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An acoustic emission characterization of the failure process of shallow foundation resting on sandy soils
Ultrasonics 93 (2019) 107–111
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short communication
An acoustic emission characterization of the failure process of shallow foundation resting on sandy soils
T
⁎
Wuwei Maoa,b, , Yang Yangb, Wenli Linc a
Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, China Key Laboratory of Geotechnical and Underground Engineering, Ministry of Education, Tongji University, Shanghai 200092, China c Department of Civil Engineering, The University of Tokyo, Tokyo 113-8656, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acoustic emission Shallow foundation Failure Sand Kaiser Effect
Shallow foundation is a common foundation type that is usually used for small to medium size structures. The bearing ability and the failure mechanism of shallow foundation are the fundamental concerns for geotechnical engineers, and the demand for new insights into the relevant issue is still increasing. This paper presents an acoustic emission (AE) characterization of the failure process of shallow foundation, with the aim of revealing the fundamental information on AE signals associated with shallow foundation loading as well as its connection with the ground bearing behavior. Experiments were carried out to model the failure process of shallow foundation resting on sandy ground with different densities (i.e. loose and dense) and subjected to different loading conditions (i.e. monotonic and cyclic loading). Comparisons between AE activities and ground bearing behavior are presented. The feasibility of using AE for stability monitoring of shallow foundation is revealed and discussed.
1. Introduction Shallow foundation is widely used for small to medium size structures. In order to ensure the safety of the structure, its foundation is expected to meet the strength conditions. That is, the required foundation should has sufficient ability to bear the load to resist sliding damage due to insufficient shear strength. Accordingly, determination of the bearing capacity of the foundation is of fundamental importance for foundation design, and the clarification of the ground failure mechanism as well as the evaluation of shallow foundation bearing capacity have been of great interest among researchers [1–7]. Compared with the damage or failure assessment of the upper structures, the foundation failure is often difficult to be detected since it is hidden under the ground. Up to now, accurate determination of bearing capacity of foundations remains an intractable problem in the field of geotechnical engineering [8], and there are ongoing demands to have more insights into the ground bearing behavior and its failure mechanism [9,10]. Usually, the ultimate bearing capacity of a foundation is the major concern for the engineering designers. However, apart from the foundation’s bearing behavior during the service stage, the stability of a foundation during the construction operations also deserves close attention. In a sense, the foundation during the construction period may
⁎
experience the most critical condition in its life cycle. One example is that a 13-storey apartment building toppled over almost intact due to the foundation failure in Shanghai, China, which is believed to be caused by a combination of the temporary dumping of the dug-out soil against one side and the excavation of an underground garage on the other [11]. Therefore, the status of foundation during construction process should also be properly monitored. Load and displacement are routinely used parameters for the severability monitoring of upper structures. Regarding shallow foundations, it should be noted that the bearing load monitoring on site is relatively difficult, while the displacement monitoring of ground surface or foundation itself may also not reveal the ground failure mechanism. Accordingly, this study introduces the acoustic emission method to monitoring the failure process of shallow foundations. As a non-intrusive method, the AE method has been demonstrated to have the ability to detect the impending failure of rock, concrete and composite materials [12–14]. However, it has seldom been used for monitoring of granular soils due to the complex nature of the soil materials. The AE signals could experience severe attenuation in a granular system, which brings measurement and interpretation difficulties. Nevertheless, the feasibility of the AE method applied to soil materials and its benefits have long been evidenced [15–20]. The earlier works mainly focused on revealing the qualitative relations between the AE
Corresponding author at: Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, China. E-mail address: [email protected] (W. Mao).
https://doi.org/10.1016/j.ultras.2018.11.007 Received 9 June 2018; Received in revised form 22 October 2018; Accepted 20 November 2018 Available online 22 November 2018 0041-624X/ © 2018 Elsevier B.V. All rights reserved.
Ultrasonics 93 (2019) 107–111
W. Mao et al.
Fig. 1. Typical failure pattern and the relevant load-displacement curves for shallow foundations resting on soil.
