I know the Strata but the 'N' Values are more than erratic
I know the Strata but the 'N' Values are more than erratic
(OP)
I am carrying out a design for work in the louisville area and have begun to examine soil reports however the strata that I seem to encounter is the same the 'N' values vary anywhere from 7 to 70 for a poorly graded sand. Is there a method other than just averages that can give me a reliable figure to base my design on?





RE: I know the Strata but the 'N' Values are more than erratic
First off, please provide some added information, if possible:
1. Nature of building/structure to be designed. Heavy loads, light loads, settlement sensitive, etc.
2. Surely there must be some layerings whereby certain zones appear to be looser or denser than others. Is there any reasonable stratigraphy?
3. Where is the groundwater?
4. Are the sands stratified? This might have important implications in any open cut below the water table (e.g., deep sewer lines.)
5. You indicate that you are reviewing several soil reports - I take it that you do not have site specific information?? If you don't; you should consider using the available information to formulate a well planned geo-investigation and then undertake it.
That's it for tonight!
RE: I know the Strata but the 'N' Values are more than erratic
A site specific investigation would be the best approach to determine if the soils underlying your site are closer to the 7 or 70 SPT blows.
One item which may narrow your 7-70 spread is the SPT energy values and the type of drop hammer which was used.
Most SPT research papers cover drop hammer types and the related energys as well as the factors affecting SPT values.
Best of luck
Coneboy
RE: I know the Strata but the 'N' Values are more than erratic
RE: I know the Strata but the 'N' Values are more than erratic
Best regards and great New Years to all.
RE: I know the Strata but the 'N' Values are more than erratic
In reference to the idea of keeping a head of water in HSA, I prefer to just switch to casing if the water is relatively shallow.
On the topic of blaming the soils vs the drilling method, one more to blame is the driller/inspector combination. Many driller's in my experience will try to stretch the requirements for sampling.
Erich
RE: I know the Strata but the 'N' Values are more than erratic
I've tried adding water nside the HSA to offset the piezometric head by maintaining the fluid level at the top of the auger. The result? Only limited success: I now only use wash rotary in water-bearing sands.
I recently agreed to hire a contract driller who claimed that he had the "right" equipment after I told him what I needed. He showed up with HSA instead of flight auger equipment; when I (politely) asked him why he pulled the "bait and switch" on me, he told me that he knew what I needed better than I did! Needless to say, I kicked them off the job forthwith. (I had a similar experience with the same drilling company 10 years ago, and had decided that it was time to give them another chance. Guess I'll have to wait 20 years this time!)
As a side note: all of the (other) geotechnical drillers in our area primarily use flight augers for geotechnical drilling. Water is generally deep, and we don't have many "running" soils or borehole collapse problems in the local geology. This driller seems to have been doing environmental drilling exclusively for the last five years, and only has one rig (out of six) equipped with flight auger.
RE: I know the Strata but the 'N' Values are more than erratic
RE: I know the Strata but the 'N' Values are more than erratic
1. It is locally available.
2. The rig is heavy enough (assuming the sand is dense and/or has some fine gravels within the profile.)
3. It is a static, not dynamic, cone (pushed, not driven.)
4. The cone tip is appropriate for the strength range to be tested. (The Louisiana Highway Department (through a research project at LSU) had problems with Hogentogler cone results about 15 years ago. I don't know whether Hogentogler redesigned their cone tips to address the issue.)
However, CPT is not a complete substitute for soil borings. If you are going to use CPT, you will still need standard borings.
RE: I know the Strata but the 'N' Values are more than erratic
I saw a Hogentogler Electronic cone testing system in action last year. What ever problems/limitation you saw have obviously been cleared up as the equipment I observed was very rugged and had no problems penetrating to depths of over 100 feet. The sands ranged from 10 to SPT blow count equivalents. Of course, it dosen't matter how strong or reliable the CPT equipment is if the drill rig or pushing truck is not up to par.
Coneboy
RE: I know the Strata but the 'N' Values are more than erratic
RE: I know the Strata but the 'N' Values are more than erratic
Zdinak
RE: I know the Strata but the 'N' Values are more than erratic