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A new mini-grating absolute displacement measuring system for static and dynamic geomechanical model tests
Measurement 105 (2017) 25–33
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Measurement journal homepage: www.elsevier.com/locate/measurement
A new mini-grating absolute displacement measuring system for static and dynamic geomechanical model tests Shucai Li a,1, Hanpeng Wang a,1, Yong Li a,b,c,⇑, Qingchuan Li a, Bing Zhang a, Haiyang Zhu a a
Geotechnical & Structural Engineering Research Center, Shandong University, Jinan, Shandong Province 250061, PR China School of Civil Engineering, Shandong University, Jinan, Shandong Province 250061, PR China c State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou, Jiangsu Province 221116, PR China b
a r t i c l e
i n f o
Article history: Received 22 November 2016 Received in revised form 19 February 2017 Accepted 3 April 2017 Available online 5 April 2017 Keywords: Absolute deformation measurement Grating data acquisition instrument Micro multi-point extensometer Static and dynamic test Geomechanical model test
a b s t r a c t In this paper, a mini-grating displacement measuring system is developed for geomechanical physical model tests. This system consists of a portable multi-channel grating data acquisition instrument, a grating ruler, a micro multi-point extensometer, and other related accessories. The motherboard of the portable multi-channel grating data acquisition instrument uses a self-developed circuit board and all channels can collect high-speed and high-precision grating displacement signals. The micro multipoint extensometer has advantages of micro-flexibility and small interference in physical model tests. Embedded measuring points in tunnel surrounding rock connect with the grating ruler which is fixed on an independent frame outside of steel wires and connects to the data acquisition system with wire lines. The system is applied to a static and dynamic geomechanical model test on a tunnel. Testing results prove that the system can be satisfactorily applied in geomechanical model tests. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Displacement is a very important testing parameter in geotechnical engineering field practices and laboratory geomechanical model tests. Especially, excavation-induced settlements may cause serious damages to nearby structures [1], therefore the deformation of surrounding rock is a key parameter to measure during tunnel excavation and construction processes [2]. In general, geomechanical model test reflects the spatial structural relations and the experimental process more realistically, which is a unique and indispensable method to investigate the stability of underground surrounding rock, and displacement value is much smaller than that of the prototype model. In order to ensure the accuracy of the displacement measurement, it is critical to develop a highprecision and low-disturbing collection and analysis system [3–8]. In previous studies, dial indicator and digital photogrammetry have been widely utilized in the world [9–12]. Dial indicator has high precision but can be only used in measuring model surface displacement [13–15]. Digital photogrammetry cannot be used to ⇑ Corresponding author at: Geotechnical & Structural Engineering Research Center, Shandong University, No. 17923, Jingshi Road, Jinan, Shandong 250061, China. E-mail address: [email protected] (Y. Li). 1 The first two authors contributed equally to this work and should be considered co-first authors. http://dx.doi.org/10.1016/j.measurement.2017.04.002 0263-2241/Ó 2017 Elsevier Ltd. All rights reserved.
test the internal displacement of the physical model neither and the testing precision is greatly related to the camera resolution, photograph distance and light intensity, which make the testing precision difficult to be guaranteed. Fiber-optic microdisplacement sensors have also attracted great interest due to their inherent advantages, and several fiber micro-displacement sensors are designed based on fiber Bragg grating. The disadvantages of fiber-optic sensors include high cost, non-reusable, and especially easy to damage during testing process which leads to the failure of the data acquisition system [16–18]. In addition, a large amount of numerical simulation have been used to calculate and analyze the deformation of underground structures, but the highly nonlinear characteristics of geotechnical engineering and the complexity of constitutive equations making the results very difficult to intepret [19–22]. In order to realize high precision of the internal displacement measurement and reduce the influences of surrounding environmental factors on measuring results, Tsinghua University developed a resistive miniature multi-point extensometer which was applied to internal displacement model test of large underground caverns [23]. The multi-point extensometer passes the displacement variable to resistance transducer by thin steel wires in a protecting tube, but its resistance transducer is strongly influenced by external electromagnetic fields. A new mini-grating displacement measuring system is originally developed in this work, which mainly consists of a grating multipoint displacement and a
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DSPL transfer of inner model
Micro multi-point DSPL meter
Tunnel
High accuracy DSPL sensor Hardware and software
Grating scale
DSPL signal
Static test
No
Grating signal acquisition instrument
High speed
PC corresponding software
Parameter setting
Yes Dynamic test
Display and analysis
Fig. 1. Schematic of the grating displacement acquisition system.
