Understanding Geotechnical Testing Data
a) What is the unconfined compression strength of the clay sample?
b) What is the shear strength of the tested sample?
c) What is the area ratio of the SPT sampler?
d) What is the SPT value?
e) What is the N60 value?
a) Unconfined Compression Strength:
The unconfined compression strength of a clay sample is the maximum stress that the sample can withstand without any lateral confinement.
b) Shear Strength:
The shear strength of a clay sample can be determined from the major and minor principal stresses at failure in a unconsolidated-undrained triaxial test.
c) Area Ratio of SPT Sampler:
The area ratio of a Standard Penetration Test (SPT) sampler is the ratio of the cross-sectional area of the sampler to the cross-sectional area of the drill rod.
d) SPT Value:
The Standard Penetration Test (SPT) value is a measure of the resistance of soil to penetration by a standard sampler driven by a standard hammer.
e) N60 Value:
The N60 value is an adjusted SPT value that represents the number of blows required for standard penetration of a soil sample over a 12-inch length after correction for hammer efficiency.
a) Unconfined Compression Strength:
Given: Major stress at failure (σ1) = 3500 psf
The unconfined compression strength is equal to the major stress at failure.
Unconfined Compression Strength = Major stress at failure
Unconfined Compression Strength = 3500 psf
b) Shear Strength:
Given: Major principal stress at failure (σ1) = 3500 psf, Minor principal stress at failure (σ3) = 500 psf
The shear strength (τ) is given by the difference between the major and minor principal stresses at failure.
Shear Strength = Major principal stress at failure - Minor principal stress at failure
Shear Strength = 3500 psf - 500 psf
Shear Strength = 3000 psf
c) Area Ratio of SPT Sampler:
The area ratio of a Standard Penetration Test (SPT) sampler is the ratio of the cross-sectional area of the sampler to the cross-sectional area of the drill rod.
d) SPT Value:
Given: Blow counts = 6, 10, 15
The SPT value is the sum of the blow counts for the first 12 inches of penetration, also known as the "N value".
SPT Value = Sum of blow counts for first 12 inches
SPT Value = 6 + 10 + 15
SPT Value = 31
e) N60 Value:
Given: Hammer efficiency = 85%, SPT Value = 31
The N60 value can be calculated by multiplying the SPT value by the hammer efficiency and rounding it to the nearest integer.
N60 Value = SPT Value * Hammer efficiency
N60 Value = 31 * 85%
N60 Value ≈ 26 (rounded to nearest integer)