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Technical NoteEffect of strain rate on the mechanical properties of salt rockW.G. Lianga,n, Y.S. Zhaoa, S.G. Xua, M.B. DusseaultbaMining Technology Institute, Taiyuan University of Technology, Taiyuan, Shanxi 030024, ChinabEarth and Environmental Sciences Department, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1a r t i c l e i n f oArticle history:Received 19 December 2009Received in revised form24 April 2010Accepted 15 June 2010Available online 1 July 20101. IntroductionSalt rock is a special material in rock engineering. With itsphysical properties of tight fabric and low permeability, as well as itsmechanical properties of low strength and ductility, it has become afavored medium for waste disposal and oil and gas storage sinceearly in the last century, when it began to be thoroughly studied.More recently, achievements documented in the six conferences onthe mechanical behavior of salt since 1984 and other publicationshave greatly increased knowledge of salt rock behavior.As the strength of salt rock is somewhat low, generally theloading rate for compression tests is also slow. However, loadingstrain rate (_e) affects the mechanical properties of the salt rock.Studies have been carried out on many different rock typesexamining the effect of_eon rock strength and deformationcharacteristics. For example,_eeffects on dynamic tensile strengthof Inada granite and Tage tuff were studied by Cho et al. 1;loading_evalues for the granite and the tuff were 4.2413.18 and0.466.82/s, respectively. It was found that the dynamic tensilestrength of the two rock types increased rapidly with_e.After studying the porosity change of Mugla marble by heattreatment,compressiontestsunder_evaluesof2?10?55?10?7/s were conducted by Mahmutoglu 2. It was found thatthe compressive strength decreased sharply with strain ratedecrease. For dry specimens, the percentage of the strengthdecrease is about 44% whereas the relative strength decrease forsaturated specimens is much larger. It was suggested by Qi et al.3 that the deformation and failure of rock under low strain rateswere controlled mainly by thermally activated mechanisms. Withstrain rate increases, a phonon damping mechanism appears andgradually plays a dominant role in the deformation process. Li andWang 4 studied the fracture toughness of marble under highloading rates using the Hopkinson bar method and found that thetoughness increased significantly with the loading rate. Yang et al.5 carried out tests on limestone under different loading ratesand found that the peak strength increased with the loading rate;at the same time, the strain at peak strength increased linearlywith strain rate. He also found that the failure mode of the rockseldom changed with the loading rate.There are also many other studies 613 of the strain rateeffect on mechanical properties of rocks; however, reports ofstrain rate effects on mechanical properties of salt rock are lesswell documented. To investigate this in the laboratory, weconducted uniaxial compression tests on salt rock (includinghalite and thenardite) under loading_evalues of 2?10?5,2?10?4, and 2?10?3/s. These results were generated as partof a more general investigation of the performance of gas storageoperations in salt caverns.2. Sampling and methodology2.1. SamplesIn this paper, rock salt refers to rock with a dominantmineral component of halite (NaCl), whereas salt rock is a moregeneral designation for all rocks with a dominant mineralcomponent of highly soluble salts generally of evaporitic originincluding NaCl, KCl, carnallite, bischofite, tachyhydrite, thenar-dite, mirabilite, glauberite, anhydrite, and so on.Samples were cored from two evaporite deposits in JiangsuProvince, China. The evaporites are saline lake sediments ofTertiary age. One type of lithology is rock salt (halite), with themajor deposit located 9001100 m deep; the other salt rocklithology is dominated by the mineral thenardite (Na2SO4sodium sulfate), and is buried about 2000 m deep. Both are ofevaporative crystallization origin and have the desired propertiesContents lists available at ScienceDirectjournal homepage: /locate/ijrmmsInternational Journal ofRock Mechanics & Mining Sciences1365-1609/$-see front matter & 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijrmms.2010.06.012nCorresponding author. Tel./fax: +86 351 6014865.E-mail address: master_lwg (W.G. Liang).International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167of fabric tightness and low permeability for fluid storage or wastedisposal. Some silt fillings were occasionally found in the rock saltsamples, and the intergranular insoluble silt content is less than10% in weight; however, the thenardite samples were pureand homogeneous. Samples were transported to the TaiyuanUniversity of Technology (TYUT) laboratory after careful packaging.In the laboratory, samples were processed according to ISRMsuggested methods. Nine cylindrical specimens with aspect ratiosof 2:1 were eventually prepared, eight of which were successfullytested, six rock salt specimens and two thenardite specimens. Forthe six rock salt specimens, uniaxial compression tests with threedifferent loading rates were conducted while the two thenarditespecimens were tested with two lower loading rates. Generally,35 specimens are suggested to be prepared for each test toguarantee reliability, but we were hampered by a lack ofacceptable quality core. To compare loading rate differences withthe limited specimen set, only two specimens for each test couldrealistically be assigned. The reason we nevertheless choose topresent our test results here is that they were found to bereasonably consistent and hence are likely to be reliable.2.2. Test and equipmentThe main purpose of the test was to study the loading rate effecton mechanical properties of salt rock during uniaxial compression.All the experiments were performed on servo-controlled mechan-ical test equipment TYT-600 (Fig. 1), manufactured by Jilin Jinli TestTechnology Co. Ltd (Changchun, China). The maximum loadcapacity of the frame is 6.0?104kg, the minimum displacementrate is 1.0?10?3mm/s and loading rates can be varied with theservo-control system.2.3. MethodologyFor the experiment, three uniaxial compression displacementrates were chosen, leading to_evalues of 2.0?10?5, 2.0?10?4,and 2.0?10?3/s, a factor of 100. As mentioned before, two each ofthe six rock salt specimens were assigned to be tested under oneloading strain rate, and the two thenardite specimens weretested under the two lower strain rates of 2.0?10?5and2.0?10?4/s.FollowingISRMsuggestedmethods14,theuniaxial compression tests for each strain rate were carried outon the same mechanical test equipment.3. Experimental resultsThe uniaxial compression test results with different strain ratesare listed in Table 1; Figs. 26 are the resulting stressstrain (se)curves from the specimens during the tests. It was found that thesecurves are comb-shaped and that stress fluctuations in therock salt were observed at the lowest_eof 2.0?10?5/s. The twosecurves for the specimens tested at this lowest_evalue areshown in Figs. 2 and 3.3.1. Relationship between strength and strain rateFrom Table 1 and thesecurves, it is found that the peakstrength of rock salt is little affected by the strain rate (within thechosen range for_e). Average UCS values are 13.6, 13.9 and12.4 MPa, respectively, for_evalues of 2.0?10?5to 2.0?10?4and 2.0?10?3/s. Also, the UCS of the tested rock salt is consistentlylower than that of ten natural rock salts in the U.S.A. 15.We believe the reason for the lower strength is that the Jiangsusalt deposits were formed in a different style and at a differentgeologic time. Most salt deposits in North America were formed inthe Jurassic, Permian or Devonian periods; some bedded saltshave been recrystallized; and, salt from salt domes has undergoneextensive recrystallization, deformation, and porosity reduction(domal salts are less porous than bedded salts, and older saltsare less porous than younger salts in general). The salt deposits inJiangsu, China were salt lake deposits formed in the Tertiary(Eocene) and have not undergone repeated recrystallizationorlargescalecreepdeformationassociatedwithtectonicmobilization or diapirism. All things being equal, salt from ayounger deposit should be somewhat weaker and less rigidthan salt from an older deposit. However, the strain rate effect onthe salt strength is similar to that reported by Farmer and Gilbert16 at_evalues of 5.0?10?3and 2.0?10?4/s, where theyalso report that thesecurves for their rock salt under confiningstresses of 3.5, 7, 14, 35, 42 MPa were quite similar. This is anaspect of rock salt behavior that is substantially differentOil supplying system Data collection and controlling centerUplifting and descending platformTested specimen Test equipment frameFig. 