Apparatus and method for cyclic simple shear testing of soil samples
BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     According to the present invention, soil is subjected to cyclic simple shear testing by placing similar soil samples on opposite sides of a loading plate. Each sample should be confined in such a manner that it can be deformed about an axis normal to the plate. A first load is applied to the samples by pressing them toward the loading plate, while the plate is moved transversely in a cyclic manner to apply a second load. 
     As the testing proceeds, the loads are applied simultaneously while both loads and the transverse plate displacement are monitored, preferably using a conventional arrangement of transducers for this purpose. It should be noted that no carriage or equivalent travel-permitting mechanism is required to accommodate the desired loading plate movement and deformation of the samples. Both samples remain fixed at their outer ends and their inner ends move with the loading plate as sample deformation takes place. 
     To apply the first load, the samples and loading plate can be positioned between a piston and a stationary member. In a preferred arrangement, the samples are confined and positioned with respect to the loading plate by stacks of rings, each ring being free to slide on the others to permit deformation of the samples. 
     Other features and advantages of both the apparatus and method of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, principles of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of apparatus constructed in accordance with the invention, for cyclic simple shear testing of soil samples, portions of the apparatus being shown in section; and 
     FIG. 2 is an enlarged cross-sectional view taken horizontally through one of the soil samples and its confining structure, taken along the line 2--2 of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An apparatus 10 that embodies many aspects of the present invention, illustrated in FIG. 1 of the accompaying drawings, utilizes two similar samples 12 and 14 of the soil to be subjected to cyclic simple shear testing. In general, it comprises a loading plate 16 located between the samples 12 and 14, an actuator 18 for applying a dynamic load to the sample, a piston 20, and a stationary member 22 that opposes the piston so that a static normal load can be applied to the samples. 
     The loading plate 16 is generally disk-shaped with its flat loading surfaces 24 and 26 oriented horizontally. The samples 12 and 14 are also disk-shaped, as shown in FIG. 2, and are located on opposite sides of the loading plate 16, bearing against its flat surfaces 24 and 26. 
     Supporting the lower sample 14 is the stationary member 22 which has a horizontal top surface, so that the sample is positioned vertically between the stationary member and the lower loading plate surface 26. The upper sample 12 is held between the horizontal bottom surface of the piston 20 and the top 24 of the loading plate 16. The piston 20 is slidably received for vertical movement within a pneumatic cylinder 28. When air is admitted to the cylinder 28, through an inlet 30, the piston 20 is urged downwardly toward the stationary member 22 and both samples 12 and 14 are pressed against the loading plate 16. In this manner, a vertical static load, normal to the loading plate 16, is applied to the samples 12 and 14. A transducer 32 attached to the stationary member 22 senses the magnitude of this static load. 
     The actuator 18, in this exemplary embodiment, includes a double acting piston 34 movable horizontally within a cylinder 36 and connected to the loading plate 16 by a rod 38. The piston 34 reciprocates within the cylinder 36 as fluid is admitted through inlets 40 and 42 on opposite sides. The fluid flow can be controlled by a conventional flapper valve (not shown). 
     Reciprocation of the actuator piston 34 causes transverse cyclical movement of the loading plate 16 in a horizontal plane normal to the direction of the static load. The magnitude of this dynamic load, or shear load, is sensed by a transducer 44 positioned between piston 34 and the rod 38. Transverse movement of the loading plate 16 in response to the dynamic load, i.e., shear strain, is sensed by a transducer 46 disposed between a fixed support 48 and the loading plate 16 on the opposite side of the plate from the actuator 18. 
     During the test, the samples 12 and 14 are positioned adjacent the top and bottom surfaces 24 and 26 of the loading plate 16 but are each permitted to deform about a vertical axis normal to the loading plate as the plate moves transversely. The surfaces 24 and 26 of the loading plate, the stationary member 22 and the piston 20 that contact the ends of the samples 12 and 14 are roughened so that the sample is frictionally held against free transverse movement during testing. 
     Each sample 12, 14 is confined within a stack of Teflon coated steel rings 50. As the loading plate 16 moves transversely, the rings 50 slide on each other to permit deformation of the samples 12 and 14. At the inner ends of the samples 12 and 14, the rings 50a closest to the loading plate 16 are positioned by short annular walls 52 and 54 that surround the upper and lower loading plate surfaces 24 and 26. The rings 50b at the outer ends of the stacks are positioned by similar annular walls 56 and 58 carried by the piston 20 and the stationary member 22, respectively. The rings 50a and 50b interlock loosely with the annular walls 52, 54, 56 and 58, permitting sufficient transverse movement that the samples 12 and 14 are held during testing by friction alone. 
     It will be apparent that the readings obtained by monitoring the outputs of the transducers 32, 44 and 46 give an indication of soil response that is substantially uneffected by the characteristics of any carriage or other moving part supporting the samples, all transverse movement of the loading plate 16 being permitted by the deformation of the samples 12 and 14 themselves. The transducer outputs may be recorded graphically or displayed as digital readouts, as desired. Considerations of surface finish do not prevent testing at low horizontal displacement levels. In fact, a low displacement, high frequency, electro-magnetic actuator can be substituted for the hydraulic actuator 18 described above. There is no variation in the distance across which the static load is applied, even at high displacement levels. The apparatus 10 is also simple, reliable and convenient to use. 
     While a particular form of the invention has been illustrated and described, it will also be apparent that various modifications can be made without departing from the spirit and scope of the invention.