Study on the influence of composite soil on the slope stability of farmland during in land consolidation

To improve the stability of sloping farmland, the optimum ratio for parameter of composite soils was determined. Plain and mixed composite soils were prepared with three improved materials of glutinous rice glue, wood fibres and coarse sand for laboratory to tests and experiments. For different moisture and soil contents, the shear strength, cohesion and internal friction angle of the composite soils were analysed and the proportions were adjusted based on the suitability of plant growth. Slope stability analysis was performed in combination with the case of slope farmland improvement. The results show that the soil improvement materials were affected by the different moisture content, the degree of influence of moisture content on soil shear strength and parameters: wood fibre composite soil > glutinous rice glue composite soil > coarse sand composite soil > plain soil. The soil amendments affected the soil mechanical properties and effectively improved soil stability, the degree of influence of soil amendments on the soil shear strength and parameters: wood fibre composite soil > glutinous rice glue composite soil > coarse sand composite soil. According to the shear strength and total biomass, the optimum contents of glutinous rice glue, wood fibre and coarse sand in the mixed composite soil were 1.5%, 2.5% and 15%. The case application shows that the minimum safety factor of the mixed composite soil was higher than the plain composite soil at slopes of 10°, 15° and 20°. The overall slope stability of the mixed composite soil was improved with different moisture contents, which is suitable for plant growth.


Introduction
Soil loss on sloping farmland is a common problem in hilly areas. Farmland can have steep slopes, loose soil and high erodibility. During the rainy season, soil has a high moisture content and strong viscosity, which is not conducive to moisture infiltration. When the moisture content of soil is high, the soil will crack and break and fine grains will gradually be lost; thereby, causing the degradation of sloping farmland and its ecological function. Relevant studies have shown that soil improvement due to the addition of a certain amount of material can increase the shear strength of soil, increase the moisture content, improve the soil structure and reduce soil erosion (Xiao et al. 2017;Le et al. 2020;Liu et al. 2012). Therefore, improving the soil structure, stabilizing slopes and promoting crop growth are particularly important in sloping farmland consolidation.
Research on soil characteristics, especially research on the effects of different materials on soil stability in the fields of agricultural engineering and geotechnical engineering, has attracted widespread attention from scholars domestically and abroad. In the field of agricultural engineering, some scholars have used biochar (Khorram et al. 2016), carbon-based materials (Cho 2008), soil conditioners (such as ferrous sulfate FES, aluminium sulfate ALS and polyacrylamide PAM) and soil disinfectants (pentachloronitro benzene PCNB) (Renner 2008), combined soil conditioners (Kadajan and Jankowskie 2021), earthworm manure (Lachnicht et al. 1997), organic materials (such as straw mulch, biomass charcoal and pig manure) (Truong and Marschner 2018) and other soil amendments to study the effect of soil agglomeration on sloping farms. The body stability, soil agglomeration, organic carbon content, soil moisture holding capacity, effective soil reservoir capacity, soil infiltration rate, moisture holding capacity, moisture retention, sand control and fertilizer control have been shown to have good control effects, but few studies have considered the effect of soil amendments on the mechanical properties of the soil of sloping farmland. Research on such properties has mainly focused on the effects of different land use types , farming methods (Du et al. 2018), rainfall intensities and slopes (Yuan et al. 2015), soil particles (Fan 2019), soil bulk densities and moisture contents (Zhang et al. 2020), plants (Ding et al. 2017;Pu et al. 2014;Shi et al. 2016) and other influencing factors on the shear strength of soil, without considering the effects of soil amendments on the mechanical properties of soil. In the field of geotechnical engineering, some scholars have found that the influence of the ratio of rock blocks on the shear strength of cemented soil-rock mixtures depends on the combination of the skeletal effect of rock blocks and the cementation effect of the mixture Getahun et al. 2016); for the resistance of soil-rock mixtures, the shear strength largely depends on the characteristics of its internal stone content. As the stone content increases, the internal friction angle increases greatly, while the cohesive decreases. The cohesive depends on mainly the internal fine particles of the components (Xu et al. 2011;Abdi et al. 2021;Tu et al. 2021); When using the curing agent PX to study the strength of sand solidification, a gel is formed, which changes the adhesion strength between the soil particles, thereby resulting in an increase in the overall strength. However, there is an optimum addition amount (Chen et al. 2015;Mahmood et al. 2021). When using the SH curing agent to strengthen the loess, the curing shear strength is effectively improved. The internal friction angle and cohesive of the solidified loess increase as the content, dry density and age increase. The elongation shows an increasing trend, but it decreases significantly as the moisture content increases (Cheng 2014); when adding coconut shell fibre to clay, increasing the fibre content gradually increases the principal stress if the reinforced soil is damaged (Maliakal et al. 2013); using discrete palm fibre reduces the preconsolidation pressure of soil and increases the shear strength and friction angle (Kar and Pradhan 2011). The incorporation of fibre increases the toughness of soil and the shear strength does not immediately decrease, but maintains a certain strength for a period of time and then decreases. Owing to the connection of fibres, the residual strength of damaged soil is still larger than that of plain soil (Makone and Wekesa 2021).
In summary, a substantial amount of research has been conducted on improving the soil properties of slopes with a single material, while few studies have assessed improvements with multiple materials. In the field of agricultural engineering, performance improvements related to soil moisture and fertilizer retention have been the main focus of research; soil stability improvements have largely been ignored and research on the slope stability of soil and the benefits for plant growth is lacking. In addition, in the field of geotechnical engineering, enhancing the mechanical properties of soil has been the focus of research; in this context, the greater the shear strength is, the better the mechanical performance is, but this situation is not conducive to plant growth. Therefore, in this study, three materials, glutinous rice glue, wood fibre, and coarse sand, were separately prepared and combined with different moisture contents and in different ratios. In addition, direct shear tests were performed on an unsaturated composite soil to study the shear strength characteristics of the soils and their changes. The optimum proportions for the composite soils were determined and improvements to sloping farmland were taken as an example to analyse the stability of the composite soil layer. The research results provide a theoretical basis and application foundation for land improvement projects.

