Oedometer

from Wikipedia, the free encyclopedia

An oedometer or "one-dimensional compression device " is a soil mechanical experimental device in which the compressibility of soils can be examined. The test according to DIN 18135 is called "one-dimensional compression test". A soil sample is loaded vertically, whereby the side stretching is prevented. From this experiment, the stress-dependent stiffness ( stiffness modulus ) and, in the case of low permeability, fine-grained soils, parameters that determine the consolidation properties of the soil can be determined.

Experimental device

The soil sample has a cylindrical shape and, in a standard device, an area of ​​40 cm² with a diameter of slightly over 70 mm and a height of 20 mm. However, other sample dimensions are also possible. Larger sample heights and diameters are selected in particular for the investigation of coarse-grained soils (sand and gravel). According to DIN 18135, the diameter of the largest grain should be smaller than 10 times the sample height. The height of the sample to the diameter of the oedometer should not be less than 1: 5, in order to minimize the adverse effects of friction losses. The soil sample can be produced in the oedometer by kneading or brushing (cohesive soils) or trickling in (non-cohesive soils) (also known as “reconstituted samples”). In the case of non-cohesive, coarse-grained soils, a certain storage density or, in the case of cohesive, fine-grained soils, a consistency , according to which the installation or sample production is to be carried out, is often defined as the initial state . Alternatively, a soil sample from a direct subsurface exposure can be used. For this purpose, the sample is as undisturbed as possible with a metal ring. B. worked out from a drill core. The end surfaces of the sample are flat and there are filter plates with which the emerging pore water is drained off.

Test execution

Carrying out the test in accordance with DIN 18135 provides for the soil sample to be loaded in stages in the axial direction. It is suggested that the axial tension be doubled every 24 hours. In order to be able to make statements about the mechanical behavior of the soil during relieving and reloading, the sample can be relieved and reloaded several times if necessary. In standard tests, the time-dependent axial deformation of the specimen is recorded over 24 hours for each load level. The information on the time-dependent course of deformation is particularly important for evaluating the consolidation properties and the viscosity of the soil. If the soil is very permeable and only information about the stress dependency of the deformation is to be obtained in the experiment, the sequence of loading steps can also take place faster than 24 hours. However, you should always wait for the end of the primary settlements. During tests with unloading and reloading cycles, the corresponding subsidence and uplift of the specimen are recorded.

In the area of ​​scientific investigations, the implementation of deformation or strain rate-controlled oedometer tests is also widespread. In these tests, a constant rate of compression (or rate of expansion) is applied to the sample. In the case of poorly permeable soils, it is important that the pore water pressure in the sample remains small compared to the effective stresses. This specification is usually checked by measuring the pore water pressure at the base of the sample. The consolidation thus takes place on one side towards the top of the sample. For the exact measurement of the pore water pressure, the forced saturation by a background pressure (saturation pressure or back pressure) is necessary. This makes a closed and pressure-tight construction of an oedometer cell necessary. The implementation of deformation-controlled oedometer tests offers, in addition to the often faster execution of the test, the possibility of directly identifying viscous influences on the deformation resistance of the sample. This requires the step-wise abrupt change in the applied strain rate. Depending on its viscosity, a soil sample reacts with an increase in the expansion rate with a higher resistance to deformation, which decreases accordingly when the expansion rate is reduced.

In addition to the investigation of soil behavior under stress-dependent and time-dependent deformations, the change in shape of a soil sample in the oedometer with a change in the water content (or saturation) and / or the suction tension is observed in special experiments. Experiments on the swelling behavior of a soil (swelling or swelling pressure test) and the detection of the potential for subsidence in the event of saturation can serve as examples. Furthermore, the coupled mechanical and hydraulic behavior of partially saturated soils is examined in special oedometer apparatus. It is usually necessary to measure the amount of water emerging from the sample or absorbed by the sample.

evaluation

The pressure-settlement diagram or the “working line” of the soil is drawn from the measured settlement at various associated vertical stresses. The slope of the tangent on the pressure-settlement curve is the stiffness modulus. It is usually greater when a sample is loaded again than when it was first loaded. In addition, the following can be derived from the experiment: compaction coefficient , compaction coefficient , compression coefficient , threshold coefficient , recompression coefficient and soil permeability . The creep coefficient and the consolidation coefficient can also be determined with the aid of the temporal course of settlement during the load levels.

See also

Web links