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Are small rocks bad for building steep terrace? (before I make a huge mistake!)

 
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Hi.  I live at 4b-5a Colorado in the foothills of the rockies on a North facing slope.  The slope is fairly steep and I want to terrace it for cultivation.  I've done one small section - using 2x2"s cut into 4ft lengths I pounded them into the ground on the contour line (found using a A-frame level).  I then used branches and downed trees to form the 'dam' of the terrace.  I cut into the area above to pull soil down to form the terrace, and received some free waddles (sediment logs) which I used as the berm (for now).  I am afraid that once the wood breaks down the terrace may fail.  I've planted some perennial shrubs in hopes to lock the soil together near the berm.

Below this section I'm constructing a much longer version.  My neighbor is asphalting his driveway and they need to remove all the 3/4-1" rocks previously laid down for his driveway.  I was thinking about taking all this rock and using it against the "dam" made again of various thicknesses of downed tree limbs which I placed along the 2x2" posts.  These posts are about 5' apart.

Would these rocks help me or hurt me in the long run?  If I've learned one thing about rocks, it's removing them is a GIANT pain.  I would want to avoid this if this is a mistake.  If not, is it best I use the rock as a base layer or do I pile them up leaning against the "dam"?

Please excuse me calling this a "dam".  I originally wanted to get an excavator down there and properly dig a terrace with berm, but the slope is too dangerous and I can't afford a more experienced operator.  I'm afraid building a terrace this way may be my only option (unless others say otherwise)

Much appreciated!
 
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How to you envision using the rocks?   Rocks that small can't be stacked, so they are going to tend to spread out to achieve the "angle of repose" which will make them spill off the edge of the terrace and/or take up most of the space you might want for improved soil.  Personally I would not use small rocks or logs in a terrace wall.  I think it would be asking for collapse.
 
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I agree with Tyler, I don't think you can make anything stable out of such little rocks.

That said, what is your soil like (sand, clay)? How steep is the slope you want to terrace? How big and high are the terraces? From your description it sounds like probably 3 feet high or less?

We have heavy clay soils and our terraces are pretty stable by themselves. If you can make sure that excess rainfall flows to the back of the terraces (instead of over the edge) and has a controlled way to exit the terraces, risks of collapse can be made very small.
 
Seth Marshall
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Tyler Ludens wrote:How to you envision using the rocks?

 
Thanks for your response.  I am new to this so advice is greatly appreciated.  Firstly, I wanted to make sure the "dam" I created is strong.  It is currently just tree limbs of varying thicknesses supported by those 2x2" posts I drove in the ground.  The posts are sticking up about 2.5 feet which I thought would be the height of this "retaining-wall" portion of the terrace.  I thought maybe the rocks could be useful as a small base layer against these limbs - and I could be very wrong.

My terrace is probably 50 feet long and I would have many yards of rocks at my disposal if I needed.  Since it's available and free I thought I could at least use some as filler before I backfill with the soil from above.  But I also thought if used correctly maybe it could strengthen the terrace itself.


Tyler Ludens wrote:Rocks that small can't be stacked, so they are going to tend to spread out to achieve the "angle of repose" which will make them spill off the edge of the terrace and/or take up most of the space you might want for improved soil.


These rocks are ~1" and I wasn't thinking to pile it so high it that it would spread out considerably, especially to the point where it would spill off the edge considering I hope to backfill this with over 2 feet of material to form this terrace.

 

Tyler Ludens wrote: Personally I would not use small rocks or logs in a terrace wall.  I think it would be asking for collapse.

Forgive me, I thought logs were a good way to start these walls.  Can you tell me what you would prefer the wall be made out of?  Regarding logs, not only did I plan to use them in the wall itself, but with so many logs around my property I wanted to use them as part of my filler material to eventually decompose.  My hope is it would act as a hugelculture while I wait for it to turn into the soil.  

Considering my slope is so steep it wouldn't be easy to purchase soil or compost to be used as fill material, and backfilling with the soil from above may not be enough.  I was also considering wood chips (which at least is easier to move).  
 
Seth Marshall
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Rene Nijstad wrote:I agree with Tyler, I don't think you can make anything stable out of such little rocks.


Thank you as well for your reply.  Just to clarify, these rocks are the type you put down on a driveway, so they aren't tiny.

Rene Nijstad wrote: That said, what is your soil like (sand, clay)? How steep is the slope you want to terrace? How big and high are the terraces? From your description it sounds like probably 3 feet high or less?  


