The Standard Penetration Test (SPT) is a widely used in-situ testing method to determine the geotechnical engineering properties of subsurface soils. The test involves driving a standard thick-walled sample tube into the ground at the bottom of a borehole by blows from a slide hammer with standard weight and falling height. The number of blows required for the tube to penetrate each 6 inches (150 mm) of the subsurface soil is recorded and reported as the Standard Penetration Test Value (SPT-N value). This value is an indicator of the density of the subsurface soil and can be used to determine the soil's engineering properties. The SPT Value gives an indication of how strong or stiff the soil is, which helps to determine its suitability for construction purposes.
The Friction Angle of
soil is an important concept in civil engineering, and its importance cannot be
overstated. This angle, which is also known as the internal angle of friction,
is a measure of the resistance of soil to shear stress. It is a critical factor
in determining the stability of a soil mass, and is used to calculate the
bearing capacity of a soil foundation.
Friction angle, also
known as angle of internal friction, is a measure of the strength of a soil's
resistance to sliding along a plane. It is an important parameter in
geotechnical engineering and soil mechanics, as it is used to calculate the
bearing capacity of soils and the stability of slopes. The friction angle is
the angle between the plane of the soil and the normal force on the soil. It is
usually determined by performing a triaxial shear test on a sample of soil. The
friction angle of a soil is affected by its composition, structure, and water
content.
The Friction Angle of
soil is determined by a number of factors, including the type of soil, its
composition, and the amount of water present. Generally, the higher the
Friction Angle, the greater the resistance of the soil to shear stress. This is
why it is important for civil engineers to understand this concept and use it
when designing structures.
SPT (N) value and
friction angle are two important parameters that must be taken into
consideration when designing piles in sandy soils. SPT (N) value is a measure
of the energy required to drive a standard penetration test (SPT) sampler into
the soil, and is used to assess the density of the soil. The friction angle, on
the other hand, is a measure of the internal angle of shear resistance of the
soil, and is used to determine the ultimate capacity of the pile. Both of these
parameters are essential in the design of piles in sandy soils, as they provide
important information about the soil's strength and stability.
The
following table provides guidelines for obtaining the friction angle using SPT
values.
Table: Friction angle, SPT (N) values and relative density (Bowles 2004)
Table: SPT (N) value and soil consistency
Bearing capacity
The bearing capacity of soil is the maximum load
that a soil can withstand without experiencing shear failure
or excessive settlement.
Allowable bearing
pressure
Allowable bearing
pressure is the maximum pressure that can be imposed by a foundation onto soil
or rock supporting the foundation. It is derived from experience and general
usage, and provides an adequate factor of safety against shear failure and
excessive settlement.
Allowable bearing capacity
The maximum allowable bearing pressure for the design of
foundations.
Ultimate bearing capacity
The bearing pressure that causes failure of the soil or rock
supporting the foundation.
Bearing capacity failure
A foundation failure that occurs when the shear stresses in
the adjacent soil exceed the shear strength.
Presumptive bearing capacity
Presumptive bearing
capacity is an estimated value of the allowable bearing capacity of soil,
typically based on visual classification of surface soil and empirical
relationships. It is used for preliminary design purposes or for small,
unimportant structures where a detailed geotechnical investigation is not
warranted.
Presumptive bearing capacity values
are typically tabulated in building codes and other design standards. These
values are based on experience with other structures already built on similar
soils.
presumptive bearing capacity values are a useful
tool for preliminary design purposes, but they should not be used for final
design of important structures. A site-specific geotechnical investigation or
shallow foundation bearing capacity tests should be conducted to determine the
allowable bearing capacity of soil for these structures.
Presumptive Bearing Capacities for Foundations in Granular Soils Based on SPT Data (at a Minimum Depth of 0.75 m Below Ground Level)
Note: The water table is assumed not to be above the base of foundation. Presumed bearing values for pad foundations up to 3 m wide are approximately twice the above values.
Source: From Tomlinson, M.J. and Boorman, R.,
1995, Foundation Design and Construction, Longman Scientific and Technical,
Brunthill, Harlow, England.
