Limit on Passive Pressure

[moe333]

I often see geotechnical reports that recommend upper limits on passive pressure, and I cannot find a good reason for this limit in the literature. It is often in relation to soldier pile walls where an equivalent passive fluid pressure is provided, for example; "allowable passive pressure of 250 psf per foot of depth up to a maximum of 4,000 psf". Any ideas on where this maximum limit comes from? Theoretically, I think the passive pressure should keep increasing with depth.

 

[GeoPaveTraffic]

At some point the passive resistance that can be mobilized is controlled by the shear strength of the soil. This is why we generally place a limiting value on passive resistance.

Mike Lambert

 

[moe333]

Mike, thanks for the comment. The shear strength is defined by the Kp value that is assigned, and that value should be chosen appropriately. Gamma is easy, but the other component of passive pressure; H is just a function of the embedded depth of wall or soldier pile. The deeper the embedment, the larger the passive pressure should be? I don't see why there would be a limit on KpxGammaxH.

 

[PEinc]

250 psf max. passive resistance seems pretty conservative unless the soil is very soft (slop), the soil below the excavation is buoyant, or the ground surface is sloping downward. IMHO, most geotech report recommendations for earth pressures for shoring are very conservative and are specified by engineers with little or no experience designing or building retaining walls. I rarely use the earth pressure coefficients recommended in the geotech report.

http://www.PeirceEngineering.com

 

[moe333]

The 250 psf value was just an example, and I agree it is a low value for level ground that is not slop. My real question is how can the passive pressure be capped at some value, say 4,000 or 6,000 psf for example. If the structural element the passive pressure is acting on continues to a depth of say 30 feet, the passive pressure should also continue to that depth. Per my original post, I often see this value capped at a depth that is above the bottom of the structural element. Any ideas why?

 

[PEinc]

I have never seen passive resistance capped at a maximum. I have seen jobs where the active and passive earth pressure coefficients were recommended and/or required. Most geotech reports recommend very conservative earth pressure coefficients. I suspect that a geotech who is capping the passive resistance is inexperienced in design, or is worried about liability coming back to him, or both.

http://www.PeirceEngineering.com

 

[moe333]

GeoPaveTraffic; How do you calculate the limiting value you place on passive resistance?

 

[GeoPaveTraffic]

Passive pressure is related to soil strength, generally the drained soil strength. As you know, some soils can behave in an undrained manner and that strength might be lower. Another reason to cap the pressure is weaker soils with depth or soils that don't increase in strength with depth.

I've also heard people say they cap the pressure due to the amount of movement that is required to mobilize the full pressure. Don't know that I really agree with that, but I have heard it.

I'm not sure most structural engineers understand how much movement can be required to mobilize full passive pressure and most geotechnical reports that I've seen do not do a good job of discussing the subject.

Mike Lambert

 

[FixedEarth]

If we are using the Net earth pressure method distribution on a cantilever pile, approx. 0.7H distance below base of excavation is the point of zero deflection. At this this point, the Passive resistance zone ends and the triangular net earth pressure jogs back to the Active zone. Therefore, we have to stop the use of our passive resistance at this depth. However, if we are using the Gross earth pressure method, this will not be the case.

http://www.soilstructure.com/

 

[Doctormo]

Going back to the original question, I would guess that someone might be thinking that if they had provided a maximum bearing capacity (say 2,000-4,000 psf) then they might want to limit the passive pressure in a similar manner since the concepts are similar (design resistances). Not sure that there was any science behind this recommendation just like their may not be any behind a 3,000 psf recommendation for maximum bearing capacity at any foundation depth in a report but who knows. One may able to calculate a much higher bearing capacity for a specific situation but a limit would be placed in a report (ie: 2,500 psf) that requires special analysis to exceed this value.

Many recommendation are simplistic and conservative due to the nature of geotechnical engineering and the minimal amount of knowledge of specific soil conditions, strengths, etc all obtained from a very limited scope of work that is all the owner is willing to pay for. All the reasons stated above make sense to me and if one is expecting to rely on large amounts of passive resistance, they may want to look at their specific methods and the quality of the soil information obtained before relying a simple calculated value with no limits. In other words, I don't really have a problem with it being expressed that way as a general report recommendation.

 

[moe333]

Doctormo, I think you nailed it. I was looking for some science behind it, but I don't think there is any. I think if you have good specific knowledge of the soil conditions, there shouldn't be an arbitrary limit on the passive pressure.

 

[fattdad]

it's possibly related to strain compatibility. It takes more movement to mobilize passive earth pressures than active earth pressures.

then again. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

With passive resistance - usually in front of a wall - one never knows down the line when someone will come and dig a longitudinal trench parallel to the wall to install a watermain, sewer or simply erosion . . . Science - not - but I know a lot of the old timers would limit kp to 3.

 

[moe333]

My original question was regarding placing a limit on the passive pressure, not on the passive pressure coefficient kp. I agree kp needs to be selected carefully. But I see no reason to limit the passive pressure to some arbitrary depth above the bottom of the embedded structural element that is being designed.

 

[fattdad]

. . . and maybe we haven't seen that either? I have not seen a truncation on the Kp*gamma product either.

I have seen folks design earth pressures on retaining walls using K-naught on both sides. Now I know that is not the OP question, just another variation on folks and their engineering approaches. For the case of K-naught, the engineer just didn't want the retaining wall to move at all!

f-d

ípapß gordo ainÆt no madre flaca!