Site Investigation Flashcards
What are benefits and use cases of the shell and augur method?
Cheap
Easily installed
Used in stiff clays
Installation of equipment
How are undisturbed cores obtained?
Rotary core drilling
Why are results obtained from coring often slightly inaccurate?
Drilling induces shear strains and external stresses, and the extraction affects water content
When is driving (percussive blows) preferred to single push methods?
In fibrous material such as peat
What is the preferred method for obtaining high quality samples of soft clay?
Piston sampling
How are coarse grained soils characterised?
Particle size grading
How are fine grained soils characterised?
Plasticity and liquidity
How is the liquid limit of a soil determined?
By performing a falling come test. When the cone is able to fall 20mm in 5 seconds this is the limit
How is the plastic limit of a soil determined?
Rolling a sample down to 3mm diameter and checking for the appearance of cracks
How does a standard penetration test characterise soil strength?
By recording the number of blows required to drive the test unit a specific distance, usually 300mm
What are the conditions of the SPT N value?
The number of blows by a 63.5kg hammer through 760mm required to drive the sampler 300mm deep
What are the 3 values which can be obtained from a cone penetration test?
Cone tip pressure
Pore water pressure
Sleeve friction
What are 3 advantages of a CPT?
Accurate and repeatable
Easy delineation of strata
Fast setup and operation
What is a pressure meter test used for?
Measuring soil stiffness
When is a ring shear test used?
To analyse very large displacements for residual strength testing
What does a direct shear box test measure?
Total stress only
How does a shear vane test measure shear strength?
The torque required to turn the blades to failure is related to shear strength
Why does the friction sleeve on a CPT affect pore pressure measurements?
The sleeve position is flexible so the size of the pressure meter intake can change
What are the typical properties of coarse grained soils?
High strength
High permeability
Poor compressibility
How can soil under a CPT be assumed to fail?
Similarly to a shallow foundation
What is the equation for relative density?
Max Void ratio - current vr
/
Max vr - min vr
What a are the factors that relate torque of a shear vane to undrained strength?
Vane width and vane height
What are the required assumptions for the shear vane test?
Minimal disturbance
No drainage
Homogeneity
The formation of a cylindrical surface
Uniform stress distribution
No progressive failure
Where is the shear vane unsuitable?
In soil with strength over 100kPa
What soil type can the self boring pressuremeter not be used in?
Gravels
What is the initial stress state in a pressuremeter?
Radial stress = hoop stress = lateral stress
How do radial and hoop stresses change as excess pressure is applied inside the pressuremeter?
Change in Radial = A+(B/r2)
Change in Hoop = A-(B/r2)
A and B are site specific contestants
What must the change in pressure be if A=0?
B/R2
How does hoop strain: 1/E (Δσθ - ν * Δσr) become: -Δp/2G?
Δσθ = -Δp
Δσr = Δp
(1 + ν)/E = 1/2G
For small strain cavity expansion, what is the equation for pressure p?
p = σho + 2Gεc
Where εc = -εθ
What is the equation for the state-dependent liquidity index IL?
w-wp
———
wL-wp
What is the relationship between liquidity index and undrained strength?
Su = 2 * 100^(1-IL)
In the equation qt = NktSu + σvo, what do each of the terms mean?
qt: cone tip resistance
Nkt: empirical cone factor
Su: undrained strength
σvo: initial vertical stress
What factors are used to calculate torque of a shear vane test?
Moment of shear resistance on the side and moment of shear resistance at the ends
What is the moment of shear resistance at the side of a shear vane?
Circumference * height * radius * undrained strength
What is the moment of shear resistance at the ends of a shear vane?
2 * int(2 pi r2 su)dr
The integral is from 0 to r
What is the plasticity index?
The water content at liquid limit - water content at plastic limit
What numbers do you need to calculate water content?
Weight of soil (minus water and sample tin)
Weight of water
What should you calculate to quantitatively classify soils from a CPT?
Normalised cone resistance
Normalised friction ratio
Pore pressure parameter
What is the equation for normalised cone resistance?
Qt = (qt-σv)/σ’v
What is the equation for normalised friction ratio?
Fr = (fs/(qt-σv))100%
What is the equation for pore pressure parameter?
Bq = Δu/(qt-σv)
After Qt, Fr and Bq are found, how are they used to classify soils?
Soil behaviour type graphs designate regions for specific soil types based on these values
How do you reduce the disturbance of a thin walled sampler?
Decrease the ratio of width to thickness
What are the pressuremeter boundary conditions?
A=0
B=delta p R^2
