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April 2017 Meeting of the Chapter

***Wednesday, April 19, 2017 ***

Topic: "Going Three-Rounds with an Active Landslide", Hume Road Landslide, Malibu, CA

Speaker: Greg Johnson, PG, CEG
Los Angeles County Department of Public Works

Date: Wednesday, April 19, 2017
6:00pm Social Hour
7:00pm Dinner
7:45pm Lecture

Location: Pinnacle Peak Steakhouse
2533 La Cadena Drive South
Colton, CA

RSVP: RSVP by Monday 4/16 at

In April 2005, two medium-sized, recently reactivated landslides, situated mostly on private property, destroyed a residential driveway and a 340-foot-long portion of Hume Road, as well as its intersections with Briarbluff Drive and Castlewood Drive. Poised in a steep canyon, the landslide debris was considered a potential debris flow hazard during winter months to Las Flores Canyon Road, 375 vertical feet below the landslide crowns.

Successful emergency mitigation of the hazard was accomplished from November 2005 to January 2006 by grading that removed ~45,000 cubic yards of loose debris and by construction of an impact wall along Las Flores Canyon Road. The late winter rainfall of 2005-06 caused additional movement of the remaining landslide masses, again resulting in a potential debris flow hazard by early summer 2006. Emergency grading began again in September 2006, resulting in reactivation of part of the landslides on October 9, 2006. Subsequently, it was decided to remediate both active landslides. Grading, completed in March 2007, was conducted to excavate the active landslides, remove the accumulated debris, and construct stabilization fills. The fills are intended to support the steep slopes resulting from grading and not the currently-inactive landslides behind them. Hume Road was not rebuilt, but was reconnected to Briarbluff Drive and Castlewood Drive via construction of two hairpin turns.

The basal shear zones of the landslide complexes commonly followed the Las Flores Thrust Fault that placed Coal Canyon Formation over the younger Sespe Formation. Shear zone geometry ranged from simple and circular to extremely complex. The shear gouge ranged from soft and wet to stiff and dry, clay to gravelly clay. Shear zone thickness ranged from ˝-inch to four feet, but was commonly 4-6 inches. The multi-phased geotechnical investigation included 27 borings using four different types of drill rigs, installation of inclinometers and piezometers, observations over 1 ˝ years, and downhole oriented-hand-sampling and repeated direct shear testing of basal shear zone gouge.

The investigation cost about 4% of this 11 million dollar project. The opinions expressed are those of the authors, and do not reflect the opinions of, nor endorsement by the LACDPW, its employees or agents.
Speaker Biography:
Greg Johnson received a B.S. Degree in Geology from Sonoma State University in 1985. Greg is currently a Supervising Engineering Geologist III with the Los Angeles County Department of Public Works in Alhambra, where he has been employed since 1990. During that time, he has held several positions including that of a regulatory reviewer for the County Local Oversight Program involving review and response to environmental reports prepared by private consultant’s reports; District Geologist for the Santa Monica Mtns/Malibu area of the Development Review Section involving technical review of consultants reports and grading/building plans with respect to geologic and hydrogeologic hazards affecting proposed residential and commercial development; Engineering Geologist in the Geology Investigations Section supervising 4 engineering geologists involved in the assessment of existing and potential geologic hazards affecting engineered projects including storm drains, dams, roads, groundwater injection barriers and other structures for numerous cities and County departments; and supervisor of over 20 employees in the County’s Materials Testing Lab which ensures the wide variety of materials utilized in County construction projects meet project specifications by testing these materials in accordance with ASTM International, California Test Method, and AASHTO test methods. In addition, Greg is currently President of Inland Geological Society and Chair of the Association of Engineering Geologists-Inland Empire Chapter while maintaining memberships in GSA and NGWA.