Estimating 1 Class - Question 2

[krichard78]

Hello...

I am trying to understand the different types of soil, and how to read a boring log.

I would like to know what has a better bearing material and why???

Light brown medium-fine SAND, trace silt at 9' below surface, at 37 blows per 6 inches.

OR

Light yellow brown coarse-fine SAND, trace silt, trace fine gravel, at 19' below surface , at 129 blow count.

Is the soil at 9' better bearing material than the soil at 19' ft??

 

[BigH]

A simplistic answer: if you remember your introductory soils course, well, a soil with higher N values has, typically, higher allowable bearing pressures - assuming that these N values are representative of the materials. It is denser. Now, a caveat. You said that the second material has trace of fine gravel - depending on the size of the gravel, it could plug the spoon tip and affect N values - or gravel during advancement could affect the N values. This is a major reason for granular soils to have unreasonably high N values. One would look at the N values just above and just below the 19ft level, too. (I haven't gone through all of the "corrections" to N values but the disparity between 37 and 129 wouldn't make any differnce.) - You might found a spread footing at 9 ft (basement, still cheaper than piles, etc) but you wouldn't found one at 19ft so . . .

 

[krichard78]

Hello...

Thank you for your help... I never took an intro to soils course, this is my first night class at Drexel. I just started as an estimator for light gauge metal, GWB, etc. This soil thing is all new to me!!

I am not sure what you mean when you say higher N values, but I get that you are saying it allows a higher bearing pressure.

 

[BigH]

I'd suggest you go to the library at Drexel and borrow an introductory soils book - I'd suggest that it be either Bowles (not that I like it that much but because it is fairly well organized and easy to find things) - or my favourite - Terzaghi and Peck. You will find both in the geotechnical section of the civil engr books.
The N value is the number of blows it takes to drive a 2 inch OD (1-3/8" ID) tube into the ground for 12 inches (actually, you drive it 18 inches but you ignore the first 6). You use a standard weight (140 lbs) dropping 30 inches to provide the driving energy. So, as you can see, the harder it is to drive in the tube (greater number of blows), the denser or harder the soil will be. As indicated, there are other factors that an experienced geotech will take into account - but for the first go, this should be sufficient intro.

 

[krichard78]

Thank you for your help!