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FW: soils pressure

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Joe,

regarding the lateral earth pressures to use for design, you should really
get help from a geotech with experience in this type of problems. Let's
assume the rock would not exert any pressure on the basement wall (this may
not be true). As the width of the gap between the wall and the rock face
increases (filled with free-draining material), the pressures on the wall
will increase. The maximum pressure (either active or repose) depending on
the stiffness of the wall) will be achieved when the gap reaches a width of
roughly half of the wall height. You cannot interpolate lineraly between
these two conditions. This also assumes that the backfill is really draining
freely.

On the other hand, the rock can exert pressure depending on the joint and
bedding pattern, type of rock, etc, and it would be recommended to assume a
minimum rock pressure in any case.

Jesús Gómez, Ph.D., P.E. 
Associate                                          
Schnabel Engineering 
510 East Gay Street 
West Chester, PA 19380 
jgomez(--nospam--at)schnabel-eng.com 


-----Original Message-----
From: John P. Riley
To: seaint(--nospam--at)seaint.org
Sent: 4/21/04 11:16 PM
Subject: RE: soils pressure

Couple of shots from the hip:
 
1.  Make sure that the "free draining fill" can freely drain; the water
needs an exit path with adequate capacity.  If water accummulates in the
voids, the lateral pressure is the sum of the hydrostatic pressure and
the equivalent fluid pressure of the buoyant fill.  And the upward
pressure on the floor slab is 62.4H.  (To help prevent it from filling,
you might want to seal the fill on top with clay.)
 
2.  In all my born days, I have never seen a spec house that has
blocking perpendicular to the floor joists; the blocking transfers the
lateral wall load to the diaphragm.  In a small ranch home, the floor
beam restrains the wall at midpoint, but it is often not enough.  Even
in larger homes that are "engineered" the top-of-wall details are
sometimes lacking.  Draw a free-body diagram of the end wall, including
the rim joist.  How does the lateral load get from the wall to the
sheathing?
 
 
 



I am told the cut will be in rock and will be a vertical rock face.  The
actual condition remains to be seen.  I also understand that the purpose
of the gravel fill is to relieve the hydrostatic pressure.  I to have
designed many basement walls. My question (which I just talked to a
geotech. about) is...with the vertical rock face about 24" (give or take)
behind the basement wall, what lateral soil pressures will the wall
design be required to resist?  I have never had this situation before.

 

My other problem after the soil pressure question is answered (by the
way, the geotech. said I "done good") is that I did all the required
design for the connections to the diaphragm and the diaphragm check for
restraining the top of the wall, and the contractors are telling the
architect that I am crazy and that they have never seen that sort of
requirements before.  Of course the architects are agreeing with the
contractors.

 

Another problem is I now live in an area where the typical construction
for basement walls is primarily CMU.  I am seeing some deep cuts,
therefore some large restraint forces at the top of the wall to deal
with and reinforcing that seems heavy to them.  I am new to this area,
so I can only guess that other engineers around here in the past have
not looked at the diaphragm and the connections to transfer the
restraint forces to the diaphragm as closely as I do.  They are also
totally against using full retaining walls since they don't want to use
concrete.

 

-----Original Message-----
From: hadiprawira djohan [mailto:hadiprawira(--nospam--at)yahoo.com] 
Sent: Wednesday, April 21, 2004 3:58 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: soils pressure

 

If i understand your question correctly, the purpose of free draining
fill, which usually wrapped around by mirafi filter fabric is only to
release hydrostatic pressure. It will not counteract  full lateral soil
pressure against the wall.

In the past, I have designed many retaining wall as a basement wall by
assuming additional support at the top of the wall. To determine if the
top support (floor diaphragm) is sufficient, it really depends on the
condition of the other end of the room. Keep in mind that the pressure
at the top of wall is less than on the bottom of wall due to triangular
soil loading.

I would be skeptical if the soil would be able to stand almost straight
up for 12' (+ footing thickness) without any means of shoring during the
duration of the construction. 

I hope this help. 

Hadi Djohan, P.E.

Brooker Engineering, NY

Joe Grill <jgrill(--nospam--at)swiaz.com> wrote:

I have been asked to design a basement wall with a 12' retained height
of
soil. There has been no geotechnical review of the residence, but the
architect, who has done other work in the area, tells me the cut will
probably stand almost vertical. If this near vertical face is just a
couple
of feet away from the wall and the backfill is a free draining fill,
will
the fill be able to achieve full lateral soil pressure against the wall.
The architect/contractor/owner wants to use the floor diaphragm to
restrain
the top of the wall. If the full lateral soil pressure is achieved then
the
diaphragm doesn't have the capacity to restrain the wall. I have
suggested
a geotech be brought in to look at the excavation and make
recommendations
at that point, but of course that hasn't been received very well. If the
full soil pressure is not activated will there be any pressure at all
and
how is it calculated.

Any ideas?

Joseph R. Grill


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