Abstract:
The micropile design is carried out through semi-empirical equations developed from field trials and previous experiences. The design represents an approximation of the real soil-micropile behavior, but during the design according to the AASHTO LRFD 2014, wide ranges of grout to ground bond ultimate strengths are presented, which depend on the type of micropiles and the type of soil. This range of values to be considered generates uncertainty for the calculating engineer when considering an appropriate micropile grout to ground bond ultimate strengths value for the design; therefore, load tests are performed to validate the design. In Cochabamba city - Bolivia, the construction of 234 postgrate micropiles was carried out with a design load of 490 kN belonging to the foundation of a building. Three tension load tests were executed on different micropiles, all of the same length (11 meters) and diameter (30 centimeters). The soil study made up to 20.45 meters determined that the predominant soil is low plasticity clay interspersed with thin beds of silt and sand and the water table is 1.50 meters from the surface. During the design of these micropiles with the project conditions, the value of the grout to ground bond resistance is between 50 and 145 kPa, the average of these was considered for the initial design. The load tests carried out followed the procedure mentioned in the FHWA NHI-05-039 micropile standard and their results are load vs. displacement curves. 800 kN were applied as maximum load during the load test with displacements between 6 and 8 millimeters. The analysis of these results validated the design fulfilling the acceptance criteria and verified the design load of 490 kN which was used to estimate a grout to ground bond resistance of 118Â kPa for Type D micropiles in clay soils.