The soil here is Colorado sandy-compost.  I imagine mostly from decaying vegetation and decomposed granite.  I'm not a soil expert, but doing the "glass jar" experiment it seems to be mostly sand.
The terrace posts are sticking up 2.5 feet so I hope to bring it up to at least that height (or greater if I can build that ability into my retaining wall".  This section of the terrace is currently 50 feet long but will ultimately be 80 feet when I expand to the other side of my yard (separated by a corner of my house).  My house is on a steep slope.  I was able to bring in an excavator to salvage a portion as a flat area for my kids to play, and want to use this previously un-used area for cultivation.  

Rene Nijstad wrote:  We have heavy clay soils and our terraces are pretty stable by themselves. If you can make sure that excess rainfall flows to the back of the terraces (instead of over the edge) and has a controlled way to exit the terraces, risks of collapse can be made very small.


Very good point about excess rainfall flowing back.  I most certainly need to make the wall high enough to allow this.  I would imagine 4 feet high may even be better-- the slope is pretty steep.  To allow for a controlled exit, would you build in some drains?  If so perhaps that's what my rocks could be used for?
 
Tyler Ludens
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Seth Marshall wrote:Forgive me, I thought logs were a good way to start these walls.  Can you tell me what you would prefer the wall be made out of?  Regarding logs, not only did I plan to use them in the wall itself, but with so many logs around my property I wanted to use them as part of my filler material to eventually decompose.  My hope is it would act as a hugelculture while I wait for it to turn into the soil.  



I believe logs will decompose and become unstable.  Water-saturated soil and logs can wash downhill, doing great damage.  https://permaculturenews.org/2015/11/06/dont-try-building-hugel-swales-this-is-a-very-and-i-mean-very-bad-idea/

Personally I would be very hesitant to build terraces on a steep slope unless I really knew what I was doing.  Which I don't.  Terracing a steep slope requires knowledge of soil properties, physics, hydrology, etc.  If I was dead-set on terracing, I would consult an expert onsite.

I believe stone is used for most permanent terrace walls.


 
Rene Nijstad
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Sand, I do think you need a retaining wall to keep a sand terrace stable, because sand doesn't glue together like clay does. A retaining wall can be made from big rocks, that have so much weight each (!) that they cannot be pushed out of place. That's why 1" rocks are too small. When force gets applied to them they can move, then tumble over each other and that simply won't hold. The higher the wall, the greater the force at the bottom. I personally would not dare to build terraces over 4 feet high on a steep slope. If I would have to do that I would bring in an engineer and the engineer would probably want to use concrete with steel.

Alternative for the 1" rocks, use big bags, fill them with the rocks, stack the bags as the wall as they now perform like big stones. You probably would have to cover the bags when you're done to prevent the sun from breaking them down. Maybe this helps?

A picture below of our terraces. They're 3 to 4 feet high mostly, and the stair like rocks are both walkways as well as overflows for excess water. As I said it's clay soil we have so it's pretty stable by itself.



small_terraces3.JPG
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Seth Marshall
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All good advice here.  Thanks to everyone for taking the time to give me your feedback!
 
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If you created gabions with wire mesh and filled those with the pebbles you may be able to create terraces which will last a long time.
Why did you purchase the block, knowing this work was needed?
 
Seth Marshall
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John C Daley wrote:If you created gabions with wire mesh and filled those with the pebbles you may be able to create terraces which will last a long time.
Why did you purchase the block, knowing this work was needed?


Thanks, gabions came to mind.  And they would work with the larger rocks.  It's probably best I keep 1/2" or smaller rocks out.
As for why I moved here.  The view is incredible, and the steep slope is what allows it-- here's a picture.  You can see my small terrace with a garden in it, and the new terrace I'm creating about 20 feet further below.  I imagine I'll create another one between the two, I've read you need to make the terraces less wide for steeper slopes.  Because it's a panarama (I couldn't fit it all in a normal photo) it's hard to get a sense of it.  The area on the left is where I excavated for play area for the kids, but I can't afford to get an excavator on the steeper areas.  The flat area will ultimately become a food forest region too as the kids get older.
terrace.jpg
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Hey, Seth!

The most amazing terrace farming I've seen was in the mountains of Oaxaca, one day when I went hiking between several villages up in the mountains with a local guide whose brain I picked about local agriculture (the ways they combine livestock and crop farming on the terraces -- I didn't have the Spanish vocabulary to ask about how they constructed the terraces), and another day when I took a van from Oaxaca City to the state's Pacific coast on a long, steep, winding road through and down the mountains among tons and tons of terraces growing corn, beans, etc. Wondering now what their methods and materials are for building and maintaining these terraces (some of which have been there for thousands of years, if I understand right, and are still maintained, while others are built fresh, and none of this in areas where it's easy or even possible to have materials or equipment brought in), I found this article (I was able to download it with a free Academia account I've had for a while). It says:

