Bousinesq lateral loading debate 1

[Kevin9679]

This has probably been treaded before. For infinite load surcharge, we often take an equivalent height of soil, aka active wedge method. For finite strip loads, we're told by every textbook to use 2q in soils. But that leads to a maximum pressure that is 3x as much as with the Active Wedge method. In many cases, the entirety of the load acting on the wall/whatever is much higher than that calculated by the Active Wedge method. This would especially be true if it was an infinite strip calculated using Bousinesq.

In Geotech we have equations, but in real life the "most realistic" answer doesn't vary by a factor of 3 for the exact same condition. I would have to believe that many people have done full-scale load cell measurements. Has anyone seen that first hand or in papers, and like to chime in?

 

[Guest090822]

The problem with Bousinesq is that most of his work was done based of a very rigid wall. A lot of engineers (and software) has "wall factors" built in to reduce the pressure. It's common to see reduction factors varying from 1 to 2 depending on if a wall is rigid, semi-rigid or flexible.

Some applications take it even further if your strip loading parallel to a wall is not infinite. The Shoring Suite Software further reduces the pressure using the following:

I guess this same question was asked on this forum back in 2009. The answer below explains it, and I was able to match the program - directly from the software vendor:

We use the following equations:


1. Pstrip = k* Q/pi * (beta-sin(beta)*cos(2alpha))

where k=1 for very flexible wall. k=2 for rigid wall

This equation can be found in P16 of USS manual. It is modified Boussinesq equation and widely used for shoring design. It called Wayne & Teng Equation. Wayne & Teng Equation is widely used in shoring design. We found that using Boussinesq equation to calculate Area Loading and Pint Loading do not match the results from strip loading calculation of Wayne & Teng Equation. We have to modify Wayne & Teng Equation for area loading to keep the results consistent.

Following are our equation:

2. Parea = f * Pstrip
Where f is length factor:
f = 1- 1/(0.25 * L/(X+1) +1)
L - length of area loading; X - Distance to the wall.
When L is infinitive, f = 1, Parea=Pstrip

We plotted curve from Boussinesq equation and use this curve to scale down to fit Wayne & Teng Equation. Then we get the above equation.

3. Ppoint = Parea when Length=1 and Width=1.

 

[PEinc]

Using CivilTech Shoring Suite for non-gravity retaining walls, I usually reduce the Boussinesq lateral surcharge pressure due to the wall's flexibility. However, when designing walls along railroad tracks, some railroads require the rigid wall analysis where the full 2q is in the lateral pressure equation.

http://www.PeirceEngineering.com

 

[Guest090822]

PEinc - I think you and I have discussed this before. I have reduced the loading based on a wall factor and have gotten approval on jobs near a railroad. It depends on who is reviewing it as I’ve also been forced to go the unreasonable route. I’ve found most “reviewers” have never actually designed anything and are just checking off the boxes.

I’ve never had a wall I designed have an issue.

Like the OP states, big difference between code/theory and reality. I wish they would teach that on day 1 in engineering schools.

 

[oldestguy]

This OG has measured pressures on a wall of a multi-story parking garage access ramp, some 30 feet high, as it was backfilled with wet sand. Compaction was done on all backfill, in layers and the reading of many cases taken often. The effect of compactor distance from the wall became clear, so location of no compactors given distance from the wall was watched and found to be at least 3 feet to minimize the effect. That still showed equivalent fluid at about 30 #/cf. Interesting that even some 28 feet above the sensor, increases in pressure were noted down at the sensor.

 

[Kevin9679]

OG, would you say based on the readings that when the compactor was 3ft from the wall, there was something like a uniform 30psf/ft lateral pressure up to a depth of 28ft? Or was 30psf/ft the maximum lateral pressure?

 

[oldestguy]

This post had some errors. See later post farther down. Sorry about confusion.

 

[Kevin9679]

What was the contact pressure and width of the compactor?

 

[oldestguy]

Hey that was some time ago. This was a big walk behind. My bet is a few hundred pounds load. The typical "good"" one, not any light weight compactor. Probably about 2.5 feet wide. I'd call it a typical compactor where it does the job. Besides this sand relatively uniform, meaning a low range between loose and dense. Compactor types probably all get near the same density in this stuff. If you are trying to justify a lower density number, not on that site.

 

[Kevin9679]

Something like this? I'm really surprised the weight was felt even 30ft down.
Link

At first I thought you were talking about a roller compactor.

 

[oldestguy]

I'll add I see the number I gave was before they kept the compactors from near the wall. After moving to 3 ft. the apparent fluid density was about 30 pcf.