The Liquefaction of Los Osos


Liquefaction caused widespread damage in the recent New Zealand and Japan earthquakes, but despite compelling evidence of the serious hazards, it has been ignored by SLO County in planning the $200 million Los Osos Wastewater Project in a high-risk liquefaction zone. Experts agree that the County needs to face the facts and take action now before it’s too late.


Liquefaction caused widespread damage in the recent New Zealand and Japan earthquakes, but despite compelling evidence of the serious hazards, it has been ignored by SLO County in planning the $200 million Los Osos Wastewater Project in a high-risk liquefaction zone. Experts agree that the County needs to face the facts and take action now before it’s too late.

The images on the evening news spilled out of our TVs for weeks, almost filling our living rooms with sand and water, yet the lessons of the New Zealand and Japan earthquakes appear to have been lost on San Luis Obispo County officials ramming through the huge Los Osos Wastewater Project, the biggest and most expensive in County history.

Despite the clear warnings, the County in its rush to push through the project has largely ignored the hazards of constructing a gravity collection system in the loose, sandy soils of Los Osos’ County-documented liquefaction zone, which lies on known and recently discovered quake fault lines.

Consider this basic scenario: A future earthquake of unknown magnitude striking the region turns Los Osos’ “Prohibition Zone,” the area to be sewered with large-diameter gravity pipes, into the liquefaction zone that overlaps it, potentially devastating Los Osos like it did Christchurch, New Zealand, and wide areas of Japan following their quakes. Along with major structural damage to homes and utility connections, sewer lines would likely be broken, and the upheaval of the County’s deep-buried, sloping gravity pipes and hundreds of manholes will be difficult, costly and time-consuming to repair or replace. Undetected 25 feet or more underground, the damage to the public water supply by injecting it with raw sewage and pathogens from broken gravity pipes could pose a serious health threat to the community and its drinkable water supply. There is no “soft fix” for multiple non-functioning gravity sewer lines, only red-tagged houses served by inoperable sewers and neighborhood-by-neighborhood evacuation.

The severe, widespread liquefaction caused by the recent massive earthquakes in New Zealand and Japan surprised even seasoned disaster researchers who surveyed the extensive damage first-hand.

Scott Ashford
, a professor of geotechnical engineering at Oregon State University and part of the Geotechnical Extreme Events Reconnaissance (GEER) advance team performing reconnaissance in Japan (, reported that much of the damage from the earthquake was the result of liquefaction: “The shifts in soil destroyed water, sewer and gas pipelines, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as much as four feet.

“Buildings that are built on soils vulnerable to liquefaction not only tend to sink or tilt during an earthquake, but slide downhill if there’s any slope, like towards a nearby river,” Ashford said. “This is called lateral spreading. … (T)his sideways sliding of more than four feet in some cases (is) more than enough to tear apart buildings and buried pipelines.”

Liquefaction is a critical issue in coastal, low-lying Los Osos with its high groundwater and sandy soils. It poses a significant threat of serious damage to homes, water, gas and sewer lines, yet the County has failed to conduct the geotechnical review required by the Environmental Impact Report (EIR). Post-quake analysis of liquefaction from the recent earthquakes raises serious questions about whether San Luis Obispo’s County’s existing project plan, building codes and performance standards, and critical choice of technology are adequate to account for the potential of this destructive phenomenon to impact the liquefaction zone of Los Osos.

Thomas L. Holzer, Ph.D., CEG, with the Earthquake Science Center, U.S. Geological Survey, in Menlo Park, California, documented liquefaction in nearby Oceano during the 2003 San Simeon magnitude (M)6.5 earthquake (

“In general in my experience,” Holzer told The Rock, “gravity-based waste-water collection and treatment systems do not fare well if they are impacted directly by significant liquefaction. This was demonstrated recently in the 2011 M6.3 aftershock in Christchurch, NZ, in which the sewer lines were not only damaged but clogged with liquefied sand. (

“In the 1995 Kobe, Japan, M6.8 earthquake,” said Holzer, “sewage treatment plants in Kobe were so disabled by liquefaction-related settlements that untreated sewage was discharged into Osaka Bay until repairs could be made ( Note that both of these earthquakes had magnitudes much smaller than M7.5.”

“Government agencies would be well advised to consider the strong extensional and shear forces that are exerted on below-ground structures when soils liquefy,” said Dr. John Claque, Shrum Professor of Science, Centre for Natural Hazard Research at Simon Fraser University, Burnaby, B.C. “A magnitude-7.5 earthquake within ca. 50 km of the site, assuming the soils have high liquefaction potential, would certainly induce liquefaction.

