SCHMERTMANN’S PROCEDURE FOR ESTIMATING IMMEDIATE SETTLEMENTS
(USED FOR IMMEDIATE/DRAINED SETTLEMENTS IN SANDS AND SILTS)
(Last Modified 22 February 2010)
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For the problem geometry, define the width of foundation (B), length of foundation (L), depth of foundation (D
F
), and depth to “rigid” foundation (T).
(2)
For the foundation L/B, estimate the Depth of Influence (Z
I
) for Schmertmann’s Influence Diagram.
NOTES:
(a)
The ordinate z measures depth below the existing ground surface (as would be used to correct SPT data).
The ordinate z* measures depth
below the bottom of the footing (as is required for estimating influence values).
The two ordinates are related to each other through:
z* = z  D
F
or through the inverse relationship:
z = z* + D
F
(b)
Schmertmann’s Influence Diagram is scaled in terms of the normalized depth below the footing, z*/Z
I
.
This ratio simply indicates the depth
below the footing (z*) relative to Schmertmann’s Depth of Influence (Z
I
).
(c)
Schmertmann’s Depth of Influence is calibrated to a specific set of experimental observations for use within this particular calculation procedure.
This depth should not be taken as the “depth of influence for stresses” for any calculations other than those associated with Schmertmann’s
Method.
(d)
According to Schmertmann’s recommendations, L/B is only allowed to vary between L/B=1 (a square or circular footing) and L/B=10 (a strip
footing).
If L/B is greater than 10, simply assume the foundation behaves as a strip foundation with L/B=10 so that Z
I
= 4B.
(e)
If Z
I
> T, then simply truncate the diagram at z* = T, ignoring all parts of the diagram for which z* > T
(3)
For all available information between D
F
≤
z
≤
D
F
+Z
I
, place stratum delineators.
This establishes the incremental thickness
Δ
z* for strata between the
bottom of the footing (z* = 0) and the depth of stress influence (z* = Z
I
).
NOTES:
(a)
The stratum delineators are generally placed at points equidistant from available test data (e.g., halfway between the locations of SPT data) or
to coincide with clearly defined breaks in the site lithology (e.g., as may be determined from CPT testing).
Once the stratum delineations have
been placed, the number of strata between the bottom of the footing (z = D
F
, where z*=0) and the depth of stress influence (z = D
F
+Z
I
, where
z*=Z
I
) is known as “N
S
”.
Subsequent summations will be made using an index, i, to identify each stratum and this index will vary from 1 to N
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 Spring '10
 ChrisSaucier
 footing

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