help with storage tanks on a slope 4

[upcountry99]

We need to install two 10' dia x 8'h polypro water storage tanks on a 40% slope. Thinking we can make a 4' cut for a bench and compacted gravel pad. This is bearing on sandstone bedrock below a ridgeline; there is no groundwater.

Is there any issue with the loading of the filled tanks causing slope failure ? basically ~500 pounds/sf and 75sf per tank. Thinking a soldier pile wall to retain the cut above/behind the tanks. The plumbing contractor says go for it; the structural engineer isn't sure. A topo sketch is attached (2' contours) and the soil parameters are
Backfill Soil: Phi = 32 degrees Unit Weight = 125 pcf
Retained Soil: Phi = 32 degrees Unit Weight = 125 pcf
Foundation Soil: Phi = 36 degrees Unit Weight = 120 pcf

Part 2 is, if the above is stable on a 4' cut pad on the native slope, would there be any issue with adding a 3'h wood landscape wall 3-4' below the toe of the tanks to hold the excavated fill material ? Any insights appreciated. This is supposed to be a 'simple' project to get an affordable home site up and going quickly and we can't seem to get agreement from the engineers involved.

 

[DaveAtkins]

Slope stability - that should be checked, but you are not altering the slope, so I don't know why it should be an issue (unless it already does not work).

Soldier pile wall with timber lagging - yes, this is a common way to do a retaining wall, although perhaps not the most cost effective for a residential situation. I am thinking a gravity retaining wall might be more appropriate, such as Redi-Rock.

 

[upcountry99]

Slope stability - that should be checked, but you are not altering the slope, so I don't know why it should be an issue (unless it already does not work).

Soldier pile wall with timber lagging - yes, this is a common way to do a retaining wall, although perhaps not the most cost effective for a residential situation. I am thinking a gravity retaining wall might be more appropriate, such as Redi-Rock.

Thanks Dave for the reply but we are looking for specific, conclusive evaluation. Is there any problem with cutting a 4’ bench to set the tanks on ? The wall is only to retain the upslope cut, not the tanks. We have a geotech report and parameters. The slope is considered stable as-is. So the only question is if the load of the tanks 500 psf would change that ? What other data or parameters are needed to evaluate this quantitatively ? Thanks.

 

[geotechguy1]

If you are going to mess around with a slope you need to get a geotech assessment done. You are modifying the slope so further assessment of stability must be done

 

[upcountry99]

If you are going to mess around with a slope you need to get a geotech assessment done. You are modifying the slope so further assessment of stability must be done

We've got a geotech report, cores, sections etc. for the site. It doesn't specifically address cutting a bench into this slope. The bench would set on resistant bedrock. The only question is the weight of the filled tanks. How would you quantitatively evaluate the effect of the weight on the slope stability ? That is the only question. 500 psf doesn't seem like much. Certainly the soil bearing capacity is greater than that and the report says that foundation grid footings can be designed to 2000 psf and slabs to 500 psf. So is there obviously no issue or how would we evaluate it ?

 

[geotechguy1]

You shouldn't evaluate it you should to back to whoever wrote the report or another geotech and get them to evaluate it

 

[Retrograde]

I do not think anyone is going to be able to provide meaningful comment without first reading the geotechnical site investigation report and being provided more details (your sketch and descriptions are vague).

 

[upcountry99]

I do not think anyone is going to be able to provide meaningful comment without first reading the geotechnical site investigation report and being provided more details (your sketch and descriptions are vague).

What other data or parameters are needed from the geotech report to analyze other than the soil parameters provided in the question?

The sketch (topo plan view) is simple. It's cutting a 10' wide 4' deep (at the upslope side) level bench to set the two poly tanks on. The toe of the tanks would be at the existing grade. A steel H beam pier & wood lath wall would retain the cut behind the tanks. But the question is simply whether the slope would support the weight of the filled tanks (500 psf x 75 sf per tank) ? There is 'resilient bedrock' just below the toe of the tanks and the cut and base of the tanks would be setting on sandstone bedrock. The geotech report provides design criteria e.g. for a mat slab on grade for a habitable structure that is 500 psf. So shouldn't that be just fine for two non-habitable storage tanks ?

I've attached a section view to further clarify. What other info is needed to evaluate it ? We think this is simple and just want a clear engineering explanation before we start digging.

 

[Retrograde]

What other data or parameters are needed from the geotech report to analyze other than the soil parameters provided in the question?

When looking at the stability of a rock slope the presence of any joints is an important consideration - particularly the strike and dip of the joints (ie. the direction of the joint).

We think this is simple and just want a clear engineering explanation before we start digging.

It may not be as simple as you think.

