soil bearing capacity in an area

[schreibs]

I am building a home. TO calculate the foundation base width I need an estimate, at least, of the soil bearing capacity. The ground is frozen here in Minnesota and the site is not ready. I cannot do an soil penetration testing etc. so I'm wondering where I can get data from some source . . . FOR bearing capacity in the area from tests done by others building homes. Suggestions on who or what organization to call? sandy soil, but I can get the official type also. (easy guys, I'm a mech engineer). THANKS!

 

[Doctormo]

I do not think house construction has bearing capacity problems for the most part. Loads are low so a classical bearing capacity failure is quite rare. Minimum footing widths probably control the standard recommendations more than any calculation.

Settlement is the principal issue and the presence of undocumented fill where new homes are proposed is quite common in the vast swamp land called Minnesota. I would focus on that and get a local soils engineer to evaluate the soils when the basement/foundations are dug. Boring are nice also to see what is under the home location prior to starting any excavation. You could be lucky and be in an all sand area north of the Twin Cities or in a man made level lot with clay fill next to a wetland.

 

[dicksewerrat]

See if the county has well drilling logs. If there is a well near your property the soil profile may be there. Talk to the well drillers and contractors in the area.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[schreibs]

Thanks Guys. This morning I called the local Menard's building estimator and he agreed I could likely use an 8 x 16 footer based on home plans he has seen. Suggested an 8 x 20 if brick veneer face. I drew in 8x16 for now even though brick face. I will refine once I get the septic boring and consult with a soils engineer. It is really sandy soil, so probably no problem at all. As usual, I'm overthinking the problem! Plus, the foundation contractor will have an opinion. . . thanks again.

 

[oldestguy]

I'd call the local building inspector. You probably will have to get his approval of your plans to then get a building permit. Might as well call him before the plans are drawn to avoid wasted time. These standard usually require PE to justify any change, such as reducing the wall thickness from their "standards'. The verifying that the footing standard is OK and that the site meets it, using a geotech is a good idea, especially if the excavation runs into rock of any kind. I have seen the excavation of rock leave loose conditions that later resulted in problems. Watch for loose backfill to utility trenches also.

 

[schreibs]

thanks Old Guy. The Building Inspector was the first call I made. He was out and I went off on a tangent, this website being one of them! I will call him, you bet. Also, since I am not an architect or structural engineer I plan to have one review my plans and esp the I joist and LVL choices not to mention resolving the footing. However, I sure don't want to put in a WRONG footing and then be required to get a PE to sign off for a simple change.

Can you please expand on your point about watching for loose backfill in utility trenches? thanks.

 

[dicksewerrat]

All utility trenches are a little softer than virgin soils. Even 30 to 50 years after the trench is dug. As you dig the new trench and cross the old one, the soils will cave in at a different angle than virgin soil. This is one way to find the other utility without ripping it out. You will also see a difference in color and makeup of soils in the old trench. But you have to call Gopher State One Call before you dig and most buried utilities will be located.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Doctormo]

I will reiterate my point about undocumented fill on sites. Utility trench backfill is just a subset of this. There are very few virgin sites left in metro areas where one can safely assume untouched foundation conditions. It is far too common to dig for a foundation and find a bunch of buried organics, rubble, or ash that requires removal and replacement prior to placing foundations. The problem is not discovering the unsuitable material when excavating as much as it can wreck the budget before the first nail is pounded. It is not always possible to discover this situation with a couple of borings as you have to be lucky (or skilled) when you locate them.

One good thing in MN is that footings will be relatively deep so the surficial soils will not be a major problem. However, it is common to find poor soil conditions at frost depth as the result of someone filling in a hole years ago or leveling a sloping lot.

 

[oldestguy]

The comment about building where there has been disturbance as at old former buildings, dumps, and such is important. However if this just a farm field and no significant site grading for a development, probably not a problem.

As to what to do about that utility trench, well probably nothing needed. If it is typically only a few feet deeper than the footing grade and maybe bucket width, the wall will span it. The footing will do a little, but mainly the wall is the "beam".

I like to put some steel Re-bar inte wall mainly to control an possible cracks that occur later (concrete shrinks you know). I have them add at lest one re bar top and another ner the bottom of wall. Re-bar in footing helps, but beam depth is so much ess than the wall.

I like to check that the foundation soil si firm using a1/2" rod shoved in by hand. If it goes easy, that is bad news. Having a local geotech to check this is good insurance however.

 

[oldestguy]

OG again. Wife pestered me to get going for a walk so I didn't get a chance to edit the post above. How do you find a geotech engineer? Yellow pages for foundation engineer, soil borings, etc. I'd ask for a site visit before digging and another at time excavation is open and they are getting ready to place footing Concrete. I doubt that test borings are needed unless you really have a difficult site, such as near a swamp where site may be underlain by peat. That's where the first site visit comes in to decide that move. If the site is really a great one but has bad conditions, then the regular test boring program and geotech analysis is needed, with engineering possibly continuing into the construction phase. We can build on crappy ground, but it takes some careful construction control. Cost can be quite high. If the geotech thinks some sort of site investigation is needed, it may be done with test borings, but many a building site has been checked using test pits dug OUTSIDE the proposed building limits. Don't dig inside since fixing that disturbance then will be needed. Call diggers hot line first.

