Overcoring Methods
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Transcript of Overcoring Methods
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Overcoring methods
Overcoring methods are measuring in situ stress based on the stress relief around the borehole.The relief of external forces by overcoring causes the changes in
borehole diameter for "USBM deformation gage";
strain on the borehole wall for "Triaxial strain cell", or
changes in strain on the flat end of the borehole for "Doorstopper".
If the elastic properties of the rock are known, the changes in borehole diameter or strains can beconverted to in situ stress in the rock.
USBM deformation gage - method
The USBM three-component borehole deformation gage is designed to measure diametraldeformations of an EX-size (1.5" in diameter) borehole during overcoring a concentric borehole (6"in diameter). The diametral deformations are measured in three directions (60 degree apart) in thesame diametral plane.
The field procedures consist of drilling a concentric EX-size borehole, installation of the deformationgage, and overcoring a stress relief borehole.
Click the above picture to get the full size drawing.
USBM deformation gage - instrument
The borehole deformation gage consists of six strain-gaged cantilever transducers made out ofheat-treated beryllium copper. At the free end of each cantilever, tungsten carbide button ismounted to make a contact with the borehole wall. A pair of transducers, placed 180 degree apart,measure diametral deformation of the borehole.
http://www.hydrofrac.com/deformation_gage.pdf -
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USBM Deformation gage
USBM deformation gage - theory
If the borehole is oriented in 0y direction and the xz plane is perpendicular to the borehole, the
diametral deformation d at (angle measured counterclockwise from 0x oriented with the a pair ofbuttons) can be represented as follows based on the theory of elasticity and the Kirsch solution.
The diametral deformations at =0, 60, and 120 are measured from field testing. Young's
modulus E and Poisson's ratio are measured from the extracted core. If the stress component, y,parallel to the borehole is known (usually assumed to be zero for shallow measurements), thestress components in xz plane can be calculated as follows.
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Three channels of diametral deformations logged continuously as the deformation gage isovercored. As the overcoring bit passes through the plane of measurements the stresses
are relieved and the results are shown as diametral deformations (After Fisher, 1982).
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El ensayo de overcoring CSIRO Triaxial HI-CELLes el metodo ms preciso para la medicin delEstado
de Esfuerzo Naturalde la roca.
Respecto a la Fracturacin Hidrulica, el campo de aplicacin es ms restringido, debido ad algunas
limitaciones inherentes:
la maxima profundidad de prueba es limitada a 25-30 m, necesidad que es inpulsada por el exito dela union de la celda extensimetrica a la roca por pegamento qumico;
la roca tiene ques ser poco fracturada; con esta tcnica se necesita una cierta facilidad de obtenerlargas muestras de roca, para permitir el overcoring sin daos a la celda extensimetrica
el sondaje puede tener orientacion de horizontal a vertical, pero debe apuntar hacia el alto, debidoa la necesidad de limpiar y secar perfectamente el barreno piloto donde se pega la celda
Este ensayo es particularmente adecuada a medio-pequeos tunenels y nichos subteraneos, etc.
Encuentra aplicacin tambien en estructuras de gran tamao (excavacione mineras de cielo abierto, cuevas,
etc.); en estos casos, pero, el resultado de la medicion puede ser un estrado tensional local, influenciado por
la proximidd con la excavacin; se debe por lo tanto ejecutr una back-analysis del resultado de los
ensayos CSIRO, desarrolando un modelo matematico de la excavacin para obtener el estado tensional
natural. Esta operacion es relativamente simple en caso de una excavacin que no soportada, en una roca a
comportamiento elastico. Puede volverse muy pesada en de una excavacin que soportada, y ubicada en un
macizo rocoso a comportameinto plastico.
