Feasibility analysis of loess slope improved by lime pile

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1 Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing Feasibility analysis of loess slope improved by lime pile Wanjiong Wu, Shouyun Liang, Fanyu Zhang, Xude Yan, Jianji Zhou Abstract Lime has powerful characteristics of hygroscopicity, expansibility and gelation with soil, so slope can be strengthened by filling into the lime as form of pile. In this paper, experiments have been performed to investigate the diffusion characteristic of lime in loess, and the influence of lime to the physical and mechanical properties of loess also has been examined, in order to verify the feasibility of using pile to improve the stability of slope. The results show that, it is feasible to improve stability of slope by lime piles. And pile spacing should be set about 8.5 times as many as the diameter of the pile when the pile holes arranged according to the plum flower form from this test. Moreover, lime piles strengthen the loess slope mainly by gathering and cementing the clay grains in loess. Keywords Lime piles, Loess slop, diffusion, landslide prevention Introduction Loess is widely distributed in the northwest of China, and it has stable structure, affluent vertical joints and obvious collapsibility (Wang and Gao 2012). The loess landscape such as loess tablelands, loess ridges, loess hills and loess gullies are shaped due to the dual function of accumulation and erosion, and always form high and craggedness natural slope (Liu 2007). However, a lot of loess slopes have been generated during the construction of infrastructures, like highways or railways etc. as a result posing threat to people s lives and properties (Yang et al. 2000). So it is very important to improve the stable of loess slope in loess area. Statistics show that loess landslides and debris flow are major geohazards in Lanzhou City, which account for more than 60% of its total geohazards (Ding and Li 2009). Lime has used to strengthen soft soil foundation in early time (Mathew and Narasimha 1997), owing to its stronger water-absorbing capacity, expansibility and physical absorption reaction between lime and soil grains. Specifically, it increases the soil strength and water stability by improving the engineering properties through physical and chemical reactions. Later, lime piles were used to solidify foundation by punching in foundation soil, then inject lime powder to form lime piles (Wang Cheng 1994). Because of lime hydration expansion of compaction, exothermic dehydration, ion exchange, gelling effect and coupled with the effect of displacement of pile group or compaction due to punching. As a consequence it greatly improves the foundation bearing capacity. At present, using lime to strengthen loess has been studied, and mainly used in geotechnical engineering field such as embankment, foundation treatment (Metelková et al. 2012; Lin et al. 2007). Lime is added in the loess and produces a series of physical and chemical reactions with clay soil grains (Lin and Liu 2003). As a result, loess is solidified by changing its engineer properties. Since loess landslides occurred frequently in the northwest of China (Zhang et al. 2013), if we put the method that lime piles solidify loess into improving the stability of loess slope, it will be a new method of landslide prevention and a new attempt of landslide treating. At the same time, lime piles can function as anti-slide piles. Physical experiment is performed to simulate the diffusion process of lime pile in loess and the physical, chemical and mechanical properties of loess are tested to illustrate the solidification phenomenon of loess surrounded on the lime pile, and the characteristics of solidified loess are analyzed as well. Moreover, the primary reasons that how lime pile solidifies loess from the perspective of diffusion mechanism has been discussed. So that provides guidance for further application of lime pile to improve stability of loess slope successfully. Materials and Methods Materials Loess tested in lab is part of the Malan loess stratum at Jiuzhou platform, Lanzhou city, Gansu province. The Malan loess stratum as the main place of living and production activities has significant meaning to study, and its thickness is over 30m (Li et al. 2007). The physical and chemical properties are shown in Tab. 1. Lime used in lab is produced by Tianjin Guangfu

2 W J. Wu, S Y. Liang, F Y. Zhang Loess Slope Improved Exquisite Chemical Research Institute, the content of activity CaO over 90%, burning weight loss about 2%. In addition, it contains a small amount of insoluble acetic acid, chloride, sulfate, nitrate, iron, lead, alkali metal and magnesium and other impurities. It satisfies the requirements of JC/T standard for building quick lime. Table 1 Physical and chemical properties of loess Physical properties Chemical properties G s 2.73 g/cm 3 SSA 29.97m 2 /g I P CEC 4.01cmol/kg LL The home-made square steel box is employed (450*450*300mm) to maintain the soil sample (Fig.1a). In the middle of box a movable rigid plastic tube (External diameter D=40mm) is arranged in order to form lime pile by filling lime (Fig.1b). corresponding distilled water solution, make sure the soil and water mixed completely. Then the plastic tube was installed and surrounded with the mixed soil which was filled in the steel box piecemeal, and compaction soil layer hierarchically as to fill up maintain box. Then slowly rotated out of the plastic tube, at the same time, packing the pile hole with lime powder (moisture content ω=15%). Finally sealed and maintained 22 days. After completion of the maintenance, scrape off the surface soil layer 50 mm in order to weaken the influence of boundary conditions. Then flat the surface and conduct miniature vane shear test and hardness test. Once again scrape off disturbed soil layer 30 mm, and carve four concentric rings spacing for 50 mm centered on pile axial (Fig.2a). Then with a small ring knives (inner diameter d=28mm) take samples as shown in Fig.2b. Test Physical property based on GB/T , test the specific surface area (SSA) and cation exchange capacity (CEC) with methylene blue adsorption method (Santamarina et al 2002; Savant 1994). (a) Maintain box (a) Physical map (b) Sketch map of equipment Fig.1 Map of test equipment (b) Sketch map Fig.2 Location of sample Methods and procedure Drying the loess under 105 for 24 hours, remove the clastic impurities from loess by screening soil over sieve 0.5mm after the loess cooling to room temperature. According to the design dry density (ρ d =1.5g/cm 3 ) and optimum moisture content (ω opt =15%), calculate and weighing a certain amount of soil and distilled water that test needed. Then divided the soil into 15 shares, respectively loading into the black plastic bag and sealed for 24 hours after evenly mixing with the Results and discussion The soil samples are marked as Rx (x=50,100,150 and 200) in order to collect data and perform analysis easily. Soil in box is assumed as homogeneous and share same physical and mechanical properties before test. Lime can absorb moisture and diffuses into loess because of its tiny particles, strong water imbibitions. As a result, the physical and mechanical properties of loess will changed due to the transform of structure. So we can 2

