During the earlier days, sites used to be discarded as
unfit, if the soil at the site was found unsuitable. At present,
industrialization and urbanization are resulting in more and more demand for
sites for constructional activities. However, due to scarcity of lands, the
sites that have been considered to be unfit earlier cannot be abandoned
altogether. Even soft sedimented soils present in the water front areas like
sea and lakes are being reclaimed. Example:

·
Changi Airport, Singapore.

·
Tokyo International Airport,
Japan.

·
Kansai International Airport,
Japan.

·
Replacement Airport at
Cheklapkok, Hong Kong.

·
Uddevalla Shipyard, Sweden.

·
Fertilizer plant at Ashuganj,
Bangladesh.

·
J. N. P. T. area, Mumbai,
India.

·
Deep water Port, Kakinada,
India.

·
Salt Lake City, Calcutta,
India.

Consolidation is the property of soil mass pertaining to
its susceptibility to time dependent decrease in volume under pressure.
According to Terzaghi; “

*every process involving decrease in the water content of a saturated soil with out replacement of the water by air is called the process of consolidation*.”
During the natural sedimentation process, soft sediments
are formed. For the effective utilization of lands containing soft sediments,
it is very much essential to understand the engineering characteristics of
these sediments. Soft sediments are characterised by very high water contents.

The compressibility behaviour of soft sediments cannot
be determined in the laboratory by the conventional oedometer test due to
following problems:

·
Undisturbed sampling of such
soft sediments is impossible.

·
Normally employed mechanical
loading cannot be applied on such sediments due to the soft consistency of the
sediment.

To over come these problems, Simplified seepage
consolidation testing procedure was developed by Sridharan & Prakash (1999)
for conducting consolidation tests on soft sediments at low effective stresses.
This is very simple and does not require sophisticated instrumentation.

#
Simplified Seepage Consolidation
Test

When water is made
to flow though a soft sediment, due to the difference between the hydraulic
heads on the upstream and down stream sides of the sediment, the seepage force
exerted by the flowing water consolidates every element in the sediment. Let γ

_{w}, γ_{sat}and γ_{sub}represent the unit weight of water, the saturated and submerged densities of the soil mass respectively. If*d*σ' is the change in effective stress,*du*is the change in pore water pressure due to the seepage of water from the top of a specimen of thickness*dz*downwards, then:*d*σ' = γ

_{sat}

*dz*–

*du*(1)

Change in
hydraulic head between the top and the bottom surfaces of the specimen is given
by:

*dh*= 1/ γ

_{w}(

*du*- γ

_{w}

*dz*) (2)

Substitution of
equation 2 in equation 1 results in:

(

*d*σ' /*dz*)*= - γ*_{w}(*dh/dz*) + γ_{sub}_{ }(3)
Seepage force is
defined as the force exerted by the flowing water per unit volume of soil and
is given by:

j = γ

_{w}i = - γ_{w}(*dh/dz*) (4)
Hence, equation 3
can be written as:

(

*d*σ' /*dz*)*= j + γ*_{sub}(5)
Equation 5
indicates that the seepage force and submerged weight of the soil cause an effective
stress gradient. That is, the seepage force is converted into effective
consolidation pressure.

Fig 1

*Typical total stress, pore pressure and effective stress diagrams for two cases of soil mass subjected to steady state seepage.*
Figure 1a shows a soil mass of uniform cross-sectional
area, A, length, L, subjected to a water head h

_{1}at its top and h_{d}at its bottom. Figure 1b, 1c, and 1d show the total stress, pore water pressure, and effective stress diagrams for the system shown in Fig. 1a. The pore water pressure at the base of the specimen shown in Fig 1c is due to the standing water column of height h_{d}connected to the base of the specimen.
For the case of
steady flow, the effective stress at the base of the sediment can be shown to
be equal to

σ' = γ

_{sub}L + γ_{w}h (6)
where h is the head causing the flow. The above equation has two
components, one due to the buoyant weight of the soil and the other due to the
action of seepage force.

Figures 1a and 1e
show two typical systems. For the system shown in Fig 1a the pore water
pressure at the base of the sediment is positive, whereas it is negative for
the system shown in Fig 1e, as the base of the sediment in this case is
subjected to a suction head equal to h

_{d}. Hence, the head causing the flow is the difference between the upstream and downstream heads (i. e., h = h_{u}- h_{d}) in the first case, while it is numerically the sum of the upstream and downstream heads in the second case [i. e., h = h_{u}– (-h_{d}) = h_{u}+ h_{d}].
It has been observed that the
homogenous sediment result without any grain size sorting when the initial
water content of the soil slurry under going sedimentation is about 1.3 – 1.5
times of liquid limit of the concerned soils (Sridharan & Prakash, 1997)

.