activity (mostly represented by the ring-down count) and different types/conditions of stressed soils; while more recent advances have been made on establishing a potential approach toward solving a specific geotechnical problem, and the interpretation of AE have been extended to a variety of parameters including spectrum analysis. Typical examples of such applications include the AE monitoring system for early warning of soil slope failure [21,22], breakage behavior of soil particle [23], seepage of soils [24], erosion of soils [25], pile bearing behavior in soils [26,27], and etc. This study presents a follow-up effort of such applications regarding AE generation in a shallow foundation situation. When a shallow foundation is subjected to load, the potential internal damage in the ground body would generate AE signals, propagate to the ground surface and be picked up by the AE sensor, as schematically illustrated in Fig. 1. According to the previous studies [1,28], the failure mode of shallow foundations can be generally divided into three categories, i.e. general shear failure, local shear failure and punching failure, as shown in Fig. 1. The general shear failure is usually seen in stiff or dense soil, characterized by a well-defined failure surface extended to the ground surface, and a pronounced peak in the load displacement curve. The local shear failure is seen in relatively loose soils and the failure surface is not well developed and does not expose to the ground surface. Slight bulging of ground is expected and there is no well-defined peak in the load displacement curve. The punching shear failure is usually seen in very soft ground and is characterized by significant large settlement. Ideally, no increase in load is expected when the displacement is continuously increasing for punching shear failure. In this study, loading tests were examined on shallow foundations resting on two typical types of the above sand grounds, i.e. a dense ground and a relatively loose ground. The AE signals were detected in the respective loading tests, and the data of AE activity and the ground bearing load were synchronized to verify the feasibility of the AE method applied to shallow ground monitoring.
Fig. 2. Layout of shallow foundation loading system equipped with AE sensor.
2. Experimental arrangements A schematic illustration of the test setup is shown in Fig. 2. The loading tests were conducted in a conventional motor loading frame assembly. The sands were prepared in a steel soil tank with internal dimensions 600 mm × 600 mm × 500 mm. The model footing used for the tests was made of rigid steel, square in shape, and measured 40 × 62 mm in the bottom surface. The tests were performed by a displacement controlled manner with a constant loading rate of 1 mm/ min. Load cell and displacement meter were used to capture the loadsettlement behavior of the footing. The AE signals were detected using AE sensor attached to the side of the footing. The piezo-ceramic type AE sensor, manufactured by Fuji Ceramics Corporation, has an optimum operating frequency range of 10 kHz–2 MHz. A main amplifier was used to enhance the AE signals with a gain of 115 dB when cooperated with the head amplifier embedded within the sensor. The sampling rate of AE data recording system was set at 2 Mps. Silica sand No. 5 was used as the model ground material. This sand is classified as the poorly-graded sand according to the Unified Soils
Fig. 3. Grain size distribution of the tested sand.
Classification System [29] and has specific gravity (Gs) of 2.65, maximum void ratio (emax) of 1.09, minimum void ratio (emin) of 0.66, uniformity coefficient (Cu) of 1.692, and mean grain size (D50) of 0.557 mm. The grain size distribution of the sand is shown in Fig. 3. The air pluviation method, i.e. sprinkling the sand from the air at a constant falling height, was used for preparing the model ground. In the current study, two types of ground conditions were made, which were the dense case and relatively loose case. For dense condition, the model ground was further compacted after every 30 mm deposition by dropping a 10 kg-weight at a height of about 10 cm 20 times all over the 108
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Fig. 5. AE count, AE amplitude and ground resistance as a function of footing displacement: (a) loose ground; (b) dense ground.
Fig. 4. AE energy and ground resistance as a function of footing displacement: (a) loose ground; (b) dense ground.
be potentially used for investigation of the ground bearing behavior. Apart from the AE energy, the AE count and amplitude are also regarded as important parameters for analysis of AE signals. The mean AE amplitude, calculated by averaging the AE event amplitude of each 10 s interval, is used to represent the AE amplitude behavior in the current study. It can be seen from Fig. 5 that for loose ground, the AE amplitude is rather fluctuating, while for dense ground, it is more stabilized. Overall, the values of the mean AE amplitude do not show much difference between the loose and dense conditons. However, the AE count in case of loose ground is almost two orders of magnitude smaller than that of dense ground. In particular, it is notable that the AE count was not significant until the footing displacement was larger than ∼25 mm for loose ground. Meanwhile, the corresponding mean AE amplitude also appeared to be less fluctuated after similar depth of footing settlement. On the other hand, the AE amplitude and event count in case of dense ground demonstrated more obvious relevance with the ground resistance, especially during the first 15 mm footing settlement where a rising or decreasing in the ground resistance corresponded to the rising or decreasing of AE count and mean amplitude. However, when the ground displacement was larger than 15 mm, the mean amplitude did not show any significant change, while the AE count kept increasing continuously, similar with the increasing trend of the ground resistance. Compared with AE energy, it seems that the AE count and amplitude have less correspondences with the ground resistance, especially when the ground is relatively loose. Therefore, it is recommended to use AE energy as the identification parameter of AE analyses. The two loading tests described above demonstrated distinct AE characteristics for different ground conditions. For dense ground expected to experience general failure mode, a pronounced peak in the load and AE curve was observed. While for relatively loose ground expected to experience local failure mode, the load and AE kept
