A MODEL STUDY OF PILE GROUP RELAXATION FOR PILES BEARING IN DISCONTINUOUS ROCK

This thesis investigates the relaxation of pile groups driven into bedrock using model simulation. Model piles were driven into a simulated discontinuous rock mass with an impact hammer. Relaxations were measured both as a loss of load capacity under static loading, and also as additional penetration during redriving. Similitude of the model experiments with the full-scale condition was verified using dimensional analysis and by comparison with actual field data.;The effects of several experimental parameters on the degree of relaxation were examined for pipe piles. Correlations with driving order, relative tip penetration, and vertical heave were developed. The degree of relaxation was found to be inversely proportional to pile spacing and group size. The maximum radius of influence for a five pile group was estimated by extrapolation to be 8 to 10 pile diameters. Group efficiencies were computed taking into account the observed relaxations. Up to 40% reductions in group load capacity were estimated for relaxed pile groups compared with unrelaxed groups.;A limited amount of testing was performed with model H-piles. H-piles were found to be much less susceptible to relaxation than pipe piles, but they exhibited significantly less ultimate load capacity.;The primary mechanisms of relaxation in the rock mass were identified as: (1) fracturing and pulverization; (2) lateral and vertical displacement; (3) reduction in lateral stress; and (4) vibration.;The model piles had nominal diameters of 1 inch and the simulated rock blocks had an average compressive strength of 3200 psi. Test measurements were made electronically and recorded on a microcomputer controlled data acquisition system.;The dynamic bearing capacity of the pipe piles during driving was evaluated using both force-velocity integrations and the wave equation. The observed dynamic capacity ranged between 2.0 and 3.0 times the unconfined compressive strength of the model rock mass.;Recommendations for detecting and remediating relaxation in the field are presented based on the case histories and the results of the experimental study. Suggestions are made for future research in the areas of pile group relaxation and rock bearing capacity.