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RE: Retaining Wall

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The ditch design flow rate Q is irrelevant to the structural design of the
wall. If you expect to have surface drainage running along the top of the
backfill, the safe thing to do is design for a groundwater load to the top
of the backfill and call it a day. For groundwater loading, assuming an
active earth pressure situation, I would design either for a wet but drained
soil or a submerged soil condition. 

For the wet soil, equivalent fluid loading would be equal to the wet weight
of the soil (say 140 pcf for illustration)times the active earth coefficient
(say .33), for a ballpark number of 46 pcf equivalent. For the submerged
case, the equivalent fluid loading would be equal to the unit weight of
water (63 pcf)plus the buoyant weight of the soil (say 75 pcf) times the
active earth coefficient, or about 88 pcf. It should be greater than the
density of water. 

This is for flat backfill. The numbers would increase due to the sloping
backfill, or for soils with greater densities or lower internal friction
angles, or be less if the reverse were true. Your geotechnical engineer's
numbers do not seem out of line.



-----Original Message-----
From: Madden, Gerard [mailto:Gerard_Madden(--nospam--at)enron.com]
Sent: Monday, July 10, 2000 10:55 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Retaining Wall


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


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