Retaining Wall Design

Concrete has little tensile strength; thus the need for reinforcement in a retaining wall subject to bending (no problem for the part that is in compression). Twelve feet is a substantial height.

Your architect has rendered poor advise.

12' is quite a height for a cantilever retaining wall. 8" thick does not sound sufficient to me but this does depend on the soil you have to retain. You cannot get good advice here without a considerable amount of extra information. It is best to get an engineer who has some experience in this type of structure to do the study. The concrete blocks do not sound a good idea.

not to forget the horizontal force due to the push from the earth which is resisted by the use of a toe which gives passive or active resistance to the push.

also note that you need to design for very small movements or you will get cracking in your house walls. 12' is a high wall and deflections will be key.

Indeed you need a local engineer to design your wall. It would be difficult at best and likely impossible to build that wall with CMUs. If you are not going to pick up any strength with your diaphragm you will need more rebar than the blocks will hold. See the ACI standards for retaining walls built without reinforcement. I seriously think you should re-visit the overall structure, calculate the strength generated by the second floor and build appropriately. Any competent engineer will pay for himself on this project. And think about firing your architect. If you don't have anyone available to form the wall you can pour it with foam blocks and still get a vastly superior product.

Years ago I designed a RC "counterfort retaining wall" for a similar condition. No big deal.

Have a structural engineer design it.

Cheers

Vince

Hello Flyinghigh,

My advice to you is go with a "Cantilevered" reinforced concrete retaining wall instead of a reinforced CMU retaining wall. The footing and the wall stem construction will have to be done consecutively, not monolithically. Additionally, it's easier to design and construct that say a Counterfort retaining wall, which is not really required for a wall only 12-feet in height. Also, slope the retained soil mass surface AWAY from the wall & house (@ 2% minimum gradient) so as to promote good drainage practice and lessen the lateral earth loading placed upon the retaining wall. And don't forget to install a positive draining wall drain (and surrounding gravel pack inside a filter fabric envelope) at the base of the wall stem....make sure the ends daylight, or at least terminate into a storm sewer catch basin or storm drainage manhole if necessary due to actual field conditions!

Don't get me wrong, a CMU can be designed and be more economical (sometimes). Only problem I see with the CMU type wall is groundwater intrusion (if seasonal high GW is present) as well as significant cracking occurring at the mortar joints (due to poor tensile properties thereof) and at the wall corners and at any intermediate CMU wall juncture. I'll echo others statements here by saying that your Architect friend/buddy is ill-advising you and doesn't have the necessary structural engineering background and expertise to make those sorts of recommendation.....actually he/she is suggesting something outside their core of professional experience, resulting in what could be referred to as professional malpractice. The state licensing board for Architects may in fact look dimly at his/her actions.

I don't know your engineering background, but if you have never designed a reinforced concrete retaining wall before don't do so now, as you may make a huge mistake in your design and it could potentially cost you in terms of property damage (that your insurance carrier will not cover due to a bad design) and possibly bodily harm or death(s) of family members. RetainWall program is a great design aid, but do not rely on it solely for your design. A lot of the final design of any retaining wall will have to be informed engineering judgement calls resulting from design RC experience....you could possibly miss something very important, thereby relying on erroneous data and producing a bad design, especially when reviewing what numbers and recommendations the computer software spits out. I've seen this over and over again with "Greenhorn" engineering grads, and yes, even Architects and Civil Engineers not properly or adequately schooled in Structural Engineering, let alone formal SE experience and practice.

If you have never design a cantilevered retaining wall, then obtain the services of an experienced Licensed Professional Engineer locally who specializes in the Structural Engineering discipline. Do not rely on some fly-by-night Civil Engineer, even if they are a PE, who is "moonlighting" to make a fast buck. First off, they may not be qualified to perform a design of this nature, and secondly they may not be insured to cover any potential damages resulting in a structural design related defect or a wall failure.

Lastly, I wouldn't go with a battered wall as it's damn difficult to erect the form work effectively and get adequate concrete placement. Also, this type of wall form work if more expensive. My advice to you is to provide one or two "step-outs" or ledges where you progressively increase the wall thickness as you go deeper....for example (do not rely on these numbers as they didn't result from an actual engineering design and analysis), 6-inches thick at upper extent of wall, then 8-inch thick and middle segment of the wall, and lastly 10 or 12-inches thick at wall base. Newer versions of RetainWall and other structural engineering software platforms do allow for this approach. You'll save money with materials, but there will be a slight increase in form works. All-in-all, it's a very good approach to take.

I also strongly advise that you have the soil mass behind and under the retaining wall investigated by a Licensed Professional Engineer who specializes in Geotechnical Engineering. This is insurance for you and the wall designer to get in right and avoid a failure sometime into the future during the service life of the wall. You most likely need to have a couple of soil borings done in the soil mass, conducted by a certified testing agency/lab and directed by the Geo. Eng., who will later issue a Geotechnical Engineering Report with design and construction recommendations as well as soils data.

