Backfill slope 6

[Dave169]

I am designing a 10' cantilevered retaining wall.

I have a geotechnical report on the soil. They provided an equivalent fluid pressure of 47 pcf for level back fill and 78 pcf for a backfill sloping up to 2:1.

The retained soil will be level for the first 10 feet then slope up the hill. Can I consider this a level backfill situation and use the lower equivalent fluid pressure or should I use the more conservative EFP for sloped backfill?

To accurately model slopes what horizontal distance from the wall should be used to determine the slope?

Thanks for your help.


David

 

[BigH]

Why don't you forget the EFP approach and carry out a Culmann graphical method to determine the wall pressures/force? Or since the horizontal slope is the same as the height - e.g., 45deg angle and active pressure is less, you can use Coulomb's active earth pressure coefficients which can be obtained from any geotechnical textbook.

 

[SlideRuleEra]

BigH's suggestion is probably better, but if you want to work with the EFP, there is a semi-graphical way. See my comments at thread255-104164

The position of the overlay is important, you don't want to neglect any loading nor to count it twice. With backfill slope of 2 Horizontal: 1 Vertical and 10' Horizontal setback to toe of backfill, I get a maximum pressure of 625 Lb/SF (at the toe of the 10' high wall).
Total pressure on the 10' high wall comes out at 2738 Lb/Ft of Wall Length. Let me know if you can't confirm.

http://www.SlideRuleEra.net

 

[fndn]

The fact that EFP of level backfill is rather high for active condition, leads me to think you have low friction angle soils. So the reduced phi means your active wedge catches the horizontal backfill at 7 or 8 feet. So you may use the "lower" EFP.

 

[Dave169]

Thanks for your replies. The geotech. who did the soils report confirmed I could use the lower EFP values since a level condition will exist behind the retaining wall for 10 feet (the height of the wall).

With the lower # I have a more econimical design.

David

 

[Boonie]

Alternatively, you can find the middle ground by using the Coulomb's with the "broken back" angle of slope. It will provide active pressure coefficients somewhere in the middle. A 10' flat area with a 10' 2:1 will generate an 18.4 degree beta angle for the Coulomb's equation. You will also need the phi angles from your Geotech.

 

[BigH]

Boonie, I am sorry that I don't understand your need for the analysis of a 'broken back' slope angle. The wall is 10 ft high. The flat portion behind the wall is 10 ft. high. Taking a line of 45 deg from the base 'just' touches the toe of the slope (worse case if fill is cohesive, but hopefully it will not be).

However, for the active pressure, the angle of the 'sliding' wedge is less than the 45 deg angle (taken from the vertical). If phi = 30 to 35 deg, then the angle is (90 - (45 + phi/2)) or about (90 - (45+17,say) or 28deg from the vertical (or approximately at an angle of 2V:1H - see Tomlinson's Foundation Design and Construction book - 6th Edition, Figure 5.10 which gives a set-back of 2V:1H for which no underpinning would be normally required). The 'sliding wedge' will not intersect the slope and therefore does not come into play in the active earth pressure on the retaining wall. However, the slope may very well have an influence on the global stability - which needs to be checked as part of any retaining wall design (and many times is not).

 

[BigH]

Okay - Boonie is right depending on the size of the heel - but if from the heel, the 2V:1H line is still within the flat portion behind the wall, then the slope doesn't affect. If it does, then suggest the Culmann graphical method - it is easy and quick to perform.

 

[Boonie]

Point taken. But what happens when the site contractor decides he has too much dirt on site and "cheats" the slope out to about 5 or 6 feet from the wall. Rarely have I seen a slope constructed exactly as drawn on the plans. There are plenty of contractors willing to "field fit" to meet their needs without consulting the design engineer. A slightly more conservative design can reduce the risk of problems caused by misguided contractors.

 

[BigH]

Well, then - you issue a non-comformance against the contractor and he must remove or remediate. If the non-conformance causes a problem, that is his problem. You know that this 'may' be a problem - then you will need to keep an eye on it to nip it in the bud - and I know from experience too how difficult this is. I agree that a slightly more conservative design is an approach - for how well do we really know 'phi' of the soil fill - or the unit weight? These are the unknowns of the design - I do believe that, even as difficult as it is, we must not let design be ruled by the possibility that contractors will not build to the plans.
As an aside - like the Pinebrook Dam (link in another of the geotechnical threads) - they have a web cam installed that updates every 5 minutes. Interesting for monitoring purposes, perhaps!

 

[shockstressed]

Inadequate seems to be agreed with by the fact that most of the local Motorway slopes are being removed under contract
and the sites cleared back to prevent such load calculations being effective, that from many slips encountered.
If the contractor knows you have placed a design that might incur contract implementation and force at later date and the authority may come after him for remediation
he may sue you for damage
I would
and as the authority I would prosecute you
and that could include criminal damage and loss fo life prosecution.
Calculations are for checking things when they are partial as all these are
and well dated and unreliable.
Convenience does not avoid the responsibility for detailing the site. The client is going to be much happier and the authority satisfied if the site stays up, if it falls down, its your neck, that is why we design and anybody suggesting you should dump this on the contractor when the contractor does not know the site and structures should be dumped, as you will be from Institution if you agree with them !