effective stress law for the permeability of clay rich sandstones 1 two models of clay rich sandstones are analyzed to explain the relative sensitivity of permeability to pore pressure and confining pressure in one model the clay lines the entire pore wall in a layer of uniform thickness and in the second model the clay is distributed in the form of particles that are only weakly coupled to the pore walls equations of elasticity and fluid flow are solved for both models giving expressions for the effective stress coefficients in terms of clay content and the elastic moduli of the rock and clay both models predict that the permeability will be much more sensitive to changes in pore pressure than to changes in confining pressure the clay particle model gives somewhat better agreement with data from the literature and with new data on a stainton sandstone having a solid volume fraction of 8 clay

experimental study on progressive failure process and permeability characteristics of red sandstone under seepage pressure abstract rock masses in underground spaces often experience in situ stress field and seepage field coupling which gives them complex mechanical behavioural and permeability characteristics in this study a high pressure triaxial automated system for rock hptas is employed in triaxial compression testing of red sandstone under different seepage pressures and the variation laws of strength deformation axial strain stiffness and permeability during rock failure are analysed the results show that with changes in the seepage pressure there are also changes in the rock s strength and ability to resist deformation and the stress threshold decreases with an increase in the seepage pressure in addition seepage pressure does not affect the strain stiffness trend but it affects the value throughout the process of stress and strain the permeability curve firstly decreases and then increases rapidly before finally stabilizing and the peak value of the permeability curve lags behind that of the stress strain curve according to the law of permeability evolution during the pre peak progressive failure process a piecewise functional relationship model between permeability and stress is established in this study and theoretical values agree well with experimental results these experimental results can be used as a reference for monitoring and controlling rock stability during geological engineering when a seepage stress coupling action is applied

the application of high resolution x ray computed tomography on naturally deformed rock salt multi scale investigations of the structural inventory abstract x ray computed tomography ct represents a useful technique providing new perspectives and insights for the structural investigation of naturally deformed rock salt several samples of permian rock salt from gorleben asse and teutschenthal germany were investigated by exploiting the non destructive nature of ct and nct techniques particularly for salt rocks ct imaging enabled the visualization and quantification of anhydrite impurities pore space and fluid phases located along grain boundaries or trapped as intracrystalline inclusions disseminated grains and aggregates of anhydrite in rock salt of the gorleben salt dome have been visualized and quantified by ct for the first time in order to portray their spatial occurrence the visualization of anhydrite aggregates and pore space shows no correlation between their spatial distributions this independence excludes the anhydrite to be responsible for elevated porosity 0 87 0 07 vol high resolution nct scans 1 m voxel size of single intracrystalline and grain boundary fluid inclusions from gorleben and asse rock salt allowed the visualization and quantification of their various phase components a major achievement is the detailed description of the morphology and shape of the fluid inclusions and of their phase components which has not been conducted in rock salt research by high resolution x ray ct imaging before

comparison of flood hazard assessments on desert piedmonts and playas a case study in ivanpah valley nevada abstract accurate and realistic characterizations of flood hazards on desert piedmonts and playas are increasingly important given the rapid urbanization of arid regions flood behavior in arid fluvial systems differs greatly from that of the perennial rivers upon which most conventional flood hazard assessment methods are based additionally hazard assessments may vary widely between studies or even contradict other maps this study s chief objective was to compare and evaluate landscape interpretation and hazard assessment between types of maps depicting assessments of flood risk in ivanpah valley nv as a case study as a secondary goal we explain likely causes of discrepancy between data sets to ameliorate confusion for map users four maps including three different flood hazard assessments of ivanpah valley nv were compared i a regulatory map prepared by fema ii a soil survey map prepared by nrcs iii a surficial geologic map and iv a flood hazard map derived from the surficial geologic map both of which were prepared by nbmg gis comparisons revealed that only 3 4 33 9 km 2 of ivanpah valley was found to lie within a fema floodplain while the geologic flood hazard map indicated that 44 of ivanpah valley runs some risk of flooding fig 2d due to differences in mapping methodology and scale nrcs data could not be quantitatively compared and other comparisons were complicated by differences in flood hazard class criteria and terminology between maps owing to its scale and scope of attribute data the surficial geologic map provides the most useful information on flood hazards for land use planning this research has implications for future soil geomorphic mapping and flood risk mitigation on desert piedmonts and playas the ivanpah valley study area also includes the location of a planned new international airport thus this study has immediate implications for urban development and land use planning near las vegas nv

