THE NX-BOREHOLE JACK: A LESSON IN TRIALS AND ERRORS
The NX-borehole jack is a widely used tool for estimating the deformability of rock masses insitu. It applies uni-directional pressure to the walls of a rock borehole by means of two steel platens. In theory, the analysis of test data is straightforward; the modulus estimate requires a record of applied hydraulic pressure versus borehole diameter change, and a knowledge of the rock's Poisson's ratio. In practice, the above procedure, using the original theoretical formula, frequently has resulted in a materials modulus being computed which was demonstrably too low. This paper describes attempts to explain the discrepancy. One or more of the following factors can provide a low modulus estimate: (a) the longitudinal outward bending of the jack plates which gives artificially high readings of the displacement gauges; (b) the mismatch between borehole and jack platen radii, which would negate the full platen/rock contact assumed in the original analysis; and (c) tensile cracking of the intact rock, or tensile opening of natural joints around the borehole under jack loading. The jack's theory was introduced in 1968. The initial analysis included an evaluation of the effect of tensile cracking, perpendicular to the direction of loading, on the calculated modulus. The effect of discrete joints intersecting the borehole at various angles, at the locations of the jack, was examined in 1971 and 1972, as well as the influence of a rock mass lacking tensile strength. The longitudinal bending hypothesis was proposed and quantified in 1976. The effect of radius mismatch was first mentioned in 1976 and correctly quantified in 1981 for oversize holes. This article relates the latest developments in correcting the original borehole jack analysis and in deriving the proper effect of radius mismatch for undersize holes. The paper is intended to augment the building block process for the borehole jack data analysis, as well as to be an example of the not-so-simple solution to a "simple" rock mechanics problem. (TRRL)
-
Availability:
- Find a library where document is available. Order URL: http://worldcat.org/issn/01489062
-
Corporate Authors:
Pergamon Press, Incorporated
Headington Hill Hall
Oxford OX30BW, -
Authors:
- HEUZE, F E
- Amadei, B
- Publication Date: 1985-4
Media Info
- Features: Figures; Photos; References; Tables;
- Pagination: p. 105-112
-
Serial:
- Intl J of Rock Mech & Mining Sci & Geomechanic Abs
- Volume: 22
- Issue Number: 2
- Publisher: Pergamon Press, Incorporated
- ISSN: 0148-9062
Subject/Index Terms
- TRT Terms: Boreholes; Errors; Fracture mechanics; Information processing; Jacks; Modulus; Modulus of deformation; Poisson ratio; Rock jointing; Tensile strength; Water pressure
- Old TRIS Terms: Borehole jacks; Poissons ratio; Rock joints
- Subject Areas: Geotechnology; Highways;
Filing Info
- Accession Number: 00458395
- Record Type: Publication
- Source Agency: Transport Research Laboratory
- Files: ITRD, TRIS
- Created Date: Sep 30 1986 12:00AM