 

[schreibs]

Hey Guys, thanks for all the input. I will keep a Geo tech engineer in mind but probably won't be in need of one for soil concerns. Here's why. I am building on virgin land NE of Elk River on a hill top covered with 100 year old trees. Nothing ever built on this land in the middle of 73 acres overlooking the Elk River. I DID get hold of the building inspector and he confirmed two things: it is typical to use 2,000 lbf/ft of bearing capacity in this area and my planned 16" footer is just fine.

I spent all day yesterday till dark clearing trees, brush, and thorn bushes-- beat and scratched to hell today! I appreciate all your input! Regards, Brian

 

[oldestguy]

Sounds like the correct move. The only thing I'd watch for is if rock has to be excavated. In cases like that (Wisconsin) I think hand clean up of loosened rock is needed. This is especially needed if any blasting is done. Blasters sometimes loosen deeper than needed.

 

[schreibs]

Noted: Watch out for big rocks. much thanks oldest guy!

 

[oldestguy]

OG once more. I took a look at maps (on line) of Minnesota glacial deposits as well as bedrock and it is a real complicated situation. Bedrock could be very hard igneous or sedimentary which varies. Glaciers dumped a lot of varied material and even left some clay filled lakes. If you excavate boulders, that zone should be backfilled with compacted soil, in layers. For a hill site, I'd be inclined to expect it has shallow bed rock under it.

In glacial country I know of two cases where "a boulder was moved". Later foundations there settled, indicating they didn't replace that zone with compacted soil.

 

[schreibs]

Hey OG, interesting, I have done construction (for work) where I had to build and compact in 4 or 6" lifts, so I will watch closely during excavation and make sure remediation is done correctly. . . Here is the GPS of my home site. 45.344999° -93.691491° from Google Earth. It's about 7 miles NW of Elk River. There is a gravel pit mining operation on the north side of my property. I have not looked it over for boulders but walked it once and don't recall any. . . Generally the land around here is very sandy and gravelly. The forest appears to be nice black humous dirt on top and I was expect gravelly under the top soil. . . but, who knows. home site is about 20 ft higher than lower ag land and 25 ft above the river. The ag field area between home site and river is pretty sandy. Also, not far away is the Sherburne Wildlife refuge.

 

[oldestguy]

It should be great stuff to build on assuming same as gravel pit. If lots of boulders, you would see a disposal pile somewhere, but if not much gravel content, they may even crush them to get that fraction for concrete or road gravel. If you will have a septic tank and tile field, it should infiltrate well.

Sorry, I'm not much into GPS. However, do that Google search for Elk River geology and maps come up with general descriptions of what the map symbols have. There even is a many page pdf file of the glacial geology of that area.

 

[Doctormo]

You should be in a primarily sand area NE of Elk River. The gravel pits should be indicative of that and foundation problems are not typical in that area. However, keep an eye out for fill if the lot was leveled up at one time.

 

[schreibs]

thanks again Guys!

 

[aeoliantexan]

Google Earth indicates your site is about one-third of the way down a 75-foot high hill, and the slope is approximately 2-1/2 horizontal to 1 vertical. I could be wrong if the airphoto is reading the treetops rather than the ground. If not, building on a slope that steep involves various hazards. Both cuts and fills may cause sliding. Filling the low side of the building pad needs proper preparation and compaction. The top few feet of soil may be loosened by creep. Are the tree trunks straight and vertical, or curved from tilting downhill?

Many houses built on sidehills are done wrong and suffer problems that are hard to correct. You need a good geotechnical engineer. Definitely not the cheapest one available.

 

[schreibs]

naaw. I'm building on the top of the hill and, the hill is 20 to 25 ft tall and slopes to the south in 40 or 50 ft of run. Here is a link to a file in Dropbox that gives the Geology. thanks to OG.

https://www.dropbox.com/s/4c3wicjql5rb3sk/Geology of Farm area.png?dl=0

NOTE the area where the hill lies in labelled GC from the website. (

http://conservancy.umn.edu/handle/11299/715)

It appears the house will be sitting about 40 ft North of the southern edge of the Superior Lobe-- ice sheet furthest advance. Here is explanation of the ice sheets:

https://www.dropbox.com/s/92zs7sghxp0mmku/ice sheets.png?dl=0

Here is a sat view of the property, too. With the home placement shown:

https://www.dropbox.com/s/a77dd00h9t3uags/home placement.jpg?dl=0

Really sandy soil beneath the upper humous, black top soil in the forest. Hill is forest, slopes down to grassland.

According to the U of MN website (Oldest Guy guided me to) the bottom of this 20 ft hill is apparently at the interface of where two Pleistocene ice sheets met. Here is the details of the materials on my land on the hill, the Superior Lobe:
Till/sand complex (Pleistocene)—A mixture of sediments including till, ice- contact sand and gravel, and minor lacustrine deposits; modified by subglacial processes. Superior lobe deposits are mixed with, or thinly overlain by, sediments of the Grantsburg sublobe. Topography is collapsed and irregular; abundant elongate ridges.

And the Des Moines Lobe left this stuff:
Sediment deposited by ice of the northwest-source Des Moines lobe—Deposits contain abundant gray, siliceous shale fragments. The till color is variable but typically is yellow-brown where oxidized.

MORE details: Yesterday I was meeting with the gravel pit owner ON SITE and he had a drilling crew mapping out his gravel in the forest just NORTH of me. They found there is about 15 ft of clay covering his 25 ft thick gravel source. On the ground at the bore hole was a big pile of the cleanest, purest sand. Nice and moist, uniform sand. Likely between the clay and the just hit gravel they were pounding into.