Fracturing behaviors of soil subjected to monotonic and fatigue pneumatic loading
Introduction
Pneumatic fracturing was developed by HSMRC (Hazardous Substance Management Research Center) of NJIT (New Jersey Institute of Technology) in 1988 (Ding et al., 1999). Pneumatic fracturing is a method used to mechanically create fractures in low-permeability rocks and soils by injecting pressurized air for enhancing permeability. It was widely applied in improving the efficiency of pollutant removal. The fractures reduce the flow distance of pollutants or groundwater through increasing seepage channels. The forming process and characters of fractures are the essential issues of pneumatic fracturing. In pneumatic fracturing, it is important to determine the pressure of creating initiation fracture, the approaches of evaluating initial pressure were concluded by Alfaro and Wong (2001). He also found that the injection pressure required to initiate fracture in the injection well could be reduced due to the presence of initial fracture slots through hydraulic and pneumatic fracturing tests. However, the initiate fractures did not define the orientation of propagating of fractures, which were influenced by the stress field.
Pneumatic fracturing technology was usually applied to enhance in-situ bio-remediation. Venkatraman et al. (1998) carried out a project in a gasoline-contaminated and low permeability site, in order to enhance the air flow and transport rate in subsurface. It was found that the permeability of the site increased by nearly 36 times after pneumatic fracturing. There are lots of factors influencing the form and propagation of pneumatic fractures. According to extensive field experiments concluded by Ding et al. (2000), the aperture of fractures for over consolidated formations was between 10 mm and 40 mm during fracturing and 0.5–5 mm during vapor extraction after fracturing. In foundation treatment, Liu et al. (2016) combined the pneumatic fracturing with conventional vacuum preloading method to improve the consolidation of soft soil. The results show that the pneumatic fractures increased the dissipating speed of pore pressure significantly. Christiansen et al. (2008) concluded 3 principles of fractures induced in low-permeability soil. Firstly, the fractures form perpendicular to the least principal stress. Secondly, the fracture usually takes on the shape of steep sided bowl, which depends on the over consolidation degree. Finally, the propagation directions of the fractures are influenced by geological weaknesses, for example, the existing fractures. King (1993) divided the process of pneumatic fracturing into 5 stages: initial fracturing, fracture propagation, fracture maintenance, residual fracture, and re-fracturing. Fracture dimensions determined by the flow of pressurized air (Wishart et al., 2009). Compared with rock fracturing, in a porous medium such as soil, air dissipation has a great influence on the maintenance and expansion of fractures. Eriksen et al. (2018) simulated the diffusion of the fluid over-pressure into the medium and indicated that the diffusing pressure field was similar to the Laplace solution.
Valuable results of laboratory and in-situ tests of pneumatic fracturing were obtained. However, the number of the fractures created by pressurized air is still limited and there is great potential to improve the permeability by making multiple tightly-spaced fractures through fatigue pneumatic fracturing, a particular kind of multi-stage stimulation method (MSM). MSM has been widely used in tight reservoirs, for example, shale gas development (Johri and Zoback, 2013). It is known that local damage accumulation in rocks under cyclic loading will cause strength decrease, thus the pressure and flow of the fracturing fluid are adjusted so that the reservoir can be formed step by step to avoid the seismicity risks of conventional hydraulic fracturing (Giardini, 2009, Ellsworth, 2013, Meier et al., 2015, Zimmermann et al., 2015, Rubinstein and Mahani, 2015). Fatigue hydraulic fracturing aims to vary the effective stress magnitudes at the tip of fractures to optimize initiation and propagation of fractures. The optimization process includes lowering seismic radiated energy and generating fracture networks with various geometry and permeability (Zang et al., 2013, Zang et al., 2017). Kwiatek et al. (2017) analyzed induced seismicity during fatigue hydraulic fracturing in-situ test. Xu et al. (2019) derived an equation of staged fracturing to study the residual strength of the rocks by laboratory test. Zang et al. (2017) found that the breakdown pressure of formation was lower in fatigue fracturing compared to the conventional hydraulic fracturing through in-situ experiment. Zeng et al. (2020) demonstrated the same phenomenon from time-dependent static fatigue, especially in brittle rocks with natural fractures. However, it can’t promote multi-hydraulic fractures initiations directly.
Compared with rocks, the mechanical properties of soil are more complex for its porous, loose, plastic and multiphase. Therefore, the mechanism of rock hydraulic fatigue fracturing is not completely applicable to pneumatic fatigue fracturing of soil. In this paper, aiming at pneumatic fatigue fracturing of soil, laboratory experiment was carried out under different conditions.
Section snippets
Soil sample preparation
The soil was sieved, remolded, and stratified into a model box of 30 cm × 30 cm × 2 cm. The total mass of the soil was 2700 g. The particle size distribution curve of soil is shown in Fig. 1 and the main geotechnical parameters are shown in Table 1. In order to ensure the homogeneity, the soil was filled into the model box step by step and the process was presented in Fig. 2.
Testing apparatus
The laboratory experiments were conducted for monotonic and cyclic pneumatic loading. During monotonic loading, the air
Earth pressures under monotonic pneumatic loading
The variations of the earth pressures at each monitor are presented in Fig. 26, Fig. 27, Fig. 28, Fig. 29, Fig. 30. The maximum earth pressure values under different PPA are illustrated in Fig. 31.
It can be seen from Fig. 26, Fig. 27, Fig. 28, Fig. 29, Fig. 30 that under monotonic pressure, the earth pressures rose rapidly and then remained relatively stable. When the monotonic pressure was low, the soil deformation was elastic and earth pressures were affected by the interference of the
Conclusions
This work considers the mechanical behaviors of soil samples subjected to monotonic and cyclic pneumatic fracturing in the laboratory. The evolution of fractures, air pressures and earth pressures were monitored and discussed. The following conclusions can be drawn:
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For monotonic loading, the pressurized air pressures and earth pressures reached stable values in a few seconds, which meant the formation and development of the fractures was also completed in a short time. The key factor of the
CRediT authorship contribution statement
Feng Huang: Conceptualization, Methodology, Program. Tian Huang: Experimental device, Implementing experiment. Kun Chen: Implementing experiment. Hongyan Liu: Software, Data analysis. Hongyan Xu: Soil samples, Writing − review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are supported by the National Natural Science Foundation of China (Grant No. 51991364, No. 41807230) and Fundamental Research Funds for the Central Universities (Grant No. 2–9-2018–090).
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2023, Tunnelling and Underground Space TechnologyCitation Excerpt :Based on the experimental results, the initiation pressure of fracturing in this experiment was determined as between 0.05 and 0.10 MPa. The earth pressures at points A, B, C, D, and E under instant pneumatic loading are displayed in Fig. 7(a)-11(a) (Huang, et al., 2022), while those under gradual pneumatic loading are presented in Fig. 7–11(b). The soil was disturbed and fractured by the pressurized air, which caused the earth pressure to rise rapidly.