Conditions Prior to 2010, Springhill Landslide
Landslide Movement
The landslide is currently active and has been continuously moving at a very slow rate since at least January 2008. Measurements indicate that different parts of the landslide are moving at slightly different rates. The landslide is moving to the northwest, toward Valley View Drive. In 2008, the landslide moved approximately 7 to 9 inches. In 2009, the landslide moved 10 to 12.5 inches.
In 2008 and 2009, the UGS monitored ground deformation at several locations on the landslide to estimate approximate movement amounts. In the head of the landslide (uppermost part), stretching (points on the ground get farther apart) occurs due to landslide movement. In the toe of the landslide (lowermost part), shortening (points on the ground get closer together) occurs.
The two plots above show that about 8.9 inches (at station SHC1) and 12.5 inches (at station SHC5) of cumulative stretching (the amount that occurs over a period of time from an initial measurement date) occurred across the main scarp zone (head) of the landslide in 2008 and 2009, respectively. The amount of cumulative stretching is likely a close estimate of the total movement of the head of the landslide in 2008. For 2009, the amount of cumulative stretching recorded is likely an underestimate of the total movement of the head of the landslide, because measurements did not begin until late February, 2009. Note that measurements across the head of the landslide in 2008 and 2009 were made at different stations. Station SHC1 was abandoned in 2009 due to difficulty in measuring across the large main scarp zone. Station SHC5 was established in early 2009, at another location across the main scarp to replace the abandoned SHC1 station.
The two plots above show that over 3.4 inches and approximately 9.7 inches of cumulative shortening (the amount that occurs over a period of time from an initial measurement date) occurred across Lot 157, situated near the toe of the landslide in 2008 and 2009, respectively. Measurements in 2008 did not begin until early April. Therefore, more than 3.4 inches of shortening likely occurred in 2008.
Change in the Rate of Movement
At station SHC1, the rate of landslide movement changed abruptly three times during 2008, increasing in late winter, slowing in late spring, and increasing in fall. The rate of movement was relatively constant in between these brief periods in which the rate of movement changed.
Between May and September 2008, the rate of movement in the head of the landslide averaged approximately 0.12 inch per week and was relatively constant. In September, the average rate of movement increased to 0.23 inch per week, increasing by a factor of about 2.
At the toe of the landslide, the average rate of movement was about 0.05 inch per week between May and December 2008, a period of about 210 days. However, during about a 50-day period between December 2008 and January 2009, the average rate of movement was about 0.17 inch per week. Thus, the average rate of movement increased by more than a factor of 3 beginning in December.
At station SHC5 near the head, the rate of landslide movement changed three times during 2009, slowing in late spring, slowing again in mid-summer, and slightly increasing in fall. The 2009 landslide movement pattern is more consistent with other Wasatch Front landslides. Generally, landslides along the Wasatch Front experience most of the damaging landslide movement in the spring and early summer months when precipitation is generally the greatest.
Between February (when measurements for station SHC5 began) and late April 2009, the rate of movement in the head of the landslide averaged 1.4 inches per week, and reached a maximum rate in late April (approximately 2 inches per week). From April to July, the rate of landslide movement decreased to an average of 0.4 inch per week. From July to September the rate of landslide movement remained relatively constant at an average rate of 0.02 inch per week. From late September to the end of the year, the average rate of landslide movement increased to 0.07 inch per week.
At the toe of the landslide, the rate of landslide movement from January to April 2009 averaged 0.4 inch per week. In late April the rate of landslide movement reached a rate of 0.84 inch per week (the maximum rate of landslide movement experienced in 2009). From April to July, the average rate of landslide movement decreased slightly from 0.4 inch per week to 0.3 inch per week. From July to the end of the year, the rate of landslide movement decreased again to an average of 0.03 inch per week.
Although the rate of landslide movement decreased throughout 2009, landslide movement did not cease. As of December 15, 2009, the head of the landslide was moving approximately 0.04-0.06 inch per week and the toe was moving approximately 0.08 inch per week.
Ground-Water Levels
UGS geologists last measured ground-water levels in December 2009. In three of the four observation wells (P-1, P-3 and P-5) ground-water levels were rising in late 2009. Of these three wells, P-1 was at the highest level recorded for 2009. The other two wells, P-3 and P-5, as well as observation well P-4, were within 0.8 foot of the previously recorded highs for each well. Observation well P-5, in the toe of the landslide, has experienced persistently high ground-water levels since early 2008. The ground-water level in this well has risen approximately 21 feet since March 2001, a significant rise for which the cause remains unknown.
Fluctuations in the seasonal peak ground-water level (SPGWL) provide a relative measure of the landslide’s stability. The plot above shows the fluctuations in the SPGWL in observation wells P-3 and P-4 (both in Springhill Drive) since 1999. The high SPGWLs since 2005 coincide with an increase in annual movement, which ranges from about 4 (in 2005) to 12.5 inches (in 2009).