“The Christchurch experience is informative,” said Clague. “The February earthquake had a magnitude of only 6.3. Although the centre of the city was only 7 km from the epicentre, the amount of liquefaction was stunning and the damage correspondingly high.”

Said Clague, “The bottom line is that the design of the facility must be done with liquefaction in mind.”

“(A) reasonable goal,” according an Oregon State University statement from Ashford’s GEER research group, “is to at least anticipate the damage – to know what will probably be destroyed, make contingency plans for what will be needed to implement repairs, and design ways to help protect and care for residents until services can be restored.”

“One does not need to look at the Japan earthquake to make inferences to (the) issue (in Los Osos),” said Ellen Rathje, Ph.D., P.E., University of Texas at Austin, citing Los Osos’ “high water table due to proximity to the water.

“Liquefaction will be a concern for all infrastructure in the area, including single family homes. The Christchurch earthquake is the closer analogy – alluvial plain, moderate earthquake, liquefaction everywhere. However, infrastructure can be constructed to withstand earthquakes and liquefaction – it just takes the right technology and money.”

The Politics of Quake Safety

On April 14 State Sen. Sam Blakeslee called for “a suspension of the licensing effort under way at Diablo Canyon nuclear power plant until a myriad of questions regarding the seismic setting at the facility are answered.” Yet Blakeslee has remained silent on asking for a suspension of engineering contracts on the nearby Los Osos Wastewater project, which he went so far as to write a state law to build, and which parallels Diablo Canyon’s earthquake vulnerabilities.

So far, Blakeslee has not asked the County to suspend work on the LOWWP until an updated liquefaction analysis — including new information from New Zealand and Japan — can answer a similar “myriad of questions regarding the seismic setting” for the gravity collection system in a high liquefaction zone.

Blakeslee’s approach to local earthquake hazards has been inconsistent. A geophysicist with a doctorate in earthquake studies from UC Santa Barbara, he seeks a seismic risk assessment for Diablo Canyon prior to relicensing but not a parallel liquefaction study for Los Osos prior to engineering the sewer project, or a thorough evaluation of the gravity system with respect to local hazards such as earthquakes, liquefaction and flooding. Such an evaluation is necessary to preserve the intent of the California Coastal Act – and protect Los Osos “Prohibition Zone” homeowners’ Prop 218 sewer assessment of $25,000 per home.

While Diablo Canyon is a “hot” national political springboard, and Blakeslee didn’t build the reactors, his “Blakeslee Bill” seized control of the sewer from the Los Osos Community Service District and gave it to the County to build a gravity system, which will own and operate it, and this may have something to do with why Blakeslee hasn’t addressed liquefaction in Los Osos.

The County did not include any liquefaction update in its RFP to CDM, and CDM did not include liquefaction in its proposal to the County. In fact, the technical portion of CDM’s Feb. 24, 2011 proposal did not include the word “liquefaction,” although it may not have been included because the proposal was prepared prior to the March 11 Japan event. Nevertheless, the EIR classified the Los Osos area with high liquefaction potential, and CDM should have addressed it somewhere in their technical proposal, but did not. The Rock contacted CDM but they declined to explain.

The County’s Expanded Geology Analysis in the EIR cites: “Within the Los Osos area… geologic units such as beach sand, dune sand, and younger alluvial deposits as having a high potential to contain sediments that may be prone to liquefaction.

“Portions of the collection system network traverse areas having a relatively high potential for liquefaction. The potential for liquefaction and seismic settlement to impact pipelines may be governed by the depth of the pipeline relative to the depth of liquefiable soils. The proposed collection system for Proposed Project 1 may experience significant liquefaction impacts. Furthermore, this potential significant impact could result in pipeline breaks and release of untreated and/or treated effluent along the proposed collection/conveyance system, including within Los Osos Creek and Warden Creek.”

As a “mitigation measure” the EIR calls for a geotechnical report: “Prior to approval of the improvement plans for the proposed facilities that are part of the collection system and at the treatment plant site, a geotechnical report that addresses liquefaction hazards shall be prepared and approved by the County of San Luis Obispo. The geotechnical report shall state the recommended actions for the collection system and treatment plant site so that potential impacts from seismically-induced liquefaction would be reduced to less than significant.”

The EIR requires the liquefaction study, yet EIR consultant Michael Brandman Associates and County Public Works have brushed off the liquefaction issue and the importance of the analysis to be done before CDM begins redesigning elements of the collection system, not after.