1. Schematic diagram of the test equipment.W.G. Liang et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167162from brittle rocks such as sandstone, marble, and granite oversimilar strain rates.For example, Ray et al. 10 found that the UCS of Chunarsandstone increased from 64.0 to 75.1 and 99.5 MPa with_eincreases from 2.5?10?1, to 2.5?100and to 2.5?101/s. Fukuiet al. 11 also reported that shear strength of Sanjome andesite isdependent on loading rate; if the loading rate was increased by anorder of magnitude, the apparent cohesion increased by 6.1%, andthis ratio was about the same for the UCS, uniaxial tensilestrength,indirecttensilestrength,andfracturetoughness.Similarly, Jeong et al. 12 note that the UCS of Kumamotoandesiteincreasedlinearlywithdecreasingwatervaporpressure for the same_eand with increasing_ein the range0.95?10?61.91?10?4/s. Li et al. 13 conducted dynamictriaxial compression tests for the Bukit Timah granite with fourstrain rates (10?4/s, 10?3/s, 10?1/s, 100/s) and six confiningpressures. It was concluded that the compressive strengthgenerally increases with increasing strain rate and confiningstress. Also, the rate of increase of compressive strength withstrain rate is lower at higher confining pressure.The_e-effect can be explained by energy theory. Damage ofmaterial is the result of development of through-going micro-fractures, and during a short but rapid loading process with alarge_e, the development of micro-fractures in the material lagsthe increment of loading. This means that the response anddeformation speed of grains in the material is slower than thefaster loading rate, just as we note with deformation hysteresiswhen we compare rock response with the rate of load decreaseduring the unloading process of a compressed material. BecauseTable 1Experimental results for the tested specimens.SpecimenLoadingrate (s)Ultimatestrength (MPa)Elasticmodulus (GPa)Poissons ratioDeformationmodulus (GPa)Stress of volumeexpanding (MPa)Lithology#12.0?10?513.273.540.150.9712.75Rock salt#22.0?10?513.8912.59Rock salt#32.0?10?415.363.420.281.9010.02Rock salt#42.0?10?412.492.590.241.099.19Rock salt#52.0?10?312.404.540.142.3811.6Rock salt#62.0?10?312.292.280.181.4512.24Rock salt#72.0?10?515.2615.20.3012.82Thenardite#82.0?10?414.694.610.1914.45Thenardite0246810121400.511.522.5Axial strain/%Stress/MPaPeak strength 13.27MPaPlasticdeformation Fig. 2. Stressstrain curve of halite with loading_eof 2.0?10?5/s (specimen#1).024681012141600.511.522.53Stress/MPaPeak strength13.83MPa Plasticdeformation Axial strain/%Fig. 3. Stressstrain curve of halite with loading_eof 2.0?10?5/s (specimen#2).02468101214161800.511.522.5Axial strain / %Stress / MPa3#4#Peak strength15.36MPaPeak strength12.49MPa Plastic deformationFig. 4. Stressstrain curve of halite with loading_eof 2.0?10?4/s.0246810121400.511.522.5Axial strain / %Stress / MPa5#6#Peak strength 12.40MPaPeak strength 12.29MPaPlastic deformationFig. 5. Stressstrain curve of halite with loading_eof 2.0?10?3/s.W.G. Liang et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167163of this hysteresis, the energy absorbed by the material cannotbe consumed or released fully in the short time span bydevelopment of micro-fractures, and it is temporarily stored asmaterial compression, thus the strength of the material issomewhat enhanced.In comparison to igneous rocks and many other sedimentaryrocks, salt rock is classified as a soft rock, and it possessescharacteristic mechanical properties of low strength and strongrheology. Many studies 1822 demonstrate that the UCS of saltrock is in range 12.020.0 MPa, and the rheology of salt rocktypically arises as a combination of elastic and viscoplasticdeformation 2327, the ratio of which is a function of theloading rate and confining stress. Large viscous strains (deforma-tions) can be achieved with long load times, even under the actionof relatively modest deviatoric stresses. This large viscous strainincludes axial and lateral deformations such that under slowdeformation conditions the material behaves incompressibly(non-dilatant). In the non-dilatant viscoplastic strain rate regime,the Poissons ratio of rock salt is essentially 0.5, almost an idealplastic deformation condition with a zero plasticity angle. Ofcourse,athigherstrainrates,saltdisplaysdilatancy,themagnitude of which is also a function of_eand the confiningstress.In any case, the UCS of salt rock is not enhanced by anincreasing strain rate as much as for brittle rocks. The energyaccumulated