Rock shed is widely used in traffic lines against rockfall. In order to cushion rockfall impact and dissipate impact energy, cushion layer is usually adopted in rock shed. Used tire cushion layer is proposed in this paper and it can cushion rockfall impact utilizing large radial deformation of tire. Reinforced concrete structure model is built with used tire cushion layer and artificial rockfall test is carried out. Twelve tests are divided into 4 sets with different rockfall mass, rockfall height, and tire filling material. Simplified calculation model with spring-damper is derived from radial repeated compression test of used tire, which improves the calculation efficiency. Test and numerical simulation show that application of used tire cushion layer in rock shed can cushion rockfall impact and effectively reduce peak acceleration and the maximum impact force. Filling sand and gravel in tire can improve tire stiffness and energy absorption capacity but will decrease cushion effect due to its large density. With the same impact energy, light rockfall is more destructive than weight rockfall for used tire cushion layer.
In mountainous region, frequent occurrence of geological disasters poses a serious threat to buildings and public infrastructures. Roads and railways that pass through mountains or valley are usually exposed to rockfall hazards and thus people will suffer from traffic suspension, vehicle damage, and personal injury. In recent years, scholars worldwide carried out extensive studies on rockfall hazards, proposing many rockfall hazard protection measures to reduce accidents and losses caused by rockfall disasters. Rockfall protection measures can be divided into active protection measures and passive protection measures. The passive protection measures contain many structure types such as stone-blocking fences, flexible protection system, rock shed, retaining wall, and dam [
Energy absorption capacity of different rockfall protection structure.
In order to mitigate the rockfall impact on rock shed, energy dissipation components or cushion layer are necessary. Delhomme et al. [
Sandwich cushion structure.
A cushion layer with combination of used tires, wire, and sand bags (Figure
Schematic of used tire cushion layer.
The used tires are radial type in all tests. The thread is nearly worn flat without belt damage or any mechanical damage, cracks, or holes in the tire. As shown in Figure
Used tire cushion layer for test.
Test configuration.
Reinforced concrete structure shown in Figure
Rockfall tests are divided into 4 sets based on different tire filling contents and rockfall mass as shown in Table
Test conditions.
Set number. | Serial number | Filling contents | Artificial rockfall | ||
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Impact energy/kJ | |||
A | 1# | Sand | 100 | 1 | 1 |
2# | Sand | 100 | 2 | 2 | |
3# | Sand | 100 | 3 | 3 | |
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B | 4# | Sand | 200 | 0.5 | 1 |
5# | Sand | 200 | 1 | 2 | |
6# | Sand | 200 | 1.5 | 3 | |
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C | 7# | Gravel | 100 | 1 | 1 |
8# | Gravel | 100 | 2 | 2 | |
9# | Gravel | 100 | 3 | 3 | |
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D | 10# | None | 100 | 1 | 1 |
11# | None | 100 | 2 | 2 | |
12# | None | 100 | 3 | 3 |
Rockfall height refers to distance between lower face of rockfall and top surface of cushion layer. Rockfall impact energy is calculated with gravity 10 m/s2. The calculation is based on hypothesis that potential energy is converted into kinetic energy without considering air resistance and others.
The decoupling device is connected to the crane and rockfall and then crane will lift rockfall to a set height through automatic unhooking device. TST6200 and high-speed camera are turned on. Switch the decoupling device to release rockfall. TST6200 and high-speed camera will record data as configured. Tests are carried out in accordance with the order of serial number in Table
As observed in the tests, rockfall will impact tire cushion layer or concrete structure roof once or more after first impact on and rebound from tire cushion layer (as shown in Figure
The rockfall impact tack captured by camera in test 2#.
