Study On Progressive Failure Of Loess And Loess Slope Based On Damage Theory

The deformation and failure analysis about loess started from the strength characteristics, which now has turned to be comprehensive with consideration of the whole failure process. Damage theory has been widely adopted by numerous researchers as an appropriate way describing such a process. This paper, based on interpretation of macroscopic triaxial test results, aims to solve such a problem, and has proposed many methods of describing failure process from different aspects. New ideas about the loess are proposed seeing loess as bulk material. The damping macroscopic parameters are adopted to establish damage evolution equation, which is also deduced from abstraction of failure pattern. Macroscopically, damage emerges from transition from brittleness to plasticity of loess, while meso-scopically bond degradation, friction, and breakage between and inside particles all contribute to damage. The change of shearing parameters c and ? during loading is quantified. Some corrections are proposed about the traditional strength reduction method after a re-evaluation of current acknowledgement. Plastic zones are calculated and studied by FEM with newly-proposed damage-reduction method. The distribution of stress and strain of the slope is investigated to seek their connection with damage zone. Then some aspects not included in this paper is discussed and the main work includes:1. Increase in shearing rate can enhance the probability of softening while higher consolidation pressure can delay the occurrence of softening. 18% and 26% are two transition points where initial structure last maintains and sensitivity to water vanishes respectively. The original structure has the highest sensitivity to shearing rate which suggest shearing rates has different impact on different structure. Friction angles continuously decrease with ever-decreasing shearing rate. The sequence of development of cohesion and friction angle during shearing is clearly affected by shearing rate.2. Initial tangential modulus drops steadily when water content increases, slower at higher water content. Only or mainly the elastic strain emerges under small strain condition. The elastic strain energy of different initial structures has strong connection with the damage of initial structure. The mean energy dissipation rate is higher for loess with lower water content, which means it has stronger structure property than those with higher water content.3. Cohesion and friction angle develop differently. At small strain stage, both parameters develop fast though cohesion is faster. Cohesion almost stays at the highest level after reaching climax value though sometimes drops little. Friction angle continues increasing with strain. But they are with utterly different inside mechanism. Shearing strength develops and reaches climax soon but changes little before failure.4. The damage of loess under shearing force can be seen as process where structural phase continuously generates elastic strain energy upon external force and this energy transfers to plastic dissipation energy producing damage phase producing same elastic strain energy till all energy is consumed. Based on this hypothesis and other appropriate ones, damage evolution equation is deduced. This model has theoretical basis but needs more verification. Water content decides initial structure, which determines damage initiation. When water content is lower or the consolidation is higher, speed of evolution can be faster.5. The essence of damage of loess in fact is reflected by cohesion and friction angle, the former one of which is irreversible and the latter one can be seen as stable at maximum value if there is no directional arrangement. So reduction of cohesion can be attributed to damage while friction will not be reduced.6. When adopting unequal proportion reduction method, with proportion parameters growing bigger, the reduction coefficient for friction angle is becoming smaller while the parameter for cohesion is becoming bigger.7. The damage of loess up a slope is visualized through programming to characterize the damage level of slope. Reduction based on damage is proposed to substitute current safety factor method.8. The failure of a slope may originate from the lower part where high horizontal stress is densely distributed which is the main cause of yield of soil there. Shear failure may not be the main cause of the lower part’s failure. Irrespective of the failure modes(softening or hardening), the shearing parameter will eventually degrade with deformation. Then the stress in the upper part redistributes till balance is reached again or calculation inconvergence. It is speculated that creep behavior or condition near the toe of slope controls the stability of whole. Damage zones contain more information than plastic strain and it can better characterize the damage level of slope. At last, the critical damage is smaller when the water content is higher.