturated soil mass, the entire load is carried by
pore water in the beginning. As the water starts escaping from the voids, the
hydrostatic pressure in water gets gradually dissipated and the load is shifted
to the soil solids which increases effective on them, as a result the soil mass
decrease in volume. The rate of escape of water depends on the permeability of
the soil.
1)
From the sample tube, eject the sample into the consolidation ring. The
sample should project about one cm from outer ring. Trim the sample smooth and
flush with top and bottom of the ring by using a knife. Clean the ring from
outside and keep it ready from weighing.
2)
Remoulded sample :
a)
Choose the density and water content at which samples has to be compacted
from the moisture density relationship.
b)
Calculate the quantity of soil and water required to mix and compact.
c)
Compact the specimen in compaction mould in three layers using the
standard rammers.
d)
Eject the specimen from the mould using the sample extractor.
1.
Saturate two porous stones either by boiling in distilled water about 15
minute or by keeping them submerged in the distilled water for 4 to 8 hrs. Wipe
away excess water. Fittings of the consolidometer which is to be enclosed shall
be moistened.
a.
0.1, 0.2, 0.5, 1, 2, 4 and 8 kg/cm2.
OBSERVATION
AND READING
Data
and observation sheet for consolidation test pressure, compression and time.
Project
: Name of the project
Borehole no. : 1
Depth
of the sample : 2m
Description
of soil :
Empty
weight of ring : 635 gm
Area of ring : 4560 mm2 (45.60 cm2)
Diameter
of ring : 76.2 mm (7.62 cm)
Volume of ring : 115.82 cm3
Dial Gauge = 0.0127 mm (least count)
| Pressure Intensity
(Kg/cm2) |
0.1 0.2 0.5 1 2 4 8 |
| Elapsed Time
0.25 1 2.5 4 6.25 9 16 25 30 1 hr 2 hrs 4 hrs 8 hrs 24 hrs |
Observation
Sheet for Consolidation Test : Pressure Voids Ratio
| Applied pressure | Final dial reading | Dial change | Specimen hieght | Height solids | Height of voids | Void ration |
| 0
0.1 0.2 0.5 1.0 2.0 4.0 8.0 4.0 2.0 1.0 0.5 0.2 0.1 |
CALCULATIONS
1.
Height of solids (HS) is calculated from the equation
HS = WS/G ´ A
2.
Void ratio. Voids ratio at the end of various pressures are calculated
from equation
e = (H – HS)/HS
3.
Coefficient of consolidation. The Coefficient of consolidation at each
pressures increment is calculated by using the following equations :
In the log fitting method, a plot is made between dial
reading and logarithmic of time, the time corresponding to 50% consolidation is
determined.
In the square root fitting method, a plot is made
between dial readings and square root of time and the time corresponding to 90%
consolidation is determined. The values of Cv are recorded in table II.
4.
Compression Index.
To determine the compression index, a plot of voids ratio (e) Vs logt
is made. The initial compression curve would be a straight line and the slope of
this line would give the compression index Cc.
5.
Coefficient of compressibility. It is calculated as follows
av = 0.435 Cc/Avg. pressure for the increment
where
Cc = Coefficient of compressibility
6.
Coefficient of permeability.
It is calculated as follows
K = Cv.av *(unit weight of water)/(1+e).
Graphs
1. Dial reading VS log of time or
Dial
reading VS square root of time.
2. Voids ratio VS logs¢
(average pressure for the increment).
General Remarks
1. While preparing the specimen, attempts has to be
made to have the soil strata orientated in the same direction in the
consolidation apparatus.
2. During trimming care should be taken in handling
the soil specimen with least pressure.
3. Smaller increments of sequential loading have to
be adopted for soft soils.