in salt rock during rapid compression during ourexperiments is absorbed and consumed by its strong rheologicaldeformation (i.e. energy dissipation or relaxation mechanisms),and the pseudo-confining stress effect noted in brittle rock isabsent or weak. This seems to be a rational interpretation for thenear-invariability of rock salt strength we observed with a strainrate increase (limited to our range of_e). There are other factorswhich also lead to the different mechanical response of salt rockcompared to brittle rocks in the rapid loading domain. Forexample, effects of mineralogy, mineral grain size, and mineralfabric differences arise in siliceous and igneous rocks, and thoughimportant, these additional factors are not discussed extensivelyherein.The elastic modulus of the rock salt specimens is calculated tobe 2.6, 3.0, and 3.4 GPa, respectively, with_eincreasing from2.0?10?5to 2.0?10?4and 2.0?10?3/s. The elastic modulusseems to show a trend of slow increase with strain rate. This trendis similar to that of Three Gorges granite 17, for example.The UCS of thenardite is somewhat larger than that of rock saltunder the same strain rate conditions. Apart from the difference ofcrystal lattice arrangement, in our opinion it is mainly theconsequence of the grain size difference of the two crystallites.Fig. 7 shows damaged specimens of halite and thenardite wherewe can see that the halite grain size is ?1?10 mm, whereas thethenardite grain size is 1?3 mm. The smaller the grain size, themore difficult it is to propagate planar intercrystalline micro-fractures to achieve the critical fracture size associated with peakstrength. In addition, the two thenardite specimens in theexperiment are pure and contain no impurities, but the rock saltspecimens contain silt at the grain boundaries. The differentstiffness of the impurity represented by the silt will act as a stressconcentrator for shear, and the weaker bonding between the siltand the rock salt makes tensile parting easier; these are importantfactors affecting the UCS. Finally, the thenardite was buried1000 m deeper than the rock salt, and was thus more compressed(i.e. lower porosity) and hence stronger.3.2. Relationship between deformation/failure and strain rateThe strain rate also has an evident relationship to the strain-to-failure of the rock salt: with an increasing_evalue, the strain at peakstrength is less. For example, the strain-to-failure is 1.31.7%(specimens#1,#2) when_e 2:0 ? 10?5=s; however, it decreases to0.30.7% (specimens#5,#6) when_eis increased to 2.0?10?3/s. Ifwe define a normalized secant deformation modulus E0as the valueof axial strain at the point of peak strength divided by the peakstrength, then on average E00.93, 1.50, and 1.92 GPa for the threeprogressively more rapid_evalues. Obviously, the deformationmodulus is increasing with_ein a manner similar to the elasticmodulus. However, the deformation modulus increment with strainrate increase is larger than that of the elastic modulus. Thedeformation modulus increases by 52% and 28% with_eincreasingfrom 2.0?10?5to 2.0?10?3/s, whereas elastic modulus increasesby 15% and 14%, respectively.The difference is because of plastic deformation of thespecimen; the more rapid the strain rate, the less the plasticdeformation compared to elastic deformation. Empirically, using alogarithmic expression to link the strain rate and deformationmodulus, E0 0:2ln_e3:2. This equation is sketched in Fig. 8,giving a reasonable fit over the_erange used.Poissons ratio relationships are less clear;nis found todecrease with strain rate with the exceptions of specimens#1 and#2 which were tested at_e 2:0 ? 10?5=s. On average,nin the rocksalt specimens decreases from 0.26 to 0.16 as_eincreases from2.0?10?4to 2.0?10?3/s, and a similar trend is found forthenardite (ndecreases from 0.30 to 0.15 as_eincreases from2.0?10?5to 2.0?10?3/s). As these are low values compared to02468101214161800.511.5Axial strain / %Stress / MPa7#8#Peak strength 15.26MPaPeak strength 14.69MPaPlastic deformationFig. 6. Stressstrain curve of thenardite under uniaxial compression.(loading strain rate of 7# is 2.0 10-5/s and 8# is 2.010-4/s )Shear plane of thenarditeGrains of thenarditeGrains of haliteFig. 