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Shallow strip foundation depth

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Shallow strip foundation depth: IBC-2009, BS8004 and BR 2010.

Shallow foundations are taken to be those where the depth below finished ground level is less than 3 m and include strip, pad, raft and slab foundations. The choice of 3 m is arbitrary.

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The depth to which shallow foundations should be carried depends on three principal factors:
1) reaching an adequate bearing stratum;
2) in the case of clay soils, penetration with suitable precautions below the zone where shrinkage and swelling due to seasonal weather changes, trees, shrubs and other vegetation are likely to cause appreciable movement.
3) penetration below the zone in which trouble may be expected from frost.

Other factors such as ground movements, changes in groundwater conditions, long-term stability and heat transmitted from building and shallow foundation to the supporting ground may be important. Shallow foundations are particularly vulnerable to certain soil conditions, e.g. sensitive clays, loose water-bearing sands and soils that change structure when loaded. Specialist advice should be sought where such conditions are indicated by ground investigation.

Shallow strip foundations are appropriate choice where column loads are comparatively small and closely spaced or where walls are heavy or heavily loaded. Where the use of ordinary shallow strip foundations would overstress the bearing strata, wide strip shallow foundations designed to transmit the foundation loads across the full width of the strip may be used. The depth below the finished ground level should be the same as for ordinary strip shallow foundations.

Selecting the miniumum strip shallow foundation depth:

The British Building Regulations 2010, A1/2, section 2E4:
Minimum depth of shallow strip foundations except where strip foundations are founded on rock, the shallow strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground. In clay soils subject to volume change on drying (‘shrinkable clays’, with Plasticity Index greater than or equal to 10%), shallow strip foundations should be taken to a depth where anticipated ground movements will not impair the stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of shallow foundations on clay soils should not be less than 0.75m, although this depth will commonly need to be increased in order to transfer the loading onto satisfactory ground.

International Building code IBC-2009, section 1809.4 Depth and width of footings (shallow strip foundations).
The minimum depth of shallow foundations below the undisturbed ground surface shall be 12 inches (305 mm). Where applicable, the requirements of Section 1809.5 shall also be satisfied. The minimum width of footings shall be 12 inches (305 mm).
IBC-2009 1809.5 Shallow foundation frost protection. Except where otherwise protected from frost, shallow foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:
1. Extending below the frost line of the locality;
2. Constructing in accordance with ASCE 32 (ASCE 32-01 Design and construction of frost protected shallow foundations); or
3. Erecting on solid rock.
Exception: Free-standing buildings meeting all of the following conditions shall not be required to be protected:
a). Assigned to Occupancy Category I, in accordance with Section 1604.5 i.e.: Buildings and other structures that represent a low hazard to human life n the event of failure, including but not limited to: agricultural facilities, certain temporary facilities, minor storage facilities.
b). Area of 600 square feet (56 m2) or less for light-frame construction or 400 square feet (37 m2) or less for other than light-frame construction; and
c) Eave height of 10 feet (3048 mm) or less.
Shallow foundations shall not bear on frozen soil unless such frozen condition is of a permanent character.

Russian Building code SNiP 2.02.01-83 «Buildings foundations»:
Table: Shallow stripe foundaions depth for buildings with unheated ventilated crowl space. *


Shallow strip foundations depth

Foundation depth where water table is above the frost line +2 m

Foundation depth where water table is below the frost line + 2 m

Rocky soils, with coarse sand aggregate, large and medium size gravelly sand

not depend on the frost line

not depend on the frost line

Fine sand

equal to frost line depth

not depend on the frost line

Sandy loam

equal to frost line depth

not depend on the frost line

Loams, clay and coarse clastic soils with silty-clay aggregate

equal to frost line depth

not less than ½ of frost line

* Table №2  ch. 2.30 SNiP 2.02.01-83 «Buildings foundations»

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