Detection of fracture dilatancy on the cliff top using the azimuthal apparent resistivity technique
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Abstract:
Hard rock cliffs erode through an initial catastrophic collapse along pre-existing discontinuities
in the rock mass. These may be ancient faults or fractures, orientated at a variety of angles to the
cliff face, or relatively new tension fractures formed during cycles of cliff recession, sub-parallel
to the cliff face. It is likely that an approaching cliff fall will be associated with increasing
fracture dilatancy within the fracture network. Hence if the change of dilatancy can be measured
then it may be possible to generate alerts of impending cliff collapse.
Since fractures often occur in sets with a preferred orientation they impose anisotropic physical
properties on the rock mass. Hence, the apparent resistivity of the rock will vary with azimuth
reflecting the dominant fracture orientation. Measures of anisotropy can be calculated from the
measurements and would be expected to vary with time if the fractures are dilating.
Work package one of the 5th Framework co-funded project ‘PROTECT’ (PRediction Of The
Erosion of Cliffed Terrains) was to detect fracture dilatancy. Azimuthal apparent resistivity data
were collected at five research sites in the UK, France and Denmark, all situated on outcropping
chalk. At each research site, data were collected with the Square array at three locations near the
cliff edge and at a Control site set back from the cliff edge by about 50 m. Data were collected
approximately every two months for two years to create a temporal data set. After processing the
data to remove the effect of the infinite resistance afforded by the cliff face, the data were fitted
to an ellipse in order to test for anisotropy. Measures of anisotropy were then calculated from
these data.
The anisotropy has been interpreted as fracturing and indicates a number of tectonic fracture
orientations that agree with geological mapping. At several of the research sites a cliff-parallel
fracture set was identified in a zone 10 to 20 m wide adjacent to the cliff edge. It is assumed that
this fracture set develops in response to the stress relief at the cliff face. At the Birling Gap
research site a cliff collapse within the zone of resistivity measurements produced a dramatic
drop in the magnitude of the post-collapse calculated measures of anisotropy. However, other
cliff falls that occurred outside of the immediate zone of resistivity measurements did not
generate appreciable changes in the calculated measures of anisotropy. It appears that the
tectonic fractures that limit the lateral extent of the cliff fall may also limit the fracture dilatancy
within the cliff parallel fracture set. At some sites there was a seasonal variation in the measures
of anisotropy with peaks in the summer and troughs in the winter. It appears that the most likely
driver for these variations is rock temperature that is itself controlled by the external air
temperature.
Overall, the research has been successful in establishing that there are measurable changes in the
rock mass prior to a collapse. However, the methodology is not yet advanced enough to be able
to develop technology for the reliable early warning of a cliff fall. The next stage of any research
would be to install a system for continuous monitoring in order to establish the magnitude of the
changes in the measures of anisotropy immediately prior to a cliff collapse.Keywords:
Cliff
Classification of discontinuities
Outcrop
Dilatant
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Abstract By integrating multiple datasets with relevant theory, covering fluid injection and fracturing, a conceptual model has been developed for the fracture development and induced seismicity associated with the fracking in 2011 of the Carboniferous Bowland Shale in the Preese Hall-1 well in Lancashire, NW England. Key features of this model include the steep fault that has been recognized adjoining this well, which slipped in the largest induced earthquakes, and the presence of a weak subhorizontal ‘flat’ within the depth range of the fluid injection, which was ‘opened’ by this injection. Taking account of the geometry of the fault and the orientation of the local stress field, the model predicts that the induced seismicity was concentrated approximately 700 m SSE of the Preese Hall-1 wellhead, in roughly the place where microseismic investigations have established that this activity was located. A further key observation, critical to explaining the subsequent sequence of events, is the recognition that the fluid injection during stage 2 of this fracking took place at a high net pressure, approximately 17 MPa larger than necessary. As a result, the fluid injection ‘opened’ a patch of the ‘flat’, making a hydraulic connection with the fracture network already created during stage 1. Continued fluid injection thus enlarged the latter fracture network, which ultimately extended southwards far enough to intersect the steep part of the fault and induce the largest earthquake of the sequence there. Subsequent fluid injection during fracking stages 3 and 4 added to the complexity of this interconnected fracture network, in part due to the injection during stage 4 being again under high net pressure. This model can account for many aspects of the Preese Hall record, notably how it was possible for the induced fracture network to intersect the seismogenic fault so far from the injection point: the interconnection between fractures meant that the stage 1 fracture continued to grow during stage 2, rather than two separate smaller fractures, isolated from each other, being created. Calculations indicate that, despite the high net pressure, the project only ‘went wrong’ by a narrow margin: had the net pressure been approximately 15 MPa rather than approximately 17 MPa the induced seismicity would not have occurred. The model also predicts that some of the smaller induced earthquakes had tensile or ‘hybrid’ focal mechanisms; this would have been testable had any seismographs been deployed locally to monitor the activity. The analysis emphasizes the undesirability of injecting fracking fluid under high net pressure in this region, where flat patches of fault and/or subhorizontal structural discontinuities are present. Recommendations follow for future ‘best practice’ or regulatory guidelines. Supplementary material: Background information on the stratigraphy, structural geology, rock-mechanical properties of the study region and its state of stress, as well as theory for fluid injection, hydraulic fracturing and Coulomb failure analysis, is available at https://doi.org/10.6084/m9.figshare.c.3781121
