Seismic Retrofit Blog

What Is Liquefaction?

What is liquefaction?

Liquefaction can be explained as the phenomena when soil behaves as a liquid when there is seismic activity occurring in that area.

How does liquid behave?

Liquid flows down and causes the soil particles to separate and lose contact.

What areas are at risk?

An area is at risk for liquefaction if the soil underground is saturated or if man-made landfill is utilized. Loose, young soils are also susceptible to this phenomena.

According to the Earthquake Engineering Research Institute, the effects of liquefaction can take a number of different forms. Among them are: flow failures, lateral spreads, ground oscillation, and loss of bearing strength- among others.(

Flow Failures

This refers to the displacement of masses of soil. This may include soil that is fully liquefied as well as portions of material that are not liquified, in addition to the liquefied soil. Flow failures are common when “relatively steep slopes” are present. The instability caused by liquefaction pushes the liquefied soil down the slope.

Lateral Spreads

Lateral refers to the side or sides and displacement refers to the moving of something from its position. A lateral spread refers to the displacement of blocks of soil, resulting from the forces created by an earthquake. As compared to flow failures, lateral spreads most often occur on gentle slopes. This can affect building foundations as well as pipelines.

Examples of Lateral Spreads In Past Earthquakes

During the 1906 San Francisco earthquake, lateral spreading had a devastating effect on the pipelines in the affected areas. Due to the damaged pipelines, the fires burned for much longer than they would have had there been access to water.

In the 1964 Alaska Earthquake, bridges were severely damaged and in some cases destroyed due to the effects of lateral spreading.

Ground Oscillation

The Earthquake Engineering Research Institute explains that if the ground is too flat or the slope is too gentle for lateral displacement to occur, the resulting effect is ground oscillation. During the 1989 Loma Prieta Earthquake, the effects of ground oscillation were apparent in the sidewalks that were buckled as a result of the soil oscillation.

An example of ground oscillation in the Loma Prieta earthquake.

 Photo Courtesy of USGS


A modern day example of liquefaction is the infamous Millennium Tower in San Francisco, which as many people know is sinking and tilting. This issue can be explained by the soil that millennium stands on, largely man-made fill- which is prone to liquefaction.

Liquefaction prone areas can be identified, but when exactly it will happen cannot be predicted. Researchers have made strides in identifying areas with the potential for liquefaction which has helped provide insight into hazard areas.





September 18, 2017

8.1 Magnitude Earthquake Hits Mexico

Mexico was hit with a massive 8.1 earthquake late Thursday night at 11:49 pm. This quake is the strongest to hit Mexico since the 8.1 magnitude quake quake that struck Las Primaveras over 85 years ago (  The epicenter of the quake was about 51 miles from El Palmarcito- Earthquake Track reports. An 8.1 magnitude earthquake is at the top of the earthquake magnitude scale, which defines an earthquake of magnitude 8 or more as “great” and capable of total destruction in the areas nearest the epicenter ( Approximately 50 million people throughout Mexico reportedly felt the strong earthquake, sending those present into shock (

Photo Accessed Via ABC News

This devastating event has taken the lives of more than 32 individuals in Mexico thus far. Hundreds of buildings were destroyed in the midst of this quake and more than 1.8 million people were left without power. The governor of Chiapas, Manuel Velasco, spoke out about the damages done to schools and hospitals. Due to loss of electricity, there was a tragic loss of a young child’s life in a Tabasco hospital.

The devastation and loss of life caused by this earthquake is indescribably tragic. Although seismologists, scientists, and many others are aware of certain locations susceptibility to earthquakes- it is a fact that is often ignored. Earthquakes are a terrifying reality of areas located on or near a fault. Temblor reports that this earthquake “can be considered relatively surprising”, due to the fact that the Global Earthquake Activity Rate (GEAR) forecasted a likely 7.25 earthquake.

CoreLogic has created an earthquake risk analysis to estimate damages to both southern and northern California if “the big one” was to hit. The scenario describes a magnitude 8.3 earthquake situation- the impact of which is more extensive than once believed. It is expected that more than 3.5 million homes could be affected- which could cost an estimated $289 billion in repairs.

The uncertainty of when, where, and at what magnitude an earthquake can strike is the most unsettling part about earthquake forecasting. For example, we know that the San Andreas Fault is long overdue for a large earthquake- however we do not know when it is coming. Seismologists can speculate that it will occur within the next 30 years but the truth is that we do not know exactly when or where it will hit. With this in mind, proper preparation to combat the effects of an earthquake of this magnitude to the best of our ability are vital.




September 8, 2017

Private School Seismic Retrofitting in SF

St. Anne’s Catholic School in San Francisco is undergoing seismic retrofitting.

Photo by Valerie Schmalz/Catholic San Francisco

St. Anne’s is the first of the four Catholic schools in San Francisco that will be retrofitted by the end this year.  These projects came in response to an ordinance that was passed in 2014. This ordinance required that all private school buildings must be evaluated in compliance with a “life-safety” standard. In other words, it must be determined as strong enough to allow for everyone in the building to safely exit the building in the case of an earthquake. This ordinance does not “require” the actual retrofit- just the evaluation, but it is a top priority of school officials to complete the retrofit in its entirety. It was signed into law in 2014 by Mayor Ed Lee. The Archdiocese of San Francisco is passionate about the seismic retrofit project for the safety of their students, present and future.

