Building Design and Construction Handbook - Merritt - Ventech!

6.60 SECTION SIX
For overconsolidated soil, there are two possible cases that can be used to calculate
the amount of settlement. The first case occurs when the existing vertical
effective stress (� ) plus the increase in vertical stress (�� v) due to the proposed
�vo
building weight does not exceed the maximum past pressure (� ). For this first
�vm
case, there will only be recompression of the cohesive soil.
For the second case, the sum of the existing vertical effective stress (� ) plus
�vo
the increase in vertical stress (�� v) due to the proposed building weight exceeds
the maximum past pressure (� ). For the second case, there will be virgin con-
�vm
solidation of the cohesive soil. Given the same cohesive soil and identical field
conditions, the settlement due to the second case will be significantly more than
the first case.
As previously mentioned, primary consolidation is a time-dependent process that
can take many years to complete. The rate of consolidation can be estimated using
the Terzaghi theory of consolidation (see K. Terzaghi and R. B. Peck, ‘‘Soil Mechanics
in Engineering Practice,’’ John Wiley & Sons, Inc., New York):
Secondary Compression. The final component of settlement is due to secondary
compression, which is that part of the settlement that occurs after essentially all of
the excess pore water pressures have dissipated (i.e., settlement that occurs at constant
effective stress). The usual assumption is that secondary compression does not
start until after primary consolidation is complete. The amount of secondary compression
is often neglected because it is rather small compared to the primary
consolidation settlement. However, secondary compression can constitute a major
part of the total settlement for peat or other highly organic soil (see R. D. Holtz
and W. D. Kovacs, ‘‘An Introduction to Geotechnical Engineering,’’ Prentice-Hall,
Inc., Englewood Cliffs, NJ).
The final calculation for estimating the maximum settlement (� max) of the insitu
cohesive soil would be to add together the three components of settlement, or:
� � s � s � s (6.23)
max i c s
where � max � maximum settlement over the life of the structure
s i � immediate settlement
s c � primary consolidation settlement
s s � secondary compression settlement
6.5.7 Settlement of Granular Soil
A major difference between saturated cohesive soil and granular soil is that the
settlement of cohesionless soil is not time dependent. Because of the generally high
permeability of granular soil, the settlement usually occurs as the load is applied
during the construction of the building. Many different methods can be used to
determine the settlement of granular soil, such as plate load tests, laboratory testing
of undisturbed soil samples, equations based on the theory of elasticity, and empirical
correlations. For example, Fig. 6.23 shows a chart that presents an empirical
correlation between the measured N value (obtained from the Standard Penetration
Test, see Art. 6.2.4) and the allowable soil pressure (tsf) that will produce a settlement
of the footing of 1 in (2.5 cm).
As an example of the use of Fig. 6.23, suppose a site contains a sand deposit
and the proposed structure can be subjected to a maximum settlement (� max) of
1.0 in (2.5 cm). If the measured N value from the Standard Penetration Test � 10
and the width of the proposed footings � 5 ft (1.5 m), then the allowable soil
pressure � 1 tsf (100 kPa).