Research On Compaction Technique Of Expressway Subgrade Filled By Lime Metamorphic Soft Rock

Roads in mountainous areas are usually filled by different types of soft rocks with different degree of weathering. The thesis analyzed the road performance of soft rock and revealed its compaction mechanism to evaluate the engineering stability and compaction property of subgrade filler. Construction procedure, construction technique and construction quality-control index were studied to guide the construction of soft rock subgrade on site. All the research contents above are important theories and engineering projects of subgrade construction in mountainous areas.On the basis of former research findings, the thesis studied physical and mechanical properties of lime metamorphic soft rock by laboratory experiments and compaction features of such soft rock by use of self-manufacturing large-scale compact device to reveal the compaction mechanism. Finally, through the research results, the thesis wanted to evaluate the road performance of lime metamorphic soft rock, and then proposed the proper construction techniques.Based on field tests, compaction techniques of impact rolling and vibration rolling were studied so that construction parameters, such as, compaction thickness and number of rolling, can be determined by analyzing compaction effects under different construction technique. By examining all the index of test roads, the thesis analyzed the relationship among compactness, porosity, settlement difference and resilient modulus in order to propose the appropriate construction quality-control index of lime metamorphic soft rock subgrade.According to rock structure, mineral components and saturated uniaxial compressive strength, such soft rock can be defined as lime metamorphic soft rock. Such soft rock’s free water absorption is3.53%, saturation coefficient is0.9, and softening coefficient is0.37. In addition, its resilient modulus and CBR decreased40%-50%and37%-49%separately after soaked by water. What is more, lime metamorphic soft rock’s uniaxial compressive strength decreased10.26%after8dry-wet cycles and decreased58.62%after15freeze-thaw cycles. All the test results showed that such soft rock’s water stability and temperature stability were very poor and subgrade strength was greatly influenced by dry-wet cycle, immersion and freeze-thaw cycle.Compaction tests with different compaction cylinder diameter and compaction energy indicated that the compaction procedure of such soft rock was a dynamic process consisting of particle crushing, filing and interlocking and compaction cylinder diameter was the important factor of soft rock’s maximum dry density. The tests results also showed that conventional compaction test method was difficult to meet the requirements of construction quality control of lime metamorphic soft rock subgrade. Further, the maximum dry density of such soft rock reached the critical state with growth rate of nearly zero when compaction energy was3762kJ/m3.When the compactness was93%,94%and96%separately, such soft rock’s CBRs met the strength requirements of the highway subgrade fillers and when the compactness increased from94%to100%, such soft rock’s resilient modulus and CBR increased70%and50%separately, which showed that compactness had a great influence on strength and ability to resist deformation of such soft rock.The field test showed that (1) roller must have great height(about22t), suitable vibration frequency (28Hz to30Hz), high amplitude (above1.8mm) in vibration rolling, and the slackly lay thickness can be70cm, and the proper number of rolling was5, with2mm as settlement difference criteria.(2)This thesis suggested the impact rolling had better to use the impact roller with standard potential energy of25kJ, and the slackly lay thickness was1.1m with14as the best number of rolling.The research results can be directly used to guide the construction of lime metamorphic soft rock subgrade, which can significantly reduce construction costs of such soft rock subgrade and can also improve subgrade durability and service life. Generally, the research results have great engineering and economic significance.