If I understand you correctly, the distance between the edge of the footing
and the retaining wall is about 17 feet and the bottom of the new footing is
about 2 feet above the road level. Generally, for a continuous footing, if
you drop a line from the edge of the footing at somewhere between a 1H:1V
and 2H:1V (depending on the type of soil) and it lies below the level of the
grade on the low side of the retaining wall, you will have little, if any,
effect on the wall. At 5H:1V as I interpret your situation, it would not
seem to be a concern. For Isolated spread footings, the load will also
spread laterally, making it even less critical.
I would be more concerned about bearing on a shallow thickness of soil
between the old and new footing as it seems your contractor is proposing
(i.e., 5.5 ft below grade plus 16" = 4'-2" below grade, proposed bottom of
new footing = 3'-0" below grades, depth of intervening soil = 4'-2" minus
3'-0" =1'2"). Sounds like some corners are being cut to me.
Bill Cain, SE
From: Alex C. Nacionales [SMTP:alexcnac(--nospam--at)easycom.net]
Sent: Friday, May 14, 1999 14:37 PM
Subject: building on elevated ground
I designed a three storey RC school building
on site which is an elevated natural ground 5' above the road. The
retainwall is about
20' from the building line. My design uses a 9' wide continuous
footing located 5'6" below
natural ground. The old two story building has concrete columns
which is also the location
of my new columns. The old footings is 4'-6" x 4-6" wide and 16"
thick. The contractor
proposed to place the new footing 3' below natural ground which
will be on top of the old spread
footings and offered to deduct demolition and excavation costs from
I will have to check the Beaing capacity at 3' level. My problem is
how to check for
the effect of seismic load on the retaining wall due to new building
because the footing will be
2' above the road elevation. The overburden load will be neglibible.
I would appreciate any help or suggestions. Thanks in advance.
Alex Nacionales, C.E.