Physics of Terracing

GardenShananigansJune 18, 2012

If you have a bank on a hill with a a 4-foot rock wall at the bottom, will the lateral pressure on the existing rock wall increase if rock wall terraces are built into the bank above it? Or, will the lateral pressure be decreased by the introduction of a terraced structure because each terrace absorbs some of the pressure that would have otherwise accumulated in the existing wall at the bottom? While soil type surely has a bearing on the stability of the bank, does soil type change the physics of the scenario?

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bahia(SF Bay Area)

You are describing a situation where advice solicited over the internet is a poor substitute for getting professional local advice that can evaluate your on-site conditions. The design and condition of the existing wall as well as soil type and drainage are key factors. It might be helpful to know that walls over 3 feet tall generally require engineering to be accepted by most cities across the country. Hire a local qualified consultant to get targeted specific advice.

    Bookmark   June 18, 2012 at 8:31AM
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lazy_gardens

It all depends on how you build the walls above the bottom of the bank. And the soil type. And the local rainfall. And your drainage provisions.

"Properly" done, it will make the pressure on the lowest wall lessen. Improperly done it can take out the entire hillside and the houses on it.

I am not an engineer dnd cannot define what is proper. In other words, this is a job for a local civil engineering firm that specializes in cuts and banks and retaining walls.

    Bookmark   June 18, 2012 at 9:40AM
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GardenShananigans

Thanks for the posts so far. Anyone out there know the physics involved? And by "physics" I do not mean "ask a professional" please. I am interested in learning about the physics of terracing hence the post title. Anyone with knowledge in physics -- your insights would be much appreciated.

    Bookmark   June 18, 2012 at 7:08PM
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tanowicki

Can you give complete soil properties of the particular site including the relative density or consistency, moisture content, geology, united soil classification, as well as measured site map with elevation contours, description of the current retaining wall and it's construction including footer and backfill, future use of the terrace including likely water infiltration, then perhaps we could give you a yes or no on the effects of terracing on increase in lateral bearing pressure of a wall. All that affects the answer. Until then, ask a professional who's visited the site.

    Bookmark   June 18, 2012 at 8:18PM
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GardenShananigans

Tano: See "physics" in my last post.

    Bookmark   June 18, 2012 at 8:26PM
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karinl(BC Z8)

Quality in = quality out.

I don't think anyone is going to write you a comprehensive essay on the physics of walls.

But if you post a little more information about what kind of walls you are dealing with, and ideally a photo of the relevant area as well as what is above and below it, someone might be able to give you some relevant information about what issues are most germane to your plans.

If what you do want is just a theoretical essay about the physics of terracing, then it's not a forum you want. You should google around for some articles with some background info on soils and terracing.

Karin L

    Bookmark   June 18, 2012 at 9:05PM
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GardenShananigans

Thanks, Karin. I think you're right. I am probably asking this question in the wrong forum. I am new to Gardenweb, and as I look out at the other posts most of them are specific to someone's project and they want specific advice. I am looking for explanation of a general concept. Thanks anyway to those who weighed in!

    Bookmark   June 18, 2012 at 9:34PM
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karinl(BC Z8)

Well, it's funny you should say that. This is not, in fact, an advice forum.

This is supposed to be a discussion forum, much as our discussions often revolve around advice. We would discuss how soils affect the physics of walls, for example, in the context of someone asking something like "will a wall solve this problem that I am having?" or "how can I achieve this objective in this setting?"

A request for just an explanation is not something that turns up here all that often. But an explanation is not a discussion either. It is sort of at the other end of the "discussion" spectrum from advice.

So I don't think the issue is that you are in the "wrong" forum. I don't think you want a forum at all. I suspect that the people who have the ability to provide the explanation you want do not dispense it through forums: they teach classes or write books.

So you want a textbook, or whatever the on-line equivalent of that is. And I actually don't think it would be a physics textbook. Soils and hydrology are specific fields unto themselves.

