Necessity of residential Geotechnical Survey + borings

Question

Necessity of residential Geotechnical Survey + borings

Alaskapilot (Aerospace)

(OP)

18 Dec 23 20:45

We're building our house in Alaska and looking for some recommendations on whether it is worth paying for a geotech boring and subsurface investigation report prior to construction versus just using typical design values common to the soils in the area. The purpose of the borings would be to verify the stability of the fill to support foundations and determine the closest possible building setback from the slope. Attached are pictures that show the knoll being built on with topo and slope shading, as well as a test pit that was dug prior to excavation. The building pad was prepared and compacted using gravel from the site, which is composed mostly of gravelly and cobbly glacial till. It was built up about 10' on the south and southwest side in order to level the site. This site is located in Palmer Alaska, so it is subject to extreme freezing temperatures as well as being earthquake prone (seismic category D2).

https://files.engineering.com/getfile.aspx?folder=...
https://files.engineering.com/getfile.aspx?folder=...
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https://files.engineering.com/getfile.aspx?folder=...

https://files.engineering.com/getfile.aspx?folder=04eea173-bbb6-4039-9294-2

dik (Structural) · Answer 1 · 2023-12-18 22:05:03

It is dependent on how sensitive the soil is and if it is consistent in the same area... talk to local contractors or geotekkies to see what you are dealing with. Is permafrost an issue... will it be an issue in 20 years? You need to find out some fundamental info.

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-Dik

GeoEnvGuy (Geotechnical) · Answer 2 · 2023-12-22 18:06:29

Your question is of is it worth it to pay for soil boring and a report. Follow up questions and some cheeky answers happy holidays

Are the houses built in the same geological area performing well? hopefully yes
Is the house builder familiar with the surrounding houses typical foundation designs? the contractor builds most of the houses in the area
Is this in the middle of nowhere where there is no houses to compare to? buts its in the same geologic formation as the closest ones
Does the municipality require a report? No requirements for a individual house of x footprint
Any issues with obtaining home insurance? No issues if the municipality approves the building permit

The soils result will tell you the foundation options based on the results of the borings. You could also just call the engineer and ask them if they think its worth it and what options will be typically proposed for the area if they do the investigation and which one is more conservative. They would be in a better position to explain the risks with the different foundation options in the area.

theCorkster (Geotechnical) · Answer 3 · 2023-12-22 20:45:25

A prime concern to me would be the potential for differential fill thickness beneath the structure and whether post-construction settlement will distress the improvements. Fill slopes constructed without plans that are not monitored during construction leaves the user at the whim of the contractor and gravity. Consult a geotech.

TigerGuy (Geotechnical) · Answer 4 · 2023-12-22 21:55:31

What does the local building department have to say? Will they issue a certificate of occupancy if you don't have this report?

There are neighborhoods where one house performs quite badly, but the contractor next door knew ahead of time what he needed to do when he built his.

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Necessity of residential Geotechnical Survey + borings