Max. total load on
load bearing wall (kN/m)
20 30 40 50 60 70Ground type Ground
condition
Field
testMinimum width (mm)Rock Not inferior
to sandstone,
limestone or
firm chalk.
Requires a
mechanical
device to
excavate.At least equal to
the width of the wallGravel Medium
density
Pick required
to excavate.
Sand Compact 50 mm square
peg hard to
drive beyond
150 mm.
250 300 400 500 600 650Clay
Sandy clay
Stiff
StiffRequires pick
or mechanical
device to aid
removal. Can
be indented
slightly with
thumb.250 300 400 500 600 650Clay
Sandy clay
Firm
FirmCan be moulded
under
substantial
pressure by
fingers.300 350 450 600 750 850Sand
Silty sand
Clayey sand
Loose
Loose
LooseCan be
excavated by
spade. 50 mm
square peg
easily driven.400 600Silt Soft
Clay Soft 450 650
Sandy clay Soft
Silty clay Soft
Silt Very soft
Clay Very soft
Sandy clay Very soft
Silty clay Very soft
Adapted from Table 10 in the Bldg. Regs., A.D: A † Structure.
Conventional strip
foundations unsuitable
for a total load
exceeding 30 kN/m.Finger pushed
in up to 10 mm.
Easily moulded
with fingers.
Finger easily
pushed in up
to 25 mm. Wet
sample exudes
between fingers
when squeezed.Conventional strip inappropriate.
Steel reinforced wide strip, deep
strip or piled foundation selected
subject to specialist advice.Guide to Strip Foundation Width