Skin Friction Pile shall be terminated in which layer?

[ndjiji]

Hi,
if someone could help me to understand this problem better.

1. For example if i have 8m stiff clay with underlying 10m loose sand.(only example)

The pile will be mainly on skin friction. So let say i want to terminate the pile 9m b.e.g.l (including 8m into stiff clay and 1m into loose sand). is it possible? skin friction will be higher but what about the settlement of pile under loading? because if it is rock, it wont settle that much, but because it is in loose sand, the settlement could be higher although the calculated/mobilised skin friction is higher(??). So should the skin friction pile only terminated in stiff/hard/medium dense/dense layer not in loose sand layer?

2. let say on top of the those layers in problem no.1 is 5m soft clays. So, does it make any different for me to terminate the pile in the stiff clay or still can be terminated in the loose sand layer.

TQ

 

[oldestguy]

Is this an actual case or something to think about? I only know of one similar actual case, some 80 feet in sand overlying a buried peat layer where settlement occurred. That's within my 58 years of experience. Very rare.

 

[ndjiji]

Hi oldestguy,

I made up the example. However, i'm sort of calculating a skin friction pile right now. So i need a good insight for my calculation and overall understanding of the skin friction pile. The borehole shows stiff clay overlying the loose sand layer. however, above the stiff clay there's also other layer (sand and clay). Here in my question i wanted to simplify the problem. but still revolving the layer in which we can terminate the skin friction pile.

Can we terminate the skin friction pile in loose sand/soft clay? because i checked the examples in Tomlinson's book, like most piles terminated in at least medium dense sand/stiff or firm clay/.

Help me pls

 

[fattdad]

it takes more strain to mobilize skin friction than end bearing. In some practices, if the pile is terminating on rock (or very dense soil), skin friction would be ignored as there just won't be enough settlement to mobilize the skin friction that you calculate. Now I'm not saying I always agtee wtih this, but it's a point to consider. How to calculate settlement of a pile group is to translate the area of the pile group 67 percent down the axial length, calculating the areal load and using conventional soil consolidation or compression theory.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Chances are out on the job if the computed pile position has it stopping a few feet or so above weaker material (or any material), the disturbance during driving will temporarily show a lower apparent friction capability. Then the pile driving will not stop there (using typical procedures measuring set, etc.). the end result will be a deeper penetration than computed. Write the spec that the tip elevation has to be below XXX elevation to be sure.

 

[ndjiji]

Hey fattdad i read about that too. but sometimes i include the skin friction value as well when the pile set on weak rock with RQD=0%.

Oldestguy, are you implying that i should only stop in stiff clay (stronger layer), but keep in mind the mobilised skin friction will be lower than the computed value?but u dont take set value for skin friction right. once the pile reach the anticipated depth, u will stop the pile driving right?

But still the main question,Is it still possible for the skin friction tip to be terminated in this weaker material?. so let say 8m stiff sand overlying 5m loose sand and i can terminate the pile at 10m below ground level in loose sand. i still don't get this. sorry

My other colleague say we can but never explain to me. personally i feel it should just stop in stronger layer regardless of the overall skin friction is high or not.

 

[Mccoy]

it takes more strain to mobilize skin friction than end bearing.

FD, I actually believed the opposite, but yours is probably an example of very clean bottom hole in rock, which would make up a rigid constraint.

In soils, as far as I know (this behaviour is observed in instrumented piles), skin friction is fully mobilized for small settlements, let's say in the order of 1/4", 1/2". Base resistance is fully mobilized for values of settlements usually expressed in function of pile diameter (5%, usually of it).

So if there is a high enough safety factor for lateral shear resistance, base resistance is never fully mobilized. The practice of making sure there is a rigid base layer when lateral resistance is more than enough is actually an emergency plan to avoid plunging of the pile in case something goes wrong.

Bottom line: if the succession is mostly clays with a decent Su (which will be reduced somewhat in the calculations), then the base resistance should not be really vital (of course you loose the back-up plan against plunging). Load tests should suggest this aspect is the one which governs, with very little settlement.

In sands it's maybe harder to achieve a good lateral resistance, unless piles are driven where sand is densified and K is higher. Friction angles are usually not the peak ones.

 

[oldestguy]

Perhaps let me explain what I do. I estimate the depth to which the pile should or may go, depending on what soil data I have. I do not specify that they do stop at that depth. My estimate is just that. On the job the piles are driven and stopped only when the pile driving formula computations says to. One might use a dynamic analyzer, but my jobs have not been that elaborate. BUT, if I know of a weak layer below the computed tip elevation, I then specify that the piles must not stop until that weaker layer is by passed regardless of good friction above. . One example in Central Wisconsin that came to my attention, by a fortunate "accident". Borings stopped a short distance in sandstone rock, so that was the assumption for where to stop. Piles were assumed to stop on that layer of sandstone rock. H piles did some perpetration of the sandstone continued down to a layer of weathered granite below (severely weathered to a soft clay condition). So, after that, any jobs in that transition area of bedrock had the added requirement that they do not stop in the sandstone above such layers of weathered granite. What I am getting to is computed penetration lengths are only that, so the on-the job pile driving situation always governs. In the specific area mentioned I do not know of any specific problem where piles stopped just above the soft clay (weathered from granite), but I would allow that if I knew such a situation was present, to be safe. On your job I'd specify by passing the lower weaker layer. The reasoning is that the pile foundation is likely to add pressures on the weak zone and cause settlement of the foundation system. Exactly how much may be argued, but don't take a chance.

 

[Mccoy]

Oldest guy, of course what you say is perfectly sensible from the technical standpoint and in the jobs I've seen the pile base Always went down to the stiff layer. I must also say though, that I used to work for a piling contractor so it was in his interest to drill the piles as long as possible.
If we want to try a design optimization though, if that's necessary, some experimentation may be carried out by testing piles before the real job is started. If the settlements keep very low, then there is no load transfer to the base hence there can be (theoretically) no settlement at the base.
One issue you may rise is long-term base settlements and that might be a concern if lateral friction is lost to an extent after time.
Again, a stiff base layer makes you sleep well at night but, if sufficient proof of stability is given in the absence of a base stiff layer (calculations supported by load tests) would you be all right? I'm speaking of non-displacement piles here, just drilled piers.

 

[oldestguy]

McCoy: 80% of jobs were (AND EVEN NOW) bridges with driven piles, H or displacement. Old time DOT bridge engineers usually required piles even if there is no stream, as at grade separations. A few auger-cast grout pile jobs and a few caisson (3' max. shaft) belled jobs. Predominately were mostly end-bearing types. Was involved with a failed friction type where artesian conditions removed materials providing that friction and thereafter saw sites where that was anticipated so piles were sent to end bearing if possible.

 

[Mccoy]

Was involved with a failed friction type where artesian conditions removed materials providing that friction and thereafter saw sites where that was anticipated so piles were sent to end bearing if possible.

That's one situation where the Emergency exit of a base bearing layer is definitively valuable! In displacement piles the conditions are obviously different since the soil is densified by the driving energy if granular, also remoulded if cohesive, but I would have to reason about the overall effect since I've only dealt with non-displacement piles, usually large diameter.