Applying MEMS Accelerometers to Measure Ground Vibrations and to Characterize Landslide Initiation Features in Laboratory Flume Test G. L. Ooi1, and Y.H. Wang2 1
Research Student, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, HKSAR, China. (Email:
[email protected]) 2 Associate Professor, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, HKSAR, China. (Email:
[email protected]) ABSTRACT: As global warming is altering the environments and precipitation patterns, the situation calls for small, low cost and accurate sensors in large deployment scale to monitor hazardous natural terrains such as slopes and essential infrastructures at risk. Advancement of technologies in the past decade offers us MEMS (Micro-Electro-Mechanical-Systems) sensors that fulfill the roles mentioned. In this paper, 2-axis and 3-axis sensing MEMS accelerometers are applied in: (1) an exploratory field measurement of ground surface vibration during an underground blasting, and (2) the characterizations of how the shear zone in a soil mass evolves both spatially and temporally before developing into a full-scale flow landslide in the laboratory flume. The MEMS accelerometers demonstrate high sensitivity and accuracy in this study, in addition to its low price (~USD$20 per piece) and miniature size, which is about a quarter of fingernail large. In turn, the results from the laboratory flume tests too impart insights on the initiation mechanisms of flow landslides prior to fluidization, and also the possibility of monitoring hazardous slopes economically in the future. INTRODUCTION The coming decade poses huge challenges as global warming brings in increasing precipitation in major cities in the tropics and higher latitudes (Westra et al. 2013). Of all types of landslides, the flow-like landslides demonstrate threatening devastations of lives and economic losses (Hungr et al. 2001). Flow landslide could be separated into three stages: (1) initiation, (2) flow, and (3) deposition. The mechanics of flow and deposition, along with the corresponding countermeasures are widely reported in the literature but how exactly a soil mass is initiated into flow is still imperfectly grasped (Iverson et al. 2000; Wang 1994; Wang and Sassa 2003). The measurement of the evolution of the properties of soil mass over the course of rainfall infiltration is essential to assess the possibility of the soil mass being in the
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process of initiation or transition into a flow landslide. Such efforts require an economical and yet small enough sensor by scale to characterize the local movement behavior of soil in both laboratory and field settings. The acceleration of the batch production technology of semiconductor chips in the past few decades leads to the emergence of the MEMS (Micro-Electro-Mechanical-Systems) technology; this manufacturing technology miniaturizes the conventional sensors operating by the method of spring-suspended masses that measure vibrations (e.g. geophone), which are of the size of a closed fist, to that of a quarter of fingernail (Ooi 2012). It is due to the light weight and the tiny physical dimension that the difficulties met in laboratory settings can be hurdled over; simultaneously, the low cost (~USD$20 per piece) and accuracy also warrant its application in large-scale field monitoring. This paper describes the applications of two Analog Devices MEMS accelerometers, the analog 2-axis ADXL203CE and the analog 3-axis ADXL335 in: (1) the laboratory calibrations and comparisons with the specifications in the datasheet; (2) an exploratory field measurement of soil vibration excited by nearby underground blasting; (3) the characterization of local soil movement behaviors inside the shear zone in one of the experiments conducted using well-instrumented laboratory water flume, and finally (4) vibration response of a stationary MEMS accelerometer prior to full-scale fluidization of the soil mass in the same laboratory flume test. DETAILS OF THE MEMS ACCELEROMETERS General View MEMS (Micro-Electro-Mechanical-Systems) technology specifically refers to the process of fabricating the miniature sensors (various devices have been produced since, namely accelerometer, gyroscope and temperature gauge), a basic technique which evolves from the fabrication technique of semiconductor device. This allows for batch-wise etching production, thereby minimizes manufacturing cost and eliminates the need for stringent piecewise calibration of the sensor. MEMS sensors, due