In-situ testing Flashcards
Most common type of in situ test and how its carried out
Penetration test: Pushing or hammering a rod or probe into ground and measuring resistance.
for stiffness or strength.
Types of penetration tests
Standard Penetration Test (SPT)
Cone Penetration Testing (CPT)
Self Boring Pressuremeter (SBP)
Why is SPT test used? (its advantage over the other tests)
sample of the soil is recovered, so that the type of soil the test was carried out in can be observed.
What does the SPT consist of and how is it carried out>
Consists of a thick walled sampler (450mm) driven into soil at bottom of borehole using hammer (63.5kg) falling at 0.76m generally.
N of blows reuqired to drive from 150mm to 450mm (final 300mm is counted).
this is spt blow count N
Why correct the SPT N-value?
hammer energy is lost. For sand N60 = (ERr/60)N. Thus uk, N = N60.
N60 used for equopment in uk and china
Other ways in which N value needs corrected
length of rod
overburden pressure
Correction of SPT value in sand equation:
N60 = (ERr/60) × λ × CN × N
CPT equiptment (Cone penetrometer, advantages?
Fast, efficient doesn’t disturb soil as much.
How is a CPT test carried out?
Push rod 10cm^2 into soil at rate of 2omm/s using drill rig or truck.
end of rod fitted with cone with point angle 60 degrees. A load cell behind this measures force that soil excerts.
force divided by cross sectional area to give cone tip reisstance (Qc)
Second cell measure sfriction force, surface area 150cm^2, giving sleeve friction. (fs)
what do these symbols represent? (Fs) (Qc)
Fs = sleeve friction Qc = cone tip resistance
disadvantageS of CPT
NO SAMPLE OBTAINED.
CPT can deduct what soil parameters:
soil classification
shear strength
stiffness
pile bearing capacity
what is a pressuremeter?
an expanding cylindrical probe in soil, measuring pressure and amount of expansion.
disadvantages
slow, expensive complex.
what does pressuremeter give?
eleastic modulud (G), strength paraeters (Cu in cohesive, f’ in sands) and horizontal stress in the ground.
Why use a vane shear test?
useful in clayey soils with shear strength les than 75 kPa
How Vane shear test is carried out?
vane pushed into ground from surface or bottom of borehole.
vane is turned at set rate, torque recorded.
relationship between torque and shear stnregth is obtained by assuming failure at surface is cylindrical.
vane typically 150mm high 75mm wide for soil w/ 50kpa and 100mm by 50 wide for soil with soil strength 50-75kpa.
shear vane equations:
H = 2D
Cu = M/K (kpa)
M = torque in Nm
K = 3.66*D^3 * 10^-6
what parameters can vane test find
determine peak and residual shear strength
rotation at 6 - 12 per minute
important to keep speed constant
Plate bearing test: what and how is used?
stiff steel plate placed at surface or base of borehole, loaded to failure.
load settlement curve is recorded.
E or G can be determine dfrom inital slope of this curve.
soil strength from Ultimate bearing capacity.
