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Help With Load Distribution

02/21/2017 9:23 AM

Hello, I'm having trouble identifying the load distribution on a sump pump buried in gravel which is surrounded by soil. My question is:

a) How does the load distribution look like without the force F? Im thinking about the pressure distribution on the sump pump due to the pressure from the soil and gravel. The top of the sump pump is barely beneath the surface.

b) How will the load distribution look like with the force F?

c) Does anybody know how to calculate stresses on certain points of the sump pump?

All help will be much appreciated, feel free to discuss!

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#1

Re: Help with load distribution!

02/21/2017 9:46 AM

What did the pump manufacturer say during the recent telephone call on the topic?

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#2

Re: Help With Load Distribution

02/21/2017 10:13 AM

A solid (soil, gravel) can support itself, which is why you can dig a hole without it disappearing.

It's not my field, but my guess is that you should consider the worst case situation, where the solid "relaxes" against the buried object and behaves like a liquid of the same density.

Here is a study that might help:

https://eng.auburn.edu/files/centers/hrc/IR-02-03-3.pdf

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#3

Re: Help With Load Distribution

02/21/2017 10:20 AM

Logic would suggest that the stress that the pump casing is able to withstand exceeds that presented by the submersion depth being equal to the maximum lift that the pump can produce. Otherwise, it would collapse when it switches off.

Assuming it is currently operable, the suggestion in #2⇑ applies in the absence of manufacturer's information giving a higher figure. The density of solids can be found in many places, including Kempe's Engineers' Yearbook, any edition. Estimating the stress at any depth is then a matter of simple arithmetic.

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#4

Re: Help With Load Distribution

02/21/2017 10:34 AM

You also need to evaluate the pressure caused by the saturation of the soil, known as the Hydrostatic Pressure.

To answer your questions is not an easy task, unless you have taken (or are experienced in) a Soils Mechanics course, as well as a Groundwater Soils Mechanics course.

There are several equations available to determine the overall lateral pressure distribution acting on the vertical surface of the sump pump. But I do warn you that you need to know (through laboratory testing) a host of factors and coefficients of the soils, whether they be cohesive or non-cohesive soils.

It's not that simple to determine the lateral pressures you seek. A good starting point is Joseph E. Bowles's Soil Mechanics textbook.

Why are you partially burying the pump? Usually sump pumps are installed in a concrete sump or perforated ABS or PVC pipe section so you can pull it for maintenance and cleaning. If you have wheel loadings present, you install a grate of significant strength atop the sump pit.

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#5

Re: Help With Load Distribution

02/21/2017 11:39 AM

Are you talking about the sump pit that holds the pump and the force acting on the walls and bottom of the pit itself?

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#6

Re: Help With Load Distribution

02/22/2017 12:20 AM

Wouldn't this be difficult, if not completely possible to estimate.

I am not a soils engineer, but I'm guessing you need some preliminary information, like :

A. What are the physical dimensions of the sump pump, including inlet and outlet size, head pressure etc. Including the type of material the pump is constructed from.

B. What is the size of the gravel, crushed, pea, slate,etc

C. Describe the area of the gravel, size, shape, depth etc.

D. What type of soil, clay, loam, sandy etc.

E. What type of liquid will the pump be transferring, water, sludge, etc

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#7

Re: Help With Load Distribution

02/22/2017 6:49 AM

Thank you for your replies,

I am currently writing by bachelor's thesis within the field of mechanical engineering, therefore soil mechanics is somewhat outside my field of study and help is needed

I am conducting a strength analysis in Ansys as part of my bachelor's thesis. I am also planning an experimental test which aims to verify these digitally aquired data.

I will be needing information to correctly apply forces to the sump i order to simulate the pressure on the walls due to the weight of the surrounding material as well as the point load. The magnitude and proximity of the point load will be used as variables to assess how close a car can pass the sump without the sump collapsing or no longer function. In case of positive initial findings; will the sump be able to withstand the pressure of a truck, a tractor etc.

In sum; I guess I need some kind of function, formula or model that takes magnitude and proximity of the point load as inputs, and gives forces or pressure on the sump wall as output while simplifying and standardizing soil- and gravel-constants

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#8
In reply to #7

Re: Help With Load Distribution

02/22/2017 8:42 AM

The engineering term that should give you search results is surcharge, this might be a good start

Surcharge Lateral Earth Pressure

https://www.webpages.uidaho.edu/ce115/resources/Thomas%20Weaver/Surcharge%20Lateral%20Earth%20Pressure.pdf

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#9

Re: Help With Load Distribution

02/22/2017 11:22 AM

As I have stated before, you need to obtain a copy of Joesph E. Bowles textbook entitled "Foundation Analysis and Design". It is currently in it's 5th Edition. You can download it for free at this link, but beware it is a huge PDF file (1231 pages):

http://us.cdn.persiangig.com/download/zA3jtD/dl

You will need to perform two separate analysis to determine the worse case lateral load scenario. First, lateral load distribution when the soil mass is SATURATED. In your case I would include a water depth (head) above the top of the pump pit rim, just to be conservative. Secondly, the other analysis would be the concentrated load from the vehicle tire load. Utilize the Boussinesq equation to determine the lateral load distribution. The results of each computation are indeed additive.

Frankly, since you are a ME student I believe that you will need to seek the advice of a Civil Engineering Dept. Professor or a Grad Student how to perform the necessary computations, as well as choosing the correct coefficients and other factors.

I cannot understand for the life of me WHY a BSME candidate would chose to undertake a Thesis Paper that involves complex soil mechanics when one does not have any formal soil mechanics courses under their belt. Bad choice of Thesis subject Bro. Solving this problem is complex and not an easy task. You need to have very good, even great, engineering judgement in making your assumptions on the various soils coefficients based on the types of soils.

You may be able to locate the above referenced textbook in your University library.

Signed,

CaptMoosie, PhD, P.E.

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