Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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| 12.3.2 Internal instrumentation Internal instrumentation for lateral load tests includes the following Vibrating wire strain gauges • Inclinometer casings • In-place inclinometer arrays (down-hole accelerometers) Shape accelerator arrays (SAAs) Strain gauges, typically of the vibrating wire sister bar type, are attached to the longitudinal reinforcement in drilled shafts in pairs, one gauge located on either side of the cage aligned parallel to the direction of applied load (Figure 12-5). One gauge in the pair will measure the compressive strain and the other gauge will measure the tensile strain. Data from strain gauges are plotted as micro strain versus load, and the data are used to compute bending curvature with depth, from which estimates of the bending moments in the drilled shaft can be computed, see Section 12.7. On steel piles (e.g., pipe piles, H-piles), the strain gauges are fixed to the face of the pile for these applications, it is prudent to provide a protective shield to the gauges by welding on steel angles over the gauges to protect them from damage during pile installation. To measure the bending curvature of the drilled shaft or pile accurately, from which p-y curves are subsequently derived, it is necessary to have multiple levels of strain gauge pairs. Typically, vertical spacing of strain gauge pairs is in the order of 5 to 10 feet. Closer spacing is preferred, although wider spacing maybe acceptable in very long elements in relatively uniform strata, provided the intended minimum data coverage is met. The cable from each gauge is routed to the ground surface in an organized manner. Labeling the cables of each pair at the ground surface with colored tape to differentiate the different gauge depth and position is a convenient approach. On reinforcement cages, it is recommended that the cables be attached to the longitudinal steel using plastic cable ties (Figure 12-5). Some protection is provided if the cable is positioned in the corner joint between the longitudinal steel and the spiral reinforcement (e.g., Hayes and Simmonds 2002). 188 Conventional grooved inclinometer casings area cost-effective addition for measuring lateral deformations along the foundation element in a lateral load test. The inclinometer probe is rundown the casing prior to the load application, and baseline readings collected at intervals from the bottom upwards. It is preferable to ensure that the tip of the casing is sufficiently deep so that it will not deflect (i.e., below the deepest point of zero deflection of the foundation element. If this cannot be achieved, it is important to survey the horizontal position of the top of the casing as a baseline to which the subsequent data are then referenced. It is important that during installation of the inclinometer casing that one set of grooves be aligned in the direction of the applied load. It is also preferred to position the inclinometer casing close to the neutral axis, to minimize axial strain induced effects on the inclinometer casing. It is good practice to measure the casing cumulative deformations at the end of each load interval, although this can add significant time to each test. Lowering an inclinometer probe to the bottom of atypical drilled shaft and measuring deformations at conventional intervals in both groove sets of the casing may take 30 minutes to 1 hour depending on the length of the shaft and the number of rounds of readings taken. With 10 load increments in a standard loading schedule per Procedure A in ASTM D, it is apparent that the time required fora well instrumented and monitored lateral load test can increase significantly beyond the time required for application of the load increments alone. If time is limited, then the inclinometer readings should be made at a minimum at the beginning of the test prior to the first load increment being applied, then again when the design load (service limit) has been applied (or when the service limit head deflection criterion has been reached, and finally when the maximum load is applied (or when the strength limit head deflection criterion has been reached. It is recommended that the readings betaken in both the A and B axes of the casing, and a spiral survey of the casing is also recommended. The cumulative deformation profile of the inclinometer casing is a useful check on the shape of the deflection profile that maybe evaluated from the strain gauges. In the event that some strain gauges malfunction during the test, the inclinometer data serves as a useful and cost- effective backup. Moreover, the data are useful for the identification of the depth of maximum moment. The deflection data from inclinometers may not be accurate enough to compute bending moments for derivation of p-y curves but it is certainly beneficial to utilize the data in the overall evaluation of the test, see for example Pincus et al. (1994) and Sinnreich and Ayithi (2014). The depth of maximum moment is important to know for reinforcement design. |