5 - In-situ testing and settlement in sands Flashcards
(41 cards)
Why is in-situ testing often used for sands instead of lab testing?
Because sand samples are difficult to retrieve undisturbed, and even minor disturbance can affect lab test results.
What are the two main in-situ tests used to determine sand properties?
The Standard Penetration Test (SPT) and the Cone Penetration Test (CPT).
What does the Standard Penetration Test (SPT) measure?
The resistance of the ground to penetration by measuring the energy required to drive a sampler into the soil.
Describe the basic procedure of an SPT.
A 65 kg hammer is dropped from 760 mm to drive a split-barrel sampler 300 mm into the ground after an initial 150 mm seating drive.
What is the uncorrected blow count in an SPT called?
The N-value.
How is the SPT N-value typically recorded?
In increments of 75 mm during the 300 mm penetration after seating.
Why might SPT results vary between different equipment setups?
Because different hammers and borehole conditions can affect the energy transferred to the ground.
What does N60 represent in SPT testing?
The N-value corrected to 60% energy efficiency.
Which standard provides guidance on SPT corrections for energy efficiency?
BS EN ISO 22476-3:2005.
Is N60 used in this course to estimate settlement or φ′max?
No, this course uses the N-value directly and does not cover φ′max estimation.
What relationship did Burland and Burbidge (1985) establish using SPT data?
A method to estimate the settlement of shallow foundations in sands and gravels based on the SPT N-value.
How is the N-value corrected for silty/fine sands below the water table (when N > 15)?
N
′
=15+
2
1
(N−15)
What is the correction for gravelly soils below the water table?
N
′
=1.25N
How do you determine the depth of influence
𝑧
𝐼
z
I
if N is constant or increasing with depth?
zI =B^0.75
Q: What does CPT stand for in geotechnical testing?
A: Cone Penetration Test
Q: What is the standard angle of the cone in a CPT?
A: 60 degrees
Q: At what rate is the CPT cone pushed into the ground?
A: 20 mm/s
Q: What is the typical truck weight used for CPT, and how deep can it push the cone?
A: A 20-tonne truck can push the cone up to 30 m in dense sands or stiff clays
Q: What does the standard electric cone in a CPT measure?
A: Tip resistance (qc) and sleeve friction (fs)
Q: What additional measurement does a piezocone test (CPTU) provide?
A: Pore water pressure around the cone
Q: Why is CPT considered more ‘scientific’ than SPT?
A: It provides continuous data and a broader range of soil parameters, including stratigraphy and pore pressures
Q: What does erratic cone resistance and low pore pressure in CPTU data suggest?
A: Dense granular material where the cone punches through layers mobilizing peak strengths
