Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers

Transcription

1 Evlution of Undrined Sher Strength Using Full-Flow Penetrometers Json T. DeJong, M.ASCE 1 ; Nichols J. Yfrte, M.ASCE 2 ; nd Don J. DeGroot, M.ASCE 3 Abstrct: Full-flow penetrometers the T-br nd bll re incresingly used on sites with thick deposits of soft clys, prticulrly prevlent offshore. Full-flow penetrtion tests were performed t five interntionl well-chrcterized soft cly test sites to ssess the use of full-flow penetrometers to estimte undrined sher strength. Field vne sher dt were used s the reference undrined strength. Sttisticl nlyses of strength fctors indictes tht full-flow penetrometers provide n estimte of undrined sher strength t similr level of relibility compred to the piezocone. Reltionships for estimting the strength fctor nd soil sensitivity using only full-flow penetrometer dt obtined during initil penetrtion nd extrction re developed. A strong dependence of the strength fctor on sensitivity ws identified nd cn be used for the estimtion of undrined strength. The effectiveness nd use of the developed correltions re demonstrted through their ppliction t n dditionl test site. DOI: / ASCE GT CE Dtbse subject hedings: Sher strength; Sensitivity nlysis; Flow. Author keywords: Full-flow penetrometer; T-br; Bll; CPT; In situ testing; Undrined strength; Sensitivity. Introduction Full-flow penetrometers T-br nd bll re incresingly used in offshore site investigtions to obtin ccurte intct nd remolded undrined strength profiles of soft sediments. In concept, penetrtion through soft soil results in viscous fluid-like flow round the penetrometers. The T-br penetrometer Fig. 1 ws initilly developed for profiling soft cly centrifuge smples Stewrt nd Rndolph 1991 nd hs been pplied in situ since the 1990s Stewrt nd Rndolph The concept ws extended to sphericl penetrometer bll to reduce lod cell bending nd llow for deployment in smller dimeter drill csing DeJong et l. 2004; Rndolph Advntges of full-flow penetrometers compred to the more conventionl piezocone penetrtion test CPTu nd field vne tests FVTs hve been described in literture Rndolph 2004; Einv nd Rndolph 2005; Yfrte et l nd include Improved ccurcy in very soft soils since lrger volume of soil is engged during penetrtion s compred to the CPTu ; Corrections for overburden re minimized since the overburden stress is nerly equl bove nd below full-flow penetrometers; 1 Associte Professor, Dept. of Civil nd Environmentl Engineering, Univ. of Cliforni Dvis, One Shields Ave., Dvis, CA corresponding uthor. E-mil: jdejong@ucdvis.edu 2 Senior Stff Engineer, Geosyntec Consultnts, Acton, MA 01720; formerly, Doctorl Student, Univ. of Cliforni Dvis, One Shields Ave., Dvis, CA E-mil: nyfrte@geosyntec.com 3 Professor, Dept. of Civil nd Environmentl Engineering, Univ. of Msschusetts Amherst, Amherst, MA E-mil: degroot@ecs. umss.edu Note. This mnuscript ws submitted on April 20, 2009; pproved on My 18, 2010; published online on December 15, Discussion period open until June 1, 2011; seprte discussions must be submitted for individul ppers. This pper is prt of the Journl of Geotechnicl nd Geoenvironmentl Engineering, Vol. 137, No. 1, Jnury 1, ASCE, ISSN /2011/ /$ Penetrtion resistnce is less ffected by soil rigidity nd stress nisotropy becuse the resistnce mesured is result of soil flow round the probe rther thn of complete soil displcement s is the cse with the cone penetrtion test CPT ; Well-defined filure mechnisms, plne strin flow round the T-br, nd xisymmetric flow round the bll, llow for sound theoreticl nlysis of penetrtion resistnce nd sher strength; nd Remolded strength cn be quickly reltive to FVT nd ccurtely estimted by cycling the penetrometer bout desired depth. This pper presents n evlution of estimting the undrined sher strength using full-flow penetrometers. The nlysis uses high qulity dtbse of in situ test mesurements collected t well-chrcterized soft cly sites. Yfrte et l described procedure for evluting the remolded undrined sher strength, sensitivity, nd penetrtion resistnce degrdtion using cyclic full-flow penetrometer testing. This pper extends these findings nd presents complimentry method for estimting the intct undrined sher s u from full-flow penetrometer dt. A sttisticl nlysis of full-flow nd piezocone dt illustrtes tht fullflow penetrometers provide estimtes of undrined sher strength tht re potentilly more relible thn estimtes by the piezocone. Finlly, the use nd potentil ppliction of the reltionships for estimting undrined strength using full-flow penetrometer dt, prticulrly erly in site investigtion before other reference dt e.g., FVT or lbortory dt my be vilble, is demonstrted for n dditionl soft cly test site. Bckground Full-Flow Penetrometer Design Penetrtion of full-flow penetrometers in soft cly forces soil to flow round the penetrometers in viscous fluidlike mnner. The penetrometer geometry nd surfce roughness influence soil flow 14 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

