Mining Foundations

Arctic Aggregate

​Finding aggregate near North Slope projects actually starts at a desktop far from the Arctic cold.

“The geologist will try to interpret events in history that would indicate high-energy transport events, which would cause a movement of coarser materials or terrain features where those coarser materials, as they are being transported, get trapped in terrain traps and get deposited,” says Torsten Mayrberger, principal and geotechnical engineer at PND Engineers, which establishes aggregate mines for road-building projects in the far north.

Often the next step in identifying these quarries is using satellite imagery and filters to define different types of vegetation, as certain vegetation is associated with areas with high-drainage soils, Mayrberger says.

“So you bring all this desktop work together and then you go out—typically in helicopters—and you start looking for clues usually found in exposures or outcrops,” Mayrberger says.

“Once a general area has been defined as having promise for gravel deposits, as well as being close to construction, we start subsurface exploration with geotechnical drilling equipment until we discover gravel.”

In some cases, the gravel layers are too thin, making them economically unviable. In other cases, there is too much overburden—usually silts and clays that have been accumulating on top of the gravel since the Ice Age.

“You don’t want too much overburden; you don’t want the gravel to be too deep [underground]. Otherwise, it’s not economically feasible,” Mayrberger says. “Typically, you’re looking for about a 70-foot deep pit, which works out pretty well up on the Slope. And so that ends up being like three benches, one overburden lift followed by two yield layers.

“Once it’s discovered, then, of course, it goes through a fairly lengthy environmental impact review,” Mayrberger continues. “Also, a part of the design is the mitigation or remediation of that mine when it’s finished.”

Often the overburden is used in the remediation process, backfilling the affected area at slopes graded to three-to-one in general, but five-to-one in areas that need to accommodate caribou access, Mayrberger says.

“Or it’s used to create little islands for bird habitat,” he says.

Rock in Southeast

​Down in Juneau, it’s not as hard to access rock resources—though before the quarries were established in the area, aggregate was brought by barge from Canada. The city now owns a couple of different rock sources on the island.

“The thing that makes Juneau perhaps unique is that we have so much high-quality rock available locally, whereas some other communities just don’t have it and they have to import it,” Chaney says. “Or, if they have rock, it’s not suitable for surfacing material, for example, so they’re always going to have to import material. In that respect, we’re really lucky here.”

Aggregate in Juneau comes from Stabler Point Quarry, which yields high-quality hard rock, suitable for a variety of construction needs, including road surfacing.

Though the City and Borough of Juneau is the owner of the quarry, it does not mine the aggregate.

“We have private companies come in and do the quarrying, and then they make some of their products available to the public through their operations,” Chaney says. “So, it’s a little bit of a hybrid between private sector and public sector.”

The blocks of rock—still in the ground—are sold to the companies, who then come in and blast them free and process them.

“We have general management oversight, and we also do all the permitting/planning for the future and that sort of thing,” Chaney says.

The Right Product

​Unlike the operation in Juneau, Anchorage Sand & Gravel is a vertically designed company.

“We’re extremely vertically integrated because we take the product from the ground and make it into a finished product,” Zins explains.

These finished products include aggregate materials (such as pit run/borrow A gravel, landscaping rocks, and sand products), ready-mix concrete, precast concrete, and a variety of masonry products.

Building aggregates, such as pit run, are in consistent demand, Zins says.

“Even pit run requires processing to meet specifications. It takes a lot of effort to provide a high quality product to our customers to ensure success on the jobsite,” Zins says, noting that they do gradations to ensure that the pit run is not mostly comprised of just large or small rocks but a good mix of the two.

Pit run is used as the foundation for most construction from roads and parking lots to buildings.

“The reason you want to do that is you want to get good compaction and a good platform so that when you put a building on it or a road or whatever, you don’t get a lot of rising or settling,” Zins says.

Though the brown, rounded stones typically found in alluvial plains and other areas targeted by gravel mining companies in Alaska provided good drainage, there are many aggregate products that require fractured edges.

“When you’re looking for compaction for D1 or any hot mix asphalt product, you want that fracture so that they kind of lock into each other as you run weights over the top of it and compact,” Zins says.

To create the fracturing, mined rock is run through one of several types of crushers depending on the desired product.

“Basically, you take a big rock, you smash it into smaller rock. And, as you’re doing that, you’re creating other products or smaller products,” Zins says. “There are certain products where you may need a fracture on at least two sides and you may need that on 98 percent of your overall product.”

This is where the company’s quality control department comes into play. The department conducts daily sampling of products as they come through the screens and onto the belts or as it’s being processed, Zins says.

“They make sure that we’re adjusting our processes so that we stay within those specifications,” Zins says. “We don’t get out of whack when our product comes out. It’s a product that needs to be within those bands for whatever application somebody is using it for.”

Costs and Concerns

However, one of the largest expenses in the sand and gravel sector is not the processing or quality control—it’s the transportation.

“One of the big issues with rock is how far you have to haul it,” Chaney says. “There’s a lot of friction in terms of sending a rock over the roads. It costs a lot for trucks and fuel and drivers and all that.”

Chaney points out that there is an almost gravitational field around a quarry pulling development closer to it because those projects that are closer to the rock source are more economically viable.

However, people rarely want to live near gravel quarries.

“Rock quarries are notoriously loud. They’re dusty. Even if dust management is well-managed, they tend to be a dusty operation. Neighborhoods don’t like them in general,” Chaney says. “Another big factor is water quality, as there is generally a lot of sediment created. If you manage the quarry correctly, there’s a lot of ways to get that sediment out of the runoff.”

Limiting sediment being released into the watershed using settling ponds at the quarries is essential due to the detrimental impacts it can have on anadromous fish streams.

“You have to be careful how you design your quarry so you’re not putting lots of sediment into salmon streams,” Chaney says, noting that constant sediment trap monitoring is important to prevent overflow.

With quarry operators following best practices, they continue to attempt to minimize their impact on the environment while providing a resource that is fundamental in today’s world.

While gold will always appear more glamorous and hold a special place in Alaska’s legacy, it’s the hard rock and gravel that will significantly impact every Alaskan’s life every day of the year for the foreseeable future.