D210

Insight Into Granular Flow Dynamics and Its Generated Seismic Signals: An Experimental Study〇Wei LI, Gonghui WANG, Doi Issei, Dongpo WANG

1. Introduction

The images recorded by high-speed cameras wereLandslides, debris flows and snow or rock

preprocessed using PIV to obtain flow velocity (Fig. 1).

avalanches are examples of geophysical granular flows

The average values of the basal force and flow depththat commonly occur on steep terrain where they

were obtained by moving average filtering (Fig. 2). The

represent significant natural hazards to life and property.

fluctuating force was obtained by subtracting the mean

Better monitoring of hazard-prone areas and better

value of the basal force. We calculated the root-mean-understanding the dynamic processes involved andsquares (RMSs) of fluctuating force and acceleration

predicting their characteristics are key to the reduction

for each time window (twin = 0.1 s, no overlaps), andof natural hazards, and the use of seismic signals is atreat them as the typical magnitude of basal fluctuatingpromising tool toward these aims. The characterization

forces and seismic signals (Fig. 3).

of granular flows from the seismic signals generated bytheir movements remains an open question, despite theincreasing number of experimental, field and numericalstudies of granular flow dynamics. Here, a series oflaboratory flume experiments designed with differentconditions were conducted to investigate the granular
flow dynamics. Basal force characteristics and seismic
signal signatures are analyzed, and the relationshipbetween them and the bulk and basal flow propertiesare discussed.
2. Methodology
The experimental device consisted of a 300 cm long,Fig. 1 PIV analysis for the granular flow velocity.

30 cm wide and 40 cm high chute whose upper part wasa reservoir from which particles were released togenerate granular flows. The particles were glass beadsof diameter d = 2, 4 or 6 mm, density ρ = 2500 kg/m3
with mass of 5, 10, 20 or 40 kg. The slope angle of thechannel θ that varied between 18°and 22°. A highspeed camera, two laser displacement sensors, two
three-component force sensors and three threecomponent accelerometers were utilized to monitor theflow’s dynamic characteristics, e.g., flow depths, flow
velocities, basal forces and acceleration signals.

Fig. 2 Mean values of basal force and flow depth.

After understanding the dynamic characteristics ofgranular flow under different experiment conditions,
we further explored the relationships between the basal
fluctuating force and seismic signatures and bulk flow
properties. Fig. 4 shows the relationships under typicalexperiment condition (d = 2 mm, m = 20 kg, θ = 20°).
The results show that there is a strong correlation
between them. It seems that there is a positive linear
correlation between the basal fluctuating force and theaverage values of basal force and flow depth (Fig. 4(a)),
Fig. 3 RMSs of fluctuating force and seismic signal.

and the same is true for the RMS of seismic signals (Fig.

3. Results and discussion4(c)). RMSs of fluctuating force and seismic signals

The experimental results show that the flow mass has

also show a positive correlation with surface velocity

a great influence on the dynamic characteristics. Withand flow rate (Fig. 4(b) and (d)), but different from thethe increase of flow mass, the average flow depth, mean

former, it seems that the power law can better fit thisbasal force, the RMSs of basal force fluctuation andrelationship. Nsav is considered to be a key parameter toacceleration signal increase continuously. This shows

characterize the bulk flow regimes. It was believed thatthat the flow mass is an important factor that causes theinertial stresses associated with grain collision prevailchange of the overall and local flow characteristics. In

over the frictional stresses associated with persistentaddition, the results show that the variation of particle

contacts in granular flows when Nsav﹥0.1. Figure 4(e)size has little effect on the overall flow properties, butshows that the inertial stress dominates the analyzed

significantly affects local instantaneous characteristics

flow. There is also a strong positive linear correlationof the flow. The variation of slope angle has little effectbetween the RMS of ∆Fz and ∆az (Fig. 4(f)), whichon the above dynamic characteristics, but the larger theindicates that it is promising to analyze the dynamicangle, the faster the flow velocity.

characteristics based on seismic signals.

Fig. 4 Relationship between normal fluctuating forces (∆Fz) and accelerations (∆az) and flow properties (meanbasal force 𝐹̅𝑧 , flow depth ℎ̅, surface velocity vs, flow rate Q and Savage number Nsav) under the typical condition.