Test materials
Red loam soil, which was derived from sloping farmland in the valley area of Binchuan County, was used in this study. The parent material of the soil formation is slope sedimentation and alluvial sedimentation. Owing to the short time of soil formation, the degree of maturation is lower and the soil profile experiences substantial leaching. In the mineral layer, the soil fertility is relatively low and the texture is light (Table 1). Glutinous rice glue (Fig. 1a) is an environmentally friendly adhesive made from glutinous rice starch, that has a wide range of applications and a high viscosity. The rice glue used in this study was produced by Tianyi Wallpaper Co., Ltd. Wood fibre (Fig. 1b) is a natural cellulose fibre with an irregular fan-shaped structure, superhydrophilic properties, fast moisture absorption, a large moisture absorption capacity, a 99% organic content and an 8% ash content. The fibre length in this study range from 3 to10 mm and the pH was 6, the fibre was produced by Southeast Wood Fibre Technology Co., Ltd. The test sand (Fig. 1c) was machinemade sand, with rock particles with a particles smaller than 4.75 mm that were processed by soil removal, broken by machinery and sieved. The shape of the particles was generally triangular, rectangular and square. The surface was rough and angular.

Sample preparation
Glutinous rice glue contents of 0.5%, 1.5% and 2.5% were used; wood fibre contents of 1%, 2.5% and 5% were used; and coarse sand contents of 5%, 10%, 15% and 20% were used. The gradients were mixed with red soil and plain soil was used as the control group. In total, four composite soil samples with moisture contents of 30%, 32%, 34% and 36% were produced. However, the experiment showed that glutinous rice glue and wood fibre had a high moisture absorption capacity. An excessively low moisture content is not conducive to the full mixing of soil particles. Therefore, more moisture was used for mixing, but the composite soil formed by mixing was due to moisture. If the direct shear test is performed directly, the data will be inconsistent, the law will not be obvious and the test results will not be inaccurate. Therefore, the air-drying method was used to control the moisture content of the composite soil and to produce multiple samples with different moisture contents. The moisture content of the glutinous rice glue composite soil was controlled at seven, corresponding to rice glue contents of 3%, 5%, 7%, 9%, 11%, 13% and 15%, The moisture content of the wood fibre composite soil was controlled at seven, corresponding to wood fibre contents of 7%, 14%, 21%, 28% and 35%. The glutinous rice glue, wood fibre, coarse sand and plain soil were combined into 21, 15, 16 and 4 different treatments, respectively, for a total of 56 treatments and each treatment was repeated four times.

