ASCE Los Angeles Section

San Pedro District of City of Los Angeles

On November 20, 2011 the White Point Landslide collapsed a 600-foot section of the bluff along Paseo Del Mar within the City of Los Angeles.  The City’s Bureau of Engineering in coordination with the Los Angeles County and local neighborhood council, and using its geotechnical engineering consultants and emergency contractors, designed and installed 20 Horizontal Directionally Drilled drains and 18 post-tensioned tieback anchors to reduce the driving hydrostatic pressure and stabilize the bluff.

The White Point Landslide was first detected in summer of 2011 at which point the City began to monitor its movement, and subsequently closed the roadway to through traffic.  Affected utility owners were notified and they subsequently relocated, rerouted, or took their installations offline in anticipation of further land movement.  On November 20, 2011 the landslide experienced major movement towards the ocean and collapsed an approximate 600-foot section of the Paseo Del Mar roadway.  The City, through creative and innovative construction methods and use of its available resources, including pre-qualified specialty contractors and geotechnical consultants, successfully installed measures to stabilize the slide and protect infrastructure and adjacent homes.

The Bureau of Engineering, with the assistance of its Geotechnical Engineering Consultant, Shannon & Wilson, Inc., immediately began plans to mitigate movement of the landslide in an effort to preserve remaining infrastructure and the adjacent nature preserve.  This phase of work consisted of demolition and cleanup of the collapsed infrastructure and grading of the landslide scarp and graben to facilitate drainage of stormwater to the ocean.  This phase of work was advertised to the City’s Emergency Geotechnical/ Structural Projects On-Call Contractors List on a fast tracked schedule and was awarded as a lump sum contract to John S. Meek Company, Inc.

The landslide occurred on a section of Paseo Del Mar approximately 400-feet west of the adjacent neighborhood.  After further geotechnical investigation it was determined that hydrostatic pressure created by a high water table was a major contributor to the instability of the bluff.  In an effort to lower the water table, an innovative system of 20 dewatering drains, each 600-feet in length, was designed and installed with the creative use of Horizontal Directional Drilling technology.  Hayward Baker Inc., was selected from the City’s On-call list of emergency contractors to design and install this system.  Due to the urgency of the project, a lengthy design phase with lump sum project cost was substituted with a design/build type contract on a Time and Materials basis.  As would be expected with a Time and Materials contract of this magnitude, the City’s project management team worked tirelessly with the contractor and inspection team to track costs and ensure the contractor was compensated accordingly.

To further stabilize the bluff adjacent to nearby homes, 18 post-tensioned tieback anchors with isolated concrete reaction pads were designed and installed between the dewatering drains.  Hayward Baker, Inc., was again retained for this phase since they had the expertise to perform this type of work and the experience and familiarity with the site gained during dewatering drain installation.  Although tie-back anchors are commonly used for slope stabilization, this project site posed a substantial challenge as the anchors were installed at the top of a 120-foot bluff.  A creative solution to installing the anchors at such a precarious location was to design a cantilevered drilling platform that was anchored to the bluff using four 45-foot long micro-piles per anchor location.  This innovative combination of air-rotary drill mast attached to the custom made cantilevered drilling platform performed flawlessly.  Again, due to the complexity and urgency of the project, the work was executed on a time and materials basis.  Anchor Installation was completed in July 2014.

Because the landslide took place on portions of land owned by the City of Los Angeles, County of Los Angeles, and a section of land managed by the Palos Verdes Peninsula Land Conservancy, project managers conducted extensive coordination meetings to ensure all interests were protected.  In addition, with the project being located on the coastline and within the jurisdiction of the California Coastal Commission (CCC) and the Los Angeles Regional Water Quality Control Board (RWQCB), the project was required to comply with strict regulations.  The final and most important group that had a stake in the project was of course the community, represented by City Council member Joe Buscaino as well as the Coastal San Pedro Neighborhood Council.  Councilmember Buscaino formed an Ad-Hoc Community Advisory Committee at which project managers and City officials regularly briefed interested community members and the public on the progress of the project and incorporated their feedback into the project design.  In light of the emergency at hand, all parties cooperated, and the project was successfully completed in July 2014.

