Stiffness and cyclic response of clayey fine sands

A substantial programme of cyclic testing has been carried out on a variable silty/clayey sand deposit from the central North Sea. The test programme included triaxial and simple shear tests, using intact and reconstituted specimens and different triaxial specimen sizes. Some tests included pre-shearing while others did not. The testing was carried out in three phases. In the first two phases of testing, large variations were found in the cyclic response of the soil. The third phase was initiated to address some of the questions raised by the first two phases. It comprised triaxial tests in which all but one specimen had internal axial strain measurement. This paper presents the principal results of the testing work. It is argued that the most important factor governing test specimen response is the shear stiffness of the specimen. The correlation between relative density (which was used as the criterion for reconstitution of specimens) and shear modulus was found to be poor. The variability of stiffness within specimens is a further important factor. Careful analysis of the specimen stiffnesses in the final series of tests has shown the importance of shear modulus and, more particularly, variations in shear modulus within the test specimen, in determining the cyclic response of the soil. In every case where closely similar soil specimens were compared, the softer sample developed excess pore pressure more rapidly than the stiffer sample. It is proposed that the cyclic response of laboratory specimens can be related to the in situ behaviour of the stratum by means of a comparison between the laboratory and in situ shear moduli. Changes to testing techniques are proposed to improve the quality of laboratory data. Relationships between pore pressure, number of cycles and shear stress ratio are presented for the reference site.