ABSTRACT:

A criminal investigation required rapid drawdown of the water in an abandoned quarry. One wall of the quarry is 230 feet high, and the face overhangs with a dip of 65 degrees. This wall presented a potentially unstable condition during rapid drawdown. Failure of this wall would bury the crime scene and prevent retrieving evidence. Our objective was to maximize the drawdown rate without failing the quarry wall. This paper describes our evaluation of the wall stability, the maximum safe rate of drawdown, the instrumentation used to monitor the wall stability, and the procedures used and strategic changes made during drawdown.

INTRODUCTION

Granite Rail Quarry is in Quincy, Massachusetts, eight miles south of Boston. The site is within the Quarry Park of the Metropolitan District Commission (MDC), consisting of many abandoned granite quarries. Granite Rail Quart3 r is approximately 1.9 acres in area and was filled with water. The normal pool level in the quart3 r is El. 163 feet National Geodetic Vertical Datum (NGVD). The deepest part of the quarry is El. -47 NGVD, which is 210 feet below the normal pool level, based on a bathymetric survey of the quarry bottom.

Near-vertical walls surround the north, south, and east sides of the quarry. The rim of the south and north sides is El. 221, or 58 feet above the normal pool level. On the west side of the quarry, the rim is at the normal pool level.

A State Police criminal investigation required the MDC to lower the quarry water level 115 feet to allow police divers to recover evidence at the bottom of the quarry. To accomplish this, at least 71 million gallons of water had to be pumped ?om the quarry. The MDC selected GEl Consultants, Inc. (GEl) to determine the maximum rate of water level drawdown that would avoid a wall collapse. A wall collapse would generate large waves in the quarry that could: 1) shift objects submerged in the quarry, 2) endanger the lives of divers in the quarry and rescue workers on the rim, and 3) bury the evidence the State Police were interested in recovering. The south wall of the adjacent Swingle's Quarry failed during uncontrolled dewatering in 1983 and permanently buried the drowning victims thought to be lost in that quarry.

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