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 dσ' 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:

dσ' = 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:

(dσ' / 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:

(dσ' / 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 h₁ 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)

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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_v, for any given pressure increment can be calculated using:

c_v = k / m_v γ_w                                                                       (7)

where m_vis 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.

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 L₂ such that L₁ = 2L₂.

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_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.

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.

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 L₁ and L₂are 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.

Figures 4 illustrates the compatibility between the results from the simplified seepage consolidation test and oedometer test in terms of m_v, for the samples of black cotton soil. Based on extrapolation, the data trends appears to be consistent. This can be expected since 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_v and 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_vvalues exhibits a decreasing trend with an increase in the effective pressure.

The black cotton soil, being a typical montmorillonitic soil, exhibits decreasing c_v with an increase in the effective consolidation pressure.

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_v), coefficient of permeability (k), and coefficient of consolidation (c_v) with changes in effective stress levels.