SPT N - required fundation 1

[Xpres]

Depth (m) Seating Seating N
1m 1 1 2 3 4 2 ?
2m 2 4 5 6 4 7 ?

I have non experiance with SPT, so i need some help. Here are some results from SPT. How can i determine the size of a foundation reguired to suport a column with a load of 500kN?

THX

 

[oldestguy]

It would be my advice not to try it, but to get the assistance of an experienced geotech engineer. Using STP is just a rough guide and several other factors are needed before deciding the foundation size.

However, some writers have given their opinions as to bearing capacity of differnt soils with varying STP figures.

As you can see these numbers are quite rough to begin with, depending on which set you use. It looks like you are in loose soils. That shoud tell you something.

 

[BigH]

Not sure what "seating" refers to - are these "blow counts per 150mm? or what? You determine the N value based on the 2nd and 3rd 150mm blows of a split spoon - the first 150 blows is seating.

In looking at the numbers, though, they may be taking the lead of Tomlinson who a long time ago suggested that you count the number of blows per each 75mm - presuming this to be the case (in that you give two seating blow count values) then the first two would be the seating (2x75=150) and the second pair is the first 150 and the third pair is the second 150 of the 300 - so, if these are blows per 75 mm, then your N value would be 11 at 1m and 22 at 2m. Average these out to say 15; go to Bowles and find the bearing pressure that would be for 25mm of settlement (chart of Size of footing vs allowable bearing pressure for different "N" values. Knowing the allowable bearing pressure, divide the load by it to get the footing area required. Hope your class assignment goes well.

 

[Xpres]

THX BigH. You helped me alot. I hope assigment goes well , I hope, i'm on the right way now.