Strain Gauges in Micropiles - Nicholson Construction Company

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MICROSTRAIN-700-600-500-400-300-200-100010020030040050060070080090010001100120013001000000800000600000PILE CROSS SECTION AE ( kips)4000002000000-200000-400000-600000-8000005 ft THEOR STEEL ONLY THEOR STL & GROUT FIT USEDFigure 7 MicroStrain versus Measured AEThe measured AE approached the theoretical line for steel only suggesting that thegrout had little contribution. On this particular test, this has been attributed to the bargoing through the bearing plate without a nut on the bar below the bearing plate andpossibly a low grout level. Therefore, the load was applied to the casing at the top ofthe pile and had to be transferred through bond and shear to the grout and the bar.The best fit line in Figure 7 was used to represent the change in pile modulusthroughout the tests and was therefore used to interpret the pile loads from strainmeasurements at depths of 50.3 and 95.0 feet. The gauge at a depth of 113.5 feet is inthe bond zone such that the cross-section of the pile is different and the backcalculated modulus is not applicable. Accordingly, a theoretical estimate of themodulus was used at this location. Using these interpretations of pile modulus, thedistribution of load within the pile is shown in Figure 8 for several specific pile headloads.
LOAD (kips)-300 -200 -100 0 100 200 300 400 50002040DEPTH (ft)6080100120140Compression Design 1.5 Design 2.0 DesignCompression Unload Tension Start Tension Design2.0 Tension Tension Unload Top of BondFigure 8. Interpreted load distribution within pile at various applied head loads.At the design compressive load of 200 kips, approximately 125 kips are transmitted tothe bond zone while the remaining 75 kips are transferred to the overburden. At themaximum compressive load of 400 kips, approximately 250 kips are carried in thebond zone. In tension, approximately 75 kips are carried in the bond zone at thedesign load of 100 kips.The test pile was first loaded in compression and then tension (Figure 5). Figure 6 and8 illustrate that the strain gauges did not return to "zero" after unloading the pile incompression. Residual strain is present within the pile because the overburdenprovides shear resistance during the unload process. Accordingly, the tension portionof the tests is starting from a residual compression condition. For future discussionon this topic, see Richards (2005).The strain gauge readings can also be used to approximate the elastic movementversus depth by using the following formulas:δ e = ε ⋅Land considering versus unit length:δ e∫ε⋅ dz= Dzwhere:z = depth of a particular gaugeD = bottom of pile
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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 and 8: logger cycles through each of the s
Page 9: Microstrain (in/in x 10 -6 )500-100
Page 13: Tension in Upper Gauges after Unloa

Strain Gauges in Micropiles - Nicholson Construction Company

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