Study Of The Strength Characteristics And Deformation Behavior Of Unsaturated Soil

This paper describes the results of a laboratory research program designed toestablish the strength characteristics for compacted fly-ash. The strengthcharacteristics ~nd deformation behavior of unsaturated fly-ash were studied byusing a modified triaxial apparatus. The pore-water pressure and pore-air pressurewere measured independently. Three types of test ways were conducted, i.e. triaxialundrained shear test, constant water content test and triaxial drained but unventilatedshear test. All specimens were consolidated. The results show that unsaturated fly-ash, as a kind of sand, has significantly different characteristics comparing withunsaturated clay. After consolidated, the matrix suction of unsaturated fly-ash isapproximately a constant. It has little relation to the value of cell pressure. The valueof ~ which is the friction angle associated with suction increases with suction, butit is always less than the effective an~1e of internal friction of saturated soil. Thefriction angle associated with net normal stress and calculated by net mean stressand deviatoric stress is similar to effective angle of internal friction of saturated soil.Another important achievement of this paper is the proposed new shear strengthequation of an unsaturated soil. Two stress state variables for strength of unsaturatedsoil were used. One of them is named suction strength. The logarithm of it versusthat of suction relationship is linear. The value of c and ~ can be calculated by therelationship of q1 `,Finally, the deformation property of unsaturated fly-ash based on the modifiedapparatus is investigated and the relation between the strength and deformation ofunsaturated soil and saturated soil is studied. Under the same initial conditions, thestrength of unsaturated soil is higher than that of saturated soil. If this relation isignored in the design of engineering practice. it will not be economical. The volumechange of unsaturated soil is bigger than that of saturated soil, and the differencebetween them increases with the cell pressure. If this characteristic is not beconsidered in the fly-ash dam design, the total deformation of fly-ash will be smallerthan the real value of itself, and it will affect the safety of the engineering.