surface. This procedure was accurate enough as the repeatability of results can be easily realized. While for loose condition, no further compaction was performed. The final relative density of the ground was 97% and 35% for dense and loose cases respectively. 3. Results and discussions 3.1. Ground resistance and AE activity Fig. 4 shows the trends of ground resistance and AE energy development with respect to the footing displacement. The AE energy here is calculated by the integration of squared AE signal voltage within the signal duration [30]. Distinct differences between the dense and loose ground conditions can be noticed. For loose case, the ground resistance increased gradually with the depth of footing settlement throughout the whole loading process. In view of AE, it was detected throughout the above process as well, and consequently the accumulative AE energy kept rising as the footing displacement increased. In general, the AE energy rate, represented by the summation of AE energy of each 10 s interval, increased with the footing displacement. For dense case, the ground resistance was firstly increased rapidly to a peak value, followed by a substantial reduction and then increased again, but with a lower rate. The AE activity was also detected throughout the whole loading process, except that at the very beginning of loading, the AE was much less significant. The trend of AE rate shared similar tendency with the development of ground resistance. For example, the parabolic feature of ground resistance in the first 15 mm displacement can be well captured. A rising or decreasing in the ground resistance corresponded to the rising or decreasing of AE energy rate. In particular, notable rising of AE is detected when the ground resistance is approaching its peak at around 5 mm displacement. This suggests that the AE monitoring could 109
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Fig. 6. Ground resistance and AE activity in case of cyclic loading condition.
increasing throughout the loading process. It is therefore suggested that the AE could capture the failure mode of the shallow foundation as traditionally defined by the load-displacement relations. Fig. 7. Accumulative AE history plot demonstrating Kaiser Effect.
3.2. AE activity during cyclic loading
which consequently generates AE signals. Such process is different from the mechanism of AE generation in metal or rock materials where the crack formation or growth is the main AE sources. If the applied load is smaller than the previous maximum load, the crack will not suffer further growth and the AE is expected to be dim. It is worth noting that after global failure of the ground, the ground resistance experienced a gradual reduction period. However, the AE during this period remained quite active. This is possibly due to the fact that after the global failure, the shear band is well formed below the foundation, and the particles within the shear band are continually subjected to relative displacement during loading process. Consequently, despite that the ground resistance is reducing, the AE signals are constantly generated. Therefore, the Kaiser Effect is not applicable for characterization of ground situation after global failure. Considering that the ground is usually functioning before the general ground failure, it is suggested that AE could still be used as a useful tool for evaluation of ground status. It is worth noting that the current study presents the results of AE monitoring in a laboratory modelling scale. For potential applications of the AE instrumentation in the field, the foundation could have a more significant length and may have other interactions with the soil in place, including the existence of water, ground anisotropy, considerable attenuation of AE signals and etc. These involve the problems of many new aspects and further investigations deserve to be proceeded. According to the existing field applications, the above issues may not change the validity of AE monitoring [21,31]. Although the intensity or property of the AE signals might be easily affected by different field conditions, the evolution tendency could remain similar, which is more important for field engineers to assess the initiation of damage or the impending failure of the ground.
Regarding the AE activity of stressed materials, it is generally recognized that when the material is loaded and then unloaded, the corresponding AE response is different upon reloading compared to the original loading. This is due to the well-known Kaiser Effect. Consequently, shallow foundation subjected to historical load may result in different AE activity. Therefore, an additional test was performed to investigate the AE behavior of shallow foundation under pre-loaded condition. 