to its tiny size, generally have a large surface area to volume ratio. Because of this, major MEMS production companies like Analog Devices and Endevco usually place extreme care to ensure that the geometrical symmetry with respect to the axis of sensitivity of the sensor is achieved during fabrication (Acar and Shkel 2003); this is to avoid surface effects such as electrostatics in affecting the performance of the sensor. After such care has been undertaken, the volume effects such as inertia and thermal mass are in fact negligible at the size scale of 3 to 5 mm. This appeal is of direct importance to the understanding of local soil characteristics inside a soil mass prior to full-scale fluidization, particularly the progressive evolution of shear zone in the process. Fig. 1 shows the MEMS accelerometers: (a) the 2-axis ADXL203CE (Analog Devices, United States) on the left with the corresponding soldered connections and also the positive directions of accelerations; (b) the 3-axis ADXL335 (Analog Devices, United States) on the right, with similar descriptions but note that the original size is 4 mm and the sensor is surface mounted on an in-house designed PCB since soldering by hand is impossible for the tiny pins. The supply voltage is 5V for ADXL203CE and 3V for ADXL335. Important specifications such as the range, the sensitivity and the
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bandwidth of each axis are summarized in Table 1. Both sensors weigh less than 1 gram. (a)
+y
(b)
+z
+x
Vs
+x
5 mm
Xout
Common
11 mm
4 mm Yout
+y
5 mm
Vs Xout Yout Zout Common
11 mm
FIG. 1. The analog MEMS accelerometers (Analog Devices, United States) used: (a) ADXL203CE, 2-axis sensing with a sensitivity of 1000 mV/g; (b) ADXL335, 3-axis sensing with a sensitivity of 300 mV/g. Although ADXL335 is able to sense accelerations in all 3 axes, the sensitivity is about 3 times lower. This is mainly due to the larger range of accelerations it can capture comparing to ADXL203CE; applications in civil engineering that require a precise travel time between signals, e.g. wave propagation in structural elements during earthquake, call for high sensitivity and narrow acceleration range. Since the signal output is in the analog format, a proper design of analog-to-digital converting circuit (ADC) befitting the resolution needed can be done by referring to Equation 1:
Resolution =
[Voltage] (2 − 1) [counts]
(1)
n
in which the [Voltage] would be the standard DC input as recommended in the datasheet for the MEMS accelerometer while n is the number of bits of the ADC. Typically 8 or 10 bits are used, but for this particular study, 16-bit National Instruments' NI-USB 6353 is adopted for the high speed sampling at 10,000 samples per second and also the high resolution. Table 1. Selected Specifications of ADXL203CE and ADXL335
ADXL203CE
Range ± 1.7 g
Size (mm) 5 × 5 × 2.00
Sensitivity (mV/g) 960 ~ 1040
ADXL335
± 3.0 g
4 × 4 × 1.45
270 ~ 330
Bandwidth (Hz) 0.5 ~ 2500 0.5 ~ 1600 (X & Y) 0.5 ~ 550 (Z)
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Calibration Verification of ADXL203CE
To facilitate measurements in both positive and negative directions, most MEMS accelerometers like ADXL203CE and ADXL335 have a bias offset at 0 g level for every axis. Mass production of MEMS accelerometers through batch etching processes usually guarantees the functional characteristics of the sensors; the distribution results offered by the factory in the datasheet on how the zero bias and the sensitivity of each axis vary with temperature are usually compiled from the performances of more than 1000 pieces of sensors. In this study, to further verify the performance stability, we had carried out frequency response calibrations of both ADXL203CE and ADXL335 in the range from 6 to 128 Hz. The frequency spectrum was divided into two frequency ranges for the test. The range of 6 to 20 Hz was done on a shaking table along with the low-noise Episensor ES-U2 (Kinemetrics, United States) in the low frequency sensing range, and that from 20 Hz to 128 Hz was done on a 4808 Magnetic Vibration Exciter (Brüel & Kjær, Denmark) with a signal generator along with the DeltaTron accelerometer type 4396 (Brüel & Kjær, Denmark) that maintained a constant phase stability up to 250 Hz. Four ADXL203CE were subjected to the calibrations and since both the X and Y axes were reported to be in the same sensitivity range, the results were compiled together with the mean and ±1 standard deviation at each frequency point, which is plotted in Fig. 2.
