| Gypsum or anhydrite, as a common sedimentary composition in China, almost exists in every age from the Cambrian period of Paleozoic to the Tertiary of Neozoic. And it is often consist in sedimentary rock in the form of its intergrowth with other carbonate sediment. In this paper, this sedimentary rock is called gypsum rock. In the natural conditions, the engineering properties of gypsum rock are apparently well and its strength is high. However, after contacted with water, its strength properties will be worse due to the solution of gypsum or anhydrite in the gypsum rock which generates sulfate ion erosive for concrete. Moreover, volume expansion will be aroused by the hydration of anhydrite. In that case, if tunnel construction through these kind sediments, the lack of knowledge in properties often causes tunnel disaster. Therefore, research on the engineering properties of gypsum rock of Yi-ba highway is conducted in this paper to deepen the understanding of the nature of this rock and this research is engineering guidance, which has a certain engineering significance.Based on the collection of strata material in the study area and the disaster characteristics in gypsum rock tunnel, a series of laboratory experiments including X-ray diffraction mineral appraisal test, ion chromatography test, scanning electron microscope test, remodeling sample expansion test, corrosion experiment and the leaching characteristic experiment, uniaxial compression test, triaxial compression test, sulfate erosion test on concrete and others are conducted to study the mineral composition and microstructure of gypsum rock, the concentration of dissolving sulfate ion, the corrosion and leaching characteristic, the hydration expansion of anhydrite and its strength properties under different moisture contents and times of drying and wetting cycles. Based on the datas of test, the internal and external sulfate erosion test were conducted to simulate erosion degradation of gypsum rock to the concrete structure. Through the above research on the engineering properties of gypsum rock, the main results are presented as follows.(1)Based on the analysis of the regional geological data and engineering geological investigation, we realize the formation environment and distribution characteristics of gypsum rock in the research area. We also know the existence forms of gypsum rock. The mineral and chemical composition of gypsum rock and the respective proportions of gypsum and anhydrite in gypsum rock were effectively identified by X-ray diffraction mineral appraisal test, chemical overall analysis test and hydration degree analysis test.(2) The expansion test of remodeling samples of gypsum rock was conducted to discuss the expansion properties of gypsum rock. We can conclude that the remodeling samples of gypsum rock cause certain expansion deformation and expansion force when in water. According to the expansion analysis of remodeling samples, the expansion rate of samples can reach6%and the size of expansion rate changes with the initial dry density of remodeling samples. The expansibility of gypsum rock is in the range from572kPa to824kPa. Moreover, its expansibility will increase with the increase of the initial dry density and water absorption. Then, the influence factors on the expansion of gypsum rock such as environmental humidity, the stress state of surrounding rock, the anhydrite content and the structural characteristics of gypsum rock are summarized with the properties of gypsum rock and other references. The expansion of gypsum rock will cause the lateral deformation of tunnel engineering concrete structure, which results in local stress concentration of concrete structure, and the structural failure. All these can influence the durability of concrete structures, which may take certain hazards to the tunnel engineering.(3)The corrosion experiment and the leaching characteristic experiment of gypsum rock indicate that there are both mechanical corrosion and chemical solution effect when the gypsum rock in static water environment. But the chemical solution plays the leading role. There is a dissolving balance in static corrosion, in the process of which the corrosion quality increases at first and reaches to a balance point at last. When the gypsum rock is in dynamic water environment, the mechanical dissolution plays a principal role. The corrosion quality continues to reduce and the structure of gypsum rock mass is destroyed ultimately. For the dynamic corrosion, corrosion speed is controlled by the length of corrosion time, the area of gypsum rock contacting with water and flow velocity. The corrosion effect of gypsum rock is affected by the solution with different ph conditions and ion components. The solubility of gypsum rock is affected by temperature, thus the gypsum rock corrosion quantity and rate are affected by it too. When gypsum rock is in corrosional condition, it can dissolve out a mass of sulfate ion which is highly corrosive for the tunnel reinforced concrete lining structure. The maximum concentration of sulfate-ion was1530mg/L. At the same time, by observing the microstructure photographs through scanning electron microscope (SEM) before or after the corrosion of gypsum rock and then comparing them, we notice that before the corrosion crystal plane of gypsum rock is intact, ridge is fine, the contact between crystal planes is close. But after, it is that some substances between the lattice is dissolved obviously. The distance of crystal planes of gypsum rock is enlarged, the edge of the crystal becomes to be shaped, joint plane is no longer apparent. In natural environment, there is static corrosion or dynamic corrosion when gypsum rock meets with water. But no matter which corrosion gypsum rock will dissolve out a mass of sulfate ion into the solution, which will contact the tunnel lining structure in a long-term. The sulfate ion can cause corrosion to lining concrete structure and some steel members of the supporting structure. At the same time due to the corrosion effect, gypsum rock is carried away by water, which will cause certain adverse effect and damage to tunnel pavement foundation and supporting structure.