Elsevier

Engineering Geology

Volume 287, 20 June 2021, 106114
Engineering Geology

Characterization of artificially reconstructed clayey soil treated by polyol prepolymer for rock-slope topsoil erosion control

https://doi.org/10.1016/j.enggeo.2021.106114Get rights and content

Highlights

  • Chemical material improves the soil mechanical property and erosion resistance.

  • Polyol prepolymer is an effective material in rock-slope topsoil erosion control.

  • The germination and growth of vegetation is promoted at low content.

  • The reticular membrane structures enhance the integrity of clayey soil.

Abstract

Steep gradient, bad natural ecological conditions, and lack of nutrient conditions necessary for vegetation growth led a large number of rock-slopes exposed and serious consequences felt with respect to ecological environment. The chemical material was used to improve the mechanical properties, erosion resistance and integrity of clayey soil, which could be the main materials in the artificial reconstruction of rock-slope topsoil. In the present study, polyol prepolymer is used as a chemical material added to clayey soil to achieve rock-slope topsoil erosion control. A series of laboratory tests were performed on clayey soil with different polyol prepolymer contents and also compared to a fallow one by which the effect on mechanical properties, anti-freeze-thaw behavior, evaporation, erosion resistance and vegetation growth were investigated. The results indicated that polyol prepolymer could increase the mechanical properties, which could be subdivided into improvements in cohesion and slight changes in internal friction angle. Meanwhile, the anti-freeze-thaw behavior, evaporation and erosion resistance of clayey soil also increased by the presence of polyol prepolymer. Low content of polyol prepolymer could promote the germination and growth of vegetation. The effectiveness of the clayey soil treated by polyol prepolymer in the restoration of rock-slope was verified in the field test.

Introduction

With rapid industrialization and urbanization, human activities are accelerating slope evolution processes through disordered mining and engineered slope constructions (Guerra et al., 2017). A large number of rock-slopes are exposed in the process, which led to serious and disastrous consequences with respect to ecological environment (Krautblatter and Moore, 2014; Wu et al., 2017). The slope vegetation was seriously damaged, and the topsoil of the rock-slope was lost under the effect of precipitation and other factors due to the lack of stability measures. This would further lead to the extinction of slope vegetation and form an irreversible ecological damage cycle (Lee et al., 2007; Lee et al., 2013; Zhang et al., 2019). In order to reduce the destruction of this cycle, rock-slope restoration efforts have been mounted worldwide, especially the rock-slope with steep gradient (i.e., >60°). But most of the engineering measures lack in considering the improvement of intrinsic topsoil properties and environment protection.

The main characteristics of rock-slope are related to three sources: steep gradient, bad natural ecological conditions, and lack of soil and nutrient conditions necessary for vegetation growth. Due to the steep gradient, the topsoil of rock-slope was prone to large-scale loss under the action of erosion (e.g., wind erosion, precipitation erosion), which made the topsoil of rock-slope sparse and lack of nutrients (Lin and Lo, 2018). In addition, due to the poor water storage capacity of rocks and the high temperature on rock surface caused by direct sunlight, it was difficult for vegetation seeds to germinate and grow. Therefore, many rock-slope restoration measures have been designed. One of these measures was to use physical retaining structure (e.g., concrete-anchor bars, frame beams, retaining walls, etc.) to prevent the loss of topsoil (Wu et al., 2011; Shi et al., 2019; Zhang et al., 2020), and the other was to manually reconstruct the topsoil. These measures could achieve certain restoration of the current situation of lack of vegetation on rock-slope. However, most of the physical structures were made of reinforced concrete. Under the influence of external erosion and time, the structural strength of reinforced concrete would gradually decrease, which would lead to the failure of both physical structure and rock-slope restoration. Therefore, more research efforts were focused on the artificial reconstruction of rock-slope topsoil. Some detailed methods have also been proposed, such as external-soil spray seeding, turf slope protection, planting bags and so on (Li et al., 2020). Out of these methods, the external-soil spray seeding could not only reduce the workload of rock-slope treatment, but also meet the dual requirements of ecological restoration and slope protection, which made it an important research direction in rock-slope restoration (Li et al., 2017; Xu et al., 2017). But some limitations have also been reported that the soil loss still occurred due to the poor erosion resistance of the external-soil itself, which caused more large-scale environmental damage due to more serious erosion (Gao et al., 2007). This phenomenon was more obvious when the external-soil was clayey soil, which was the most widely used in case of external-soil spray seeding. In this situation, some chemical materials were used to change the clayey soil properties to maintain the mechanical strength and stability under large-scale erosion, which became noteworthy of the developed concept.

