Nonrotating, self-centering anchor assembly for anchoring a bolt in a borehole

An expandable anchor assembly for insertion in a borehole drilled in a rock formation to anchor within the borehole a threaded end portion of an elongated bolt extending into the borehole, said anchoring assembly comprising:

• (a) a hollow inner expander sleeve having a wall with an inner threaded cylindrical section, said wall decreasing in thickness from a leading end to a trailing end of said inner sleeve to provide an outer surface tapering outwardly from said leading end to said trailing end, said inner sleeve in response to insertion of the threaded end portion of the elongated bolt into said inner treaded cylindrical section of said inner sleeve being operable to radially expand over and clamp around the threaded end portion of the elongated bolt such that said inner sleeve is axially movable with the bolt relative to the borehole;
• and
• (b) a hollow outer sleeve having a cylindrical outer surface and a wall increasing in thickness from a leading end to a trailing end of said sleeve to provide an inner surface tapering inwardly from said leading end to said trailing end, said outer sleeve having gripping teeth provided on said outer cylindrical surface of said outer sleeve, a plurality of longitudinal segmented wall portions symmetrically arranged about a longitudinally extending central axis and means for yieldably retaining said plurality of longitudinal segmented wall portions such that said outer sleeve retains and captures said inner expander sleeve therewithin as said expandable anchor assembly is inserted into the borehole but permits said segmented wall portions to expand radially outwardly from said central axis, said inner expander sleeve being disposed within said outer sleeve with said inwardly tapered inner surface of said outer sleeve overlying said outwardly tapered outer surface of said inner sleeve and said inner sleeve being axially moveable relative to said outer sleeve, said outer sleeve in response to axial movement of said inner sleeve from said leading end toward said trailing end of said outer sleeve being operable to expand radially outwardly causing said gripping teeth thereon to grip a sidewall of the borehole;
• whereby once the threaded end portion of the elongated bolt is inserted into said inner threaded cylindrical section of said inner sleeve from said trailing end to said leading end thereof such that said inner sleeve has expanded over and clamped around the threaded end portion of the elongated bolt, then thereafter at least the partial withdrawal of the elongated bolt from the borehole causes the inner sleeve to axially move relative to said outer sleeve from said leading end toward said trailing end of said outer sleeve and thereby cause radial expansion of said outer sleeve and engagement of said gripping teeth of said outer sleeve with the sidewall of the borehole to thereby secure said expandable anchor assembly and therewith the threaded end portion of the elongated bolt within the borehole.