P19

Revealing a Possible Shear Strength Weakening Mechanism Based on Microscopic Insights fromRing Shear Tests〇Shengshan WU, Gonghui WANG
velocity controlled method, in which the samples were

1. Introduction

initially sheared slowly to reach the residual state andthen sheared by increasing the velocity to a target value

Rock avalanches are normally featured by large
volume, high running velocity, and long traveling(0.1 ~ 220 cm/s). To better compare these shear

distance, and then frequently result in greater damage(Heim,1932;

hypermobility

Legros,ofrock2002).

However,avalanches

remains

behaviors, the stress ratio (μ = monitored sharresistance against normal stress) as a common index istheaused in this study.

challenging research issue in geological disasters.
Recently, nanoparticles (with particle diameters
ranging from 1 to 100 nanometers) were found in rockavalanches and natural faults (Green et al., 2015;
McSaveney and Hu, 2022). Much research supported
that the nanoparticles could play as a lubricant andcause the ultra-low shear resistance of the materials
Fig. 1. (a) Pure SN; (b) TEM image of SN.

along the shear surface (i.e. Han et al., 2010, 2011;
Reches and Lockner, 2010; Tisato et al., 2012)

3. Results

However, the mechanism of shear strength weakening
resulting from nanoparticles is unclear. To address thisThe results of these tests are depicted in Fig. 2a inissue, a serais of ring shear test were performed on

the form of the shear stress ratio against the shear

nanoparticles to check their shear behavior at first.

displacement. At normal stresses of 107, 161, and 215

2. Materials and Methods

kPa, the stress ratios at the residual state are greaterthan 0.53. These values are close to the shear strength

To investigate whether the nanoparticles can work asof typical natural soils (Keaton, 2017), suggesting thatlubricants, resulting in a significant reduction in theshear strength of geomaterials, we utilized Silicon
dioxide (SiO2) nanoparticles (SN). SN consists of

SN itself possesses high shear strength. However, it

nano-scale particles with a spherical microstructure,normal stresses of 619 and 1030 kPa, respectively. Fig.

each having a diameter of approximately 50
nanometers (Fig. 1). Under natural dry conditions,

2b presents the time series data of stress ratio and shear

some ring shear tests were performed by the ring shear

can be noticed that after the occurrence of shear failure,decreased to 0.34 and 0.25 approximately under thevelocity for one test under normal stress of 161 kPa. It

apparatus of DPRI-5 and DPRI-6. We used the shearstress controlled method, in which shear stress isthe stress ratio remains relatively consistent (around0.52~ 0.63) and presents no significant variation evenincreased at a rate of 1 kPa/s until the sample fails toreach the residual state under different normal stresses(sn = 107 kPa, 161 kPa, 215 kPa, 619 kPa, and 1030

under the shear velocity of 2.1 m/s. Fig. 2c shows thatkPa, respectively). Besides, we also used the shear-

SN at residual state presents strong dependence on thein the tested range of shear velocities, the stress ratio of1

normal stress, but at each normal stress level (161 kPa,619 kPa, and 1030 kPa), it does not show significant
variation with the shear velocity. As shown in Fig. 3,
we also took sample blocks from the shear zone andconducted SEM observation on the sliding surface aftertests. SN particles aggregate to form distinctive powder
rolls on the shear surface with the long-axis directionof these rolls being approximately perpendicular to theshear direction. It is observed that the powder rolls havecharacteristics in common with the debris rolls thatFig. 3. Post-test photo and SEM images from a test

have been reported in the shear tests on different

conducted on SN under the normal stress of 1030 kPa. (a)materials for the studies of fault dynamics (Chen et al.,Shear surface after the test. (b), (c), (d) SEM images
obtained by scanning the shear surface. The arrow

2017a, 2017b). It is inferred that the rolling of powder

indicates the shear direction.

rolls facilitates fault slip along the smooth patches ofthe principal shear zone, contributing to overall

4. Summary

weakening and reduced friction.

To clarify the role of nanoparticles in shear behavior
and investigate their potential as lubricants to weaken
shear strength during high-velocity shearing, weconducted a series of ring shear tests on SN undervarying normal stresses and shear velocities. There aresome summaries for this study:a) The shear resistance of pure nanomaterial SN
exhibits high and stable at lower normal stress levels
but decreases significantly with the increase of applied
normal stress.
b) The shear strength of SN does not present asignificant change with the shear velocity, even though
the shear velocity reaches approximately 2.1 m/s.
c) It is inferred that the significantly lowered shear
strength of SN under higher normal stress results fromthe formation of these powder rolls, which enable theparticle movement mode from sliding to rolling.
Nevertheless, some other possible effects resulting
from the formation of nanoparticles, such asFig. 2. Results of ring shear tests on SN: (a) Results of thedehydration, and flash heating, may play key roles inshear stress-controlled method; (b) Result of high shear-

the shear behavior. Concerning this, further study by

velocity ring shear test on SN under normal stress of 161

using different types of nanoparticles will be necessarykPa; (c) The residual stress ratio of SN at different shear

and are undergoing.

velocities and different normal stresses.

2