collection-analysis system, which has a strong anti-interference ability, but collection-analysis system adopted the programmable logic controller whose volume is large. Although the collection precision is high (resolution ratio is 0.001 mm), it cannot obtain high speed acquisition of displacement and be applied in dynamic physical model tests. We attempted to perform preliminary deformation-measuring tests using this mini-grating displacement measuring system in some geomechanical model tests and the tests results showed that it was reliable in deformation measurement [24–26]. In summary, to achieve high precision and portability of model internal displacement collection, a new mini-grating displacement measuring system is then developed and successfully applied to dynamic and static model tests, especially it can satisfy the high-speed absolute displacement test requirements in dynamic loading tests. 2. A grating mini-type displacement acquisition system 2.1. Displacement test requirements of internal model According to the displacement test needs, and to obtain high precision displacement results in real time, the test should meet the following requirements. (1) Test results should be absolute displacement, but not relative displacement; (2) Internal measure point should be as small as possible to minimize the disturbance of model; (3) High sampling frequency to satisfy the need of instantaneous measurement; (4) High resolution and strong anti-interference ability; (5) Apply to both dynamic and static test with reliable stability; (6) Low cost. 2.2. System composition The self-developed system is mainly composed of a micro multi-point extensometer, a grating ruler, a multi-channel grating signal acquisition instrument, a testing software and other related accessories as shown in Fig. 1. In Fig. 1, the multi-point extensometers transfer the displacement signals of key points in the physical model to the high
Grating scale
Accessory box
Multi-grating data acquisition instrument
Micro multi-point extensometer
Fig. 2. A photo of the grating displacement acquisition system.
accuracy displacement sensor-grating ruler, here displacement will be converted to digital signals, then the acquisition instrument collects digital signals from the grating ruler, setting the parameters of testing software such as sampling frequency and displays the results and analysis. The system has a small volume and is convenient to be carried as shown in Fig. 2.
2.3. The principle of the system Portable multichannel grating data acquisition instrument is the core part of the system, connecting with grating ruler and computer, respectively, by conductors to collect displacement signals from grating ruler and transfer the data to the computer. Its principle is shown in Fig. 3. The motherboard of the grating data acquisition instrument is the integrated circuit plate mainly including four parts, which are the integral of grating signal collection, grating signal count, direction judgment, and frequency doubling part, ARM data communication parts and communication parts between the main chip and the PC. The integral part of grating signal collection adopts high-speed optocouplers to collect grating signal (cut-off frequency up to 20 MHz), and then further integer of grating signals are made through the inverter, so that it can ensure the grating signals clean and complete before entering into the ARM high speed data
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1# grang scale 2# grang scale 3# grang scale
Integer part of grang signal collecon Grang signal count, Direcon judgment and Frequency doubling part ARM data communicaon parts
N# grang scale
Grang signal output secon
Data communicaon part between ARM chip and PC
Data collecon and processing soware
Mul-channel grang signal acquision instrument
PC
Fig. 3. A diagram of the portable multi-channel grating data acquisition instrument.
Table 1 Main performance index of the instrument.
Anchoring head
Items
Technical parameters
Channels Sampling frequency Resolution ratio Accuracy Measuring range Precision error Maximum Operating speed Interface Machine size
32-channel, 16-channel 1 Hz–512 kHz Depend on the grating ruler, minimum 0.1 lm Related to the grating ruler, ±3 lm 100 mm to +100 mm ±0.02% 20–120 m/min
Working environment
Gigabit Ethernet Network 350 mm (Width) 150 mm (Height) 420 mm (Length) Temperature: 10 to 50 °C, Relative humidity: 10–85%
acquisition module. It can greatly improve the reliability and antiinterference of signal acquisition. ARM data communication parts: Every ARM chip transmits the data to the main ARM chip via SSI (synchronous serial interface). Data communication part between the main ARM chip and the PC: the main ARM chip communicates via recommended standard Gigabit Ethernet network interface and the data acquisition software of the PC.
PTFE supporting Protection tube
Steel wire
Pulley
Drop-hammer
Fig. 4. Parts of micro multi-point extensometer.
2.4. Main performance and function of the instrument 2.4.1. Instrument main performance index The main performance index of SD-6 type portable multichannel grating data acquisition instrument is shown in Table 1.
Fig. 5. Embedment of the mini multi-point extensometer.
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Moveable ruler
Stable pulley Fixed ruler Air plug
2.4.2. Instrument function Acquisition instrument is connected to the grating ruler via an aviation plug. The ARM high-speed processing module with high sampling rate can capture the instant displacement. Acquisition instrument can be set up with chassis number sequence, where a computer is connected with more than one collection devices in series at the same time. Acquisition instrument can be simultaneously connected to different accuracy grating rulers. Special collection and analysis software can complete the test parameter settings (sampling frequency and channel number) and has advantages of amicable program interface, data automatic collecting, data acquisition and real-time display of displacement curves, and convenient data export.
Fig. 6. Grating displacement sensor.
Model test system Measuring anchor head Protecting tube Thrust plate
PTFE support ring
Measuring wire
Micro multi-point DSPL meter
Pulley
Tunnel
Grating scale Drop hammer Test model
Independent frame ACQ instrument
(a) The system application schematic of the physical model and the relevant measuring instruments Outlet Grating scale
Hydro-cylinder
Pulley
Steel wire Independent frame Drop-hammer
(b) Photo of micro multi-point extensometer Fig. 7. The system application in geomechanical model test.
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through a PVC protection tube with a diameter of 6 mm. In the protection tube, three strands of thin steel wire pass through the PTFE support ring which can ensure noninterference and reduce the friction. Thin steel wire with a diameter of 0.3 mm is a specific type of fine stainless steel wire which has high strength and good flexibility. The drop-hammer has two types of weight: 120 g and 250 g, which can guarantee the steel wires to be straight. In the process of the construction of the physical model, the multi-point extensometers need to be embedded in the surrounding rock mass of the tunnel, as shown in Fig. 5. In order to reduce the influences on each anchor head, the spacing can be adjusted freely according to the requirements, 20 mm to 50 mm as a general.