 

[upcountry99]

When looking at the stability of a rock slope the presence of any joints is an important consideration - particularly the strike and dip of the joints (ie. the direction of the joint).


It may not be as simple as you think.

On the other hand, it may be that simple. I'm looking for a basic quantitative approach based on the soil parameters, the slope and the weight and area of the tanks. Let's assume there are no abnormalities or fissures in the bedrock at that area of the slope. See attached geologic map ~ we are setting the tanks in the flattish area on the section A line, so there is "resistant bedrock" just below the toe of the tanks and 1-2' of loose soil cover over the bedrock. Also attached is the boring log for geologic section A ~ that B2 boring extends 21' from the top of the site through the upper 'resistant bedrock'. The tanks will be set just above the lower resistant bedrock. There's also data from Trench 1. Lmk if you would need any other soil data to sketch a quantitative analysis for the static case.

 

[geotechguy1]

Since reason wont convince you, call your insurance and explain the situation and where you plan to get advice before proceeding and whether they would cover you. Then call the disciplinary committee in your state or province and pose the same questions.

 

[upcountry99]

Since reason wont convince you, call your insurance and explain the situation and where you plan to get advice before proceeding and whether they would cover you. Then call the disciplinary committee in your state or province and pose the same questions.

I've yet to hear any reason, logic, method or analysis from you ~ just unhelpful trolling. So I certainly won't be hiring you. Hopefully a qualified engineering member here can demonstrate some knowledge. I have a graduate engineering degree myself ~ and years of computer science background ~ so I understand dynamic modeling and mathematics. And I have experience as a builder. I just don't know anything about soils or geology or how to approach this quantitatively. It seems like a lot of the slope stability modeling is ad hoc and empirical. This seems like a very simple static situation to cut a 4'h x 10'w bench on a moderate slope. So I'm sure someone who knows what they're doing and wants to be helpful can chime in. Just trying to compare the load of the tanks, 500psf, with the resistance of the slope, or how to frame the question quantitatively. Thanks.

 

[LittleInch]

Up country.

Be careful about calling people trolls or any other negative comments when they are experienced engineers offering an opinion, Tends to just annoy people and stop them responding.

The key sentence in your post number 5 about your geotech report is " It doesn't specifically address cutting a bench into this slope. ". In my opinion it needs to.
This is what most people are telling you need to do. The risk is not only movement of the tanks down the hill, but whether there would be any settlement of the tanks and how best to hold the rest of the hillside back. Maybe a bunch of stone gabions would do (7 degree angle backwards). Think also about drainage and water flow.

It would seem to be a steep slope based on firm rock, but that's where the skill of a geotechnical engineer comes into play looking at the specifics of your site.

If it really is a simple as you hope then it won't cost you more than a days work for the geotech engineer to assess it and confirm this and then everyone is happy.

Don't want to spend the money and just start digging and the tank starts moving or tipping when you fill it with water and you've got a huge problem. Your risk.

 

[upcountry99]

Up country.

Be careful about calling people trolls or any other negative comments when they are experienced engineers offering an opinion, Tends to just annoy people and stop them responding.

The key sentence in your post number 5 about your geotech report is " It doesn't specifically address cutting a bench into this slope. ". In my opinion it needs to.
This is what most people are telling you need to do. The risk is not only movement of the tanks down the hill, but whether there would be any settlement of the tanks and how best to hold the rest of the hillside back. Maybe a bunch of stone gabions would do (7 degree angle backwards). Think also about drainage and water flow.

It would seem to be a steep slope based on firm rock, but that's where the skill of a geotechnical engineer comes into play looking at the specifics of your site.

If it really is a simple as you hope then it won't cost you more than a days work for the geotech engineer to assess it and confirm this and then everyone is happy.

Don't want to spend the money and just start digging and the tank starts moving or tipping when you fill it with water and you've got a huge problem. Your risk.

OK thanks but so far no one has addressed the simple question: what is the quantitative approach to analyzing the static case ? I'm looking for a mathematical answer not simply advice to get our geotech to address it. It's not about the cost of that work or the insurance it's about getting a scientific answer based on a mathematical model. Your suggestion of gabions also does not make sense ~ are you saying to use that to retain the upslope 4' cut ? That's not the issue.

 

[fel3]

What other data or parameters are needed from the geotech report to analyze other than the soil parameters provided in the question?

