PBS Coals

Problem

In 2013 the state of Pennsylvania required the Cambria Site 93 refuse pile to be closed using a geosynthetic membrane. The site is located approximately 7 miles east of Somerset, Pennsylvania and one mile north of the Pennsylvania Turnpike (Route 76). Aggressive milestones were put into place by the State. The first 25% of the cap was required to be completed through the entire cross section (liner, soil, seed) by May 6, 2014; a challenge in Somerset County considering May snow is not uncommon.

The coal refuse pile to be closed encompassed an area of approximately 30 acres. The side slopes were steep with average slopes of 2.5H:1V. The pile was designed with these slopes to allow for maximum material storage capacity within the available footprint. In addition to the steep slopes, a material referred to as “Rec-Mix” was placed over the coal refuse as alkaline addition and used as the subbase layer for the geomembrane. The Rec-Mix presented additional design challenges due to its variable characteristics along with considerations of frost upheaval over the winter months. The main design challenges for this system included the steep slopes, accelerated project schedule, and challenging subgrade characteristics.

An initial attempt during 2011 to close the facility with a geosynthetic cap system was not successful. The closure profile consisted of (bottom to top): a 6 oz/sy non-woven geotextile, 30 mil LLDPE geomembrane, 6 oz/sy non-woven geotextile and 12 inches of topsoil. The initial closure plan was not an engineered solution and subsequently did not provide adequate shear strengths on the side slopes. The geomembrane and geotextile interface was likely not of sufficient strength for the application in addition to a lack of drainage for the cover soil material above the geotextile layer. The cover soil and top layer of geotextile experienced shear failure and the project was put on hold.

Solution

For the second attempt, the project was bid in a design/build scenario during the winter months of 2013. The bid process produced a team with two design options, construction, quality assurance and certification of the closure project. Two geosynthetic options were thoroughly tested and evaluated for all of the on-site challenges presented by this closure. The selected solution utilized a unique structured geomembrane product that excels in steep slope applications and provides an integrated drainage system to accommodate and remove the water that drains through the cover soil from the closure system. Construction occurred for approximately four months and the closure was completed ahead of the prescribed deadline.

In October 2013, PADEP and Corsa completed their COA negotiations which ultimately limited the cap system redesign options and included the following construction milestones, all on a finished area basis including topsoil placement: 25% completion by mid-May 2014; 50% completion by mid-June 2014; and 100% completion by mid-August 2014. In order to meet the requirements of the COA, Corsa solicited design/build/certify proposals from teams of geosynthetic installers and engineering firms to develop cap system alternatives that would deliver acceptable short- and long-term sliding (veneer) stability while providing Corsa with options to balance performance, cost, and regulatory approval. The team of American Environmental Group (AEG) and Tetra Tech (Tt), utilizing geosynthetics manufactured by Agru America (Agru), was selected to complete the project with notice-to-proceed (NTP) issued in late January 2014.

Immediately upon NTP, a laboratory testing program commenced to evaluate the candidate geosynthetics, stockpiled topsoil resources, and Rec-Mix for a variety of index and engineering design properties. Concurrent with the laboratory testing program, design analyses were prepared using presumptive engineering properties that were adjusted as testing was completed. The laboratory testing and engineering design activities were completed by late February and submitted to PADEP for expedited review and approval. The redesigned cap system included the following components, listed from the bottom up: recompacted Rec-Mix; a 50-mil high-density polyethylene (HDPE) structured geomembrane, Agru Super Gripnet; an 8 oz/sy nonwoven, needle-punched (NWNP) geotextile; and 12 inches of screened (4-inch minus) topsoil.

Outcome

PADEP approval was received in mid-March and the geosynthetic materials were immediately manufactured and shipped to the site to support a mid-April construction start date. Construction commenced on time and the 25% and 50% completion milestones were both achieved one week ahead of schedule, and the 100% completion milestone was achieved four weeks ahead of schedule.

The installed cap system met the unique and challenging design, permitting, and construction requirements presented by the site and project schedule. High interface shear strengths were provided by the geosynthetic components, thereby avoiding significant regrading of the landfill side slopes which would have added additional cost and work days to the project. The Rec-Mix material was able to be incorporated into the design, utilizing the results of site-specific laboratory testing with the proposed geosynthetic materials. The project was completed within the specified timeframe, even while facing a late snow melt and wet start to the spring season. An additional closure project at a nearby Corsa facility will be completed utilizing a similar cap system during the summer of 2015.