Geomechanics
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In fractured bedrock of steep alpine permafrost rock faces, cleft dynamics, creep and stability of rock masses is influenced by the thermal conditions within and around discontinuities. An increasing number of documented rock falls from periglacial environments in the last decade raise the question, how and where this thermal influence becomes a controlling factor of rock fall activity. The mechanics of permafrost bedrock containing ice-filled clefts has rarely been investigated and only qualitative understanding of the processes interlinking temperature and stability in these situations exist. Here we present temperature, dilatation and translation measurements with high temporal resolution from six clefts at MatterhornHornligrat (3500m a.s.l.; Swiss Alps). Cleft opening / closing is recorded during cooling / warming at subzero temperatures in the upper cleft. This reversible dilatation is commonly explained by segregation ice formation within the cleft (cryogenic opening). Once temperatures reach the melting point (indicated by a zero curtain in cleft temperature records) an accelerated opening or shearing (depending on the geometrical setting) of the cleft takes place. We attribute this second slow mass movement to a sliding at a basal fracture plane of the rock mass that is introduced by melting conditions. We suspect this sliding mechanism to potentially culminate in the triggering of rock fall or rock avalanches. The response time of the cleft movements to temperature changes is on the order of some hours for both cases (cryogenic opening and sliding). This is surprisingly short for the dimensions of the surveyed rock masses and can not be explained by conductive heat transport between the rock surface and the ice-rock interface within the cleft. Dilatation-temperature plots with the lower cleft temperatures show even a time lag of the cleft temperature in comparison to the movement. This finding is little understood at present but indicates the possible importance of rock hydrology (pore water pressure/expulsion and advective heat transport) as a coupling mechanism between the meteorological conditions and the mechanics of the fractures.
Bedrock
Rockfall
Classification of discontinuities
Shearing (physics)
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Abstract To investigate the zonal disintegration form of the surrounding rock in deep tunnels, model tests were performed in the simulation set-up of fracture mechanism and support technology of surrounding rock in deep tunnel. The test results illustrate that the first fracture of the surrounding rock occurred at the intersection of the tunnel floor and the side wall. After more serious destruction, the side wall and the vault were destroyed. Although the fracture width of each surrounding rock mass was distinct, they were relatively uniform with a nearly continuous fracture form. The width of the split bodies of the model tunnels (i.e., the annular zonal disintegration area) developed with an increasing load. It was observed from the fitting curves of the data that all radial strain values of the surrounding rock were more symmetric with a smooth fitting curve, and the maximum value occurred near the tunnel wall before reducing instantly. The circumferential strain values were dispersed and the data were inconsistent with the fitting curve, which caused some data to be unreliable. The phenomenon of zonal disintegration was primarily caused by radial tension strain of the surrounding rock. This phenomenon would not extend indefinitely as the rupture range would be limited to a certain extent, because the maximum radial tension strain of the surrounding rock was less than the limiting value.
Tension (geology)
Plane stress
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Abstract : Modeling the failure surface of a landslide as a three-dimensional shear fracture with an elliptical perimeter provides insight into incipient sliding process. The elastic boundary element model developed accounts for failure surface geometry, topographic slope, and gravity. As a region of sliding at depth grows, increasingly large stress concentrations develop near its perimeter. Once the slip surface length becomes several times greater than its depth, tensile stress concentrations near its head help drive the slip surface up towards the topographic surface to form a steep scarp; from there the failure surface unzips down along the flanks. The model predicts opening of an arcuate pattern of echelon fractures at the surface at the slide head, normal faulting near the head, and thrust faulting at the toe, as is observed. Three other predictions stand out. First, notches are preferred sites for slide nucleation and should be avoided on slide-prone slopes. Second, arcuate cracks on a slope indicates that it is on the verge of failure. Third, crack patterns on the seafloor can be used to estimate the area, thickness and volume of potential slide masses, parameters important in assessing tsunami hazards due to submarine landslides.
Submarine landslide
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In the Minami Nagaoka gas field, north-northeast to south-southwest trending break-out dominate. The trend of maximum horizontal stress is vertically crossing the break-out that means west-northwest to east-southeast. The minimum horizontal stress value estimated from hydraulic fracturing method is slightly smaller than overburden pressure. The maximum horizontal stress is 1.58 times larger than the minimum horizontal stress. The Green Tuff in this field is expected to be in a strike-slip faulting regime, and the estimated differential stress at the depth of 4, 300m is about 490ksc. The Green Tuff exposed to tectonic stress largely exceeding the horizontal stress of a standard overburden pressure state. Therefore the dominant trend of opening fractures is assessed to be controlled by subsurface stress and to be parallel to the maximum horizontal stress.The results of triaxial compression test show that different lithofacies exhibit different stress-strain profile. This implies that the Green Tuff has the inner stress distribution controlled by lithological variation. Therefore, it is highly possible that the difference in deformation behavior, such as brittleness or ductility of the lithofacies in reservoir creates the environment in which remnant stress and local opening fractures are maintained in the Green Tuff reservoir.The combination of the regional tectonic stress and local stress distribution caused by lithofacies's variation is likely to control the distribution of opening fractures and therefore quality of the Green Tuff reservoir.