2 Fig. 1. Conventionl 15-cm 2 CPTu dimeter=43.7 mm with 100 cm 2 bll dimeter=115 mm nd 100 cm 2 T-br length =250 mm, dimeter=40 mm penetrometers configured for 10 cm 2 dimeter=35.7 mm push rods during penetrtion. A bll penetrometer induces xisymmetric soil flow while T-br induces plne strin soil flow. Both mesure soil tht undergoes nonloclized turbulent shering flow wheres the FVT mesures strength on defined sher plne. Full-flow penetrometers re typiclly sndblsted to mintin rough interfce condition tht prevents slippge t the soilpenetrometer interfce. Tretment to crete rough interfce condition is bsed on observtions tht smooth surfce results in 5% lower penetrtion resistnce thn rough surfce Chung nd Rndolph 2004 nd n increse in the interfce friction rtio defined s the rtio of interfce friction to undrined sher strength results in n increse in penetrtion resistnce Einv nd Rndolph 2005; Rndolph nd Andersen The projected re of the conventionl full-flow penetrometer is 100 cm 2. This corresponds to n re rtio defined s the projected penetrometer re to the projected push rod re A R =A p /A s of 10:1 when using conventionl 10 cm 2 CPT push rods. Although smller penetrometer size is more desirble for ese of deployment, the full-flow mechnism will only occur if reltively lrge re rtio exists since the presence of the push rod forces some permnent displcement of soil similr to CPTu. This lters the soil deformtion pttern from those round n infinitely long cylinder T-br or sphere bll. As the re rtio decreses, soil displcement resulting from the presence of the push rod bove the penetrometer hs n incresing effect on the overll flow mechnism nd mesured resistnce. Yfrte et l showed tht penetrtion resistnce decreses with incresing re rtio, nd stbilizes t n re rtio of bout 10:1. Chung nd Rndolph 2004, Yfrte et l. 2007, nd DeJong et l concluded tht probes with n re rtio of 5:1 or lrger yield similr penetrtion resistnce with less thn 10% vribility. Conventionl Full-Flow Penetrometer Test Procedure Procedures for full-flow penetrometer testing re similr to conventionl piezocone testing procedures. Full-flow penetrometer tips cn simply replce the conventionl conicl tip in piezocone system. This prctice is sufficient in very soft deposits, but in stiffer soils custom lod cell my be desirble since the T-br geometry is prticulrly susceptible to inducing lrge bending Fig. 2. Mesured penetrtion resistnce; b cyclic degrdtion curve with the bll penetrometer t the Onsøy test site moments tht cn dmge the lod cell. A penetrtion rte of 20 mm/s is commonly used. Mesurement of tip resistnce is obtined continully during penetrtion nd extrction. Remolded strength cn be evluted by cycling the penetrometer verticlly bout the desired depth Fig. 2, Yfrte nd DeJong 2005 nd cyclic dt re commonly presented s cyclic degrdtion curve Fig. 2 b. Remolded penetrtion resistnce is typiclly defined s the penetrtion resistnce fter 10 cycles Yfrte et l Correction of Penetrtion Resistnce Mesured penetrtion resistnce is the result of compressive nd shering forces cting on the penetrometer s soil is forced to flow round the probe s it is dvnced Rndolph nd Andersen The overburden stress bove nd below the probes is nerly equl, unlike the CPTu where overburden stress cts only on the projected re of the cone tip. To correct for the imblnce in force bove nd below full-flow probes due to the push rod, Rndolph 2004 recommended the following correction to compute net resistnce: JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 15

3 q net = q m v0 u 1 A s A p where q m =mesured penetrtion resistnce; vo =totl verticl overburden stress; u = pore-wter pressure; = lod cell re rtio Lunne et l ; nd A s nd A p =projected res of the push rod typiclly 10 cm 2 nd penetrometer typiclly 100 cm 2, respectively. If the pore pressure in the u 2 position t the trnsition from the full-flow probe to the push rods is not mesured, u 0, the hydrosttic wter pressure, cn be used. The use of the hydrosttic pressure u 0 in lieu of the mesured pressure u 2 results in reltively smll error since the correction is multiplied by the inverse of the re rtio typiclly 0.1. The use of u 0 results in smller correction to q t nd hence the N fctors thn when the u 2 mesurement is used, since positive excess pore pressures re generlly induced in soft sediments during penetrometer penetrtion. An dditionl