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January 12, 2019

Structures on Helical Piles Withstand Alaska Earthquakes

On November 30, 2018, back-to-back earthquakes and aftershocks of 7.0 and 5.7 magnitudes respectively shook Anchorage, Alaska, damaging roads, public buildings, houses, and caused significant foundation settlement that damaged many structures. What remained solid and strong were structures built on Techno Metal Post (TMP) helical piers.

The following is an explanation of how helical piers can be used to successfully repair these foundations that settled and prevent future damage, but we first need to understand what occurred. In many cases the ground beneath the foundations settled during the earthquake and the two main causes were:

  1. Soil liquefaction – that occurs when a saturated, or partially saturated soil substantially loses strength and stiffness in response to an applied stress (such as shaking during an earthquake or other sudden change in stress conditions), in which material that is ordinarily a solid behaves more like a liquid.
  2. Improper and/or inadequate foundation footing preparation or placement of inadequate fill that is not properly compacted during the original construction.

 

 

As an example of settled foundations and surrounding soils in Anchorage after the Nov 30, 2018 earthquake – in the picture above, notice the settlement expands beyond the footprint of the building, but not beyond the likely area of original excavation.

 

How helical piers resolve this issue and prevent future foundation settlement

Techno Metal Post helical piers are like giant steel screws that are designed, engineered, and tested to the highest quality and standards, thus guaranteeing the durability, security, and sustainability of a structure’s foundation. They are installed by TMP’s certified technicians using proprietary hydraulic machinery. The piers are screwed into the ground until the desired bearing capacity is achieved and then they are considered a “deep foundation.”

The principle behind helical piers is to bypass weak soil and find deeper, more dense soils to adequately support the weight. With TMP’s powerful, compact installation equipment, their certified installers can install helical piers deep into competent soils, passing through weaker soils (such as peat, soft clay, or fill) that are subject to failure under weight or in events such as earthquakes. The installation into competent soils allows the piers to support heavy loads such as a house. The installation process is engineered, meaning that when piers are installed, the exact weight the pier can support in that particular soil is known. If the pier needs to support more weight, it is simply installed deeper into more dense soils.

How did TMP helical piers perform in the earthquakes on November 30, 2018 and January 24, 2016?

After the November 2018 earthquake, TMP Alaska took elevation measurements of one of its recent foundation projects and saw that the house foundation remained level and did not suffer any settlement during the quake. They also checked similar projects and discovered the same result: structures supported by TMP helical piers were not affected by the quake and remained the same as the day they were installed. TMP Alaska has almost 10,000 helical piers installed throughout the Anchorage and Mat-Su area and have had no reports of failures.

Two-story house intact and the deck piers used also remained stable.

In January 24, 2016, another earthquake (7.1 magnitude) rocked South Central Alaska and was felt as far north as Fairbanks. TMP Alaska has always been confident our helical pier foundations are suitable for earthquake regions, and an informal post-quake survey of TMP Alaska’s completed projects demonstrated this to be true. After the January 24 quake, there were no reports of any issues of settlement or other damage due to the more than 20 seconds of shaking. After inspecting many finished projects, there were no signs of settlement, no pier damage or movement, and no drywall cracking. Several houses and cabins on the Kenai Peninsula remained fully supported by TMP piers.

 

House on helical piers in Big Lake, Alaska.

 

What do we know about helical piers and earthquakes?

While it is known and widely accepted that helical piers preform better than traditional foundations in earthquakes, it was not quite known why. Until recently, helical piers have not undergone extensive testing during seismic events. That changed when the University of Oklahoma’s Amy Cerato Ph.D., P.E. decided to study and test the performance of helical piers subjected to earthquake loads. In her abstract, Dr. Cerato begins by mentioning the 2011 earthquake in Christchurch, New Zealand:

“After the series of earthquakes in 2011, the city of Christchurch was surveyed, and it was found that all buildings/infrastructure constructed on helical piers sustained minimal structural damage, however, a large majority of the condemned buildings were constructed on other foundation types. The international community has qualitative proof that helical piers perform well in earthquake prone areas, but engineers have not quantified “why” those piers are superior foundation elements, and unfortunately, helical pier use in seismically active areas within the United States remains minimal. Therefore, this project seeks to find out ‘why’ helical piers seem to behave so well in seismic regions by subjecting them to earthquake loads in the University of California – San Diego’s Large Shake Table.”

Now, through testing, and “real world” seismic events like the ones in Alaska, Dr. Cerato’s hypothesis continues to be accurately supported: “The international community has qualitative proof that helical piers perform well in earthquake prone areas.” The seismic testing of helical piers was carried out in 2016 at the University of California, San Diego Jacobs School of Engineering, on the largest outdoor shake table in the world. The results of the tests were very positive, demonstrating what we have always believed: that helical piers remain strong and stable during seismic activity. Dr. Cerato’s research continues and her early work indicates that her hypothesis (and TMP’s assumption) is correct, finally helping to explain “why” helical piers perform so well in earthquakes.

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