Model test of soil deformation response to draining-recharging conditions based on DFOS
Introduction
Excessive groundwater pumping cause the land subsidence due to consolidation of multi-layer clayey and sandy soil, which seriously influences human life and infrastructure construction. Land subsidence has become a worldwide geological hazards problem because it may cause geological, environmental hydrogeological and economic impacts (Mousavi et al., 2000, Sato et al., 2003, Phien-wej et al., 2006, Tosi et al., 2006, Wang et al., 2009b, Tung and Hu, 2012, Pacheco-martínez et al., 2013).
Over the past few decades, scholars have conducted research on land subsidence problems caused by groundwater extraction. Poland and Davis (1969) demonstrated that the centers of subsidence in the Santa Clara valley, California, USA, coincided with the major pumping centers, and due to aquifer pumping, the water head dropped and the effective stress increased, which led to ground subsidence. In 1972, Ramnarong (1989) injected storm water into an underground storage in Bangkok and verified that artificial recharge of groundwater was an effective control measure for land subsidence. Numerous studies have demonstrated that the variation of land subsidence is related to the fluctuation of ground water table (Chang et al., 2004, Chen et al., 2007, Zhang et al., 2015, Chai et al., 2017). To investigate soil compaction and rebound mechanisms, a large number of field and laboratory tests focused on soil deformation characteristics in the process of land subsidence. Based on monitoring data of strata deformation in Shanghai, Zhang et al. (2006) found that the deformation characteristics of soil layers under the different pressure conditions varied. Wang et al. (2009b) noted that strata compression in Changzhou, China, varied significantly and was strongly influenced by groundwater drawdown. Land subsidence was mainly attributed to consolidation of the thick aquitard. Hung et al. (2010) deployed a multi-sensor monitoring system to monitor the aquifer-system compaction within a depth of 300 m and showed that compaction depths were in accordance with pump depths. However, due to complicated geological conditions and interference factors in subsidence areas, these field studies have mainly focused on total ground settlement. For laboratory tests on land subsidence, Kitaro (1969) performed the most representative large-scale land subsidence model test, studying the relationship between land subsidence and groundwater extraction in aquifers. Xu et al. (2011) conducted drainage experiments of clayey and sandy soil under different soil combination conditions. Their results showed that the clay layer was the primary cause of soil compaction. However, bubbles in the soil body easily affected the water tube observation data in model tests. Moreover, because of the production process, installation restrictions and limited monitoring points, coupling changes in deformation and water content of the entire soil profile were not obtainable. Both laboratory-made extensometers and borehole extensometers can only reflect deformation and displacement of limited stratum due to point number and distribution limits and this fact leads to urgent need of distributed monitoring technologies.
Fiber Optic Sensing (FOS) technologies have been developed since 1980s and they allow the monitoring of one-dimensional structural physical field along entire optical fiber (Udd, 1995, Grattan and Sun, 2000, Lee, 2003, Barrias et al., 2016). Based on the sensing technique and principle, the optical fiber sensors can be categorized into different types including Fiber Bragg Grating (FBG) Sensors, Extrinsic Fabry-Perot Interferometric (EFPI) Sensors, Optical Time-Domain Reflectometry (OTDR) Sensors, etc. (Park et al., 2006, Galindez-Jamioy and L'opez-Higuera, 2012, Palmieri and Schenato, 2013). Recently, Distributed Fiber Optic Sensing (DFOS) technologies are widely applied in structure health monitoring due to many advantages over other conventional sensors (Ye et al., 2013, Rodríguez et al., 2015). Sensing cables with both sensing and transmission functions are slender and flexible and hence are suited to bound to the surface or embed inside the geological bodies and related structures without affecting the measured object's structure to monitor strain, temperature, displacement, moisture, seepage and related parameters using a variety of equivalent transformations (Sun et al., 2014, Zhu et al., 2014a). The stability and safety monitoring based on DFOS technologies of civil engineering structures such as pipelines, dams, tunnels, bridges and historic buildings has been extensively discussed in the last decade (Bastianini et al., 2005, Matta et al., 2008, Rajeev et al., 2013, Gue et al., 2015, Lim et al., 2016). More recently, the application of DFOS for geotechnical structures monitoring were in-depth studied as an urgent problem. The efficiency of monitoring slope stability through BOTDA sensing technology in a media-sized model of soil nailed slop in laboratory was proved by Zhu et al. (2014b). Sun et al. (2014) systematically studied the multi-field information from slopes using DFOS technologies and emphasized its advantages for multi-field information monitoring, including stress, temperature, seepage and deformation in rock-soil mass. Kunisue and Kokubo (2010) have demonstrated the feasibility of using optical fibers to monitor deformation at different depths in boreholes. Finally, Wu et al. (2015) successfully applied DFOS technologies to field monitoring of land subsidence. According to the monitoring data, significant compressive strain in the aquitard overlying the pumping aquifers was found and the deformation of aquitard was delayed in comparison with the fluctuation of water table in the monitored adjacent aquifer. However, it is hard to obtain the water level and water content changes in each soil layer in the field due to the limit of monitoring technology. Hence, synchronous monitoring of soil deformation and water content of land subsidence has not been reported and the deformation responses of the whole soil profile to water level changes have not been well explained.