Overburden
Overburden pressure
Stress field
Differential stress
Brittleness
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Research Article| March 01, 2002 Gravitational Failure of Sea Cliffs in Weakly Lithified Sediment MONTY A. HAMPTON MONTY A. HAMPTON 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information MONTY A. HAMPTON 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 Publisher: Association of Environmental & Engineering Geologists First Online: 02 Mar 2017 Online ISSN: 1558-9161 Print ISSN: 1078-7275 Copyright © 2002 Geological Society of America Environmental & Engineering Geoscience (2002) 8 (3): 175–191. https://doi.org/10.2113/8.3.175 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation MONTY A. HAMPTON; Gravitational Failure of Sea Cliffs in Weakly Lithified Sediment. Environmental & Engineering Geoscience 2002;; 8 (3): 175–191. doi: https://doi.org/10.2113/8.3.175 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract Gravitational failure of sea cliffs eroded into weakly lithified sediment at several sites in California involves episodic stress-release fracturing and cantilevered block falls. The principal variables that influence the gravitational stability are tensional stresses generated during the release of horizontal confining stress and weakening of the sediment with increased saturation levels. Individual failures typically comprise less than a cubic meter of sediment, but large areas of a cliff face can be affected by sustained instability over a period of several days. Typically, only the outer meter or so of sediment is removed during a failure episode. In-place sediment saturation levels vary over time and space, generally being higher during the rainy season but moderate to high year-round. Laboratory direct-shear tests show that sediment cohesion decreases abruptly with increasing saturation level; the decrease is similar for all tested sediment if the cohesion is normalized by the maximum, dry-sediment cohesion. Large failures that extend over most or all of the height of the sea cliff are uncommon, but a few large wedge-shaped failures sometimes occur, as does separation of large blocks at sea cliff–gully intersections. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Lithification
Geological survey
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The approximate physical distribution and condition of displaced and ruptured rock materials in the Rainier and Neptune areas were established by geologic observation of structural and lithologic details, and stratigraphic correlations. The effects of the detonations were found to be related to the rock types considered from the standpoint of engineering materials as well as to their structural positioning. Gross displacements and fracturing are in apparent accord with Mohr theory of rupture stress orientation. Factual data find explanation on the fundamental basis of primary (blast) and secondary (gravity) induced principal stresses. The 90 plus or minus 20-ton Neptune explosion disaggregated and displaced rock materials to a radial distance which ranged between 45 and 100 feet from ground zero depending on the orientation of bedding plane weaknesses and proximity of the ground surface to the point of detonation. Primary fracturing extended from ground zero to a radial distance which ranged from 55 feet to an estimated 145 feet depending on the favorable distribution of geologic structural weaknesses. Neptune was found to exhibit primary (blast) and secondary (gravity) rupture features which are fundamentally the same as those found in thc Rainier area. The 1700-ton Rainier shot initiallyformed a fused- rock-lined cavity of an average 62 plus or minus 10foot radius below ground zero and an undetermined radius above ground zero Gross primary rock displacement and gain disaggregation took place to a radiul distance of from 80 to 130 feet from ground zero. Visible primary fracturing extended to a radial distance which ranges between 150 and 220 feet depending on the proximity of hard brittle rock (welded tuff) which, it is concluded, transmitted rupture stresses to a much greater distance than the "punky" granular tuff. Collapse of rock material into the initial cavity was favored by primary shear fractures, which developed prominently to a radius of 150 feet from ground zero. The collapsed rock defines a 100-foot-diameter cylindrical zone. This zone is presumed to extend for 388 feet upward from ground zero. A dome-shaped top is postulated on the basis of Mohr stress theory. It should be noted that, with the exception of drill hole G, there has been no drilling or underground working above the Raise'' Drift. Cavities, distributed around the perimeter of the cylinder, are attributed to variations in the coherence of the rock strata which were disrupted during collapse. The collapse block has remained essentially intact. This fact, coupled with the virtual absence of radioactive fission products above ground zero and nearly complete pulverization of the central collapse block, leads to the conclusion that underground nuclear explosions are applicable to mining by block-caving methods. Furthermore, material within the crushed zone should be mineable without the use of additional explosives. The mineability of material within the fracture limit is a question that is difficult to answer at present, as the degree of fracturing has not been established. Perhaps the direct approach of experimental excavation would be the best solution to the problem. The conclusions presented should be considered as hypotheses awaiting verification. There is still much to be learned from the Rainier explosion. (auth)
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Anticline
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Discontinuity (linguistics)
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