huangyhg

hydrostactic presure to retaining walls
morning my structural minded colleges i have what will hopefully be an easy question for you.
residential blocks with 1.5% fall across them with 1.0m retaining wall between them. in a major storm the water runs down through every ones blocks and out into a field. not a problem yards are graded to pass water through and way from houses.
how ever my neighbour on the high side of me has build a garden along her wall for the entire wall length (25m) now in a major storm her yard has a weir effect and dams 150mm of water for about 8 鈥?10hrs before it will dissipates. the wall has 100mm agg pipe behind it to drain small amounts of water away but can not cope with the sort of discharge.
our dividing wall is 1.0m high has a 2.0m fence on top of it.
its design data is
-allowed for 125kpa safe bearing pressure
-47.1m/s wind speed
-and allows for 5kpa surcharge.
should i be worried about hydrostatic pressure behind this wall and if failing and her yard ending up in mine. or is this wall to small to really worry about that. the wall is 6yrs old and is in good condition.
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where are you located? soil conditions?
i have a hard time getting excited about 150 mm of water; but a full meter of impoundment would be a very different story. in some ways your neighbor has done you a favor by slowing her runoff - she created on-site retention of storm water.
have you had an engineer look at the wall? what did s/he say?
you need local help - hire a local engineer to look at your concern -
assuming 1m of water behind the wall, the total soil + water loading would be approximately 95psf resulting in a thrust of wx^2/2 = 430# and a moment of wx^3/6 = 430 ft-lbs.
the wind loading will cause more moment than the water impoundment. if the effects are long term, the thrust coult tilt the wall some.
as one who has been down this road as an engineer and surveyor, get yourself some good documentation, pictures andwritten data, in case the wall tilts and you want remedial action.
austdesign,
i am an australian structural engineer and tend to do a few of these types of retaining walls.
from my knowledge i wouldn't get too exited about the water as it is banking up against a garden away from the top of the wall and then slowly filtering through the soil to the ag drain. you should also have a layer of open graded stone behind the wall for the full height, the ag drain will be at the base of this. the stone will relieve some of the hydrostatic pressure as the water will infiltrate through this to the ag drain at a reasonable rate.
if you like you can send me a dimensioned sketch showing elevation and section (i need to know what the footing and reiforcement is if possible) and i will give it a quick check if you like.
regards
sc
while the moment due to one meter of water isn't very large, garden walls (in my area, anyway) are seldom built with a strip footing beneath them for support - and aren't embedded more than about 150 mm. if we assume the wall is about 200 mm wide, then a combination of shallow bearing level, water infiltration / saturated ground, a very narrow footing - and overturning moment due to one meter of impoundment could cause problems.
of course, this is only hypothetical - since the water is reportedly only 150 mm deep...
i have seen three feet retaining walls failed, most likely due to hydrostatic pressure as a result of poor drainage. if you see the failure of such wall not resulting in any further major damage to neighboring structures, there should not be much concern. what type of soil is located just below the top soil? sometimes layer of clayer soils are placed just below the top soil to reduce infiltration which then to reduce infiltration behind to wall in order to prevent significant hydrostatic pressure build-up. this may help to magnify the actual hydrostatic pressure behind the wall.
however, if you have the feeling that something is not right have an experience engineer (geotechnical/structural) check the wall.

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