Karin L

    Bookmark   June 19, 2012 at 1:26AM
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lazy_gardens

GardenShenanagans - The core physics of terracing? Newtonian physics, specifically "gravity".

And then "angle of repose" for a given soil type, which is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material.

And it's affected by hydraulics as soon as you add rain or ground water in the slope. And that is affected by the geology of the slope. There are the famous "slippery layers" in the California coastal hills that are fine until you make a cut through them, then the clay layers start sliding.

It's all an attempt to slow down or stop "mass wasting" ... material eroding and sliding and falling down hills:
http://en.wikipedia.org/wiki/Mass_wasting

http://geology.cnsm.ad.csulb.edu/people/bperry/Mass%20Wasting/Introduction_to_Mass_Wasting.htm

And that is why we have Civil Engineers and geologists.

    Bookmark   June 19, 2012 at 5:56AM
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shaxhome

The added weight of the rocks in your proposed terraced walls above, is heavier than the existing soil. They will not defy gravity unless each is based solidly at the bottom ground level...
I've done a bit of this work over the years.

Your walls will fall.
Get an engineer to convince you.

Here is a link that might be useful: Stone Walls

    Bookmark   June 19, 2012 at 9:53AM
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gardengal48

To add to lazygardens' post, the other elements needed to be considered are force and direction - ie. vector analysis. Not at all a topic the general layperson, landscape background or not, is very well-versed in and not a topic very well-suited to discussion on this forum - they offer entire college level courses on the subject. Not to mention all the specifics related to soils, geology and hydrology that are necessary to factor into the equation.

In short, not at all a '25 words or less' type of response. In this case, you DO need a professional.....and maybe one with a teaching certificate as well as an engineering degree ;-)

    Bookmark   June 19, 2012 at 4:13PM
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yardvaark

There are innumerable variables that one would need to take into account when designing retaining walls. I think soil type would have everything to do with the physics of your hypothetical question. There are some soils that exist in a state that is nearly like soft, dry rock. They can be carved away and will stand for a long time without the assistance of a retaining wall. Other soils are like heavy mush with high water content and behave more like a slow-flowing glacier. And there is everything in between. Angle of repose would matter much as would wall structural details. The idea of a series of smaller walls vs. one larger wall is sometimes the preferred, more practical solution, but making it work also depends on sufficient depth of wall, adequate footing and reinforcements as per the specific situation. Best rule of thumb on retaining walls: overbuild.

None of what I've said should be construed as construction advice or encouragement. :-)

    Bookmark   June 20, 2012 at 9:06AM
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pls8xx

I would have to agree with what others have said on the complexity of slopes and multiple walls, or even a single wall with a continued up-slope.

But for most forum readers a more useful question might have been ... "Is there some rule of thumb that a homeowner can follow for slope design that will generally prevent a disaster." For my area there is an answer to that. It limits the possibilities to something much less than an engineer might approve, but could be a suitable solution for the homeowner.

    Bookmark   June 20, 2012 at 12:57PM
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frankielynnsie(7B)

Could we do a new discussion on pls8xx last entry? As home owners with limited funds 20 years ago we terraced a 5 foot bank that formed an 'L' around the back and one side of our home with rail road ties. We made three 4 foot wide beds about 2 feet tall. We need to replace a few ties but overall the terracing has held up well and has brought my flowers close to the house and easy to care for. We just plunged in without any real idea of what we were doing. It would be nice to have a little more knowledge for future projects.

    Bookmark   June 20, 2012 at 2:10PM
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marcinde(7)

What most homeowners don't realize is that, in many areas, your local building office offers typical retaining wall details that provide a recipe to follow. Are they over-engineered for many applications? Absolutely, because they're designed as a one size fits most solution. And that's a good thing.

Here in northern Virginia, most counties provide specs for retaining walls from 2' in height up to 5' in height, built from 6x6 timbers, formed and poured concrete, and CMU. Some manufacturers' systems of segmental retaining wall block are provided as well, but most jurisdictions have only certified one, maybe two manufacturers (most likely because there are so many out there).

