Landslides on the Peninsula

There are 5 well known landslides on the Peninsula that have resulted in significant expense or loss of improvements:

1) Portuguese Bend / Abalone Cove in Rancho Palos Verdes
2) Ocean Trails (golf course hole 18) in Rancho Palos Verdes
3) Flying Triangle in Rolling Hills
4) Bluff Cove in Palos Verdes Estates
5) Indian Peak / Deep Valley in Rolling Hills Estates

To a lesser extent localized erosion and collapse of bluff faces have occurred in several locations on the Peninsula.

The current status of the above landslides:

1) Portuguese Bend / Abalone Cove remains active with building restricted.
2) Ocean Trails has been mitigated and is in full use.
3) Flying Triangle is active with building restricted.
4) Bluff Cove has been stabilized and most blufftop homes were bought by city.
5) Indian Peak is in planning for stabilization and re-development.

Palos Verdes Geology

The topography of the peninsula is generally hilly, ranging from gently rolling to steep. A bluff exists along the coastline, varying between 100 and 200 feet in height above sea level, and elevations rise to 1500 feet within 3 miles of the coast. As ocean waves eroded the coast, they carved relatively flat surfaces out of the bedrock. Tectonic forces  then uplifted the marine platforms, forming marine terraces that rest high above sea level. Palos Verdes Peninsula has been steadily pushed out of the Pacific Ocean over the past 1.5 or so million years, producing numerous marine terraces that can be seen on all sides of the peninsula. This process has produced a complex series of terraces that appear to be grouped into 13 major levels. However, within landslide areas, terraces have been disrupted and the terrain now appears hummocky and irregular. Large scarps occur near the head of slide masses.

The colorful stripes in the cliffs are called sedimentary rock. Millions of years ago each stripe was the top layer of the ocean floor, just like the sand at the beach today. As sand and other particles (sediment) suspended in the water sank to the bottom, they gradually built up the thickness of the ocean floor. Different kinds of sediment left different colors of rock.

Within one mile of the coastline, the subsurface consists of volcanic and sedimentary rocks that dip toward the sea at about the same angle as the average slope of the land surface. Landslide slip surfaces occur near the base of a volcanic rock unit known as the Portuguese Tuff. This rock unit includes a layer of bentonite, a clay mineral that forms from the weathering of volcanic ash and is capable of absorbing large amounts of water. 
 
Smuggler's Cove is nestled between Inspiration Point to the East and Portuguese Point toward the West. A minor fault runs through the rocks at Portuguese Point. This fracture in the bedrock is where local sections of Earth's crust have moved past one other, grinding rocks at the contact into a fine powder called fault gouge. Fault gouge is easily removed by erosion. At Portuguese Point, waves eroded some of the fault gouge, creating a tunnel through the base of the point.

Livingston Quarry

This quarry operated from 1945 to 1956 and was located in the Portuguese Bend slide area. It extracted minerals such as barite, quartz, gypsum and dolomite, as well as basalt that were used for railroad bedding. It also left a large scar on the hillside that is still visible today. The minerals extracted from Livingston Quarry formed from a process called secondary mineralization. This means that the minerals formed inside rocks. This differs from primary mineralization, where the minerals crystallize from a molten liquid, like cooling lava or magma. The minerals at Livingston Quarry formed in cracks in the rocks. Groundwater saturated with elements moved through the rocks and formed crystals where cracks formed.

The quarry had three sections. The western section is where most of the barite was located. There is still an abundance of dolomite. Some concrete from the quarry's buildings is still present. For a detail presentation go to http://seis.natsci.csulb.edu/VIRTUAL_FIELD/Palos_Verdes/pvquarry.htm

Portuguese Bend Landslides

The Portuguese Bend Landslide is a translational slide and actually occurred in two pulses. The first began about 37,000 years ago. The actual cause of this weakness is not well understood. Total displacement is also not known. This movement, called the "Ancient Landslide,” is no longer active.  Land flow and resulting conditions are at http://seis.natsci.csulb.edu/bperry/Mass%20Wasting/PortugueseBendEarthFlow.htm

After this movement ceased, a new, shallower plane of weakness formed. An ancient volcanic eruption produced the geologic layer that developed the recent plane of weakness. The thick volcanic ash and subsequent thinner layers altered to a clay called bentonite. Bentonite absorbs much water and loses its cohesiveness, forming slip planes along which the land can slide.

The initial movement is evidenced by Long Valley, which runs parallel to Crenshaw Blvd. in the city of Rolling Hills. This area is called the Crenshaw Extension. The recent slide has been moving sporadically over the last several thousand years. However, renewed activity occurred in 1956, when a portion of the southeast end of the slide showed signs of movement. The most active part of the slide is in the southeast corner at Klondike Canyon.

The Abalone Cove landslide movement began in 1978 and is located on the West border of the Portuguese Bend landslide and has a similar mechanism (movement along seaward-dipping bentonite tuff beds). The 600-acre Abalone Cove landslide, affected over 100 homes.

