SPT of zero 1

[ukengineer58]

Hi Interesting - or not, question. If you had a SPT of zero quoted for loose sands at depth (several metres down but in the influence zone), how would you correlate that to a friction angle or allowable bearing pressure ultimately? its a simple pad, nothing serious on it in our case. How would you go about solving that? As we cant even correct the N value for overburden - but it has several metres of soil above it of more reasonable strength.

 

[MTNClimber]

EireChch - I use what I feel is appropriate in each situation. I have never used a friction angle less than 24 degrees for a sand in any of my analyses. Maybe I should explore that topic more, but honestly the resistance that I obtain from that layer is usually nothing compared to what I’m getting from everything else and is not worth worrying about.

I don’t think anyone does a great job addressing this, in text or in this forum (no one has specifically given a general rule of thumb), for a reason! It’s hard to nail down an actual value that you can argue is even likely 80% correct. Those formulas are all over the place. All you can is play with the value, see how it effects your analysis, evaluate it’s weight in your design, and make an educated decision.

 

[geotechguy1]

The NZ market is sort of unique because the HQ coring for soil is the normal borehole drilling technique (I'm honestly not certain why this is - maybe it's an offshoot of the canterbury earthquake or because the industry here is geologist / engineering geologist dominanted or perhaps because there are so many fiddly soils here that continuous coring is viewed as justifiable). This means that one 15m borehole costs $5000-$7000 NZD whereas a 20m CPT might cost $500-$700 / CPT if you do 5 - 7 of them. A common site investigation approach for a normal sized commercial development say might be one borehole per 5 to 7 CPTs and a scattering of scalas and hand augers. As opposed to Canada - Unfortunately I didn't write down any pricing but from memory I'm reasonably sure we might have done something like 5x9m boreholes for the same type of job back in Canada using a solid stem auger technique at a cost of 1k-1.5k a hole.