Anything over 5' requires an engineer. I think it's also important to note that if you have a 4' wall, 4' of bed behind that, and another 4' wall, you need an engineer because you functionally have an 8' wall in the eyes of the county... and physics.

While I appreciate the OP's desire to understand the physics of how walls work, etc, there are just way too many variables for a lay person to pick up from an internet post (never mind that if someone was bored, they could troll you and make up formulae - welcome to the web). And, it's way more work than anyone needs to go through. As a professional who designs several walls a month, here's my process: Check to see if the wall falls within spec for typical details. If it does, design the wall to those. If it does not, draw up the walls where and want them and how high, and email the file to my engineer. It's much easier, and it keeps things from falling down and going boom.

    Bookmark   June 20, 2012 at 5:29PM
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karinl(BC Z8)

As a person who has achieved a high level of lay understanding of some fields, and who also has profession-level knowledge in others, I don't buy the line of argument that says anything is too complicated for a lay person to understand.

I think what usually differentiates a trained expert from a well-studied lay person is the ability to identify variables and assess risk, as well as to understand where they sit on the learning curve. The greatest danger is a layperson who THINKS they are up the learning curve, but who in reality is missing a key body of knowledge or piece of fundamental information, and thus one element in their analysis.

So the advice to engage an expert should not be read as "you can't possibly learn this yourself." It should be seen as a way to manage risk. And there, it matters what the risk is. If this wall is in the middle of nowhere with no risk to any place or thing, then well and good. But if it is at the top of a cliff with homes or a sensitive environment down below, you cannot just go "oops" when it fails. A wall failure that causes property damage, loss of life, or flooding because a layperson really thought they had thought of everything cannot be considered a good learning experience.

I usually post the local media story about a backyard pool that fell down a cliff in a rainstorm, taking out one house, damaging others, and killing one and injuring others. I don't have time to find it just now, but tragically it is not a unique story.

That's why the key question about whether to call a professional or try to learn it all has to be "Well, what is at risk?"

Ah well, Gardenshananigans has likely shenaniganed off and we are talking to ourselves anyway. Perhaps this will be useful for others.

Karin L

PS FrankieLynn, start a new thread by all means.

    Bookmark   June 20, 2012 at 6:27PM
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gardengal48

I don't think anyone said this subject was too complicated for a layperson to understand.....only that it was specialized enough that few laypeople would be able to converse knowledgeably about it. Heck, with a degree in architecture and multiple quarters of structural engineering under my belt, I wouldn't consider myself the average layperson and even I am not comfortable discussing the subject at length. I farm all my structural jobs out to engineers myself.

Areas requiring technical expertise should be left to those who have that expertise and use it daily - IOW, a professional.

    Bookmark   June 20, 2012 at 8:26PM
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deviant-deziner(Oh zone)

Frankly, the question isn't 'what is the risk', but what is required by law.

I appreciate and applaud people who want to learn how structures work but when it comes to engineering retainment walls , a lay person needs to know their limits.

There is a reason why I pay my engineer $ 150.oo per hour to detail and provide the calculations for the retaining walls we build - it's the law and its a matter of public safety and honestly a trained structural and geotechnical engineer/s are the only professionals that are recognized by the building department to submit the calcs.
Even landscape architects with a Phd or masters do not possess the legal accreditation to submit drawings and the calcs for a retaining wall - they require an ' licensed, bonded, insured engineer'.
These engineers didn't spend 4 to 6 years in college and walk away empty headed.
It is doubtful that any layperson could understand the calculation sequence that is required to obtain a building permit.
I'd be happy to share a few calculations and would bet that it would look like chinese characters to most.
Don't diss the engineers, they are what make our structures safe.

    Bookmark   June 20, 2012 at 8:40PM
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loreleie

I haven't read through all the responses and Im not even sure this will help but here's a link to an article with some useful information about stabilizing a slope.

Here is a link that might be useful: Simple erosion control for a hillside or garden slope

    Bookmark   June 21, 2012 at 4:34AM
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pls8xx

deviant-deziner said:

"Frankly, the question isn't 'what is the risk', but what is required by law.

