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Re: Retaining Wall

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In a message dated 7/10/2000 11:05:04 AM Pacific Daylight Time, 
Gerard_Madden(--nospam--at)enron.com writes:

<< I have designed a 350ft masonry retaining wall adjacent to a property line
 near the top of a hill. The plan checker scribbled on a piece of paper and
 gave it to my client at the plan check counter. 
 
 No structural comments.
 
 Grading Comments: 
 Calculate Qmax + Safety factor and size the V-ditch at the top of the wall
 for this value.
 
 This seems silly to me. The V-ditch at the top of the retained soil in my
 drawing is at least 6" wide and is 6" below the top of the wall. The
 backfill slopes 2:1 for about 7 feet back to the property line. This the
 hill slopes at about 4:1 after the property line and probably peaks about 6
 feet above the top of soil @ the property line.
 
 I have long forgotten my water resources engineering coursework but in the
 books I have looked into, in order to calculate Q, I need to know the area
 (in acres) draining to the wall. Since the area that is draining is not on
 the property, the owner has no survey or map that allows me to figure this
 area out. I have followed all of the requirements of the soils report
 (Including designing the wall for 70pcf (More than water ??? Come on !))
 
 My questions are:
 1.Is this really necessary? The client is improving the situation and the
 wall has drain rock, perforated pipe, and the v-ditch at the top. I don't
 want to relearn this stuff and still be unsure of what I am doing.
 
 2. If I do calculate Q, I assume that I divide Q by the width of my ditch
 and the velocity or the runoff along the ditch to figure out how deep the
 water gets in the ditch so it doesn't spill over the front of the wall.
 Also, what is a reasonable V to assume, I was thinking 3-5 ft/sec.
 
 3. Is this really necessary???
 
 Any advice is welcomed.
 -Gerard
  >>

This has nothing to do with the structural strength of the wall, unless you 
have debris to consider acting on the freeboard.

I believe the plancheker is concerned about the interruption of the natural 
flow of runoff. I would use a minimum 12" wide "v" ditch 6" deep with a 
minimum of 2% slope behind the wall to a discharge location outside the 
developed area.

You could get a general drainage map for the area from your local building 
department, which would be adequate in most cases, to figure out the 
tributary area to your ditch. Then, based on the rainfall in the area and 
soil type, you find Q (Q=CIA). From Manning's formula (Open Channel 
Hydraulics), you check the capacity of the "v" ditch and adjust accordingly. 
If mud flow is a problem, you need to increase the slope to at least 4%.

Hope this helps.

Oshin Tosounian, S.E.
Los Angeles, CA

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