Study On Continuum Damage Of The Tunnel Brittle-Quasibrittle Surrounding Rocks

With the development of the economic, traffic engineeringmake a large progress. As the key of the tunnel, stability analysisis very important for the designing and construction. Now, Chinahas more tunnels than any other in the world, at the same time, itcause some trouble during the procedure. There are so manytunnels that results many engineering accident because of the lowtechnical standard, out –of-date construction technology, inferiorquality and inadequate structure. Of all the factors, the most is thetheory of the surrounding rocks does not catch up with the demandof the engineering development, which leads to the deviation ofthe cognition. With the external loading, cracks gradually expandand transfix, finally lead to failure because there are manymicroscopic and macroscopic deficiency included in thesurrounding rocks. Usually, evaluation of the stability and supportsystem must base on the correct cognition of the surroundingrocks. But current evaluation system and support method does notreflect the practical engineering geology characteristic of thesurrounding rocks. So, it present a problem that how to describethe damage mechanism, and how to use a quantitative parameterto characterize the cracks generating and expanding. Of all, mostimportant is how to determine the mechanical behavior on thecondition of the different work condition such as blasting damageand unloading damage. How to disclosure the evolutional damagemechanism by combining the microscopic breakage mechanicsand macroscopic characteristic and how to give a unifiedparameter to describe the evolution of the damage based onstrength theory, fracture theory and damage theory. With these, wecan give a appropriate evaluation of the stability and effectivesupport system. The author take the example of the Longgangtunnel to do some research on the continuum damage of brittleand quais-brittle surrounding rocks.The damage of the surrounding rocks reflects deterioration ofthe mechanical state. On the condition of the "bore-blastingmethod", the damage of the surrounding rocks includes blastingdamage and unloading damage. That means surrounding damageessentially means a coupling of the blast damage and unloadingdamage. This paper presents the use the partition to evaluate theengineering geology distinction; with this, we can give the differentconstitutive equation to set forth the surrounding rocks damageBlasting damage is the importance factor affecting the stabilityof the tunnel surrounding rocks. Damage at the basting stage isthe base of the unloading damage, and the same as crack does.Blasting damage including macroscopical damage andmicrocosmic damage, and it reflect the coupling influence on thestability of the surrounding rocks.There come into being three regions such as blastingneighboring region(crush region), (fracture region)and blastingremote region(stretch region)during the blasting stage, and thedifference of the damage is unlike. At the neighboring region, thevalue of the damage D can be considered as 1,and at the blastingmidst region , the value of the damage D must be 0 < D <1,whileat the remote region ,D can be seem as 0. Blasting damage showitself as the cracks enlarging. The rule of the cracks enlargingincluding the followings: adopting dynamic compression strengthas the rule of the blast wave; adopting brittle damage breakage asthe stress wave and adopting quasi-brittle damage breakage asthe quasi-static. The crush region does not change largeaccording to the type of the surrounding rocks, and the range ofthe cracks usually as the same as the diameter of the blast hole.Of the fracture region, the range of the cracks changes accordingto the type of the surrounding rocks. Of the type IV-V, the value ofthe range usually is 1.8m; of the type III is 0.8m and 0.5m in thetype II. The change of the damage range follows the same rule. Sowe can draw a conclusion that the range of the surroundingdamage usually is 0.7-2.2m. On the condition of unloading, the failure of the surroundingrocks is the accumulation of the damage. As the unloading, stressfield gradually adjust on the bases of the "initial damage", at thesame time, cracks generate which lead to the failure of thesurrounding rocks. The damage includes tensile damage andshear damage on the unloading condition. We can use themaximal tensile strain rule and Mohr-Coulomb rule to define thetensile damage and shear damage. At the same time, the crackgenerating includes macroscopic and microscopic expanding.Macroscopic expanding includes press-shear and tensile-shearexpanding. Microscopic crack expanding means to be formed thesub-damage on the tip of the crack, and we can use theDrucker-Prage rule to determine the shape and dimension of thedamage field. On the base of the blast damage and unloading damage, wecan draw a conclusion that surrounding damage essentiallymeans a coupling damage of the blast damage and unloaddamage. We can use the acoustic wave velocity to define thedamage parameter of the coupling field. The test data show thatthe state of the surrounding rocks seems to be deteriorative. Thevalue of the wave velocity is usually below 2000m/s and when weadopt the combined bolting and shot-concreting support,the valuegradually increases, that means the damage get the improvement,but there is a delay as 5-10 days after the combined support. Thetendency of the value when we use the full face excavationmethod change very small and when use the sub-step method toexcavate ,the value decreases with the reduce between theworking face and the measuring point, the value change obviouslywhen the test point position lies within ten meters and doesn'tchange over ten meters. Cross-hole testing data show the range ofthe damage between 1.2m to 1.6m. This paper has several originality innovations as followings:①use the partition to evaluate the engineering geology distinction;with this, we can give the different constitutive equation to set forththe surrounding rocks damage; ②use the macroscopic damageand microscopic damage to attribute the change of thesurrounding rocks, and then use the dynamic compressionstrength rule, brittle breakage rule and quasi-brittle breakage ruleto describe the cracks; ③present the damage constitutiveequation and cracks expanding regularity; ④draw a conclusionthat the damage of the surrounding roots in the coupling of theblast damage and unload damage; ⑤use the acoustic wavevelocity to define the damage parameter of the coupling field, thenwe can obtain the relationship such as damage D versus time T,damage D versus distance between test point and working facesand the damage evolution when use different excavation method...