In February 2014, shortly after installation of the dewatering drains, the project received an Engineering Excellence Merit Award from the American Council of Engineering Companies (ACEC) for outstanding achievement in engineering and land surveying.

The dewatering drains are composed of 4-inch HDPE pipe transversely slotted at 0.060-inch increments.  Drains were installed in 6-inch diameter holes drilled using Puregold-Cleandrill drilling fluid and a Ditch Witch JT100 Mach1 directional drill.  The 600-foot length pipes are composed of fused 40-ft lengths. Tracking of drain alignments was performed using microcoil surveying equipment.  A set of upper and lower drains, between 10 to 100 ft bgs, were installed to drain both confined and unconfined aquifers.  Dewatering drains are spaced 20 feet apart and the target exit/breakout windows were only 5-ft square after 600-ft to allow future installation of ground anchors between drain alignments.  Anchors were installed perpendicular to dewatering drains within the 20-foot spacing leaving only a 10-foot clearance and in some cases less due to initial learning curve.  Anchor alignment accuracy was paramount to avoid anchors striking dewatering drains and to reduce possibility of grout migrating into drain slots.

Anchors consist of 0.6-inch diameter 6 strand bundles with lengths of 165-feet and only the bottom 35-ft considered as bonded sections.  The anchors were installed to a depth of 165-feet to penetrate the landslide slip plane which was determined to have occurred on a layer of bentonite located at approximately 90-ft bgs. Anchor installation was performed using a Hennessy International Inc. duplex HD180B drilling rig and custom fabricated drilling platform.  Due to the anticipated caving conditions a duplex drilling system was selected.  To ensure a tolerance of +/- 2.5 degrees and that the anchors did not come within 5-feet of the adjacent HDPE dewatering drains, a Reflex Gyro system was used to track the hole alignment.  This system consists of a silicon sensor chip and integrated circuit, assembled in a ceramic (non-magnetic) package, and incorporates an integrated Azimuth Pointing System (APS) that can indicate True North azimuth, a GPS position, and a degree of inclination in order to accurately locate itself within the bore path alignment.  Two of the 18 installed anchors are equipped with DYNA Force stress monitoring systems used to monitor the force exerted during the stressing process as well as monitoring the forces during the service life of the ground anchor.

In November 2011 a large landslide collapsed a 600-foot section of bluff along a major coastline road within the City of Los Angeles.  The City’s Bureau of Engineering, supported by prequalified geotechnical engineering consultants and specialized contractors, successfully designed and installed a dewatering drain and post tensioned tieback anchor system to reduce the driving hydrostatic pressure and stabilize the bluff.

On November 20, 2011 the White Point Landslide collapsed a 600-foot section of the bluff along Paseo Del Mar within the City of Los Angeles.  The City’s, Bureau of Engineering in coordination with the County of Los Angeles and local community, and using its geotechnical engineering consultants and emergency contractors, designed and installed 20 Horizontal Directionally Drilled drains and18 post-tensioned tieback anchors to reduce the driving hydrostatic pressure and stabilize the bluff.

Due to the urgency of this project, the Bureau of Engineering utilized its pre-qualified geotechnical consultants and emergency contractors to expedite the mitigation measures.  Shannon and Wilson Inc. was selected as the Geotechnical Engineering Consultant and Hayward Baker Inc. (HBI) was selected as the specialty contractor to install the dewatering drains and ground anchors.  The initial grading and demolition work was fast-tracked and awarded as a lump sum emergency contract to John S. Meek Company, Inc.  Since the drain and anchor installation required specialized skills and experience, it was awarded as a design build construction order on a time and materials basis.  HBI, with creative use of current horizontal directional drilling technology, successfully installed 20 dewatering drains, each 600-feet in length.  The installation of 18 tieback anchors atop a 120-foot bluff required an innovative marriage of an air rotary drill rig to a custom fabricated cantilevered drilling platform to successfully complete.  With the design responsibility assigned to the contractor the potential for large change orders was reduced and it allowed for a more streamlined project.