Soil Consolidation and Oedometer Test

What is Soil Consolidation?

Soil Consolidation refers to the process in which the volume of a saturated (partially or fully) soil decreases due to an applied stress. The term was introduced by Karl von Terzaghi also known as the "father of soil mechanics and geotechnical engineering". Terzaghi established the one-dimensional consolidation theory and changed the definition of the term since it was previously associated (and still is, in geosciences) with the compaction of clay sediments that formed shales.

When a load is applied in a low permeability soil, it is initially carried by the water that exists in the porous of a saturated soil resulting in a rapid increase of pore water pressure. This excess pore water pressure is dissipated as water drains away from the soil’s voids and the pressure is transferred to the soil skeleton which is gradually compressed, resulting in settlements. The consolidation procedure lasts until the excess pore water pressure is dissipated.

The increment of applied stress that causes consolidation may be due to either natural loads (e.g. sedimentation processes), or human-made loads (e.g. the construction of a building or an embankment above a soil mass) or even the decrease of the ground water table.

Duration of Consolidation

The duration of the consolidation process is a critical issue and highly depends on the permeability of the soil subjected to the load and on the drainage paths. In general, consolidation in sandy soils is a quick process (occurring possibly immediately during construction) whereas the process may last for many years or even decades in clay soils. 

The consolidation procedure is commonly separated into 3 stages:

1. Initial consolidation: A quick volume loss of the soil mass associated with the application of external stress that compresses the air inside the soil’s voids.
2. Primary consolidation: Soil settlement during which the excess pore water pressure is transferred to the soil’s skeleton
3. Secondary consolidation: A subsequent settlement procedure that occurs after primary consolidation and is associated with internal changes in the soil’s structure while subjected to nearly constant load. This process is commonly referred to as creep.

The Oedometer Test

The simplest case of consolidation examined is the one-dimensional consolidation. In this case, the lateral strain of the soil mass is neglected. The testing procedure to quantify the critical soil properties associated with soil consolidation is the Oedometer Test. The term “Oedometer” derives from the Ancient Greek language and means “to swell”. The test is one of the most commonly conducted, and important, laboratory tests in geotechnical engineering. The Oedometer Test aims at measuring the vertical displacement of a cylindrical, saturated soil sample subjected to a vertical load while it is radially constrained. In the subsequent test, the incremental loading consolidation test is described. Note that there is also a constant rate of strain (CRS) test, that nowadays is becoming more popular

Test Set-up Components

A typical Oedometer test set-up, shown in Figure 1, is composed of: i) a consolidation cell, ii) a loading frame, and iii) a deformation measurement mechanism.

The consolidation cell consists of the following components:

• Confining ring, placed circumferentially around the sample to restrict the lateral displacement
• Loading cap, to transfer the load to the soil specimen
• Reservoir, filled with water to ensure that the soil remains essentially saturated
• Porous stones, which are several orders of magnitude more permeable than typical samples of fine-grained soil. These stones enable the drainage of the water from the top and bottom of the specimen
• Filter papers, placed between stone and soil sample to prevent soil from clogging the pores of the stone

Typical diameter (D) to height (H) ratios of the soil samples are D/H = 3 - 4. The cross-section area of the soil specimen may be 20, 35 or 50 cm² (D = 5 - 8 cm) and its height is H = 2 - 2.5 cm.

The loading frame configuration is composed of a loading beam and dead weights. The configuration allows for a constant load to be maintained indefinitely. The application of the load causes deformation of the loading frame, the porous stones and the soil sample. Since the test is intended to measure only the deformation of the soil, the other movements (machine deflections) must be measured and later subtracted from the total deformation. This is achieved by measuring the deflection of the set up using an aluminum sample, which is characterized by linear elastic, and thus known, response.

The vertical deformation measurements of the soil specimen is performed using a dial gauge (most often) or an electronic instrument.

Figure 1: Typical Oedometer test set-up (photo from the National Technical University of Athens)

Testing Procedure

The typical testing procedure consists of the following steps:

1. Position the dial gauge (or electronic instrument)
2. Measure weight, height, diameter of the confining ring
3. Measure height (H) and diameter (D) of aluminum sample
4. Trim specimen into the confining ring
5. Take water content measurement from the trimmings
6. Weigh soil sample and confining ring
7. Soak porous stones and filter papers
8. Place the consolidation cell in the loading frame and adjust height. The loading beam should be almost horizontal.
9. Take initial reading (R_i – reading will be subtracted from all measurements)
10. Place seating load
11. Add water to the reservoir

The load is maintained for a period of 24 hours (in certain clays the required time is 48 hours) during which the soil consolidates with drainage from the porous stones. Afterwards, the applied load is increased incrementally by doubling the applied stress at each stage. The number of the load stages and the maximum stress applied depends on the stress range of interest. During the loading process, water is provided into the cell so that the specimen remains fully saturated. At each loading stage, readings of deformation are taken systematically to develop a time-settlement curve. That is, after the application of each load, the deformation is measured at 6, 15, 30 seconds, then at 1, 2, 4, 8, 16, 30 min and at 1, 2, 4, 8 and 24 hours, respectively. When the maximum load is reached, and possibly in a load increment in between, an unloading stage is introduced that may be conducted in one or multiple steps; typically, the load is reduced by a factor of 4 at each step. When the test is completed, the final height of the sample and its water content are measured.

