10th Asian Regional Conference of IAEG (2015)

Influence of particle size and shear velocity on the frictional properties and slip instabilities of sheared granular materials
Yao JIANG(1), Gonghui WANG(2) and Toshitaka KAMAI(2)(1) Graduate School of Science, Kyoto University, Uji, Kyoto, 611-0011, Japan,
E-mail: jiang.yao.26z@st.kyoto-u.ac.jp(2) Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, 611-0011, Japan
Abstract
Granular friction plays a significant role in controlling diverse styles of movement
for landslides. Field observations also demonstrate that landslides may exhibit slow, stablecreep, periodicity stick-slip or accelerative sliding, which are similar to the diverse styles of
motion inferred for faults and subduction zones. Laboratory friction experiments provide
insight into the mechanisms of different spatiotemporal patterns of failure for theses catastrophic events. However, the effects of particle characteristics and mechanical conditions
on the strength and stability are not well understood. In the present research, we report onlaboratory experiments designed to investigate the frictional behaviors with particular emphasis on the influence of particle size and shear velocity on the strength and stability insheared granular materials. The experiments employed the dynamic ring shear apparatus
and were run at room temperature and humidity. We mainly find that: (1) the particle sizeand shear velocity have profound influence on the strength and instability of sheared granular materials; (2) within a regime of low shear velocities and large particles, the ultralow
shear resistance will appears to be associated with localized shearing; (3) the granular assemblies composed of small particles will show stable sliding with varying the shear velocity. Here we suggest that: (1) frictional behaviors depend on the number of force chains(particle size effects) and contact time of inter-particles (shear velocity effects); (2) the failure of arch jamming leads to the dramatic destabilization in locally sheared particles; (3) theformation of longer force chains will result in more compliant granular deformation. Our
results highlight the fundamental role of particle size and shear velocity on the frictional
properties and instability of granular materials, and provide important constraints to understand the diversity and complex movement for landslides.
Keywords: granular materials; particle size; shear velocity; frictional properties; slip instability; locally shearing
1.

Introduction

Petley et al., 2005; Wang et al., 2010). Generally,
despite the great diversity and complexity, the frictional strength of these gouge zones will gradually
increase and dynamically drop to a low value, inferred as stick-slip instability, which has been recognized as an analogy phenomenon of an earthquake,and also indicates many similarities of the reactivatedlarge landslides (Brace and Byerlee, 1966; Scholz,
2002; Clague and Stead, 2012). Source parameters
for such kind of instabilities, such as the magnitude ofstress drops and recurrence intervals of catastrophic
failures, are systematically varied and vital for predicting their occurrences and characters, and also forassessing resulting hazards (Scholz, 2002; Clague andStead, 2012). It is worth mentioning that the existingstudies did effectively offer abundant evidence ofFrictional properties provide the fundamental underpinnings of many geophysical processes, ranging
from landslides to earthquakes, which are often adequately described as frictional instabilities alongpreexisting basal shear zones or fault zones (Harp andJibson, 1996; Parise and Wasowski, 1999; Chambon
et al., 2006; Chigira and Yagi, 2006; Friedmann et al.,2006; Kaproth and Marone, 2013; Schulz and Wang,2014). These locally deformed zones are composed ofcomminution, disaggregation, and alignment granularmaterials that play a significant role in controllingand determining the diverse styles of movement, suchas continuous slow-stable creep, periodic stick-slip or
accelerative sliding (Fleming and Johnson, 1989;1

10th Asian Regional Conference of IAEG (2015)

frictional instability and proved particularly fruitful.
Neverthless, it remains elusive how intrinsic characteristics of grains directly affect the instability of locally sheared granular materials under different mechanic conditions.
Similar to previous studies, we sought to evaluate
the frictional behaviors and slip instability for locallysheared granular materials with particular emphasis
the intrinsic properties of particle size and shear velocity. We presented specific laboratory experiments
aiming at simulating a thick granular layer by using
dry cohesionless glass beads with uniform particle
size to rule out the influence of particle shape. We
stepped the shear velocity sequence from low to high,and described our results by involving the frameworkof granular micromechanical.
2.

chemical processes of grain-scale frictional contact(Scuderi et al., 2014). Annular specimens were constructed at 3 cm wide (within an inner diameter of 12
cm and outer diameter of 18 cm) and about 5 cm high.
The laboratory experiments were conducted at velocity range of 0.005~1mm/s, and normal stress of 200
kPa, and data were recorded at sampling rate between
100 and 1000 Hz. Other experimental conditions arepresented in Table1.