"Farmers described a process in which green and dry brush is cut and piled in mountain drainages in a cross-channel direction to retain soil and sediments washed down from the hills during the rainy season. Called bordos, these brush barriers are lined in the front with stones found nearby to create a permeable retention wall. The stones are carefully fitted like a jigsaw puzzle, but they are not cut or bound with mortar. The placement of stones is progressive and begins after the brush bordos have already begun to collect sediment. Low, vertical rows of stone are positioned in front of the brush bordos to create the base of a terrace wall. As the rains continue to transport more sediment, additional stone rows are placed slightly upslope and at a tilt, increasing terrace height. The result is a sloping, porous wall that allows excess water to filter through. This mode of construction protects crops from oversaturation and prevents wall collapse, since strong currents of water are filtered through the walls. Terrace maintenance is a continuous, yearly process."


Then, later:

"The contemporary model of bordo terrace construction corresponds well with the stratigraphic profiles of Prehispanic terraces exposed in erosion cuts around Cerro Jazmın and in the lama-bordo terraces excavated at the sites of Yucundaa and Nicayuju (Perez Rodrıguez 2006, 2014; Perez Rodrıguez et al. 2011). At these sites, the initial stage of the terrace wall consisted of a trapezoidal pile of uncut stone (Figure 3). These piles were up to 50 centimeters wide and had no mortar, and were similar in shape to the brush barriers (bordos) described in the ethnographic interviews. The barriers exposed by the excavations were then topped by narrower terrace walls of one- or two-stone rows. Based on the stratigraphy preserved behind terrace walls, subsequent rows were added as the sediments accumulated during rainfall events."


I'm attaching that Figure 3 mentioned, as well as Figure 2 of contemporary terraces. I'm no expert on this, and I don't know what size stones they use, but I could try to find out more if you're interested! (I am interested, but may not be able to get to it right away.)

Your place is indeed beautiful! Wishing you all the best there with your family.
Figure-2.png
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Lama-bordo terraces in Cerro Volado in San Martín Huamelulpan
Figure-3.png
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Excavated lama-bordo terrace at Yucundaa, Pueblo Viejo de San Pedro y San Pablo Teposcolula
 
Seth Marshall
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Hi Beth!  That's very interesting and definitely similar to what I'm doing.  It was hard to visualize what they meant about placing these stones in front of the green and dry brush.  Do you think they meant they placed these stones "on top of and at the front of" the piles of brush?  That's the only thing that would make sense to me.  I would certainly love to learn more, thanks for sharing this!
 
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Seth, I can understand why you re putting up with the inconvenience of slopes.
I collect all sorts of things when I travel around my area. I have a ute I can throw anything I can manage into and I take them home for reuse.
I get lots of rocks and over time you will be surprised at how the numbers creep up,
Have you thought about asking or advertising around that you want rocks?
Some terraces in either Italy or Greece are worth looking at also.
I would suggest any timber use is unwise, since they will rot way.
May be steel rail way line sections could be possible.
Are there many rocks in the area, sometimes they are around road cuttings etc.
 
Seth Marshall
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John C Daley wrote:
Are there many rocks in the area, sometimes they are around road cuttings etc.


There is certainly no shortage of rocks in my area!  I think I'll start placing them in front and leaning against my existing terrace construction.  These would be much larger rocks then the ones I was originally asking about, more like rip-rap or boulders.   Perhaps I'll pile up the smaller rocks on the upslope side of my terrace and the larger rocks on against the downhill side.  Unless someone chimes in saying these smaller rocks important I will probably not use the small rocks from my neighbors driveway.  But I didn't mention that I'm currently removing rock from my house as well.  I hate it that the previous owner used rock.  So I need to get rid of this and maybe it'll help fill in these terraces a bit.
 
Beth Wilder
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Seth Marshall wrote:Hi Beth!  That's very interesting and definitely similar to what I'm doing.  It was hard to visualize what they meant about placing these stones in front of the green and dry brush.  Do you think they meant they placed these stones "on top of and at the front of" the piles of brush?  That's the only thing that would make sense to me.  I would certainly love to learn more, thanks for sharing this!


Good question. By context -- reading the second quote as well -- I think that "in the front" actually means downslope, because later it specifies that "additional stone rows are placed slightly upslope"; and in the second quote, it calls the prehispanic stone walls both "trapezoidal" and "similar in shape to the brush barriers (bordos) described in the ethnographic interviews." So my read is that basically the brush and sediment are getting enclosed within stone walls that are trapezoidal on the cross-section, i.e. wider at the bottom, and more stone is added as more sediment is accumulated. Placing stone on the downslope side first would make good sense to me, given that the walls would need to be taller on that downslope side than on the upslope side in order to be vertical and inward sloping rather than sloping downhill (which would collapse at some point, probably sooner rather than later).