Test method
Studies have shown that if the vertical pressure is more than 400 kPa, the vertical pressure and the shear strength of soil will exhibit a linear relationship. If the vertical pressure is below 100 kPa, the friction between the shear boxes will increase, which will affect the accuracy of the test (Graham 2006;Jiang et al. 2003;Yang 2021). Therefore, this study used the ZJ strain control direct shear instrument to perform the shear tests and the shear rate was 2.4 mm/min. To obtain a complete shear strength curve, vertical pressures of 100 kPa, 200 kPa, 300 kPa and 400 kPa were used for each processed sample. In strict accordance with the "Geotechnical Test Method Standards", for the fast shear test (Nanjing Hydraulic Research Institute 1999), the Coulomb formula was used to obtain the cohesion and internal friction angle of the composite soil under different conditions: In the formula, is the shear strength of the soil (kPa); is the vertical pressure (kPa); is the angle of internal friction (°); and c is the cohesion (kPa).

Data analysis and processing
Origin 2017 was used to process and analyse the measurement data and to generate charts. Geostudio software is a specialized software for slope stability analysis. The Morgenstern-Price method was used to calculate and analyse the slope stability. In the model simulation the slope was according to the actual case. During the model simulation according to the internal friction angle and cohesion under different moisture content conditions, the minimum safety factor under the same slope was calculated.

Analysis of the shear strength of composite soils with different moisture contents
When the moisture content of the composite soil changed, its shear strength also changed and the composite soils made from different materials exhibited different performances (Table 2 and Fig. 2). As the moisture content increased, the shear strength of the plain soil was relatively stable when the moisture content was in the range of 30% to 32% and then slowly decreased. The internal friction angle fluctuated slightly, the floating range was between 22.06° ~ 22.4°, the change rule was not obvious and the cohesion continued to decrease. The shear strength and internal friction angle of the glutinous rice glue composite soil showed a trend of first decreasing, then increasing and then decreasing. The cohesion increased slowly at moisture contents of 3-7% and from 7 to 15%, it increased rapidly and then decreased rapidly. When the moisture content was 3%, the glutinous rice glue was dehydrated and solidified and mostly existed in the form of solid particles in the composite soil. When the moisture increased, the glutinous rice glue gradually absorbed moisture and softened, becoming "gelatinous" consequently, the cementing effect was enhanced and the cohesive increased. The internal friction angle decreased. When the moisture content was 11%, the shear strength, internal friction angle and cohesive all reached maximum values. The shear strength and cohesive of the wood fibre composite soil showed a significant decline after the moisture content increased to 14%. When the moisture content was 21%, the internal friction angle reached a maximum value; when the moisture content increased to 28%, a rapid decreasing trend was observed. The hysteresis phenomenon of the internal friction angle may have been due to the increase in the soil moisture. The structure between the wood fibre and the soil particles and the structure between the soil particles and the soil particles changed due to moisture and reoccluding was observed; therefore, the contact area changed, resulting in a certain internal friction angle. The moisture cut interval increased slightly first in the moisture interval of 14-21%. The shear strength, internal friction angle and cohesion of the coarse sand composite soil increased and then decreased. When the moisture content was 32%, the three values were the largest. The internal friction angle and cohesive were affected by changes in the moisture and were basically the same; in addition, no hysteresis was observed (Fig. 2). Because the soil amendments and soil particles combined to form a certain strength, the effect of the moisture content on the soil was reduced. Figure 3 and Table 3 show that in the vertical pressure range of 100-400 kPa, the shear strengths of the composite soils with different contents of glutinous rice glue, wood fibre, and coarse sand, were greater than that of the plain soil. As the glue content increased, the shear strength and internal friction angle of the glutinous rice glue composite soil first increased and then decreased. When the glue content was 1.5%, the two values were the largest. Among them, the cohesive showed a continuous increasing trend. However, the rate of increase in the high-content stage (1.5-2.5%) was slower than that in the low-content stage (0.5-1.5%) because the addition of the glue to the soil promoted the agglomeration of soil particles through the coagulation of the glue and improved the cohesion and internal friction angle. However, when the amount of glue was too high, the proportion of soft glue increased to form a glue network, which separated and wrapped the soil particles, reduced the contact area between the soil particles and caused the soil to form a "soft plastic" shape. Therefore, the shear strength was reduced. As the fibre content increased, the shear strength, internal friction angle and cohesive of the wood fibre composite soil showed a continuous upward trend. When the fibre content was 5%, the three values were relatively large and the maximum value was not reached. As the sand content increased, the shear strength and internal friction angle of the coarse sand composite soil gradually increased, thereby reaching a maximum value at a content of 15% and then beginning to show a downward trend. When the sand content ranged from 5 to 15%, the coarse sand contacted the soil particles and the internal friction angle increased; because the particle size of the coarse sand itself was much larger than that of the soil particles, when the sand content exceeded 15%, the mass and volume of the coarse sand in the composite soil increased and the sand particles interacted with each other. It was difficult for the particles to come into close contact with each other; therefore, a certain void structure was formed, the internal friction angle began to decrease and the cohesive was negatively related to the sand content. The greater the sand content was the lower the cohesive.

