Civil and Building Engineering.

To determine what would happen in the field, foundation engineers obtain soil samples from the field and test the samples in the laboratory. Because the problem involves only one-dimensional deformation and one-dimensional water flow, a one-dimensional consolidation test in the laboratory is appro-
priate.
The laboratory apparatus used for this purpose is called a one-dimensional consolidation cell or oedometer. An example of a consolidation cell is shown in Figure 3.6. The soil sample that is tested is a circular disk with a diameter about two to four times its thickness. Diameters of testing specimens are typically between 2 and 4.5 in. (5 to 10 cm). Such small diameters are used to reduce the cost of drilling, sampling, and laboratory testing. However, samples with larger diameters are preferred so that the properties measured in the laboratory are more representative of the soil deposit in the field.
The soil is encased in a rigid metal ring to prevent deformations due to lateral squeezing of the soil and to force the flow of water in the vertical direction. The soil sample is covered with a loading cap, and vertical loads are applied to represent applied vertical stresses in the field. The original vertical load applied on the sample in the field depends on the depth from which the sample was retrieved and the weight of the soil and fill. Since the weight and thickness of the fill may not be known during the testing phase (and for other reasons as well), a wide range of stresses will be applied in the laboratory to obtain a stress-strain curve that can be used for a range of loads in the field.
The soil is assumed to be in equilibrium under an applied load in the laboratory and that an additional load is suddenly applied. Also, this additional load is assumed to represent the weight of the load in the field. Settle-
ment of the top cap in the consolidation cell is measured using either a dial gauge or a linear transducer, with an accuracy of 0.0001 in. (0.0025 mm).
When a compressive load is applied to saturated clay, the test operator discovers that the settlement is initially very small; in fact, the initial magnitude of the settlement approximately equals the settlement that would develop if

Figure 3.6 Computer-controlled, one-dimensional consolidation testing equipment (photograph courtesy of Trautwein Soil Testing Equipment Company).


the soil sample was replaced with a rigid metal disc. This instantaneous settlement is caused by compression of the porous stones and other mechanical parts of the testing apparatus because the soil is nearly incompressible during the short interval after the application of stress. If the applied load is sustained, the test specimen compresses and the amount of compression change with time in a manner similar to that shown
in Figure 3.7. After a period of time, the compression essentially ceases and the soil is again in equilibrium. Similar settlement curves versus time are obtained for each level of loading, and the load is increased again. After the maximum consolidation stress of interest has been reached, the level of consolidation stress is lowered in stages, and the rebound of the specimen under each stage is measured, until the test specimen is unloaded completely.
For samples of usual dimensions in the laboratory, the time needed to achieve equilibrium varies from a few minutes for sands to several days for some clays. The delay is caused by the time needed for water to squeeze out of the specimen of soil. The more pervious soils come to equilibrium quickly, whereas relatively impervious clays are comparatively slow to equilibrate.
As in pipe-flow hydraulics, where the rate of water flow in a pipe depends on the diameter of the pipe, the rate at which water can be squeezed from a saturated specimen of soil depends on the sizes of the openings between the soil particles. Also, as in hydraulics, the rate of flow for a given pore (pipe) size decreases as the total flow distance increases, so the time required for equilibrium for a thick layer of soil in the field is much longer than for a

small specimen of the same soil in the laboratory. Delayed settlement can be a serious problem for the design engineer if the settlement is still continuing after a structure has been completed on a stratum of consolidating soil.