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RE: 20' foundation walls

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I think buoyancy is a major concern here. A twenty feet deep basement 
constructed in water-bearing soil will tend to move upwards in the ground. 
This tendency must be encountered by ensuring that the weight of the 
basement structure is greater than the uplift equal to the weight of the 
ground water displaced by the basement.

The safety margin required is a matter for the judgment. Dependent on the 
certainty with which the ground water level is known, the factor for safety 
 may vary between 1.10 and 1.25.

Also, the floor must be designed against the uplift due to the ground water 
 pressure.

Mansoor Khan

-----Original Message-----
From: seaint-seaosc(--nospam--at)mail-list.com [mailto:seaint-seaosc(--nospam--at)mail-list.com] On 
Behalf Of Will Haynes Sent: Monday, October 22, 2012 5:28 PM
To: seaint-seaosc(--nospam--at)mail-list.com
Subject: Re: [SEAINT-SEAOSC] 20' foundation walls

The water table may fluctuate so it would be good to have a geotech's 
opinion for the area but I am figuring a report may not be available. Even 
if there is adequate drainage behind the wall, if the high seasonal water 
table is above any drain elevation there will be saturated soil pressure on 
 the wall and I think following ACI 350 is a good idea.

I would add a water stop to any construction joints in a basement wall 
regardless. Self consolidating concrete can help with larger pour heights 
as mentioned, the pressure on the formwork will be quite high if pouring 
from 20 feet.

Will H

On Sun, Oct 21, 2012 at 1:46 AM, Valerie Eskelsen < 
v.t.eskelsen(--nospam--at)sbcglobal.net> wrote:

> Dear List,
> 
> Designing 20' deep, restrained foundation walls for a residential
> structure. The water table will not be known until the dig is begun.

Truncated 420 characters in the previous message to save energy.

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