djustment is sometimes pplied bsed on the cyclic offset Yfrte et l Under idel conditions i.e., perfect flow round the penetrometer in the bsence of push rod, penetrtion nd extrction resistnce re expected to be of equl mgnitude once the soil is remolded. An offset difference in mgnitude of less thn 15 kp in net penetrtion resistnce is common, however. The source of the offset is uncler, but is likely influenced by the ltertion of the flow pttern of soil round the probe during extrction due to the push rod. The men of the offsets from depths t which cyclic testing is performed is sometimes pplied s correction to the entire profile. The mgnitude of offset should be smll less thn 15 kp nd comprble mong ll depths within given soil deposit t which cycling ws performed if the offset is to be pplied to the entire profile. Significnt chnges in the offset between cyclic depths my be the result of zero drift in the lod cell, indicting n equipment problem. The cyclic offset is commonly pplied to the entire cyclic dt set to crete smooth degrdtion curve such s the dt presented s q cyc in Fig. 2 b s this is used to estimte the rte of strin softening in the soil i.e., rte of resistnce degrdtion with continued cycles nd soil sensitivity i.e., rtio of initil penetrtion resistnce nd remolded resistnce Yfrte et l No significnt chnges to the profiles or correltions presented subsequently were observed by djusting the dt set considered herein by the verge offset. Influence of Penetrtion Rte 1 Tble 1. Empiriclly Estimted N Fctors for Previous In Situ Full-Flow Investigtions Site N T-br-DSS, verge N T-br-DSS, rnge N T-br-FVT, verge N Bll-FVT, verge Burswood Austrli 11.9 b 10.9 b Onsøy Norwy 11.9, 12.5 b b Offshore Austrli 12.4 b 11.3 b Offshore West Afric 12.2 b 12.7 b Wtchet Hrbor Cnd 13.0 Colebrook Cnd 10.3 c Richmond Cnd 11.0 c Note: DSS is direct simple sher nd FVT is field vne sher test. Norwegin Geotechnicl Institute tests: Lunne et l b University of Western Austrli tests: Rndolph c Soft Cndin clys: Weemees et l The effect of penetrtion rte on undrined strength cn be evluted through full-flow penetrtion testing t vrible penetrtion rtes. This enbles site-specific evlution of the sensitivity of undrined strength to penetrtion rte. During undrined penetrtion, the resistnce increses s the penetrtion rte is incresed due to viscous effects. As the penetrtion rte is slowed, prtil dringe nd consolidtion cn occur, resulting in n increse in penetrtion resistnce with decresing penetrtion rte. A minimum penetrtion resistnce occurs t the rte t which conditions trnsition from completely undrined to prtilly drined Bemben nd Myers 1974; Chung et l The undrined sher strength increses t bout 10 to 20% per log cycle increse in the strin rte in lbortory specimen testing Kulhwy nd Myne 1990 nd t bout 13%+ / 3% for vne sher testing Peuchen nd Myne 2007 through it cn rnge from 1 to 60% Biscontin nd Pestn Becuse strin rte cnnot be quntified during full-flow penetrtion testing, the following reltionship which utilizes normlized penetrtion rte velocity my be used Yfrte nd DeJong 2007 q = + q ref log /d /d ref 2 where v=penetrometer velocity; d=penetrometer dimeter; =rte prmeter; nd =constnt typiclly equl to unity. The subscript ref refers to penetrtion mesurements obtined t reference penetrtion rte typiclly equl to 20 mm/s. Yfrte nd DeJong 2007 reported the rte prmeter,, to be 0.11 nd 0.12 for the bll nd T-br on verge, but vrying significntly mong test sites Estimtion of Undrined Sher Strength Undrined sher strength cn be estimted s the rtio of net penetrtion resistnce q net to the strength fctor, N, with s u = q tnet = q net = q net 3 N kt N T-br N Bll where N kt, N T-br, nd N Bll =strength fctors for the CPTu, T-br nd bll, respectively. Empiricl reltionships nd theoreticl/ numericl solutions hve been developed to determine pproprite N fctors for estimting undrined sher strength Rndolph 2004; Einv nd Rndolph 2005; Rndolph nd Andersen 2006; Zhou et l. 2008; Zhou nd Rndolph Theoreticl solutions hve provided significnt insight into soil flow round full-flow penetrometers nd give cler guidnce in how to estimte undrined strength bsed on penetrometer mesurements. However, these models cnnot entirely cpture ll fctors which influence the mesured response in situ. For tht reson, site-specific empiricl reltionships clibrted ginst pproprite reference undrined sher strength fctors re generlly necessry to develop confidence in undrined strength fctors predicted from full-flow penetrometer dt. A representtive rnge of empiriclly defined N fctors from previous full-flow studies is presented in Tble 1. Using field vne dt s the reference strength, the rnge in N T-br fctors is The bll penetrometer hs seen limited prior use in situ, but t the two vilble sites listed in Tble 1, N Bll fctors re 10.3 nd 11. All fctors previously published nd presented in Tble 1 re from sites with soil sensitivities between 2 nd 8, where sensitivity is defined s 16 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