Good luck with your project! If you should have any additional questions pertaining to this matter, please feel free to ask questions.

Signed,

CaptMoosie, NYS LPE / PhD

Civil, Structural & Environmental Engineer

Hi CaptMoosie,

I understood that Flyinghigh is cutting into the hill and that would give the wall a surcharge with surface sloping upward from the crest of the wall...

ergo I advised a counterfort retaining wall.

As per the old engineering adage "When in doubt use big bloody nails"

Cheers

Vince

Depending on the actual existing conditions occurring on and in that hillside, such as slope severity, depth of groundwater if present and the make-up of the soil mass, a counterfort retaining wall may not be necessary. Even a wall that is 12-foot in height, a cantilevered RC retaining wall may be more then adequate to resist the lateral loading due to the sloped soil mass,

If need be, a cantilevered retaining wall can be a "modified cantilevered retaining wall", where the wall stem is designed as a "PROPPED" span, meaning that top of the wall stem is laterally supported with horizontally-oriented steel struts, say spaced @ 48-inches o/c and terminating at a centralized RC core. This would resist the horizontal thrusts (reactions) imparted upon them and also lessen the Design Bending Moments and shears acting withing the wall stem,and hence reduce the amount of steel reinforcement for both bending and shear. It would also lessen the horizontal reactions (and sliding) at the base of the wall, although most or all of the force can be taken-up by the basement slab on grade. The aforementioned horizontal struts could be either TS_x_x_ (?) square or rectangular structural steel tubing, or if needed, wide flange steel beams. These struts can be anchor-bolted to the opposing RC walls using steel plate end connections. Adjustments at the ends of the struts can be made using horizontal slotted bolt holes in the strut WF webs.

I would NOT recommend using any wood framing and floor sheathing to help resist the lateral wall reactions resulting at the top of the propped cantilevered retaining wall. The reason why is that the imparted loads would most likely be too great and overstress the wood framing in the first floor. If the first floor framing featured structural steel framing members, metal floor deck and a reinforced concrete structural suspended floor slab, then it may be possible to use the floor diaphragm to resist the lateral wall loads. The only problem of this approach is the cost + the need to provide either steel columns or another reinforced concrete wall in the opposing wall to pickup and distribute the those loads to the foundation below.

Another 2 Cents added.....

Dear, Sir

If there any software program to design retaining wall free

Not really. I haven't found any "free" retaining wall programs worth their weight and effort yet.

You may want to purchase a software package known as RETAINWALL, as developed by JAVASOFT. It is a very good program and very flexible regarding analysis and design of masonry and reinforced concrete retaining walls. It's also reasonably priced ($100.00 USD) and will pay for itself the first time you employ it for a project.

I'm not sure if the latest version deals yet with Segmental Concrete Masonry Retaining Wall (concrete blocks w/ Geotextile fabric anchoring.....basically "reinforced earth" technology) design and analysis as well as "GLOBAL STABILITY ANALYSIS". Some concrete block manufacturer's in the USA and Canada have their own programs......so does TENSAR.

Here's a link for a downloadable TRIAL version of RETAINWALL 2.60:

http://www.javasoft-softwares.com/retainwall.htm

NOTE: Remember, this Trial version is only good for 14 days and has limited utility and restraints.

First I would pat the architect on the head n' give him a couple cookies to occupy him. secondly I would prefer a poured wall w/ several rods anchored deep into the hillside tied in but since this is not your course of action I would have a network of 1" tied rebar in the floor pour, 2' of which would be bent up to receive the 10" block work(being safe rather than sorry). This would help the "shear factor" of the hillside. Depending on the length of this wall I would tie in pile aster's every 4', drop in rebar every 2' of the completed wall then pour it solid. I would backfill against the wall w/backhoe using '57' limestone (limestone "locks" together, gravel never will) lay your covered perforated pipe w/correct drop around the base making provisions for the down spouting. Just make sure to check your local building codes and save the aspirin for later n' good luck.

Don't neglect side walls. These will greatly strengthen the whole assembly. After all of this and that, the biggest problem is keeping the whole damned hillside from falling off. Can you say "Califonia?"

Hmm, A 12 foot high wall is a relatively minor design, though definitely structural. Typically stem walls on retaining structure thickness run about 0.08 to 0.12 times the height of the wall. It would be extremely common for such a common structure to have the stem wall constructed of CMU. CMU walls at that height have to be reinforced just like concrete. You might need a deeper CMU block than standard 8-inch, fortunately 12-inch deep CMU is also really common. There really is nothing particularly distinctive between using CMU and concrete, as CMU is made of concrete and filled with concrete. You just might not be able to specify extremely high strength material. however, you probably would not plan on this anyways, as the quality control becomes a huge issues on common simple construction projects. As you don't want the wall to move laterally, you should use at a minimum the at-rest lateral earth pressure. You should evaluate how much you can excavate backbehind the wall before construction to develop the heel, and how much room is allowed in front for the toe of the foundation for the structure. This is really a fairly straight forward design, most Civil engineering student would do something like this as a test question on a exam.

