Experimental Study On Composite Solidfication Of Saline Soil Slopes With Sulfur-free Lignin Synergistic Hydrophobic Materials Under Rainfall Conditions

Ensuring food security is a crucial requirement for the nation.In the execution of the latest initiative to boost grain production capacity by 100 billion kilogrammes,there is a growing emphasis on utilising saline soil as a supplementary resource.In response,Jilin Province is now implementing land consolidation and saline-alkali land transformation initiatives in the western Songnen Plain.Additionally,the province intends to initiate a range of complementary water conservancy engineering projects.Due to the high water sensitivity of the local saline soils,they are extremely vulnerable to structural collapse and weakening when contacting with water.As a result,the slopes of water channels in irrigation areas are easily damaged during rainfall and other weather conditions.Simultaneously,Jilin Province,as a major base for commercial food production in China,possesses abundant maize stover resources.However,effectively utilising these straw resources in this location poses a challenge.Hence,the efficient utilisation of sulfur-free lignin,a byproduct of the bioethanol industry,to enhance the quality of local channel slope soils will result in a mutually beneficial outcome of improving saline land transformation and maximising the use of straw resources.Sulfur-free lignin has demonstrated efficacy in enhancing the strength of saline soils.However,its hydrophilic nature restricts its application in dry-wet cycling conditions.Hydrophobic materials containing hydroxysiloxane-based polymers have diverse applications in light industry and other domains.They effectively prevent the adhesion and penetration of liquids.The engineering properties of the channel slopes can be enhanced in terms of mechanical strength and water stability properties by combining them to improve the local saline soils through the composite curing method.This method involves internally mixing lignin and externally spraying a hydrophobic agent.Based on the aforementioned objectives,this paper proposes the use of a lignin-hydrophobic-composite solidifcation method to improve the saline soil of the channel slope,focuses on the factors influencing the damage of the natural soil slope under rainfall conditions,and carries out the following aspects of work.First,the impact of particle size of sulfur-free lignin on the curing effect was examined in terms of microstructure,structural connection,and pore characteristics.This analysis serves as a foundation for selecting lignin species and conducting further investigation into their water-sensitive disaster-causing properties.Furthermore,the study investigates the penetration strength of the slope surface and the shear strength of the slope body,with a specific emphasis on analysing the impact of dry-wet cycle effects.Subsequently,due to the sporadic instances of decentralised disintegration near the slope foot,cyclic disintegration tests in dry-wet cycling settings were conducted.Additionally,intermittent rainfall physical simulation tests were conducted to study the progressive water-sensitive flooding process of natural soil slopes and evaluate the feasibility of the composite solidifcation method.These tests aimed to understand the cumulative erosion phenomena that occur on channel slopes due to rainfall during rainy periods,similar to dry-wet cycling conditions.Finally,the composite curing mechanism and the dry-wet cycle deterioration mechanism were investigated based on these results.Below are the main findings:(1)The effect of lignin particle size on the curing effect of saline soils was clarified.Compared with powdered lignin,granular lignin has the same chemical properties and larger particle size.In terms of microstructure type,powdered lignin is easy to directly attach to the surface of powder particles to form a skeleton-aggregate structure with powder particles as the skeleton,while granular lignin is better at agglomerating fine particles to form an agglomerated-skeleton structure with agglomerates as the skeleton.In terms of structural linkage differences,the differences in structural linkages between the two lignin-cured soils were mainly in the deterioration of water-binding linkages by the free lignin powder and the significant reinforcement provided by the coarse lignin particles.In terms of pore characteristics,differences in lignin particle size distribution mainly affected the pore distribution characteristics and had little effect on porosity.Powdered lignin had a better cementing effect on intra-aggregate pores and inter-aggeragate pores with smaller particle sizes(<8.66μm),resulting in larger dominant pores due to the adjustment of particle alignment,etc.,which is detrimental to the water stability of the soil samples.In conclusion,granular lignin consolidated soils have better engineering geotechnical properties and hence were selected for subsequent studies.(2)The mechanically optimal composite solidification scheme was preliminarily determined,and the effect of dry-wet cycle on the shear strength attenuation of composite cured soil slope was discussed through the establishment of the Duncan-Chang modified model considering dry-wet cycles.Based on the results of penetration strength and shear strength tests,the optimal composite solidification scheme considering economic benefits was initially determined to be 10%lignin dosage and0.5 g/cm~2 hydrophobic polymer dosage.Lignin can effectively improve soil shear strength,in which the cohesive force and internal friction angle increase and then decrease with the increase of lignin dosage,and the optimal dosage is 10%.However,both cohesive force and internal friction angle gradually decreased with the increase of the number of dry-wet cycles.In order to deeply investigate the effects of dry-wet cycles,a modified Duncan-Chang model was established based on the number of dry-wet cycles for natural and consolidated soils(optimal dosage of 10%).The results show that the elastic modulus and shear strength indexes of the natural soil and consolidated soil change from a rapid decrease to a slow decrease after 10 dry-wet cycles.After 30cycles,all the mechanical parameters enter into the stage of imminent destruction.Therefore,from the point of view of engineering practice,it is considered that lignin-cured soil slopes should be regularly monitored after 2 years of construction(10 dry-wet cycles),and should be reinforced again after 6 years(30 dry-wet cycles)depending on the actual situation..