IRC require mitgation of settlement due to liquefaction?

[DM2222]

To my knowledge, the word "liquefaction" is not in the IRC. In my area, Geotech investigations are not required unless the building official requires it. Most homes are built without one unless the owner desires to have one completed. If one is completed, it is usually because of concerns about poor soils on low-lying lots. So it is standard practice for engineers to design homes without a Geotech report using a presumptive bearing capacity. My concern is when a Geotech report is completed and it reveals potential large settlements from liquefaction, but otherwise suitable soils for shallow foundations. I know there is a lot of subjectivity with liquefaction settlement estimates, and there never seems to be a consensus on it. (I'm not a Geotech - I'm structural) But my question is: Although the code does not require the report, once the report is completed, is the owner forced to mitigate settlement? It seems to be the normal opinion among engineers that the owner may accept the risk of damage from liquefaction if they want. I honestly think this is reasonable given the costs and relative risks associated with potential settlement in a potential earthquake. (I am sure many may disagree, but I have some reasoning for that opinion) But aside from seeming reasonable: Do you know of any code requirements that support or debunk this practice of "accepting risk" and moving on?

I know the conservative approach is to require some type of mitigation whether through ground modification, deep foundations or strengthened foundations. But is seems that more and more (most) of the Geotech reports are including large estimates of potential settlements, and taking that conservative of an approach would "kill" the viability of most projects. My point is that if a Geotech report were completed on every house I would suppose (based on the reports I see) that a majority would require mitigation. My concern is that owners forgo a Geotech investigation because of fears of having to mitigate for liquefaction, and end up building on a lot with underlying poor soils, and the house sinks. Gravity works 100% of the time, whether we have an earthquake every 100 years or not.

Thoughts?

 

[MTNClimber]

It's my opinion that it's up to the engineer to make the owner aware of the risk, even if the IRC doesn't ask for it, and it's up to the owner to accept or mitigate the risk. That's how things are handled in Colorado. Geotechnical reports are common for Colorado homes. Their are several geologic hazards throughout the state including collapse-prone soil, expansive soil and bedrock, sinkholes from water dissolvable deposits, mine collapses, landslides, avalanches, debris flow, and more. Owners are constantly accepting or mitigating risk. People will build trophy homes within a potential avalanche slide path and certain towns will allow it as long as the owner has signed off on the accepted risk. Developers will install deep foundations for homes or even excavate 18 feet of soil beneath lots to mitigate expansive soil.

As a geotechnical engineer, I'm getting paid to make the owner aware of the potential issues but not always requiring them to protect themselves from an event that may or may not happen. I'd require mitigation if the risk is pretty certain and their will be high probability of an issue. Liquefaction may or may not happen. The IRC may not require deep foundations for liquifiable soils but I'll be sure to tell the owner that it's a possible hazard and leave it up to them to decide.

 

[DM2222]

I agree that is a reasonable way to manage the risk, and seems to be the norm to let the owner accept the risk when it is brought up. I just don't know what literature, code, guidance etc. to point to that allows the owner to choose not to mitigate a structural failure. Thank you for your comments and time!

 

[GeoEnvGuy]

I work mostly regarding liquefaction to dams but i thought i might share this photo showing the possible effects of liquefaction to structures.

The building on the left is on piles, while the building on the right settled due to liquefaction. Left building probably would have less internal damage but require work to match the new street grade and possibly severed utility connections. Building on the right may have gotten lucky or had internal damage or could have tilted.

Liquefaction effects vary widely according to the material uniformity, relative density and the size of the earthquake. It is costly to assess liquefaction and even more so to design around it. For houses the footprint is small and the bearing pressures are pretty small and uniform. Liquefaction settlements should be more uniform over the larger region.

Unless regional studies are undertaken by the developers or municipality to provide blanket mitigation over an area investigated, I don't think house foundations will ever be designed for liquefaction. But it is still the geotechnical engineers job to warn the house designer of the possible risks.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.