Determination of the optimum ratio parameters of the mixed material composite soil
According to Sect. 2.2, when the shear strength of the three glutinous rice glue, wood fibre and coarse sand materials was the highest, the corresponding optimum contents were 1.5%, 52.5% and 15%, respectively. Optimum amounts of the three materials were mixed with the red soil to form a new mixed composite soil and the shear strength of these mixtures was compared with the shear strength of the palin composite soils (Fig. 4). Under different moisture contents, the shear strength of the mixed composite soil was greater than the shear strength of the plain composite soils with the optimum moisture and soil contents and the shear strength decreased slowly as the moisture content increased. The internal friction angle of the mixed composite soil first increased and then decreased, which was mainly because of moisture on the glutinous rice glue. In the moisture content range of 11-14%, the glutinous rice glue was gradually diluted and transformed between the colloidal state and the fluid state due to soil structure. The cohesive of the mixed composite soil showed a continuous decreasing trend. When the moisture content was 11%, the cementation effect of the glutinous rice glue was fully exerted and the cohesive was the largest; when the moisture content was 14%, the cohesive of the mixed composite soil was lower than that of the wood fibre composite soil, possibly, because the glutinous rice glue in the soil occupied a certain proportion and was diluted by the moisture and the binding effect of the wood fibre was affected. Overall, the internal friction angle and cohesion of the mixed composite soil were better than those of the plain composite soils. When improving sloping farmland, in addition to considering the shear strength of the slope, the suitability of plant growth should be considered. During the experiment (Fig. 3b), as the content of wood fibre increased, the shear strength and cohesive continued to increase. The contribution to the soil consolidation ability was greater than that of the glutinous rice glue and coarse sand, but an excessively large wood fibre content caused soil compaction, which is not conducive to plant growth. Therefore, wood fibre contents of 1.5%, 2.5%, 4% and 5% in the abovementioned mixed composite soil were used in pot experiments to test the suitability of plant growth and the shear strength, as shown in Fig. 5 (an image was taken after Cynodon dactylon had grown for approximately 3 months). According to the test results, the total biomasses of A:B 1 :C, A:B 2 :C, A:B 3 :C and A:B 4 :C were 153.06 g/m 2 , 158.22 g/m 2 , 37.65 g/ m 2 and 11.42 g/m 2 , respectively. A:B 1 :C and A:B 2 :C significantly increased the total biomass, the value of A:B 2 :C was slightly higher than that of A:B 1 :C and the plants grew well (Table 4).

