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.

 

[TigerGuy]

If the material in question is several meters down and under a simple pad, does it really matter? How far below structural load is this zone? How large are the loads and load footprints? I've encountered "weight of hammer" blow counts many times. They rarely affect the project design and construction, maybe a delay to allow for settlement, but that's it.

We need more information.

 

[Francisco A.]

I would assign a friction angle of 28 degrees to that material, which is the minimum angle for loose sand, and run a bearing capacity analysis considering this strength property.

 

[MTNClimber]

Typically settlement controls allowable bearing pressures. If the foundation loads are provided by the structural engineer, you can size the foundation, run the settlement analysis to determine an allowable bearing pressure. Just note that settlement analysis can be hit or miss. It's always a good approach to provide a conservative value.

 

[ukengineer58]

Hi all. Thanks for responses so far. Sorry been offline. So this case, it’s very lightly loaded equipment slab. Settlement
Isn’t a really a concern specifically. Pressure applied at ground only around 30kpi. The zero spt is about 5-6m Down. I don’t really have huge concerns on this one I suppose. But it was more of general question when we get these readings. For simple things like this we use simple correlation charts, which typically give an increase in stress for 25mm settlement. Albeit we can accept more. And for things like this we may only have a borehole with spt readings.

 

[TigerGuy]

You are a strange person. You actually want to think ahead on this type of situation?

On most sites where I have encountered this condition, the recommendation has been either to surcharge the site and wait for settlement to occur, or to use a deep foundation system or ground improvement. All options come with a cost, that the owner usually does not like.

 

[EireChch]

I agree with Fran, I think 28 deg is the lowest for loose SAND. In this instance you should either do a layered bearing capacity assessment as I assume you have stronger material over weaker material. Or alternatively just assign 28 deg for everything within zone of influence.

Also, MTN is correct, settlement usually governs. If this is a large wide slab then your bearing pressure is likely pretty high. Your bearing pressure which results in 25mm settlement will be your minimum of the 2.

 

[ukengineer58]

Tiger guy. Yes I like to think ahead. Not sure why the judgemental tone. It was just a question. And it would be very strange to surcharge Ground or pile for a small equipment slab where settlement is if little consequence. Each project and circumstance needs to be considered on its own merits does it not?
For others Maybe I needed to be clearer in my question. My question really was assigning parameters from spt tests when we get a reading of zero. Principally in deeper soils. And what approach you to take. They doesn’t involve expensive needless piling or unfeasible ground improvement. Obviously I am not talking about multi-storey buildings here.

 

[MTNClimber]

It depends on the material. What kind of sand? Fine? Fine to coarse? How much silt and clay (if any).

 

[GeoEnvGuy]

Stop assigning a friction angle to N-values below 4 in saturated very loose sands. In the chart below from Peck, where geotechnical engineers are getting 28 degrees, it is not applicable the line ends at very loose. Determine the undrained strength and if this material is susceptible to liquefaction displacements.

 

[ukengineer58]

Hi - not in a seismic area (UK). So how would you go about determining the undrained strength? I assume you are referring to carrying out a shear box test?

 

[TigerGuy]

Uke,

Not judgmental, just sarcastic. We all see too many engineers that don't actually look at the situation in front of them.

 

[MTNClimber]

28 degrees is used in a couple of references, but shouldn't be used blindly. My approach is looking at the material type and assigning a friction angle that makes sense. Silty fine sand will be lower than well-graded fine to coarse sand. Dig through the piles of information on this subject and make an educated decision. UFC 3-22-10 (DM7.1) has some great information to dig through.

 

[SlideRuleEra]

ukengineer58 - Is the soil with zero SPT below the water table? That could make a big difference.

 

[ukengineer58]

Thanks for the info guys! In this specific case it’s not in the water table. But I’m interested in the general thoughts around this subject as well as the specifics for this case. Which is really less of a concern.

 

[GeoEnvGuy]

So how would you go about determining the undrained strength? I assume you are referring to carrying out a shear box test?

Shear box is one way, triaxial testing is another, the issue with lab methods is you can get a shelby tube of silty fine sand but its not truly undisturbed, so you would most likely reconstitute the sample to the void ratio you measured. A field vane with increased rate of rotation is another way in granular soils.

I work with liquefiable silty sand tailings deposits and the tailings industry is starting to understand, due to all the failures of late, that you can't assign a friction angle to saturated very loose granular material. A comparison for the shear strength of granular materials related to relative density is illustrated in the chart below.

 

[SlideRuleEra]

But I’m interested in the general thoughts around this subject...

Two things to consider when working with this type soil:

1) Say a "long" point-bearing piling can support very heavy loads because surrounding soil provides lateral bracing along length of the pile.

Would you consider this pile to be laterally braced for the portion of it's length that passes through a "thick" layer of soil where N = 0?

I would say "no", regardless of that layer's other soil properties.

2) If an underground layer of soil with N=0 was below the water table, it is likely more of an incompressible, viscous pressurized fluid with friction angle essentially zero.

The incompressible pressurization supports TigerGuy's experience that this layer does not cause problems (for reasonable applied loads) except for slow settlement (as increased pressure caused by the applied load reaches equilibrium with soils that contain it).

 

[geotechguy1]

You could push a CPT as an option. In New Zealand I regularly deal with sites where SPTs in HQ cored boreholes (they use the rock coring tooling from North America to drill in soil here for Geotech instead of solid stem / hollow stem augers) are zero, but the adjacent CPTs show reasonable strength and stiffness properties in these layers. It's a regular cause of debate. To me, SPTs are a very crude and conservative technique and knowledge of how crude and unreliable those friction angle - SPT correlations are and how dubious some of the datasets are used to derive them seems to be lost. The industry it seems has played a hundred year game of telephone with these correlations.

 

[EireChch]

If I have a PSD that confirms that the material is granular, i.e less than 30% fines and Atterberg's confirm that the little fines there is are plotting as a silt and not a highly plastic CLAY, then I blindly assign 28 degs. My reasoning is as per the graphs above, many of the greats did not include lower than 28 deg in their graphs.

MTN your reference shows for an SPT of 1 in rounded uniform sand, Dunham would give a phi of 23 degress = (12*1)^0.5+20. Do you honestly believe that?

GG1 nailed it, I think the issue really stems from the inadequacy of SPT to determine the strength of very loose material. These materials need CPTs.

 

[ukengineer58]

cheers again all. As usual lots of great information... So GG1 - how would you specify geo surveys in such circumstances? I am referring here to pretty simple schemes, and low risk, so typically there will be a borehole with SPT, and some samples taken in case testing is needed. So would you specify that if the SPT blows are below a certain figure then they then do a CPT? As typically we have one hit at the GI survey on these specific jobs.