ABSTRACT

Foroptimaldesignofrock-socketedshaftsusedtosupportaxialloading,theendbearing resistance should be considered. The existing empirical methods for determining the end bearing capacityqmaxofrock-socketedshaftsuseempiricalrelationsbetweenqmaxandtheunconfined compressive strength of intact rock, σc. Since rock-socketed shafts are supported by the rock mass (bothintactrockblocksanddiscontinuitiesseparatingthem)notjustbytheintactrock,one should consider not only the intact rock properties but also the influence of discontinuities when determiningqmax. Inthispaper,a databaseconsistingof25testshaftswith RQD (rockquality designation) value available is developed. Using the developed database, a new empirical relation betweenqmaxandtheunconfinedcompressivestrengthofrock mass, σcm,isderived. The new empirical relation explicitly considers the effect of discontinuities by using σcm, which is directly relatedto RQD. Finally, an example is presented to show the application ofthe newly derived empirical relation. Theresultsindicatethatthenewempiricalrelationbetweenqmaxandσcmprovides more accurate prediction of qmax than the old empirical relations between qmax and σc.

1 INTRODUCTION

Drilled shafts socketed into rock are nowadays amongst thewidelyusedvarietyofdeepfoundations. Loads appliedtotheshaftsaresupportedbytherocksocket throughthesideshearresistanceandtheendbearing resistance(Horvathetal.1983). although "There are significant advantagesin designingtoinclude a base[or end bearing] resistance component" (Williams and Pells 1981), the end bearing resistance is often ignored in current design practice(Crapps and Schmertmann 2002; Turner 2004). According to Crapps and Schmertmann (2002), the most common reasons cited by designers for neglecting endbearingresistanceindesigninclude settled slurrysuspension,reluctancetoinspectbottom, concern forunderlyingcavities,andunknownor uncertain endbearingresistance. Obviously, neglecting the endbearingresistanceindesignwillresultin excessive rocksocketlengths. Due to the high cost of shaft construction in rock, an over-designof sock length will leadtoagreatwasteofmoney. Crapps and Schmertmann (2002) suggested that accounting for end bearing resistanceindesignandusingappropriate construction andinspectiontechniquestoensurequality base conditionsisa better approachthanneglecting end bearing resistance.

To include the end bearing resistance in design, it is necessary todeterminetheendbearingcapacityfirst. Although some methods are available for predict

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