Project area overview
The project area is located in Binchuan County, which is part of the dry-hot Jinsha River valley in China (Fig. 6). The study site is located between 100°16′ ~ 100°59′ east longitude and 25°32′ ~ 26°12′ north latitude. The altitude is 1410-1558 m and the landform is a red soil hilly area. This area has a semiarid monsoon climate and is in the southern subtropical zone. The average rainfall is 560.9 mm, the annual average temperature is 17.9 °C and the frost-free

Analysis of the application results
According to the actual situation of the project area and the optimum ratio of 1.5%:2.5%:15% for the glutinous rice glue, wood fibre and coarse sand, a terrace embankment slope treatment was performed. At 10°, 15° and 20°, Geostudio software  (Table 5). Under different slopes, the minimum safety factor of the slope for the three single composite soils showed an upward trend before reaching the optimum moisture content and a downward trend after the optimum moisture content was achieved. At the optimum moisture content, the minimum safety factor of the slope was ranked in the following order: wood fibre composite soil > glutinous rice glue composite soil > coarse sand composite soil. The minimum safety factor of the slope of the mixed composite soil for the three improved materials increased as the moisture content increased, it was the largest when the moisture content was 11% and then it gradually decreased. When the moisture content was 14%, the minimum slope of the mixed composite soil was achieved, the safety factor was slightly lower than that of the wood fibre composite soil and it was higher than that of the plain composite soils under other moisture content conditions. However, when the moisture content increased to 28%, the minimum safety factor of the slope of the composite soil was less than that of the wood fibre soil and the overall stability was better. The mixed composite soil (1.5% glutinous rice glue, 2.5% wood fibre and 15% coarse sand) was applied to the sloping surface of the farmland, which effectively enhanced the stability of the slope (Fig. 7).

Conclusion
(1) Different moisture contents have different effects on the composite soil. With the increase in soil moisture content, the internal friction angle of plain soil and composite soil were from increased to decreased; the shear strength and cohesion of plain soil and wood fibre composite soil were decreased; and the shear strength and cohesion of sand composite soil was from increased to decreased. The degree of influence of moisture content on soil shear strength and parameters: wood fibre composite soil > glutinous rice glue composite soil > coarse sand composite soil > plain soil.
(2) The soil amendments affected the soil mechanical properties and effectively improved soil stability. With the increase in soil amendment content, the shear strength and internal friction angle of glutinous rice glue composite soil and coarse sand composite soil were from increased to decreased. The shear strength, internal friction angle and cohesion of wood fibre composite soil were followed; and the cohesion of the glutinous rice glue composite soil was increased. The coarse sand composite soil was decreased. The degree of influence of soil amendments on the soil shear strength and parameters: wood fibre composite soil > glutinous rice glue composite soil > coarse sand composite soil. (3) In this paper, the optimum contents of glutinous rice glue, wood fibre and coarse sand in the mixed composite soil were 1.5%, 2.5% and 15%. The shear strength and total biomass of the mixed composite soil was greater than plain composite soil. In the case of slope farmland consolidation, the minimum safety factor   of the mixed composite soil was better than the three slopes of 10°, 15° and 20°. The slope of the mixed composite soils had a better overall stability than the plain composite soil under different moisture contents, which is suitable for plant growth.
Fuding Funding were provided by the Natural Science Foundation, China (Grant Nos. 41867038, 41662021 and 32160304), the ministry of natural resources "Technical Innovation Team for land allocation and ecological consolidation in Southwest diversified regions", China (Grant No. YNTD2018KF05), and the Yunnan province Natural Science Foundation, China (Grant No. 202001AU070114 and 202101AT070754). For help with the collection data and revise this article, we would like to thanks professor Jianxin Yu of Yunnan Agricultural University.