DISCUSSION OF LOAD-DEFORMATION MECHANISM FOR BORED PILES

THE AUTHORS HAVE MADE SIGNIFICANT CONTRIBUTION TO THE ANALYSIS OF DEEP FOUNDATIONS BY APPLYING THE NONLINEAR FINITE ELEMENT METHOD TO A CLASS OF PROBLEMS WHICH HERETOFORE HAS BEEN APPROACHED ONLY BY LESS RIGOROUS TECHNIQUES. THE STUDIES OF BORED PILES IN STIFF CLAY PROVIDE A GOOD OPPORTUNITY FOR ASSESSING THE VALIDITY OF THE FINITE ELEMENT FOR ANALYZING DEEP FOUNDATION BEHAVIOR, BECAUSE THE STRESS-STRAIN CHARACTERISTICS OF THE SOIL-CONCRETE SYSTEM AND THE STATES OF STRESS AT THE BEGINNING OF BOTH THE FIELD LOAD TEST AND THE ANALYSIS ARE RELATIVELY WELL KNOWN. SUCH IS NOT GENERALLY TRUE FOR DRIVEN PILES, IN WHICH RELATIVELY LARGE AND INDETERMINATE STRESSES EXIST IN THE PILE AND IN THE SOIL SURROUNDING THE PILE PRIOR TO LOADING. SEVERAL OF THE RESULTS PRESENTED SUGGEST THAT THE FINITE ELEMENT METHOD DOES PROVIDE AN ACCURATE MATHEMATICAL MODEL OF THE MANNER IN WHICH THE SOIL AND PILE INTERACT, WHEN THE SPRING-ELEMENT-TYPE GRID PROPOSED BY THE AUTHORS IS EMPLOYED. FIRST, THE SIDES WERE SHOWN TO PICK UP LOAD AT A MUCH HIGHER RATE THAN THE BASE. THIS PHENOMENON IS WELL KNOWN FOR NUMEROUS LOAD TESTS ON INSTRUMENTED BORED AND DRIVEN FRICTION PILES. SECOND, SHEAR FAILURE WAS SHOWN TO PROPAGATE FROM BOTH THE TOP AND BOTTOM OF THE PILE TOWARD THE CENTER. SUCH BEHAVIOR HAS BEEN OBSERVED BY THE WRITER IN FIELD TESTS ON INSTRUMENTED BORED PILES IN STIFF CLAY. THIRD, THE BASE, SIDE, AND TOTAL LOAD-DEFORMATION RELATIONSHIPS PRODUCED BY THE FINITE ELEMENT METHOD VERY CLOSELY TRACKED THOSE OBTAINED IN THE FIELD STUDY ANALYZED. THE FINITE ELEMENT METHOD, THEREFORE, APPEARS TO BE A USEFUL TOOL IN EVALUATING THE INFLUENCE OF PARAMETERS SUCH AS SOIL STRENGTH, SOIL MODULUS, AND PILE GEOMETRY ON LOAD-SETTLEMENT BEHAVIOR. HOWEVER, FOR TWO REASONS IT CANNOT YET BE USED AS A SELF-SUFFICIENT PROCEDURE TO PREDICT LOAD-SETTLEMENT AND LOAD TRANSFER BEHAVIOR FOR DESIGN PURPOSES. FIRST, THE MAGNITUDE AND VERTICAL VARIATION OF THE ALPHA FACTOR MUST BE KNOWN IN ADVANCE OF Q-TYPE ANALYTICAL SOLUTION FOR PILES IN CLAY. THE SOLUTION FOR LOAD SETTLEMENT IS STRONGLY DEPENDENT ON THE VALUE ASSUMED FOR ALPHA, AND THE FINAL SHAPES OF THE LOAD TRANSFER CURVES DEPEND ALMOST ENTIRELY ON THE ASSUMED VARIATION OF ALPHA WITH DEPTH. SECOND, ANY PLANE STRAIN OR AXISYMMETRIC FINITE ELEMENT SOLUTION IS EXTREMELY SENSITIVE TO POISSON'S RATIO WHEN THE VALUE OF POISSON'S RATIO IS NEAR 0.5. THEREFORE, THE VALUE OF POISSON'S RATIO TO BE USED IN THE SOLUTION MUST BE CHOSEN CAREFULLY TO BE REPRESENTATIVE OF THE SOIL AND CONDITIONS OF THE TEST. THE METHOD OF ANALYSIS PRESENTED BY THE AUTHORS IS USEFUL BECAUSE IT IS GENERAL. IT ALLOWS ALPHA AND POISSON'S RATIO TO BE VARIED, BUT INDEPENDENT DETERMINATIONS OF THE VALUES OF THOSE PARAMETERS MUST BE MADE FOR EACH PROBLEM BEFORE THE METHOD CAN BE USED FOR DIRECT MODELING IN DESIGN-RELATED APPLICATIONS. /DISCUSSER/

  • Supplemental Notes:
    • Vol 97, No SM9, PP 1350-1351
  • Authors:
    • O'Neill, M W
  • Publication Date: 1971-9

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  • Accession Number: 00236287
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Dec 20 1971 12:00AM