gas migration of coal in front of a mining face considering crack evolution the analysis of gradual mechanical responses of coal and gas migration within the coal in front of a mining face is crucial to coal and gas outburst prevention and control in the present paper the stress distribution of an entire stope is numerically studied by flac3d the stress state and its development within coal in front of a mining face are analyzed and a simplified physics model is developed based on the simulation results the crack expansion process of a coal specimen under a quasi plane strain condition is studied using a coal solid gas coupled meso mechanics system that is imported into comsol for simulating the gas migration of coal in front of a mining face a self established solid gas coupling constitutive model which considers the gas pressure and gas adsorption is used the gas pressure gradient and speed of gas migration reach maximum values near the free sidewall which is in agreement with the field test results

mechanical properties of abandoned and closed roadways in the kushiro coal mine japan abstract the objective of this research is to clarify the mechanical properties and self healing ability of the excavation damaged zone edz around rock caverns in clastic rock observations of nearly one hundred closed roadways up to fifty years old which can be regarded as very severe edzs with no initial sealability and are up to 300 m deep in clastic rock were made at the kushiro coal mine japan to accomplish the objective most old roadways were closed though a few remain open closure of old roadways was mainly due to roof deflection and or floor heave large plastic deformations dominated however severe fractures were seldom observed in closed old roadways rayleigh wave velocity and hydraulic conductivity in the closed old roadways were in the range of 0 3 1 2 km s and 5 10 7 1 10 7 m s respectively and those in edz and edz excavation disturbed zone around recently excavated roadways were 1 1 1 8 km s and 1 10 8 5 10 8 m s respectively the extent of edz around the present tailgate was in the range of 1 5 m mechanical excavation and prevention from water are suggested as the key points for long term maintenance of rock repositories pressurization from inside the cavern to decrease the permeability of edz is proposed for maintenance of rock repositories in medium hard clastic rock masses at similar depths for long periods

slake durability and mineralogical properties of some pyroclastic and sedimentary rocks abstract slake durability of rocks is an important property of rock mass and rock materials in geotechnical practice the slake durability of rocks is closely related to their mineralogical composition in this paper mineralogical examinations and slake durability tests for argillaceous clastic rocks especially pyroclastic rocks sandstones and mudstones of neogene tertiary age from japan were performed in order to assess the slake durability and rock alteration process of these rocks as well as to understand the relationship between mineralogy and durability the mineral composition and textural features of the rocks were studied by means of optical microscopy om x ray diffractometry xrd electron microprobe analysis epma and scanning electron microscopy sem in addition the slake durability test was carried out by using the standard testing method of isrm int j rock mech min sci 16 1979 148 in distilled water and in the aqueous solutions with dissolved electrolytes of nacl and cacl 2 the pyroclastic rocks and tuffaceous sandstone rich in di octahedral and tri octahedral fe smectite respectively show distinctively different slaking behaviors the pyroclastic rocks show relatively high slaking id 2 55 5 and id 10 10 5 than the tuffaceous sandstone id 2 94 1 and id 10 87 8 refer to text for id 2 and id 10 this difference in the slake durability observed in these rocks is due to the microscopic occurrences of smectite present in the interspaces between the particles pyroclastic rocks and zeolite cementing the interspaces tuffaceous sandstone as alteration minerals in addition the durability results of tuffaceous sandstone show that the slake durability decreases as the degree of weathering increases weathered material id 2 88 7 and id 10 65 3 furthermore two mudstones of miocene and pliocene ages having different clay mineral compositions smectite vs illite chlorite show the lowest and the highest slake durability among the tested clastic rocks hard mudstone shows the highest id 2 98 1 and id 10 95 5 while the soft mudstone shows the lowest id 2 33 9 and id 6 0 4 slake durability thus the slake durability of pyroclastic and sedimentary rocks is greatly affected by their mineral composition and texture and is closely related to their alteration history slake durability is also affected by the kind of dissolved electrolyte and its concentration in the aqueous solution providing some useful information for geotechnical practice

