One-Dimensional Consolidation (Oedometer test)

Consolidation is the gradual reduction in the volume of a fully saturated soil of low permeability due to drainage of some of the pore water, the process continuing until the excess pore water pressure set up by an increase in total stress has completely dissipated; the simplest case is that of one-dimensional consolidation, in which a condition of zero lateral strain is implicit. The process of swelling, the reverse of consolidation, is the gradual increase in the volume of soil under negative excess pore water pressure.

The oedometer consolidation test is used for the determination of consolidation characteristics of low-permeability soils when subjected to vertical loads. The results may be used to calculate and estimate settlements of structural foundations when placed on the ground. The two parameters normally required are:

•        The compressibility of the soil: Coefficient of volume compressibility, m_v

•        The time-related parameter: Coefficient of consolidation, C_v

When structures are built on saturated soils, the load is presumed to be carried initially by incompressible water within the soil voids. Due to the additional load on the soil, water will tend to be squeezed out from the voids causing a reduction in void volume and consequently settlement of the structure.

In soils of high permeability (coarse-grained soils), this process takes a relatively short time for completion, with the result that almost all of the settlement will occur during the construction period. These rarely cause major problems. In low permeability soils (clays), this process takes place slowly and continuously over a long period of time – months, years, and even decades – after completion of construction.

This method covers the determination of the magnitude and rate of the consolidation of a saturated or near-saturated specimen of soil in the form of a disc confined laterally, subjected to vertical axial pressure, and allowed to drain freely from the top and bottom surfaces. The method is concerned mainly with the primary consolidation phase, but it can also be used to determine secondary compression characteristics.

In this test, the soil specimen is loaded axially in increments of applied stress. Each stress increment is held constant until the primary consolidation has ceased. During this process water drains out of the specimen, resulting in a decrease in height which is measured at suitable intervals. These measurements are used for the determination of the relationship between compression (or strain) or voids ratio and effective stress, and for the calculation of parameters that describes the amount of compression and the rate at which it takes place.

Apparatus

The consolidation apparatus, known as the oedometer, shall be of the fixed ring type.

Preparation of sample

The sample may be built in (extruded into the consolidation ring) from the following type of samples:

•        Cylinder samples, U100 or 54 mm samples

•        Block samples cut from test pits

•        In-situ specimen built in directly from the bottom of test pits.

The soil is carefully trimmed away outside the consolidation ring. Check that there is no gap between the ring and the sample. If there is, a new sample should be prepared.

Specimen measurements

1. Measure the height of the specimen to 0.05 mm (H₀) in its ring.
2. Place the specimen in its ring on the watch glass or tray and weigh immediately to 0.1 g, m₁. Determine the initial mass of the specimen, m₀       m₀ = m₁ – m_ring  – m_tray
3. Take a sample of soil similar to that in the ring for the determination of initial moisture content, and if required, the particle density. The trimmings from the sample preparation are suitable.

Preparation and assembly of apparatus

Preparation of the porous plates

1. Clean the surface using a natural bristle or nylon brush.
2. Ensure that the pores are not clogged by fine soil particles and that the plates are readily permeable to water.
3. Saturate the pores by boiling in distilled water for at least 20 min, either overheating at atmospheric pressure, or in a vacuum desiccator in which the pressure has been reduced to about 20 mm of mercury.
4.  For saturated soils, or for soils that do not exhibit a high affinity for water, keep the plates saturated in de-aerated water until required for use. Immediately before assembly in the consolidation cell remove free surface water with a tissue, ensuring that the pores remain saturated.
5. For soils that readily absorb water, allow the plates to air dry.

Assembly of consolidation cell

1. Place the bottom porous plate centrally in the consolidation cell.
2. Place the specimen contained in its ring centrally on top of the porous plate.
3. Assemble the cell components so that the consolidation ring is laterally confined and in correct alignment.
4. Place the top porous plate and loading cap centrally on top of the specimen.

Assembly in the load frame

1. Place the consolidation cell in position on the bed of the loading apparatus.
2. Adjust the counterbalanced loading beam so that when the load- transmitting members just make contact with the loading cap the beam is slightly above horizontal position.
3. Add a small weight to the beam hangar, sufficient to maintain contact between the load-transmitting members while final adjustments are made. The resulting seating pressure on the specimen shall not exceed 2 kN/m² (kPa).
4. Clamp the compression gauge securely into position so that it can measure the relative movement between the loading cap and the base of the cell. Arrange the gauge to allow for the measurement of a small amount of swelling of the specimen, while the greater part of the range of travel allows for compression. Record the initial reading of the gauge.

Test procedure

Loading sequence. A range of pressures selected from the following sequence has been found to be satisfactory.

6, 12, 25, 50, 100, 200, 400, 800, 1600, 3200 kN/m² (kPa).

The loadings of 1600 and 3200 kN/m² should be considered only for stiff and overconsolidated clays.

A typical test comprises four to six increments of loading, each held constant for 1 h, and each applied stress being double that of the previous stage.

The last increment of loading shall be held for 24 hours.

Application of pressure

1. Record the compression gauge reading as the initial reading for the load increment stage di.
2. Apply the required pressure to the specimen at a convenient moment (zero time) by adding the appropriate weights to the beam hanger without jolting. Remove the weight used for the seating load.
3. Fill the consolidation cell with water after applying the pressure. If the specimen begins to swell, or if the compression virtually ceases within a short time, proceed to the next higher pressure. Alternatively, if required, determine the swelling pressure.
4. Take readings of the compression gauge at suitable intervals of time. The following periods of elapsed time from zero are convenient.

