GEOLOGY, MOVEMENT HISTORY, AND EFFECTS OF THE HEATHER DRIVE LANDSLIDE, LAYTON, UTAH

Movement of the Heather Drive landslide in late August 2001 forced homeowners to evacuate their houses and caused significant economic losses. The landslide is a reactivation of a prehistoric landslide in lacustrine silt and clay of the late Pleistocene Lake Bonneville Weber River delta. The landslide is 1,030 feet wide and 450 feet long, and lies on a relatively gentle 20 to 28 percent slope above South Fork Kays Creek in Layton, Utah. The landslide movement history indicates a gradual reactivation followed by acceleration to a slow rate and then an abrupt stop. The majority of movement occurred between August 20 and 29, 2001, and formed a main scarp up to 9.5 feet high that crossed eight residential lots. The lower part of the landslide had a minimum of 7.1 feet of horizontal displacement and the toe had a minimum of 4.1 feet of horizontal displacement into the creek, restricting water flow.

The cause(s) of landslide movement in late summer 2001 is uncertain, but homeowner observations indicate that movement likely started in 1997 or 1998 when other nearby landslides reactivated following several years of above-normal precipitation. No documented changes in slope configuration or shallow ground-water conditions preceded accelerated movement in 2001, although little ground-water data exist. The overall movement pattern indicates the landslide probably moved intermittently at an extremely slow to very slow rate since 1997 or 1998, perhaps until the slip surface developed and the soil strength was reduced sufficiently to cause the accelerated movement in August 2001.

Of the six houses on the landslide, two straddled the main scarp, three were on the head, and one was on the right flank. Three houses were moved off the landslide and three were demolished due to landslide-related building damage. Preliminary estimates of homeowner equity losses, lending institution losses, costs to Layton City, and costs to relocate utility lines exceed $1 million. Future landslide movement could enlarge the landslide, placing additional houses, streets and underlying utilities, and South Fork Kays Creek at risk.