Design walkways on a steep slope 4

[ANgK]

I am not sure if this is the correct forum to ask this question.

We are trying to figure out a solution for walking access of a steep slope (20-40 degrees, mostly 25 degrees). The plan is to be able to plant rows of shrubs along the contour of the land and watering them.


Initially, we intended to terracing the plot to make small parallel walkways along the contour of the slope for access. However, the rockery retaining walls would take up a lot of space and we may need riprap between them.

Another options would be wood boardwalks along the contour to provide access.

Is there any other solutions that we didn't think of?

Thanks!

 

[geov86]

A crib retaining structure will also be a good idea, especially if you want to combine it with shrubs.Do you know anything about the site's structural geology? Knowledge of the site's structural geology and stability is crucial.

 

[ANgK]

This is what I got from the report:

The underlying material consists of Stodick very stony loam, 15 to 30 percent slopes (# 730). This
shallow, well-drained soil is on back slopes and side slopes of pediments. It formed in residuum and
alluvium derived dominantly from lacustrine sedimentary rocks.
Typically, 3 to 5 percent of the surface is covered with stones. The surface layer is light brownish gray
very stony loam about 4 inches thick. The subsoil is light yellowish brown very gravelly clay loam
about 10 inches thick. Interbedded tuff, mudstone, and sandstone are at a depth of 14 inches. Depth
to sedimentary bedrock ranges from 14 to 20 inches. Permeability is moderately slow. Effective
rooting depth is 14 to 20 inches. Runoff is medium, and the hazard of water erosion is moderate. The
hazard of soil blowing is slight. Limitations for shallow excavations are severe due to rock, and slope.
The frequency of flooding is none. Depth to high water table is greater
than 6.0 feet. Depth to bedrock (soft bedrock) is 14 to 20 inches.


Our subsurface exploration confirms, in general, the soil and geologic mapping with the native
surface soil consisting of dark brown clay with sand, gravel and cobbles (CH) that is medium stiff and
dry. This soil is underlain by light brown silty sand with gravel (SM), that is medium dense and dry.
This soil is further underlain by light brown layered siltstone and sandstone that has little fracturing,
low hardness, moderate strength and is moderately weathered to the depth explored. This native soil
is overlain by approximately two to five feet of fill material that consists of medium dense silty sand
(SM). A portion of the native soil is overlain by about four feet of fill material that consists of layered
silty sand with gravel (SM) and sandy clay with gravel (CH) that are loose and dry.
At the time of our subsurface exploration (August 31, 2017), no free water or evidence of previous
water was noted. The native soils are in a compact or firm density state, exhibits a very low (silty sand and bedrock) to high (clay) potential for expansion.

 

[dik]

Can you use gabion baskets? and, can you use vegetation like Japanese Knotweed? (not this because it's invasive)

Dik

 

[ANgK]

We did think about Gabion basket.

The slope is very stable right now. We hope to minimize the excavation to prevent soil instability. We think the footprint of the gabion is too big and will cause more instability upstream.

One idea floating around is a timber wall with post/spike and back filled with gravel but there is concern about the stability of this type of construction:

 

[geov86]

Ok, here are some things to have in mind. Loam as a soil generally is not the most water friendly geomaterial, which means that if for some reason a strong rainfall occurs (i.e 400mm overall precipitation height), let's say for two or three days, your slope will probably have a problem. I have faced a case where a slope started to slide after 12 hours of heavy rainfall. Also, the report states that there was no water, however the measurements were taken in August which should be true, due to the weather conditions, however no one can be sure that this will be the same during November, December or January. And your soil,from the classification you wrote at your post, needs some "good care".

I don't have the complete report and the laboratory tests to assess the soil more properly, so I will just give an estimation. Run a good stability analysis and take into account the complete hydrogeological history of the area, both in terms of groundwater hydraulics and of precipitation. If in the worst case scenario the stability is low, I'd go for a crib wall along with plants (i.e shrubs) which can provide a good drainage as well, and since you also want to create a walking access. I wouldn't use just wood boardwalks on this slope unless if the slope stability analysis provided me with very good results. Gabion walls (if you are referring to them by saying rockery retaining walls) could also be an option, but again, in all cases, you must examine where your slip surface is going to be. You don't want you slope to slide along with your everything on it. And, generally, vegetation alone is used in cases where shallow slip surfaces exist. For a case where a deeper slip surface exists, vegetation is not enough. Therefore, to conclude, your type of soil needs more careful examination, and a good slope stability analysis by taking into consideration the complete hydrogeological conditions and history.