The sketch (topo plan view) is simple. It's cutting a 10' wide 4' deep (at the upslope side) level bench to set the two poly tanks on. The toe of the tanks would be at the existing grade. A steel H beam pier & wood lath wall would retain the cut behind the tanks. But the question is simply whether the slope would support the weight of the filled tanks (500 psf x 75 sf per tank) ? There is 'resilient bedrock' just below the toe of the tanks and the cut and base of the tanks would be setting on sandstone bedrock. The geotech report provides design criteria e.g. for a mat slab on grade for a habitable structure that is 500 psf. So shouldn't that be just fine for two non-habitable storage tanks ?

I've attached a section view to further clarify. What other info is needed to evaluate it ? We think this is simple and just want a clear engineering explanation before we start digging.

Have you compared the weight of the proposed tank when full to the weight of the soil to be removed? I did a quick back-of-the-envelope calculation, and it appears to me that the full tank will be somewhat to significantly heavier than the soil that will be removed. This will increase the load that the slope below will experience. That's why you need to have your geotech evaluate the proposed design for slope stability. It may be just fine, but you won't know for sure until it's properly evaluated.

 

[upcountry99]

Have you compared the weight of the proposed tank when full to the weight of the soil to be removed? I did a quick back-of-the-envelope calculation, and it appears to me that the full tank will be somewhat to significantly heavier than the soil that will be removed. This will increase the load that the slope below will experience. That's why you need to have your geotech evaluate the proposed design for slope stability. It may be just fine, but you won't know for sure until it's properly evaluated.

Yes that is a good thought and as a ballpark say it's 100 pounds/cu ft for the soil removed ~ a 4'h x 10'w wedge x 20' l. Say that's 40k pounds of soil. The filled tanks would be 10k gallons ~ 80k pounds. So the difference is only a few hundred psf. But again I'm looking for a mathematical analysis of the weight / forces on the slope... is there a "surcharge" of the tanks on the slope below the toe of the bench and round tanks ? etc.

 

[geotechguy1]

The quantitative analysis you want is a slope stability analysis...that is what a geotech should do. It may be a LEM model (SlopeW, Slide). It may be a FEM model (Plaxis, RS2 etc). It may be a hand calc (there are lots of chart based methods although they probably don't apply in this case), or it may be the geotech is an oldtimer that's done a hundred of these tanks in your region. Even if the tank were less in weight than the removed soil you still want a geotech to tell you that the proposal is OK (for liability / insurance reasons) - as soon as you start messing around with a slope like that you may end up triggering something else. The slope could be 'globally' stable but you make it unstable upslope and downslope by doing the cut / fill and adding the tank. The cut/fill form that you show casually applied is the cause of long-term stability issues in many regions of the world by being improperly assessed and implemented.

There are also regulatory considerations which vary by area - if the slope is marginally stable as-is, a government authority may or may not accept maintaining the existing stability as acceptable; they may want you to achieve a specific FoS of 1.3 or 1.5 which could require improvement beyond the existing condition.

 

[upcountry99]

The quantitative analysis you want is a slope stability analysis...that is what a geotech should do. It may be a LEM model (SlopeW, Slide). It may be a FEM model (Plaxis, RS2 etc). It may be a hand calc (there are lots of chart based methods although they probably don't apply in this case), or it may be the geotech is an oldtimer that's done a hundred of these tanks in your region. Even if the tank were less in weight than the removed soil you still want a geotech to tell you that the proposal is OK (for liability / insurance reasons) - as soon as you start messing around with a slope like that you may end up triggering something else. The slope could be 'globally' stable but you make it unstable upslope and downslope by doing the cut / fill and adding the tank. The cut/fill form that you show casually applied is the cause of long-term stability issues in many regions of the world by being improperly assessed and implemented.

There are also regulatory considerations which vary by area - if the slope is marginally stable as-is, a government authority may or may not accept maintaining the existing stability as acceptable; they may want you to achieve a specific FoS of 1.3 or 1.5 which could require improvement beyond the existing condition.

OK that's more useful. What would a hand calc look like for this situation ?
The report did do a monte carlo simulation of shallow slope failure for the opposite side of the knoll (same section line) which is where there would be a habitable structure and the FoS is 1.1 below the 1.5:1 line at the top of the site. So there is a slope remediation for that. But this is for water storage tanks, not habitable, on the opposite and less steep side of the knoll. So there is no local code reqmt for any slope improvements for the tanks, other than an 'engineered' retaining wall for the cut. If they slide, they slide. That would be inconvenient but would not cause catastrophic loss of life or downslope property damage. The question is how to formulate a calculation (not a simulation) to establish whether making the cut and adding the slight weight of the filled tanks would 'likely' lead to a slide or would be 'unlikely' to affect the stability of that portion of the slope (on 'resistant bedrock') below the tanks.

 

[Retrograde]

The "Bishop method of slices" is one form of hand calculation that can be used for slope stability analysis.