0, 8, 15, 30 seconds

1, 2, 4, 8, 15, 30 minutes, and 1 hour

2, 4, and 24 hours for the last load increment

5. Plot the compression gauge readings against square root time, while the test is in progress, either manually or by an automatic recorder.
6. Maintain the pressure for 1 hour and plot the readings to confirm that t₉₀ has been reached.
7. Record the time and compression gauge reading at the termination of the load increment stage d_i. This reading becomes the initial reading for the next stage.
8. Increase the pressure to the next value in the selected sequence.
9. Repeat further stages of the sequence of loading, making at least four stages in all. The maximum pressure applied to the specimen shall be greater than the effective pressure which will occur in situ due to the overburden and proposed construction. The last stage of load increment shall stay on for 24 hours, and readings are taken at suitable intervals including 2, 4, and 24 hours.

Unloading

Normally unloading is done in one step. Record and plot the final reading, and proceed to “Dismantling”.

If the unloading curve is required, the specimen shall be unloaded from the maximum pressure in steps as follows:

Reduce the pressure to a value not less than the last but one value of the loading sequence at a convenient moment (zero time).

Record the reading of the compression gauge at convenient intervals.

Plot the reading so that the completion of swelling can be identified. Step 4: Record the final reading of the compression gauge.

Repeat points 1 to 4 at least twice more, finishing with an applied pressure equal to the swelling pressure (if applicable) or to the initially applied pressure.

When the compression gauge indicates that equilibrium under the final pressure has been reached, proceed to “Dismantling”.

Dismantling

Drain off the water from the cell. Allow to stand for 15 min to enable free water to drain from the porous plates.

Mop up any excess water from within the cell.

Remove the load from the specimen and remove the consolidation cell from the apparatus.

Dismantle the cell, and weigh the specimen in its ring on the weighed watch glass or tray.

Transfer the specimen and ring on the watch glass or tray to the oven maintained at 105^oC to 110^oC, dry the specimen to constant mass and determine the dry mass of the specimen to 0.1 g (m_d).

Calculations and plotting

1)       Calculate the initial moisture content, w₀ (in %), from the specimen trimmings.

2)       Calculate the initial bulk density, ρ, from the equation

Where

 m₀     is the initial mass of the specimen (in g):

A        is the area of the specimen (in mm²):

H₀      is the initial height of the specimen (in mm).

3)       Calculate the initial dry density, ρ_d, from the equation

4)       If it is required to plot the void ratio against pressure calculate the initial voids ratio e₀ from the equation

 Where

ρ_s  is the particle density

5)       The initial degree of saturation S₀ (%) may be calculated as a percentage from the equation

This value can be used to indicate whether the test specimen is fully saturated initially.

Compressibility characteristics

The compressibility characteristics may be illustrated by plotting the compression of the specimen as ordinate on a linear scale against the corresponding applied pressure p (in kPa), as abscissa on a logarithmic scale. Compression is usually indicated in terms of voids ratio, but the actual thickness of the specimen or the strain expressed as a percentage reduction in thickness referred to the initial thickness, may be used as alternatives.

1)       Calculate the equivalent height of solid particles, H_s (in mm), from the equation:

H =   H₀

2)       Calculate the height of the specimen, H (in mm), at the end of each loading or unloading stage from the equation

H = H₀ – ΔH

Where

ΔH      is the cumulative compression of the specimen (reduction in height) from the initial height as recorded by the compression gauge:

3)       Calculate the Voids ratio, e, at the end of each loading or unloading stage, if required, from the equation

4)       Calculate the Coefficient of volume compressibility, m_v (in m₂/MN), for each loading increment from the equation

Where

H₁      is the height of the specimen at the start of a loading increment (in mm):

H₂      is the height of the specimen at the end of that increment (in mm):

P₁       is the pressure applied to the specimen for the previous loading stage (in kN/m² = kPa).

 P₂      is the pressure applied to the specimen for the loading stage being considered (in kN/m² = kPa).

5)       If required plot values of voids ratio as ordinate against applied pressure on a logarithmic scale as abscissa. Draw smooth curves through the points for both the loading and the unloading portions. If the swelling pressure was measured, the curves will start and terminate at the swelling pressure. Indicate the value of the initial voids ratio, e₀, on the vertical axis.

Coefficient of consolidation

 General. Two curve fitting methods are recognized for evaluating the Coefficient of consolidation, C_v, namely

•        The logarithm-of-time curve-fitting method, and

•        The square root time curve-fitting method.

Square root time curve-fitting method

1.       Draw the straight line of best fit to the early portion of curve (usually within the first 50% of compression) and extend it to intersect the ordinate of zero time. This intersection represents the corrected zero point, denoted by d₀.

2.       Draw the straight line through the d₀ point which at all points has abscissa 1.15 times as great as those on the best fit line. The intersection of this line with the laboratory curve gives the 90% compression point d₉₀.

3.       Read off the value of t₉₀ from the laboratory curve corresponding to the d₉₀ point and calculate the value of C_v (in m²/year), from the equation

C_v = 0.112 H² /t₉₀     [m²/year]

Where

H is the average specimen thickness for the relevant load increment (in mm), i.e.

t₉₀ is expressed in minutes.

Coefficient of secondary compression

The Coefficient of secondary compression, C_sec, may be derived from the laboratory logarithmic of the time curve. The duration of the load increments may have to be extended to up to 1 week, however. The derivation is not included in this procedure, ref. BS1377 for description.