POSSIBLE MECHANISMS FOR ROCK FALL EVENTS ALONG PA STATE ROUTE 11/15, SOUTH OF LIVERPOOL, PA

Massive rock falls occur along the southbound lanes of PA State Route 11/15, near Liverpool, PA, where lithologically heterogeneous mudstone, shale, sandstone and conglomerates from the Pocono Formation (Mississippian) are intruded by diabase (Jurassic). Some of the rock falls have temporarily closed the 2 traffic lanes, and a recent rock fall has resulted in the current closure of 1 lane.

The diabase intrusion cross cuts a succession of sandstone, shaley-sandstone, mudstone, and coal that are dipping 70˚N; this succession occurs both above and below the intrusion. The thickness of the intrusion varies from ~6 m at the top of the exposure to 13m its base. The intrusion exhibits columnar jointing, and the surrounding bedrock contains southward dipping, E-W oriented fractures. The quartz grains in the quartz arentite contain abundant gas vacuoles that exhibit an upward flow dispersal. Vacuoles are more abundant and densely spaced in the sandstones up dip of the intrusion. This tends to be the rock involved in the rock falls.

Rockfalls at this particular location may be due to several contributing factors related to weathering, fracture sets, and vacuole concentration. Differential erosion of the shale and mudstone located both above and below the intrusion, surrounding the diabase intrusion, weather more quickly than the diabase and overlying sandstone, producing a thick sandstone overhang that is prone to falling. The sandstone is fractured, causing it to be weak along several fracture sets. Additionally, the quartz grains in the overlying sandstone contains abundant gas vacuoles related to the emplacement of the intrusion. The increased abundance and density of gas vacuoles in the sandstone, result in a weakened rock.

Rock falls at this location might be mitigated by drilling and blasting away the sandstone overlying the mudstone and coal. Terracing the diabase at this location might also decrease the amount of falling debris capable of reaching the road. Careful attention needs to be paid to the strength of the rock that tends to fall, which are composed of lithologically heterogeneous quartz and lithic wackes.