Method and apparatus for measuring in situ earthen stresses and properties using a borehole probe

An apparatus for measuring material properties and ambient stresses in ground media surrounding a borehole, including;

• a tubular cylindrical member adapted to contain high pressure fluid in the range of 10,000 psi, and to expand elastically in response to said pressure;
• a pair of end cap assemblies joined to said cylindrical member to limit expansion of the ends thereof and to contain said high pressure fluid;
• diameter measuring means for determining the expansion of the outer surface of said cylindrical member as a function of loading pressure;
• wherein the opposed ends of said cylindrical member are tapered in diameter, said end cap assemblies are received and secured about said tapered ends;
• further including a large plurality of bridging blocks arrayed annularly about said tapered ends, each block extending generally longitudinally with respect to said cylindrical member, said array of bridging blocks being disposed to contain said high pressure fluid at the junction of said end cap assemblies and the expandable cylindrical member of said apparatus;
• means for joining said bridging blocks to each of said end cap assemblies with limited rotational freedom to permit the bridging blocks to pivot outwardly in umbrella fashion as said cylindrical member expands under high inflating pressure, each end cap assembly including a steel cylinder secured about the distal portion of each of said tapered ends;
• wherein said means for joining said bridging blocks to said end cap assembly includes an outwardly flared groove in the proximal end surface of each of said steel cylinders, said bridging blocks each including a rounded end adapted to be received in said groove in smoothly rotatably fashion to permit the bridging blocks to pivot outwardly in umbrella fashion as said cylindrical member expands under high inflating pressure;
• and a pair of concentric, sleeve-like layers of high strength steel cables secured about each of said tapered ends to prevent extrusion of said cylindrical member through said array of bridging blocks under high inflating pressure.

A method for determining the material properties and ambient stress field of a portion of ground media surrounding a borehole, the ground media having a tensile strength and a existing tangential stress about the borehole, comprising the steps of:

• inserting a stress-materials probe into said borehole,
• expanding said probe under high pressure to impinge upon and deform the borehole wall,
• increasing said pressure until the tensile strength and tangential stress of the media are overcome and the media begins to fracture,
• increasing said pressure further to expand said fracture and destroy the tensile strength of the media,
• reducing said pressure to substantially zero to deflate said probe,
• increasing said pressure again to expand said probe until said tangential stress is exceeded and said fracture begins to re-expand,
• while simultaneously measuring the pressure and the diameter of the probe, and
• comparing the pressure levels with the initiation of fracture and the reinitiation of fracture to determine the ambient tangential stress, tensile strength, compressive strength, and Young's modulus.