I'm building a retaining wall in Tucson, AZ, it is about 6' tall and
70' long. I had a structural engineer do all of the design (but he is
almost impossible to get a hold of for these kinds of questions). I've
been told that I should waterproof the side which will be backfilled
otherwise the wall will not last long and the other side will show ugly
white stains right away. I bought some Xypex at the local Home Depot
and plan to use that.
I was told that I only need to coat the area which is above the
finished grade on the low side of the wall (the side that isn't being
filled). Does that sound right?
The engineer called for 3" PVC every 8', at 8" above finished grade.
I read somewhere in this newsgroup that the wall needs to let the water
through or you'll have so much presure that the wall will burst. So do
I really want to waterproof this thing... Isn't that going to cause
extra presure if I try and hold back the moisture instead of let it
p.s.-This soil is typical for Tucson, AZ, hard as rock and not much
It seems your engineer is providing appropriate drainage for future water
which may collect behind the wall for a number of reasons which may, or may
not, occur. The preventative measures are cheap insurance. Generally, the
sealant on the back side of the wall will reduce the chance of staining of
the wall while the PVC drains will prevent dangerous buildup of water
pressure behind the wall. I presume you were given instructions to place a
rock or gravel filter along the level of the drains (back side of course).
Be sure to follow those instructions.
That 3" pvc is in a gravel french drain on the back of the wall
with ground cloth covering the gravel, yes?
This is a poured concrete wall isn't it? Or is this a CMU wall?
Xypex is a good product, though I don't think you would ever rot
out a concrete retaining wall in yours or my lifetimes.
Keep the whole world singing . . . .
DanG (remove the sevens)
Thanks for all the answers, and to answer all of the questions:
1. Yes, the engineer called for crushed rock on the fill side of the
wall, around the PVC pipes. I haven't planned how I'll place this yet,
I figure I'll do the back-fill to within a couple inches of the pipe,
then place the crushed rock, then do the rest of the fill. If I'm
renting a tractor or paying helpers by the hour I'll probably figure
out a way to avoid the 3 step process by placing the crushed rock
before any fill.
2. No, I'm not using perforated pipe, I'm using the gray 3" PVC that is
made for direct sunlight, the kind you find in the electrical section
of a hardware store.
3. I wasn't planing on having the PVC stick out very far on either
side, probably just a couple inches.
4. It is a CMU wall, 8" bond beam, but 100% filled with concrete and
rebar in all directions. #5 @ 8" vert, plus #5 cont/horz every 24".
Also, even with the Xypex/waterproofer I'll still see lime on the
outside of the wall? I don't have to look at it, and my neighbors
could always do something to hide it, but I'm mostly just wondering so
that I don't freak out when I start to see it and think that I did
it's a matter of degree. the process is called effloresence, and some amount
of it is almost unavoidable in a masonry wall that is not completely
protected from the weather. what is happening is that water is migrating
through the porous cement, picking up some of the salts that are a byproduct
of the cement/mortar hardening process, and depositing them on the outside
as it evaporates off the front side of the wall. in and of itself,
efflorescence does not harm anything, nor is it necessarily the sign of a
bad installation. it can be scrubbed off, or left to fade with time. it is
very typical that a new installation will effloresce for a while and then
stop, as the salts that are not bonded are "washed" out.
the thing to remember is that efflorescence is a sign that water is moving
through a masonry wall. thus it is a great early warning sign when an old
brick wall starts efflorescing - it is telling you that there's a leak, and
usually it shows up long before other visible signs become apparent. (thus
giving you a chance to fix the problem before it does major damage.)
so, the reason xypex was recommended is that it will stop the flow of water
from the soil through the wall. no matter how dry your climate, there will
always be more water in the soil than on the outside face of a wall, so
without the xypex, there will be a flow. since the wall is outside, there is
no real reason why moisture moving through the wall is a bad thing, except
for the appearance. the xypex will greatly reduce, but not completly
eliminate, the presence of moisture in your wall.
bottom line is, yes, a certain amount of "blooming" should be expected.
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