HEADER HERE CONTRIBUTIONS PROFESSIONAL relatively high density of mapped karst features, and sinkholes have caused not only road closures but also the condemnation and demolition of several residences (Kochanov, 1989; Kochanov and Reese, 2003; Miller, 2014; Barr, 2016; Moyer, 2018). The prevalence of sinkhole activity has led some residents to call Palmyra “the sinkhole capital of Pennsylvania” (Kochanov, 1995). About 0.25 mile north of US 422, lies the Millard Quarry, which at more than 1,000 acres, is one of the largest active surface mining operations in Pennsylvania. Originally known as the Clear Spring Quarry when it opened in 1880, the quarry was acquired by Jacob Millard in the 1890s and operated by the Millard family until the mid-1960s. After changing hands several times, including a period of ownership by the Bethlehem Steel Corporation from 1966 to 1988, Pennsy Supply (a subsidiary of Oldcastle Materials) acquired the quarry in 2001 (Faiola, 2014; Fullmer, 2013). Underground mining began at the Millard Quarry circa 2013. The quarry extracts dolomite from the Ontelaunee Formation and high-calcium limestone from the Annville Formation (Sims, 1968). The dolomite is used for construction products (hot-mix asphalt and ready-mix concrete), and the high-calcium limestone is used for steel making, industrial minerals, and cement. The quarry has been excavated from 450 feet above to 85 feet below sea level, extends three miles along bedrock strike and about one-half mile across strike. Much of the water pumped from the quarry is used in the manufacturing process and for dust-control (Faiola, 2014). Susquehanna River Basin Commission (2021) records indicate dockets are approved for consumptive water use at the Millard Quarry by Pennsy Supply (555,000 gallons/day) and Carmeuse Lime, Inc. (100,000 gallons/day).
Previous Sinkhole Activity The project site in Palmyra has a long history of sinkhole activity with sinkhole records dating back to the 1950s. A sinkhole closed the center lane of the roadway in 1979, and sinkholes occurred along the shoulder in 1982. A sinkhole at US 422’s intersection with South Green Street was repaired with concrete and aggregate in 1992. New sinkholes occurred adjacent to the roadway in 1993, and the roadway was closed in 2009 to repair sinkholes again with concrete and aggregate. When sinkholes closed US 422 in 2014, a concrete-columnsupported bridge was constructed at grade in the affected area (Figure 3). The at-grade-bridge consisted of a 17-inch-thick structural concrete slab resting on a five-sided polygon of 40inch-wide by 38-inch-deep concrete beams supported by structural concrete caissons having a diameter of 30 inches in soil and 24 inches in rock and installed 5 feet into rock. The design called for five caissons, but during construction, a sinkhole was encountered at one of the caisson locations, so two additional caissons were installed. In 2017, sinkholes developed in the shoulder of US 422 adjacent to the area of the at-grade-bridge. PennDOT opted to repair the sinkholes with a 30-inch-thick flexible sinkhole safety net consisting of multiple layers of geogrids, geotextiles, and 20
Figure 3. Schematic section showing 2014, 2017, and 2019 repairs of active sinkhole zone affecting roadway
soil (Figure 3). In 2019, a depression was observed to be developing in the westbound lane of US 422 in the area of the sinkhole safety net and adjacent to the 2014 at-grade-bridge. PennDOT was again forced to close the roadway in June 2019.
2019 Sinkhole Repair Design After closing the roadway, PennDOT engaged Gannett Fleming, Inc. on July 2 to assist with the design of a solution. Due to the tight time frame for reopening the road, a standard subsurface investigation (e.g., test borings and lab testing of samples) was not deemed feasible. The design team had to rely on limited air track drilling data and geophysical information available from previous repair efforts. Within five days, including working through the July 4 holiday and weekend, the project team determined that the safest solution for the traveling public was a structural concrete slab supported by micropiles. The selected solution offered the advantages of easy installation, redundancy, and a low likelihood of catastrophic failure due to future sinkhole activity. The micropiles were designed to meet American Association of State Highway Transportation Officials (AASHTO) and PennDOT Load and Resistance Factor Design (LRFD) standards. Design details are shown in Figure 4. The design assumed the micropiles have a minimum 5-foot bond length in rock and are subject to compression loading only with no lateral loading and with no group effects or uplift on the micropiles. Material assumptions are listed in Table 1. Based on the design assumptions, the material assumptions, and parameters specified in the design references, the geotechnical resistance was calculated to be 363 kips, and the structural resistance was calculated as 701 kips for the cased length and 327 kips for the uncased length, so the uncased structural resistance controlled the design. The design called for 84 micropiles in 21 rows, spaced about 15 feet apart, with four micropiles spaced about 10 feet apart in each row. The micropile grid was designed to support
AEG NEWS 64(2)
Spring 2021
