Estimating Pile Tip Elevations based on SPT N values 1

[OSUCivlEng]

I am used to reviewing Texas Cone Penetrometer (TCP) values to estimate pile tip elevations. However, I find myself working in a different state and the report I was given used the Standard Penetration Test method and the logs list the blow count every 5 feet (blows/foot). I am using steel H-piles (12x53) that will be driven into weathered shale.

Does anyone have any material I can refer to that discusses how to estimate pile tip elevations based on SPT values? Or a rule of thumb about how well steel piles can penetrate materials based on the SPT blow count. The geotech report recommended elevations show as much as 40' penetration into the shale. That seems a little suspect, but I am also asking them to review as well.

Thanks

 

[EireChch]

I wouldnt use SPT data alone to design piles.

In saying that, there is a method by Meyerhoff in Tomlinsons book for capacity based on SPT N.

 

[OSUCivlEng]

These are end bearing piles, that will bear in shale. There will be plenty of capacity. I'm not trying to design the piles, just estimate a pile tip elevation based on the N values.

 

[PEinc]

The piles would probably hit refusal just a couple of feet below where the weathered rock has N = 50 blows per couple of inches.
Run a wave equation drivability analysis. Model the soil layers and properties and enter a specific pile hammer.

http://www.PeirceEngineering.com

 

[MTNClimber]

What kind of blow counts were they getting in the shale?

 

[OSUCivlEng]

The SPT values were recorded every 5 feet.

The first boring was 43, 48, 62, 60, 60, 50, 50/5". The pile tip elevation was estimated to be at the 50/5" value.

The second boring was 32, 61, 44, 50/4", 70, 50/5", 50, 32, 50/4", 50/1". The pile tip was estimated to be 5 feet below the last value, which was also the bottom of the boring.

The third boring was 18, 33, 37, 62, 50/6", 50/3", 50/4". Then they proceeded to core the shale and reported RQD values of 97 and 82. They estimated the pile tip to be in the middle of the two corings.

The fourth boring was 17, 70, 18, 50/3", 50/5.5". They estimated the pile tip elevation to be at the bottom of the boring which is the 50/5/5" value.

I don't think the pile tips are going to be 20' to 30' below SPT values of 60. The geotech says they are being conservative in estimating pile lengths.

 

[SlideRuleEra]

Agree with PEinc, a couple of feet, or so, into weathered rock.

If the project uses a large quantity of piling, taking time to drive an index pile will provide info on necessary pile tip elevation before finalizing pile length.

http://www.SlideRuleEra.net

 

[PEinc]

The pile tip penetration will also depend on the capacity to which the piles need to be driven and on the pile hammer that is used. From the N values you noted, it might be possible that the piles will be driven to their required resistance long before hitting the 50 blows per couple inch zone. Again, do a drivability analysis with GRL WEAP. It will tell you the pile's ultimate capacity at different driven depths.

https://www.grlengineers.com/services/grlweap/

http://www.PeirceEngineering.com

 

[SlideRuleEra]

In those kind of driving conditions, did the geotech recommend shoes for the HP 12x53?

IMHO, a second opinion from another geotech is warranted. There such a thing as being conservative, but what this geotech recommends is far beyond ultaconservative.

http://www.SlideRuleEra.net

 

[PEinc]

An HP12x53 is not a particularly beefy pile section. Your N values are very high - either cobbles and boulders, cemented granular soils, or weathered rock. You may need pile points as SRE mentioned or maybe a heavier pile section. What is the required ultimate capacity to which the piles need to be driven? It may be impossible to drive to bedrock without damaging the pile or pile hammer. You might develop the required pile capacity well before hitting "soil" with N = 50/x" or bedrock. You also need to know what number of pile hammer blows per inch will be considered to be pile "refusal." Again, you need a WEAP analysis and need to know the soil and rock properties and elevations, what pile hammer and energy will be used, what size pile, and if there will be pile points. I am surprised that the geotech did not run this analysis and, instead, said to figure driving 40' into shale bedrock. (You need a different geotech.)

http://www.PeirceEngineering.com

 

[MTNClimber]

If there's a lot riding on these estimated pile tip elevations then I would recommend doing an indicator pile program. I've seen HP 12x74's driven 20' into weathered shale. Pile capacities were 300-320 tons based on SLT. The blow counts you provided would make me suspect there could be a lot of variability in pile tip elevations.

A WEAP analysis could help but I've seen them be way off in these cases. I'll also echo SRE and PEinc, these pile should have drive shoes.

 

[OSUCivlEng]

Pile points will definitely be used.

Ultimate (factored) pile loads will be around 75-80 tons for the pier bents, around 55 tons for the abutments.

Now I'm wondering if pilot holes will be needed on one of the bents, as rock is fairly close to the surface and I want at least 15' minimum embedment.

I am not very happy with this particular geotech right now, as I thought his numbers were highly suspect and everything I am reading is confirming that. I'm starting to think these guys give ridiculously conservative piling lengths and think the PDA will figure everything out in the field.

 

[PEinc]

You will learn more talking to the company that will do the PDA than you will talking to the current geotech.

http://www.PeirceEngineering.com