In the past decades, some conventional chemical materials (e.g., cement, lime and fly ash, etc.) have been widely added to the clayey soil to enhance the integrity and mechanical properties of clayey soil. A large number of indoor and field studies showed that these conventional chemical materials could effectively improve the mechanical properties, erosion resistance and integrity of clayey soil (Chian et al., 2017; Abd El-Aziz and Abo-Hashema, 2013; Kolias et al., 2005). But these materials also had some disadvantages, such as strong alkalinity and biological toxicity (Golewski, 2018; Imbabi et al., 2012). In addition, the treated clayey soil was completely filled and could not allow vegetation to grow, which caused great environmental pollution. Non-traditional chemical materials (e.g., ions and ionic groups, microorganism and organic polymer, etc.) were considered as effective substitutes for traditional chemical materials in recent research. These non-traditional materials could not only effectively improve the mechanical properties, erosion resistance and integrity of the clayey soil, but also will not cause pollution and damage to the environment (Tingle et al., 2007; Sukmak et al., 2013; Liu et al., 2020). As an important kind of non-traditional chemical material, organic polymer material had the advantages of less dosage and convenient transportation, and had broad application potential in the field of rock-slope restoration. Relevant studies also showed that organic polymer materials could effectively enhance the structural relationship between clayey soil particles and enhance the integrity of clayey soil. At the same time, it has good resistance to external erosion, especially for the precipitation erosion (Liu et al., 2011; Buchmann et al., 2020). This undoubtedly provides foundation for rock-slope restoration by using organic polymer.

In the present study, polyol prepolymer is used as a chemical material and added to the clayey soil to act as the main material in the artificial reconstruction of rock-slope topsoil. A series of experimental tests were performed comprehensively to obtain the effect of the clayey soil added with polyol prepolymer in rock-slope topsoil erosion control. The mechanical properties, anti-freeze-thaw behavior, evaporation and erosion resistance of clayey soil under different concentrations of polyol prepolymer were tested. Experiments on the germination and growth of vegetation under different concentrations were also carried out. In addition, a rock-slope was selected as test site for field application and result validation purpose to verify the effectiveness of the clayey soil treated by polyol prepolymer in the rock-slope restoration.

Section snippets

Materials

The clayey soil used for this study was taken from the slope of Nanjing Qixia Mountain, China, which was also selected as the test site for field application and validation. The soil was air-dried, crushed into powder and passed through the 2 mm sieve (Fig. 1). The physical and mechanical properties of clayey soil were obtained according to the American Society for Testing and Materials (ASTM) standards and are summarized in Table 1. The result of the standard Proctor test according to ASTM

Mechanical property test

The representative curve of the relationship between axial strain and principal stress difference of specimens with different polyol prepolymer contents under confining pressure of 50 kPa is shown in Fig. 3. The peak and residual value of specimen under 50 kPa confining pressure is shown in Table 3. As shown in Fig. 3, the specimens in Group I and II showed a different relationship between axial strain and principal stress difference.

For the specimens in Group I (Fig. 3(a)), the axial strain

Field application contrast test

In order to verify the effectiveness of the clayey soil treated by polyol prepolymer in the restoration of rock-slope, a rock-slope was selected for field application contrast test. The rock-slope is located in Nanjing Qixia Mountain, China, and the soil used in the laboratory tests was brought from this location (Fig. 19). The rock-slope was affected due to the excavation for surrounding buildings. The gradient of the slope was uneven and large of the order of 65°-70°. Under the condition of

Conclusion

A series of experimental tests were performed to determine the mechanical properties, anti-freeze-thaw behavior, evaporation and erosion resistance of clayey soil added with polyol prepolymer, which could be the main material in the artificial reconstruction of rock-slope topsoil. Experiments on the germination and growth of vegetation under different contents were also carried out. In addition, a rock-slope was selected for field application contrast test to verify the effectiveness of the

Author statement

All authors contributed equally to the preparation of this work.

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.

Acknowledgements

This research was financially supported by the Open Project of Technology Innovation Center of Land (Cultivated Land) Ecological Monitoring and Restoration Project of The Ministry of Natural Resources (Grant No. 2020), Natural Resources Science and Technology Project of Jiangsu Province (Grant No. KJXM2019028), Fundamental Research Funds for the Central Universities (Grant Nos. B200203112 and B200202013) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No.

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