Milivolt / acceleration in g
1500 1400 1300 1200 1100
Max = 1040 mV / g
1000 Min = 960 mV / g
900 800
Mean = 1000 mV / g
700 600 500 0
20
40
60
80
100
120
140
Frequency (Hz) FIG. 2. The compiled calibration results of four ADXL203CE sensors in the frequency range from 6 to 128 Hz, with the bar of mean and ±1 standard deviation of the sensitivity at each frequency point.
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Apart from the inherent instrumental errors involved in the measurement process, e.g. the noise and fluctuations of the equipment benchmarks (oscilloscope, signal generator etc.), ADXL203CE seems to fall within the range of the minimum and maximum value given in the datasheet under 64 Hz. The mean seems to shift after that range; although some of the sensors still fall within factory specifications, the standard deviation is growing larger. Similar finding for ADXL335 agrees with the aforementioned. In the preliminary flume experiments, the maximum low pass cutoff frequency where the signals still remain undistorted is around 125 Hz, thus converting the values captured simply by following that prescribed in the datasheet should suffice. Suggestion of a Simple Calibration Method
The datasheet usually provides a zero g bias offset value, which falls within a distribution. Under a static condition, one could convert the acquired voltage from the ADC (Vcurrent) by: a X/Y/Z =
Vcurrent − VX/Y/Z , off S X /Y / Z
⋅g
(2)
in which the VX/Y/Z, off is the offset bias voltage, S X / Y / Z is the sensitivity and g is the Earth's standard average gravity, while the subscripts X/Y/Z refer to the corresponding axis of interest. Due to the temperature fluctuation and the fabrication anomaly, it is expected that the zero g bias offset of each axis of each sensor will certainly deviate; on the other hand, sensitivity usually would stay constant over the range of -30°C to 35°C. To resolve this problem, one could first place the sensor in a relatively low noise environment in three different static positions. Since in any alignment or tilted position, if the environment is in room temperature and the sensor static, the resultant or Euclidean vector magnitude of the mean offset values of each axis will always yield 1 g. Thus one can search for the combination of the matching correct bias offset from the distributions detailed in the datasheet. APPLICATIONS OF THE MEMS ACCELEROMETERS Surface Wave Characterization during Underground Blasting
On 30th November 2011, two MEMS accelerometers ADXL203CE were installed near the blast radius of underground blasting at the Anderson Road development zone, Hong Kong, for a preliminary field measurement of the magnitude of surface waves propagating from the source of blast. Fig. 3 shows the acceleration of local ground response perpendicular to the surface of a measurement point about 1 kilometer away, sampled at 10,000 samples per second. The event captured spans about 1 second and the peak acceleration captured is 4.3 m/s or 0.438 g. The test itself was carried out on the spot thus the coupling of the sensor with the ground was done simply by inserting into the ground and then laid over with a
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bag of sand. However, ADXL203CE still can capture clear vibration events with similar signal pattern as the geophone installed on site, hence verifying that the sensor itself is sensitive enough and on par with the conventional method. However, it should be noted that the geophones and the MEMS accelerometers respond to different types of excitations, i.e. the former records velocity in a higher frequency range and the latter records acceleration in an all inclusive frequency range. The numerical data from the geophone is not shown here because it is a property of Hong Kong government. Nevertheless, ADXL203CE costs only about USD$20; the small size and also the light weight of it allows the measurement point to be truly local in the field. 5 4 Peak Acceleration = 0.438 g
Acceleration (m/s)
3 2 1 0 -1 -2 -3
