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RE: Expansive Soil

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Depends on the specifics of the soil. In general, in our local highly
expansive soils (Texas gulf coast), a two story building does not have
enough weight to stop expansion. OTOH, if you building is very heavy, it
might.

Some factors that could matter include: How deep are the grade beams and how
much steel do they have? what is the total building weight? what is the PI
of the soil? what is the primary clay mineral?

btw, what exactly do you mean by "40% openings?" Grade beams cover 60% of
the area? That would be a honkin' stiff slab if the grade beams were
reasonably deep.


Eric Green, PE

-----Original Message-----
From: Syed A Masroor [mailto:smasroor(--nospam--at)gem.net.pk]
Sent: Thursday, April 26, 2001 2:03 PM
To: seaint(--nospam--at)seaint.org
Subject: Expansive Soil


    I am designing a two-storied religious building (14000 sft each
floor) in concrete.  Foundation is on extremely plastic soil with heave
potential.  The slabs are to be flat type and the spans are nearly 30ft
x 30ft in the main hall.
    The soil engineer asked for a stiff footing system so I went for mat
type footing with 40% openings.  Now that the foundations are
constructed, the outer retaining walls (no basement, plinth of the
building is 4ft above EGL) have cracked at the middle.  I assured the
owner that once the building is constructed, its weight will counteract
the ground forces and the cracks will go away.
    The owners, are worried that if its cracking before construction,
God knows what may happen later.
    Any Help?

S A Masroor
Cons Str Engr
Karachi, Pakistan


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