top soil bearing capacity 7

[gmcnair]

I have designed an 8' high, concrete block wind blocking wall for a client on 24" of dark, undistrubed top soil that lays on a damp, gray clay layer. The footing is 4' wide and will go 16" below grade. The footing has all the necessary steel and a building permit has been issued.

I dug down 24" with post hole diggers, and the soil is a firm dark loamy, top soil. My reference books say this soil should bear at least 2000 lbs. psf.

A local fellow with some engineering experience told the client that the soil will bear 500 pounds per square foot after he pushed a 3/8" rod down about 20" by hand. He said the 4' footing needs 4' deep holes dug to bed rock and flled with concrete.

Does this sound right?


gmcnair

 

[Ron]

What the "local fellow with some engineering experience" did was not appropriate to determine the "bearing capacity" of the soil. His "quick and dirty" method assesses the consistency of the near surface soil, and to some degree its compaction.

I suspect you have a compaction issue; however, if you have a high percentage of organic material in this topsoil (greater than about 5% by weight), then you will likely get some settlement of the soil from decay.

500 psf is a very low bearing capacity, usually indicative of very soft clays or highly organic silts.

Consult with a local geotechnical consultant. It will be worth it.

 

[gmcnair]

Dear Ron:

Thanks for the reply. The owner sees me as the expert since I'm a landscape architect. The soil appears to be normal topsoil with a small percentage of organic material that is visable. 5% organic by weight would be pretty visable with a hand lense since dried wood and bark is light compared to pebbles. It hard to justify geotects on this small wall that is 8' high by 75 feet long in a u shaped layout. But then again, my name isn't going on the project if it looks too questionable.

If the wall evenly settled 1/2" and the footing was super strong would that mean the wall would tilt or just go down evenly if the soil is homogenous?

What about me doing a timed load test with a four legged table like stand with four 1 1/2" legs? Each leg would be loaded with an equivalent 2000lbs by placing 125 lbs in the middle of the table. The legs would be level at 16" below grade. The test would see how much settlement occured in 24 hours. If it looks bad then the owner hires the geotech.

thanks......George

 

[dicksewerrat]

I'd wait at least through one rain event. Or saturate the soil first.

 

[BigH]

gmcnair - first off, as a matter of course (unless you carry out extensive testing and know what you are doing), I wouldn't put a foundation on topsoil. You have indicated 24inches of it and yet designed the footing to go down only 16 inches. Why no have put the footing down the 24 inches - and then, what is the frost depth of your site? This needs to be addressed as well. You've indicated the underlying material to be a damp grey clay. The greyness tells me that it is below the groundwater level at the site - as a general rule for if clays were above they would be brownish in colour or desiccated. Now the grey clay may be pretty good stuff or pretty poor stuff. I have "damp grey clay" on one of my sites with only 300psf undrained shear strength. I've had other sites where "damp grey clay" has undrained shear strengths greater than 1000 psf. Your friend pushed his rod through the topsoil - what do you expect other than easy pushing? The point is, that you:
(1) have to provide for frost protection (3 to 4 ft unless you are below mid-level USA - can be confirmed with your local building department or through general textbooks (viz., Bowles);
(2) you need to have proper bearing values determined of the material that your foundation will sit on - and for a four foot wide footing, you should confirm to depths of about 3 to 4 ft. below the footing founding level.
(3) you need to ensure that you have some embedment of the footing - topsoil will not give you much, if any lateral embedment protection. A footing founded at ground surface has better chance of tilting than one embedded.

If it were I, I would suggest you get some labourers - young kids - have then dig a hole about 1m by 1m to the top of the "damp grey clay". Carefully trim to this level. Then you can use the "thumb" test to see if the material is "soft", "firm" or "stiff". (this info is also in OSHA excavation literature). Then, I would dig down another foot and repeat. Do this to about 3 feet into the "damp grey clay." If the "thumb tests shows you are in firm soil, then with Su in the order of 700psf, you should be able to get 1500psf bearing pressure. But, if you are not comfortable in this - by all means get a geotechnical engineer involved. Remember your reputation is on the line.

Are you a PE?? How did this get by the building department for a building permit without a proper estimation of the soil's bearing ability? If you are not a PE, how can you design the wall? You see all the time where engineer's are brought up to task because of things like this - and if you are not a PE, be very careful.

 

[GeoPaveTraffic]

gmcnair,

You obviously do not have the experience to design a retaining wall and should not be doing it, PE or landscape architect or whatever. You are not an expert and should not allow your client to view you as one.

Hire an engineer to review the soild conditions and provide a properly designed retaining wall.

 

[BigH]

GeoPaveTraffic - I agree but I think that this is just a wind-break, not a retaining wall. Still, it has loads and sits on a foundation. One more point to the originator in addition to those made earlier - did he take into account wind loading?

 

[GeoPaveTraffic]

BigH,

You are correct, I miss read the post. The loads will still be significant, but not as much as I was assuming.

 

[connect2]

wouldn't build anything on topsoil, especially a masonry wall, they really won't tolerate differential settlement very well.

 

[Cthornton]

I agree with GeoPaveTraffic 100%

gcmnair you need some help - find a PE who is willing to give you a hand / advice.

 

[cbosy]

I wouldn't put a foundation on any soil with over 3% organics.

 

[gmcnair]

Thank-you for the help people. I will hire a geotechnical engineer for this project. The site plan is done so he can calculate the footing.

Here in Oregon, the frost depth is 12" and most of our soils out here are a sandy loam topsoil. Two story homes require a 15" wide footing 12" deep on this type of soil. I did a test with a jar, soil and water, and 40% of the soil is sand, 40% silt, 20% clay and very little floating organic material. My chart claims clay will hold 2000lbs/sf and alluvium-silt 5000lbs/sf. Some of your comments about topsoil and clays have really shaken my confidence in my charts.

Thanks for the help.

 

[Riggly]

gmc,
You have to be careful with these charts. There must be other conditions upon which the stated capacities of the clay was mentioned; soil behavior varies widely - to say the clay soil will support 2000psf is very superficial. A site-specific evaluation of the soil is the most reliable. Aparently, you are not very comfortable with the condition at the site because of the soil conditions, and as you were advised already, seek help from a local geotechnical engineer.