Experimental method

We performed a series of tests by employing a servo-hydraulic controlled ring shear apparatus (Figure1). The geometry has two independent servo controlled load feedback firmware to maintain constantnormal stress and shear stress (or shear velocity),
respectively. The rotating (lower) part of the chamber
is forced by a servo controlled ram at different constant displacement rate, while the stationary (upper)
opposes shearing through two arms with integrated
force sensors. Such fixed shear plane results in theformation of a horizontal shear zone near where theupper and lower cylinder parts meet. Sample thickness changes are continuously monitored by using adisplacement transducer. A set of rubber gaskets between the upper and lower cylinder pairs is designed
to prevent leakage of sample and pore fluid. Extension work has been done to investigate the shear behaviors for landslide materials using this assembly(Sassa et al., 2004; Wang and Sassa, 2009; Wang etal., 2010; Schulz and Wang, 2014) and additional
details concerning this intelligent device are reported
by Sassa et al. (2003).
The discrete landslide materials or weared granular materials in shear zone might be displaced underwide range of rates, meanwhile the material properties also could dictate the frictional strength and instability, via the grain size or particle shape. To further investigate such kind of effect of particle characteristics and mechanical conditions on friction properties in locally sheared granular materials, we usedspherical glass beads as an ideal analogy materials for
exploring the role of particle size and ruling out thelarge uncertainly associated with the particle shapeeffects (Yang and Wei, 2012). In the present study,
four uniform grain sizes (ranging from ~0.1-5 mm)
were used. All dry spherical samples were sheared
under room temperature and humidity (about ~20°C
and ~50~70%, respectively) in order to exclude thepossible influence of hydrothermal and physico-Figure 1. Schematic diagram of the ring shear apparatus. Left figure shows cross section of the entire
specimen configuration. Normal stress is applied vertically, and shear velocity is driven by rotating thelower part. The horizontal shear band will formed
where the upper and lower cylinder parts meet.
Table 1. Experiment conditions
Particlesize

Sample height

Bulk volume

(mm)

(cm)

(cm3)

4.7 – 5.3

4.82

681.41

Stick-slip

1.0 – 1.41

4.79

677.17

Stick-slip

0.5 – 0.71

4.78

675.76

Stick-slip

0.106 – 0.149

4.80

678.59Stable sliding

Sliding type

Note: All experiments were performed under a constant normal
stress (200 kPa) and shear velocity sequences were gradually
stepped from 0.005 mm/s to 1.0 mm/s at room temperature(~20°C) and humidity (~50 – 70%).

3.

Results

3.1 Characteristics of frictional instability events
Fig. 2 presents a selected section of data from anexperiment to reveal the dynamic instabilities forsamples with different particle sizes under the sameshear velocity (VL = 0.01 mm/s). In response to avariation of particle size, three general observations
can be made. First, the sample layers composed oflarger particle sizes revolves unstable stick-slip behavior (Fig. 2a-c), whereas the smallest particle size2

10th Asian Regional Conference of IAEG (2015)

exhibits an almost stable-sliding character (Fig. 2d).
Second, the failures for granular materials (i.e.,
stick-slip instabilities) vary from relative regular toirregular (Fig. 2), and such instabilities are largest for
samples within the largest particles. Third, the frequency of such instability events increase with decreasing of particle size, and the longer stick-slip recurrence interval occur at larger particle size. It isnoteworthy that such kind of stick-slip characteristics
has also been reported in previous work (Mair et al.,
2002; Anthony and Marone, 2005), and our results
suggest that particle size has a fundamental effect on
the frictional instability within localized shear zones.
Upon an increase in shear velocity, we find thatfluctuations in shear stress still can be observed
among samples with larger particle size throughout(Fig 2a-c and Fig. 3). Fig. 3 provides a selected detailof the dynamic instabilities against shear velocity forthe same particle size (1.0~1.41mm). The duration
periods of repetitive stick-slip instabilities are sensitive to the shear velocity, which exhibits an increasein stick-slip event frequency. For the velocity step
sequence from 0.005 mm/s to 0.01 mm/s, the peakstrength has a slight decrease, but the variation ofminimum shear strength experiences minor changes.