Portuguese Bend Landslide and geology is also documented, in detail, at  http://seis.natsci.csulb.edu/VIRTUAL_FIELD/Palos_Verdes/pvportuguese.htm

Landslide Movement

Landslides have been active here for thousands of years, but recent landslide activity has been attributed in part to human actions. The rates of slippage have varied through time, initially moving up to 5 inches/day in the first two years, and then diminishing to less than .4 in/day in recent years. By 1961, more than 150 homes had been destroyed or seriously damaged by the slide. Since then, many additional homes have been affected to some degree, and Palos Verdes Drive South has been in constant need of repair. All underground utilities have been moved above ground into steel pipes with flexible couplings.

Corrective Actions

An early attempt to prevent further slope movement was made by installing pre-cast concrete pins through the slide mass in 1957. These pins, measuring 4 ft in diameter and 20 ft in length, extended 10 ft into the underlying bedrock. Initially they slowed the rate of slope movement by 50 percent; however, after five months, the pins failed and movement accelerated.

The first of a three-phase stabilization project involved the installation of eight de-watering wells in the slide mass during the mid-1980s. Phase 2, completed during the late 1980s, included road relocation, surface drainage improvements, re-grading the seaward side of the slide mass, and the addition of five more wells. The canyons were lined with impermeable materials and runoff directed to the sea to prevent absorption into the subsoil. In the early 1990s, revetments were placed at the base of the bluffs to protect them from wave erosion. Following the completion of this third phase, parts of the landslide was largely stabilized. The most recent mitigating measure was the installation of a sewer system replacing the septic systems throughout the area. These measures may have greatly diminished the land movement, however the lack of heavy rainfall in recent years could also be a major factor.

Class Action Lawsuit

 This Portuguese Bend landslide is notable for causing the first major class action lawsuit brought about as a result of a geologic event.  It was also the impetus for California to pass the nation's first law requiring geologists to have a professional license to provide consulting services.

 In 1961, Portuguese Bend homeowners filed a class action suit in state court against the County of Los Angeles. This suit sought compensation for damages to 160 homes affected by a landslide encompassing an area of roughly 270 acres, which amounted to nearly $10 million in 1960 dollars. The homeowners alleged that this landslide was caused by road construction along Crenshaw Boulevard, which added weight to the upper slopes of the slide mass in the form of artificial fill. The lawsuit also alleged fraud and negligence on the part of the developers for participating with the county in road construction in a geologically unstable area. Although negligence could not be established, the county was found liable by the presiding judge using inverse condemnation.

By the time of the 1956 Portuguese Bend landslide, more than 100 homes had been built within the slide area, most of them south of Palos Verdes Drive. All of these houses were constructed with individual septic systems, generally consisting of septic tanks and seepage pits.

Arguments by experts for the county brought several facts to light that were ultimately ignored in the judgment. These included: (1) the amount of artificial fill used in constructing Crenshaw Boulevard equaled only 0.5 percent of the total landslide mass in terms of weight, (2) the water added through septic system discharge and lawn irrigation probably caused groundwater levels in the slide mass to rise, and (3) detergents in the septic system effluent would be expected to have a negative effect on the strength of saturated bentonite clays.

Ocean Trails Landslide

Just prior to the grand opening of the Ocean Trails golf course on the ocean bluff in the southeast area of Rancho Palos Verdes, the eighteenth hole collapsed into the ocean. The project descended into foreclosure and was purchased by the Trump organization. The entire land mass was removed and rebuilt removing the slide formation.

Flying Triangle Landslide

The flying triangle slide is located in the gated city of Rolling Hills at the Southern end of Portuguese Bend Road and high above the Portuguese Bend slide. This slide began in the last 15 years and affects the ranch style homes in this very rural area. These homes are on large lots (generally over 1 acre) and are on septic systems. There are no major mitigating measures in place other then minimization of watering and slow movement continues. Building is restricted primarily to maintenance issues.

Bluff Cove Landslide

This landslide is located in the bluff cove area of Palos Verdes Estates and generally only affects the 12 bluff top homes on the West side of Palos Verdes Drive West above the ocean. This slide was blamed on a leaking city storm drain, which undermined the soil and caused damage to these homes. The storm drain was replaced and the ground movement minimized. In subsequent legal action by the homeowners, the city purchased 10 of the homes. Some of the homes are now housing for city employees as part of their compensation package. Two of the homes were demolished and the land became open space.

Indian Peak Landslide

The Indian Peak landslide involved the collapse of a hillside on Indian Peak Road above Deep Valley Drive that was developed with commercial buildings. Most of the structures were destroyed and commercial buildings below were damaged as well. This slide occurred in the past decade and plans are underway to mitigate the slide and rebuild on the property.

Other Landslides

Throughout the Peninsula there are other localized instances of small hillside and bluff failures. It is imperative that geologic considerations are addressed in hillside locations to prevent damage to property and homes. Homes built along the bluffs over the ocean are usually built on caissons in newer construction. These caissons can be required to go to sea level in some instances resulting in drilling to depths of over 100 feet. Foundations are generally required to be into bedrock for construction projects.