Triaxial Experimental Study On Improved Soil Mixed With Lignin Fiber And Fly Ash

Nowadays, there exists a very serious problem in cold region subgrade construction, and that is the damage of embankment due to freeze-thaw cycles. Millions of researchers devoted themselves into this area, and so many achievements are made, but the problem still can’t be settled properly. Among all researches, improved soils seem to be favored, such as fly-ash improved soil, cement improved soil, lime improved soil and so on.However, insufficiency of freeze-thaw resistance lies a great problem for subgrade mixed with inorganic matters. Therefore, a new kind of improved soil with both good freeze-thaw resistance as well as sufficient strength for construction needs to be discovered.The application possibilities of improved soil mixed with lignin fiber and fly ash for construction are analyzed by conducting triaxial tests in this article, in order to find the proper proportion of lignin fiber which meets both demand of strength and cost control.Specifically, a series of single-variable repeated UU(unconsolidated-undrained) tests are conducted, which contain five fiber contents, three curing conditions, three moisture contents and three confining pressures, so there are 135 groups of tests totally. The test results are studied, and several conclusions are drew below:(1) Stress-strain characteristics of samples in same condition of lignin fiber content, moisture content and curing but in different confining pressures are analyzed. And it comes to that 1)those samples are strain-so fting; 2)stress-strain curves of three different confining pressures coincide, where the axial strain is relatively small, thus the accuracy of tests is verified.(2) The ultimate strengths of all samples are analyzed by confining pressure, fiber content, moisture content and curing and it comes to that 1) the higher moisture content is, the lower ultimate strength becomes, where other conditions remain the same; 2) placing curing and freeze-thaw cycles both decrease ultimate strength; 3) improved soils with fiber content of 0.5% and 1%are of maximum strength in placing curing and freeze-thaw cycles, respectively.(3) Shear strength and elastic modulus of these two construction-applicable samples are studied, and the function relationships of both and moisture content are obtained, which reveal that 1)with addition of lignin fiber content, moisture content of corresponding extremum of internal friction angle, cohesion and elastic modulus increases; 2) placing curing increases elastic modulus of samples, and on the contrary, freeze-thaw cycles draw it down.Via tests above, it can be summarized that the application of improved soil mixed with lignin fiber and fly ash for construction is possible. It is suggested that fly-ash improved soil with lignin fiber content of 0.5% and 1% could be applied in non-cold region and cold region, respectively.