Table 2 Main physico-mechanical parameters. Items
Values 3
Density (g/cm ) Elasticity modulus (MPa) Poisson’s ratio Cohesion (MPa) Internal friction angle (°) Uniaxial compressive strength (MPa)
1480 18 0.25 0.6 40 0.76
3. Micro multi-point extensometer and grating ruler 3.1. Micro multi-point extensometer Micro multi-point extensometer is a linkage between the grating ruler and the measuring point in the physical model, with the characteristics of small volume, flexible, and bending buried [1]. It is mainly composed of a measuring point anchor head, an extra thin steel wire, a protection tube, a Polytetrafluoroethylene (PTFE) support ring, a pulley and a drop hammer as shown in Fig. 4. Each multi-point extensometer has three points of anchor head, and the head is connected to the outside of the physical model
3.2. Grating ruler Grating ruler is a type of high-precision micro displacement sensor, based on the physics principle of moire fringe, which mainly consists of light source, two pieces of long grating (moveable and fixed rulers), and the optoelectronic detector. Grating ruler outputs electrical signal and it will change a cycle with the moveable ruler moving a lattice spacing, then obtain the relative displacement
Multipoint DSPL meter Strain gauge Pressure cell Vibration pick-up
Upper bench
Middle bench
Bottom bench
(a) Model tunnel section and measuring points layout (Unit: mm) 400
400
A
500
B
800 Entire Section Excavation
C 800 Layer-step Excavation
D 900 Retained
(b) Longitudinal diagram of the physical tunnel model (Unit: mm) Fig. 8. Physical model test of a tunnel.
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Multipoint DSPL meter Tunnel excavation
(a) Static model test (tunnel excavation)
Dynamic loading
Multipoint DSPL meter
(b) Dynamic model test Fig. 9. Photos of the static model test and dynamic model test.
between moveable and fixed grating ruler through the signal changing cycle. The resolution of the grating ruler can reach up to 0.1 lm, with high precision, high speed and undisturbed by external electromagnetic interference. The bottom of customized moveable ruler installs 4 stable pulleys according to the test requirement, which can ensure the only relative axial movement without side movement between moveable and fixed rulers, and the data line is also in the air plug form, as shown in Fig. 6. 4. Application in static-dynamic test 4.1. System application An application of the grating mini-type displacement acquisition system in model test is shown in Fig. 7(a). Similar materials spread in layers in the model test device. When the construction process of the physical model reached to the key positions such as haunch and vault, mini multi-point extensometers would be embedded. Three
points of anchor head of each multi-point extensometer would be embedded according to the pre-set spacing. The protection tube would be connected to outside along the outlet and fixed on the moveable ruler of the grating ruler which is fixed on an independent frame. The moveable ruler connects with pulleys and hung hammer by steel wire, and all of these elements are to guarantee the transmitting precision of the measuring point displacement in the model for the grating ruler. The installation photos of mini multi-point extensometers are shown in Fig. 7(b). The displacement of the measuring point is passed to the grating ruler without interference, and then the multi-channel grating signal acquisition instrument would obtain high speed and high precision real-time acquisition and data analysis of the measuring displacement. 4.2. Testing system and the physical model The physical model test is performed on a large-scale staticdynamic coupling loading geomechanical model test system,
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SectionA
SectionB
SectionC
Overloading
0
Displacement (mm)
-1 -2 -3 -4 -5
section A -6
section B -7
section C
-8 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36
Excavation step
haunch displacement numerical
haunch displacement test
vault displacement numerical
vault displacement test
vault stress numerical
vault stress test
0
0.00
-1
-0.05
-2
-0.10 -0.15
-3
-0.20 -4 -0.25 -5 -0.30 -6
-0.35
-7
-0.40
Overloading
Section A
-8
-0.45 -0.50
-9 0
4
8
12
16
20
24
28
32
36
Excavation step
(b) Comparison curves on the Section A displacement: test measured and numerical results Fig. 10. Displacement results of the static model test.
450 400
Dynamic loading (kN)
350 300 250 200 150 100 50 0 0
10
20
30
40
50
60
Time (s) Fig. 11. Force-time curve of the vibration generator.
70
80
Stress (MPa)
Displacement (mm)
(a) Vault displacement curves in the excavation-overloading process
S. Li et al. / Measurement 105 (2017) 25–33
0.30 0.25
Displacement (mm)
mainly consisting of a counter-force apparatus system, a static and dynamic hydraulic loading system and a high-speed dataacquisition system, which can be used to physically simulate true triaxial static loading test and dynamic response of surrounding rock subject to dynamic loading. In order to further verify the reliability and accuracy of the testing system, we select a certain tunnel as the testing prototype. According to similarity theory, the geometric similarity scale of the model test is 1:50. Two types of typical yellow sand and loess are selected as the similar material which are mixed together with the volume proportion of 1:6. The physico-mechanical parameters of the prototype rock and similar material are shown in Table 2. Three-centered arch type is selected as the tunnel model, 302 mm in height and 355 mm in width. The mini multi-point extensometer, pressure cell, strain gauges, vibration pick-up are embedded 20 mm away from the tunnel surface, as shown in Fig. 8(a). Among them, the multi-point extensometers are placed on the right side of haunch and the vault, respectively, and the spacing of three measuring points is 20 mm. The excavation methods of the tunnel include the following three: entire section excavation, layer-step excavation, and retained section. The length of the first two excavation methods is 800 mm, and the retained section is 900 mm. The layer-step excavation includes three steps. And there are three monitoring sections, namely Sections A, B and C, as shown in Fig. 8(b).