19 cycles of loading with maximum ground resistance of about 40 kPa (Loading Stage 1) and subsequent 7 cycles of loading with maximum ground resistance of about 60 kPa (Loading Stage 2) were applied, followed by a monotonic loading until ground failure (Loading Stage 3). The ground was prepared under the same condition of the above dense case, and accordingly, the applied cyclic loading stresses were believed to be smaller than the peak failure stress of the ground. Fig. 6 shows the time history of the ground resistance and AE activity. One notable issue is that there existed stress plateau zones when the ground resistance approached around 20–25 kPa. This might be attributed to the local failure of the ground. Another important issue is that the Kaiser Effect did demonstrate its influence on AE activity. Although the AE activity did not turn to be zero during reloading steps, the amount of the detected AE during the reloading steps reduced dramatically. When the stress exceeded the previous maximum value, e.g. during the 20th loading cycle or after 26th loading cycle, the AE activity became significant again. Fig. 7 shows the accumulative AE and the ground resistance relationship. It is clearly demonstrated that during unloading and reloading process, the accumulative AE did not increase notably if the ground resistance did not exceed the previous maximum value (i.e. from 1st cycle to 19th cycle during Loading Stage 1 and 20th cycle to 26th cycle during Stage 2). On the other hand, notable increment of AE can be observed once the applied stress was larger than the previous maximum value (i.e. from Loading Stage 1 to Stage 2 and from Stage 2 to Stage 3). For materials such as metal, rock, and concrete, the AE activity is generally considered to be faint if the current stress is smaller than the historical maximum stress due to the Kaiser Effect (or similarly the Felicity Effect). Nevertheless, for granular soils in the current study, although the AE become less significant during reloading process, it did occur throughout the whole process. This is because the soils consist of numerous granular particles, and any disturbance due to foundation settlement could result in the rearrangement of the granular assembles
4. Conclusion This study presents an experimental investigation of acoustic emission during the failure process of shallow foundation. The major findings of this study can be summarized as following: (1) The AE is well observed throughout the failure process of the shallow foundation. The rate of AE activity is related with the ground density and the loading procedure. In general, the evolving trend of AE rate shared similar tendency with the development of ground resistance. 110
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(2) The energy of AE signals can be used as a better identification parameter for characterizing the behavior of the shallow foundation compared with AE count or amplitude. (3) The influence of Kaiser Effect is observed during cyclic loading of shallow foundation. AE becomes less significant during reloading process when the applied stress does not exceed the previous maximum stress. However, it did occurred throughout the whole loading process, demonstrating certain difference compare with those observed from metal or rock materials. (4) After the global failure of the ground, the Kaiser Effect is not applicable since the AE may continuously increase upon decreasing ground resistance.
[11] J. Chai, S. Shen, W. Ding, H. Zhu, J. Carter, Numerical investigation of the failure of a building in Shanghai, China, Comput. Geotech. 55 (2014) 482–493. [12] Z. Moradian, H.H. Einstein, G. Ballivy, Detection of cracking levels in brittle rocks by parametric analysis of the acoustic emission signals, Rock Mech. Rock Eng. 49 (3) (2016) 785–800. [13] A. Behnia, H.K. Chai, T. Shiotani, Advanced structural health monitoring of concrete structures with the aid of acoustic emission, Constr. Build. Mater. 65 (2014) 282–302. [14] V. Malolan, G. Wuriti, A.S. Gopal, T. Thomas, Comparison of acoustic emission parameters for fiber breakage and de-lamination failure mechanisms in carbon epoxy composites, J. Eng. Technol. Res. 8 (3) (2016) 21–30. [15] R.M. Koerner, A.E. Lord Jr, Acoustic emissions in stressed soil samples, J. Acoust. Soc. Am. 56 (6) (1974) 1924–1927. [16] A.E. Lord Jr, R.M. Koerner, Acoustic emissions in soils and their use in assessing earth dam stability, J. Acoust. Soc. Am. 57 (2) (1975) 516–519. [17] R.M. Koerner, W.M. McCabe, A.E. Lord, Acoustic emission behavior and monitoring of soils, Acoustic Emissions in Geotechnical Engineering Practice, a symposium, 750 ASTM STP, 1981, pp. 93–141. [18] R.M. Koerner, A.E. Lord Jr, W.L. Deutsch, Determination of prestress in granular soils using AE, J. Geotech. Eng.-ASCE. 110 (3) (1984) 346–358. [19] K. Tanimoto, Y. Tanaka, Yielding of soil as determined by acoustic emission, Soils Found. 26 (3) (1986) 69–80. [20] J. Chen, H. Wang, Y. Yao, Experimental study of nonlinear ultrasonic behavior of soil materials during the compaction, Ultrasonics 69 (2016) 19–24. [21] A. Smith, N. Dixon, P. Meldrum, E. Haslam, J. Chambers, Acoustic emission monitoring of a soil slope: comparisons with continuous deformation measurements, Geotech. Lett. 4 (4) (2014) 255–261. [22] A. Zaki, H.K. Chai, H.A. Razak, T. Shiotani, Monitoring and evaluating the stability of soil slopes: a review on various available methods and feasibility of acoustic emission technique, C. R. Geosci. 346 (9–10) (2014) 223–232. [23] W. Mao, I. Towhata, Monitoring of single-particle fragmentation process under static loading using acoustic emission, Appl. Acoust. 94 (2015) 39–45. [24] M.H. Hung, G.C. Lauchle, M.C. Wang, Seepage-induced acoustic emission in granular soils, J. Geotech. Geoenviron. 135 (4) (2009) 566–572. [25] Z. Lu, G.V. Wilson, Acoustic measurements of soil pipe flow and internal erosion, Soil Sci. Soc. Am. J. 76 (3) (2012) 853–866. [26] W. Mao, S. Aoyama, I. Towhata, Feasibility study of using acoustic emission signals for investigation of pile spacing effect on group pile behavior, Appl. Acoust. 139 (2018) 189–202. [27] W. Mao, Y. Yang, W. Lin, S. Aoyama, I. Towhata, High frequency acoustic emissions observed during model pile penetration in sand and implications for particle breakage behavior, Int. J. Geomech. 18 (11) (2018) 04018143. [28] A.S. Vesic, Analysis of ultimate loads of shallow foundations, J. Soil Mech. Found. Div. 99 (sm1) (1973) 45–73. [29] ASTM, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), D2487-11, West Conshohocken, PA, 2011. [30] R.K. Miller, E.V.K. Hill, P.O. Moore (Eds.), Nondestructive Testing Handbook, third ed., vol. 6, Acoustic Emission Testing, 2005. [31] N.W. Xu, C.A. Tang, L.C. Li, Z. Zhou, C. Sha, Z.Z. Liang, J.Y. Yang, Microseismic monitoring and stability analysis of the left bank slope in Jinping first stage hydropower station in southwestern, China, Int. J. Rock Mech. Min. 48 (6) (2011) 950–963.