The last geotechnical report for the LOWWP was prepared in March 2004 by Fugro West in San Luis Obispo for Montgomery Watson Harza’s halted midtown project. That report identified high liquefaction areas but did not flag liquefaction on Broderson and on the collection system in their report, and did not address the pipelines themselves or any of the approximately 800-plus manholes. Based on current experience in New Zealand and Japan, this is a major omission in their prior work.  Contacted by The Rock, Fugro declined to explain the potential impact of liquefaction on about 200 home downgrade from the Broderson leach fields, below what will be fully saturated, shallow, sandy soils receiving 800,000 gallons per day on seven acres with a 10% slope.

In June 2010, Michael Saunders of Orenco Systems Inc., the largest manufacturer of STEP systems in the world, complained to the California Coastal Commission that “incorrect data and conclusions presented in County documents have, in our opinion, often been misleading or incorrect,” clearing the way for the County’s pre-selected gravity collection system — without comparing the hazard risks and repair/replacement costs to any other system — and for the Coastal Commission to permit it.

“According to (August 2005) Los Osos Community Services District Hazard Mitigation Plan, the probability of a severe earthquake in Los Osos is medium to high in terms of severity and a high probability of occurring. Despite this the fine screening mentions the word earthquake once and never mentions it with respect to the comparison of wastewater collection alternatives. The EIR talks about earthquakes but only with respect to the County’s preferred project. The NEPA document never mentions earthquakes. Liquefaction isn’t mentioned in any of the documents.”

The County’s historical bias toward gravity collection technology has been and continues to be a nagging problem for informed geologists and engineers, as well as for the portion of the community selected to pay the inflated bills. The County’s Rough and Fine Screening analysis completely ignored risks associated with naturally-occurring hazards such as earthquakes, floods, storm events and liquefaction. The EIR does not compare the impacts of liquefaction on a gravity sewer system versus any other available system, only on the County’s project. And the evaluation of each technology should have included local environmental hazards, but it didn’t happen. Instead, the County declared gravity the environmentally-preferred project without an updated geotechnical report to determine what magnitude of earthquake and ground-shaking should be used as basis-of-design.

Construction methods and techniques may reduce or prevent damage, experts report. Jeanne Perkins, former Earthquake Program Manager for the Association of Bay Area Governments (ABAG) and now a consultant to ABAG, said: “In general, waste water collection systems can be designed to be safe. The most vulnerable portions of the system tend to be the ‘connections’ in the system where pipes go into and out of, for example, pumping stations, so those connections need special design.  Another vulnerable area is the ‘edge’ of liquefaction areas because differential settlement and movement of the sewer lines can break them.  However, specific engineered fill in the trenches of the lines and special pipe designs can work. “

The County may simply believe that a M7.5 earthquake would break any sewer line, gravity or STEP collection, but this was not the case in New Zealand where HDPE and MDPE pipes used in STEP collection held up to the extreme rigors of multiple M7.1 2 (2010) and M6.3 (2011) quakes. Comparing system repair and replacement costs, generally, repair costs for the STEP collection system run a fraction of the cost to repair gravity sewer line, a maximum of 15% if it needs 100% replacement relative to a gravity sewer, STEP experts told The Rock. The County rejected STEP collection in April 2007 without comparing STEP costs and risks to gravity, as initially promised by the County for the Proposition 218 vote. That vote passed, in part, because alternative technologies were to be compared to gravity in a co-equal analysis, but they weren’t.

Geologist Alfred Hochstaedter, a professor at Monterey Peninsula College in Monterey, California, agrees that Los Osos could be very prone to liquefaction during a large earthquake. “It looks like Los Osos would indeed be prone to liquefaction hazard.

“Then the geologic question reverts back to the likelihood of a large earthquake in the vicinity of Los Osos to trigger the liquefaction. Is there a chance of this happening? Yes. Is there a *high* probability of this happening in the next 30 years? Probably not. But nor was there a high probability of the Japanese tsunami happening either. Did geologists know that things like this occurred in Japan in the past? Absolutely. Did geologists think that this kind of thing could happen in Japan in the future? Absolutely. Is there a *high* probability of something this catastrophic happening in any one place on any human time scale? No.

“The timing of large, catastrophic events is very difficult to predict,” said Hochstaedter. “The places where they will occur is easier to predict. The probability of them happening in a particular place in any human time scale (tens of years) is low. So how do municipalities allocate funds to ensure that infrastructure will withstand very low probability events? That’s a very difficult question.”

The GEER-OSU release states that “some degree of soil liquefaction is common in almost any major earthquake. It’s a phenomenon in which saturated soils, particularly recent sediments, sand, gravel or fill, can lose much of their strength and flow during an earthquake. This can allow structures to shift or sink and significantly magnify the structural damage produced by the shaking itself.”

GEER research also points out that most earthquakes are much shorter in duration than the recent Japan quake, which may have lasted as long as five minutes. “The length of the Japanese earthquake, as much as five minutes, may force researchers to reconsider the extent of liquefaction damage possible in situations such as this,” said the report.

