Strain Gauges in Micropiles - Nicholson Construction Company

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Example Data from a Real Load TestAn example problem is used to illustrate interpretation ofsister bar strain gauges. The ground conditions were silts,clays and loose sands to a depth of approximately 100 feetunderlain by very dense till which served as the bondzone. Five strain gauges were installed (Fig. 4): a set ofparallel strain gauges at the pile head to calculate theaverage pile modulus; one gauge approximately midwaythrough the overburden; one gauge just above the bondzone; and one gauge near the base of the bond zone.Figure 5 shows the measurements of strain versusnormalized time for each gauge. Superimposed on thisfigure is the load applied to the pile head. Strains aremeasured from the left y-axis, while load is measuredfrom the right.The measured strain versus pile head load is shown inFigure 6.StrainGaugesBondZoneFigure 4. Layout ofstrain gauges in examplecase.NORMALIZED TIME15000204060805.0 ft. 50.25 ft. 96.5 ft. 113.5 ft. LOAD600MICROSTRAIN (in/in x 10 -6)10005000-5004002000-200LOAD (kips)-1000-400Figure 5. Measurements of strain at each gauge versus normalized time.
Microstrain (in/in x 10 -6 )500-1000-500050010001500400300200Load (kips)1000-100-200-3005.0 ft. 50.25 ft. 96.5 ft. 113.5 ft.Figure 6. Measured strain versus pile head load.The average Young's modulus of the pile can be estimated using the strain data fromthe gauges located five feet below the pile head assuming that 100% of the jack load isapplied to this section. These data are summarized in Figure 7. The solid squaresrepresent the results obtained from the average of two gauges. Two gauges are used inorder to compensate for possible eccentric loading conditions. The data show that themodulus at lower pile loads is greater than at larger pile loads. For comparison,"theoretical" average moduli are superimposed on the data considering: i) thecomposite pile; and ii) only the steel section. Under compression loading the truemodulus decreases by approximately 50 percent, while in tension the decrease isapproximately 20 percent.
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Page 3 and 4: Comparison to Tell-talesHistoricall
Page 5 and 6: Typical layout of the gauges is sho
Page 7: logger cycles through each of the s
Page 11 and 12: LOAD (kips)-300 -200 -100 0 100 200
Page 13: Tension in Upper Gauges after Unloa

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