 

[ANgK]

Thanks!
This area is very arid, less than 7 inches per year. Raining is usually light and there is generally no thunderstorm.

There will be only walkways on this slope. No housing or any structure.

The shrubs will be planted directly into the slope.

Our concern about Rockery, crib wall, gabion that the footprint of the wall will be very big and involve lot of excavation ( 3-4 walls running along the contour to provide adequate access to most of the land) and will cause instability of the slope.

Ultimately, we hope the slope will have multiple row of shrub like this ( certainly not as manicured) but that would required walkway access to plant, trim and water these shrub:

 

[geov86]

First of all, where is the timber wall going to be fixed? Within the CH, SM or the bedrock? The third case,and with a right dimensioning, may be regarded as a possible scenario. However, I wouldn't advice the wall to be constructed within the CH or SM soil at all.

 

[ANgK]

There is shallow sandstone bedrock (14-20 inches deep)that the post will be fixed.

Can you give me more detail about the crib wall for walkway? So we excavate the area to the bed rock, put in the crib wall and the walkway is the top of the crib wall?

 

[geov86]

Well, given that it is very arid and that no structure will be constructed on the,and since that the slope's stability analysis shows that it is very stable, I think that a pile wall fixed within the bedrock will do the job.

 

[geov86]

It depends on how you want to shape your slope, how many walkways you want to build etc. Yes, the way you mentioned is one way. Another way is to build more than one crib walls

 

[geov86]

 

[ANgK]

Thanks!
That would be too big vertical wall and not the aesthetics we are shooting for.
We want to minimize the land and excavation for the walkways and use most of the land for planting.

 

[dik]

Can you disturb some of the soil and use gabions with geogrid? Segmental precast?

Dik

 

[geov86]

The photo I uploaded was because I would like to show another way of building crib walls. In your case the "one crib wall" way you mention would be the way if a crib wall structure was your final decision. Now, as for the planting, I think crib walls can provide you with some options.

In any case, given the hydrogeological and geological conditions you mentioned before, and just for walkway use, I think that if you study well a pile wall within the sandstone bedrock in an appropriate depth, you won't have any problem.

 

[dik]

geov86: Much appreciated; I thought it looked sharp and thought something like your later picture was possible... though it could even get 'bushier' and be totally obscured.

Dik

 

[geov86]

Dik: I imagined that I haven't managed to give the whole aspect of the crib wall's function, that's why I uploaded the last photo. However, it depends on the budget and on the site's regeneration scope,as well as on a good agronomist. In this case, in which ANgK wants to construct only a simple walkway,I suppose something like the last picture would be expenses-for-no-reason.

 

[dik]

Glad you did

Dik

 

[ANgK]

Thank for the input!
I think the soldier pile timmber wall to create walkways would be the most economical way to achieve what we want.

So I guess we anchor the soldier pile directly onto the bedrock, put in the timber wall,then back fill the area with gravel. We would walk on top of the gravel area. Does that sound reasonable?

Do we need any excavation?

 

[Mad Mike]

From a look at the concept image, I'm amazed that a structural solution is the first option for walkways between plantation shrubbery. Admittedly I am only considering slope stability, not aesthetics.

If your bedrock is within 20 inches of the surface, why not just cut the foot-paths in vertically to the narrowest acceptable width?

If the paths are kept to 2 or 3 feet wide, the cuttings made into the slope will be nominal. The geology doesn't sound problematic, but ultimately the stability would be most affected by the amount of water added through irrigation.

I've walked through hundreds of plantation roads/paths on steep slopes where thick soils are cut vertically- they often maintain themselves surprisingly well...of course there'll be the odd bit of maintenance but then again you've got hardly any soil on your site.

All the best,
Mike