John C Daley wrote:I would suggest any timber use is unwise, since they will rot way.


As I said, I am very far from an expert and certainly not an engineer or builder, but it seems to me like the builders of these ancient-and-contemporary stone-walled terraces plan(ned) for the wood rotting away. The brush is used to accumulate sediment, and then the brush and sediment are used to support the stone as it is built up into a trapezoidal shape around it. If the wall collapses a little as the wood rots away, the trapezoidal shape -- if done right -- should collapse inward to form a denser stone wall rather than down the slope.

But yes, Seth, I would say that your larger rocks should maybe go at the base/below the brush dams. I would guess that you could use those smaller rocks on the upslope side [ETA:] or as part of higher additional layers later, on top of larger stones. What do the rest of you think?

[Edited to add another thought about those smaller stones.]
 
Beth Wilder
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Seth, I found this more recent article by the same author as well (that link may automatically begin a PDF download). It gives more detail:

"Farmers report cutting green and dry brush and piling it up in drainages to create barriers that will stop the soils and deposits that wash down naturally from the adjoining hills during the rainy season. From these brush barriers bordos or barriers of soil form. Often, these soil barriers are lined with uncut stone found nearby to create a façade. These stones are carefully fitted together but they are not bound with mortar. Since the soil bordos are the first to form, short and very vertical, single rows of stone are then placed in front to create the base of a terrace wall. As the rains continue to wash down sediment, additional stone rows are placed on top, making the terrace wall grow up and back. The result is a talud or sloped wall. Terrace walls are porous allowing excess water to simply filter out, which prevents crop damage from oversaturation and rotting."


Later, there's also this describing the ancient walls, which isn't quite as clear to me:

"Lama-bordo stratigraphic profiles show that terrace walls were essentially rock piles of uncut stone with no mortar. Around Cerro Jazmín we identified a number of tall terrace walls that consisted of single rows of stones that could not have been freestanding walls (Fig. 3). These start as a very short and vertical single-row stone wall, about 30 to 50 centimeters tall. This initial wall is at an almost 90° angle from the ground. After this vertical segment, single stone rows were continuously placed as the sediments filled in the terrace and supported the stones. The top rows, however, were no longer vertical but tilted towards the back of the terrace to support the slope created by the terrace sediment as it collected.
"The excavated terraces at Yucundaa and Nicayuju showed a slightly different pattern of construction. The initial stage of the terrace wall consisted of building a trapezoidal pile of uncut stone (Fig. 4). These piles, some up to 50-centimeters wide, perhaps mimic the shape of the brush barriers described in the interviews. The barriers exposed in the profiles of terrace excavations are then topped by narrower terrace walls of one or two stone rows. It appears, based on the stratigraphy retained behind these walls, that these subsequent rows were added as the sediments washed during the rains."


I've attached the referenced Fig. 4. Fig. 3 is a distant picture of a red cliff that I can't make that much sense of.

As a point of interest, towards the end of the article/chapter, it says:

"The narrow Prehispanic terraces have been modified and widened to accommodate draft animals or occasionally tractors. Manure and fertilizers are used to enrich terrace soils and different crops are cultivated, including GMO tomatoes that are grown in elaborate government-subsidized greenhouses."

Fig-4.png
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Lama-bordo terrace built using the pile-of-rock method
 
Beth Wilder
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Further info. is in this earlier (2003) article by the same author (link begins a PDF download). A couple of pictures (Fig's 4 and 9) from this are attached.

This one is in Spanish (link begins a PDF download) -- the year is unclear -- but has a potentially useful image, which is also attached (Fig. 8).

This 2018 study (again, link begins a PDF download) analyzes the soil in these Mixteca terraces. I don't see a discussion of construction methods, but it could be interesting for how the terraces may have changed the soil over time.

I think I don't have time for this at the moment, but another idea I have is to try to find again the communally-owned and -run organization that organized the hiking tour I took in the Sierra near Oaxaca City years ago and see if they could either answer any questions themselves or connect us to other folks who could. Unfortunately I don't think I kept notes about the name of this group; I just remember that it had a small office in the city and that the place where they had us start the tour for breakfast in the first mountain village had the most delicious chilaquiles I've had in my life.

That's it for now!
Fig-4.png
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Lama-bordo terraces in Nicayuhu
Fig-9.png
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Lama-bordo terrace archaeological trench
Fig-8.png
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Lama-bordo terraces in Asunción Nochixtlán
 
Seth Marshall
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Hi Beth, thank you so much for taking the time to share all this info. It was a lot to type up and I really appreciate it.

I must admit I’m not quite understanding the nuances the writer is trying to describe between various methods of building. But I’m sure I can get close by building the trapezoidal style rock retaining wall.
 
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