Doctoral Dissertations


"Although rock tunneling machines are being used extensively with performance and cost advantages over traditional drill and blast methods, their usefulness for hard rock excavation is far from being practical due mainly to the inability of present day mechanical cutters to penetrate these rocks economically. Among various novel techniques that are being investigated, internal heating methods seem to be the most promising. This investigation is concerned with a feasibility study of thermal rock fragmentation using heat to create in-depth thermal inclusions.

The three-dimensional problem of in situ rock fragmentation involves parallel rows of equidistant holes drilled to a constant depth. Thermal inclusions are created at the bottoms of these holes. The nature of the temperature and resulting stress field is such that the rock is first fractured along the line of a series of holes. A second and very important fracture occurs on a plane perpendicular to the hole axes passing through the thermal inclusion. This fracture is parallel to the working face and makes possible the removal of a layer equal to the depth of the thermal inclusions.

Two two-dimensional models were obtained by passing cutting planes through and perpendicular to the hole axes. These models were used to study the process parameters; hole diameter, hole spacing and hole depth. Hard rock was characterized as a linearly elastic, homogeneous, isotropic brittle material, and the problem was formulated within the framework of the linear, uncoupled theory of thermoelasticity. For the temperature analysis, average thermal properties were used, whereas thermoelastic properties for stress analysis were allowed to vary with the temperature. Temperature and stress results were obtained through finite element approximations. A finite element code was developed for the transient thermal stress studies. Fracture predictions are based on the Griffith and the McClintock-Walsh modified Griffith fracture criteria.

Hole spacing and the melt-free depth were found to be the most influencial parameters governing the fracture. Also, the optimum melt-free depth was found to be related to the hole spacing and thus, the fragmentation configuration can be optimized by a proper choice of the single parameter, the hole spacing.

The optimum location of the subsurface fracture parallel to the working face and the fracture time were found to be associated with hole depth at least equal to half the difference between the hole spacing and the hole diameter with the thermal inclusions concentrated at the very base of the holes. Although any further increase in the hole depth was found to have a negligible effect on the location and the fracture time of the parallel cracks, it will mean that the heat source will have a greater burden against which to open cracks between the holes. The optimum hole depth therefore seems to be one associated with very small melt length and a melt-free depth equal to approximately half the difference between the hole spacing and the hole diameter.

For the optimum location of the parallel fracture of Dresser basalt, the dimensionless fracture time ratio, t*f, was found to be related to the dimensionless fracture length ratio, L*, according to the equation, t*f = L*2.7.

This power relationship suggests that the optimum fragmentation configuration should involve small hole spacings.

The theoretically predicted fracture patterns and the fracture length - fracture time relationships were found to be in good agreement with those observed in field tests "--Abstract, pages ii-iv.


Lehnhoff, T. F., 1939-

Committee Member(s)

Barker, Clark R.
Clark, George Bromley, 1912-
Hansen, Peter G., 1927-2010
Penico, Anthony J., 1923-2011


Mechanical and Aerospace Engineering

Degree Name

Ph. D. in Mechanical Engineering


University of Missouri--Rolla

Publication Date



xii, 157 pages

Note about bibliography

Includes bibliographical references (pages 109-113).


© 1973 Mahendrakumar Ramkrishna Patel, All rights reserved.

Document Type

Dissertation - Open Access

File Type




Subject Headings

Fracture mechanics
Rock mechanics -- data processing
Rocks -- cleavage

Thesis Number

T 2997

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