in situ experiments on an excavation disturbed zone induced by mechanical excavation in neogene sedimentary rock at tono mine central japan abstract the excavation disturbed zone edz is defined as the rock zone where rock properties and conditions have been changed due to the processes induced by excavation such as fracturing stress redistribution and desaturation based on our results of the in situ blasting excavation disturbance experiment at the tono mines the rock mass around the drift behaved elastically and the excavation damage induced by blasting was considered to be a primary factor in causing a change in rock properties the excavation damage was considered to depend on the excavation method so we performed an excavation disturbance experiment with mechanical excavation using a boom header in the neogene sedimentary soft rock to compare the rock property change and width of the edz induced by blasting geological observation rock mass displacement and vibration measurement were performed during the drift excavation seismic refraction and tomographic surveys borehole expansion test and hydraulic test were performed before and after the drift excavation a low seismic velocity layer of the edz with a width of ca 0 3 m and a seismic velocity of 65 70 of the intact rock was detected by a seismic refraction survey along the drift wall deformability and hydraulic conductivity were not changed in the rock mass up to 0 3 m from the drift wall the change in rock properties and width of the edz was shown to depend on the excavation method in this geological setting the mechanical excavation method is more effective in limiting the extent of the edz and change in rock mass properties

geomorphic controls on biological soil crust distribution a conceptual model from the mojave desert usa abstract biological soil crusts bscs are bio sedimentary features that play critical geomorphic and ecological roles in arid environments extensive mapping surface characterization gis overlays and statistical analyses explored relationships among bscs geomorphology and soil characteristics in a portion of the mojave desert usa these results were used to develop a conceptual model that explains the spatial distribution of bscs in this model geologic and geomorphic processes control the ratio of fine sand to rocks which constrains the development of three surface cover types and biogeomorphic feedbacks across intermontane basins 1 cyanobacteria crusts grow where abundant fine sand and negligible rocks form saltating sand sheets cyanobacteria facilitate moderate sand sheet activity that reduces growth potential of mosses and lichens 2 extensive tall moss lichen pinnacled crusts are favored on early to late holocene surfaces composed of mixed rock and fine sand moss lichen crusts induce a dust capture feedback mechanism that promotes further crust propagation and forms biologically mediated vesicular av horizons the presence of thick biogenic vesicular horizons supports the interpretation that bscs are long lived surface features 3 low to moderate density moss lichen crusts grow on early holocene and older geomorphic surfaces that display high rock cover and negligible surficial fine sand desert pavement processes and abiotic vesicular horizon formation dominate these surfaces and minimize bioturbation potential the biogeomorphic interactions that sustain these three surface cover trajectories support unique biological communities and soil conditions thereby sustaining ecological stability the proposed conceptual model helps predict bsc distribution within intermontane basins to identify biologically sensitive areas set reference conditions for ecological restoration and potentially enhance arid landscape models as scientists address impacts of climate change and anthropogenic disturbances

