Skin Friction Piles - Seismic Case

[GeoGrouting]

For transient wind and seismic loads in spread footings,the maximum allowable bearing pressure may be increased by one-third (1/3) at the toe for a triangular pressure distribution against the underside of the footing.

How the above principal is applied to skin friction piles, i.e. could one increase the friction by 1/3?

Thank you.

 

[Qshake]

I'd be interested in someone citing a code or textbook on this subject.

Spread footings, especially those on engineered fill, is one thing while deep foundations subject to soft soils and or liquefaction is quite another.

Unless it was proven that the underlying soils were predominately sandy or sand with silt and a low water table, I would not entertain an increase.

Also under some of the current codes, displacement may govern over strength issues.

Regards,
Qshake

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[GeoGrouting]

Dear Qshake

It is not clear to me how sandy silty soils are relevant to my question. One would think that the 30% increase is because of dynamic loading (or in fact higher strain rate) and a material shows higher strength if fails/shears under a higher strain. As such one could hear that we could increase cohesion value by 30%. I request comments from others as it became confusing for me. Thank you.

 

[ishvaaag]

When the action of loads over the foundation produced due to eccentricity nonuniform pressures on the soil, the old MV 101 code in Spain allowed a 25% increase over the allowable stress defined at some 8.1 table, as long as the pressure at the center of gravity of the support surface didn't exceed the allowable pressure on the soil by the table.

Code NBE AE-88 that superseded the former retained the same respect another table also labeled 8.1 (likely a direct port of the former specs) and was vigent till 2005.

Current CTE (as per 2009 release) has eliminated every mention to this issue, yet for "Direct Foundations", i.e., footings, it is stated you can take the maximum cobaricentrical area of support within the foundation base for the checks, and this to some extent is an allowance somewhat pointing in the same direction.

When referring to combinations to be considered at the checks, normal and transient loads (say gravity and wind) are just given the same coefficients, and only for extraordinary (say earthquake) loads other coefficients tantamount to allowing bigger nominal stresses are stated. This is as per table 2.1.

For earthquake loads it is customary here to study the "extraordinary" loadcases involving earthquake with the loads (not the strengths) stated at their probabilistic values (well, with all the response factor issues subsumed, of course), i.e., with the estimated actual values having effect on the structure, and then foundation. Pressures at the foundation interface were and still are checked at service level for whatever the loadcase, i.e., against surmised actual loads, and so any further allowance (by a division by a lesser factor -from 3 to 2, and from 3.5 to 2.3- to get the design strength) is tantamount to admit the soil be stressed at the extraordinary earthquake cases what is here 1.5 times what for gravity and/or wind.

So as a recension, the customary allowance here was being made on eccentricity, and not cause of the load. Otherwise said, was allowed on an allowed plastic distribution of the loading and then stresses, and not cause of the load. It is clear the intent was for footings, and somewhat of this is retained by the current code on the use of the cobaricentrical area for design; even if quoted for footings, by the criterium of the designer equal loads to the piles might be considered in a plasticity of response way and that would be something in the same intent; depending on how the response develops this could be valid enough for strength purposes (accurate is a too strong word for foundations), and a bit more imprecise for settlement issues at support points (which likely will be more affected by the number included or not in the cobaricentrical area than for use of the criterium itself).

It is also seen that in Spain the further allowance for earthquake stressing not only respect soil loading but structure itself is a division by 1.5 of the otherwise applied loads (consistent with a reduction of a characteristical loading to a probabilistic one). For both the structure and the soil, this means they have to take the expected actual probabilistic loads with only its material strength (reduction) factors still afoot. Concrete and structural steel so will have say still 1.6 or 1.35 times the loading to wear, and the soil will have a lesser safety factor against the loading, where it had 3.5 for wind and gravity at service level (i.e., actual loads) would have only 2 when earthquake strikes (still, then, expected actual loads).

In the soil aspects, it is clear that:
-it is unwanted to exact that the earthquake cases push up the design to what must be being presently considered to be uneconomical levels
-that the current statement of the specs for seismic cases must be seen consistent enough with the structural safety above, and both able to warrant what it is by now considered a reasonable standard against seismic risk
-if the allowed reduction in safety factor bears direct relationship (in any other way thant the 2 factor over the characteristical strength being considered still enough) with the behaviour of the foundation-structure interface or soil mass in dynamic excitation. For wind it is clear not, for the code makes no allowance, and my guess is that so is the case with earthquake loads).

See here the quoted table 2.1 of CTE

http://www.codigotecnico.org/web/recursos/documentos/dbse/se4/030.html

 

[ishvaaag]

errata

-if the allowed reduction in safety factor bears direct relationship (in any other way thant the 2 factor over the characteristical strength being considered still enough) with the behaviour of the foundation-structure interface or soil mass in dynamic excitation. For wind it is clear not, for the code makes no allowance, and my guess is that so is the case with earthquake loads).

should be read

-remains unclear to me if the allowed reduction in safety factor bear any direct relationship (in any other way thant the 2 factor over the characteristical strength being considered still enough) with the behaviour of the foundation-structure interface or soil mass in dynamic excitation. For wind it is clear not, for the code makes no allowance, and my guess is that so is the case with earthquake loads).

 

[Qshake]

geogrouting - the point was sandy or sandy-silty soils with a low water table. For the seismic case with sandy soils and a relatively high water table it is likely to promote liquefaction, you will lose shear strength of the soil. So for that case how can you justify the increase in strength?

Since liquefaction is more common in sandy soil and less common in silty and clay soil it matters what you are driving into. And while silty and clay soils are less prone to loss of shear strength, silty soils are weak and friction pile may turn out to be bearing pile if bedrock is shallower than 80 or so feet.

so yes, it seems to me that the answer is dependent on the soil and application.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Mccoy]

Geogrouting,
as far as I know, usually the increase in strenght displayed by a generic soil during transient or dynamic loading (except if very brittle or sensitive) is neglected in favour of safety.
Unless possibly you have dynamically tested samples so you know exactly what's the (certified) strenght in similar conditions. IF the lab conditions are really applicable to the real world.
Is your source a code, or some guidelines?
Theoretically the same principle should hold for shallow and deep foundations, but I would be very careful to apply it since so many other variables contribute to the overall result, as said by Qshake, and sone of'em may be unfavourable.

 

[irawanfirmansyah]

Actually, it is not increasing the allowable bearing pressure, but lowering the safety factor from 3 to 2, considering the load is temporary.