In 1925, Terzaghi proposed a theoretical approach to the consolidation process, and he had already designed the first consolidation
apparatus which he named an ‘oedometer’ (from the Greek oidema, swelling).

In the early 1930s, consolidation tests on specimens of various sizes were carried out in the USA and were reported by Casagrande (1932), Gilboy (1936), and Rutledge (1935). The mathematical theory of consolidation was published by Terzaghi and Frohlich in 1936.

In 1938, Skempton at Imperial College, London, developed an oedometer for a 1 in thick specimen based on the Casagrande principle, using a bicycle wheel to support the beam counterbalance weight. A more compact oedometer, for a specimen 3 in diameter and 3/4 in
high, was designed by Nixon in 1945, and four of these were mounted on one bench. Other machines, based on the same principle, were developed by the leading manufacturers of testing equipment, and many are still in use today.

When oedometer consolidation testing became recognised as a standard laboratory procedure after 1945, two types of oedometer cell were developed, known as the fixed-ring cell and the floating-ring cell.

In the fixed-ring cell, the mould into which the specimen was transferred from the cutting ring was clamped in the cell on top of a lower porous disc having a diameter larger than that of the specimen. The upper porous disc, immediately beneath the loading cap, was fractionally smaller so that it could enter the ring as the specimen consolidated. Only the top surface of the specimen was displaced during consolidation.

In the floating-ring cell the ring into which the specimen was initially trimmed was used to hold the specimen during the test, but it was supported only by friction from the specimen itself. Both the upper and lower porous discs were slightly smaller than the inside diameter of the ring, so that the specimen was compressed about equally from top and bottom, and it was claimed that the amount of side friction was half that of a fixed ring cell (Lambe, 1951). The floating ring cell was cheaper than the fixed-ring type, and another advantage was that specimen disturbance was less because transfer from ring to mould was eliminated. Its disadvantages were that only a light ring could be used to hold the specimen, and therefore it was likely to undergo some lateral deformation under high pressure; and that the weight of the ring could cause some disturbance to soft clays. In addition this type of cell could not be adapted for making direct measurements of permeability.

References:

Head, K. H. (1994). Manual of soil laboratory testing. Volume 2: Permeability, Shear Strength and Compressibility Tests (No. Ed. 2). John Wiley & Sons.