The
normal stress of the constant normal stiffness direct shear apparatus is
applied by weights through a lever, and the normal stiffness is
controlled by a normal spring connected to the lever. Different normal
stiffness boundary conditions can be achieved by replacing springs with
different stiffnesses. During shearing, the shear stress at the
interface is controlled by a micro-motor in the system, with a maximum
shear stress of 10kN. The measurement and control system can
simultaneously measure and output the interfacial shear displacement and
normal displacement in real-time. Data can be directly collected to the
control computer through the data acquisition system. The working
principle of the direct shear apparatus during the shearing process is
shown in Figure 5a. By measuring the stiffness of soils with different
properties as required by the experiment, different stiffness values can
be controlled for the experiment. The maximum and minimum stiffness
values obtained are 1100kPa/mm and 200kPa/mm, respectively. Two
intermediate values are then selected, and all springs are uniformly
processed using 65Mn carbon steel (Figure 5b). The internal effective
dimensions of the shear box are 100mm × 100mm × 50mm, with both the
upper and lower shear boxes having a height of 25mm. The sample consists
of a 100mm × 100mm × 25mm soil layer on top and a 100mm × 100mm × 25mm
mortar cube below. The sample is tightly attached to the shear box to
reduce shear stress errors during preloading. A schematic diagram of the
force applied to the sample is shown in Figure 5c.。
FIGURE 5 (a) Direct shear instrument (b) Spring selection (c)
Specimen stress diagram
2.5 Testing Process
Boundary conditions are crucial factors that influence the mechanical
response of soil. During the shearing process, changes in the normal
stress applied to the interface due to volume changes in the shear zone
can lead to alterations in shear stress. Existing research has shown
that constant normal stiffness (CNS) conditions are closer to actual
engineering conditions compared to constant normal stress conditions38. Under constant normal stiffness conditions, the
change in normal stress at the interface is denoted as,and the change
in normal displacement at the interface is denoted as , the normal
stiffness at this time is represented by k =constant.
The process of interfacial shear testing includes the following steps:
(1) Calculate the weight of the soil sample required for a compaction
rate of 95%. Install the prepared mortar block into the mold, and then
fill the soil sample into the mold evenly in three layers. Fabricate the
interfacial shear sample using the layered compaction method and wrap it
in plastic film to prevent moisture loss during the freezing process.
(2) Place the interfacial shear sample in a -20℃ freezer for rapid
freezing for 12 hours to prevent moisture migration during the freezing
process. (3) After rapid freezing, remove the mold from the solidified
sample, wrap it in a new plastic film, and transfer it to a constant
temperature freezer. Maintain the sample at the target experimental
temperature for 24 hours. (4) Adjust the environmental temperature and
instruments for the test, ensure the sealing of the freezer to minimize
any potential errors, install the sample on the shear apparatus, and
begin the interfacial shear test. (5) After the shear test is completed,
take photos of the sheared interface and record the test details.
The experiment was conducted using a rapid shear mode with a shear rate
of 0.8mm/min. The shearing duration was set to 15 minutes, resulting in
a shear displacement of 12mm. The experimental conditions considered
during the test included various normal stiffnesses (200kPa/mm、500
kPa/mm、800kPa/mm、1100 kPa/mm),normal
stress(40kPa、80kPa、120kPa、160kPa),water content of
soil(10%、13%、16%)and testing temperature(20℃、-2℃、-4℃、-6℃).