The present invention relates to soil testing, and more particularly to a method and apparatus for cyclic simple shear testing.
There are many situations in which it is desired to test accurately and reliably the ability of soil samples to withstand shear producing dynamic loads. Such tests can be of crucial importance in, for example, seismic risk studies and the design of foundations for gravity-type offshore platforms and compressors. Soil responses that can be investigated in this way include cyclic deformation, compaction, liquefaction, and permanent deformation.
It is generally agreed that simple shear testing, in which uniform shear is applied over the full height of a sample, is preferable to direct shear testing, in which the shear is confined to a single plane. It should not be confused with triaxial testing in which shear is induced by compression or extension loadings. Triaxial testing has generally been conceded to be less representative of actual conditions but has been used, with correction factors applied, because triaxial equipment has been more convenient to operate or more readily available.
Cyclic simple shear testing that has been done in the recent past has involved the use of a horizontally movable carriage on which the sample to be tested is positioned. The carriage is generally mounted on wheels, but vertical legs that flex to permit horizontal movement can also be used. The sample is confined while deformation is permitted by a stack of metal rings that can slide horizontally on each other or by a wire-reinforced membrane. A piston then applies a vertical load to the top of the sample which is moved horizontally in a cyclic manner while the loads and displacement are measured electronically.
Carriage movement required by this known arrangement necessarily introduces errors, due in large measure to the friction of the carriage wheels when high vertical loads are imposed. Moreover, there is a lower limit of horizontal displacement, usually about one ten-thousandth of an inch, below which surface finish limitations prevent meaningful readings from being obtained. If flexible legs are substituted for the carriage wheels, large horizontal displacements result in distortion due to changes in the height of the sample with corresponding changes in the vertical load. Under some circumstances, a reduction in height of one thousandths of an inch can cause the vertical load to fall to zero. Other distortions present in readings conventionally obtained by cyclic simple shear testing are attributable to the inertia of the carriage which must have substantial mass if it is to withstand the loads.
It is a primary objective of the present invention to provide an improved method and apparatus for cyclic simple shear testing that is free of many of the above-mentioned disadvantages and limitations inherent in previously known techniques.