Water-filled Cracks Research Articles

Magnetic resonance imaging of crack formation in hydrated cement paste materials

A magnetic resonance imaging (MRI) methodology is presented to spatially resolve cracking in hydrated cement paste. The method is based on the relatively long spin–lattice ( T 1) and spin–spin relaxation ( T 2) times (magnetic resonance signal lifetimes) associated with water-filled cracks. Water-filled capillary and gel pores have much shorter signal lifetimes. As a noninvasive method, the technique may be used to study crack formation and propagation in well-controlled laboratory test samples. The method is proven to be capable of spatially resolving crack structures with widths of tens of micrometers.

Characteristics of pressure pulses propagating through water-filled cracks

A series of experiments was conducted at Queen's University Belfast with a specially designed experimental apparatus in order to investigate the characteristics of, and the effect of crack geometry on, impact-induced pressure pulses propagating through water-filled cracks. The pressure pulses propagated with speeds of ranging from 50 to 300 m/s, whereby the speed increased with increasing crack width. It was found that changes in crack geometry such as right angle corners do not affect the propagation of the pressure pulse. The slow speed of propagation was attributed to the aeration of the water, which changes the water into a two-phase medium with dramatically increased compressibility. The pressure pulses also attenuated; the slowest most quickly with higher frequencies being damped out preferentially, whereby the rate of attenuation appeared to be mainly a function of time. A previously proposed theoretical model, which was based on the assumption of a two-phase medium, was improved and shown to model the experimental results satisfactorily. The experimental and numerical results suggested that, within the very short time scale of the pulse propagation, the water inside of a crack constitutes an elastic medium with viscoelastic damping.

WAVE IMPACT PRESSURE PROPAGATION INTO CRACKS.

The propagation of wave impact pressures into cracks and fissures of man-made or natural coastal structures has been held responsible for damage to coastal structures. In order to establish whether wave impact pressures can propagate into water-filled cracks, model scale experiments were conducted at Queen's University Belfast. Wave impact pressures were measured on a vertical sea wall and in a water-filled model of a crack inserted into the wall. It was found that impact pressures can propagate into water-filled cracks which are only open to the front. The pressures and propagation velocities measured in the experiment gave some indication about the generationi mechanism of wave impact pressures.

Impulse radar reflection waveforms of simulated reinforced concrete bridge decks : Chen, H.L.R.; Halabe, U.B.; Sami, Z.; Bhandarkar, V. Materials Evaluation, Vol. 52, No. 12, pp. 1382–1388 (Dec. 1994)

This paper presents the findings of an experimental study on the use of ground penetrating radar (GPR) for nondestructive evaluation of reinforced concrete bridge descks. An impluse radar PS-24 system was used for this study. Fifteen concrete bridge deck specimens were cast in three batches. The internal conditions were varied as follows : with steel reinforcement and unreinforced, with air cracks, with plain water-filled cracks, and with saline water-filled cracks.

Viability of shear-wave amplitude versus offset studies in anisotropic media

SUMMARY The study of shear-wave splitting can yield information about crack strike, crack density, and other parameters of the cracks and aligned inclusions within the in situ rockmass. Such information is important for reservoir characterization and other hydrocarbon production applications. Crack density is usually inferred from the percentage shear-wave anisotropy which is measured from time delays between the two split shear waves. Reservoirs may be too thin for discernible time delays to build up, and the time delay (and crack density) in a reservoir layer may be unresolvable in conventional arrival-time analysis of reflection surveys or vertical seismic profiles. Shear-wave amplitude versus offset (AVO) techniques are studied to see if they provide a more viable method of determining anisotropic parameters in thin reservoir layers. The behaviour of reflected shear-wave amplitudes with angle of incidence is investigated in simple two-layer models. The variations in shear-wave AVO are calculated for a range of percentage shear-wave anisotropies for water-filled cracks and dry cracks in the second layer (representing a reservoir) to see if characteristic information about reservoir properties can be extracted from the shear-wave AVO signatures. The shear-wave AVO curves, for thin cracks, are sensitive to changes in anisotropy and crack content. Most of the information about crack content is contained in wide offset reflection data which suggests applications in crosshole monitoring of enhanced oil recovery. The variations in shear-wave AVO may be distorted by the acquisition system. Most of the information about the percentage anisotropy is contained in the amplitudes of shear waves reflected at near-vertical angles of incidence. Vertical incidence reflection amplitude methods are reviewed and a simple graphical procedure is suggested for determining their viability in different reservoir environments.