They feel that the seismic retrofit projects are of the utmost importance if these schools are to continue educating children for years to come. Although St. Anne’s was not damaged during the Loma Prieta earthquake in 1989, nor did it experience damage in any other quakes, it doesn’t mean that they weren’t at risk. Had the Loma Prieta quake “zigged instead of zagged” the outcome would have been much different due to the difference between the wooden floors and the concrete walls (

St. Anne’s is very fortunate to be funding this retrofit largely with money that has been accruing from their endowment as well as through loans and funds raised by alumni and parents. It is estimated that the other 24 schools recommended to retrofit will be completed within five to eight years, after they have raised funds and completed the engineering and construction process. To avoid disrupting the student’s and faculty, most of the retrofit work will be done during summer breaks. Many of San Francisco’s Catholic schools were built following the tragic 1906 earthquake in San Francisco- making some of them nearly 90 years old. The city of San Francisco has more private schools than any other majority city in California- many of which are within historic buildings. Private schools and public schools have different seismic safety requirements per the Field Act.  Any building constructed after 1978 is at significantly higher risk, so these retrofits are certainly necessary.



September 1, 2017

Why Choose Penhall?

Why choose Penhall?

A big company means big support,  customer service means everything!


It may be easy to assume that when working with a large company you will not receive the same intimate level of customer service that a smaller company could provide you. However, at Penhall Company, we feel that our growth has not taken away from our ability to interact closely with customers, but rather has added to it.  

We have been in business for 60 years and in that time we have become known as the most trusted provider of concrete cutting, coring, demolition, excavation, highway and bridge services, GPR scanning, X-Ray and seismic retrofit services in North America. Over the years, while our service line has expanded our commitment to excellence and quality customer service has remained the priority of our company.

In our seismic retrofit department, we take our customer’s satisfaction very seriously. There is no project too big or too small for our seismic retrofit team.  Each member of our team serves a pivotal role in your project’s process. Whether it is through our project managers or client support team – we make every effort to provide service that exceeds your expectations.Your questions and concerns are valid and extremely important to us, which is why our team makes themselves available to you around the clock to ensure that your project is taken care of.  

We are proud of our company’s history and are extremely excited to be apart of the seismic retrofit process in California and beyond.  Above all, it is our firm belief that seismic retrofitting is an extremely important process that should only be carried out with the utmost expertise and care- which is why we like to say that we retrofit with a purpose. Every single job we do is important to us and we hope that you will keep Penhall Company Seismic Retrofit Services in mind for your design-build retrofitting needs.

August 25, 2017

The Differences between Ordinary Moment Frames and Special Moment Frames

Due to the latest seismic ordinances passed in Santa Monica, there has been a lot of talk around different seismic solutions and options, moment frames being one of those options.  There are three different types of moment frames: ordinary, intermediate, and special. After reading this short article, it will only take a moment for you to understand the fundamental differences between Ordinary Moment Frames and Special Moment Frames.

Seismic Retrofit building structure

Ordinary Moment Frame- Seismic Retrofit

To begin, a standard moment frame is defined as a system of columns and beams which are connected by either fully or partially restrained moment connections.  As one might expect based on its name, “ordinary” moment frames, one of the three types of moment frames,  are generally found in areas described as non/low-seismic regions. The expectation is that ordinary moment frames are able to endure limited inelastic deformations caused by lateral forces. Ordinary moment frames only require onsite welding, which differs from the requirements for special moment frames. Special Moment Frames, another type of moment frame, are “special” in that they are expected to endure more significant inelastic deformation than ordinary moment frames. In addition, Special Moment Frames utilize different connections than Ordinary Moment Frames and they do not require field welding. Generally even with on-site welding the cost of Ordinary Moment Frames won’t go over that of a Special Moment Frame. These connections must be prequalified, as stipulated by the American Institute of Steel Construction.

Since Special Moment Frames can endure more deformation, they are often found in areas described as mid to high-seismic regions. Seismic retrofits constructed utilizing moment frames can sometimes be more costly and time consuming because moment frames often require more welding on site than other frames do. Traditionally, special moment frames are more expensive than ordinary moment frames because these types of frames are often found in mid/high-seismic regions. Your engineer may decide whether or not moment frames are right for your particular project but they are preferred under certain circumstances. If your building includes large open areas within your structure, such as a tuck-under parking garage, moment frames would often be the building solution that is preferred.  Considering the latest ordinances regarding soft story seismic retrofits, the Santa Monica area will be seeing moment frames of all types over the course of the next five years.

Seismic Retrofit Structure to decrease earthquake damage

Photo courtesy of Hardy Frame by MiTek

For more helpful information on seismic retrofit services that will fit your individual projects needs, keep an eye out for more blog postings on our website-

References inlcude: (by Dylan Richard and Walter Moore) 

August 4, 2017

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