In order to use the advantages of unconfined compression tests, a new testing procedure in the field is proposed and a new portable unconfined compression test apparatus with suction measurement is outlined. From laboratory data, the effect of specimen size on unconfined compressive strength properties for natural clay deposits are discussed. In an engineering sense, it was proved that there was no difference in shear strength and deformation characteristics between the S (or Small size) specimen (15 mm in diameter (d) and 35 mm in height (h)) and the O (or Ordinary size) specimen (35 mm d and 80 mm h), which were examined for soils having plasticity indexes (I p) from 17 % to 150 % and unconfined compressive strengths (q u) from 20 kPa to 1000 kPa and at nineteen different sites in the United Kingdom, Korea and Japan.


The unconfined compressive strength (q u) is widely used for the stability analysis of clay foundations under undrained condition in Japan. This is mainly because the average value of q u/2 well describes the undrained shear strength on a failure surface in a ground in total, and in addition to this, the testing procedure for the q u-value is simple. The specimen size usually used in Japan for unconfined compression test (UCT) is the O (or Ordinary size) specimen (35 mm in diameter (d) and 80 mm in height (h)). However, for O specimens, the number of specimens required is remitted and their preparation for testing is difficult due to latent cracks or inhomogenity. Therefore, the small size specimen is better for effective use of samples because the cross sectional area of the specimen is about 10 cm2 and it is not necessary to retain it since calculations and measurements can be done via automatic electronic instruments.

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