effects of confining pressure on the permeability of three rock types under compression abstract triaxial tests were conducted on three different rocks under confining pressures of 1 15 mpa at 295 k using an ultra compact triaxial cell the samples were held for 24 h under the target consolidation pressure and then constant strain rate compression was applied measuring permeability the structural changes were measured by thin section image analysis and micro focus x ray computed tomography for the shikotsu welded tuff the permeability decreased monotonously with axial compression mainly due to pore collapse for the kimachi sandstone the permeability first decreased with increasing axial stress then began to increase when the total lateral strain recovered its value before the application of confining pressure and then maintained an almost constant value in the post peak region the minimum and final permeability decreased with increasing confining pressure mainly due to compaction and large plastic deformation of clay cementing materials for the inada granite the tendency of permeability change during axial compression was almost the same as for the kimachi sandstone the final permeability was larger than that before compression and the permeability decreased with smaller confining pressure mainly due to a decrease in the number and width of rupture planes and the absence of axial cracks from biotite the permeability however increased under larger confining pressures mainly due to the formation of subrupture planes caused by the high stress concentration at the rough and stiff rupture plane under large confining pressure

the examination of fracturing process subjected to triaxial compression test in inada granite the behavior of a granite subject to a triaxial compression test ranging from the prefailure stage to the postfailure stage was studied using a fluorescent technique from the geological point of view microscopic observations of the specimens at different stages showed changes in the failure process the start of formation of new microcracks paralleled the compression direction through their propagation until the onset of faulting and ended with the failure of shear zones after the strength failure point pores chiefly identified in the feldspar increased in length and width in the early stages but not in number it seems that the effect of pore spaces did not have any effect of failure the microcracks generated on angular edges of quartz or feldspar grains and around biotite grains with increasing compression force the phenomenon appearing on the crystal boundaries among biotite and quartz or feldspar agrees with the result calculated based on the theory on stress fields with ellipsoidal inhomogeneity suggested by eshelby

geohydromechanical processes in the excavation damaged zone in crystalline rock rock salt and indurated and plastic clays in the context of radioactive waste disposal abstract the creation of an excavation disturbed zone edz or excavation damaged zone edz is expected around all man made openings in geologic formations macro and micro fracturing and in general a redistribution of in situ stresses and rearrangement of rock structures will occur in this zone resulting in drastic changes of permeability to flow mainly through the fractures and cracks induced by excavation the edz may have significant implications for the operation and long term performance of an underground nuclear waste repository various issues of concern need to be evaluated such as processes creating fractures in the edz the degree of permeability increase and the potential for sealing or healing with permeability reduction in the zone in recent years efforts along these lines have been made for a potential repository in four rock types crystalline rock salt indurated clay and plastic clay and these efforts have involved field laboratory and theoretical studies the present work involves a synthesis of the ideas and issues that emerged from presentations and discussions on edz in these four rock types at a cluster conference and workshop held in luxembourg in november 2003 first definitions of excavation disturbed and edzs that may be applicable to all four rock types are proposed then an approach is suggested for the synthesis and intercomparison of geohydromechanical processes in the edz for crystalline rock salt indurated clay and plastic clay comparison tables of relevant processes associated factors and modelling and testing techniques are developed a discussion of the general state of the art and outstanding issues is also presented a substantial bibliography of relevant papers on the subject is supplied at the end of the paper

reservoir zonation rock typing and compartmentalization of the tortonian serravallian sequence temsah gas field offshore nile delta egypt abstract the present study aims to evaluate the petrophysical properties and to discriminate the tortonian wakar and the serravallian sidi salem formations in the offshore temsah gas field north egypt into distinctive rock types the measured conventional core data includes helium and summation fluids porosities grain and bulk densities horizontal and vertical permeabilities as well as the water saturation also the reservoir quality index rqi flow zone indicator fzi and reservoir potentiality index rpi were calculated to establish a successful reservoir zonation in the temsah field the tortonian serravallian sequence has been discriminated into four reservoir rock types rrts which are described based on its lithologic composition as 1 sandy to argillaceous carbonates rrt1 2 siliceous pebbly sandstones rrt2 and 3 argillaceous to calcareous sandstones rrt3 and 4 calcareous to dolomitic sandstones rrt4 based on the petrophysical interpretation and ranking for the studied samples samples of rrt2 have very good rank due to presence of very good porosity excellent permeability and good to very good rqi and fzi values which are assigned as the best reservoir rock type in the tortonian serravallian sequence the petrophysical behavior of rrt4 shows the lowest reservoir quality impervious rqi and poor fzi the examined sequence has been sliced into reservoir zones each zone has its hydraulic flow unit range that represents the studied rock types in each zone based on the applied pressure tools including repeat formation tester rft and the modular formation dynamic tester mdt both the wakar and sidi salem reservoirs represent completely isolated compartments therefore ranking both reservoirs and estimating their hydrocarbon potential must be done separately for each reservoir