Cracked media, Poisson's ratio and the structure of the upper oceanic crust

SUMMARY Theoretical models of water-filled cracks within a solid are in general agreement, and indicate that the Poisson’s ratio of the composite material will typically increase for thin cracks with aspect ratios less than A, but will decrease for thick cracks with aspect ratios between &, and i. Near-spherical pores cause little change in Poisson’s ratio. This is true both for randomly oriented, isotropic crack models, and, at certain directions, for aligned crack models which predict seismic anisotropy . Observed anomalous values of Poisson’s ratio in oceanic Layer 2 can often be explained using these models as an effect of cracks, and do not necessarily require the presence of rocks of unusual composition. In particular, the low Poisson’s ratios reported by Spudich & Orcutt (1980) and Au & Clowes (1984) can approximately, but not always exactly, be fitted with a model consisting of thick cracks within higher Poisson’s ratio rock more typical of the upper oceanic crust. However, this interpretation is near the limit of existing theories of cracked material, so other explanations for the anomalous observations must also be considered.

Gas-driven water volcanism and the resurfacing of Europa

We have analyzed the problem of how pathways are created for the resurfacing by water or volatiles for a Europa model in which there is an ocean underlying a thin ice shell (∼10-km thickness). Open cracks propagating downward cannot reach the ocean unless the relative tension in the ice is both very high (∼100 bar) and applied very rapidly (⪅10 4−10 5sec); there is no known way to satisfy these criteria. Water-filled cracks propagating upward can approach the surface for relative tensions of 5–10 bar, comparable to the stresses induced by the eccentricity tide. Exsolution of plausible gas species (CO 2, CO, CH 4, SO 2) can then occur at the crack tip. The application of linear elastic fracture mechanics shows that the gas-filled portion of the crack pinches off from the water-filled portion and may rise rapidly to the surface, entraining some water. Many of these cracks may end up as intrusions rather than extrusions because of partial freeze-out of CO 2 and water on the walls of the crack. The resulting eruption is initially gas dominated, but may include a subsequent less extended eruption of foam. There may not be a direct relationship between the resurfacing process described here and the currently known geologic features observed on the surface. The main problem with this or any model for tapping the ocean is the propagation of cracks through the soft ice immediately overlying the ocean.

