Retaining Wall Question

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Forum topic by TopamaxSurvivor posted 07-24-2015 05:43 AM 2209 views 0 times favorited 59 replies Add to Favorites Watch
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18286 posts in 3700 days

07-24-2015 05:43 AM

Topic tags/keywords: retaining wall earthqake

I’m wondering if I can tap the collective brain power of LJ to find a definition of the grade differential required to determine when a concrete block fence becomes a retaining wall? Obviously one cannot continue to pile dirt on one side to the top of a 6 foot fence and still call it a fence can they? What is a reasonable grade differential? The local building department does not seem know and I have not been able to find much on line about the construction of concrete block fences and the footings or foundations required.

Here in the Seattle Tacoma area, geologists have predicted the odds of a magnitude 9 or greater quake in the next 50 years at 10 to 14% and an 8 or greater at 37%. This is due to the Cascadia quakes. The last one on January 26, 1700 is thought to be #7 on the all time biggest quake list.

In the mid 90s geologists thought we are due any day because the coast line has pushed up 3.5 meters since 1700. They believed it is higher than it has ever been. When the coast from Eureka to Victoria drops 15 feet, I’m thinking they will be wondering what happened in Boise or maybe even Cheyenne when it happens.

The reason for my curiosity is a neighbor built a concrete block wall 6 feet high and piled 4’ of fill against it at one end. This wall has no footing or foundation. The based he poured to start the block work is just a few inches (less than 6) into the soil. I have no experience or expertise in this area, but we generally bury electrical poles a minimum of 25% of their height. It does not seem reasonable to me to allow a 6 foot high, 175 foot long, 35 ton concrete slab to stand on edge in a residential area between 2 houses in earth quake country expecting a 9 or greater quake any day. It would seem prudent to use a foundation or footing to prevent lateral slippage and tipping, would it not? Any comments?

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

59 replies so far

View NDakota's profile


72 posts in 1570 days

#1 posted 07-24-2015 06:13 AM

I will bet it falls over long before any earthquake hits! That is far to much fill on one side of any wall. I’v been in construction for over 30yrs, a lot of retaining walls, and I know that wouldnt pass inspection in ND.

View TopamaxSurvivor's profile


18286 posts in 3700 days

#2 posted 07-24-2015 06:24 AM

The dirt against it is only about 40’ and tapers down from 4’. I don’t see how it can be stable other than 35 tons is hard to move ;-) I’m assuming the reason I can’t find a definition for the grade differential is the code panel thinks no one would be stupid enough to allow or do this!!

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View David Grimes's profile

David Grimes

2078 posts in 2664 days

#3 posted 07-24-2015 06:30 AM

Hello, old friend. There is nothing (no aspect) right about that “retaining wall”.

Where I am (Lee County Georgia), we would be required to install steel rebar reinforced footers and vertical rebar into the block, then the block would have to be concrete filled. Codes and practices vary from one area to another, but nowhere would a non-footer hollow block without additional reinforcement be allowed or wise in any case.

Fodder for your cause (and this should be enough):

Dangerous is the word I come up with!


David Grimes

-- If you're going to stir the pot, think BIG spoon or SMALL boat paddle. David Grimes, Georgia

View TopamaxSurvivor's profile


18286 posts in 3700 days

#4 posted 07-24-2015 06:39 AM

Thanks David, I see the requirement on the second drawing is a minimum 24” deep footing. I can only believe the building inspector that talked to him was totally incompetent!

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View stefang's profile


15881 posts in 3358 days

#5 posted 07-24-2015 09:19 AM

I agree that it will eventually fall over Bob. That is a very dangerous wall. I have read in the English news several times where walls with inadequate footings have collapsed towards the sidewalk and killed people, children among them, and these were not even being used as retaining walls. You have very good reason for concern.

-- Mike, an American living in Norway.

View Redoak49's profile


3283 posts in 2012 days

#6 posted 07-24-2015 11:45 AM

I would worry about it. Can you talk to the building inspector and also find out what the code is for your area? That is the place to start.

Are you on good enough terms to talk to your neighbor? I had a 6’ by 140’ fence put in between my house and a neighbor. He was most unhappy and called everyone he could to get it taken down. It was well v within code and it was own my property.

I think checking building codes is you best chance of getting something done.

