wall design (pressure) when placing sand/silt as a slurry

[Kivi]

What do you soils/civil guys use to design retaining walls when sand/silt is placed behind as water based slurry (say at about 60 to 75% solids)? Using pure hydrostatic head is always way too high. What if the wall is permeable? sorry, a lot of questions here.

 

[jdonville]

Kivi,

What's the application? Dewatering tailings? Retaining tailings?

Is the slurry discharging off the top of the wall or at some distant point behind the wall?

How high a wall are you talking about? What kind of wall are you talking about?

Why is using pure hydrostatic head too high?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[Kivi]

Jeff, just speaking generically as underground we have been designing and building retaining walls to secure cemented backfill which could be tailings or sand under many differing conditions. it could be discharged at the wall or at some distant point. at most 20 feet tall. Field measurements show that pure hydrostatic is never achieved. What do the civil / soil people use to design something like this? What are important characteristics of the solids that should be measured to assist in estimations?

 

[jdonville]

Kivi,

I'm just talking this out, so bear with me.

It sounds like you are talking about a lean cement/grout or CLSM-like material. I haven't worked with cement-emended tailings/backfill, so I'm not personally familiar with the characteristics, but if I recall correctly, you need to keep the slump high to be able to pump the slurry (thus the 60-75 percent solids - is this by weight/mass or volume?).

The Portland Cement Association indicates that the unit weight of CLSM is usually 90 to 120 pcf (in place) and usually begins to set in 1 to 8 hours. Typical slump is greater than 10 inches for CLSM.

Unless specifically designed for, CLSM is generally denser than water, so any excess water in the slurry would tend to be displaced and rise to the surface of the fill.

Kivi, I'm thinking that what you may be seeing in your field tests of lateral pressure is that the bottom of the cemented backfill may be setting up before you get the fill level to the top of the wall. As the material sets up and becomes more self-supporting, the lateral pressure exerted on that portion of the wall goes toward zero.

I would test the speed of setting of your material - possibly by casting small cylinders. To be really cutting-edge, you could place them in molds that you could subject to axial confining pressure and plot the curing time vs axial pressure to see if the bottom layers set faster or slower due to the pressure.

What you have is a situation where the short-term pressure against the retaining structure should be similar to the in-place density. You will likely need to test the in-place density from the mix design and/or unit weight tests of cored backfill. Regardless of the rate of fill behind the retaining structure, I would still recommend that you design for at least this in-place density. However, as this is a temporary condition, you could use probably use reduced factors of safety against sliding and overturning for the design of the retaining structure.

I would check with the regulatory authority to see what reduced factors of safety are permitted (if any).

Good Luck!

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[dpa]

What I am about to say will be completely counter-intuitive.

The way I read your post is that you are using water as a medium for placing sand behind a retaining wall. Is that correct? I don't see any mention of cement or other chemical additives. So the situation you would have, at least temporarily, before settling, would be loose sand fully saturated.

In which case I have to disagree with the statement "Using pure hydrostatic head is always too high."

If you refer to "Civil Engineering Reference Manual" 6th Edition, by Michael R. Lindeburg P.E. page 10-18 Section 26 "Horizontal Pressure From Saturated Sand" and solve the equations using the values found in the sections noted below:
dry density = 90lb/cu ft Table 9-6
angle of internal friction 30 degrees
void ratio 0.85 Table 9-6
You will come up with a horizontal pressure of 81 lb/sq ft for each foot of depth. This is 30 percent higher than pure hydrostatic pressure. At phi = 36 deg horizontal pressure is still 77 lb/sq ft times depth.

That is a very surprising result but keep in mind that this book is the reference which is recommended to prepare for the Civil P.E. by the National Society of Professional Engineers so it most likely is correct.

I don't know how to insert mathcad files or I would have included the calcs so you wouldn't have to do them for yourself.

Dave Adkins P.E.

 

[Kivi]

thanks Dave,
I'll look into your post and may contact you again through the thread.
Paul

 

[dpa]

Kivi,

I sent my original post without noticing that your later post said you had run field tests that showed pure hydrostatic is never achieved. It does seem that in practice some of the loads we calculate never happen. Could you elaborate on the type of field tests or point me to a reference. I would like to use those tests on some of my own projects.

Thanks,
DPA

 

[Kivi]

Thanks dpa,
our applications have cement or may not have cement. our designers use the wet density (say at an SG of 2.0) as a worse case. for your example that equates to 124 lb/sqft compared to yours. this is a liquifaction limit. in practice with some monitoring, we do have some curing occuring that affects the results however in many cases our results on soil pressure (using earth cells) is much less. I'll look into your calculation methods to use in our application.
Thanks

 

[Kivi]

dpa,
you had suggested to use a civil textbook, I am unable to find anything in the short term. Do you know of any websites which may detail out the calculation you suggested.
Thanks,
kivi

 

[dpa]

Kivi,

It's a fairly large format book so I don't know if I can get that page to fit on my scanner. I'll give it a try. This seems to be the only source that propounds loads that high but if they are right a lot of retaining walls could be in trouble.

DPA

 

[Kivi]

dpa,
Thanks for your help. send to me or post the info.

 

[Kivi]

dpa,
see my profile for my fax and email,
Thanks,
kivi