Results and Parameters derived from the Oedometer Test

The following soil properties are derived from the Oedometer Test:

• The Pre-consolidation Pressure: The maximum effective stress that the soil specimen has sustained in its geological history.
• The Compression Index C_C: C_C is an index associated with the compressibility of the soil. In particular, it is measured as the slope of the curve between void ratio and effective stress. The void ratio is plotted in a normal scale whereas the effective stress in a logarithmic scale. A typical compression curve in terms of void ratio—effective stress is presented in Figure 2. The inclination of the “virgin” part of the curve denotes the Compression Index, C_C.

  Figure 2: Typical diagram of void ratio - effective stress correlation obtained by Oedometer Test. The Compression C_C and Recompression C_r indices are also presented.

  Therefore, C_C is:

  C_C = Δe / Δlog (σ')

  C_C usually ranges from 0.1 to 10 and has no units. For normally consolidated clays the index commonly ranges between 0.20 to 0.50 and for silts between 0.16-0.24. For sands, the index ranges between 0.01 to 0.06, although this is not a particularly meaningful parameter for a sand.

  Some empirical expressions that relate the Compression Index, C_C, with the Liquid limit (LL) and Plasticity Index (PI) of the soil, are the following:

  • C_c = 0.007(LL-10), (Skempton, 1944)
  • C_c = 0.009(LL-10), (Terzaghi and Peck, 1967)
  • C_c = 0.50×PI×G_s, (Wroth and Wood, 1978)
• The Recompression Index C_r: C_r is used to derive the compressibility of an over-consolidated soil and is derived using the slope of the rebound-recompression curve (Figure 2). For inorganic soils, C_r is 0.1-0.2 of C_C value.
• The Coefficient of Consolidation C_V: C_V is a parameterthat describes the rate at which the consolidation process evolves during a test. Typical values of the coefficient of consolidation are given in Table 1.

  Table 1: Typical values of the C_v coefficient

  SOIL	Cv (cm2/sec) x 10-4
  Soft blue clay (CL-CH) (Wallace & Otto, 1964)	1.6-26
  Chicago Silty Clay (CL) (Terzaghi & Peck, 1967)	8-11
  Mexico City Clay (MH) (Leonards & Girault, 1961)	0.9-1.5
  Organic Silts and Clays (OH) (Sivakugan,1990)	1-10

Determination of Coefficient of Consolidation, C_V

The coefficient of consolidation, C_V can readily be estimated from the time-settlement curve using graphical methods. There are two, most commonly used, methodologies:

1. Casagrande Logarithm of Time Fitting Method (Casagrande and Fadum, 1940):

   The coefficient of consolidation, C_V, is determined by estimating the time at 50% consolidation (t₅₀), as shown in the short animation/presentation below. Then, C_V can be estimated as:

   C_V = 0.917 * (H²_dr / t₅₀)

   where H_dr is the drainage path. Given the initial height of the specimen (H_i) and the compression of the soil sample at 50% consolidation (ΔΗ), the drainage path (for double drainage), H_dr, is computed as:

   H_dr = (H_i - ΔΗ) / 2

2. Taylor Square Root of Time Fitting Method (Taylor, 1948):

   In this method, the dial readings are plotted against the square root of time. The coefficient of consolidation, C_V, is determined by estimating the time at 90% consolidation (t₉₀), as shown in the short animation/presentation below. Then, C_V can be estimated as:

   C = 0.848 * (H²_dr / t₉₀)

   where H_dr is the average drainage path (typically, half the specimen height).

References

Leonards, G.A. and Girault, P. (1961). A Study of the One-Dimensional Consolidation Test, Proc. Fifth Int. Conf. on Soil Mechanics and Found. Eng., Paris, Vol. 1, 116-130.

Sivakugan, N., (1990).  Development of Marshy Areas in Colombo, Sri Lanka, Proc. Tenth Southeast Asian Geot. Conf., Taipei, Vol. 1, 469-472.

Skempton, A. W. (1944). Notes on the Compressibility of Clays. Q. J. Geol. Soc. London, 100(1-4), 119-135.

Terzaghi, K. and Peck, R.B., (1967). Soil Mechanics in Engineering Practice, John Wiley & Sons, New York, 729 pp.

Wallace, G.B and Otto, W.C. (1964). Differential Settlement at Selfridge Air Force Base, Jnl. Soil Mechanics and Found. Div., ASCE, Vol. 90, No. SM5, 197-220.

Wroth, C. P. and Wood, D. M. (1978). The correlation of Index Properties with Some Basic Engineering Properties of Soils.  Canadian Geotechnical Journal, 15, 137-145.