Rockfall entering camera capture range (34 frames)
The first time that rockfall comes in contact with tire cushion layer (
Rockfall impact to its lowest point in its track (79 frames)
The first time that rockfall rebounds apart from tire cushion layer (
Rockfall rebound to its peak in its track after first impact (
Rockfall impact cushion layer for the second time (246 frames)
According to Newton’s third law, the average impact force of rockfall applied on tire cushion layer equals average force of tire cushion layer applied on rockfall written
Take rockfall as analysis object and, according to momentum theorem, we have
Frame rate of high-speed camera is set to 250 fps considering calculation requirement and light condition. Image that corresponds with
Average force
Set number | Serial number |
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A | 1# | 172 | 222 | 267 | 1.800 | 0.200 | 4.136 | 0.12 |
2# | 59 | 110 | 163 | 2.120 | 0.204 | 5.139 | 0.37 | |
3# | 210 | 263 | 323 | 2.400 | 0.212 | 5.786 | 0.78 | |
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B | 4# | 265 | 323 | 350 | 1.080 | 0.232 | 5.657 | 0.17 |
5# | 275 | 335 | 362 | 1.080 | 0.240 | 6.627 | 0.70 | |
6# | 312 | 375 | 404 | 1.160 | 0.252 | 7.268 | 1.36 | |
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C | 7# | 69 | 116 | 151 | 1.400 | 0.188 | 4.123 | 0.20 |
8# | 195 | 244 | 284 | 1.600 | 0.196 | 5.043 | 0.51 | |
9# | 244 | 294 | 339 | 1.800 | 0.200 | 5.773 | 0.93 | |
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D | 10# | 116 | 180 | 223 | 1.720 | 0.256 | 3.419 | 0.13 |
11# | 205 | 276 | 338 | 2.480 | 0.284 | 4.100 | 0.16 | |
12# | 138 | 212 | 287 | 3.000 | 0.296 | 4.630 | 0.20 |
Within each set of tests, time duration in which rockfall contacts tire cushion layer increases with rockfall impact energy. Contact time increase step is small while tires are filled with sand or gravel. Contact time increases 2~5% with adjacent rockfall impact energy. Contact time increase step is large while tires are empty without any filling. Contact time increases 2~5% with adjacent rockfall impact energy.
Within each set of tests, peak acceleration of concrete structure slab increases with rockfall impact energy. Peak acceleration increment is large while tires are filled with sand or gravel. Peak acceleration increases 82~312% with adjacent rockfall impact energy. Peak acceleration increment is small while tires are empty without any filling. Increment increases 23~25% with adjacent rockfall impact energy.
Average force of tire cushion layer applied on rockfall and rockfall rebound velocity increase with impact energy within each set of tests.
Tires are all filled with sand in test set A and set B and rockfall mass is 100 kg and 200 kg, respectively, for test set A and set B. With the same rockfall impact energy, contact time duration, contact force, and peak acceleration of concrete slab are all bigger in set A than that in set B, but rockfall rebound velocity is opposite.
In summary it is obvious that contact time and force, rockfall rebound velocity, and peak acceleration of concrete slab increase with rockfall impact energy. But with the same impact energy, rockfall with large mass is unfavorable.
In test sets A, C, and D, rockfall masses are all 100 kg. But, with the same impact energy, tire cushion layer shows different cushion capacity due to different tire filling content.
The contact time in sets A and C is basically the same and contact time in set D is the longest among the three sets with about 8% more than sets A and C.
Average contact force in set D is the smallest among the test sets A, C, and D. Average contact force in sets A and C is basically the same with average 23% increase compared with set D.
Rockfall rebound velocity in set C is the smallest within the test sets A, C, and D which indicates that cushion layer filled with gravel shows good energy absorption property. The energy absorption capacity derives from gravel crushing and friction.
In set D, peak acceleration of slab is the smallest within the three sets of tests. Peak acceleration of slab in sets A and C is basically the same with average 197% higher than that in set D.
All in all, within the four sets of tests, used tire cushion layer without any filling performs better in cushion rockfall impact and reduces peak acceleration of rock shed slab and is lighter in weight compared with cushion layer filled with sand or gravel which is preferable. Due to the test conditions limitation, test carried out with low rockfall impact energy and used tire deformation is small. Used tire cushion layer will be compressed and flattened. Filled with sand or gravel, tire stiffness increases and tire can also absorb some impact energy from crush and friction of sand or gravel. However, different from [
Zhang [
The force-deformation curve of tire under repeated compression test is shown in Figure
The force-deformation curve of tire under repeated radial compression.
