Applying Tosoil to a Buttress

[firebird16]

Gentlemen:

The site being developed has a rock buttress on sound bedrock which is retaining a public road. Just below I am making geotechnical recommendations for construction of a home. What objections should I have to applying topsoil over the buttress (for landscaping). Any? I am troubled because I have never seen it done like this in my previous highway career.

 

[ishvaaag]

The more simple is that in adding topsoil over the buttress you may be loading it in excess of the strength it may be prepared to in accord to the codes resist. However, since you are making recommendations instead of a design, insist in that if there is such intention the adequacy of the retaining structure to its purpose in its presnt or if necessary revamped status will be fully compliant with its requirements as such structure both in construction and in its finished state.

 

[Focht3]

How much topsoil? Buttress dimensions? Proximity to existing/proposed structures?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[firebird16]

Fought3:
Buttress dimension-length: 120 ft, height: 13-15 ft, built upon l.s. bedrock, 1.5:1 slope, app. 20' width. Adequately designed by others (colleague-competition...) with proper drainage.
The undeveloped site is on a hillside lot. Owner wants to build house on downslope. The entire parcel was subject to colluvial landslide in 1998. You can imagine quite steep.
Buttress constructed for purpose of stabilizing the road only in 1999.
I have no problem with buttress itself. I am recommending quite conservative geotechnical parameters for a SE to build a basement wall on the site.
Back to the buttress. Are there any drainage consequenses I should be aware of (and are not)of landscaping soil over the graded rock.

 

[Focht3]

Well, it's not that tall of a slope (13-15 feet), so I don't think there's reason to panic. The limestone rock support of the buttress helps a lot. I'm not crazy for the 1.5:1 (H:W) slope, but understand the implications of a flatter embankment.

Keep all new construction at least 25 feet away from the buttress - preferably 50 feet, but I realize the lot may not be that deep. Construction of the basement is my biggest concern - not so much the wall pressures. You don't want to destabilize the buttress during construction... How will the contractor excavate for the basement?

With regard to the topsoil and drainage: I'd be inclined to use a lightweight geotextile and sand bed below the topsoil. This should reduce the risk of clogging the rock fill, resulting in drainage problems within the buttress. If the topsoil will cover the entire face of the buttress, you will need to provide positive drainage from behind the topsoil. (The slope appears too steep to be covered with topsoil.)



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[cphi]

Firebird16: I believe I agree with ishvaaag. I will add that the basement may cause an instability, as Focht3 mentioned. These are my comments. The topsoil cover will intrude the rockfill butress and "disappear" reducing the drainage permeability, if the rockfill is open graded. Check out geotechnical engineering texts for filter design to evaluate if the topsoil will intrude. I aggree with Focht3 that a geotextile selected for appropriate filter/separator function would be recommended, but the geotextile/rockfill/topsoil interface friction value is critical for determining sliding stability (infinite slope analysis). In other words with the 1.5H:1V slope, run the stability analysis to check if the topsoil likely will slide off the geotextile during placement, or during the first rainfall, and run "what ifs" to evaluate how to improve the factor of safety against sliding. Regarding the basement excavation, the elevations and lateral location relative to the buttress may decrease the buttress stability designed by your "colleague-competition". I recommend you contact him/her. You mentioned a previous colluvial slide. Will the basement be in the bedrock or soil? If in soil, the possibility is greater in reducing the buttress stability to less than a safe value in comparison to the excavation in bedrock. Run a slope stability analysis to determine the safe location for the basement, if any.