0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
Time (s) (a) The vault displacement response curve 25 20
Stress (kPa)
32
15 10 5
4.3. Test scheme and process 0
4.4. Results of static displacement
0
4.5. Results of dynamic displacement Fig. 11 shows the dynamic loading-time history curves at the arch crown of the physical model from the vibration generator.
20
30
40
50
60
70
80
150
2
100 50 0 -50 -100
arch bottom
-150
above the vault
-200 0
The testing curves of mini multi-point extensometers in Sections A, B and C close to tunnel surface are shown in Fig. 10(a). The comparison between measured vault and haunch displacements in section with numerical simulation results using FLAC3D is shown in Fig. 10(b). The full displacement curve is obtained in tunnel excavationoverloading process. Compared with other methods, Grating mini-type displacement acquisition system can measure the inner absolute displacement, such as the vault displacement before excavation went into Sections A, B and C. Compared with numerical results, test results have the same trend and last value, especially the displacement result. Those agreements certify that the new mini-grating absolute displacement measuring system has good performances in static model test.
10
Time (s) (b) The vault stress response curve
Acceleration (mm/s )
The test is divided into two parts: static loading test (tunnel excavation and overloading test) in true triaxial static stress state and dynamic loading test which is applied on the top surface of model after tunnel supporting. In the static loading test, the excavation footage is 50 mm, as shown in Fig. 8. Middle and bottom benches will be excavated simultaneously when the upper bench firstly reaches up to 100 mm. In the dynamic loading test, tunnel gypsum lining with a thickness of 10 mm was firstly applied, and strain gauges on the top and two sides of the lining and paste vibration pick-up at the bottom were glued. Then we removed the top beams and cylinders, and installed vibration generator and force transferring structure. Finally, dynamic loadings were applied and the high-speed data acquisition mode was turned on to collect dynamic response data. Photos of static test and dynamic test are shown in Fig. 9.
10
20
30
40
50
60
70
80
Time (s) (c) Vertical acceleration at upper 90 cm of the tunnel vault and th arch bottom Fig. 12. Displacement, strain and acceleration results of the dynamic model test of the tunnel.
The static preloading (100 kN) was firstly applied and then the dynamic load at 2 Hz as shown in Fig. 11. The general strength was increased initially, and then declined later. The maximum load is 400 kN, the minimum load is 50 kN, and the maximum difference is 350 kN. Tunnel displacement, strain and acceleration response frequency is the same with the vibration generator (see Fig. 12). The vault displacement evolution in Section A shows that the test results of the dynamic displacement is 0.053 mm which corresponds to the force 100 kN. Grating- displacement acquisition system fitted well with the curve trend from the vibration generator, which is also increased at first up to 0.24 mm then declined
S. Li et al. / Measurement 105 (2017) 25–33
gradually (see Fig. 12(a)). Moreover, the vault stress response in the same position is selected as a secondary check, as shown in Fig. 12(b). Vertical acceleration at upper 90 cm of the tunnel vault and arch bottom is shown in Fig. 12(c). Through being used in static and dynamic geomechanical model tests, the new mini-grating displacement measuring system has been proved to be an effective instrument in the absolute displacement measure of model test.
5. Conclusions (1) The developed grating mini-type displacement acquisition system includes an SD-6 type portable multi-channel grating data acquisition instrument, a grating ruler, a mini multi-point extensometer, and other related accessories. (2) The multi-channel grating signal acquisition instrument as the secondary instrument can obtain synchronous high speed and high precision acquisition, and communication of grating displacement signal. (3) The mini multi-point extensometer has the characteristics of small volume, low influence on the physical model and location flexibility. The customized grating ruler fixed on the outside of the model has the characteristics of high precision and low interference, which can achieve the high-accuracy data acquisition by the combination of above two parts. (4) The acquisition instrument and the grating ruler can be both used repeatedly, and only the mini multi-point extensometer is disposable, which is much lower in cost compared with other traditional methods. (5) Static and dynamic loading testing on the physical tunnel model prove that the system has stable performances, which can be well applied in the inner displacement high speed acquisition in the physical model test in geotechnical and underground engineering.
Acknowledgment This work was financially supported by Shijiazhuang Tiedao University, National Natural Science Foundation of China (51427804), the National Science and Technology Support Program of China (2015BAB07B05), Shandong Provincial Natural Science Foundation, China (ZR2016EEQ01), the Fundamental Research Funds of Shandong University (2016JC007) and State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining & Technology (SKLGDUEK1708). References [1] L.E. Sozi, General report: urban constraints on underground works, in: World Tunnel Congress 98 on Tunnels and Metropolises, 1998, pp. 879–897. [2] E. Fumagalli, Statical and Geomechanical Models, Springer, 1973.