(3)The study assessed natural soil and composite-modified soil disintegration under different dry-wet cycles with different dosages of lignin and hydrophobic polymer.The influence mechanism of each factor on disintegration was clarified.It was specified that a lignin dosage of 10%and a hydrophobic polymer amount of 0.5 g/cm~2were found to be the optimal composite solidification scheme for modifying saline soil in the Qian’an area.Initial humidification completely disintegrated the natural soil,making it unsuitable for dry-wet cycles.On the other hand,the lignin-modified soil exhibits enhanced water resistance as a result of the bonding between soil particles and lignin.Both the single disintegration ratio and the overall disintegration rate exhibited a tendency to initially increase and subsequently drop as the dosage of lignin increased.The infection point was a lignin dosage of 10%.The water stability of the surface hydrophobic layer significantly enhanced when the amount of the hydrophobic agent was increased to 0.5 g/cm~2.Both lignin and hydrophobic agent have dual effects on the disintegration characteristics of saline soil.Lignin improves the durability of soil samples by reinforcing the bonding between particles.However,the solubility of lignin powders also speeds up the disintegration.Hydrophobic polymer decreased the speed at which water entered the soil by creating a hydrophobic layer.However,an incompletely covered hydrophobic layer accelerates the water erosion into the soil sample.The primary consequences of dry-wet cycles on the composite-modified soils were primarily the dissolving of surface lignin,the formation of surface fissures,and the disruption of the surface hydrophobic layer.By adding 0.5 g/cm~2 of a hydrophobic agent,the surface hydrophobic layer of the composite-modified soil demonstrated resistance to dry-wet cycles for a total of four cycles.Hence,it is advisable to regularly monitor the soil displacement at the base of the slope one year after the composite-modified soil slope is finished.This will help prevent slope damage caused by soil softening at the base of the slope.(4)Physical rainfall simulation tests was utilised to uncover the gradual water-sensitive-induced-disaster process that occurs in natural soil slopes during pluvial period.Additionally,the effectiveness of the composite solidification method was assessed.The erosion characteristics of the natural soil slope can be described in five stages:"dissolution and melting-rainwater scouring-partial collapse at the foot of the slope-development of unloading fissures-expansion of the collapse surface".These stages exhibit shallow and gradual traction.Conversely,the composite-modified soil slope has higher integrity and little damage,mostly in the form of splash pits and fine gully erosion on the slope surface.Regarding the assessment of erosion amount,there is a direct correlation between the change pattern of cumulative runoff and sediment yield of the natural soil slope and the rainfall intensity.Furthermore,both the cumulative runoff and sediment yield consistently increase.As for the composite-modified soil slope,the turning point at which runoff and sediment yield transitied from increasing to decreasing occurs at a rainfall intensity of 80 mm/h.Additionally,this rainfall intensity also marks the point at which rainwater shifts from producing runoff to infiltrating the slope surface.Regarding the water spatiotemporal distribution,rainwater that seeps into the natural soil slope tends to accumulate in the shallow layer,creating a transient saturated zone.This led to a significant increase in seepage force,which in turn increased the potential danger of local landslides.Composite-modified soil slopes can mitigate the effects of seepage forces by minimising precipitation infiltration due to dense soil structure.The primary function of the hydrophobic polymer is to decrease water infiltration and prevent the formation of crust,while lignin can compact soil structure and improve soil strength.The combination of these two elements can effectively minimise the damage caused by composite-modified soil slopes.Combined with the results of erosion characteristics,assessment of erosion amount,water spatiotemporal distribution,the stability of composite-cured soil slopes was proved before 80 mm/h rainfall intensity.Given that this rainfall intensity is the largest in recent years in the study area and the rainfall does not last for 24 hours,the composite-cured method is effective.(5)The composite-cured mechanism of sulfur-free lignin and hydrophobic agent was investigated.Simultaneously,the degradation mechanism of dry-wet cycle was explained.Lignin mainly generates physical cementation,which promotes the conversion of the more hydrophilic illite/smectite mixed layer minerals to the less hydrophilic kaolinite and illite through electrostatic attraction and ion exchange.At the same time,the coarser lignin particles can form an effective three-dimensional network of agglomerated particles,which plays a reinforcing role similar to that of fibers,thus improving the strength and water resistance of the cured soil.Hydrophobic silicone-containing polymers bind to the hydroxyl groups on the surface of the particles,thereby removing the hydrophilic hydroxyl groups and forming a hydrophobic elastic membrane structure.The alkaline environment co-created by lignin and the natural soil favors the formation of hydrophobic membranes,thus weakening the water erosion of the soil.Both lignin and hydrophobic polymer do not bring heavy metal pollution to the soil,making them green curing agents.The deteriorating effect of dry-wet cycles on saline soils is mainly manifested in the destruction of the internal structure of the soil,particle fragmentation,and pore changes.The total pore volume of lignin-modified soil tend to first decrease and then increase with the increase of dosage,and the minimum value occurs when the dosage is 10%.The number of 5 dry-wet cycles is the inflection point when the change of pore distribution within the specimen becomes slower,while the number of 10 dry-wet cycles is the inflection point when the rise of total pore volume becomes slower after the relative stabilization of pore distribution in the specimen.