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Landslides are a major geologic hazard in many locations around the globe. They are considered a mass-wasting process, the most common of which are debris flows, hill slides, and rock falls. These events can take place over the course of several years of creeping but powerful movement, or in a matter of a few devastatingly destructive moments. There are several factors relating to structural geology that correlate directly to landslide occurrences. One major factor pertaining to landslides is rock strength. Rock strength is defined by stress/strain relationships, pore fluid pressure, and confining pressure. Stronger, more dense rocks are not as likely to be involved in a rock slide or landslide than porous less dense rocks that can be easily saturated with water. In Utah the groundwater level is continually fluctuating, making the area particularly susceptible to landslides. As water saturates the ground, making it softer and heavier, the stress/strain relationships experienced by the rocks increases considerably. The strength of a rock can be defined by its Mohr Circle, and its corresponding failure envelope. Once conditions are reached which place a rock over its failure envelope, it will experience deformation.
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There are two main types of deformation which rocks undergo and both are pertinent to landslides. If a rock behaves elastically and experiences micro fractures involved with tiny slip movements in the rock, cohesive strength is still somewhat maintained, and a slide might be temporarily prevented. However, if a rock undergoes brittle deformation and breaks into pieces, a landslide is much more likely to occur. Stress and strain conditions associated with rocks and their failure envelopes differ between rock types, but they have been studied extensively in laboratories because the implications of these data have relevance in every scope of geology.
Landslide events in Utah
A recent publication done by the Utah Geologic Survey reports the occurrence of over 22,000 landslide activities throughout the state over the recent geologic past. There have been several major landslide events in Utah county over the last 25 years which have caused hundreds of millions of dollars in damages and property loss. Below is information pertaining to the Thistle, Utah landslide and the Cedar Hills, Utah landslide (active).
The Thistle Landslide
The landslide in Thistle, Utah, (1983) caused over $200 million in damages and property loss. Heavy snowfall, combined with a warm, wet spring triggered the landslide which eventually reached speeds of up to 3.5 feet per hour. The slide was over 200 feet thick, 1000 feet wide, and over one mile long. The slide eventually stopped when it collided with a large sandstone cliff at the base of the mountain. The amount of material moved in the slide was enough to create a 200 foot tall dam which blocked off the Spanish Fork river, and created a large lake where the town of Thistle used to be. It also inhibited railroad traffic and covered two major highways (US6 and US89). The lake eventually drained off leaving the remnants of the town behind, 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 performed on course-grained sandstone make it easy to see why the landslide was triggered. Triaxial compression testing done for dry silty sandstone show cohesive strength integrity to 18.7 MPa, meaning that the rock will not fracture until these stress conditions are reached. For a wet sample, similar to the conditions in Thistle, the cohesive strength integrity is reduced to 15.9 MPa. These data show that when silty sandstone is 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 tremendous amounts of water, it is not surprising that Spanish Fork canyon has experienced several landslide events throughout its geologic history. The thistle slide has reactivated several times since its main activity in 1983, and will continue to do so until the ideal landslide conditions created by the local weather patterns change.
The Cedar Hills Landslide
In April 2005, an existing landslide was triggered in Cedar Hills, Utah. The landslide area last moved in 1983, in conjunction 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. Strength properties associated with wet vs. dry shale are similar to those of wet vs. dry sandstone. When shale is near the ground surface where the water content fluctuates, it weathers into a clay-rich soil where the added moisture reduces the rock strength and increasing the likelihood of a landslide event. Triaxial compression testing done on dry shale showed a mean effective stress failure above 15MPa. The Cedar Hills landslide was not as severe as the Thistle slide because the slope of the hill was not as steep, resulting in a lower normal stress. Also, shale tends to have a higher compressive strength than sandstone, so the overburden did not have as great an influence on the shale. Luckily for the residents of Cedar Hills, the slide has stopped for now, and hopefully will continue to be stagnant, though the geologic evidence shows that this is not likely.
Precautions were taken after the 2005 slide to reduce the stress on the hillside, including the construction of a retaining wall to reduce the effects of the vertical stress (sigma 1), and gravel drains to help remove water from the hillside to help rocks maintain cohesive strength. These measures have been ineffective, as the slide has moved three times since 2005, eventually completely destroying the homes in the adjacent picture.