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Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
A new mini-grating absolute displacement measuring system for static and dynamic geomechanical model tests Shucai Li a,1, Hanpeng Wang a,1, Yong Li a,b,c,⇑, Qingchuan Li a, Bing Zhang a, Haiyang Zhu a a
Geotechnical & Structural Engineering Research Center, Shandong University, Jinan, Shandong Province 250061, PR China School of Civil Engineering, Shandong University, Jinan, Shandong Province 250061, PR China c State Key Laboratory for Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou, Jiangsu Province 221116, PR China b
a r t i c l e
i n f o
Article history: Received 22 November 2016 Received in revised form 19 February 2017 Accepted 3 April 2017 Available online 5 April 2017 Keywords: Absolute deformation measurement Grating data acquisition instrument Micro multi-point extensometer Static and dynamic test Geomechanical model test
a b s t r a c t In this paper, a mini-grating displacement measuring system is developed for geomechanical physical model tests. This system consists of a portable multi-channel grating data acquisition instrument, a grating ruler, a micro multi-point extensometer, and other related accessories. The motherboard of the portable multi-channel grating data acquisition instrument uses a self-developed circuit board and all channels can collect high-speed and high-precision grating displacement signals. The micro multipoint extensometer has advantages of micro-flexibility and small interference in physical model tests. Embedded measuring points in tunnel surrounding rock connect with the grating ruler which is fixed on an independent frame outside of steel wires and connects to the data acquisition system with wire lines. The system is applied to a static and dynamic geomechanical model test on a tunnel. Testing results prove that the system can be satisfactorily applied in geomechanical model tests. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Displacement is a very important testing parameter in geotechnical engineering field practices and laboratory geomechanical model tests. Especially, excavation-induced settlements may cause serious damages to nearby structures [1], therefore the deformation of surrounding rock is a key parameter to measure during tunnel excavation and construction processes [2]. In general, geomechanical model test reflects the spatial structural relations and the experimental process more realistically, which is a unique and indispensable method to investigate the stability of underground surrounding rock, and displacement value is much smaller than that of the prototype model. In order to ensure the accuracy of the displacement measurement, it is critical to develop a highprecision and low-disturbing collection and analysis system [3–8]. In previous studies, dial indicator and digital photogrammetry have been widely utilized in the world [9–12]. Dial indicator has high precision but can be only used in measuring model surface displacement [13–15]. Digital photogrammetry cannot be used to ⇑ Corresponding author at: Geotechnical & Structural Engineering Research Center, Shandong University, No. 17923, Jingshi Road, Jinan, Shandong 250061, China. E-mail address: [email protected] (Y. Li). 1 The first two authors contributed equally to this work and should be considered co-first authors. http://dx.doi.org/10.1016/j.measurement.2017.04.002 0263-2241/Ó 2017 Elsevier Ltd. All rights reserved.
test the internal displacement of the physical model neither and the testing precision is greatly related to the camera resolution, photograph distance and light intensity, which make the testing precision difficult to be guaranteed. Fiber-optic microdisplacement sensors have also attracted great interest due to their inherent advantages, and several fiber micro-displacement sensors are designed based on fiber Bragg grating. The disadvantages of fiber-optic sensors include high cost, non-reusable, and especially easy to damage during testing process which leads to the failure of the data acquisition system [16–18]. In addition, a large amount of numerical simulation have been used to calculate and analyze the deformation of underground structures, but the highly nonlinear characteristics of geotechnical engineering and the complexity of constitutive equations making the results very difficult to intepret [19–22]. In order to realize high precision of the internal displacement measurement and reduce the influences of surrounding environmental factors on measuring results, Tsinghua University developed a resistive miniature multi-point extensometer which was applied to internal displacement model test of large underground caverns [23]. The multi-point extensometer passes the displacement variable to resistance transducer by thin steel wires in a protecting tube, but its resistance transducer is strongly influenced by external electromagnetic fields. A new mini-grating displacement measuring system is originally developed in this work, which mainly consists of a grating multipoint displacement and a
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DSPL transfer of inner model
Micro multi-point DSPL meter
Tunnel
High accuracy DSPL sensor Hardware and software
Grating scale
DSPL signal
Static test
No
Grating signal acquisition instrument
High speed
PC corresponding software
Parameter setting
Yes Dynamic test
Display and analysis
Fig. 1. Schematic of the grating displacement acquisition system.