Acknowledgment This work is supported by the Shanghai Sailing Program (Grant number 18YF1424000), the Fundamental Research Funds for the Central Universities of China (Grant number 22120170118), and Shanghai Education Commission (Peak Discipline Construction, Grant number 0200121005/052). References [1] K. Terzaghi, Theoretical Soil Mechanics, John Wiley and Sons, New York, 1943. [2] G.G. Meyerhof, The ultimate bearing capacity of foundations on slopes, Proc. 4th Int. Conf. on Soil Mechanics and Foundation Engineering, vol. 1, 1957, pp. 384–386. [3] L.Y. Wu, Y.F. Tsai, Analysis of earth pressure for retaining wall and ultimate bearing capacity for shallow foundation by variational method, J. Mech. 20 (1) (2004) 43–56. [4] M.T. Omar, B.M. Das, S.C. Yen, V.K. Puri, E.E. Cook, Ultimate bearing capacity of rectangular foundations on geogrid-reinforced sand, Geotech. Test. J. 16 (2) (1993) 246–252. [5] M. Abu-Farsakh, Q. Chen, R. Sharma, An experimental evaluation of the behavior of footings on geosynthetic-reinforced sand, Soils Found. 53 (2) (2013) 335–348. [6] M. Chakraborty, J. Kumar, Bearing capacity of circular foundations reinforced with geogrid sheets, Soils Found. 54 (4) (2014) 820–832. [7] S. Saha Roy, K. Deb, Effects of aspect ratio of footings on bearing capacity for geogrid-reinforced sand over soft soil, Geosynth. Int. 24 (4) (2017) 362–382. [8] C. Vulpe, S. Gourvenec, M. Power, A generalised failure envelope for undrained capacity of circular shallow foundations under general loading, Geotech. Lett. 4 (3) (2014) 187–196. [9] E. Conte, A. Donato, A. Troncone, Progressive failure analysis of shallow foundations on soils with strain-softening behavior, Comput. Geotech. 54 (2013) 117–124. [10] Q. Chen, M. Abu-Farsakh, Ultimate bearing capacity analysis of strip footings on reinforced soil foundation, Soils Found. 55 (1) (2015) 74–85.
111
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short communication
An acoustic emission characterization of the failure process of shallow foundation resting on sandy soils
T
⁎
Wuwei Maoa,b, , Yang Yangb, Wenli Linc a
Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, China Key Laboratory of Geotechnical and Underground Engineering, Ministry of Education, Tongji University, Shanghai 200092, China c Department of Civil Engineering, The University of Tokyo, Tokyo 113-8656, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acoustic emission Shallow foundation Failure Sand Kaiser Effect
Shallow foundation is a common foundation type that is usually used for small to medium size structures. The bearing ability and the failure mechanism of shallow foundation are the fundamental concerns for geotechnical engineers, and the demand for new insights into the relevant issue is still increasing. This paper presents an acoustic emission (AE) characterization of the failure process of shallow foundation, with the aim of revealing the fundamental information on AE signals associated with shallow foundation loading as well as its connection with the ground bearing behavior. Experiments were carried out to model the failure process of shallow foundation resting on sandy ground with different densities (i.e. loose and dense) and subjected to different loading conditions (i.e. monotonic and cyclic loading). Comparisons between AE activities and ground bearing behavior are presented. The feasibility of using AE for stability monitoring of shallow foundation is revealed and discussed.
