玫瑰海棠花
THE THIRTY-FOURTH TERZAGHI LECTURE
Prented at the American Society of Civil Engineers
1998Annual Convention
Michael W.O’Neill
I NTRODUCTION OF M ICHAEL W.O’N EILL T HIRTY-F OURTH T ERZAGHI
L ECTURER,1998,B OSTON,M ASSACHUSETTS
玻璃裂痕By Ronald E.Smith,P.E.
It is my pleasure to introduce Dr.Michael W.O’Neill as the thirty-fourth Karl Terzaghi Lecturer.Dr.O’Neill’s talk is en-titled‘‘Side Resistance in Piles and Drilled Shafts.’’This lec-ture deals with a subject on which Dr.O’Neill has become recognized as a leading expert.
exo图片Mike O’Neill was born in San Antonio,Texas,in1940and was educated at the University of Texas at Austin.He com-pleted his BSCE in1963and proceeded directly to graduate school,where he was awarded a Master of Science in Civil Engineering(Soil Mechanics)in1964.After receipt of his MSCE,he entered the U.S.Army as a Second Lieutenant and rved as Environmental Engineer at Fort Bliss,Texas.He ro to the rank of Captain and was discharged from military r-vice in1967.At that time he returned to the University of Texas at Austin,where he proceeded to obtain a PhD in Civil Engineering(Soil Mechanics)in1970.
After completing his graduate studies,Dr.O’Neill worked for one year as a Rearch Associate at the Center for Highway Rearch at the University of Texas.In1971he moved to Houston and joined Southwestern Laboratories as the Manager of Geotechnical Services,where he rved until1974.
小学英语单词汇总In1974O’Neill joined the faculty of the University of Houston as Assistant Professor of Civil Engineering.In1978 he became an Associate Professor,and in1984Professor of Civil Engineering.He rved as Chairman of the Department of Civil and Environmental Engineering,John and Rebecca Moores Professor,and Cullen Distinguished Professor.He cur-rently rves as Director of the National Geotechnical Exper-imentation Site at the University of Houston.
Professor O’Neill has devoted most of his rearch career to advancing the state of the art of design and construction of deep foundations,with much of this effort devoted to drilled shaft foundations.His joint work with Professor Lymon Ree, Construction and Design of Drilled Shafts,Second Edition, was developed for the Federal Highway Administration.This volume reprents the state of the art and state of the practice in drilled-shaft foundations.Of tremendous importance to rou-tine u of drilled-shaft foundations is the work for which O’Neill is currently rving as Principal Investigator.This FHWA Pool Fund Study address the important issue of the u of nondestructive geophysical testing techniques for eval-uating the significance of minor defects in constructed drilled-
shaft foundations.
In addition to drilled shafts,O’Neill has conducted signifi-cant rearch on continuousflight auger piles,driven open-ended piles,design of offshore piles,resistance factors for driven piles,and many other related deep-foundation topics. For his rearch efforts he has received the ASCE Huber Rearch Prize,the ASCE State of the Art in Civil Engineering Award,the ADSC Outstanding Service Award,and the ASCE Texas Section Hawley Award,and was cited by Texas DOT for one of the Top Ten Innovative Rearch Projects in 2000.
In addition to his rearch,Dr.O’Neill is often called on to consult on large,important deep-foundation projects.His re-cent projects include the Woodrow Wilson Replacement Bridge across the Potomac River outside Washington,D.C.; the H-3Viaducts in Oahu,Hawaii;the TH-36Bridge over the St.Croix River,in Stillwater,Minnesota;and the Fred Hart-mann Bridge over the Houston Ship Channel,in Texas.
For his many accomplishments and contributions to the state of the art in deep foundations,ASCE and the Geo-Institute have proudly lected Dr.Michael W.O’Neill as the thirty-fourth Karl Terzaghi Lecturer.
Obrvations,Theoretical Modeling, Design Method Development
FIG. 2.Stress and Forces Acting upon Axially Loaded Pile cidated many of the design rules that practitioners follow to-day.
