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Riprap wedge active or passive

Riprap wedge active or passive

(OP)
I have a similar question to the one that Kenny posted in thread255-367340: Passive Resistance on Sheet Pile Wall back in 2014. We are in the process of designing a sheet pile to bisect a WWTP lagoon into two sections which will allow the municipality to keep the WWTP running while they first lower the water level on one side of the sheet pile, install a liner, and then refill that side of the lagoon before doing the same procedure to the other half of the lagoon. The water level in the lagoon is 15' deep which results in significant pressures on the sheet pile. The site also has weathered rock and/or bedrock approximately 15' below grade so the depth of embedment of the sheet pile below grade is limited. We are proposing to drive the sheet pile and install class 1 riprap on the side of the lagoon that will be drained to act as a wale to laterally brace the sheet pile at a height of H/3 (5') above the base of the lagoon.

If the angle of the riprap is roughly installed at the angle of repose, can we use passive pressure to determine the resultant force from the riprap? Also as suggested in the referenced thread, should Culmann's solution be used to determine this force or another method?

Thanks.
Scott Francis

RE: Riprap wedge active or passive

I'd consider using log-spiral charts, which are in DM-7.1 or 7.2. For such rip-rap, I'd also use a friction angle of 50 degrees. Bear in mind that the development of the passive resistance will require movement of the sheets. That movement is partially controlled (and public safety too) by applying a safety factor to the passive pressure. You will get a bit number from the log spiral charts (especially for a friction angle of 50 degrees). Don't forget to apply the correction for wall friction!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Riprap wedge active or passive

You will not be able to develop full passive pressure from the riprap if you just place a triangle of material at the angle of repose. You will need to place some width of material at the 5' mark then start sloping the material.

Mike Lambert

RE: Riprap wedge active or passive

I don't understand Mike's comment. You will develop full passive resistance, but the full value of passive resistance will be lower than the typical Rankine earth pressure envelope.

Maybe I'm just being pedantic. . . ?

f-d

ípapß gordo ainÆt no madre flaca!

RE: Riprap wedge active or passive

(OP)
I think that what Mike is saying, is that if the width of the riprap is not wide enough at the top then the full passive pressure won't be realized. I found this sketch in my Principles of Foundation Engineering (Das) textbook. The red wedge that I drew in represents the riprap installed at the angle of repose, and the portion of the soil within the dashed diamonds above the red line is not there to exert pressure on the soil below it back down to the retaining wall which it would need in order to fully develop the passive pressure.

http://res.cloudinary.com/engineering-com/image/upload/v1469031001/tips/20160720104121_df7isd.pdf

Does this seem right?

RE: Riprap wedge active or passive

for horizontal earth conditions, the Rankine passive earth pressure is found using Kp=Tan^2(45+phi/2). If the slope on the passive side of a wall exists, the Rankine earth pressure is something different. It still exists though. I mean who cares about the width of the horizontal bench? The rip-rap could meet the wall at some 1-1/2H:1V inclination and you'd still have some value for passive earth pressure, eh?

Log spiral charts allow you to determine this passive resistance. My first reply suggested you look at these log-spiral charts, which are found in DM7.1 or 7.2. You'd need to take some wall friction into account - i.e., the interface friction angle between the rip-rap and the sheeting.

The friction angle of rip-rap is likely 50 degrees. The Rankine passive wedge is taken as the wedge of soil with an angle of 45+phi/2 as measured from the vertical line and as marked from the toe of the sheets. That'd be 70 degrees! If the rip rap is 5 ft up the side of the wall, you'd need a wedge of at least 14 ft (5*tan70).

Or you can just bank the rip rap on the angle of repose against the sheets and calculate the passive resistance for that non-Rankine wedge of resistance using log-spiral charts.

f-d

ípapß gordo ainÆt no madre flaca!

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