### Experimental procedure

The entire specimen can be represented by its average
voids ratio from all practical purposes when the specimen thickness is
relatively small. With this understanding, the following procedure is followed
for Simplified Seepage Consolidation test:

1.
The sediment is obtained after
the settling and self weight consolidation are practically over.

2.
The upstream and downstream
constant over flowing water levels are maintained at different elevations to
introduce a known hydraulic head difference. The height of the sediment gets
reduced due the induced seepage force.

3.
Once steady state is reached,
the height of the sediment is noted and the average void ratio is calculated.
The effective pressure at the base of the sediment is calculated using equation
6. The average void ratio is plotted against the average of the effective
pressures developed over the depth of the sediment,

*p’*.
4.
Either constant head or falling
head permeability test is conducted on the sediment that has been formed to
determine the average coefficient of permeability

*k*.
5.
Now the hydraulic head
difference is increased gradually and slowly to a new value, which in turn
increases the seepage force. This results in further consolidation of the
sediment. Corresponding to the consolidated sediment, a second set of (

*e, p’, k*) values is calculated.
The above procedure is repeated with several known values of
hydraulic head difference to get corresponding (

*e, p’, k*) values. The values of coefficient of consolidation,*c*, for any given pressure increment can be calculated using:_{v}*c*γ

_{v}= k / m_{v }_{w }(7)

where

*m*is the coefficient of volume change calculated using the average values of void ratio and effective pressure. From the e-log p’ plot, the slope of the curve Δe / Δlog p’, can be calculated. In this way, the simplified seepage consolidation test can be used to obtain all consolidation parameters of soft sediments. By suitably adjusting the hydraulic head difference, the test can be conducted with any desired pressure increment ratio._{v}### Experimental programme

The experimental setup used in conducting the simplified
seepage consolidation tests is shown in figure 2. It consists of a graduated
glass cylinder of 50mm internal diameter with an outlet at the bottom. The
glass cylinder is connected to a downstream reservoir through a flexible tube.
The graduations are such that any change in the specimen height up to 0.5mm
accuracy can be measured. The filter provided at the bottom of cylinder
comprises filter papers and coarse sand.

The soil tested was black cotton
soil having Liquid limit 76.6%, Plastic limit 35.4%, and Shrinkage limit 9.9%.
Grain size distribution was done and the sample was found to contain 55% clay
size, 32.6% silt size and 12.4% sand sized particles.

The soil of known dry weight was
mixed with water to form a slurry, and its water content was then increased to
its 1.3 to 1.5 times the liquid limit. The soil-water mixture was thoroughly
mixed and then carefully introduced into the test cylinder and allowed to settle.
Sufficiently long time was allowed for the self-weight consolidation before the
sediment could be considered to have reached the equilibrium for all practical
purposes. Normally 24-hr duration has been observed to be sufficient. Depending
upon the plasticity of soils, the initial height of the sediment formed varies
between 4 and 14 cm (say L). During the sediment formation process, the water
surface levels in the upstream and downstream reservoirs were maintained equal.
Once equilibrium was reached, the top surface of the downstream reservoir was
lowered to a level as determined from the consideration of the pressure
increment ratio adopted slowly and gradually. Even though any pressure
increment ratio can be adopted, for the testing discussed in this paper, a load
increment ratio of unity was adopted. Because of the difference in elevation
between the water levels in the glass cylinder and the downstream reservoir,
the resulting water flow through the sediment initiated the seepage
consolidation. During the seepage consolidation process, the upstream and
downstream water levels were kept overflowing.

Once steady state was reached, the height of the
sediment and hence, the compression that occurred was noted. On average, it was
observed that about 24 hr of time was required to reach this state of
equilibrium. Then, the falling head permeability test was conducted by allowing
the upstream reservoir level to reduce, keeping the downstream overflowing
reservoir level unchanged. By following this procedure, a number of average
velocities of flow and corresponding values of average hydraulic gradients were
recorded.

After the permeability measurements were over, the top
surface of the downstream reservoir level was further lowered to get a
predetermined effective stress guided by the adopted pressure increment ratio,
and the entire procedure as outlined above was repeated. Following this
procedure, the consolidation parameters were obtained in the very low to low
effective stress range of 0.1 kPa to 10 kPa, which overlaps the stages of the
standard oedometer consolidation test.