1. Introduction Shallow foundation is widely used for small to medium size structures. In order to ensure the safety of the structure, its foundation is expected to meet the strength conditions. That is, the required foundation should has sufficient ability to bear the load to resist sliding damage due to insufficient shear strength. Accordingly, determination of the bearing capacity of the foundation is of fundamental importance for foundation design, and the clarification of the ground failure mechanism as well as the evaluation of shallow foundation bearing capacity have been of great interest among researchers [1–7]. Compared with the damage or failure assessment of the upper structures, the foundation failure is often difficult to be detected since it is hidden under the ground. Up to now, accurate determination of bearing capacity of foundations remains an intractable problem in the field of geotechnical engineering [8], and there are ongoing demands to have more insights into the ground bearing behavior and its failure mechanism [9,10]. Usually, the ultimate bearing capacity of a foundation is the major concern for the engineering designers. However, apart from the foundation’s bearing behavior during the service stage, the stability of a foundation during the construction operations also deserves close attention. In a sense, the foundation during the construction period may
⁎
experience the most critical condition in its life cycle. One example is that a 13-storey apartment building toppled over almost intact due to the foundation failure in Shanghai, China, which is believed to be caused by a combination of the temporary dumping of the dug-out soil against one side and the excavation of an underground garage on the other [11]. Therefore, the status of foundation during construction process should also be properly monitored. Load and displacement are routinely used parameters for the severability monitoring of upper structures. Regarding shallow foundations, it should be noted that the bearing load monitoring on site is relatively difficult, while the displacement monitoring of ground surface or foundation itself may also not reveal the ground failure mechanism. Accordingly, this study introduces the acoustic emission method to monitoring the failure process of shallow foundations. As a non-intrusive method, the AE method has been demonstrated to have the ability to detect the impending failure of rock, concrete and composite materials [12–14]. However, it has seldom been used for monitoring of granular soils due to the complex nature of the soil materials. The AE signals could experience severe attenuation in a granular system, which brings measurement and interpretation difficulties. Nevertheless, the feasibility of the AE method applied to soil materials and its benefits have long been evidenced [15–20]. The earlier works mainly focused on revealing the qualitative relations between the AE
Corresponding author at: Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, China. E-mail address: [email protected] (W. Mao).
https://doi.org/10.1016/j.ultras.2018.11.007 Received 9 June 2018; Received in revised form 22 October 2018; Accepted 20 November 2018 Available online 22 November 2018 0041-624X/ © 2018 Elsevier B.V. All rights reserved.
Ultrasonics 93 (2019) 107–111
W. Mao et al.
Fig. 1. Typical failure pattern and the relevant load-displacement curves for shallow foundations resting on soil.
activity (mostly represented by the ring-down count) and different types/conditions of stressed soils; while more recent advances have been made on establishing a potential approach toward solving a specific geotechnical problem, and the interpretation of AE have been extended to a variety of parameters including spectrum analysis. Typical examples of such applications include the AE monitoring system for early warning of soil slope failure [21,22], breakage behavior of soil particle [23], seepage of soils [24], erosion of soils [25], pile bearing behavior in soils [26,27], and etc. This study presents a follow-up effort of such applications regarding AE generation in a shallow foundation situation. When a shallow foundation is subjected to load, the potential internal damage in the ground body would generate AE signals, propagate to the ground surface and be picked up by the AE sensor, as schematically illustrated in Fig. 1. According to the previous studies [1,28], the failure mode of shallow foundations can be generally divided into three categories, i.e. general shear failure, local shear failure and punching failure, as shown in Fig. 1. The general shear failure is usually seen in stiff or dense soil, characterized by a well-defined failure surface extended to the ground surface, and a pronounced peak in the load displacement curve. The local shear failure is seen in relatively loose soils and the failure surface is not well developed and does not expose to the ground surface. Slight bulging of ground is expected and there is no well-defined peak in the load displacement curve. The punching shear failure is usually seen in very soft ground and is characterized by significant large settlement. Ideally, no increase in load is expected when the displacement is continuously increasing for punching shear failure. In this study, loading tests were examined on shallow foundations resting on two typical types of the above sand grounds, i.e. a dense ground and a relatively loose ground. The AE signals were detected in the respective loading tests, and the data of AE activity and the ground bearing load were synchronized to verify the feasibility of the AE method applied to shallow ground monitoring.
Fig. 2. Layout of shallow foundation loading system equipped with AE sensor.
