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The City Journals

Numerous geologic hazards at gravel pit site discussed in May council meeting

Jun 08, 2021 12:16PM ● By Cassie Goff

By Cassie Goff | [email protected]

Last month, Cottonwood Heights City Journal reported a general overview of the proposed development plans for the north end of the gravel pit (6695 S. Wasatch Blvd.). Since then, the Cottonwood Heights City Council discussed the geological hazards on site in more detail. On May 4, Community and Economic Development Director Mike Johnson reported with Geostrata’s Senior Geologist Tim Thompson on their findings of the geological and seismic hazards. 

“There are numerous fault lines and slope stability issues. Hillside reclamation will be an important part of the project,” Johnson said. 

The two major geologic hazards currently identified on the site are faulting and seismically induced slope instability. The developer (Rockworth Development) has identified ways to mitigate these known hazards including construction and grading. Such mitigation will be reflected in building plans and was based on original studies and sensitive land ordinances. 

Johnson explained to the city council how city staff members have been working through the Sensitive Lands Evaluation & Development Standards (SLEDS) (Chapter 19.72 of the Cottonwood Heights Code of Ordinances) process for this proposed development. 

“We require reports for each hazard on site which are then reviewed heavily with the Development Review Committee (DRC),” Johnson said. 

The DRC includes staff members from each applicable city department. The DRC for this specific proposed development site includes members from the planning staff, fire department (UFA), and Public Works Department. The DRC also meets with Geostrata and the water and sewer district as needed. The DRC reviews reports that document and analyze sensitive land hazards. They may require additional studies and reports until hazards have been thoroughly addressed. 

As plans for the proposed development keep circulating through city leadership and being updated with more detail, the developers will have to account for more mitigation as more is learned about the hazards on site. 

Some of the potential hazards and related mitigation can be accounted for based on the current understanding of the fault lines. There are four main fault lines running through the site, including the Wasatch Fault. Geologists can make assumptions about how the fault lines interact on site by mapping the faults and studying the angles in which they maneuver through the ground. However, more data analysis and trenching will be needed for a complete understanding for this site. 

“The fault lines could be different. Sometimes, faults don’t dip in the way we anticipate them,” explained Thompson, detailing how it’s not uncommon for a fault line to be more shallow or steep than understood with surface-level data.  

Thompson emphasized the complexity of the site as he explained the history of the geology, the current understanding of the hazards on site and the developer’s incorporated mitigation, as well as additional data collection that will be needed.

All of the gravel on site was deposited by either Lake Bonneville or from out of the canyon. This created alluvial fan deposits, some of which are out in the valley now because of the previous drainage patterns. Since there are no current sources of drainage, liquification induced landslides aren’t as plausible as the geological mechanisms needed for movement aren’t the same anymore. 

“The fan is just too thick. There’s not a practical way to trench so they’ve dug as deep as they can go for carbon dating,” Thompson said. 

In order to understand the geology in more complete detail, final grading will need to be completed. After which, the foundations for each proposed building will need to be trenched and observed. 

“We just don’t know where the faults could shift over excavation of this site so it needs to be a site where setback zones and other adjustments can be made,” Thompson explained. 

If there were to be a hazard identified on site that could not be properly mitigated and made construction unfeasible, an amendment of the PDD (Planned Development District) zone ordinance and site plan would be needed and plans would return to the Cottonwood Heights Planning Commission and City Council for review.

Councilmember Tali Bruce asked if a ground motion study has been completed, as many geologists residing in the community have voiced the importance of such a study. 

“Those studies will be driven by needs,” Thompson replied. “The question is if they are going to find other hazards as they proceed with grading and final design. We need to see how they design the buildings. Those will be highly scrutinized. Geologists don’t want to be responsible for designing something that might fail.”

Based on the current understanding of the hazards and mitigation on site, Geostrata and the DRC feel comfortable moving forward with the developer on planning the site. The developer has incorporated the recommend mitigation and will be required to do so as city planners study the site more with every phase of development.