7. Damaged specimens of halite and thenardite with sloughing crystal grains(the left thenardite specimen damaged in the style of shearing along a relativelyplanar surface; the right halite specimen damaged in style of brittle tensilefracture along grains).W.G. Liang et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167164theliterature(0.300.40),theylikelyreflectcrackclosureprocesses, always an important issue in polycrystalline rockstested at low confining stress. Also, with lower_evalues, there ismore time for viscoplastic processes to act, giving more lateraldeformation, thus a higher Poissons ratio (as stated previously, atvery low_evalues and under some confining stress, salt is non-dilatant as it undergoes viscous deformation).The failure mode of the specimens was found to be consistentover the strain rate range (similar to results in Ref. 28). Thefailure of the rock salt is a clear brittle fracture process along grainboundaries with little shear; for thenardite, shear develops alonga relatively planar surface (Fig. 7). The failure style difference isbecause of different rock fabrics; the grain size of the rock saltcrystallite is larger than that of the thenardite, and the presence ofintercrystalline silt in the rock salt weakens the bonding betweenadjacent halite crystals and tensile parting can happen moreeasily in orientations close to 901 tos3. Compared with thehomogeneous thenardite with small pure grains, impure rock saltbreaks much more easily with a tensile brittle failure demon-strated as the development of approximately columnar surfacesin the unconfined condition.3.3. Volume dilation stress during compressionGenerally, the compressivesecurve of these rocks can bedivided into five stages (Fig. 9). Stage I is closing of pre-existingmicrocracks or pore space oriented at suitable angles to theapplied stress. This stage is less obvious for salt rock because of itslow porosity, but there is usually intercrystalline extensionaldamage from the sampling and de-stressing process. Stage II iselastic and largely recoverable deformation.Stage III is characterized by the onset of dilation and by a near-linear increase in volume, which is offset by the continuingcompression. It is supposed that microcrack propagation occurs ina stable manner during this stage. Its upper boundary is the pointof maximum compaction and zero volume change, and the stressat the point of zero volume change has been called the criticalstress for volume dilation; it is about 80% of peak stress for mostrock specimens. If the stress increases continuously after thecritical stress is passed, stage IV, characterized by positivedilation, takes place with an acceleration of microcracking, andthe internal structure is rendered more porous and debondeduntil stage V, the cessation of dilation occurs. What we willexamine here is the critical stress for volume dilation of thespecimen under different strain rates.It can be shown from diagrams of volumetric strain versusstress (Figs. 10 and 11) that the critical stress of volume dilation isdifferent for the two groups of specimens which were compressedunder_e 10?4and 10?3=s,respectively.Forthespecimenscompressed under_e 10?4=s, the critical stress for volumedilation is 9.6 and 8.6 MPa for specimens#3 and#4, about 63%and 69% of their peak stress. However, this increases to 94% and96% for specimens#5 and#6 when_eincreases to 10?3/s;obviously, the stress triggering fast microcrack development isless under a low strain rate than at a high strain rate. This may bey = 0.203Ln (x) + 3.190900.511.522.500.00050.0010.00150.0020.0025Deformation modulus / GPaStrain rate / sFig. 8. Deformation modulus vs. loading_eof specimens.IIIIIIIVVOABCDFig. 9. Common stressstrain curve of compression for rock materials.4#02468101214-5-4-3-2-1012volume strain/%stress / MPaCritical stressAxial strain / %Fig. 10. Volume strain and axial strain versus stress during uniaxial compressiontest with loading_eof 2?10?4/s.6#0246810121416-5-4-3-2-1012Volume strain / %Stress / MPaAxial strain / %Critical stressFig. 11. Volume strain and axial strain versus stress during uniaxial compressiontest with loading_eof 2?10?3/s.W.G. Liang et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167165explained by the deformation or response hysteresis effect at arapid strain rate discussed previously. It can be concluded that thehigher the strain rate, the larger the critical stress needed to dilatethe rock salt.4. Stress fluctuation phenomenon under low strain rateDuring the uniaxial compression test for halite at_e 2:0?10?5=s, a stress fluctuation phenomenon was observed. When theloading stress reaches about 8.0 MPa (Figs. 2 and 3), about 60% of thepeak strength, plastic deformation began in the specimen, defined asthe threshold value of plastic deformation. When the loading stressrose to about 10.0 MPa, about 75% of the peak strength, the stresssuddenly dropped to a value around the threshold of the plasticdeformation and then recovered to the original stress or slightlyhigher; this stress fluctuation phenomenon persisted till thespecimen underwent strain-weakening.This phenomenon was also noted in Ref. 29 during mechan-ical tests on specimens of salt rock comprised of laminated haliteand non-salt interlayers. At_e 1 ? 