(4) The strength test research results indicate that the gypsum rock whose changes of strength and deformation index with moisture content are very sensitive, is soft rock with low compressive strength and its softening characteristics is obvious in water. The values of physical mechanics parameters of samples reduce with the increase of moisture contents in rock, including uniaxial compressive strength, elastic modulus, deformation modulus and poisson’s ratio. Poisson’s ratio increases with the increase of moisture contents. The drying and wetting cycles have a obvious effect on the strength reducing of gypsum rock.The values of physical mechanics parameters of samples reduce with the increased times of drying and wetting cycles to the rock, including uniaxial compressive strength elastic modulus, deformation modulus and poisson’s ratio. Poisson’s ratio increases with the increased times of drying and wetting cycles in a low level. Gypsum rock of triaxial compression test results’s curves show that gypsum rock are hard pressed, the elastic modulus of the rock and the yield strength with the confining pressure increases. And with the increase of confining pressure, gypsum rock’s deformation characteristics are transfer brittleness to ductility from low pressure to high pressure. And the critical point of the change are deeply reduced,which in saturated is18MPa and8MPa,but in natural is59MPa and15MPa.By contrasting values of internal friction angle and cohesive strength before and after saturated gypsum rock, it is found that the value of internal friction angle has been weaken approximately by42.6%and cohesive strength by53.97%. So we can conclude that the effect of water to gypsum rock is mainly reflected in the intercalative and interlocking binding between the particles of gypsum rock, as same as the attractive influence between the molecules of gypsum rock. Whether dry or saturation state, the poisson ratio of gypsum rock and elastic modulus are increasing with pressure.But in the same level of confining pressure, the poisson of the saturated state is smaller than the natural state, and the elastic modulus is bigger than the natural state. Through the strength properties test research, we realize that the softening property of gypsum rock is observably when in water with its deformation increasing and intensity decreasing at the same time, which makes a great impact on the stability of tunnel surrounding rock and the tunnel construction. Strengthen support and timely lining engineering measures should be taken in tunnel construction combined with engineering characteristics and requirements.(5) The indoor sulfate erosion to the concrete test was conducted to simulate the degradation effect of gypsum rock to tunnel concrete structure. According to the test results we can realize that whether the external corrosion of sulfate ion leaching from gypsum rock to concrete structure or the internal corrosion of gypsum rock to concrete structure when is used as concrete aggregate, a series of physical and chemical reactions between gypsum rock composition and concrete materials occur, which eventually make concrete structure intumescent, strength reduced, structure damaged, performance degenerated. Based on the test data, the work of high performance concrete designing was completed to prevent sulfate erosion of gypsum rock. External sulfate erosion concrete test, the long-term strength and quality changes of the mortar and concrete blocks were measured in saturated solution with Na2SO4erosion solution, acid environment Na2SO4solution in as erosion solution. The results show that the concrete blocks in the two kinds of solution both occur erosion damage. The strength change of concrete block soaked in saturated gypsum rock solution is not big, which shows that the erosion ability of static saturated gypsum rack solution is limited. It will take a long time to appear the erosion hazard. In internal sulfate erosion to the concrete test, by cement net pulp test, cement mortar test, crushed stone concrete test,it shows that blocks occur expansion damage when the gypsum rock whether is used as a block embedded into cement mortar stone, or as concrete coarse and fine aggregate mixed into cement mortar and concrete rubble into concrete. The damage degree increases with the dosage. Using the gypsum rock as fine aggregate into concrete can make great affect on expansion rate and strength of concrete blocks. No matter what kind of erosion above, a series of physical and chemical reactions between gypsum rock composition and concrete materials will occur. By electron microscope, the ettringite could be seen visibly in mortar or concrete block, especially the gypsum rock is used as aggregate, the ettringite of contact surface is very big, which directly destroys the contact surface in result of the adverse impact on concrete body. According to the test results, we put forward the scheme ratio of high performance concrete to prevent sulfate erosion and study the raw material variety, mix design scheme and configuration of the tunnel secondary lining waterproof concrete. Technical measures should be taken to strengthen the concrete strength grade, improve its crack resistance, permeability and resistance to sulfate erosion performance.(6)According to the field investigation and indoor test research, we summary the harms of gypsum surrounding rock to tunnel construction as follows. The expansion harm of gypsum rock and volume expansion appear when the gypsum mass rock meet with water, which cause lateral expansion force and the extrusion effects to tunnel concrete lining structure. All these result in the local stress concentration in concrete structure and leading to structural damage, which can influence the durability of the concrete structure. The softening harm of gypsum rock. The softening characteristics of gypsum rock is significant when in water with its deformation increasing, strength decreasing. The plastic deformation will be more obvious under the action of stress, which makes a influence on the stability of tunnel surrounding rock. Leaching harm of gypsum rock. The erosion can easily appear when gypsum rock with water. A drip phenomenon on tunnel top could occur when the gypsum rock is hollowed out by water, which has a mechanical dissolution effect on gypsum rock. At the same time if gypsum mass rock is corroded, the surround rock mass structure will destroy with its strength and the stability of surrounding rock tunnel reduced. When gypsum rock is in corrosional condition, it will dissolve out a mass of sulfate ion which is highly corrosive for the tunnel reinforced concrete lining structure. In this case, the concrete strength of tunnel support structure gradually decreases and eventually loses its bearing capacity. The harm of gypsum rock used as concrete aggregate. The gypsum mineral of aggregate surface participated in hydration reaction of cement when the gypsum rock is used as aggregate concrete or the concrete contacts to gypsum rock, which will react with calcium aluminate, iron calcium aluminate minerals of cement and hydrated calcium aluminate or sulphoaluminate (AFm) of single sulfur type to generate a swelling material called hydrated sulphoaluminate (AFt) of three sulfur type. When expansion stress of the aggregate-slurry interface reaches a certain degree, the concrete will craze with the result of reducing its strength. This is a kind of sulfate erosion (internal erosion) hazard of gypsum rock used as concrete aggregate.(7) It will cause many diseases when the gypsum rock exposed in tunnel surrounding rock. In view of complex, compound diseases of gypsum rock, some opinions and suggestions are put forward from the tunnel survey, design and construction of the tunnel and the later operation maintenance to the tunnel in the article. |
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