collection-analysis system, which has a strong anti-interference ability, but collection-analysis system adopted the programmable logic controller whose volume is large. Although the collection precision is high (resolution ratio is 0.001 mm), it cannot obtain high speed acquisition of displacement and be applied in dynamic physical model tests. We attempted to perform preliminary deformation-measuring tests using this mini-grating displacement measuring system in some geomechanical model tests and the tests results showed that it was reliable in deformation measurement [24–26]. In summary, to achieve high precision and portability of model internal displacement collection, a new mini-grating displacement measuring system is then developed and successfully applied to dynamic and static model tests, especially it can satisfy the high-speed absolute displacement test requirements in dynamic loading tests. 2. A grating mini-type displacement acquisition system 2.1. Displacement test requirements of internal model According to the displacement test needs, and to obtain high precision displacement results in real time, the test should meet the following requirements. (1) Test results should be absolute displacement, but not relative displacement; (2) Internal measure point should be as small as possible to minimize the disturbance of model; (3) High sampling frequency to satisfy the need of instantaneous measurement; (4) High resolution and strong anti-interference ability; (5) Apply to both dynamic and static test with reliable stability; (6) Low cost. 2.2. System composition The self-developed system is mainly composed of a micro multi-point extensometer, a grating ruler, a multi-channel grating signal acquisition instrument, a testing software and other related accessories as shown in Fig. 1. In Fig. 1, the multi-point extensometers transfer the displacement signals of key points in the physical model to the high
Grating scale
Accessory box
Multi-grating data acquisition instrument
Micro multi-point extensometer
Fig. 2. A photo of the grating displacement acquisition system.
accuracy displacement sensor-grating ruler, here displacement will be converted to digital signals, then the acquisition instrument collects digital signals from the grating ruler, setting the parameters of testing software such as sampling frequency and displays the results and analysis. The system has a small volume and is convenient to be carried as shown in Fig. 2.
2.3. The principle of the system Portable multichannel grating data acquisition instrument is the core part of the system, connecting with grating ruler and computer, respectively, by conductors to collect displacement signals from grating ruler and transfer the data to the computer. Its principle is shown in Fig. 3. The motherboard of the grating data acquisition instrument is the integrated circuit plate mainly including four parts, which are the integral of grating signal collection, grating signal count, direction judgment, and frequency doubling part, ARM data communication parts and communication parts between the main chip and the PC. The integral part of grating signal collection adopts high-speed optocouplers to collect grating signal (cut-off frequency up to 20 MHz), and then further integer of grating signals are made through the inverter, so that it can ensure the grating signals clean and complete before entering into the ARM high speed data
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1# grang scale 2# grang scale 3# grang scale
Integer part of grang signal collecon Grang signal count, Direcon judgment and Frequency doubling part ARM data communicaon parts
N# grang scale
Grang signal output secon
Data communicaon part between ARM chip and PC
Data collecon and processing soware
Mul-channel grang signal acquision instrument
PC
Fig. 3. A diagram of the portable multi-channel grating data acquisition instrument.
Table 1 Main performance index of the instrument.
Anchoring head
Items
Technical parameters
Channels Sampling frequency Resolution ratio Accuracy Measuring range Precision error Maximum Operating speed Interface Machine size
32-channel, 16-channel 1 Hz–512 kHz Depend on the grating ruler, minimum 0.1 lm Related to the grating ruler, ±3 lm 100 mm to +100 mm ±0.02% 20–120 m/min
Working environment
Gigabit Ethernet Network 350 mm (Width) 150 mm (Height) 420 mm (Length) Temperature: 10 to 50 °C, Relative humidity: 10–85%
acquisition module. It can greatly improve the reliability and antiinterference of signal acquisition. ARM data communication parts: Every ARM chip transmits the data to the main ARM chip via SSI (synchronous serial interface). Data communication part between the main ARM chip and the PC: the main ARM chip communicates via recommended standard Gigabit Ethernet network interface and the data acquisition software of the PC.
PTFE supporting Protection tube
Steel wire
Pulley
Drop-hammer
Fig. 4. Parts of micro multi-point extensometer.
2.4. Main performance and function of the instrument 2.4.1. Instrument main performance index The main performance index of SD-6 type portable multichannel grating data acquisition instrument is shown in Table 1.
Fig. 5. Embedment of the mini multi-point extensometer.
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Moveable ruler
Stable pulley Fixed ruler Air plug
2.4.2. Instrument function Acquisition instrument is connected to the grating ruler via an aviation plug. The ARM high-speed processing module with high sampling rate can capture the instant displacement. Acquisition instrument can be set up with chassis number sequence, where a computer is connected with more than one collection devices in series at the same time. Acquisition instrument can be simultaneously connected to different accuracy grating rulers. Special collection and analysis software can complete the test parameter settings (sampling frequency and channel number) and has advantages of amicable program interface, data automatic collecting, data acquisition and real-time display of displacement curves, and convenient data export.
Fig. 6. Grating displacement sensor.
Model test system Measuring anchor head Protecting tube Thrust plate
PTFE support ring
Measuring wire
Micro multi-point DSPL meter
Pulley
Tunnel
Grating scale Drop hammer Test model
Independent frame ACQ instrument
(a) The system application schematic of the physical model and the relevant measuring instruments Outlet Grating scale
Hydro-cylinder
Pulley
Steel wire Independent frame Drop-hammer
(b) Photo of micro multi-point extensometer Fig. 7. The system application in geomechanical model test.
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through a PVC protection tube with a diameter of 6 mm. In the protection tube, three strands of thin steel wire pass through the PTFE support ring which can ensure noninterference and reduce the friction. Thin steel wire with a diameter of 0.3 mm is a specific type of fine stainless steel wire which has high strength and good flexibility. The drop-hammer has two types of weight: 120 g and 250 g, which can guarantee the steel wires to be straight. In the process of the construction of the physical model, the multi-point extensometers need to be embedded in the surrounding rock mass of the tunnel, as shown in Fig. 5. In order to reduce the influences on each anchor head, the spacing can be adjusted freely according to the requirements, 20 mm to 50 mm as a general.
