SPT's & Groundwater 1

[Ihatelawyers]

Hi,
My first boss (about 30 years ago) pointed out his observation that SPT values drop significantly in immediate proximity to the GWS only to increase again on either side. (For instance N values of 10, 5, and 12 at depths of 1, 6, and 11 (respectively) with groundwater around 6 feet)

Well ... I've been watching and have observed the same behavior ... anyone else notice this behavior? any ideas as to cause?

When performing companion DCP testing (which I don't always do ... so I don't have an exhaustive comparison library) ... a corresponding drop in blow counts is rarely there (in fact ... I can't remember when it was).

Which brings me to the meatier question ... if the drop is noticeable in SPT, but not DCP ... would you consider the SPT drop in a liquefaction assessment? Or would you be more inclined to assume it's a localize anomaly due to gw and assume N values consistent with those above and below?

I think I would be more inclined to ignore the anomaly ... hmmmmm ... but I'm only right until I'm not.

Thanks in advance.

 

[GeoPaveTraffic]

One question for you about running the SPTs.

How is the boring being advanced as it approaches groundwater? Hollow stem, flight auger, mud rotary?

Mike Lambert

 

[fattdad]

yes, it's the drilling method that causes this observation (typically). If you are using hollow-stem augers in certain sands, you'll affect the soil density by the drilling method. Pulling the plug, suction, critical gradients, and other factors will influence the density of the sand below the auger bit.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ihatelawyers]

Hi,
Thank you... it's typically hollow stem ...

I appreciate the potential 'disturbance' factor ... however ... I can't understand why the disturbance would be so critical at the GWS interface and not so critical in the sample immediately beneath it (sampling interval ~ 2 1/2 feet). I would expect the mechanism driving the behavior to extend deeper than just at the gws (assuming all the caveats ... same soil, same deposition, same etc.).

I have two hypotheses:
1 - GWS fluctuations over the years removes the fines from the interval, leaving a higher void structure that would be more sensitive to the dynamics of driving the SPT...(not readily discernible by adjacent gradations - maybe hydrometer would identify - but that gets to be an expensive 'what if?').

2 - The nonsaturated zone allows for a 'quickening' of the soils to occur at the interface, whereas deeper would it would take more cycles because of the surrounding saturated soils (kind of like in an eq ... where we need a certain number of cycles before the soils become quick).

Anyway ... I was just wondering if anyone else had noticed a similar behavior pattern ... or am I just seeing what I was cautioned to look out for?

Thank you!

 

[oldestguy]

You can get reduced blows count if you merely advance the spoon as the drilling method. I put the reason in the category of effects from the submerged unit weight, likely affecting the intergranular stress, even perhaps increasing the pore water pressure, due to the displacement effect of the spoon. I don't see the effect in clays as noticeable, if at all. So, using SPT numbers alone can be meaningless.

 

[GeoPaveTraffic]

In my experience, if you drill with mud rotary technquics; you don't see any effect around the groundwater level.

I can't say why you aren't seeing it a few feet below be the goundwater level.

If you care about SPT values, the best practice is to switch to mud rotary before you get to the groundwater.

When I'm reviewing reports, any SPT taken in hollow stem augers below the groundwater is questionable.

Mike Lambert