The collapse of diaphragm walls retaining clay

Five centrifugal model tests are reported which illustrate aspects of the collapse of stiff cantilever retaining walls embedded in overconsolidated clay. The drainage of a heavy fluid in flight was used to simulate the effects of excavation, following the establishment of a high initial groundwater level. Two modes of collapse were observed with unpropped walls. The temporary stability of walls with small penetration was interrupted by the hydraulic action of a water-filled crack opening on the retained side of the wall. The long-term rotational failure of walls of deeper penetration was also observed, involving distributed strains in ‘active’ and ‘passive’ zones which could lead ultimately to sliding on shear rupture surfaces. An analysis was developed based on admissible stress fields, with active and passive zones switching about a pivot point, so that the unpropped wall could satisfy the conditions of both moment and force equilibrium. A back analysis of the two sudden failures using an undrained strength based on the overconsolidation ratio was successful in matching the critical penetration ratio and pivot position observed in the tests. A drained analysis using φ′ derived from triaxial and plane strain tests was equally successful in comparison with the data of long-term failure. A similar stress analysis for a wall propped at the top was shown to be conservative. This was thought to be due to the kinematic restraint of the prop which produced a rupture surface on the active side which was much steeper than those observed before. A back analysis of the observed failure mechanism generated a credible value of mobilized soil friction close to the peak observed in soil tests. This value also gave a consistent match for the bending moments and propping force measured in the test. Care must be taken to account for the possible effects of progressive failure. Critical state soil angles, with fully mobilized wall friction, can be anticipated to relate to the gross long-term deformation of walls. Loss of retained height and heave in the exacavation lead eventually to self-stabilization. L'article décrit cinq essais de centrifugeuse effectués sur des modèles qui illustrent des aspects de la rupture de murs de soutènement cantilever rigides encastéb dans de l'argile surconsolidée Le drainage d'un fluide lourd en vol fut employé pour simuler les effets de léexcavation, suivant l'etablissement d'un haut niveau initial d'eau souterraine. Deux modes de rupture furent observés avec des murs sans appui. La stabilité temporaire des murs avec peu de pénétration fut interrompue par l'action hydraulique d'une fente remplie d'eau qui s'ouvrait sur le côte amont du mur. On observa aussi la rotation à long terme des murs de pénétration plus profonde, conduisant à des déformations distribuées dans des zones ‘actives’ et ‘passives’ qui pourraient mener à la longue au glissement sur des surfaces de rupture de cisaillement. Une analyse fut développée basée sur des champs de contrainte admissibles, avec des zones actives et passives alternant autour de quelque point de pivotement, de sorte que le mur sans appui pouvait satisfaire aux conditions de l'équilibre des moments et des forces. Une analyse régressive des deux ruptures brutales, employant une résistance non-drainée basée sur le rapport de surconsolidation réussit à s'accorder au rapport critique de pénétration et à la position de pivotement observés au tours des essais. Une analyse drainée employant φ′ dérivé d'essais triaxiaux et de déformation plane réussit également en comparaison avec les données de la rupture à long terme. Une analyse de contrainte analogue pour un mur appuyé en haut se montra un peu en deça de la realité, apparemment en raison de la restriction cinétique de l'appui, qui produisit sur le côté actif une surface de rupture beaucoup plus raide que celle observée auparavant. Une analyse régressive du mécanisme de rupture observé donna une valeur fiable du frottement du sol en mouvement ne différant pas beaucoup du pic observé lors des essais de sol. Cette valeur s'accorda bien aussi aux moments de flexion et à la force d'appui mesurés au cours de l'essai. Les champs de contrainte d'équilibre donnent une solution qui est en général sûre et fiable aux problèmes de l'instabilité des murs rigides encastrés. Les effets possibles de la rupture progressive doivent être pris en considération. On peut s'attendre à ce que les angles du sol dans l'état critique aient une relation avec la déformation totale à long terme des murs lorsque le frottement centre les murs est maximum.

Evidence for aligned cracks in the Earth's crust

Three-component records of shear waves from small earthquakes recorded within the shear-wave window at the surface and shear waves recorded below the surface in three-component VSPs almost routinely display shear-wave splitting indicating some form of effective anisotropy. This splitting appears to be the result of liquid-filled cracks throughout at least the brittle, top 10-20 km of the crust, where the cracks are aligned with respect to the direction of the principal axes of stress. Liquid-filled cracks with small aspect ratios have very little effect on P-wave propagation, but may have major effects on the polarisations of shear-waves. Fluids are a very common constituent of the crustal rock mass (Fyfe, Price and Thompson 1978). Water is contained in the pore space of sedimentary rocks when they are first deposited, and every prograde metamorphic process releases chemically-bound water into microcracks of the otherwise intact rock mass. Deep wells (Kozlovsky 1984) find water-filled fractures deep in the crust, and even upper mantle xenoliths are pervaded by fluid-filled microcracks, where the fluid is largely CO2 (Andersen, O'Reilly and Griffin 1984). We shall usually refer to such cracks in the top half of the crust as 'waterfilled' but recognise that they may be oil-filled in hydrocarbon reservoirs. The major difficulty in accepting the proposition that such cracks are common is that seismic investigations, which are the most direct way we have of evaluating the properties of rocks in situ, have previously displayed little evidence for widespread cracking. The reason most seismic investigations in the past, including the detailed surveys by the hydrocarbon industry, have not found evidence for widespread cracking is because these studies have usually been confined to P-waves, and water-filled cracks have little effect on P-wave propagation. In contrast, the behaviour of shear-wave particle displacements (polarisations) is very sensitive to crack distributions whenever the cracks display preferred orientations. This paper reports recent observations of shear waves in seismic zones by my colleagues and myself at the British Geological Survey (Crampin et al. 1980; Booth et al. 1985; Crampin and Booth 1985; Crampin et al. 1985b) and in stable continental areas by the hydrocarbon industry, that display a number of phenomena which suggest that distributions of aligned water-filled cracks are widespread in (at least) the brittle, top 10 to 20 km of the crust. Monitoring the behaviour of such extensive dilatancy anisotropy (EDA) by the analysis of shearwave polarisations has been suggested as a way of monitoring the build-up of stress before earthquakes (Crampin et al. 1984).

An investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 N, Mid-Atlantic Ridge

Summary A detailed seismic refraction survey using three 3-component ocean bottom seismographs over young (1.1-3.4 Ma old) Mid-Atlantic Ridge at 45°N recorded compressional wave velocity anisotropy in the upper crust of about ±5 per cent and associated particle motion deviations of up to ±8°. The magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near-vertical water-filled cracks in layer 2. The directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions, in the range ±(30–60)° with respect to the ridge axis. We explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis. The principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis. The apparently negligible seismic influence exerted by ridge-parallel sets of (shallow?) cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge-normal compression. We find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust. Lateral variability across the 25 km square survey is limited in magnitude (typically ±0.05 s variations in delay times), and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography.

Resonant scattering by fluid-filled cavities

abstract Interaction of seismic energy with a fluid-filled cavity results both in scattering by the cavity and in excitation of its normal modes. The normal modes contain information about the properties of the fluid that fills the cavity and its size. In the frequency domain, each mode in the cavity response can be decomposed into a sum of two spectra; the “background” scattering from an empty cavity and the resonant coupling between the fluid cavity filler and the surrounding matrix. Synthetic seismograms are computed by Fourier transformation and summing modes. The theory is applied to a water-filled crack in granite, a model for the core of Mars, and a magma chamber.

Processes during the Matsushiro, Japan, Earthquake Swarm as Revealed by Leveling, Gravity, and Spring-Flow Observations

Recently revised leveling data and a more complete set of gravity data show that the uplift accompanying the later part of the Matsushiro swarm in central Honshu can be explained as due to dilatant expansion of the source region during the formation of a new fault. The local dilatancy occurred between the second and third peaks of seismic activity, although some preswarm dilatancy may have taken place in the region. It is likely that water inflow kept the medium saturated during expansion, except possibly for the times of most rapid uplift, when the drop in seismic activity indicates dilatancy hardening. A reasonable amount of horizontal expansion can explain the observed rate of change of gravity during uplift. The gravity measurements carry uncertainties of the same order as the observed changes, but the ensemble of data indicates clear trends during uplift and subsidence. The volume of land subsidence following the peak uplift agrees very closely with the best estimates of anomalous outflow of warm mineral water from springs during the last part of the swarm and supports the conclusion that subsidence was due to closing of water-filled cracks. The local rate of change of gravity during subsidence is very high, 1 mgal/m, and can only partly be accounted for by density increase due to crack closure.

Factors Influencing Release of Fenac from Polyethylene Matrices

Fenac [(2,3,6-trichlorophenyl)acetic acid] and its sodium salt were incorporated in polyethylene matrices under varied conditions to afford a series of herbicide-polyethylene formulations. Diffusion within the matrix determined the rates at which the herbicides were released from the formulations into water. Release rates of the herbicides were altered significantly if (a) the formulation curing conditions were varied, (b) the initial amount of herbicide in the matrix was varied, (c) the solubility of the herbicide was changed, or (d) if an additive was used. The results indicate that herbicide diffuses from the matrix through water-filled cracks and capillary channels.