retaining wall question

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bgaviator

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Is it possibly to have a retaining wall just at the top or middle section of a hillside, rather than having to start tiers at the bottoms? Could you dig down pretty deep, maybe two rows thick with those stackable wall blocks in the side of the hill for stability, or would it just topple over eventually? I want to replace these wooden walls the previous owner put in place, as they really aren't doing anything for erosion control. In fact, I'd like to make a wall high enough to be able to make this grass section level, rather than the immediate slope it has right next to the house. It's a pain to mow, and I think my house would benefit from having a little more flat ground to sit on.

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As long as the base is prepared properly you could do the wall right where the wooden one is and take it up high enough to make the ground near the house more level. You'd still want some slope for drainage but you can certainly make it flatter than it is today.

Check the manufacturers specs for how high the wall blocks can go. Also, if you go too high you'll need a barrier to keep people from falling.
 
If you are going vertically with cement block you will need a lot of strengthening. The footer has to be deep enough to protect against frost heave. If the wall exceeds 4', you must use rebar and fill the voids with cement. Additionally, you must provide drainage via a weeping tile (perforated pipe) to minimize the hydraulic back-pressure against the wall or it will crack.

Given the possibility of legal issues with your neighbor from drainage or wall collapse, I would get permits for this job ...

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Do you have any ides of the foundation type and depth? A house can exert considerable laeral forces on any type wall, depending on the depth. If the foundation is deeper, the loads will be minimal.

The segmental retaining walls (SRWs) do not require concrete footings, steel and reinforcement because they are able to withstand the loads. Usually, an engineer is required to protect from yourself on walls over 4' or 5' or tiered walls to get it planned right.

You could also do a single wall at the bottom, but would have to use some "geogrid" (nylon mesh soil reinforcement) at several levels.



Dick

You wall will have an end or two or corner and this is easily handle with SWRs since they can be curved and tapered up or down.

Your situation is ideal for a tiers system with a lower wall

The Allan Block system is very similar to the other 3 major systems available in the U.S. and most foreign countries and have complete information for design most walls. I have seem walls (inside and outside curves) several miles long along freeways varying in heights from 1' to 30' high in Spain, so they are not unusual or impossible.

Dick
 
I would need a wall about 4 ft high, or just slightly higher to bring the dirt more level in this section. Now does the wall height include what gets buried, or only what's visible above the buried layer? The reason I asked about only having a wall where those timbers are is to minimize cost. We are going to be doing the back of the house with probably 2-3 levels of retaining walls, and early estimates are $15-20k. A ton of money to me right now! And that's just for the back walls. I would ideally like to have everything done in one shot, so that's why I'm trying to minimize cost on the side of the house. The hill is even larger on the side, and to do multiple tiers would probably cost another $15k I imagine.
 
bgaviator -

Most people do not understand the difference between a "rigid" wall (wide footing and a slender reinforced vertical wall) and a "flexible" wall (small shallow footing) that is actually structural facing of a "gravity" wall. You usually do not build structures on a "flexible" wall, but fencing is commonly done. In the case of a near-by structure, the load of that is converted to a surcharge on the soil, just as if it was more gravity load for either type of wall.

The major construction difference is the amount soil involved. A "rigid" concrete wall requires a large amount of excavation and backfill because of the depth AND distance back into a hill that increases with height. A SRW wall is only about 8" deep plus and addition amount (8" or so) for compacted gravel footing that also provides a horizontal base for starting from. This dramatically decreases the volume of excavation and material movement and traffic.

A "rigid" wall uses a vertical slender reinforced wall that is tied to a wide footing with a overlapping layers of steel reinforcement. The vertical reinforcement is needed in the thin wall because the concrete is inherently weak in flexure and to make up for this.

A "gravity" wall is NOT exactly vertical. The face has a slight batter into the soil that is created by shear lugs on the bottom or top of each block, so there in no tension in the concrete, but only shear, which concrete can handle easily. The wall does move and return in place with the weather (temperature, frost and soil pressure) and provides slow uniform drainage of the usually saturated soil behind a "rigid" wall. This interlocking units still allow some curvature (inside or out side) of the walls. The individual units allow for stepping the wall height up or down at the end to match landscaping needs.

The two are totally different method and the gravity situation is obviously older and proven, but the SRW units provide a structural facing that becomes for efficient since it allow geogrid to reinforce the soil behind in the case of higher walls and in areas of poor soil.

Forgive the parenthesis around "rigid" and "flexible", but being an engineer, I am aware of the major difference in the design, construction and cost of the walls. The "flexible" walls do not move much (fractions of an inch that are usually not noticed) and can return when loads are eased off when removed. The slight movement is why "rigid" structures cannot be set on them because the structure cannot tolerate movement, but fences and landscaping can easily be put on SRW walls.

Since all the major systems were developed in the Minneapolis/St. Paul area they have been expose to severe conditions for over 20 or 30 years. The private and municipal crews that install new or replacement sidewalks usually use then for elevation and finishing problems and routinely build walls up to 4' without and off-site engineering.

Dick
 

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