Fig.2can be thought of as the‘‘problem definition’’figure. The surface of side shear failure is at or near the peripheral surface of the pile,and the shear strength of the soil there is controlled by the normal effective stress along that surface at the time of loading the pile,The difficulty comes in
Ј.h
evaluating tho effective stress,which can perhaps be es-timated from the ambient vertical effective stress,the
Ј,v o earth pressure coefficients,K i or K c,and the shear strength parameters of the soil.K i and K c,respectively,are the earth pressure coefficients related to the conditions immediately af-ter installation and to the conditions after all pore pressures have stabilized,or‘‘after consolidation.’’Knowing the pa-rameters,we can proceed to compute the peak unit side shear-ing resistance,f max,as a function of depth,and integrate that resistance over the surface of the pile to ev
aluate the total side resistance R s,assuming the pile is rigid or the soil does not undergo progressive failure.There are some complicating fac-tors,such as progressive failure inflexible piles,and that issue will be considered later.
Becau of space limitations,I have neglected the topic of toe resistance(R b in Fig.2).In some instances R b,which is addresd in numerous texts and design codes,can be of par-amount importance.In the context of a paper on side resis-tance,however,it is appropriate to point out that the direct addition of side and toe resistance to determine the total com-pressive resistance of the pile is an issue for engineering judg-ment.The maximum value of R b is reached at a toe ttlement of perhaps5%of the pile’s diameter at the toe,while the maximum value of side resistance is reached after perhaps5 to10mm(0.2to0.4in.)of local ttlement.If the soil or rock in which the pile is embedded can experience deflection softening,R s can decrea significantly after reaching its peak and before R b is fully developed.In such a ca,addition of R s to R b is clearly not advisable.Perhaps the best way to con-sider the possibility of this occurrence is through the perfor-mance of site-specific loading tests upon piles instrumented to discriminate between side and toe resistance.
DRIVEN PILES IN CLA Y UNDER MONOTONIC LOADING
One issue that was heightened by offshore development is that of making static estimates of the resistance of very long,driven steel-pipe piles in saturated cohesive soil.Static esti-mates can be verified or improved by performing stress wave analys on piles as they are driven,but pile lengths must be ordered and construction operations planned before such mea-surements are made.So,even with the availability of today’s ‘‘high-tech’’tools,the design team must still estimate R s from soil properties and good engineering judgment.
Meyerhof(1976)outlined the␣(total stress)and(effec-tive stress)methods for asssing f max(Fig.2)for driven piles in saturated clay,as described succinctly in(1)–(4):
日记大全200字
f=␣s(1)
max u
where s u=average undrained shear strength along the length of the pile,or
f=Ј(2)
max v o
where,in Meyerhof’s version of themethod,
=(1ϪsinЈ)tanЈfor normally consolidated clay(3)
0.5
=(1.5)(1ϪsinЈ)OCR tanЈfor overconsolidated clay
送水节(4) and=average vertical effective stress in the soil along the Јv o
pile before driving.
In the␣method,the changes in effective stress and soil structure brought about by driving the pile and the effective stress path that the soil along the pile follows during loading are all collapd into one factor,␣,which in both derivation and application is empirical.The␣method has been derided as inadequate becau the undrained shear strength s u is not a unique soil property but rather an artifact of the way the soil is tested,and becau the problem is an effective stress prob-lem.Nonetheless,the␣method is practical becau the soil data are easy to obtain,and it continues to be ud.
Themethod satisfies our instinct to analyze the problem as an effective stress problem.We simply multiply by the
Јv o product of an earth pressure coefficient(K c)and the inclination of the effective resultant against the pile wall,which Meyerhof took to be the tangent ofЈ.The critic would ask‘‘what Ј?’’,but we will understand it to be the value corresponding to the maximum principal stress difference in drained triaxial compression.K c for cohesive soil was taken as(1ϪsinЈ) OCR0.5,which was a curve-fitting expression from experi-mental data for K o.Meyerhof also stated that fromfield load tests he obrved that f max was higher than K o tanЈby a
Јv o
factor of1to2in overconsolidated London clay.So I have inrted the empirical multiplier1.5for overconsolidated clay. While Meyerhof’s version of themethod presumes that K cϷK o(or exceeds K o by afixed factor in OC clays),it should not be surmid that inrtion of a pile produces no changes in total or pore water stress in the soil around a pile. The driving of a displacement pile into saturated clay produces a zone of highly remolded soil around the pile,within which elevated pore water pressures exist with a high hydraulic gra-dient.Tho excess pressures dissipate,but the volume change resulting from the exiting pore water also results in a change (decrea)in total stress against the pile wall,making the an-alytical tracking of effective stress from driving to loading difficult.