2. Experimental arrangements A schematic illustration of the test setup is shown in Fig. 2. The loading tests were conducted in a conventional motor loading frame assembly. The sands were prepared in a steel soil tank with internal dimensions 600 mm × 600 mm × 500 mm. The model footing used for the tests was made of rigid steel, square in shape, and measured 40 × 62 mm in the bottom surface. The tests were performed by a displacement controlled manner with a constant loading rate of 1 mm/ min. Load cell and displacement meter were used to capture the loadsettlement behavior of the footing. The AE signals were detected using AE sensor attached to the side of the footing. The piezo-ceramic type AE sensor, manufactured by Fuji Ceramics Corporation, has an optimum operating frequency range of 10 kHz–2 MHz. A main amplifier was used to enhance the AE signals with a gain of 115 dB when cooperated with the head amplifier embedded within the sensor. The sampling rate of AE data recording system was set at 2 Mps. Silica sand No. 5 was used as the model ground material. This sand is classified as the poorly-graded sand according to the Unified Soils
Fig. 3. Grain size distribution of the tested sand.
Classification System [29] and has specific gravity (Gs) of 2.65, maximum void ratio (emax) of 1.09, minimum void ratio (emin) of 0.66, uniformity coefficient (Cu) of 1.692, and mean grain size (D50) of 0.557 mm. The grain size distribution of the sand is shown in Fig. 3. The air pluviation method, i.e. sprinkling the sand from the air at a constant falling height, was used for preparing the model ground. In the current study, two types of ground conditions were made, which were the dense case and relatively loose case. For dense condition, the model ground was further compacted after every 30 mm deposition by dropping a 10 kg-weight at a height of about 10 cm 20 times all over the 108
Ultrasonics 93 (2019) 107–111
W. Mao et al.
Fig. 5. AE count, AE amplitude and ground resistance as a function of footing displacement: (a) loose ground; (b) dense ground.
Fig. 4. AE energy and ground resistance as a function of footing displacement: (a) loose ground; (b) dense ground.
be potentially used for investigation of the ground bearing behavior. Apart from the AE energy, the AE count and amplitude are also regarded as important parameters for analysis of AE signals. The mean AE amplitude, calculated by averaging the AE event amplitude of each 10 s interval, is used to represent the AE amplitude behavior in the current study. It can be seen from Fig. 5 that for loose ground, the AE amplitude is rather fluctuating, while for dense ground, it is more stabilized. Overall, the values of the mean AE amplitude do not show much difference between the loose and dense conditons. However, the AE count in case of loose ground is almost two orders of magnitude smaller than that of dense ground. In particular, it is notable that the AE count was not significant until the footing displacement was larger than ∼25 mm for loose ground. Meanwhile, the corresponding mean AE amplitude also appeared to be less fluctuated after similar depth of footing settlement. On the other hand, the AE amplitude and event count in case of dense ground demonstrated more obvious relevance with the ground resistance, especially during the first 15 mm footing settlement where a rising or decreasing in the ground resistance corresponded to the rising or decreasing of AE count and mean amplitude. However, when the ground displacement was larger than 15 mm, the mean amplitude did not show any significant change, while the AE count kept increasing continuously, similar with the increasing trend of the ground resistance. Compared with AE energy, it seems that the AE count and amplitude have less correspondences with the ground resistance, especially when the ground is relatively loose. Therefore, it is recommended to use AE energy as the identification parameter of AE analyses. The two loading tests described above demonstrated distinct AE characteristics for different ground conditions. For dense ground expected to experience general failure mode, a pronounced peak in the load and AE curve was observed. While for relatively loose ground expected to experience local failure mode, the load and AE kept
surface. This procedure was accurate enough as the repeatability of results can be easily realized. While for loose condition, no further compaction was performed. The final relative density of the ground was 97% and 35% for dense and loose cases respectively. 3. Results and discussions 3.1. Ground resistance and AE activity Fig. 4 shows the trends of ground resistance and AE energy development with respect to the footing displacement. The AE energy here is calculated by the integration of squared AE signal voltage within the signal duration [30]. Distinct differences between the dense and loose ground conditions can be noticed. For loose case, the ground resistance increased gradually with the depth of footing settlement throughout the whole loading process. In view of AE, it was detected throughout the above process as well, and consequently the accumulative AE energy kept rising as the footing displacement increased. In general, the AE energy rate, represented by the summation of AE energy of each 10 s interval, increased with the footing displacement. For dense case, the ground resistance was firstly increased rapidly to a peak value, followed by a substantial reduction and then increased again, but with a lower rate. The AE activity was also detected throughout the whole loading process, except that at the very beginning of loading, the AE was much less significant. The trend of AE rate shared similar tendency with the development of ground resistance. For example, the parabolic feature of ground resistance in the first 15 mm displacement can be well captured. A rising or decreasing in the ground resistance corresponded to the rising or decreasing of AE energy rate. In particular, notable rising of AE is detected when the ground resistance is approaching its peak at around 5 mm displacement. This suggests that the AE monitoring could 109
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Fig. 6. Ground resistance and AE activity in case of cyclic loading condition.
increasing throughout the loading process. It is therefore suggested that the AE could capture the failure mode of the shallow foundation as traditionally defined by the load-displacement relations. Fig. 7. Accumulative AE history plot demonstrating Kaiser Effect.
