A mix of portable plant equipment helps a Pennsylvania producer overcome rock fragmentation challenges and bring a 110-foot hill to ground level.

by Rodney E. Garrett

Most stone quarries are created by starting at ground level and excavating down into the earth. Nonetheless, there are a small number of quarries that are started high above ground level on a mountain or hill. On these sites, mining advances toward the base until it is level or below the level of the surrounding grade

Such is the case at a quarry in Avondale, Pa., that is 30 miles west of Philadelphia and 12 miles northwest of Wilmington, Del. Avondale is a well-established residential community with 1,100 residents. Near the center of the town, there is a hill about 110 feet high and 20 to 25 acres wide at its base. While there were some minor excavating activities carried out in the 1980s and early 1990s, it was of little significance. One contractor stripped some overburden, and another quarried some rock.

In 2006, TechniVate Co. was contracted by the property owner, Limestone Properties, LLC, to remove a complete section of the hill to make it level with Avondale’s street grade for the purpose of building a residential area and a commercial buildings park on site. As a diversified site improvement company, TechniVate was up to the task. It employs 120 people and has a fleet of 100 pieces of major equipment. Services offered include preparing turnkey building sites and specific project activities that range from erosion control and underground utilities installation to road paving.

The Avondale project was started in April 2007, and by the beginning of November, 632,000 tons of rock and sand had been excavated and processed through two onsite portable crushing and screening systems. There is at least 2.7 million tons of rock yet to be quarried in order for the hill to be brought to the specified grade.

Challenges and opportunities

TechniVate subcontracted the rock drilling and blasting to a local contractor. Three Sandvik down-the-hole drill rigs are being used for blasthole drilling. The blasthole pattern is 8 feet by 10 feet by 30 feet, which is within the Pennsylvania Department of Environmental Protection (DEP) specified drilling-for-blasting parameters. TechniVate considers this blast design to be acceptable; however, the rock fragmentation has resulted in oversize rock jamming the jaw crushers.

Most of the rock excavated at the top fourth of the hill is mica and mica schist. Both rocks are relatively soft and are suitable for structural fill such as certain sections of berm construction as found on sanitary landfill projects. For construction purposes, the rock is classified as structural fill and is marketed as such to contractors, mostly for private construction applications, and to the Cherry Island Landfill facility in New Castle County, Del. Owned and operated by the Delaware Solid Waste Authority, the landfill is currently increasing capacity through added vertical waste containment. To that end, a berm is being built around the perimeter that is one mile in circumference and 60 feet high. It will require approximately 3 million cubic yards of fill. TechniVate anticipates supplying most of the project’s aggregates needs for the berm.

Fortunately, as the mining descends, better quality, more usable rock including sandstone, quartz, and schist is found. It is the sandstone (sedimentary) and the quartz (metamorphic) stone that has the best physical characteristics for a variety of construction applications. None of the rock types is overly abrasive. In fact, a recent Los Angeles abrasion test shows an average of 45 percent for the best rock. This degree of abrasiveness is within acceptable limits as outlined in Pennsylvania Department of Transportation (PennDOT) specifications. Both the sandstone and quartz are classified as granular fill so it can be used effectively for many different construction applications such as road base, where the structural fill cannot be used.

It is projected that the quarrying phase of this project should be completed within three years. That timetable is based, in part, on the projected quantities of both structural and granular crushed/screened fill that is to be shipped to the Cherry Island Landfill.

Versatile crushing and screening

Throughout the Avondale project, six portable crushing and screening plants — including three different brands — are being used. Having supplied four of the six plants, Extec Screens and Crushers Limited (which is now fully owned by the Sandvik Mining and Construction Group of Sandvik AB) is the primary equipment vendor.