Obrvations
Pursuing the obrvation-before-theory theme helps us gain deeper insight.In a Federal Highway Administration study some years ago,we measured earth pressure coefficients in the ground,at rest,and againstfive heavily instrumented,driven, clod-toe,steel pipe piles.The test site consisted of saturated Beaumont clay of varying plasticity that had been overcon-solidated by desiccation.
FIG. 3.Measured Trends for K o ,K i ,and K c for Clod-T oe Pipe Piles at Houston Site (after O’Neill et al.
1981)
四季桂花FIG. 4.Measured and Computed Relations for f max versus Depth at Houston Site (after O’Neill et al.
芭蕾舞简笔画1981)
FIG. 5.Measured Stress Paths during Axial Loading of Dis-placement Piles at Two Overconsolidated Clay Sites
The average measurements of earth pressure coefficients versus depth among the five instrumented piles are shown in Fig.3,extracted from data given by O’Neill et al.(1981).Clearly,‘‘K i ’’is 2to 3times K o near the middle of the piles,which is not influenced by end effects.K i is shown in quotes becau it is not a true value.The pore pressures that were generated by pile driving dissipated so quickly that by the time the instruments were read,the soil was already in a state some-where between K i and K c .K c was slightly larger than K i throughout most of the pile,indicating that the consolidation process resulted in an increa in effective stress against the pile,which would imply time-dependent tup in this rather highly overconsolidated,very stiff clay.
One is tempted to proceed directly from K c to f max ,as shown in Fig.4.For example,it might be assumed that pile instal-lation has produced a completely disturbed soil structure with-out cohesion (residual shear strength condition)and that there are no changes in the pore water pressures against piles in overconsolidated clay during slow,monotonic loading.The assumptions let us invoke the equation in the box in Fig.4,where is the residual angle of internal friction.We are Јr mildly successful in reproducing the measured values of f max by doing this,but not completely.This exercis
e,however,demonstrates the importance of predicting K c ,which is very different from K o .
The stress paths that were measured during compression loading indicated some changes in effective stress against the pile face during loading (O’Neill et al.1981),so our as-
sumption regarding changes in effective stress are not quite valid.Near the ground surface,where the clay was plastic,pore water pressures incread during shear loading,and ef-fective stress became smaller,possibly explaining,in part,the overprediction of f max by the simple equation in Fig.4.Near the bottoms of the piles,where the clay was less plastic and sandy,pore water pressures and effective stress were approximately as assumed—very little change during loading.If we expand our view,a different picture emerges.A con-sortium of oil companies sponsored a major load-testing pro-gram for deep-penetration piles in the ly (Gibbs et al.1993;Lambson et al.1993).A steel pipe pile 0.76m in diameter and driven open-ended,but plugged,to a depth of 59m was tested at an overconsolidated clay site called Til-brook Grange.Our tests on much shallower piles in Houston,in a clay formation known as the Beaumont clay,exhibited normalized stress paths that converge to a straight line at the point of slippage,denoted by the vertical arrows in Fig.5.That is,f max was proportional to the measured radial effective stress at the pile-soil interface all along the pile at the time of slip.No such proportionality existed at the measured slip points for the Tilbrook test,
however,as indicated by the mea-sured stress paths in Fig.5.Why?We do not know.There may have been a considerable difference in the effective angles of wall friction in the upper and lower formations becau of the mineralogical content of the soils,despite the cloness of their OCRs and plasticity indices.If so,this test points to the need to measure the angle of wall friction between the remolded and reconsolidated soil and the pile material in every specific geologic formation in some appropriate way.The point to this story is that the method carries with it considerable uncer-tainty in the form shown in (4)unless Јis evaluated as a residual value in which the effects of the surface texture of the pile material are considered and the term (1.5)(1Ϫsin Ј)OCR 0.5(=K o )is replaced by K c .
Another possibility is that the measurements at one or both sites were faulty.There is no evidence for this conclusion,but effective stress measurements against curved pile surfaces are difficult to make,and any analyst should be cautious when using such data to verify his or her analytical model.
To put the issue in perspective,measured values of are plotted (in Fig.6)versus depth for full-sized steel pipe piles at three normally consolidated and two overconsolidated clay sites.,which in Fig.6is a ‘‘point’’value defined at a specific depth (not an average value over the length of the pile),ap-pears to be 0.16Ϯ0.06,independent of depth,in soil classi-fied as normally consolidated clay
when the measured excess pore pressure (⌬u )has dissipated prior to loading.In overcon-solidated clay,however,does not appear to be a predictable
