Technical Study On Basalt Fibre-Cement Improvement Of Expansive Soils For Roads

Expansive soil is widely distributed in China,and traffic road construction often passes through expansive soil areas.Because expansive soil has significant water absorption expansion and water loss shrinkage characteristics,it needs to be improved before it can be used as roadbed filler.Traditional improvement techniques usually only consider whether the strength of the improved soil under conventional conditions meets the engineering requirements,while ignoring the effects of dry-wet cycle and hydrostatic soaking.However,in practical engineering,the improved expansive soil subgrade is more likely to crack after long-term dry-wet cycle or long-term hydrostatic soaking,which is particularly significant in rainy areas in the south.In view of this engineering problem,this paper takes the expansive soil in Nanning,Guangxi as the research object,and uses basalt fiber as the reinforcing material and cement as the reinforcing agent to improve the expansive soil.The optimum cement content was determined by compaction test and disintegration test.Based on the optimum cement content,the scheme of improving expansive soil by basalt fiber and cement was proposed.The disintegration test,the unconfined compressive test and the consolidated undrained triaxial compression test under dry-wet cycle and hydrostatic soaking conditions were carried out respectively.The disintegration rate F,the unconfined compressive strength value q_u,damage deviation stress（σ₁-σ₃）_f,the cohesion c and the internal friction angleφof the improved swelling soil under different dry-wet cycles and different hydrostatic immersion lengths were analysed,and the optimum ratio of cement,basalt fibre,soil and water in the improved soil was obtained.The following key findings emerged from the study:（1）Through the standard compaction test and the disintegration test under the action of dry-wet cycle and hydrostatic soaking,the influence of different cement content on the compactness and disintegration of expansive soil is studied.The results show that when the cement content is 6%,the compactness of the improved expansive soil is the largest and the disintegration rate is the smallest.（2）The unconfined compressive strength characteristics of improved expansive soil were obtained by unconfined compressive tests under conventional conditions,dry-wet cycles and static water immersion.The test results show that the unconfined compressive strength of expansive soil increases by 3.68 times after adding 6%cement,but the strength decreases greatly under the action of dry-wet cycle and static water immersion.When the basalt fiber composite is used to improve the expansive soil,the compressive strength attenuation of the soil is greatly improved.（3）triaxial shear strength properties of modified expansive soils were obtained by triaxial compression strength tests under conventional conditions,dry cycling and hydrostatic immersion.The test results show that the values of damage deviation stress（σ₁-σ₃）_f,the internal friction angleφand the cohesion c decreased rapidly before three dry-wet cycles and two-day of hydrostatic immersion and then decreased with the increase of dry-wet cycles or hydrostatic immersion time,and the rate of decrease gradually levelled off.（4）Based on the results of the disintegration test,the unconfined compressive strength test and the triaxial compression strength test,the improvement effect of the swelling soil was evaluated in terms of water stability,compressive performance and shear resistance,and the optimum compound improvement ratio for the swelling soil was obtained and the improvement mechanism was discussed and analysed.