SANTA CLARA - A portion of the Santa Clara Heights near Truman Drive has grappled with the danger of sliding land for over 30 years and despite numerous efforts by the City of Santa Clara to stop it, there is still no solution in sight.
Geologic history and cause
The Santa Clara Heights is built upon the Santa Clara Bench, a natural plateau bordered by the Santa Clara River Valley to the southwest and a small tributary stream to the northeast. Like most of Southern Utah, the soil of the Santa Clara Bench is mainly composed of petrified forest member, commonly called “blue clay,” along with sediment from the Santa Clara River. Petrified forest member is a weak rock that swells quickly and moves easily when wet.
Natural groundwater, along with runoff from developments, has saturated and softened the soil of the Santa Clara Bench over the last three decades, triggering movement. A pair of major landslides in 1992 and 2002 destroyed a portion of Truman Drive, forced the evacuation of three houses and is immediately threatening two more. Numerous efforts have tried, and failed, to slow down or stop the slide.
The city owns all of the property on which the Santa Clara Heights is built and will bear the cost of any construction.
“The city council has asked the Santa Clara Hillside Committee to come up with suggestions on how to slow down or stop the slide,” Santa Clara Public Services Director Jack Taylor said. “Every one of their ideas so far has been very expensive and we don’t know how to come up with the funds.”
- 1975 – Development in the Santa Clara Heights began near the east end of the plateau, extending to Truman Drive and beyond by the early 1980s.
- 1981 – The first geologic instability in the Heights, a minor rotational failure in the clays near Truman Drive, was reported by the Utah Geological Survey Geologic Hazards Division.
- 1992 – The first major landslide in the Heights destroyed a portion of Truman Drive and encroached on the backyards of houses on nearby Cinnamon Circle and Chapel Street.
- 1992 – The city re-graded the surface and constructed a rock berm at the base of the slide in an effort to restore the hill to its original configuration.
- 1993 – Large cement shafts were installed above the slide area, behind the affected homes in Cinnamon Circle, in an effort to isolate the slope. The shafts extended between 20 and 40 feet into the shale bedrock under the hill.
- 1994 – Additional landslides occurred west of the affected area, destroying a portion of the Santa Clara Field and Canal Company canal at the base of the hill.
- 1996 – The city installed a drain on Desert Dawn Drive in an effort to collect and redirect the hill’s groundwater.
- 2002 – Another major landslide forced residents to vacate two homes on the top of the hill and one at the base, which the city later purchased. Most of the cement shafts were also displaced and the drain pipe damaged.
- 2002 – A concrete wall was installed on Truman Drive to intercept water runoff from roads further uphill and redirect it away from the affected area.
- 2005 – Additional movement of the western slide expanded the affected area further towards Truman Drive.
- 2006 – Drains were installed in the western slide to collect and redirect groundwater.
- 2012 – Despite all past efforts, the affected area has continued expanding and the land sliding.
With no effective means currently in place to slow down or stop the slide, two houses are in immediate danger. One is abandoned and owned by the city but the other, located at 1656 Cinnamon Circle, is occupied. A portion of its backyard has already been lost to the slide and another part is collapsing; structural damage appears to be an imminent possibility.
“We are concerned for the property owners,” Santa Clara Mayor Rick Rosenberg said.
St. George News spoke with the homeowner, Judy Larsen, but she declined to comment.
In February 2013, the Santa Clara Hillside Committee met with Bill Lund, a senior scientist with the Utah Geological Survey, to inspect the affected area and discuss possible solutions. Lund conducted the original inspection of the area in 1981 and has been monitoring the slide’s progress since. After his recent visit, he sent a letter to the city detailing his recommended solutions.
“The best way to stabilize the slope is to dry it out. If it’s not dried out, it will never stop weakening and sliding,” Lund said. “The question isn’t whether it can be stopped, it’s whether the city can afford to stop it. We have to find a solution that’s within practical cost parameters.”
A community meeting was held at the Santa Clara City Hall in early March to update residents on the current status of the slide. No action based on the recommendations of Lund or the committee has been taken yet.
The issue of unstable ground is not isolated to the Truman Drive area; all of the Santa Clara Heights, especially homes situated near the edge of the bench, could potentially be threatened by landslides in the coming years. Natural groundwater and petrified forest member exist throughout the bench and as long as people live atop it, the problem will be present.
“It’s spreading along all the hills in the Heights,” Taylor said. “We don’t know what the slide will do in the future.”
In an area where petrified forest member is widespread and landslides are often a concern, it is very difficult to predict, even with advanced geologic science, when a hazard will occur. “When subdivisions are planned, they have a geotechnical study done that looks for potential issues such as landslides. If any are discovered, mitigating actions must be taken to ensure the safety of the development,” Lund said. “But obviously, no one saw this coming.”
Copyright St. George News, StGeorgeUtah.com Inc., 2013, all rights reserved.