An Alternative Source for Construction Aggregates

By: Warren R. Coalson and Danny B. Sims, PhD

Underground Mining Equipment. BY Honza Groh  (   Jagro   )

Underground Mining Equipment. BY Honza Groh (Jagro)

Construction aggregates are among the most basic of economic drivers.  Aggregates are the second most commonly consumed commodity by modern society; only water is more important to our way of life.  These commodities are responsible for building and maintaining our modern infrastructure including roads, building foundations, hospitals, bridges, pipelines, office buildings, schools and the like.  Without construction aggregates, our communities would quickly become decadent eye sores and all commerce would cease.  

California’s Shortage

California is experiencing a shortage of construction aggregates in most of the urban areas of the state.  This condition has been extensively documented by the California Geological Survey in their publication, “Aggregate Availability in California” (CGS Map No. 52).  This report identifies the anticipated demand for aggregate resources within localized market areas and then tracks the quantity of permitted resources that are available to meet this demand, over a 50-year horizon.  With limited exception, all of the urbanized areas in the state are lacking in permitted resources.  

The reasons for the shortfall are multiple.  Many of the existing aggregate production sites are operating on older permits and have been active for extended time periods.  As a result, much of the resource base for these sites is nearing depletion.  Although California has a rich resource base that is suitable for construction aggregate uses, it is located at extended distances to areas of consumption (urban populations), resulting in high shipping costs. Where it is located in close proximity to market areas, permitting of new production sites is usually fraught with significant opposition.  Aggregate companies loathe entering the permitting process for a number of reasons:

  • High cost,
  • Lengthy permit review process,
  • Damage to company image, and
  • Risk of application denial by local land use agencies.

Taken together, very few new projects are being proposed.  All proposals for aggregate development are controversial.  Public concerns include (to name a few) reduced air quality, noise, traffic congestion, habitat destruction, loss of property value, and visual impacts.  Whether these concerns are valid or not, the public expresses strong emotions when a project is proposed anywhere near their communities.

This has a domino effect on existing sites.  As resources are depleted at existing production sites, the supply of aggregate shifts to other production facilities.  This results in an increase in annual production from remaining sites, increases in truck traffic in local areas, and shorter life spans for these operations.  At some point other solutions must be affected.

As is occurring in many areas of the state, the solution involves importing aggregates from market areas with relative abundance.  Examples include San Diego County (sand), Los Angeles metro area (rock), Ventura County (rock), Orange County (everything), and all areas of the San Francisco Bay area and Sacramento (rock and sand).  However, while the short term solution is satisfied using imports of domestically produced aggregates, most of the production areas are also nearing depletion.  

This condition has opened a market niche for off-shore aggregate deliveries.  Aggregates are currently being mined in British Columbia and shipped into the San Francisco Bay area and the Port of San Pedro.  In addition, sand is transported by truck from Mexico to San Diego where it is distributed to end users from a secure yard near the border.  

Because aggregates are a high bulk low valued commodity, the cost of transportation is often a determining factor in the price end users pay for these products.  As the supply for construction aggregates diminishes and permitting of new sites continues to lag behind demand, basic economic principles dictate that the price of aggregates will increase as the supplies wane.  Unless we can permit additional resources, we can expect prices to increase dramatically over time.  In addition, high paying industry occupations are exported off shore.  As a result, our local economies suffer a double negative; we pay more for aggregate commodities and high paying jobs and support industries are lost to other areas.  On top of this, increased transportation requirements result in substantial increases in green house gas emissions and limit the capability of local governments to meet their legislatively mandated emission reduction goals.


What is the solution?  That's simple (probably not); permit more resource in close proximity to local market areas.  But, aggregate companies are adverse to this process as outlined above.  Will our future supplies be provided by sources in neighboring countries?

One possible solution might include underground mining.  While our industry would need to retool to accommodate the different mining techniques, a number of advantages are present that could help to reduce community concerns: 

  • Air quality concerns are reduced.  Mining operations would take place below the ground surface, capturing dust and equipment exhaust;
  • Heavy equipment noise would be reduced.  Mining and primary processing are completed below ground, reducing noise exposure to nearby areas.
  • Biological impacts would be eliminated or minimized;
  • Visual impacts are eliminated; and
  • Property values would be unaffected as visual blight, noise and air emissions are controlled.

For these reasons, the shift in practice is already occurring in other parts of the country.  In 2014, domestic production from 91 underground mines in 17 different states totaled 74.3 million tons of crushed stone, or about 5.5 percent of total production.  As evidenced by the Society of Mining Metallurgy & Exploration Mining Engineering Handbook’s (2011) statement that “in today’s permitting environment, at any given time there are between 20 and 40 underground aggregate mines being planned,” aggregate production from underground mines is expected to increase substantially. 

All of the active underground aggregate mines use the room and pillar mining method.  The mine may be accessed by drifting directly into a hillside, driving a vertical shaft or developing a decline ramp.  A drift is the most economical method and it can be used to access reserves adjacent to an existing quarry or in an area where topography is favorable.  A shaft or decline is more expensive but these may be used to gain access to resources that are under a quarry that can no longer be mined at the surface.  

There are many site-specific variables that must be accounted for in determining mining costs, but in some cases the cost per ton for aggregate produced from a room and pillar mine may be only 50% greater than the cost per ton from a nearby quarry.  Although this may seem like a significant pricing disadvantage, when considered against transportation costs for acquiring distant aggregate sources, underground resource recovery starts to make sense.  

It is no accident that the states that lead in underground aggregate production(i.e., Kentucky, Missouri, Illinois, Pennsylvania, and Indiana) have a long history of underground mining and a work force that is familiar with the room and pillar method.  Similar expertise is available in California and it is likely that the time has come to put it to use.