Fig. 3 Friction data taken from section of experimental data for 1.0 – 1.41 mm particles. The shear
velocities are 0.005 and 0.01 mm/s, respectively. Data show repeated stick-slip events with variations in
amplitude and frequency.
3.2 Effect of particle size and shear velocity on
stress drop
For a series of multiple stick-slip events, themagnitude of stress drop shows a strong correction
with particle size and shear velocity (Figure 4). Considering the frictional properties for smallest particle
sizes (0.106 – 0.149mm) almost show stable sliding
behaviors (Fig. 2d), we only analyzed all large instability events for the other three particle sizes in eachshear velocity. All stick-slip events were picked upby hand considering a threshold value (Δμ > 0.02) toinsure that no noise was picked up as events, reporting the mean value and statistical variability, witherror bars calculated by using a standard error of themean method (Fig. 4).
For our range of shear velocities, the changes ofmaximum friction (μmax) for largest particle size (4.7
– 5.3mm) are ~0.05, accompanying a slight decrease
below ~0.1 mm/s and not obvious change above thatvelocity. However, the μmax of the other relativesmaller particle sizes (i.e., 1.0 – 1.41mm and 0.5 –
0.71mm) gradually decrease with the logarithm ofshear velocity, and the range of variations is ~0.1 (Fig.
4a). Note that our data show that μmax is not only velocity-dependent, which is consistent with previous
work (Nasuno et al., 1998; Albert et al., 2001; Mair etal., 2002; Anthony and Marone, 2005), but also particle-size-dependent. When the shear velocities are
below at ~0.1 mm/s, the largest particle size (4.7 –
5.3mm) exhibit lower μmax than the other two relativesmaller particle sizes. Interestingly, upon the shear
velocity of ~0.1mm/s, the largest particle size (4.7 –
5.3mm) has a higher μmax due to not strongly sensitive
to shear velocity.
Furthermore, we compare the changes of minimum friction (μmin) during dynamic friction drops forour range of shear velocity. As shown in Fig. 4b, following the shear velocity sequence, the value of μmin
is general higher in sample composed of smaller particles at each given velocity, which suggests the larger particles may induce more dynamic instabilities(i.e., lower minimum friction strength). For these data
the mean value of μmin ranges from ~0.32 to ~0.38 forFig. 2 Friction data taken from sections of experimental data where the shear velocity is 0.01 mm/s fordifferent particle sizes, (a) 4.7 – 5.3 mm; (b) 1.0 –1.41 mm; (c) 0.5 – 0.71 mm; (d) 0.106 – 0.149 mm.
Stress drops decrease and event frequencies increase
with decreasing of particle sizes until stable sliding
occurring.

3

10th Asian Regional Conference of IAEG (2015)

the smallest particles and from ~0.17 to ~0.25 for thelargest particles. For each particle size, the variationsof μmin show velocity-dependent. As shear velocity
stepping from low to fast, smallest particles (i.e., 0.5– 0.71 mm) exhibit a slightly decrease. However, thisrelation is not observed for larger particles (i.e., 1.0 –1.41 mm and 4.7 – 5.3 mm). The value of μmin forintermediate size (i.e., 1.0 – 1.41 mm) shows a decreasing trend while for largest particle (i.e., 4.7 – 5.3mm) varies roughly with velocity below ~0.1 mm/s.
But both the two sizes show an increase in μmin withvelocity above ~0.1 mm/s.
We compare the variations of friction drop (Δμ)
as a function of particle size and shear velocity toemphasize the stick-slip properties for granular materials (Fig. 4c). It is note that the Δμ is always higher
for larger particles in contrast with the trend of μmin at
a given shear velocity. For each particle size, the value of Δμ is larger at lower shear velocities, according
to the tendency in relation to Δμ and shear velocity.
Moreover, it is striking that the velocity dependenceof Δμ changes slope at ~0.1 mm/s. For the intermediate size (i.e., 1.0 – 1.41 mm), the Δμ decreases more
sharply than the other two particle sizes for velocities> ~0.1 mm/s.

(b) minimum friction and (c) friction drop (Δμ = μmax
- μmin) as a function of shear velocity for each particle
size, respectively. Data points represent mean values
with error bars calculated by using a standard error of
the mean method for all events at each shear velocity.
Note that experiments in which composed of larger
particles tend to have higher friction drops thansmaller particles for our range of shear velocities.
Friction drops decrease systematically with increasing shear velocity, and the friction drops markedly
decrease when velocity increases above ~0.1 mm/s
for each particle.
4.