To check the effect of sample thickness on the
consolidation behavior, tests were also conducted on sediments formed using a
quantity of soil sufficient enough to result in sediments of initial height L

_{2}such that L_{1}= 2L_{2}.
In order to confirm the validity and applicability of
the proposed simplified seepage consolidation test, it is necessary to examine
whether the results obtained from the simplified seepage consolidation test
were reasonable or not. For this purpose, the validation checks adopted include
comparison of the e-log p’, m

_{v}- log p’, e-log k and c_{v}-log p’ relationships obtained from the simplified seepage consolidation test with those from the conventional oedometer test at overlapping effective pressure ranges.
In this context, some typical oedometer consolidation
tests were also conducted. As the water contents of the sediments of the soils
used in this is subjected to simplified seepage consolidation test and
conventional oedometer test were observed to be in the range 1.1 to 1.3 w

All oedometer tests were conducted in the
standard oedometer setup of fixed-ring, double-drainage type, equipped to
conduct the falling head permeability test also. Soil samples were mixed
thoroughly with water such that their initial water contents after moisture
content equilibration were equal to the required values. The prepared soil
samples were worked into the oedometer rings of 60mm internal diameter and 20mm
thick. Care was taken to eliminate the ring friction by using silicon grease.
The seating pressure adopted was 2.775kPa, which included the pressure on the
sample due to top porous stones, top cap, and the plunger, in addition to the
pressure due to the load placed on the loading pan. In order to avoid squeezing
out the sample from the ring, a pressure increment ratio of 0.5 was adopted up
to 6.25 kPa and unity thereafter. A normal loading period of 24 hr was adopted
per load increment. At the end of primary consolidation, due to each pressure
increment, a falling head permeability test was conducted before the next
pressure increment was applied._{L}the oedometer tests were conducted on soil samples with initial water content ranging from 1.1 to 1.3 w_{L }depending upon the soil type.**Results and discussion**

Figures 3 represent the combined

*e*-log*p’*curves for black cotton soil, obtained by combining the results of simplified seepage consolidation and oedometer consolidation tests. The seepage consolidation is due to body force, which is constant over the depth of the sediment, the oedometer consolidation is due to incremental surface force, which can be considered constant at all cross sections in the steady state. While the seepage force is gradually applied dynamic force, the incremental load applied in the oedometer test is a suddenly applied static force.
FIG-3:

*Composite e-log p’ curve for black cotton soil.*
In spite of all these independent
variables, namely the testing procedure adopted, the complex load transferring
mechanisms, the specimen thickness, and many other unknown factors, the
continuity in the

*e*-log*p’*behavior, which spans about three log-cycles of pressure range, can be observed excellent.
Figures 3 also illustrates the
effect of sample thickness observed in the simplified seepage consolidation
test on the

*e*- log*p’*curves obtained from sediments with initial sample thicknesses*L*and_{1}*L*are essentially the same both in magnitude and trend of variation. In conducting the simplified seepage consolidation test, the initial water content is of at most importance than the thickness of the sediment._{2}
Figures 4 illustrates the
compatibility between the results from the simplified seepage consolidation
test and oedometer test in terms of

*m*, for the samples of black cotton soil. Based on extrapolation, the data trends appears to be consistent. This can be expected since_{v}*e*– log*p’*plots compare well.
FIG-4:

*Compatibility between simplified seepage consolidation test and oedometer test in terms of m*_{v}for black cotton soil.

FIG-5:

*Compatibility between e-log k relationships obtained from simplified seepage consolidation test and oedometer test for black cotton soil.*
Figures 5 show typical

*e*– log*k*relationship obtained from simplified seepage consolidation test and oedometer test for the samples of black cotton soil. Considering the wide variations in the testing procedure adopted for the values of*k*, the data trend obtained from both the tests procedures can be considered satisfactorily.
FIG-6:

*Compatibility between simplified seepage consolidation test and oedometer test in terms of c*_{v}for black cotton soil.
The values of the coefficient of
consolidation obtained with the help of

*m*and_{v}*k*using equation 7 from the simplified seepage consolidation and oedometer consolidation tests are plotted together in fig 6. The agreement between the results from two entirely different testing procedure can be observed to be good for black cotton soil.*c*_{v}_{ }values exhibits a decreasing trend with an increase in the effective pressure.
The black cotton soil, being a
typical montmorillonitic soil, exhibits decreasing

*c*with an increase in the effective consolidation pressure._{v}
All the validation checks presented
above indicate that the simplified seepage consolidation test can be used to
study the compressibility and permeability characteristic of soft sediments at
low effective stress levels satisfactorily.

**Conclusion**

In order to study the
compressibility and permeability characteristics of soft sediments at low
effective stress levels, the principle of seepage-force-induced consolidation
test can be used. The test procedure involves a very simple test setup,
measurements, and calculations. The results obtained from the simplified
seepage consolidation test are in good agreement with those obtained by the
conventional oedometer consolidation test at over-lapping effective stress
levels, thus proving the validity of the method. The validity checks have been
performed in terms of changes in void ratio, coefficient of volume change (

*m*), coefficient of permeability (_{v}*k*), and coefficient of consolidation (*c*) with changes in effective stress levels._{v}