Building terraces on rocky slopes

Hi fellow permies,

My family and I have a piece of land in central inland Portugal. It is in quite a mountainous area and the land itself is one big slope with lots of smaller vallies and slopes dotted around the edges. There is a a couple of slopes in particular (one west and one north facing) that are very important in terms of water retention on our land, as the seasonal rains in the winter will simply wash down their barren rocky slopes onto the properties adjoining.

So I would like to make terraces. I have a lot of sub soil from one pond I dug at the base of one of the other vally slopes on the land, and I will dry to dig more at the bases of the west and north facing slopes as well. So I will have a wealth of soil. For this reason, I would like to use the soil in order to build my terraces but I am unsure of the best method. The idea that comes to me most readily is using the subsoil in order to make thick rammed earth walls and then filling the trough that has been created with biomass in order to create usable, deep soil.

I am unsure about some of the practicalities of this and am not having much success finding information regarding this subject online. Is it wise to build a terrace wall from rammed earth? Will it be strong enough? Will torrential seasonal rains be a problem. Should the wall be sloped at an angle, getting wider at the base, thinner at the top or can it be a standard wall dimensions? Another thing I was wondering is (assuming a rammed earth terrace is a good idea) whether I can slimply begin to build the walls onto the soil or if I should dig down to the bedrock (which is only half a foot or so down in some places) and build on that?

So many questions. Any insights valued and appreciated. Thanks.

At my place, if water overflows a terrace it will scour out rammed earth. In my climate, earthen terraces work fine as long as overflows are constructed and lined with rocks that are larger than can be carried away by the largest flows. So at my place, rammed earth terraces have a slight bund on the edge, so that they are essentially gently sloped shallow swales. So overflow is directed towards spillways. To estimate the size of rocks needed for the spillways I look at the surrounding area... What size are the largest rocks that stay on the slope without washing away? The rocks in the spillways should be larger than that. And if part of a terrace washes away, or a spillway fails, then no big deal... I look at what failed... Try to envision why... And add more and larger stones next time, or redo slopes. Scouring typically occurs first on the down-slope side of the spillways, then the stones fall into the hole and the spillway fails, so I like spillways to be on bedrock, or to be rock armored on the back end of the spillway to about 2X the height of the spillway.

Where stone is plentiful, I prefer to armor earthen terraces with a single layer of stone. At my place, anywhere the water slows gets filled with soil or gravel during the first heavy storm. I don't have to drag soil up from the lowlands, because it comes to me as a free gift from the highlands. Wetter climates might could use plant roots to stabilize the terraces and prevent downslope movement of soil.

Assuming your rocky slopes come with loose rocks in the soil, I would use the rocks to make the terraces. Slope the faces back, DO NOT try to make pretty vertical walls.

Joseph Lofthouse wrote:
At my place, if water overflows a terrace it will scour out rammed earth. In my climate, earthen terraces work fine as long as overflows are constructed and lined with rocks that are larger than can be carried away by the largest flows. So at my place, rammed earth terraces have a slight bund on the edge, so that they are essentially gently sloped shallow swales. So overflow is directed towards spillways.

Given that I would like to make at least two levels of terraces, surely the spillway from the top terrace could lead into the terrace below, to make sure they both get optimum saturation (I would like to retain as much of the winter rain as I can, because in the summer there is none); similarly that terrace below could also have a spillway for when it becomes full... And then the excess from that lower terrace could go into the pond at the base of the slope... Something like that? Apologies for asking many questions - I have little experience. Insights and affirmations are very helpful.

Glenn Herbert wrote:Assuming your rocky slopes come with loose rocks in the soil, I would use the rocks to make the terraces. Slope the faces back, DO NOT try to make pretty vertical walls.

We do have some loose surface rocks but not enough to make the size and quantity of terraces that we need (about 1m tall x 10m long x 2 levels of terraces). Making sloped faces for the walls sounds very sensible. Thank you. Maybe we could make the earthen walls more durable by using what rocks we do have to create a stone facade for the walls to protect them more against heavy rains?

Thank you so much to both of you for your input. This has been so helpful.

If you are short on rocks, use them at the most exposed areas, especially the spillways between terraces as you describe.