Discussion of “Settlement of Spread Footings on Sand”

Abstract

WAYS TO MODIFY THE TERZAGHI AND PECK METHOD FOR ESTIMATING THE SETTLEMENT OF SPREAD FOOTINGS FOUNDED ON SANDS ARE EXAMINED. THE AUTHORS INCLUDE THE EVALUATION OF THE RELATIVE DENSITY OF SAND FOUNDATIONS BY THE STANDARD SPLIT-BARREL PENETRATION TEST UTILIZING THE EFFECT OF OVERBURDEN PRESSURES. THE WRITERS' PREFER TO REGARD THIS IN TERMS OF INTERPRETATION OF THE RELATIVE DENSITY OF SANDS UNDER VARIOUS OVERBURDEN PRESSURES FROM PENETRATION TEST DATA, RATHER THAN A CORRECTION AS EXPRESSED BY THE AUTHORS AND DISCUSSERS. EXPERIENCE ON BUREAU OF RECLAMATION PROJECTS DURING THE LAST 15 YEARS HAVE PROVIDED CONFIDENCE IN THE APPLICABILITY OF THE PROCEDURE. PENETRATION TEST RESULTS MUST BE LOOKED AT FROM THE STANDPOINT OF AVERAGE VALUES AND RANGE OF VALUES. THE WRITER FEELS THAT A CORRELATION WHICH IS CLOSE TO AVERAGE TRENDS OF PENETRATION RESISTANCE AND RELATIVE DENSITY IS MOST USEFUL. CHANGES OF OVERBURDEN PRESSURE WITH RESPECT TO DEPTH IS A NECESSARY CONSIDERATION. HOLTZ AND GIBBS CONCLUDE THAT A GENERAL CORRELATION SHOULD BE AN AVERAGE REPRESENTATION FOR A BROAD VARIETY OF USES. PECK AND BAZARAA DISCUSS THE PENETRATION TEST METHODS. THEY DISCUSS THE CORRECTION OF N VALUES FOR THE EFFECT OF OVERBURDEN PRESSURES USED BY THE AUTHORS. THEY CRITICIZE THE HOLTZ AND GIBBS CORRECTION AS BEING UNREALISTIC AND INTRODUCING AN INHERRENT INCONSISTENCY INTO THE PROCEDURES. A METHOD OF IMPROVING THE GIBBS AND HOLTZ CORRECTION IS INTRODUCED. THE WRITER SUGGESTS THAT THE N VALUES SHOULD BE CORRECTED FOR THE EFFECT OF OVERBURDEN PRESSURE, PREFERABLY BY BAZARAA'S MODIFICATION OF THE GIBBS AND HOLTZ PROCEDURES. THE CORRECTED N VALUES SHOULD THAN BE INSERTED INTO THE TERZAGHI-PECK CHART TO DETERMINE THE SETTLEMENT BY THE STANDARD SPLIT-BARREL PENETRATION TEST. BOLOGNESI DISCUSSES SETTLEMENTS PREDICTIONS FROM PLATE LOAD TESTS AND THE INFLUENCE OF THE WATER TABLE ON THE SETTLEMENT OF FOUNDA- TIONS. A FIGURE IS PRESENTED WHICH SHOWS ALL THE POSSIBILITIES AND LIMITATIONS OF THE PROPOSAL THAT, ON FIRST LOADING, THE TANGENT MODULUS OF DEFORMATION OF SANDS IS A FUNCTION OF THE STRESS CONDITIONS. THE ANALYSIS PRESENTED SHOWS THAT THE MOST IMPORTANT FACTOR TO BE TAKEN INTO CONSIDERATION WHILE EXTRAPOLATING SETTLEMENTS FROM A STANDARD SIZE PLATE TO THE SETTLEMENT OF A PROTOTYPE FOOTING IS THE PRECONSOLIDATION PRESSURE, OR PRECONSOLIDATION-LIKE EFFECTS. REFERENCES: SETTLEMENT OF SPREAD FOOTINGS ON SAND, DAVID J. D'APPOLONIA, ELIO D'APPOLONIA, AND RICHARD F. BRISSETTE, ASCE PROC. PAPER 5959, MAY 1968.