Geological Report Finds Vulcan on Shaky Ground in Plan to Move Mining to Van Tassel Ridge
Vulcan Materials Company’s plan to move its mining operations to a mountain ridge above Duarte homes and schools could be a disaster waiting to happen, according to the findings of a geologic analysis of the mining site.
The mine is located in a landslide area and portions of the property are also located in a liquefaction hazard zone as designated on the state of California’s geologic map. Earthquake shaking or activities such as blasting can trigger liquefaction, a physical process that can cause soil to flow like liquid. Liquefaction has been responsible for major destruction in earthquakes including the 1989 Loma Prieta earthquake. The Van Tassel Ridge, where Vulcan proposes to move its mining activities and lower the ridgeline by 800 feet, is located within one quarter mile of the Sierra Madre Fault.
The geology and soils review conducted by A.G.I. Geotechnical, Inc., was commissioned by the City of Duarte and submitted to the City of Azusa in response to the Vulcan Environmental Impact Report (EIR) and Revised Conditional Use Permit currently under consideration for approval by the Azusa City Council.
A.G.I. Geotechnical principal geologist, Mark Swiatek addressed the Azusa City Council with his findings during the April 19 public hearing on the Vulcan mining expansion plan.
“One of the most significant areas of CEQA guidelines is that the EIR documents address the possibility of landslides on the offsite properties,” Swiatek told the Council.
Regional geologic maps indicate at least three large landslides exist on the natural slope areas immediately south of the proposed future mining area and within a quarter mile of residential neighborhoods located within the City of Duarte near the mouth of the Van Tassell Canyon. “The EIR doesn’t even mention or address any of these landslides,” said Swiatek.
“These landslides are located on 600 foot high slopes and they will tote down to the valley floor if they fail. You are going to be blasting within 300 feet of these landslide features shown on this map,” said Swiatek. The Vulcan EIR proposes to increase blasting activities from the 20 per year allowed under the current CUP to 100 times per year.
Both the Vulcan EIR and a City of Azusa commissioned 2005 report by Greystone Environmental Consultants, Inc., mention that an estimated one million ton landslide occurred in 1997 on the east quarry slope designated for reclamation.
“That landslide failed when the excavation was a total height of 700 feet. The proposed slope on the west side mining area is in excess of 1200 feet. My own stability analysis shows me that that landslide barely has a factor of safety that says in a large earthquake that slope will fail,” Swiatek said.
Swiatek told the Azusa Council that they should also be concerned that a proposed 12 acre foot detention basin with 4 million gallons of water is to be constructed on what is identified on the geologic map as a liquefaction zone.
“You have chosen not to address the liquefaction hazards in that zone because there’s no permanent structures. Yet the 12 acre basin will there for 30 years. What’s the likelihood that we are going to have a significant earthquake in the lifetime of that structure? Thirty years and it’s a 12 acre debris basin. What happens when that water comes washing down the canyon? Instantaneous failure,” said Swiatek.
In a follow-up exchange between Councilman Keith Hanks and Swiatek, Hanks asked, “how high of a slope would you need to worry about for loss of stability?” Swiatek answered that “the height of the slope is irrelevant, it’s the volume of water that is retained by the slope and its stability of the slope that’s the concern. And the stability of those slopes is what needs to be addressed. It’s a public safety issue.”
Hanks, an engineer by trade, noted, “the retention basin is basically a hole in the ground with a berm around it,” Swiatek was quick to respond. “The San Fernando reservoir in 1971 was basically a hole in the ground too, and it came within inches of killing 20,000 people when it almost failed there in the San Fernando earthquake.”
Vulcan has a permit from the City of Azusa to mine 190 acres of its 270 acre property. It is seeking approval to move its mining operations from the currently approved 80 acres on the east side of its property to 80 acres on the west above Duarte. While Vulcan’s EIR portrays the proposed project as an equal swap, the A.G.I. geologic analysis disputes that contention. The analysis determined that mining on the west side would yield 138 tons of rock plus overburden, while remaining reserves on the east side are calculated at 53 million tons, less than half of the 121.5 million tons stated in the DEIR for the 1988 Reclamation Plan.
The Azusa City Council postponed a decision on whether to allow Vulcan to move its mining operations to the west side of its property to provide its staff time to review and prepare answers to questions raised in the geology report and other recently received comments on the Vulcan EIR submitted by the South Coast Air Quality Management District, City of Hope, and the City of Duarte. The hearing will continue at a special meeting set for Monday, April 26 at 7:30 p.m.
In 2008, the Duarte City Council established a $700,000 fund entitled the “Fight Against Vulcan Expansion” fund (FAVE) in response to the anticipated expansion plans by Vulcan and its potential adverse impact on Duarte residents.
For more information about the City of Duarte’s Fight Against Vulcan Expansion activities, call Duarte Deputy City Manager, Karen Herrera at (626) 357-7931, ext. 221.