On the basis of above description, it can be concluded that deformation of soil layers are closely linked with water level changes. The purpose of this study is to understand the deformation responses of soil under drainage and recharge conditions, respectively. Pulse-PrePump BOTDA and FBG technologies were applied in a model box with sand-clay interbedded layers. The strain and water content distribution of soil profile were synchronously monitored during two drainage-recharge cycles. Finally, soil compression potential and deformation mechanism were discussed for land subsidence prediction and assessment.
Section snippets
Water content monitoring principle
Soil water content is monitored using fiber Bragg grating (FBG) technique. According to Bragg's law, a beam of white light is written in the FBG sensor, and when the light from the broadband source passes through the grating at a particular wavelength, the Bragg wavelength (λB) depend on the effective reflected index (neff) and grating period (Λ) is reflected (Leng and Asundi, 2003, Zhou et al., 2003). Any local strain or temperature changes alter the index of core refraction and the grating
Test box and materials
To study the interaction and deformation response characteristics of different soil layers during drainage and recharge in detail, a relatively small model land subsidence test was conducted in a model box with an inner diameter of 42 cm and a height of 100 cm. As shown in Fig. 4, the box was flange-connected by three organic glass cylinders and a foundation. Xiashu soil, which is widely distributed in the Nanjing area, China, was used as a model test clay layer. Physical and mechanical
Soil strain and deformation response during drainage
Fig. 7a shows the vertical soil strain at different times monitored by SSC during drainage. The initial strain data are subtracted from the later monitoring data, so compressive strain is negative while tensile strain is positive. During the first 4 h of drainage, the strain changes of each soil layer are relatively small, and compressive strain primarily occurs below a depth of 20 cm. Compressive strain increases gradually after 4 h, and the most obvious compressive strain is located at a depth
Deformation characteristics of sand and clay layers
Fig. 13 presents deformation change-time curves for Sand-1, Clay and Sand-2. We find that compression and rebound of the clay layer are significantly greater than those of the sand layers. The clay layer compression curves have obvious segmentation. At drainage onset, compression is not obvious in this slow deformation stage (Stage A); with the release of water, compression increases rapidly with increased pressure and transforms into a rapid deformation stage (Stage B). Soil compression until
Conclusions
Detailed and distributed monitoring of aquifer systems is important for understanding deformation laws in pumping and artificial groundwater recharge processes, which can serve as a guide to judge land subsidence trends. In this paper, distributed fiber optical sensing (DFOS) technologies were applied to a model test to simultaneously monitor soil strain and water content changes during drainage and recharge cycles. The response mechanisms of sand and clay layers in the process of drainage and
Acknowledgment
The authors gratefully acknowledge the financial support provided by the State Key Program of National Natural Science of China (Grant No. 41230636), the National Natural Science Foundation of China (Nos. 41372265, 41502274), the Natural Science Foundation of Jiangsu Province (No. BK20150389) and the Open Foundation of State Key Laboratory of Geohazard Prevention and Geoenvironmental Protection, Chengdu University of Technology (No. SKLGP2016K010).
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