In April 2005, an existing landslide was triggered in Cedar Hills, Utah. The landslide area last moved in 1983, along with the same heavy winter and warm, wet spring conditions that triggered the Thistle slide. The landslide is part of a larger prehistoric landslide complex associated with the Manning Canyon Shale. The active landslide is approximately 375 feet long and 150 feet wide. The rock and soil types associated with this event are clay and mud-rich shale. The strength properties associated with wet versus dry shale are similar to those of wet versus dry sandstone. When shale is close to the ground surface where water content fluctuates, it erodes into clay-rich soil where added moisture reduces the strength of the rock and increases the likelihood of a landslide event. Triaxial compression tests performed on dry shales showed a mean effective stress failure greater than 15MPa. The Cedar Hills landslide was not as severe as the Cardo landslide because the hillside was not as steep, resulting in a lower normal stress. Additionally, shale tends to have a higher compressive strength than sandstone, so overburden did not have a large influence on shale. Fortunately for Cedar Hills residents, the slide has stopped for now, and will hopefully continue to stall, although geological evidence shows this is unlikely.

Precautions were taken after the 2005 slide to reduce stress on the slope, including construction of a retaining wall to reduce the effects of vertical stress (sigma 1) and gravel drains to help remove water from the slope to help the rocks maintain their cohesive strength. These measures have been ineffective, as the slide has moved three times since 2005, eventually completely destroying the houses in the adjacent image.

The Thistle, Utah, landslide (1983) caused more than $200 million in damage and property loss. Heavy snowfall, combined with a warm, wet spring caused the landslide that eventually reached speeds of up to 3.5 feet per hour. The slide was more than 200 feet thick, 1,000 feet wide, and more than a mile long. The slide finally stopped when it collided with a large sandstone cliff at the base of the mountain. The amount of material moved on the slide was enough to create a 200 foot high dam that blocked the Spanish Fork River, and created a large lake where the town of Thistle used to be. It also inhibited rail traffic and covered two major highways (US6 and US89). The lake eventually drained leaving behind the remains of the city, which can still be seen in the canyons.

Rock and soil samples involved in the Thistle landslide were collected and studied. The main rock types are sandstone and limestone, and the soil is composed almost exclusively of quartz sand grains and clay minerals (illite). Previous strength studies conducted on grained sandstone make it easy to see why the landslide was triggered. Triaxial compression tests done for dry silty sandstone show cohesive strength integrity at 18.7 MPa, meaning the rock will not fracture until these stress conditions are achieved. For a wet sample, similar to the conditions at Thistle, the cohesive strength integrity is reduced to 15.9 MPa. These data show that when silty sandstone becomes saturated, the rocks are more susceptible to breaking, which is one way landslides can be triggered. With the porosity properties of silty sandstone and the ability of clay-rich soil to absorb enormous amounts of water, it is not surprising that Spanish Fork Canyon has experienced several landslide events throughout its geological history. The Cardo Slide has been reactivated several times since its primary operation in 1983, and will continue to do so until the ideal sliding conditions created by local weather patterns change.

In April 2005, an existing landslide was triggered in Cedar Hills, Utah. The landslide area last moved in 1983, along with the same heavy winter and warm, wet spring conditions that triggered the Thistle slide. The landslide is part of a larger prehistoric landslide complex associated with the Manning Canyon Shale. The active landslide is approximately 375 feet long and 150 feet wide. The rock and soil types associated with this event are clay and mud-rich shale. The strength properties associated with wet versus dry shale are similar to those of wet versus dry sandstone. When shale is close to the ground surface where water content fluctuates, it erodes into clay-rich soil where added moisture reduces the strength of the rock and increases the likelihood of a landslide event. Triaxial compression tests performed on dry shales showed a mean effective stress failure greater than 15MPa. The Cedar Hills landslide was not as severe as the Cardo landslide because the hillside was not as steep, resulting in a lower normal stress. Additionally, shale tends to have a higher compressive strength than sandstone, so overburden did not have a large influence on shale. Fortunately for Cedar Hills residents, the slide has stopped for now, and will hopefully continue to stall, although geological evidence shows this is unlikely.

Precautions were taken after the 2005 slide to reduce stress on the slope, including construction of a retaining wall to reduce the effects of vertical stress (sigma 1) and gravel drains to help remove water from the slope to help the rocks maintain their cohesive strength. These measures have been ineffective, as the slide has moved three times since 2005, eventually completely destroying the houses in the adjacent image.