Table 2 Main physico-mechanical parameters. Items
Values 3
Density (g/cm ) Elasticity modulus (MPa) Poisson’s ratio Cohesion (MPa) Internal friction angle (°) Uniaxial compressive strength (MPa)
1480 18 0.25 0.6 40 0.76
3. Micro multi-point extensometer and grating ruler 3.1. Micro multi-point extensometer Micro multi-point extensometer is a linkage between the grating ruler and the measuring point in the physical model, with the characteristics of small volume, flexible, and bending buried [1]. It is mainly composed of a measuring point anchor head, an extra thin steel wire, a protection tube, a Polytetrafluoroethylene (PTFE) support ring, a pulley and a drop hammer as shown in Fig. 4. Each multi-point extensometer has three points of anchor head, and the head is connected to the outside of the physical model
3.2. Grating ruler Grating ruler is a type of high-precision micro displacement sensor, based on the physics principle of moire fringe, which mainly consists of light source, two pieces of long grating (moveable and fixed rulers), and the optoelectronic detector. Grating ruler outputs electrical signal and it will change a cycle with the moveable ruler moving a lattice spacing, then obtain the relative displacement
Multipoint DSPL meter Strain gauge Pressure cell Vibration pick-up
Upper bench
Middle bench
Bottom bench
(a) Model tunnel section and measuring points layout (Unit: mm) 400
400
A
500
B
800 Entire Section Excavation
C 800 Layer-step Excavation
D 900 Retained
(b) Longitudinal diagram of the physical tunnel model (Unit: mm) Fig. 8. Physical model test of a tunnel.
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Multipoint DSPL meter Tunnel excavation
(a) Static model test (tunnel excavation)
Dynamic loading
Multipoint DSPL meter
(b) Dynamic model test Fig. 9. Photos of the static model test and dynamic model test.
between moveable and fixed grating ruler through the signal changing cycle. The resolution of the grating ruler can reach up to 0.1 lm, with high precision, high speed and undisturbed by external electromagnetic interference. The bottom of customized moveable ruler installs 4 stable pulleys according to the test requirement, which can ensure the only relative axial movement without side movement between moveable and fixed rulers, and the data line is also in the air plug form, as shown in Fig. 6. 4. Application in static-dynamic test 4.1. System application An application of the grating mini-type displacement acquisition system in model test is shown in Fig. 7(a). Similar materials spread in layers in the model test device. When the construction process of the physical model reached to the key positions such as haunch and vault, mini multi-point extensometers would be embedded. Three
points of anchor head of each multi-point extensometer would be embedded according to the pre-set spacing. The protection tube would be connected to outside along the outlet and fixed on the moveable ruler of the grating ruler which is fixed on an independent frame. The moveable ruler connects with pulleys and hung hammer by steel wire, and all of these elements are to guarantee the transmitting precision of the measuring point displacement in the model for the grating ruler. The installation photos of mini multi-point extensometers are shown in Fig. 7(b). The displacement of the measuring point is passed to the grating ruler without interference, and then the multi-channel grating signal acquisition instrument would obtain high speed and high precision real-time acquisition and data analysis of the measuring displacement. 4.2. Testing system and the physical model The physical model test is performed on a large-scale staticdynamic coupling loading geomechanical model test system,
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SectionA
SectionB
SectionC
Overloading
0
Displacement (mm)
-1 -2 -3 -4 -5
section A -6
section B -7
section C
-8 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36
Excavation step
haunch displacement numerical
haunch displacement test
vault displacement numerical
vault displacement test
vault stress numerical
vault stress test
0
0.00
-1
-0.05
-2
-0.10 -0.15
-3
-0.20 -4 -0.25 -5 -0.30 -6
-0.35
-7
-0.40
Overloading
Section A
-8
-0.45 -0.50
-9 0
4
8
12
16
20
24
28
32
36
Excavation step
(b) Comparison curves on the Section A displacement: test measured and numerical results Fig. 10. Displacement results of the static model test.
450 400
Dynamic loading (kN)
350 300 250 200 150 100 50 0 0
10
20
30
40
50
60
Time (s) Fig. 11. Force-time curve of the vibration generator.
70
80
Stress (MPa)
Displacement (mm)
(a) Vault displacement curves in the excavation-overloading process
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0.30 0.25
Displacement (mm)
mainly consisting of a counter-force apparatus system, a static and dynamic hydraulic loading system and a high-speed dataacquisition system, which can be used to physically simulate true triaxial static loading test and dynamic response of surrounding rock subject to dynamic loading. In order to further verify the reliability and accuracy of the testing system, we select a certain tunnel as the testing prototype. According to similarity theory, the geometric similarity scale of the model test is 1:50. Two types of typical yellow sand and loess are selected as the similar material which are mixed together with the volume proportion of 1:6. The physico-mechanical parameters of the prototype rock and similar material are shown in Table 2. Three-centered arch type is selected as the tunnel model, 302 mm in height and 355 mm in width. The mini multi-point extensometer, pressure cell, strain gauges, vibration pick-up are embedded 20 mm away from the tunnel surface, as shown in Fig. 8(a). Among them, the multi-point extensometers are placed on the right side of haunch and the vault, respectively, and the spacing of three measuring points is 20 mm. The excavation methods of the tunnel include the following three: entire section excavation, layer-step excavation, and retained section. The length of the first two excavation methods is 800 mm, and the retained section is 900 mm. The layer-step excavation includes three steps. And there are three monitoring sections, namely Sections A, B and C, as shown in Fig. 8(b).