16:57:09.046
16:57:08.836
16:57:08.626
16:57:08.416
16:57:08.206
16:57:07.996
16:57:07.786
16:57:07.576
16:57:07.366
-5
16:57:07.156
-4
Time FIG. 3. Ground surface vibration signal captured by the MEMS accelerometer ADXL203CE during the underground blast event. Characterization of Local Soil Movement inside Shear Zone
The understanding on how different parts of a soil mass are evolving prior to a full-scale fluidization is crucial. It provides insights on the temporal and spatial development of a shear zone under different hydrologic triggers and soil properties. In order to study this in detail, a laboratory flume was built and instrumented with 10 basal
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porewater pressure transducers (PPT) and 10 wired ADXL335 accelerometers at different lengths and depths of the flume. In addition, one of the ADXL335 accelerometers was also installed on the external side wall to measure the precursory vibrations before the soil mass develops into a flow. Fig. 4 shows the flume rack tilted at 30° along with the positions of the PPTs and MEMS accelerometers in the specimen. M9 was installed on the external wall as a stationary observer as aforementioned. S1 and S2 are locations where sudden groundwater triggers were induced after a prolonged water inflow. The soil sample measures 100 cm × 45.2 cm × 20 cm (L × W × H) and the prism encompassing it was made of acrylic for easy visualization of internal soil behaviors. The soil specimen rests on a porous stone bed that was made from similar sand cemented together to offer a boundary condition of the same friction angle and at the same time the ability to let water permeate in the slab so that the running water is not allowed to scour the soil right above the bed. However, the slab itself is impermeable at the exposed end so that the groundwater table can accrete in the soil (see Ooi and Wang 2013 for more elaborate details on the setup and measurement results).
Saturation Box
M1
S2 E2 S1
Porous Stone Bed M9
MEMS Accelerometers (ADXL335) Porewater Pressure Transducer slide direction Groundwater Supply Pipes
FIG. 4. The well-instrumented laboratory flume setup to study the precursory signs prior to the initiation of flow.
A set of results documenting a sudden groundwater inflow triggered first at S1, then at S2 (see Fig. 4 for the location and Fig. 5 for the corresponding times) after a prolonged constant groundwater accretion over the whole soil mass is presented here to detail the characteristics of soil mass captured prior to full-scale fluidization.
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The top of Fig. 5 is the response of basal PPT E2, while the middle is the Y-axis response of MEMS accelerometer M1 and the bottom is the X-axis response of MEMS accelerometer M9. Both Y-axis of M1 and X-axis of M9 point in the same direction, which are normal to the slide direction (see Fig. 4). The temporal scale for all three sensors are in the range of 1727.7 to 1741.7 seconds. The soil was loose and contained 5% of fines. This particular experiment sets out to understand the role of fines content in regulating the rate of pore-pressure dissipation and also the spatial and temporal development of the shear zone through the subsurface movement characteristics acquired. S1
40
V1
35 Buildup of excess porewater pressure
30 25 20 15 10 0.5
Acceleration (g)
V2
S2
E2
Movement in shear zone begins
Pressure Head (cm)
45
M1-Y
0.4 0.3 0.2
Fully fluidized & flow
Vertically contract then dilating as slowly gaining momentum
Bias offset due to zero g
0.1 0 -0.1
Acceleration (g)
0.78
M9-X
Signal spike as the soil mass is on the brink of moving
0.74
Continuous vibration as momentum is building in the soil mass
0.70 0.66 Bias offset due to zero g 0.62 1728
1730
1732
1734
1736
1738
1740
Time (seconds)
FIG. 5. The features of landslide initiation characteristics captured by the MEMS accelerometer M1 inside the shear zone and the stationary external observer M9 after the two sudden hydrologic triggers are invoked.
