correction for cpt fs errors due to variation in sleeve ... · to variation in sleeve diameter m....
TRANSCRIPT
Correction for CPT fs errors due to variation in sleeve diameter
M. Holtrigter & A. Thorp
Ground Investigation Ltd, Auckland, NZ
Study to investigate the effect of different diameter friction sleeves in piezocone tests
andto see if the effects can be
corrected for
Piezocone
Cone tip (qc)
Friction sleeve (fs)
Pore pressure filter (u2)
Piezocone
Cone tip (qc)
Friction sleeve (fs)
Pore pressure filter (u2)
Friction Sleeve (fs)
Piezocone
Cone tip (qc)
Friction sleeve (fs)
Pore pressure filter (u2)
Friction Sleeve (fs)
Least accurate of the three sensors
Friction Sleeve Accuracy
Reasons for lack of accuracy in fs (Lunne and Andersen (2007):1. Pore pressure effects on ends of the
sleeve2. Tolerance in dimensions between the
cone and the sleeve3. Surface roughness of the sleeve4. Load cell design and calibration
Friction Sleeve Accuracy
Reasons for lack of accuracy in fs (Lunne and Andersen (2007):1. Pore pressure effects on ends of the
sleeve2. Tolerance in dimensions between the
cone and the sleeve3. Surface roughness of the sleeve4. Load cell design and calibration
Experimental study
• Side-by-side piezocone tests with different diameter sleeves at five selected sites
• The same piezocone penetrometer was used for all tests (Pagani 10 cm2, 50 MPacompression piezocone)
• Same size cone tip for all tests (35.7 mm dia.)
• Slot filter machined to same size as cone tip
Experimental study
ds = 35.6, 35.7, 35.85, 36.05, 36.15 mm
dc = 35.7 mm
Five different size sleeve diameters
Experimental study
ds = 35.6, 35.7, 35.85, 36.05, 36.15 mm
dc = 35.7 mm
Five different size sleeve diameters
ds – dc = -0.1, 0.0, +0.15, +0.35, +0.45 mm
ASTM D5778-12 and ISO 22476-1:
ds – dc = 0.0 to 0.35 mm
Experimental study
Side-by-side testing at five sites:
1. Bower Ave, Christchurch (sand)
2. Keyes Road, Christchurch (sand)
3. Central Park Dr, Auckland (clay)
4. Huapai, Auckland (clay)
5. Herald Island, Auckland (clay)
Results – Bower Ave (sand)
Results – Keys Road (sand)
Results – Central Park Dr (clay)
Results – Huapai (clay)
Results – Herald Island (clay)
Hypothesis
Increased fs due to:
1. End resistance (qt) acting on edge of sleeve protruding from cone tip (fs(qt))
2. Increased friction acting on sides of friction sleeve due to volume expansion
fs = fs(qt) + fs(f)
Assumption: fs is ‘correct’ for ds = dc
Then: fs = fs(0)
Interpretation
Increased fs due to qt acting on edge of sleeve:fs(qt) = pqt/60 * (ds
2 – dc2)
Can then correct for this effect:fs(f) = fs – fs(qt)
Then need to adjust fs(f) to fs(0)
Done empirically by plotting fs(0)/fs(f) vs. (ds2 – dc
2)
Interpretation
fs(0) = fs(f) x [1 – 0.0084(ds2 – dc
2)]
General: fs(0) = fs(f) x [1 – mfs(ds2 – dc
2)]
Interpretation
First order approx.
fs(0) = fs(f) x [1 – 0.0084(ds2 – dc
2)]
fs(0) = {fs – [pqt/60 * (ds2 – dc
2)]} x [1 – 0.0084(ds2 – dc
2)]
General: fs(0) = {fs – [pqt/60 * (ds2 – dc
2)]} x [1 – mfs(ds2 – dc
2)]
Corrected Results – Bower Ave
Corrected Results – Keyes Rd
Corrected Results – Central Park Dr
Corrected Results – Huapai
Corrected Results – Herald Island
Summary
• Sleeve friction increases with increasing sleeve diameter (relative to cone tip dia.)
• ‘Correct’ result assumed to be when sleeve and cone tip are of the same diameter
• Correction for larger diameter sleeves is suggested
• Needs more research
• Tighter tolerances in standards?
Thank You