Slab on slope 2

[WARose]

This is a home/property project.....but I need ideas. I am thinking about pouring a slab on a slope. (See attached pic. Sorry if it looks sloppy to you.) The thing that worries me is: this is likely susceptible to a sliding failure because of the slope. (Needless to say, I haven't called a geotech on this. So I don't know what the coefficient of friction is. The numbers I've come out with put the SF at about "1")

I could live with the sliding down.....but, I really don't want the "undesirable" scenario (shown in the pic) to happen. So my question is: would having one of the edges (3 options shown) possibly insure the slab penetrates the soil at the bottom to prevent the "undesirable" scenario?

Before anyone asks: right now, changing the slope is not a option. Anchoring the slab (into the slope) is something I've considered......but right now, I'd like to focus on: what happens if it does slide down and can I control that? Thanks.

 

[JAE]

Well first off there no horizontal shear in this

Second - could you add some under-side keys to create a "grab" of the slab to the soil?
The idea is to engage some passive resistance along the slab length?

 

[MotorCity]

To address the potential for sliding, you might consider thickening the bottom edge of the slab such that its embedded some distance in the soil (like a shear key) JAE beat me to it

 

[JAE]

By a few seconds! Great minds think alike!

 

[r13]

Only provide for fun.

 

[WARose]

Well first off there no horizontal shear in this

Here we go again.

Second - could you add some under-side keys to create a "grab" of the slab to the soil?
The idea is to engage some passive resistance along the slab length?

I've thought about that.....but it would make forming a whole lot more difficult......QC on this job isn't going to be that great either. (Since it's me and a bucket.)

 

[SlideRuleEra]

On good, well compacted soil, perhaps with a build-down at the toe, slope of 20^o (2.75H:1V) should not be a problem. Slope protection slabs for overpass abutments are typically sloped 2H:1V (26.6^o).

This is a SCDOT drawing for Slope Protection Details.

http://www.SlideRuleEra.net

 

[JAE]

Well not really forming - just trenching the grade below the slab with cross trenches. Not a whole lot of cost there.

Or build a large key grade beam at the bottom but I like the multiple key idea better.

 

[WARose]

[blue](SRE)[/blue]

On good, well compacted soil,....

Not really.

[blue](JAE)[/blue]


Well not really forming - just trenching the grade below the slab with cross trenches. Not a whole lot of cost there.

Or build a large key grade beam at the bottom but I like the multiple key idea better.

I'll give it some thought.....but I normally like some rebar in the key. That adds to the difficulty of it.

 

[JAE]

Here's a detail from an article on spillway designs (

https://www.ose.state.nm.us/dams/Newsletter/Western Dam Engineering_Issue01_Vol06 - April 2018.pdf)

...and a detail from the article. This is quite extravagant and tries to mitigate upward forces from seepage under a levee/dam with the drain.
But the key detail is what I was thinking about.

 

[JAE]

Not sure what the anchor bars below are...perhaps into rock or something.
I would do what you mentioned - series of vertical bars in the keys to take the key bending.

 

[r13]

If W_conc*sin θ > 0.5*W_conc*cos θ, sliding will occur, then you need to provide a resistant force equal to, or greater than the difference. Shouldn't be large, for such mild slope.

 

[r13]

Forgot to mention safety factor for potential to lose friction if the subgrade getting wet. One experimental method is to embed short PVC pipes into the subgrade, grout them full, then cast with the pavement. The effect is the same as a key way, should work for residential small project.

 

[STrctPono]

Provide the downturn thickened edge at the base. Extend your rebar into the thickened edge. I would much rather rely on soil bearing at the base than friction along the full height.I would not even consider the mid height intermediate thickened keyways as this is unnecessary material that would slow down your pour rate (especially if you are hand mixing everything).

 

[hokie66]

If this is a survey, I'll give a vote to JAE's proposal.

 

[r13]

I can agree to his response without intermediate keys.

 

[r13]

In another note, I think, depends on detail "B", SRE provided an excellent example/solution.

 

[mfrad]

SRA's solution is very good. According to MK Hurd's "Formwork for Construction", 1H:1V slopes need a top form. There is a footnote in the text from ACI Committee 347 on formwork, that indicates that top forms should be placed on slopes over 12H:8V, but for lower slopes a low slump concrete is required and rebar helps too. Your sketch indicates 20degrees but arcsin(0.50) = 30 degrees instead. If it is 30degrees then depending on the soil properties you may have to investigate slope stability issues too and slab undermining from erosion. JAE's is interesting with the bottom drainage considered. His previous local thickening of the slab may cause some cracking since it is a slab stress raiser. I hope this is helpful, I enjoyed your previous posts over the years.

 

[GeoEnvGuy]

My last house had a steep concrete driveway that changed grade to flat at the top, the slab looked like a 5 inch with no rebar. So when I purchased the house the slab was cracked at the change in grade about 1 inch separation. They redid the water main on the road and included some blasting, the gap was about 3 inch when I sold the house.

 

[r13]

A grade beam with some rebars at the slope changes would have helped.