Discussion

The dynamical drops in stress have been recognized as an important correction to the catastrophic
failures, such as the rupture of gouge-filled faults, theinitial and movement for landslides. It has also been
documented that the observed periodic stick-slip behaviors in laboratory will be an effective way to investigate the origins and mechanisms for these events.
Many assumptions have been approached in the last
decades (Brace and Byerlee, 1966; Scholz, 2002;
Clague and Stead, 2012).
The rupture of asperity has been proposed to understand the frictional instabilities between solid surfaces (Scholz, 2002). The magnitude of asperity normally ranges about ~μm, it is also reasonable to inferthat the asperity rupture can induce the fluctuations instress. However, our observed data show that thestress drop of locally sheared granular materials are
more catastrophic, and we believe that this is not response for the stress drops of the present study.
The sheared and loaded granular medium form
layer-spanning chains of stressed particles, and granular force chains are separated by regions of spectator grains that support little to no load. Such stressed
particles cause jamming to provide resistance to deforming, and some highly stressed particles along the
network of force chains may support most externalloads. When the stored elastic energy exceeds grain
yield strength of highly stressed particles, the formed
force chains can lose by crushing (grain fracture)(Davies and McSaveney, 2002; Crosta et al., 2007;
McSaveney and Davies, 2007). The failure of force
chains can partially or fully release the elastic strain,
which result in the probability of sudden, dynamic
reduction of frictional resistance. It is note that before
the failure of force chains, the particles can suffer theexternal loads being solid condition (stick behavior);
but after the failure of force chains, the can bear nostresses briefly behaving as a liquid condition (slipbehavior), transmitting pressure to their surroundings.
As examined by laboratory tests (Mair et al., 2002),
the breakage and comminution of analogy glass beads
always occur under the higher regime of confining
stress (generally higher than ~20 MPa for granularFig. 4 Stick-slip friction parameters as a function ofparticle size and shear velocity. (a) maximum friction,4

10th Asian Regional Conference of IAEG (2015)

materials). Moreover, all samples in our study were
examined after shearing and we cannot find any
fragments. Therefore, we infer that the transition
from jamming to rolling of force chains may be induced by the relative motion among granular materials, such as inter-granular slip and grains rotation. For
a given shear velocity, the friction drops are higher
for larger particles, and the value deceases with thedecrease of particle size, until stable sliding observedin a relative small size. We suggest that the influenceof shear velocity on frictional instability is related to
the nature of contact time, while the effect of particle
size is associated with the development of contact
area and the number of force chains acting across the
granular layer. Note that the ring-shear configuration
we used in the present study has a fixed shear plane,
the failure intensity of force chains should be concentrated near this shear plane due to locally sheared. It
is should be pointed out that we do not observe theevolution of force chains and quantify the relationship between grain motions and force chains failure,
but legitimately infer how they deform on the basis of
the macroscopic frictional response. Further verification of these speculations using experimental data
should be of interest.
5.

mechanisms likely operate in nature.
Acknowledgements
Financial support for this study was provided by ascientific research grant (No. 25560181) from the
MEXT of Japan.
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Conclusions

We have shown that particle size and shear velocity have important influence on frictional properties
within locally sheared granular materials. The test
material of glass beads with uniform particle size
distribution allows a well-controlled study. We proposed the framework of granular micromechanicalwith particular emphasis on the evolution of force
chains at grain scale and macroscopic response. The
major results and findings of present study are summarized as follows.
(1) Stick-slip instabilities were observed for locally sheared granular materials, and the failure offorce chains acting at grain interactions plays theprimary role in controlling the dynamic failures
without the contribution of particle fragmentation.
(2) Magnitude and frequency of dynamical shear
resistance drops are closely relevant to particle sizeand velocity-stepping sequence. For unstablestick-slip regimes and a given shear velocity, large
particles show large stress drop and low event frequency. Within decreasing of particle size, the frictional instabilities have a transition from unstablestick-slip to stable sliding as well as increasing ofshear velocity, accompanied by the decrease of magnitude and increase of frequency.
(3) Last, our experimental results suggest thathigher-resolution studies of particle properties andmechanical conditions may aid in the search for better
understanding the movement characteristics andpromoting hazard mitigation for hazards, if similar

510th Asian Regional Conference of IAEG (2015)

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