The site for a residential apartment building overlies an abandoned iron mine in granitic gneiss in northern New Jersey. The mine stope is about 137 m (450 long) and dipping over 344m (800 feet) at 450 to 500. As the building footprint straddles the mine site needed remediation. The remediation scheme consisted of compaction grouting a minimum 10 m (30 ft.) depth of the mine stope in rock to establish a buttress for the hanging wall and allow support of the building foundation. The rock strength parameters (friction and cohesion) were established based on Hoek Geologic Strength Index (GSI). The derived strength parameters were used in the wedge analysis to simulate rock cave-in. It was concluded that a cave-in would be unlikely. Verification holes confirmed the effectiveness of grouting. Although post grouting micro gravity survey depicted a few anomalies, no anomalies were found to exist by further drilling and excavation.

INTRODUCTION

The abandoned Bull Frog Mine is located in Morris County, New Jersey, in an area where several mines were operating in the late 19th century into the 20th century. Figure 1 depicts the location of the site which borders the major interchange of I-80 in northern New Jersey. Figure 2 is extracted from USGS map (Sims and Buddington, 1958) depicts the Bull Frog Mine (No. 22) in relation to other mines in the area. The Mt. Pleasant mine (see No. 21 on Figure 2) strikes north of Bull Frog Mine and extends under Interstate 80 and has caused problems in the past with subsidence under I-80. It is believed that the Mt. Pleasant Mine ends at the shaft discovered just over the northern property line of the site in the NJDOT Right of Way at the northeast terminus of the Bull Frog Mine (See Figure 3).

The iron ore was removed by open stoping with some timbers and stulls used (Shea 1977). The iron ore was formed in foliated planes in granitic gneiss rock although in some places the ore did not occupy the entire foliated area.

After the ore was removed from such planes, small to large pieces could fall from the hanging wall that eventually may fill the narrow stope cavity and provide support (Shea 1977).

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