Fig. 2 Granules used in this study through crushing and dry-wet cycles
2.2 Test setup for limited shear
displacement
The progress and development of ring shear apparatuses have provided an
effective approach for exploring the dynamic shear process of quartz
granules under large shear displacement and high normal stress
conditions (Fukuoka et al., 2007; Kimura et al., 2019; Mair et al.,
2002; Tanikawa et al., 2012). Due to the difference in lithology between
mudstone and quartz, the shear behavior of soft interlayers may rely
more on the water content and weathering intensity rather than the shear
displacement and normal stress. To select appropriate shear box
dimensions, ASTM D 3080 (2001) suggests a minimum specimen width of at
least 10 times the maximum particle size diameter, a minimum
width-to-thickness ratio of 2, and a minimum initial specimen thickness
of six times the maximum particle diameter. With a maximum particle size
of 5 mm, a self-developed digital direct shear test apparatus (shear box
dimensions of 200*200*200 mm3) was used in this study;
the bottom of the shear box was filled with quartz grains
2~5 mm in size to ensure that the sheared samples were
40 mm thick. Two digital force sensors were equipped in the vertical and
horizontal directions, and the shear and normal forces were loaded with
a hydraulic system. With this system, the shearing rate can be
controlled in the range of 0.05~10 mm/s.
Considering the strength reduction of mudstone under the action of
water, normal stresses ranging from 200 kPa to 1400 kPa were adopted in
the direct shear tests with a constant shear speed of 0.1 mm/s until a
shear displacement of 20 mm was reached. The highest normal stress of
each group decreased with increasing dry-wet cycle number. Two
conditions, dry and wet (water content equal to 15% of the dry specimen
mass), were introduced to discuss the attenuation behavior of strength
due to water.
2.3 Testing procedure
Each specimen was filled in the shear box in three equal layers, each
subjected to 25 blows with a light compactor (2.5 kg hand rammer falling
from 305 mm) to preset the target dry density of 1.337
g/cm3. The top of each layer was loosened before the
next filling and compaction to keep the sample homogeneous. The
prespecified normal stress was applied to consolidate the specimen until
the vertical displacement was constant. After reaching the preset shear
displacement, the cutting ring (with a 61.8 mm diameter and 40 mm
height) was gently pushed into the sheared specimen from a vertical
direction to measure the permeability with the TST-55 falling-head
permeameter.
To analyze the variation in particle shape with dry-wet cycles, PCAS
software (Liu et al., 2011) was adopted to obtain the shape coefficients
by statistically analyzing 480 particles with different sizes. Five
coefficients were taken into analysis, including the uniformity
coefficient (UC), sorting coefficient (SC), curvature coefficient (CC),
average form factor (AFF), and fractal dimension (FD). Coarse granules
were broken to obtain the cross-section for observing their
microstructure in Thermo Scientific Prisma E environmental scanning
electron microscope (ESEM).