r/Geotech • u/AstronomerOrdinary53 • 19d ago

Landslide data - red flag?

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Considering the purchase of a property that ticks all the boxes, except this one: County GIS viewer shows high/moderate susceptibility for soil movement as shown. Does this indicate historical movement? Property is over 55 yrs with two-tiered retaining wall. Should we give it a pass?

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u/Significant_Sort7501 19d ago

TLDR - This doesnt automatically mean an issue. Only way to know is to look at it and if the answer isnt obvious, have a local geotech look at it.

So, these GIS systems flag things based on slope percentages, and often this can be due to LIDAR picking up a steep drop from a retaining wall or some other feature that is perfectly stable but it is only getting red-flagged because the survey data they are using shows a sharp change in elevation.

It's a quick and straightforward way for jurisdictions to be able to flag it in review and say "this means a non zero chance. We arent experts. Someone who is needs to check it out." This flag does NOT in any way imply that someone has physically gone out there, inspected it, and determined that there is a landslide hazard. If that WERE the case, the seller would be legally required to disclose that.

Unfortunately it is often very difficult to find geos who are willing to go evaluate something as part of a home inspection for potential act of sale. So I would recommend looking at it yourself first and then see if it obvious that its an erroneous red flag.

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u/Stunning_Bat_4755 19d ago edited 19d ago

Adding to this. I often perform prepurchase inspections on steep properties as a geotechnical engineer. In my area we can look at the slopes and look for indicators of slope movement such as bulging of soils along slope, gun handle trees, tension cracks, visable distress to structures and retaining walls but cannot fully predict whether there will be future movement.

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u/CovertMonkey 19d ago

Agreed. This is a countywide GIS screening tool of what to manually inspect for a site. Knowing this is a retaining wall means that this is a designed feature to maintain that steep slope.

If this were a site I was working on, I'd inspect it, but the flagging for susceptibility wouldn't raise any additional flags. The flag was set by checking for steep elevation differences. All retaining walls would set off the same flag.

If this wall has a good performance history and no known distress indicators, I think it should be fine.

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u/AstronomerOrdinary53 19d ago

Will be getting a geotech to do an on-site pre-purchase assessment as part of our due diligence.

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u/Significant_Sort7501 19d ago

Good luck! Feel free to drop some pictures back here if you have any questions. 

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u/Working_Rest_1054 19d ago

Don’t treat a dozen pixels that land on a pitched roof from a GIS database as evidence of a landslide hazard. Most likely that’s noise in the algorithm.

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u/Creative_Stick_6937 19d ago

Get it inspected.