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Observations and Nomenclature of Shear Bands Professor, Department of Civil and Environmental Engineering, University of Nevada, Nick Huydma Assistant Research Professor, Department of Civil and Environmental Engineering, Burçin Avar PhD Student, Department of Civil and Environmental Engineering, University of Nevada, |
ABSTRACT
Shear bands were observed in the granular layers of deformed specimens composed of alternating layers of granular and cohesive materials. Several purely granular specimens were prepared and deformed in a specially designed apparatus in order to observe shear band formation. During deformation, visible shear bands could be seen forming in the specimens, as documented in the included digitized video. Three categories, primary, secondary and tertiary, shear bands were identified. The names are based on the orientation and development location of the shear bands.
KEYWORDS: Shear Band, Granular Soil
INTRODUCTION
Shear bands are well defined narrow regions of intensely sheared material in which significant decreases in density have occurred (Scarpelli and Wood, 1982 and Vermeer, 1990). Recently there has been a lot of research activity in the area of granular material deformation and shear band formation. For example, Finno et. al. (1997) and Alshibli and Sture (2000) have used biaxial test equipment with video analysis and Saada et. al. (1999) used hollow cylinders with video analysis to investigate the formation of shear bands. In all three of these studies, the granular materials were emplaced in latex membranes and grids were drawn directly on the membrane (Finno et. al.,1997 and Alshibli and Sture, 2000) or directly on the specimen (Saada et. al., 1990). In these cases, the video recorded the deformation of the latex membrane against the deforming granular material.
The occurrence of shear bands within granular layers of deformed specimens composed of alternating layers of granular and cohesive materials, Figure 1, prompted the deformation of purely granular specimens to observe the formation of shear bands. The specimens were deformed in a specially designed apparatus. The apparatus allowed video recording of the specimen deformation but was not configured to record the force required to shear the specimens.
Figure 1. Shear Band Observed in Granular Layers of Laboratory Produced Layered Soil Specimens
This paper describes three categories of shear bands and a naming scheme for the shear bands observed in this study. An included digitized video shows the formation of one shear band within a deforming specimen. A layered granular specimen was deformed to identify the sense of movement that occurs along shear bands.
Two types of materials were used in this study, a granular (sand) material and a cohesive (clay) material. The granular material that was chosen for this study was Ottawa Foundry Sand F-110. It is silica sand that is available from the U.S. Silica Company. The grains are rounded and have a specific gravity of 2.65.
The classification (USCS) of the sand is SP, poorly graded sand. The grain size distribution curve is presented in Figure 2. The sand was dyed pink, gray and blue using stamp pad ink
Figure 2. Grain Size Distribution of Granular Material
The cohesive material that was used to produce the specimens was specially blended clay. The clay was a mixture of kaolin clay (50%), pyrophyllite clay (20%) and feldspar particles (30%). These materials were mixed together at a water content of approximately 23%. The clay mixture had a Liquid Limit of 42% and a Plastic Limit of 22%. The mixture produced an off-white colored cohesive material that could easily be trimmed with a wire saw.
The specimens were produced in a clear acrylic tube that was 102 mm in length and had a diameter of 50.8 mm. The specimens were tested in an upright position so each of the specimens had a bottom clay layer to hold in the sand. The sand was placed in the acrylic tube, inundated with water and the sides of the tube were tapped with a rubber hammer to increase the density of the sand.
APPARATUS
The design of the deformation apparatus is presented in Figure 3. The outer tube is a steel cylinder that has an inside diameter of 57.2 mm, is 230 mm in length and 10 mm thick. A bottom stage is connected to the plunger that ensures that the top and bottom movement of the specimen are synchronous. The bottom guide ensures that the bottom stage will remain perpendicular to the shearing direction. An elliptical viewing window has been cut into the outer tube. At its widest and longest axis, the window is measures 50.8 mm and 102 mm respectively. The centerline of the window is in line with the centerline of the specimen. The viewing window allows video recording of the specimen during deformation.