3 SSA/(m 2 /g) CEC(cmol/kg) Water content/% Void ratio 17mm liquid limit/% 17mm plastic index. Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing speculate the effect of improvement by testing the physical and mechanical properties of samples that located around lime pile. The effect on physical properties of loess by lime pile Water content will greatly influence the stability of slope, due to the strong collapsibility and disintegrative of loess. Void ratio is the crucial physical index that represents the compaction of loess. Fig.3 shows the maximum, minimum, average values of water content and void ratio change along with radius. From the chart, it is clearly shown that water content (ω) and void ratio (e) decreased compare with original soil samples (ω=15% and e=0.82). In particular, every physical indexes of improved soil samples located in R=50mm change significantly, average moisture content is 13.47% and average void radio is Meanwhile moisture content and void ratio increase along the radius direction, when R=200mm, each physical indexes are close to the corresponding value of original soil sample. It is revealed that lime piles only improve loess within a certain radius in this test, and with the increase of radius the improvement decrease gradually water content void ratio Fig.3 Change of water content and void ratio along with radius Test using loess belongs to the low liquid limit clay, and the content of clay grain in loess is very important to the engineering geological properties of loess (Lin and Liu 2003). Liquid and plastic property is an index that can reflect the content of clay grain in loess. With the elevation of plasticity index the content of clay grain increase and the engineering stability become poorer. Fig.4 shows the change of liquid and plastic properties along with radius direction. We can find some regular changes from the chart, that plastic index (I P ) of soil sample the one located close to lime pile is minimum. Due to the effect of lime diffuses into loess and react to the clay grains, I P is increases along with the radius. When R=50mm, I P =9.6, when R=200mm, I P =11.1, and the I P of original soil sample is It indicates that the content of clay grain in loess is decreased, but within the range of influence of lime pile, the content of clay grain is increases along with radius. Plastic index decreased due to diminish of liquid limit and boost of plastic limit. It is show that lime improves loess mainly by gathering and cementing the clay grains in loess, at the same time decreasing the plastic index so that increasing the water stability mm liquid limit 17mm plastic limit Fig.4 Change of liquid and plastic properties along with radius direction In order to further explore the influence of clay on the engineering properties of loess carried out the specific surface area (SSA) and cation exchange capability (CEC) test. Results of the test (Fig.5) indicate that SSA values and CEC values increase along with the radius, but less than the values of original soil sample, which mainly due to Ca(OH) 2 generated in the nitration reaction after lime absorbing water, and dissociative out high charged ion Ca 2+, then Ca 2+ exchange the low charged ions absorbed soil particle surface such as Na + K + H + etc. As a consequence, reduce the electric double layer of soil particles adsorbed on the surface, gathering and cementing clay grains. And decrease the dispersivity of the soil and strengthen its adhesiveness then formed a stable coherence structure. Afterwards recrystallization of lime reaction produces substances containing water of crystallization of calcium hydroxide: Ca(OH) 2 +nh 2 O=Ca(OH) 2 nh 2 O Crystal products combined with each other then integrate the clay grains formed eutectic. Due to further gathering clay grains, decrease the SSA and CEC values SSA CEC Fig.5 Change of SSA and CEC along with radius