10?4=s, a stress dropphenomenon was observed before the axial stress reached peakvalue. The stress drop was about 4 MPa, and it was concluded thatthe hard-brittle interlayer was suffering fracture while the haliteremained intact. The stress drop thus resulted from the mechan-ical differences between halite and the interlayer material.To explore the deformation properties at_e 2:0 ? 10?5=s, weconducted supplementary tests with another three rock saltspecimens; unfortunately, we did not have identical samples fromthe same salt deposit as the former specimens#1 and#2. Thesupplementary tests were carried out with three new specimensfrom the Hongze salt deposit, also in Jiangsu Province, but theburial depth is similar to that of the thenardite, about 2000 m. Inthe latter uniaxial compression experiment, the stress-fluctua-tion phenomenon was not found for the stressstrain curves ofnew specimens (refer to Fig. 12).In the first experiments, the stress drop and recovery tookplace continuously, and the phenomenon began at the plasticdeformation threshold. Because of this, we presume that thephenomenon is an intrinsic response of the tested rock salt underthat loading rate, and that it is related somehow to the fabric ofthe salt rock specimens. The testing device stiffness or servo-control system was excluded as sources of the response bycarrying out many tests of the same size and rate with mudstonespecimens (and the stress fluctuation phenomenon was notobserved). Also, we have not observed this behavior on previoustest programs using this equipment with various lithologies in thepast. We remain open to suggestions as to the source of thestrange response at a particular_evalue.5. DiscussionSalt rock is a special rock type with properties of low strengthand strong rheology. In rock engineering, the strength anddeformation of all rock material is affected by loading rate,temperature, moisture content, and so on, and salt rocks are noexceptions. In the 1970s, most research was focused on creep anddamage of salt rock under different stresses and temperatures forthe study of nuclear waste disposal. Most salt caverns are used forgas and hydrocarbon liquid storage, and the issue of salt caverndeformation (cavern dilation and shrinkage under various loadrates) was not addressed in as much detail as the issue of slowdeformation. Generally speaking, gas injection rates (for storage)and withdrawal rates (for consumption) are different from theloading rates associated with radioactive waste storage, and arealso different from time to time (injection is generally slowerthan withdrawal), so there is a substantial range of_evalues ingas storage, with the higher loading rates being during gaswithdrawal.From our experimental results, hopefully to be more fullyconfirmed when more core becomes available, it can be said thatthe strength of the salt rock around the cavern used for gasstorage will not differ from the fast withdrawal period to the slowinjection period, giving greater confidence in the independence ofcavern stability for the appropriate_erange. On the other hand,with the deformation modulus increase with_e, shrinkage of thesalt cavern becomes smaller during the gas withdrawal periodwhereas the dilation of the cavern during the process of gasinjection is somewhat larger than expected. Of course, overall, thecaverns continue to show long-term closure from the generalcompressive creep behavior of the salt farther from the cavern,Supplementary #105101520253000.511.522.53Strain / %00.511.522.53Strain / %Stress / MPaSupplementary #20510152025Stress / MPaSupplementary #30510152025303501234Strain / %Stress / MPaFig.12. Stressstrain curve of halite with loading strain rate of 2.0?10?5/s(supplementaryspecimens#1#3)(remarks:thesupplementarysaltrocksamples were from Hongze Salt deposit in Jiangsu about 2000 m underground, itis about 1000 m deeper than the Jintan salt