V-ditch design

[kirei17]

Hi,
I'm curious about soil pressure design for V-ditches.
What i have been thinking is that there must be a difference for lateral soil pressure if the surface is sloped than when it is vertical (no slope).
This thinking of V-ditches should not be a concern if you are imagining sideway ditches.. But imagine a v-ditch that is huge enough to drown a bus. I mean now we're talking significant soil pressure, even if it would not be as big as when the surface is vertical.

And another question suddenly come up is about... I know this is probably related to basic.. What is the relation of internal angle friction and stability of a soil or rock cut? Is there some certain formulas?

Thanks before,
Rio.

 

[aeoliantexan]

Most V-ditches (for example, drainage ditches) are cut flat enough that the slopes are stable without support. The surfaces can then be grassed or paved. The analysis would be a slope stability analysis.

If you need a steep-sided V-ditch, the sides could be considered battered retaining walls. The pressures are lower than for vertical walls. Design Manual 7.02 by the U.S. Naval Facilities Command has charts for lateral pressures against sloping walls supporting cohesionless soils, about Page 66. You can find it and download it for free. Determination of the pressure becomes more complicated for cohesive soils.

Clean, dry, granular (cohesionless) soils have an angle of repose approximately equal to the internal friction angle. For example, a sand with an internal friction angle of 35 degrees will stand barely stable at a slope of 35 degrees from horizontal. The downwind slope of a sand dune lies at such an angle, and if you try to walk up one, it is pretty unstable under foot. If seepage is present and exiting the slope, the angle of repose can reduce to about one-half the friction angle. If the soil has both friction and cohesion, it can stand at a steeper angle, depending on the overall height.

Rock cuts are not so simple.

 

[kirei17]

Aeolian,
Thanks!

I have found your recommendation, and at a glance it's quite a handy complete handbook. Lots of graphics etc.

Anyway, i still can't find the graphic for sloping wall surface lateral pressure. There are some, but the slope is in reverse side, rather than the slope that i want to learn from v-ditch design. Could i possibly adapt the formulas/graphic for a reverse slope than what it show?

 

[aeoliantexan]

Figure 5 has curves for active pressure coefficient, Ka, for both positive and negative values of Theta. For example, for a Phi angle of 30 degrees, Ka = .3 when Theta = 0,(vertical wall), but Ka = .2 for Theta = -15 degrees.
The chart is credited to "Tables for the Calculation of Passive Pressure, Active Pressure, and Bearing Capacity of Foundations, by Caquot and Kerisel. Perhaps someone has the reference and can give you the formula.

 

[kirei17]

Aeolian,
Really late response. But thanks for the explanation. I didn't see the positive negative sign before. So the limitation is -30 degree. For bigger negative slope, it is considered near flat so no need to calculate for thickness of wall (if wall is even required).