The oedometric test
The oedometric test is used to study the one-dimensional compressibility characteristics (zero lateral deformation) of a cohesive soil in the laboratory. The data obtained is of utmost importance for hypothesizing the behavior of a soil subjected to a change in stress state from the in situ state (e.g., oedometric stresses such as foundation loading).
Description and procedure
The test is performed in the oedometer, which consists of a rigid steel ring inside which a cylindrical soil specimen (undisturbed sample) is inserted. Porous septa are placed on the ends of the specimen to allow drainage of the water contained in the specimen in the vertical direction only. The whole is placed in a cell that is filled with water to constantly maintain a fully saturated condition. A rigid plate is placed over the upper porous septum of the specimen where a vertical load is applied and progressively increased. From the instant the vertical pressure is applied, the vertical settlement of the specimen is measured.
Figure 1 – Oedometer compression test equipment: A) Bishop-type oedometer; B) Oedometer cell with steel die (consisting of: lower and upper porous stone, filter paper, die containing specimen); C) Comparatorre
Since the granules of the solid part of the sample and the water occupying its interstitial spaces can be considered incompressible, the change in volume recorded during the compression step is due solely to the expulsion of the inter granular water, i.e., pore shrinkage.
For practical purposes, it is like a soil layer placed between two permeable layers subjected to a very large load (one-dimensional condition). Consequently, the oedometer can serve to reproduce in the laboratory what might happen in situ.
Charts
As part of the same test and on the same specimen, several compression steps are repeated with increasing pressures, leaving each load for the time (24-48h) required for the specimen’s settlements to stabilize. In this way, several load steps are conducted and, for each, an oedometric consolidation curve (settlement-time curve) can be determined.
Figure 2 – Oedometric consolidation curve – Semilogarithmic chart (on the x-axis is the logarithm of time)
The following data can be obtained from the oedometric consolidation curve:
- rate of consolidation
- extent of settlement
- primary consolidation coefficient Cv (Method of Casagrande and Taylor)
- secondary consolidation (Casagrande’s Method)
From the oedometric test, it is also possible to obtain the oedometric compressibility curve, which is much more interesting and useful for the purposes of analyzing the deposit’s stress history and calculating settlements. In this graphical representation on the x-axis are the vertical stresses while on the y-axis are the voids index.
Figure 3 – Oedometric compressibility curve – Semilogarithmic chart (on the x-axis is the logarithm of the pressures)
From this chart it is possible to derive the index of:
- compressibility Cc
- recompression Cr
- swelling Cs
These indices are necessary for calculating settlements/swelling and for calculating the preconsolidation pressure (σc), which gives an indication of the geologic history of the deposit.
Purposes
It is carried out on those soils subject to overloading due to foundations, road embankments, etc. because through this test we want to know the behavior of the soil following an increase in load. Specifically, oedometric tests allow us to:
- determine the stress-strain relationship
- determine the deformations over time
- reconstruct the stress history of the deposit from which the sample was taken
For which soils?
Fine-grained soils: silts and clays.
Parameters obtainable from the oedometric test
The following is a summary table within which the parameters obtainable from the charts derived from the oedometric test are shown.
Figure 4 – Parameters and charts
In addition, it is also possible to obtain the tension-strain curve chart (not shown in the article) from which the oedometric modulus Eed (kPa) and the permeability coefficient k (cm/s) can be obtained
Reference standard
- AASHTO T 213-83
- ASTM D 2435-80, D 4186-82, D 4546-85
- BS 1377: Part 5 1990
- UNI CEN ISO/TS 17892 – 5
Figure 5 – Laboratory instruments: oedometers
Other tests
The results of the oedometric test become even more interesting when combined with the results obtained from physical laboratory tests such as:
- Particle size analysis: through this the classes that make up the soil are identified, in which case it is important to understand the percentage of fine present.
- Atterberg limits: which provide the limits and indices. These, related to water content, will provide insight into the state of the soil and an understanding of the extent of the settlements.
- Volume weight of the solid (and natural volume weight) is needed to calculate the initial void index (e0) and degree of saturation (S).
GeoStru Software
Considering what has been described, in order to best deal with studies involving aspects of settlements, we particularly recommend the software: Loadcap
In addition, we can also use the Stratigrapher software that allows us to enter information from laboratory analysis performed on soil samples.
