SYNOPSIS

The information presented in this paper was obtained from laboratory tests and injection of chemical grout in fractured granite at the NORAD underground Combat Operations Center near Colorado Springs, Colorado, by the-Omaha District of the U. S. Army Corps of Engineers. The purpose of the injection of chemical grout was to stabilize and strengthen areas of faulted rock where relaxation and possible transfer of stress was causing movement. Injection of the chemical grout was also necessary to provide additional strength to the rock in order to resist possible future dynamic loading. Simple equipment was devised for injection of grout holes and for injection of grout into rock bolt holes. Movement gages installed in one area indicated that movement stopped after injection of the epoxy resin; other observations also indicated the areas were beneficiated by grouting. This report describes the first phase of the work done in one corner of a chamber intersection in June 1963 and also a second phase of the work done in November 1963. The second phase of the work consisted of injecting additional grout in the first corner and in an adjoining corner of the same chamber intersection and the chamber wall adjacent to this corner.

BACKGROUND

One corner of a chamber intersection had been a problem to stabilize since the area was first excavated. There was a difference of opinion on the proper treatment to provide stability to the badly faulted rock. One plan proposed removal of loose rock and replacement with concrete, and the other plan was for rock bolting. A decision was made to install rock bolts, but during January 1963 after the rock bolts were installed and while blasting for excavation was proceeding, excessive movements were noted on the movement gages installed in the corner and blasting was suspended. Rock movement occurred along a well-defined, existing joint and shear planes dipping into the chamber intersection. As a result of the major rock movement and the stress concentrations due to the opening, many small cracks were formed in the rock in addition to the larger voids in the joint and shear planes. Laboratory tests were started in September 1962 using core samples to determine if the special epoxy resin selected would bond rock faces which had chlorite and carbonate deposits, which were typical of those in the shear plane. The laboratory tests determined that a reasonable amount of tensile strength of approximately 1,000 psi, a compressive strength ranging from 2,450 to 8,220 psi and shear strength ranging from 1,061 to 2,642 psi could be developed across the bonded sections of cores. Inspection of exploratory holes in the corner with a strata scope indicated the presence of cracks ranging in size from hairline up to 3 inches in width. When it was established by laboratory tests that bond could be developed across joints and that there were voids in the rock which would take grout, the feasibility of epoxy resin grouting to stabilize the rock was considered to be established.

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