I appreciate and applaud people who want to learn how structures work but when it comes to engineering retainment walls , a Frankly, the question isn't 'what is the risk', but what is required by law.

I'm sure what she says is correct for her area. But in the small towns and rural areas of Arkansas, there is no regulatory law for walls, none. No permit required or inspections made. For us the "lay person needs to know their limits" is the whole ballgame.

Since there is no law requiring a wall permit, none of the engineers point their practice at wall design for residential property. We do have engineers that do wall design, but it's for commercial or highway projects where the wall cost is often a tiny fraction of the project cost. If you can find an engineer willing to do a small residential wall, the design is likely to be so over engineered as to be outrageously expensive. His engineering fee will be likewise.

    Bookmark   June 21, 2012 at 10:31AM
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deviant-deziner(Oh zone)

Pls8xx,
Good to hear from you. I have always appreciated your feedback.
I thought that every state in the US was required to recognize the IBC and the UBC ( international and unified building codes )
I know that California takes it several steps further and has adopted a Seismic code, a Green Code and a Rip all your money off Code for the shear pleasure of doing construction business with you, thank you very much code.

On top of the UBC the IBC and the supplemental Green, Seismic and FU codes we also have local ( county or town ) ordinances that stiffens up the UBC and adds another layer of Design Review in some cases.

I now have to submit a " Vegetative Manangement Plan" if a building permit is pulled for a project - even if there is not landscaping being done AS WELL as a Water Audit Report showing evaporation transpiration rates of each f'ing plant.
It is no wonder why CA is the most expensive state in the union.
I'm constantly paying education fees and spending hours just keeping up with all the certifications required to plant a stupid plant.

I can't fart and open the window here without getting a permit, ... really ... a permit for window replacement is required.
Yesterday I was sent a 'home user occupation fee/ permit application from the city to submit. What the Fu.... a revenue fee for using my house. $ 77.00 - this is not to be confused with a buisness licence fee that is an additional 150.00 .

    Bookmark   June 21, 2012 at 11:33AM
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pls8xx

We build homes here for around $110 to 140 per square ft. Such things as granite counter tops push toward the upper cost. Homes are inspected for structural integrity, electrical, plumbing, and HVAC. That's about it for regulation for much of the state.

I got caught a few years ago building a 18 x 18 DIY addition on my house without a permit (I live in town). They made me provide a copy of my lot survey and a crude sketch of my project plus $75 for the permit; no plumbing and the only electrical was relocation of an outside light to the center of the addition. They didn't ask me about the shop under the room and I didn't tell them what I had planned for the electrical down there.

    Bookmark   June 21, 2012 at 1:09PM
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deviant-deziner(Oh zone)

75 dollars for even the most minor of permit applications would be dream come true.
I shouldn't complain as much though, much of my billable time when working for a client is wading through all the administrative crap that is set up as 'protections for the public'.
You couldn't build a carport here for 140.00 a square foot. The medium average is about 275.00 sq. ft for a simple residential remodel. ... and that is if you already have a water meter on site. If you don't have a water meter you might not even be able to build on the site even if you handed over a million bucks.
I've been working on a swimming pool / pool house / retaining walls ( big ass walls ) for more than 2 years and we just submitted for 'planning comments' last week . We hope to break ground this late summer . It will be in the 400K range - and that is after we scaled the design way back - no more infinity edge pool, no shade structure, tiny equipt/ storage room . silly, especially when you consider this particular area is fairly cool in the summer months due to the coastal fog .

    Bookmark   June 21, 2012 at 2:42PM
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pls8xx

A consumer can be totally ignorant where regulatory 'protections for the public' exist. Around here people have to be on guard. It works because people learn to network. Reputation and experience means more in selecting a contractor. And no matter what the subject is, most know someone they trust to provide insight. I average getting about two calls a week.

    Bookmark   June 21, 2012 at 4:26PM
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