Thursday, April 21, 2011

Landslide Summary

Brief Historical Summary:

* In 1992, Klienfelder Engineering issued a report on Building "L" of Garden South and identified problems with expansive soil - no problems with the slope were identified.

* Approximately 10 years later, in or about 2002, Rosenberg Associates reviewed Building "L" and identified possible slope instability, though a landslide area was not specifically identified.

* In 2008 the Utah Geological Society issued a report identifying the area in and around Garden South as part of an historic landslide.

* In 2008, AGEC issued a report to Garden South and to Ence Construction confirming slope instability in the area of Buildings "L" and "M".  Evidence of separation between the fire wall partitions of building 'L' and the stress on utilities forced the city to evacuate residents and condemn the building.

* In 2011, AGEC conducted a detailed assessment of the entire area and determined the area involved in and affected by the slide, which area included Knights Inn, Dairy Queen, Pier 49, Claim Jumper, and the Gardens South Condos.  

Evidence of earth movement along the south end of the old airport hill in St. George, Utah has been visible for many years.   Recent geo-technical studies have shed additional light on the most active areas of movement which affects the following business properties located near 900 S. Bluff St.:   Knights Inn, Dairy Queen, Pier 49 Pizza, Claim Jumper Steak House, and the residential community of Garden South Condominiums.     Had this knowledge base been available 15 to 20 years ago when many of the buildings were constructed within the slide area, the construction methods would have been much different.

As a result of several geotechnical studies funded by Gardens South, the evidence suggests that the affected property owners are victims of an ancient, slowly moving slide that has existed for eons.  There appears to be two separate slide planes:  one about 10 feet below the surface and other located about 35 feet below.  The lower slide plane appears to be moving at less than 1/2 inch per year while the upper plane is moving at a faster rate.  The direction of the slide is not perpendicular to the slope of the hill, but rather at approximately a 30 degree angle to the south east.   The most active area appears to be contained within a 400 foot wide zone.

Earth movement is usually caused by the accumulation of water to unstable or layered soils.  The hydrostatic pressure increases the relative weight of soils within the unstable area forcing slope to seek equilibrium, at which time the movement stops.  During  the past five years there have been two notable floods in St. George (Jan. 2006 and Dec. 2010) with water levels in the nearby Santa Clara and Virgin Rivers reaching record highs.   Following the prolonged rains during the month of December 2010, evidence of accelerated movement within the slide area has been noted.   Underground water tables, which can fluctuate, were notably higher during the early months of 2011 (within several feet of the surface).  Stress on the utilities caused a gas line to break at the Knights Inn and two water lines at Gardens South to break in May and June of 2011.  The additional water caused the slide to move at an accelerated rate forcing the evacuation of building L residents on July 1, 2011 for fear that utilities would be further compromised.

Construction practices can also aggravate and destabilize a slide area.  Cutting out the toe of the hill or terracing can sometimes cause an imbalance in the slope.   The addition of water from heavy landscaping irrigation, broken pipes or water leaks, and poor drainage practices can add to soil weight and slope instability.   However, water can also be fed into the ground from remote sources including water that percolates into the soils from the top of the airport mesa above the slide area and the water that is permitted to drain over the cliff along the airport road.  Whatever the source, there is clearly substantial hydrostatic pressure resulting in soils movement along the slide plane.

Posted on this site are some of the geo-technical studies that have continued to evolve over the years as more scientific data is made available.  In 2008, a number of holes were drilled in the slide area to measure over time the location and rate of movement within the slide plane.  The most recent readings (April and May, 2011) were very helpful and we are now finalizing the recommendations for stabilizing the slope and attempting to prevent further property damage.

The plan forward is to:

a. Gather the scientific data to understand the extent and nature of the problem
b. Determine if it can be fixed within the scope of economic reason
c. If so, design a fix and obtain a cost estimate
d. Obtain funding, preferably via a Local Improvement District in which affected property owners vote to tax themselves over time (20 years?) to repay a general obligation bond that will be used to pay for the repairs.
e. Once funding is assured, perform the slope stabilization construction and repairs to buildings.
f. Live happily ever after!

We welcome questions and comments.   If you wish to have your questions addressed privately (not posted here publicly, use the "contact" tab and I will respond by email.

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