Experimental Study On Mechanical Properties Of Volcanic Ash Subgrade Filling And Its Improvement By Red Mud

The Party’s Twentieth National Congress proposes redoubling efforts to build China into a country with strong transportation network,pointing out the future development of the transport industry.In recent years,there has been a significant expansion in both scale and scope of road engineering construction;however,it has also brought about increasingly complex geotechnical challenges.The Changbai Mountain region in northeastern China is characterized by extensive distribution of volcanic ash deposits.Consequently,when constructing roads in this area,it becomes challenging to avoid traversing through areas covered with such deposits.The fundamental characteristics of volcanic ash differ significantly from those of typical soils,and its impact on road stability remains inadequately defined.Insufficient research has been conducted on the utilization of volcanic ash in road engineering,and there is a dearth of comprehensive and thorough investigations into the mechanical behavior of volcanic ash under both static and dynamic vehicle loading conditions,which hinders accurate prediction of deformation and stability in volcanic ash foundations.The project has not yet developed a practical and effective method to enhance the mechanical strength of volcanic ash,thereby impacting its engineering application.To elucidate the fundamental attributes of volcanic ash as subgrade filling,this study relies on the research project“Deformation and Strength Characteristics of Volcanic Mudflow Accumulated Soil and the Mechanism of Rainfall Erosion Damage”funded by the National Natural Science Foundation of China（NSFC）.The investigation focuses on volcanic ash from Changbai Mountain area as the research subject.Laboratory experiments are conducted to examine various key properties of volcanic ash.A soil enhancement technique utilizing red mud to enhance the fundamental properties of volcanic ash was also proposed.The key findings obtained are as follows:（1）XRD,XRF,sieve,and SEM tests were employed to determine the material composition and structural characteristics of volcanic ash.The test results indicate that feldspar,hematite,and magnetite dominate the mineral content of volcanic ash,with minor amounts of quartz and clay minerals.The primary oxide in volcanic ash is SiO₂,followed by Al₂O₃ and iron oxides.Volcanic ash contains about 43.6%fine particles and 56.4%sand particles,which is a fine-grained earthy sand and a poorly graded soil.The angularity of volcanic ash particles contributes to their poorly rounded shape while the soil particles exhibit an abundance of“pores”.（2）The influence of water content on the mechanical properties of volcanic ash was investigated through triaxial tests,consolidation tests,and California bearing ratio tests.The findings indicate a significant water sensitivity in volcanic ash,as evidenced by the reduction in cohesion,internal friction angle,compressive modulus,and CBR with increasing water content.（3）The long-term dynamic properties of volcanic ash were investigated through undrained dynamic triaxial tests.The shakedown types of volcanic ash under long-term cyclic loading were identified as plastic shakedown,plastic creep,and incremental collapse.It was demonstrated that the accumulated plastic strain of volcanic ash significantly increased with an increase in initial water content.When the specimen is saturated,the shakedown type observed is plastic creep at a dynamic stress level of 30kPa.As the dynamic stress increases,there is a significant increase in the plastic deformation of volcanic ash,leading to all shakedown types exhibiting incremental collapse.An empirical criterion for evaluating the shakedown types based on the plastic deformation characteristics of volcanic ash is proposed.Furthermore,this study clarifies the limits for plastic shakedown,plastic creep,and incremental collapse in volcanic ash.（4）Based on the plastic deformation parameter models proposed in MEPDG,a predictive model for accumulated plastic strain was developed,taking into account the initial water content,confining pressure,and dynamic stress.The critical dynamic stress model was refined by incorporating the peak strength from triaxial tests based on Dawson et al.’s proposed critical dynamic stress model.The refined model effectively considers the influence of specimen physical properties and stress levels on the critical dynamic stress.It has been proved that the proposed models can accurately predict the plastic deformation and dynamic strength of volcanic ash.（5）The engineering properties and environmental effects of red mud-amended volcanic ash improved soil were investigated,and the optimal proportion of red mud-amended volcanic ash admixture was determined through compaction test,triaxial shear test,consolidation test,California bearing ratio test,pH test,and TCLP test.The results suggest that the cohesion,internal friction angle,compression modulus,and CBR of the improved soil exhibit an initial increase followed by a subsequent decrease with increasing red mud content.These four mechanical parameters reach their maximum values at a red mud content of 15%.Furthermore,as long as the red mud content does not exceed 15%,both the pH value of the improved soil and heavy metal concentration in leachate remain within safe limits.Therefore,it is recommended to use a 15%proportion of red mud-amended volcanic ash for optimal performance.（6）The undrained dynamic triaxial tests were conducted to investigate the enhancement effect of red mud on the dynamic properties of volcanic ash,as well as the influence of confining pressure,dynamic stress,dynamic frequency,and waveform of dynamic load on the dynamic properties of saturated improved soil.Besides confining and dynamic stress,both frequency and load waveform can also impact the dynamic properties of improved soil.With increasing frequency,there is a tendency for plastic deformation to decrease.The accumulated plastic strain under trapezoidal wave loading in improved soil is slightly higher than that under sinusoidal wave loading.However,no consistent pattern is observed regarding the effect of frequency and load waveform on the dynamic modulus of improved soil.Dynamic stresses leading to instability damage in improved soils are significantly higher under saturated conditions.The improved soils exhibited a significantly lower accumulated plastic strain,a significantly higher dynamic modulus,and a slightly lower damping ratio compared to the volcanic ash prime soil.Therefore,red mud can effectively enhance the dynamic properties of volcanic ash.