You have made far to many assumptions to say this is a minor design.

Maybe there is water behind this wall and so crack widths and water proofing are an issue.

Maybe the soil is not very cohesive and so the push on the walls is higher than what would be "normal".

Maybe the hillside has a history of slope instability.

Maybe the two lateral walls to the house are simply masonry and so provide no lateral support and may be subject to cracking.

So many possibilities and no answers.

Not one of the answers so far presented has even touched upon the actual site conditions. The best answers have said that a structural engineer should be employed.

This is perhaps the worst example of engineering advice that I have ever seen on this CR4 site. If the advice of some of the participants was to be followed, it is possible that people may have died from the consequences.

You should actually consider the potential for submergence in design, it isn't that hard and is a well established standard of practice (However, this doesn't mean that the wall necessarily has to be designed to bear against submerged pressures, as a drainage system might have been used in design). Still a basic retaining wall design problem. We did this in intorductory geotechnical engineering class as a test question, including the reinforcement design calcs, concrete design, everything. Also, it isn't non-chesive soils that are the problem as they have the same behavioraly characteristics (angle of friction) wet or dry, cohesive soils can lose substantial cohesion when saturated and have a very low angle of internal friction thus adding substantial loads. Typically you would replace any cohesive soils that are agains a wall with non cohesive engineered fills anyways. However, in the end, even with very weak CH or OH soils, there are some limits that walls will fall within, such as stem wall thickness of 8% to 12%.

To address the other issues, it is straight forward problem even if site conditions were adverse. you address the adverse site conditions then design the retaining structure. As I indicated this is a structural problem, not simply an architectural issue. In all jurisdictions in the US, you would be required by code to have this structure designed and stamped by at least a professional civil engineer and submitted for review by the local jurisdiction. In order for a civil engineer to do the design, he is going to need some geotechnical investigation of the site conditions. This is all very routine, and I am saying this considering I have worked on a variety of retaining walls upto 40- and 50-foot high, some in in Diablo clay soils in the foothills along the California coast (those are a little more complicated, but basically still the same proportionally to the smaller structural walls though the seismic reinforcement is sometimes increased greater than standard proportion if it is a high improtance structure such as a major road protection structure). As a matter of fact some of the smaller simpler retaining walls can be designed in some more advanced jurisdictions using the jurisdictions standard details.

This is in response to our "Guest" in Post #13.

You're wall off base guy, as we have collectively answered the OP's questions to practically farthest extent possible given the limited amount of information presented to the forum membership. All you have to do is read through the responses to see that we've addressed much of what you have bitched about, especially about the need for the OP to obtain the services of two PE's (Structural & Geotechnical Engineers). We have also hit upon various options open to the OP for a final retaining wall design.

As usual, we cannot actually design this wall for the gentleman (the OP) from a forum situation due to very limited information provided, nor do we have any data relative to the Geotechnical conditions present at the OP's home site. Please let me remind you that essentially if for some reason a Licensed & Registered PE were to do so from this sort of online forum provide any sort of retaining wall design based on the limited amount of info provided, then that design professional could face potential censorship or Professional Engineering Licensing suspension or revocation for malpractice, etc. from the State Education Department (Professional Engineering Licensing Board) having jurisdiction over that PE's License & Registration.

And another thing, I and other CR4 members herein do not give much credence to persons coming in this forum as a "Guest" who do not identify themselves nor provide any sort of credentials, and collective educational, knowledge, and experience backgrounds. It's fairly easy to complain, but yet another thing for one to offer up any real world-time (as opposed to cyber-Net time) solutions.

It is really easy to be an "Armchair Quarterback or General", so do us an big favor by either putting up or shutting up....IMO, it is best for you to vamoose, unless you have sometime of relevancy to offer herein.....

Sorry for any comment that was inappropriate. It was not intended.

However, as I look back through the posts I see the following:

#3 "Shouldn't be too difficult" The advice that then follows may or may not be good advice depending wholly on the local ground conditions. The post finishes by saying that a professional should check it off which I agree with. My feeling is that the advice may well be wide of the mark and that, should someone wish to follow that advice, it could well be a very dangerous situation. The structure of the house itself and its foibles has not been considered which is also important

#7 (your own post but bear with me) Alot of good advice but I would argue that it is irrelevant until the ground conditions are known. Once again, if the OP were to follow up on any of this advice he may be doing something dangerous. I would worry that the OP were to pick and choose the advice given and not do the right thing and get a professional to advise him.

#8 Again, lots of advice which may or may not be appropriate.

#10 Again, advice which may or may not be good advice depending on the ground and the structure that is being built.

# 11 "relatively minor design" is perhaps a little optimistic given that we know so little about the site and the intended structure.

It is clear that most of the posts are covered by saying that an engineer should be used but I still worry that to give any advice at all is almost certainly bad advice.

I hope this explains my comments previously which were said, in hindsight, with a little too much haste and insensitivity. I apologise unreservedly.