| As a new type of solid waste treatment,fluid filling material has been widely used in many backfilling projects because of its self-compacting,self-leveling and strength-controllable properties.In this paper,a cement-fly ash solidified fluid filling material containing collapsible loess and gravel soil was proposed.Firstly,the workability（flowability and bleeding rate）,physical and mechanical properties（compressive strength and permeability）and collapsibility of cement-soil fluid filling material were analyzed by adjusting the fine-to-coarse aggregate ratio（F/C）,water-to-solid ratio（W/S）,binder-to-aggregate ratio（B/A）,sandy soil to fine aggregate ratio（SS/F）and fly ash-to-aggregate ratio（FA/A）,and the influence mechanisms of various factors affecting the material properties were analyzed by microscopic tests.Then,a recommended mix ratio was given and a strength prediction model for the proposed material incorporating five controlling factors was developed.Moreover,the stress-strain curves obtained from uniaxial compression tests were analyzed and a suitable uniaxial constitutive model was developed.Finally,field tests were carried out to analyze and evaluate the quality of the practical application.The main research results and conclusions of this paper are as follows:（1）The research on the workability of the mixture shows that the mixture with low F/C ratio（1.50～1.86）needs less water to meet the flowability requirement,and the increased SS/F ratio significantly improves the flowability of the mixture,but the increased B/A ratio has the opposite effect.Improvement in the flowability of the mixture by increasing the FA/A ratio is not unlimited,with the greatest improvement in flowability occurring at an FA/A ratio of 0.12.The water retention capacity of the mixture meets the requirements within the research range.（2）The research on mechanical properties of the material shows that the strength increase of the cement-soil fluid filling material can be achieved by increasing the FA/A,B/A and SS/F ratios and decreasing the F/C ratio.The strength of the proposed material can be divided into three stages with the increase of age.The addition of fly ash is beneficial to the long-term strength of the material.The addition of sandy soil is beneficial to the strength increase of the material,but an excessive SS/F ratio under a fixed B/A ratio will lead to a reduction in material strength.（3）The recommended optimum proportion was proposed,with the slump of the fresh material being around 200 mm,a strength of the specimen reaching 0.8 MPa after standard curing for 28 days and a permeability coefficient of less than 1×10-6cm/s.The collapsibility of cement-soil fluid filling material with collapsible loess which meets the requirements of flowability and strength has been completely eliminated.（4）A model for predicting the compressive strength of cement-soil fluid filling material was proposed by sorting out and fitting the strength test data,which can take into account the effects of five factors on the strength including B/A,FA/A,F/C,SS/F ratios and age.Meanwhile,the stress-strain relationship obtained from unconfined compressive strength test was used to analyze the damage process of the material after compression,and the suitable uniaxial constitutive relation was given.（5）An optimal mixture proportion was selected for field test,and the self-leveling and self-compacting properties of the fluid filling material were observed from discharge and hardening process.Field sampling test results show that the material exhibits sufficient bearing capacity and good stability under the influence of water,which verified the applicability of cement-soil fluid filling material prepared with excavated waste soil in backfilling projects.Finally,the corresponding prevention measures and suggestions are given with the practical problems that appear in the field test. |
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