Presumptive Bearing Capacities for Foundations in Clayey Soils Based on Undrained Shear Strength (at a Minimum Depth of 1 m Below Ground Level)
Source: From Tomlinson, M.J.
and Boorman, R., 1995, Foundation Design and Construction, Longman Scientific
and Technical, Brunthill, Harlow, England.
Preliminary estimate of bearing capacity
Sands
– * For Clayey Sands reduce φ by 5◦.
– * For Gravelly Sands increase φ by 5◦.
– * Water level assumed to be greater than B
(width of footing) below bottom of footing.
– * For saturated or submerged conditions – half
the value in the Table.
– Based on a foundation width greater than 1m
and settlement = 25 mm. Divide by 1.2 for strip foundation. The bearing value
in sands can be doubled, if settlement = 50 mm is acceptable.
– For B < 1 m, the bearing pressure is reduced by a ratio of B (Peck, Hanson and Thornburn, 1974).
Bearing capacity of granular soils
- •
In granular soils, the
friction angle is often determined from the SPT N – value. Methods that
directly use the N – value to obtain the bearing capacity, therefore can
provide a more direct means of obtaining that parameter.
- •
The table below assumes
the foundation is unaffected by water. Where the water is within B or less
below the foundation then the quoted values should be halved. This practice is
considered conservative as some researchers believe that effect may already be
accounted for in the N – value.
- • The allowable capacity
(FS = 3) is based on settlements no greater than 25 mm. For acceptable
settlements of 50 mm say, the capacity can be doubled while for settlements of
12 mm the allowable capacity in the Table should be halved.
- • The footing is assumed
to be at the surface. There is an increase bearing with embedment depth. This
can be up to 1/3 increase, for an embedment = Footing width (B).
- • The corrected N – value should be used.
Allowable bearing capacity of granular soils (adapted from Meyerhof, 1956)
Clay strength from SPT data
• As a first approximation Cu = 5 SPT
is commonly used. However, this correlation
is known to vary from 2 to 8.
• The overburden correction is not required for
SPT values in clays.
•
Sensitivity of clay affects the results.
Clay strength from SPT data
Clean sand strength from SPT data
• The values vary from corrected to uncorrected
N values and type of sand.
• The SPT – value can be used to determine the degree of compactness of a cohesionless soil. However, it is the soil friction angle that is used as the strength parameter.
Strength from SPT on clean medium size sands only
• Reduce φ by 5◦ for clayey sand.
•
Increase φ by 5◦ for gravely sand.
Fine and coarse sand strength from SPT data
• Fine sands have reduced values from the table
above while coarse sand has an
increased strength value.
• The corrected N value is used in the table below.
Strength from corrected
SPT value on clean fine and coarse size sands
– (No)60/D2 r = 55 for Fine Sands.
– (No)60/D2 r = 60 for Medium Sands.
–
(No)60/D2 r = 65 for Coarse Sands.
Source: Look, B.G. (2014). Handbook of Geotechnical Investigation and Design Tables: Second Edition (2nd ed.). CRC Press.
The elastic modulus of soil, also known as Young's modulus for
soil, is a measure of the soil's stiffness or resistance to deformation. It is
defined as the ratio of the stress (force per unit area) applied to the soil to
the resulting strain (deformation). In other words, it is the amount of stress
required to cause a given amount of strain in the soil.
The elastic modulus of soil is an important parameter in
geotechnical engineering, as it is used to estimate the settlement of
foundations, the stability of slopes, and the bearing capacity of soil. It is
also used to design pavements, retaining walls, and other structures that
interact with soil.
Elastic Modulus vs. SPT (N) Values
Elastic Modulus and Poisson's Ratio for Soils
Soil Elastic Moduli from In Situ Test Data
Source: From Bowles, J.E., 2002, Foundation Analysis and
Design, McGraw-Hill, New York.
Gunaratne, M. (Ed.). (2013). The
Foundation Engineering Handbook (2nd ed.). CRC Press.