“With such a long-lasting earthquake, we saw how structures that might have been okay after 30 seconds just continued to sink and tilt as the shaking continued for several more minutes,” GEER’s Ashford said. “And it was clear that younger sentiments, and especially areas built on recently filled ground, are much more vulnerable.”

Fugro’s 2004 geotechnical report partially addressed a sustained seismic event. “The manifestation and damage that can be associated with liquefaction is strongly dependent on the duration of the ground motion. Larger magnitude earthquakes typically result in longer periods of shaking. Earthquakes that occur closer to a site generally result in higher ground motions than a similar magnitude earthquake that could occur away from the site.”

The Fugro report continues: “Liquefaction can result in ground mobility that impacts pipeline grades, or results in pipelines floating out of the ground in areas of liquefaction. The soils encountered within the pipeline network vary from soils having a relatively high potential for liquefaction, to soils having a relatively low potential for liquefaction. The potentially liquefiable soils were typically encountered in areas that are either low in elevation or relative topographic relief, such as the shoreline areas along Morro Bay and interdunal depressions along Morro Avenue, Paso Robles Avenue, Santa Ynez Avenue, and Ramona Avenue-Mitchell Drive. These areas are typically characterized as being underlain by relatively loose sand and shallow groundwater.”

Wrote Sam Blakeslee on March 19: “The public counts on lawmakers and regulators to put the public’s safety ahead of the agendas of powerful interests. Our greatest risk is arrogantly asserting that California is immune to this type of disaster before obtaining the necessary scientific data to adequately understand the risks posed by the complex fault systems off our coast.”


UPDATE: On May 17 Dana Ripley of Ripley Pacific, a former consultant to the LOCSD and a consulting engineer to STEP builder WM Lyle, gave a brief summary of his presentation to the SLO County Board of Supervisors on liquefaction in Los Osos. “Liquefaction is a known quantity in Los Osos,” he said. “(The County’s 2007) liquefaction hazards map from the LOWWP EIR shows indeed that most of the collection area is designated as having a very high risk of liquefaction. The EIR was very clear that the project had a high risk of liquefaction. The awareness of earthquakes and tsunamis are very high in this county due to (nearby) Diablo Canyon (nuclear facility). That level of awareness should extend to this particular (wastewater) project as well. The position of this board should be the same as Diablo Canyon: Study the seismic risks before you do the detailed design. The gravity PVC collection pipe system needs a current budget as proposed by CDM. I would also recommend that CDM produce a budget for producing a collection system that could withstand a magnitude 8.0 earthquake-related liquefaction. Finally, I know we’ve been excluded from the table, but put the HDPE high-density polyethylene collection system back on the table for cost-comparison purposes.”

Additional links:
NPR Liquefaction Report
Sewers Float and Sewer Performance in Earthquakes
Water Pipes vs. Wastewater Pipes
Christchurch NZ Liquefaction Poster 2010

Two additional working definitions of liquefaction:

Liquefaction, which is also commonly observed during earthquakes, is a phenomenon where saturated sands lose their strength during an earthquake and become fluid-like and mobile. As a result, the ground may undergo large permanent displacements that can damage underground utilities and well-built surface structures. The type of displacement of major concern associated with liquefaction is lateral spreading because it involves displacement of large blocks of ground down gentle slopes or towards stream channels. (Source: “Liquefaction-Induced Lateral Spreading in Oceano, California, during the 2003 San Simeon Earthquake,” by Thomas L. Holzer and others, prepared by the USGS in cooperation with the San Luis Obispo County Planning and Building Department, 2004)

Liquefaction is a loss of soil strength due to a rapid increase in soil pore water pressures due to cyclic loading during a seismic event. In order for liquefaction to occur. three general geotechnical characteristics are typically present: 1) groundwater is present within the liquefiable zone; 2) the soil is granular; and 3) the soil is in a low to medium state of relative density. If those criteria are met and those soils are subjected to strong ground motions, then those soils may liquefy, depending upon the intensity and cyclic nature of the strong ground motion. Seismically induced settlement or collapse can occur in soils that are loose, soft, or that are moderately dense and weakly cemented, or in association with liquefaction. Manifestations of liquefaction can consist of sand boils, loss of bearing capacity. Manifestations of liquefaction can consist of sand boils, loss of bearing capacity, lateral spreads and slope instability, and differential and areal settlement. The severity of the consequences of liquefaction is dependent on relative density of the soil and intensity and duration of the ground motions; however, not all soils that liquefy experience the same degree mobility or ground failure. (Source: Fugro 2004 Geotechnical Report for MWH)