Good Luck with this

View firefighterontheside's profile


18351 posts in 1880 days

#7 posted 07-24-2015 11:56 AM

In this area anything over 4’ has to be engineered and have support from the back. That usually means mesh that is built into the wall and then back filled on top of it to help support the wall. Is this a cinder block wall or a wall built with actual retaining wall blocks. If it’s retaining wall blocks, I’d say it’s fine. If it’s plain old concrete blocks, I’d say it will fail pretty quickly and before the earth quake.

-- Bill M. "People change, walnut doesn't" by Gene.

View Ralph's profile


166 posts in 2157 days

#8 posted 07-24-2015 01:04 PM

My story…
In the early 70’s I built two retaining walls. One wall about 60’ long and 6’ high. Back filled one side of the wall to about 5-1/2’ high.

I pored the footings myself- reinforced with a coouple of 3/8” deformed rebars; also rebars going vertically from the footing up the wall every 16”. The first course of block was laid into the wet concrete. The rebar went through the block and the hollow of the blocks filled with mortar.

The wall was block faced with brick. I put a rebar horizontally every layer of block, plus a wire screen connecting the block and brick together.

The wall was about 60’ but the center 20’ section was offset forward about 8”. At the ends of the 20’ center section I put a buttresses going back about 2’ at the foundation with rebars going up and connecting into the wall.

I puored the footing (with my trusty Craftsman 1/2 bag cement mixer), bent the rebars myself; I was much younger then.

The second wall was longer, higher in spots, but had some curves to it.

Forty plus years later, both walls are still standing just fine here in the LI, NY area.

But unlike your neighbor’s wall, my walls have a real footing, steel reinforcing, and a couple of buttresses.

Also, I made weep holes every 16” along the base.

-- The greatest risk is not taking one...

View PaulHWood's profile


435 posts in 2277 days

#9 posted 07-24-2015 02:29 PM

IBC Chapter 18 covers retaining walls and they all need to be designed, there is no height limitation I can find. If you are over 6 feet of retained earth, several seismic items kick in if you are in SDC D through F which I am sure you are. I have designed retaining walls and planter seats as low as 2 feet tall.

Two modes of failure are sliding and overturning for both a retaining wall and a fence, one loaded with soil and hydrostatic load potentially, and the other with wind & seismic. Block fence should have a footing. It will likely fail, but usually they do not fail catastrophically at that height. The soil load will cause rotation and cracking which actually will relieve the soil stress. It could also just tip over as a unit if the block wall is reinforced.

We looked at a house where the 3 foot retaining wall had rotated and the soil opened up behind it. The builder thought the solution was to backfill it again. Yes load it up again so it can rotate more?? I told him it was what we considered a structural failure and required more than soil to fix it.

-- -Paul, South Carolina Structural Engineer by trade, Crappy Woodworker by choice

View RobS888's profile


2413 posts in 1869 days

#10 posted 07-24-2015 03:18 PM

I found this on the Keystone website. I’ve made 2 retaining walls using their standard blocks.

Keystone recommends additional trench depth for below grade placement of Keystone units on a ratio of 1” (25 mm) below grade for each 8” (200 mm) of wall height above grade (to a maximum of 3 units buried). This lowers the base course below grade locking the wall in place and also helps prevent erosion and scouring at the base of the wall. The base trench should be wide enough to allow for the Keystone unit and drainage zone. An option to a compacted, granular material leveling pad is to use a non-reinforced concrete leveling pad. In some cases, contractors find this is a time-saving approach. Walls built to a level condition on a sloping grade require a stepped base. It is best to work out the stepped base as the wall steps up in elevation. If a concrete leveling pad is used, the step-up height needs to exactly match the Keystone unit height. Do not use PEA GRAVEL for levelling pad.

I set a full course below ground and used 4’ rebar on the end blocks and 2’ rebar on each of the blocks in the base level. My wife thought I overdid it, but it hasn’t moved. They also suggest you set each course back about an inch.
I put 12 inches of gravel behind the wall with drainage at the bottom.

-- I always suspected many gun nuts were afraid of something, just never thought popcorn was on the list.

View hhhopks's profile


651 posts in 2401 days

#11 posted 07-24-2015 04:05 PM

I would recommend going to city hall and get some retaining wall reference.
I believe most larger city has a code and would have typical / guide lines to follow.
I have purchased such reference document from my city.

