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RE: soils pressure[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: soils pressure
- From: "John P. Riley" <jpriley485(--nospam--at)peoplepc.com>
- Date: Wed, 21 Apr 2004 22:16:31 -0500
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.
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.
Brooker Engineering, NY
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