relief threshold for eolian sand transport on alluvial fans 1 many arid alluvial fan terraces downwind from eolian sand sources exhibit an abrupt increase in eolian epipedon thickness and sand content below a critical elevation which varies from fan to fan above this elevation sand accumulates locally and is not transported across the fan below this elevation eolian sand from nearby playa and channel sources is readily transported across the distal fan here we test the hypothesis that these distal fan eolian corridors are controlled by a threshold fan surface relief we propose that when along strike relief falls below a critical threshold value an eolian surface of transportation or corridor develops to test this hypothesis we measured multiple along strike topographic profiles and eolian epipedon textures on two piedmonts in the ivanpah valley and adjacent hidden valley in clark county nevada both piedmonts are located near sand dominated playas and exhibit clear evidence of eolian transport across their distal fan regions the near surface boundary layer flow above each topographic profile was quantified using the multispectral finite difference msfd numerical model this model predicts the surface shear stress above complex terrain for any wind condition assuming neutrally stable flow the minimum shear stress calculated by msfd for each profile during extreme wind conditions was compared to the shear velocity threshold necessary to initiate saltation model results showed that in proximal fan areas along strike relief was large enough to prevent eolian transport of all but fine sand particles in distal fan profiles within the eolian corridor model results predict shear stresses everywhere above threshold for both fine and coarse sand

debris flows from small catchments of the ma ha tuak range metropolitan phoenix arizona abstract debris flows debauch from tiny but steep mountain catchments throughout metropolitan phoenix arizona usa urban growth in the past half decade has led to home construction directly underneath hundreds of debris flow channels but debris flows are not recognized as a potential hazard at present one of the first steps in a hazard assessment is to determine occurrence rates the north flank of the ma ha tuak range just 10 km from downtown phoenix was selected to determine the feasibility of using the varnish microlaminations vml method to date every debris flow levee from 127 catchment areas only 152 of the 780 debris flow levees yielded vml ages in a first round of sampling this high failure rate is due to erosion of vml by microcolonial fungi the temporal pattern of preserved debris flow levees indicates anomalously high production of debris flows at about 8 1 ka and about 2 8 ka corresponding to northern hemisphere climatic anomalies because many prior debris flows are obliterated by newer events the minimum overall occurrence rates of 1 3 debris flows per century for the last 60 ka 2 2 flows century for the latest pleistocene and 5 flows century for the last 8 1 ka has little meaning in assessment of a contemporary hazard this is because newer debris flows have obliterated an unknown number of past deposits more meaningful to a hazards analysis is the estimate that 56 flows have occurred in the last 100 years on the north side of the range an estimate that is consistent with direct observations of three small debris flows resulting events from a january 18 22 2010 storm producing 70 mm of precipitation in the ma ha tuak range and a 500 m long debris flow in a northern metropolitan phoenix location that received over 150 mm of precipitation in this same storm these findings support the need for a more extensive hazard assessment of debris flows in metropolitan phoenix

