Mechanics of Micropile Performance from Instrumented Load Tests

Micropiles, commonly employed for retrofit of existing foundations and new foundation construction in urban areas, present complex problems in analysis and performance prediction. This paper presents analyses of the three instrumented micropile load tests from two case histories. A total of 34 strain gauges were employed for the three load tests, consisting of spot-weldable gauges on the centralized reinforcing bars and embedment gauges within the neat cement grout surrounding the bars. Two of three test piles were tested to plunging failure and one to impending failure. The composite pile modulus was found to decrease with increasing strain levels within the pile, primarily due to the nonlinear behavior of the grout. The load distribution in the bond zone of the test piles was generally nonuniform, indicating that the mobilized unit bond stress is not constant. Significant pile tip resistance was generated as a function of total pile load and displacement for the two piles that plunged, even at loads less than those causing failure. Dissimilar strain levels measured by strain gauges within the grout and on the reinforcement indicate that perfect strain compatibility may not be present as is commonly assumed.