Active Pressure Combined with Water Pressure
Active Pressure Combined with Water Pressure
(OP)
Can someone show me an example of calculation of equivalent fluid pressure when a retaining wall is designed to support the soil + water the full height?
Thanks.
Thanks.





RE: Active Pressure Combined with Water Pressure
You really did not provide enough information to get an accurate answer. However, Earth Pressure = Depth Below Top of Wall x Equivalent Fluid Pressure.
Water Pressure = Depth below Water x Unit Weight of Water.
Don't forget to include lateral pressures from any surcharges behind the wall.
RE: Active Pressure Combined with Water Pressure
Say soil density = 115 pcf
Say H=10 ft
Water table is at top of wall
Can we say Total Equivalent Fluid pressure = 62.4 psf + (115-62.4)*0.333 = 80 psf?
RE: Active Pressure Combined with Water Pressure
DRG
RE: Active Pressure Combined with Water Pressure
Your calculation is correct.
RE: Active Pressure Combined with Water Pressure
You did not say what type of wall you have. The earth pressure distribution and magnitude could be different depending on the type of wall and compaction of backfill, if any, behind the wall.
Braced and tiedback walls do not usually have triangular earth pressure distributions behind the walls. Rigid walls may not use Ka. Compacted earth pressures may not be triangular or active.
The actual earth pressure depends on several different factors.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
Interior pit dimensions are 8'-0" X 8'-0" x Depth 10'-6".
I would consider a triangular shaped pressure on the wall, with maximum at the base, using the fully calculated equivalent fluid pressure of 80 psf. Thus, the pressure at the base would be 80psf x 10.5' =840 lb/ft, and total horizontal force would be at 1/3 from the base = 840 x 10.5' x 1/2 = 4410 lb/ft of wall.
This the way I have always calculated retaining walls.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
hokie66, the pit walls will deflect when they are being backfilled, before the concrete slab is installed. The slab will not support the walls until after the soil load is supported by the cantilevered walls. Therefore, the earth design pressure is, at best, debatable.
RE: Active Pressure Combined with Water Pressure
My comment was based on the pit being a box which will span horizontally. As it is only 8' square, there will be negligible deflection. The lid doesn't matter in this case. Structurebeton said it would act like a cantilever when it is backfilled, but he is mistaken.
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
RE: Active Pressure Combined with Water Pressure
As a structural engineer rather than a geotech, I would always consider the pressure on a box like yours as resulting from the at rest condition. I would design the walls horizontally spanning based on the pressure perhaps three feet above the base. The vertical reinforcement would take care of fixity at the base. As to arching action as PEinc suggested, sure it is there, but I wouldn't use it. With this type structure, the labor is the major item. A bit more reinforcement would be insignificant in the cost.
RE: Active Pressure Combined with Water Pressure
If fill is compacted against a stiff wall, the lateral pressure near the top can actually be higher than the simple Ko condition. (Search earlier threads. For explanation, see paper by Duncan and friends in December 1991 ASCE JGE + errata in March 1992 + discussion in July 1993.) This can have the double whammy of moving the centroid of the force higher, in addition to increasing the total force. The equivalent fluid-pressure model can't capture this (or having the water table below the ground surface).
Of course, for an 8'x8' box, you might just want to throw in some extra steel and call it good.