coupled thermal hydrological mechanical analyses of the yucca mountain drift scale test comparison of field measurements to predictions of four different numerical models abstract the yucca mountain drift scale test dst is a multiyear large scale underground heating test designed to study coupled thermal hydrological mechanical chemical behavior in unsaturated fractured and welded tuff as part of the international cooperative code comparison project development of coupled models and their validation against experiments four research teams used four different numerical models to simulate and predict coupled thermal hydrological mechanical thm processes at the dst the simulated processes included heat transfer liquid and vapor water movements rock mass stress and displacement and stress induced changes in fracture permeability model predictions were evaluated by comparison to measurements of temperature water saturation displacement and air permeability the generally good agreement between simulated and measured thm data shows that adopted continuum model approaches are adequate for simulating relevant coupled thm processes at the dst moreover thermal mechanically induced rock mass deformations were reasonably well predicted using elastic models although some individual displacements appeared to be better captured using an elasto plastic model it is concluded that fracture closure opening caused by change in normal stress across fractures is the dominant mechanism for thermal stress induced changes in intrinsic fracture permeability at the dst whereas fracture shear dilation appears to be less significant this indicates that such changes in intrinsic permeability at the dst which are within one order of magnitude are likely to be mostly reversible

an integrated approach to flood hazard assessment on alluvial fans using numerical modeling field mapping and remote sensing millions of people in the western united states live near the dynamic distributary channel networks of alluvial fans where flood behavior is complex and poorly constrained here we test a new comprehensive approach to alluvial fan flood hazard assessment that uses four complementary methods two dimensional raster based hydraulic modeling satellite image change detection field based mapping of recent flood inundation and surficial geologic mapping each of these methods provides spatial detail lacking in the standard method and each provides critical information for a comprehensive assessment our numerical model simultaneously solves the continuity equation and manning s equation chow 1959 using an implicit numerical method it provides a robust numerical tool for predicting flood flows using the large high resolution digital elevation models dems necessary to resolve the numerous small channels on the typical alluvial fan inundation extents and flow depths of historic floods can be reconstructed with the numerical model and validated against field and satellite based flood maps a probabilistic flood hazard map can also be constructed by modeling multiple flood events with a range of specified discharges this map can be used in conjunction with a surficial geologic map to further refine floodplain delineation on fans to test the accuracy of the numerical model we compared model predictions of flood inundation and flow depths against field and satellite based flood maps for two recent extreme events on the southern tortolita and harquahala piedmonts in arizona model predictions match the field and satellite based maps closely probabilistic flood hazard maps based on the 10 yr 100 yr and maximum floods were also constructed for the study areas using stream gage records and paleoflood deposits the resulting maps predict spatially complex flood hazards that strongly reflect small scale topography and are consistent with surficial geology in contrast fema flood insurance rate maps firms based on the fan model predict uniformly high flood risk across the study areas without regard for small scale topography and surficial geology

deflection of sand movement on a sinuous longitudinal seif dune use of fluorescent dye as tracer abstract sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal seif sand dune in the sinai desert the wind regime was monitored simultaneously tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps the dune was subjected to a seasonally bidirectional wind regime with the wind hitting the dune obliquely on either side on the windward flank the sand was transported parallel to the wind direction on the lee flank sand movement was deflected towards parallelism with the crest line sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank the deflection depended on the angle of incidence between the wind and the crest line when the angle of incidence was

fractured rock permeability as a function of temperature and confining pressure triaxial compression tests were carried out on shikotsu welded tuff kimachi sandstone and inada granite under confining pressures of 1 15 mpa at 295 and 353 k the permeability of the tuff declined monotonically with axial compression the post compression permeability became smaller than that before axial compression the permeability of kimachi sandstone and inada granite declined at first then began to increase before the peak load and showed values that were almost constant in the residual strength state the post compression permeability of kimachi sandstone was higher than that before axial compression under low confining pressures but lower under higher confining pressures on the other hand the permeability of inada granite was higher than that before axial compression regardless of the confining pressure values for the all rock types the post compression permeability at 353 k was lower than at 295 k and the influence of the confining pressure was less at 353 k than at 295 k the above temperature effects were observed apparently for inada granite only the latter effect was apparent for shikotsu welded tuff and they were not so obvious for kimachi sandstone the mechanisms causing the variation in rock permeability and sealability of underground openings were discussed

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out