0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
Time (s) (a) The vault displacement response curve 25 20
Stress (kPa)
32
15 10 5
4.3. Test scheme and process 0
4.4. Results of static displacement
0
4.5. Results of dynamic displacement Fig. 11 shows the dynamic loading-time history curves at the arch crown of the physical model from the vibration generator.
20
30
40
50
60
70
80
150
2
100 50 0 -50 -100
arch bottom
-150
above the vault
-200 0
The testing curves of mini multi-point extensometers in Sections A, B and C close to tunnel surface are shown in Fig. 10(a). The comparison between measured vault and haunch displacements in section with numerical simulation results using FLAC3D is shown in Fig. 10(b). The full displacement curve is obtained in tunnel excavationoverloading process. Compared with other methods, Grating mini-type displacement acquisition system can measure the inner absolute displacement, such as the vault displacement before excavation went into Sections A, B and C. Compared with numerical results, test results have the same trend and last value, especially the displacement result. Those agreements certify that the new mini-grating absolute displacement measuring system has good performances in static model test.
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Time (s) (b) The vault stress response curve
Acceleration (mm/s )
The test is divided into two parts: static loading test (tunnel excavation and overloading test) in true triaxial static stress state and dynamic loading test which is applied on the top surface of model after tunnel supporting. In the static loading test, the excavation footage is 50 mm, as shown in Fig. 8. Middle and bottom benches will be excavated simultaneously when the upper bench firstly reaches up to 100 mm. In the dynamic loading test, tunnel gypsum lining with a thickness of 10 mm was firstly applied, and strain gauges on the top and two sides of the lining and paste vibration pick-up at the bottom were glued. Then we removed the top beams and cylinders, and installed vibration generator and force transferring structure. Finally, dynamic loadings were applied and the high-speed data acquisition mode was turned on to collect dynamic response data. Photos of static test and dynamic test are shown in Fig. 9.
10
20
30
40
50
60
70
80
Time (s) (c) Vertical acceleration at upper 90 cm of the tunnel vault and th arch bottom Fig. 12. Displacement, strain and acceleration results of the dynamic model test of the tunnel.
The static preloading (100 kN) was firstly applied and then the dynamic load at 2 Hz as shown in Fig. 11. The general strength was increased initially, and then declined later. The maximum load is 400 kN, the minimum load is 50 kN, and the maximum difference is 350 kN. Tunnel displacement, strain and acceleration response frequency is the same with the vibration generator (see Fig. 12). The vault displacement evolution in Section A shows that the test results of the dynamic displacement is 0.053 mm which corresponds to the force 100 kN. Grating- displacement acquisition system fitted well with the curve trend from the vibration generator, which is also increased at first up to 0.24 mm then declined
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gradually (see Fig. 12(a)). Moreover, the vault stress response in the same position is selected as a secondary check, as shown in Fig. 12(b). Vertical acceleration at upper 90 cm of the tunnel vault and arch bottom is shown in Fig. 12(c). Through being used in static and dynamic geomechanical model tests, the new mini-grating displacement measuring system has been proved to be an effective instrument in the absolute displacement measure of model test.
5. Conclusions (1) The developed grating mini-type displacement acquisition system includes an SD-6 type portable multi-channel grating data acquisition instrument, a grating ruler, a mini multi-point extensometer, and other related accessories. (2) The multi-channel grating signal acquisition instrument as the secondary instrument can obtain synchronous high speed and high precision acquisition, and communication of grating displacement signal. (3) The mini multi-point extensometer has the characteristics of small volume, low influence on the physical model and location flexibility. The customized grating ruler fixed on the outside of the model has the characteristics of high precision and low interference, which can achieve the high-accuracy data acquisition by the combination of above two parts. (4) The acquisition instrument and the grating ruler can be both used repeatedly, and only the mini multi-point extensometer is disposable, which is much lower in cost compared with other traditional methods. (5) Static and dynamic loading testing on the physical tunnel model prove that the system has stable performances, which can be well applied in the inner displacement high speed acquisition in the physical model test in geotechnical and underground engineering.
Acknowledgment This work was financially supported by Shijiazhuang Tiedao University, National Natural Science Foundation of China (51427804), the National Science and Technology Support Program of China (2015BAB07B05), Shandong Provincial Natural Science Foundation, China (ZR2016EEQ01), the Fundamental Research Funds of Shandong University (2016JC007) and State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining & Technology (SKLGDUEK1708). References [1] L.E. Sozi, General report: urban constraints on underground works, in: World Tunnel Congress 98 on Tunnels and Metropolises, 1998, pp. 879–897. [2] E. Fumagalli, Statical and Geomechanical Models, Springer, 1973.
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