3.2. AE activity during cyclic loading
which consequently generates AE signals. Such process is different from the mechanism of AE generation in metal or rock materials where the crack formation or growth is the main AE sources. If the applied load is smaller than the previous maximum load, the crack will not suffer further growth and the AE is expected to be dim. It is worth noting that after global failure of the ground, the ground resistance experienced a gradual reduction period. However, the AE during this period remained quite active. This is possibly due to the fact that after the global failure, the shear band is well formed below the foundation, and the particles within the shear band are continually subjected to relative displacement during loading process. Consequently, despite that the ground resistance is reducing, the AE signals are constantly generated. Therefore, the Kaiser Effect is not applicable for characterization of ground situation after global failure. Considering that the ground is usually functioning before the general ground failure, it is suggested that AE could still be used as a useful tool for evaluation of ground status. It is worth noting that the current study presents the results of AE monitoring in a laboratory modelling scale. For potential applications of the AE instrumentation in the field, the foundation could have a more significant length and may have other interactions with the soil in place, including the existence of water, ground anisotropy, considerable attenuation of AE signals and etc. These involve the problems of many new aspects and further investigations deserve to be proceeded. According to the existing field applications, the above issues may not change the validity of AE monitoring [21,31]. Although the intensity or property of the AE signals might be easily affected by different field conditions, the evolution tendency could remain similar, which is more important for field engineers to assess the initiation of damage or the impending failure of the ground.
Regarding the AE activity of stressed materials, it is generally recognized that when the material is loaded and then unloaded, the corresponding AE response is different upon reloading compared to the original loading. This is due to the well-known Kaiser Effect. Consequently, shallow foundation subjected to historical load may result in different AE activity. Therefore, an additional test was performed to investigate the AE behavior of shallow foundation under pre-loaded condition. 19 cycles of loading with maximum ground resistance of about 40 kPa (Loading Stage 1) and subsequent 7 cycles of loading with maximum ground resistance of about 60 kPa (Loading Stage 2) were applied, followed by a monotonic loading until ground failure (Loading Stage 3). The ground was prepared under the same condition of the above dense case, and accordingly, the applied cyclic loading stresses were believed to be smaller than the peak failure stress of the ground. Fig. 6 shows the time history of the ground resistance and AE activity. One notable issue is that there existed stress plateau zones when the ground resistance approached around 20–25 kPa. This might be attributed to the local failure of the ground. Another important issue is that the Kaiser Effect did demonstrate its influence on AE activity. Although the AE activity did not turn to be zero during reloading steps, the amount of the detected AE during the reloading steps reduced dramatically. When the stress exceeded the previous maximum value, e.g. during the 20th loading cycle or after 26th loading cycle, the AE activity became significant again. Fig. 7 shows the accumulative AE and the ground resistance relationship. It is clearly demonstrated that during unloading and reloading process, the accumulative AE did not increase notably if the ground resistance did not exceed the previous maximum value (i.e. from 1st cycle to 19th cycle during Loading Stage 1 and 20th cycle to 26th cycle during Stage 2). On the other hand, notable increment of AE can be observed once the applied stress was larger than the previous maximum value (i.e. from Loading Stage 1 to Stage 2 and from Stage 2 to Stage 3). For materials such as metal, rock, and concrete, the AE activity is generally considered to be faint if the current stress is smaller than the historical maximum stress due to the Kaiser Effect (or similarly the Felicity Effect). Nevertheless, for granular soils in the current study, although the AE become less significant during reloading process, it did occur throughout the whole process. This is because the soils consist of numerous granular particles, and any disturbance due to foundation settlement could result in the rearrangement of the granular assembles
4. Conclusion This study presents an experimental investigation of acoustic emission during the failure process of shallow foundation. The major findings of this study can be summarized as following: (1) The AE is well observed throughout the failure process of the shallow foundation. The rate of AE activity is related with the ground density and the loading procedure. In general, the evolving trend of AE rate shared similar tendency with the development of ground resistance. 110
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(2) The energy of AE signals can be used as a better identification parameter for characterizing the behavior of the shallow foundation compared with AE count or amplitude. (3) The influence of Kaiser Effect is observed during cyclic loading of shallow foundation. AE becomes less significant during reloading process when the applied stress does not exceed the previous maximum stress. However, it did occurred throughout the whole loading process, demonstrating certain difference compare with those observed from metal or rock materials. (4) After the global failure of the ground, the Kaiser Effect is not applicable since the AE may continuously increase upon decreasing ground resistance.
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