deposit).W.G. Liang et al. / International Journal of Rock Mechanics & Mining Sciences 48 (2011) 161167166but near the walls, issues of strength and stiffness remainrelevant. The total cavern response is not simply from generalcreep, there are clearly issues of volume changes and deformationin the damaged zones where, because of more extensive crystalboundary opening, the effective confining stress can be muchsmaller, courtesy of the resulting higher permeability, leading topressure equalization in the damagedzone. Thus, uniaxialcompression conditions are approached in the skin of the cavernand remain an issue in understanding and predicting cavernbehavior.6. ConclusionsVariation of loading strain rate happens systematically in saltcavern gas storage. The mechanical properties of salt rock underdifferent strain rates are significant issues for assessing safe andstable salt cavern operation. A series of experiments of strain rateeffects on the mechanical response of salt rock was conducted inthe laboratory, and the main conclusions are as follows:The strength of salt rock is only slightly affected by loadingstrain rate. The elastic modulus slightly increases with strain rate,but the increment is small. Under the same strain rate, thestrength of thenardite is somewhat larger than rock salt, mainlyrelated to crystal grain size and fabric of the minerals.With strain rate increase, Poissons ratio of salt rock decreasessomewhat and the lateral deformation capacity is diminished.The strain rate also affects the strain of salt rock before it yields(peak strength). The strain at the point of peak strength decreaseswith strain rate increase. A logarithmic relationship betweendeformation modulus and loading strain rate was observed in ourcase: E0 0:2ln_e3:2.The failure style of salt rock does not change with strain ratevariation, and this is different from brittle rock. The failure ofhalite is mainly in the style of brittle fracture with a bit of shearfailure whereas that of thenardite is in the style of shearing alonga planar surface. The strong viscous deformation ability of saltrock makes it able to absorb energy during uniaxial compression.The resulting internal lateral compression effect in the specimenis diminished, so the strength and failure style of salt rock almostdo not change with loading strain rate.The stress to trigger fast development of microcracking islower under low strain rates than at high strain rates. The criticalstress of volume dilation is about 6369% of the peak stress under_evalues of 10?4/s, increasing to 9496% as_eincreases to 10?3/s.This demonstrates that a deformation hysteresis occurs in saltrock under high loading and strain rates.A stress fluctuation phenomenon happened during uniaxialcompression tests for rock salt under a low_evalue of 2.0?10?5/s.We believe this is an actual response and not an experimentalartifact, but still need to more deeply investigate this from theviewpoint of micro-structure deformation of crystalloids.In an actual salt storage cavern, injection and withdrawal ratesare different. Within the range of our experimental results, it canbe said that the strength of salt rock around the storage cavernwill be little affected. The whole cavern volume response is thusless affected during its operation, and a more stable caverncondition can be expected from the point of view of salt rockstrength at low confining stress and at the strain rates that mightbe applied through relatively rapid loading during gas withdrawalor injection.AcknowledgementsThe work was financed by the National Natural ScienceFoundation of China (No.50874078) and Program for New CenturyExcellent Talents in University of China (NCET-07-0594), whichare greatly appreciated.References1 Cho S, Ogata Y, Kaneko K. Strain-rate dependency of the dynamic tensilestrength of rock. Int J Rock Mech Min Sci 2003;40:76377.2 Mahmutoglu Y. The effects of strain rate and saturation on a micro-crackedmarble. Eng Geol 2006;82:13744.3 Qi C, Wang M, Qian Q. Strain rate effects on the strength and fragmentationsize of rocks. Int J Impact Eng 2009;36(12):135564.4 Li Z, Wang Q. Experimental research on effect of loading rate for dynamicfracture toughness of rock. Chin J Geotech Eng 2006;28:211620.5 Yang S, Zeng S, Wang H. Experimental analysis on mechanical effects ofloading rates on limestone. Chin J Geotech Eng 2005;27:7868.6 Liao H, Pu W, Yin J. Study on strain rate effect of soft rock. Chin J Rock MechEng 2005;24: