I know the Strata but the 'N' Values are more than erratic 2

[Russellccooper]

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?

 

[BigH]

Another interesting SPT query!!

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!

 

[coneboy]

I agree with comment #6 in BigH's posting. The soil layer which you are discussing may occur across a large area, but the relative density and SPT values can vary dramatically. Even within a small site it is quite common to get large variations in the relative density.

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

 

[Focht3]

Pay attention to the drilling equipment that was used. Hollow stem augers and waterbearing sands can combine to give decreasing SPT values with depth - the sands are going "quick" when the driller pulls the center plug. Plot the SPT results with depth for the entire site. If HSA was used and you see decreasing SPT with depth, don't blame the soils - blame the drilling method.

 

[BigH]

The French Canadian Drillers I used always referred to the "quick" sands as "live" sands. They always say that you can keep HSA (or casing) filled with water, but it is always easier said than done. Pulling out plug about same size as HSA inner dia doesn't leave much room for water to go around. Agree that sands may loosen with drilling method. One consideration is to drive the 2inch dynamic cone (same energy as SPT - if live sands are a problem, you can get good idea of constantcy of sands with this method - of course, those with the money will go with piezo-cone.

Best regards and great New Years to all.

 

[erich]

I agree with BigH's first post we need to know how the sand is poorly graded. Uniformly fine, medium, coarse or gap graded with gravels. Also you should make sure the ground water level is truly known, the SPT values can plunge at or near the water level.
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

 

[Focht3]

Along the blame the drilling method vs driller line of discussion: I did not mean to let the driller off the hook, by any means. After all, the holes don't drill themselves!

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.

 

[jdmm]

All of these uncertanties can be resolved by not using the SPT and using the CPT. Within a single geologic unit densities ranging from 7 to 70 blows usually mean poor testing practices. It is likely that the low blow counts represent slough in the hole and the high blow counts undisturbed material. In a value added project in Washington about 15 years ago we used the CPT to show that SPT values were meaningless. The site was underlain by sand with artesion pressure that caused even boreholes filled with sand to boil at the bottom of the hole. As a result the blow counts from the SPT were very low. The CPT confirmed extremely dense sand. For the cost involved I would suggest a CPT adjacent to a SPT hole to confirm the uniformity of the strength.

 

[Focht3]

CPT will work provided:

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.

 

[coneboy]

To Focht3


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

 

[Focht3]

The problem had to do with weak clays - quite common in Louisiana. As I recall, the problem began with S_u of less than 400 psf (19 kPa.) Correcting the problem would have required a complete redesign of the cone tip. It had nothing to do with the rig, electronics, etc.

 

[AZdinak]

Referencing the original post, you should expect heterogenous nature of subsurface conditions even if for a "stratum" that is "geologically" the same. We commonly complete designs for embankments and foundation elements in areas that may have N-value ranges from WOH to refusal. Many designers still approach situations like this using a "average" value approach, or try to account for minor variations by correcting the exploration approach utilized to characterize the subsurface. However, we have found that using fairly simple methods of relability analysis, we get a much better handle on the "important" parameters and what their variability really means. For example, we completed DMT testing in an area of residual silty sands and were tasked with slope design. The interpreted effective friction angles ranged from 28 degrees to nearly 40 degrees. Instead of running our slope stability analyses with a phi of 28 degrees, we varied all of the parameters that went into our analyses (stengths, unit weights, depth to water, etc). We concluded that the governing factor of slope stability was not the strength of the soils, but in fact the depth to water in the slope (imagine that). This influenced our solution to focus on controling the water in the slope and not what was the "correct" strength of the soil. Reliablity methods require about 5% more work and yield substantially more information than more traditional approaches. Zdinak

 

[BigH]

Zdinak: Your sensitivity approach is a good one and one that I quite often use. One rule of thumb that I've learnt over the years in slope design (embankments, damns, cuts) is that controlling the water can increase the stability in the order of 25% or so whereas adjustments to geometry (cut down of crest, stabilizing berm, etc. would have a much smaller impact. It is always best to control the phreatic surface as a primary goal.