Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.41
TABLE 6.12 Typical Effective Friction Angles (��) for Different
Cohesionless Soils*
Effective friction angles (��) at peak
strength
Soil types
Loose Medium Dense
Silt (nonplastic) 26 to 30� 28 to 32� 30 to 34�
Uniform fine to
medium sand
26 to 30� 30 to 34� 32 to 36�
Well-graded sand 30 to 34� 34 to 40� 38 to 46�
Sand and gravel
mixtures
32 to 36� 36 to 42� 40 to 48�
* Data from B. K. Hough, ‘‘Basic Soils Engineering,’’ 2d ed., John Wiley
& Sons, Inc., New York.
denser state the soil particles are interlocked to a higher degree and hence the
effective friction angle is greater than in a loose state. It has been observed that
in the ultimate shear strength state, the shear strength and density of a loose and
dense sand tend to approach each other.
3. Grain Size Distribution. A well-graded granular soil will usually have a higher
friction angle than a uniform soil. With more soil particles to fill in the small
spaces between soil particles, there is more interlocking and frictional resistance
developed for a well-graded than a uniform granular soil.
4. Mineral Type, Angularity, and Particle Size. Soil particles composed of quartz
tend to have a higher friction angle than soil particles composed of weak carbonate.
Angular soil particles tend to have rougher surfaces and better interlocking
ability. Larger-sized particles, such as gravel-sized particles, typically
have higher friction angles than sand.
5. Deposit Variability. Because of variations in soil types, gradations, particle arrangements,
and dry density values, the effective friction angle is rarely uniform
with depth. It takes considerable judgment and experience in selecting an effective
friction angle based on an analysis of laboratory data.
6. Indirect Methods. For many projects, the effective friction angle of the sand is
determined by indirect means, such as the Standard Penetration Test and the
Cone Penetration Test.
Cohesive Soil. The shear strength of cohesive (plastic) soil, such as silts and clays,
is much more complicated than the shear strength of granular soils. Also, in general
the shear strength of cohesive (plastic) soils tends to be lower than the shear strength
of granular soils. As a result, more shear-induced failures occur in cohesive soils,
such as clays, than in granular (nonplastic) soils.
Depending on the type of loading condition, either a total stress analysis or an
effective stress analysis could be performed for cohesive soil. In general, total stress
analysis (s u or c and �) are used for short-term conditions, such as at the end of
construction. The total stress parameters, such as the undrained shear strength (s u),
can be determined from an unconfined compression test or vane tests.
Figure 6.11 presents an example of the undrained shear strength (s u) versus
depth for Borings E1 and F1 excavated in an offshore deposit of Orinoco clay
(created by sediments from the Orinoco River, Venezuela). The Orinoco clay can
be generally classified as a clay of high plasticity (CH) and can be considered to