MEMS accelerometer M1 is selected for the discussion because it was situated right within the shear zone, which is believed to be in between the height of 4 to 8 cm. The loose slope was initially saturated by a slow and small groundwater inflow, and a fixed
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interval later a sudden rise of groundwater inflow twice the original volume was first invoked at the bottom of the slope toe (S1); seconds later the rear bottom (S2). The soil mass does not show any significant movement even though the PPT is registering a climb; moments later M1 captures a sudden contractive behavior while the whole soil mass is still visually static (see response of E2 and M1-Y in Fig. 5; be noted that there are two video cameras shooting from the top and the side throughout the experiment). The initial unbalancing occurs right after the contraction behavior transitions into a dilative behavior. Henceforth the soil mass starts to move slowly; at the time of event V2, full-scale fluidization then takes place. On the other hand, moments after M1 registers the dilative behavior, the external stationary observer M9 captured a signal spike right at the time when the soil mass is on the brink of moving. Continuous vibration signals then follows as the soil is moving slowly and building up the momentum to the eventual full-scale flow. The rich features MEMS accelerometers characterize before a soil mass builds up to fluidization warrant the potentials of the applications of the sensors in meticulous experiments in the geotechnical laboratory. The small size of the sensor and also the accuracy in measurement are the factors why MEMS sensors are convenient in this context. SUMMARY AND CONCLUSIONS
MEMS accelerometer proves to be a versatile candidate in future civil engineering applications, both in the field and the laboratory settings, for its miniature size, accuracy, high sensitivity and low cost. Although not properly coupled, the event captured in the underground blasting shows similar signal pattern when compared to the stationary geophone on site; this calls for a proper treatment of the field sensor package design as a whole, which includes mounting, data storage, power supply and data transmission protocol. The relative low cost of one sensor, which is about USD$20 each, promises a future of massive sensor deployment without the fear of losing any in the monitoring process. In the second part of this study, how MEMS accelerometers successfully characterize the subsurface local soil behaviors are demonstrated. The miniature size and the accuracy of the MEMS accelerometer allow laboratory scale wave characterization to be carried out at any location interested. Also, an external stationary sensor too is able to capture certain features of soil mass movement as a whole; that data acquired could be used as the basis of stationary vibration monitoring equipments serving to disseminate early warnings on incoming debris flow. The insights provided in this laboratory flume experiment would further illuminate how the shear zone in a soil mass evolves spatially over time. ACKNOWLEDGMENTS
This research was supported by the Hong Kong Research Grants Council (GRF 621109). The authors also appreciate the support from the Hong Kong Geotechnical Engineering Office and Ove Arup & Partners for the arrangement of the site test.
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REFERENCES
Analog Devices (2010). "ADXL335 Data Sheet, Rev. B." Available online: http://www.analog.com/static/imported-files/data_sheets/ADXL335.pdf. Analog Devices (2011). "ADXL203CE Data Sheet, Rev. D." Available online: http://www.analog.com/static/imported-files/data_sheets/ ADXL103_203.pdf. Acar, C. and Shkel, A.M. (2003). "Experimental evaluation and comparative analysis of commercial variable-capacitance MEMS accelerometers." J. Micromech. Microeng., Vol. 13: 634-645. Hungr, O., Evans, S.G., Bovis, M.J., Hutchinson, J.N. (2001). "A review of the classification of landslides of the flow type." Environmental and Engineering Geoscience, Vol. VII (3): 221-238. Iverson, R.M., Reid, M.E., Iverson, N.R., LaHusen, R.G., Logan, M., Mann, J.E. and Brien, D.L. (2000). "Acute sensitivity of landslide rates to initial soil porosity." Science, 290:513-516. Ooi, G.L. (2012). "Exploratory study on initiation mechanism of flow landslide in loose soil." Master’s Thesis, Hong Kong University of Science and Technology. Wang, Y.H. (1994). "A study on the trigger mechanisms of debris flow." Master’s Thesis, National Taiwan University. Note: original in chinese. Wang, Y.H. and Ooi, G.L. (2013). "Characterizing flow landslide initiation features with well-instrumented laboratory water flume and MEMS accelerometers." Geotechnical Testing Journal, preparation for submission. Westra, S., Alexander, L.V., and Zwiers, F.W. (2013), "Global Increasing Trends in Annual Maximum Daily Precipitation." Journal of Climate, American Meteorological Society, Vol. 26:3904–3918.
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