Figure 3. Design of Deformation Apparatus
A small load frame was used to provide the force to shear the specimens. The deformation apparatus was placed on the platen of the load frame. A variable rate motor controlled the movement of the platen, which was kept constant at 1.6 mm per minute. A digital dial indicator was attached to the load frame. The indicator measured the movement of the platen, which indirectly measured the displacement of the plunger into the soil specimen.
A camera stand was constructed and attached to the platen of the load frame. An 8-mm camcorder was used to record the specimen deformation and the digital dial indicator. The dial indicator was recorded so that deformation events could be linked to the displacement of the plunger. A circular-polarizing filter was added to the camcorder. The filter reduced or eliminated the glare produced by light reflecting from the surface and/or scratches on the acrylic tube. Figure 4 contains a sketch of the deformation apparatus, load frame, camera stand, camcorder and digital dial indicator.
Figure 4. Setup of the Deformation Apparatus, Load Frame, Camera Stand, Camcorder and Digital Dial Indicator
RESULTS
Three categories of shear bands were identified during this study. The three categories of shear bands have been named primary shear bands, secondary shear bands and tertiary shear bands. These names are based on the orientation of the shear bands and where the shear bands form within the deforming specimen.
Primary shear bands form in the direction of shearing, in this case vertically. Secondary shear bands are oriented at an acute angle, approximately 40 degrees, to the direction of shearing. Secondary shear bands are further classified as either continuous or terminating. Continuous secondary shear bands form a visible band that is continuous across the passive side of the specimen. Terminating secondary shear bands end when the shear band ceases to continue to form or the shear band encounters a clay layer. Tertiary shear bands form off of primary or secondary shear bands, typically in the direction of shearing. Figure 5 contains photographs showing the categories of shear bands identified in this study.
Figure 5. Photographs Showing the Three Catagories of Shear Bands
The deformation apparatus allows video recording of specimen deformation. Movie 1 (404 kb Quicktime movie) contains a video recording of a deforming sand specimen. In the video, the sand portion of the specimen is purple and the clay portion of the specimen is white. The digital dial indicator records the movement of the plunger into the specimen.
Due to the size of the digitized video, only small portions of the test could be presented. Movie 1 begins with the plunger displaced 2.32 mm into the specimen. One continuous secondary shear band has already formed within the specimen. As the plunger is forced into the specimen, another continuous secondary shear band begins to form.
A second video clip, Movie 2, (666 kb Quicktime movie) shows the formation of two terminating secondary shear bands. The video begins with the plunger already displaced 4.42 mm into the specimen and two continuous secondary shear bands already formed. As the plunger is forced into the specimen, another secondary shear band forms. This shear band continues to form until the plunger is approximately 5.16 mm into the specimen. After that, the shear band ceases to form. At a plunger displacement of approximately 5.42 mm, another shear band is seen forming near the plunger. This shear band forms and eventually terminates on the bottom clay layer. A granular specimen with colored layering was also deformed as part of this study.
In granular specimens that were made up of one color of sand, it is assumed that there is movement along the shear band(s) but it is impossible to determine the sense of movement. It was hoped the colored layering would help determine the sense of movement along the shear band. The deformed granular specimen with colored layering is shown in Figure 6. Movement is easily seen in the specimen because of the off-set of the sand layers along the secondary shear bands.
Figure 6. Shear Bands Indicating Sense of Movement
CONCLUSION
A deformation apparatus used for analog modeling experiments was used to deform granular specimens to observe shear band formation. The apparatus did not record the force required to shear the specimens however, it allowed the use of a video camera to capture specimen deformation. Three categories of shear bands were observed, primary shear bands which formed in the direction of primary shear, secondary shear bands which formed at an acute angle to the primary shearing direction, and tertiary shear bands which form off of primary or secondary shear bands, typically in the direction of shearing.
A digitized recording of a specimen undergoing deformation documented the formation of shear bands. Using the video recording, the sequence of shear band formation can easily be documented. A special specimen consisting of layers of different colored sand was deformed in the apparatus. This specimen showed that there was movement along the shear bands and direction of relative movement could be determined.
The authors wish to thank the US Silica Company for providing the granular material used in this study.
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