-- I'll be a woodworker when I grow up. HHHOPKS

View TopamaxSurvivor's profile


18286 posts in 3700 days

#12 posted 07-24-2015 06:21 PM

I just lost the response to most of you ;-(( ;-(( I’ll be back after a short cooling period ;-)

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View nkawtg's profile


277 posts in 1275 days

#13 posted 07-24-2015 06:30 PM

I’d bet he never got a permit to build the wall.
Call the building code enforcement division in your town and have them inspect the wall.
They may make your neighbor take it down.
Sore feelings are better than the alternative.

View TopamaxSurvivor's profile


18286 posts in 3700 days

#14 posted 07-24-2015 07:45 PM

Mike, thanks for that little tidbit. Good to know there are problems documented.

Reoak49, I‘m on speaking terms but I think he is a bit upset about us continually requesting they park so we can see to get out of the driveway. This is an arterial street with a 35 mph limit. There is a blind curve about 75 yards in the opposite direction. One needs unobstructed visibility to get out without being hit. Especially when the occasional 55-60 mph goes by ;-(( My biggest concern is not being able to see bicyclists. They have a bit of down hill grade and can easily do 35 mph. With only 40 feet to react, they don’t stand a chance when the view is blocked out to the fog line. He told my wife to go out slower. Going slower that stopped is impossible and one still cannot see through a solid object such as a Ford 150.

He doesn’t seem to care much about public safety. He placed a row of rather large rock along the edge of the pavement. The public right of way extends 20’ or more behind the row of rocks. I used to hear cars hit them occasionally, but never saw any signs of an oil pan being punctured. The city transportation dept didn’t seem to care. I used to move them out from in front of the school bus stop and he’d move them back. I finally took a picture of the kids getting off the school bus and emailed it to the city. Some of the rocks were knee high to a 5 or 6 year old! The city picked them all up after they saw that picture.

Bill, It is 8 “ concrete block with an architectural finish on both sides. The construction is consistent with what I saw block layers doing in buildings during the 45 years I worked on construction sites except for there not being any footing or foundation.

Ralph, Sound like your wall is more substantial in design and construction. What little I have learned indicated anything that deviates from a straight line adds to the stability.

Paul, thanks for the info. I believe the first and foremost issue I need to address is the definition of when a “fence” becomes a retaining wall by piling soil against it. My biggest concern is lateral sliding prior to falling over. If it slides much, it will definitely hit the house. Along the end away from the backfilled end, there are some trees planted that are about 20 feet high about 3 or 4 feet form the fence. He says they are just “shrubs”. They look like cedar trees to me. If they continue to become full grown cedars, I believe they root system will put addition rotational pressure on the wall. Do you have any opinion on that?

Rob, These are standard x 16 concrete build blocks. Nothing like the ones you referenced. .
hhhopks, Unfortunately, be there and done that. I did not file a complaint. The fellow told me if it started to lean, they would take action. My biggest concern is after a 7.5+ quake, when up to 50% of the structures in the area have collapsed, moving that 35 tons of concrete will be a very low priority. My little stick built house may very well rock and roll with flow and survive. That 35 ton vertical concrete slab sitting on top of the ground will be coming down in one form or another. Masonry structures are the first to go in big quakes ;-(

nkawtg, I’m not sure one was required for the fence he told them he was building. But he has certainly exceeded what they said he was allowed to do at the time.

I just thought I’d throw this out and get a little more info and maybe a few more brain storming ideas to present to the building department at city hall. When he started pouring the slab to base the first course of blocks, I asked what he was building. He said a retaining wall. That didn’t sound like a very good idea to me. There was no change of elevation to retain. The city told me a retaining wall required 5’ set back and a footing. They came out to see it and he told them it was just a fence. The first 20 feet at the street end could only be 4’ feet high; 6 foot for the remainder. After the house was built, he started landscaping. He added 2 more courses of block to the 4 foot end and piled 4 feet of dirt by his driveway.

Robert Frost wrote “Good fences make good neighbors.” I’m not so sure about this one? ;-((

Here is what it sets one

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View ElChe's profile


630 posts in 1360 days

#15 posted 07-24-2015 08:41 PM

Not to worry. The chain link fence will hold it up. Can I put my Jack Russell Terrorist’s dog house next to the fence? Just kidding.

-- Tom - Measure twice cut once. Then measure again. Curse. Fudge.

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