4 Shear strength/kpa Hardness value W J. Wu, S Y. Liang, F Y. Zhang Loess Slope Improved The effect on mechanical properties of loess by lime pile The mechanical strength of loess is the outward manifestation of loess improvement effect. It is also one of the most concentrated engineering properties in the landslide prophylaxis and treatment projects. Micro cross section shear apparatus and Piccolo hardness tester are innovatively used to test shear strength and tensile strength of soil indirectly. The test results are shown as Fig. 6. Shear and tensile strength have consistent change trend, similar to the change of physical and chemical properties. Compared with original sample, the values of mechanical strength increase in different scale and display a rising trend along with radius. Because of expanding of lime pile, which absorbs water afterward, it leads to forming the expansion of cracks and circular cracks, etc. within 50mm (Fig.7). So the mechanical strength of soil samples named R50 decrease to the minimum. It is easily found that lime piles should be mixed with some cement, fly ash or SH additive when they are used to improve loess (Wang et al. 2005; Guo et al. 2004), instead of using lime purely. It will weaken the expansion of lime pile, enhance the mechanical strength of pile itself and amplify the effect of anti-slide pile in the landslide prophylaxis and treatment project shear strength hardness value Fig.6 Change of shear strength and hardness along with radium and slope weak interlayer or shear zone of potential sliding surface, and make sure the lime piles perpendicular to the slope (Fig.8(a)). The water absorption and lime diffusion into surrounding loess increase its strength, so that the stability of loess slope will be improved. Based on the physical and mechanical strength characteristics of improved loess, the space between piles will be determined. From the test results, when the diameter of pile is 40mm (D=40mm), the influence radius is about 200 mm, and the influence radius is about 5 times of pile diameter. If arrange the pile holes according to the plum flower form, pile spacing can be calculated about 8.5 times the diameter of the pile based on geometric relationships of triangle and edges. For example, if the diameter of pile is 80mm, then the pile spacing will be 680mm (Fig.8 (b)). (a)location of lime piles (b)arrangement of lime piles Fig.8 Sketch map of lime piles in loess slope Conclusions Fig.7 Map of cracks reduced by lime expansion Feasibility analysis of loess slope improved by lime pile Lime piles improve the stability of loess slope mainly adopts imbedding lime piles into loess in the slope toe (a)it is preliminary demonstrated that lime piles can be used to improve the stability of loess slope, but more physical experiment or field test would be needed to verify this fact. In addition, lime piles should be mixed with some additives instead of pure lime in the practical engineering. It will make the loess slope more stable. (b) With the similar conditions to this test, the influence radius of lime pile improve loess is about 5times of piles diameter, and if arrange the pile holes according to the plum flower form, pile spacing can be calculated about 8.5 times the diameter of the pile. 4

5 Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing (c) Lime piles improve loess mainly depends on the water absorption, ion exchange reaction and recrystallization reaction. All of them make the clay grains in loess gathered and cemented, formed stable coherence structure so that the loess improved. Acknowledgments This study was supported by the National Natural Science Foundation of china (No ). The valuable comments by Binod associate professor and earnest modification by Wengui Huang are greatly appreciated. References Wang Y M, Gao L C. (2012) Experimental research on chemical improvement of loess. Journal of Engineering Geology. 20(6): Liu Y H (2007) Experimental study on properties of lime - stabilized loess embankment. MS thesis, Southwest Jiaotong University, Chengdu, China. Ding Z Q, Li Z H (2009) Geological hazard and control in Lan Zhou. Gansu Science and Technology Press, Lanzhou, China ( ). 472P. Yang X T, Dong H Y, Huang Y R (2000) Study on slop stability of expressway in loess region. Journal of Soil and Water Conservation. 14(1): Mathew P K, Narasimha R S (1997) Effect of lime on cation exchange capacity of marine clay. Geotechnical and Geoenvironmental Engineering. 123(2): Wang R, Cheng S X. (1994) Stabilizing soft foundation with quicklime piles. Rock and Soil Mechanics. 15(3): Metelková Z, Boháč J, Přikryl R (2012) Maturation of loess treated with variable lime admixture: Pore space textural evolution and related phase changes. Applied Clay Science. 61: Lin D F, Lin K L, Hung M J (2007) Sludge ash/hydrated lime on the geotechnical properties of soft soil. Hazardous Materials. 145(1-2): Lin T, Liu Z D (2003) Study on indoor tests of fly ash and quick lime improving soft soils. Rock and Soil Mechanics. 24(16): Zhang F, Wang G, Kamai T (2013) Undrained shear behavior of loess saturated with different concentrations of sodium chloride solution. Engineering Geology. 155: Li B X, Niu Y H, Miao T D (2007) Water sensitivity of Malan loess in Lanzhou. Chinese Journal of Geotechnical Engineering. 29(2): Santamarina J C, Klein K A, Wang Y H (2002) Specific surface: determination and relevance. Canadian Geotechnical Journal. 39(1): Savant N K (1994) Simplified methylene blue method for rapid determination of cation exchange capacity of mineral soils. Communications in Soil Science & Plant Analysis. 25(19-20): Wang Y M, Yang Z C, Chen W W (2005) Strength characteristics and mechanism of loess solidified with new polymer material SH. Chinese Journal of Rock Mechanics and Engineering. 24(14): Guo T T, Zhang B P, Tian Z G (2004) Study on engineering characteristic of lime-fly ash loess. Chinese Journal of Geotechnical Engineering. 26(5): Wanjiong Wu Lanzhou , China wuwj2012@lzu.edu.cn Shouyun Liang ( ) liangsy@lzu.edu.cn Fanyu Zhang zhangfy@lzu.edu.cn Xude Yan @qq.com Jianji Zhou @qq.com 5