Tires show good elastic properties in radial compression tests that are similar to mechanical properties of spring, establishing spring-damper model using COMBI165 element (spring-damper element) to simulate used tire cushion layer under rockfall impact. Take the average of the second and third compression force-deformation curves and then simplify them to broken line shown in Figure
Simplified Force-displacement curve of used tires under radial compression.
Figure
Calculation model of tire.
Tire model
Simplified calculation model
Contrast of force-time curves.
Concrete structural model is established in accordance with the actual property in rockfall impact test. Simulations were carried out with rockfall 100 kg and falling height 1 m, 2 m, and 3 m. Average impact force applied on slab and tire deformation derived from simulation with comparison with test results are listed in Tables
The contrast of the average forces in the test and numerical simulation.
Rockfall impact energy/kJ | 1 | 2 | 3 |
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Test/kN | 3.42 | 4.10 | 4.63 |
Simulation/kN | 4.28 | 5.78 | 6.90 |
The contrast of the maximal compressed length.
Rockfall impact energy/kJ | 1 | 2 | 3 |
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Test/m | 0.26 | 0.32 | 0.40 |
Simulation/m | 0.32 | 0.40 | 0.45 |
Performances of concrete slab with and without used tire cushion layer are both simulated in two series simulations as shown in Table
Rockfall configuration in simulation.
Series | Rockfall configuration |
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1 kJ | 2 kJ | 3 kJ | 5 kJ | 10 kJ | |
One |
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Two |
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Simplified model with advantage of high computational efficiency and easy access to parameters can be used to simulate the rockfall impact process and results can reflect used tire cushion layer performance. Results derived from simulation of rockfall with different mass and falling height impact slab without cushion layer given in Tables
Maximum impact force of slab without cushion layer.
Series | Maximum impact force/kN | ||||
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1 kJ | 2 kJ | 3 kJ | 5 kJ | 10 kJ | |
One | 2470 | 3369 | 4051 | 5082 | 6717 |
Two | 2774 | 3576 | 3817 | 4465 |
Maximum displacement of slab without cushion layer.
Series | Maximum displacement/mm | ||||
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1 kJ | 2 kJ | 3 kJ | 5 kJ | 10 kJ | |
One | 2.35 | 3.38 | 4.21 | 5.68 | 8.65 |
Two | 1.8 | 4.79 | 7.32 | 12.04 |
Results derived from simulation of rockfall with different mass and falling height impact slab with cushion layer given in Figure
Impact-time curve of slab.
With rockfall configurations
The process contrast of different mass of rockfall in the same energy level.
Rockfall energy/kJ | 1 | 1 | 3 | 3 |
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Rockfall mass/kg | 50 | 100 | 100 | 150 |
Tire deformation/m | 0.27 | 0.32 | 0.45 | 0.50 |
Average impact force/kN | 4 | 4.28 | 6.9 | 7.69 |
The maximum impact force applied on slab with cushion layer is much less than that without cushion layer from comparison of Table
Rock shed is commonly used in traffic line disaster protection. In order to improve rock shed performance, impact cushion and energy absorption measures are usually adopted. This paper proposes a used tire cushion layer for rock shed and carries out test and numerical simulation studies; results show that (1) used tire cushion layer can effectively cushion rockfall impact and reduce peak acceleration and maximum impact of rock shed slab; (2) cushion layer filled with sand and gravel can increase rigidity and energy absorption capacity but increases dead load of rock shed simultaneously; (3) with the same energy, rockfall with large mass is more unfavorable for cushion layer; (4) simplified calculation model can greatly improve the computational efficiency and the parameters for calculation are easy to obtain through test.
In practical engineering application, cushion layer can be set up with multilayer and filled with polyurethane foam, polystyrene, low-density industrial waste, or other lightweight materials with a cell structure in order to improve the rigidity and energy absorption capacity of cushion layer.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by grant from National Natural Science Foundation of China (nos. 51378495 and 51408602), Chongqing Natural Science Foundation (cstc2012jjA30005, cstc2012jjB30004), and Bureau of Land Resources and Housing Management of Chongqing.