4 s t = s u 4 s ur nd s ur =remolded undrined sher strength Mitchell nd Sog More recently numericl nlyzes of full-flow penetrtion by Einv nd Rndolph 2005 nd Zhou nd Rndolph 2008 hve incorported both sensitivity nd strin rte dependency to estimte undrined sher strength. The empiriclly derived N T-br fctors Tble 1 fll within the rnge predicted by Einv nd Rndolph s 2005 upper bound strin pth method of 10.8 in rpidly softening soil to 15.1 in grdully softening soil, while the field N Bll fctors re lower thn the predicted rnge of with the ssumptions of strin rte coefficient of =0.1, sensitivity of s t =5, nd penetrometer surfce roughness =1/s t =0.2; Einv nd Rndolph Testing Progrm The testing progrm ws formulted with the objective of ssessing pproprite test equipment design nd testing procedures for the evlution of undrined nd remolded sher strength of soft cly. The five test sites consist of soft to medium stiff cly nd re well chrcterized. Testing Equipment The T-br nd bll penetrometers used in this study hve projected res equl to 100 cm 2 nd re rtios, A R, equl to 10:1 A R =A p /A s. The 10:1 T-br hs 250 mm length nd 40 mm dimeter nd the bll is 113 mm in dimeter. The surfces of the penetrometers were lightly sndblsted to ensure rough interfce. The lod cell is locted directly behind the penetrometer, in line with the drill string, nd ws designed to compenste for bending nd temperture chnges DeJong et l A commercilly vilble seismic piezocone ws used t ll sites for piezocone testing. Depth mesurements were recorded with wire line potentiometer. Testing Procedure A stndrd penetrtion rte of 20 mm/s ws used for penetrtion, extrction, nd cycling. Penetrtion resistnce ws mesured throughout testing, but pore pressure mesurements were not obtined. Two millimeter mesurement intervls improved resolution reltive to stndrd piezocone testing procedures ASTM D ASTM Prior to testing, the lod cell ws equilibrted t the in situ temperture nd ll electronics operted for minimum of 30 min. Bseline mesurements under zero lod were tken prior to testing nd fter completion of ech sounding. Cycling ws performed t minimum of two depths, one ner the top nd one ner the bottom of the sounding. Cycling depths were chosen bsed on the vilbility of piezocone, FVT, nd high qulity lbortory dt dt shown in Tble 2. A minimum of 10 cycles were performed t ech test depth. The conventionl cycling intervl dopted ws 1.0 m, though intervls down to 2 3 penetrometer dimeters re sufficient Yfrte nd DeJong Testing cross rnge of penetrtion rtes ws performed with either monotonic testing where individul profiles were performed t constnt rte, cyclic rte testing where the rte ws djusted for ech cycle, or twitch testing where penetrtion rte ws reduced t penetrtion distnce intervls equl to 2 penetrometer dimeters DeJong et l A reference test ws lwys performed t 20 mm/s nd penetrtion rtes were vried between 160 nd 0.02 mm/s. The dt cquisition rte ws djusted ppropritely to obtin redings t bout 2 mm depth intervls for ll penetrtion rtes. Test Sites The testing progrm included in situ full flow nd piezocone testing t five soft cly test sites including Amherst, Msschusetts, United Sttes; Burswood, West Austrli, Austrli; Gloucester, Ontrio, Cnd; Louiseville, Québec, Cnd; nd Onsøy, Norwy. Tble 2 provides summry of the site properties nd dt from depths t which cyclic tests were performed. Unless otherwise noted, undrined nd remolded sher strength nd sensitivity were mesured with the FVT. The undrined strength rtios of lbortory trixil to the FVT in Tble 2 generlly gree with trends reported by Chndler Complete full-flow profiles of ech site re presented in Yfrte nd DeJong The Amherst, Mss. United Sttes vrved cly Connecticut Vlley Vrved Cly CVVC test site hs low over consolidtion rtio OCR medium stiff vrved cly lyer with nerly constnt undrined sher strength between 35 nd 40 kp nd sensitivity between 5 nd 25 DeGroot nd Lutenegger The Burswood test site in Perth, West Austrli Austrli hs soft to medium stiff highly plstic clyey silt deposit with incresing undrined sher strength from 20 to 30 kp nd sensitivity between 4 nd 9 Chung The Gloucester, Ontrio Cnd test site hs three lyers of soft to medium stiff cly with strength rnging from 10 to 57 kp nd sensitivity vlues between 20 nd 100 Lo et l The Louiseville, Québec Cnd site hs medium stiff homogenous cly with undrined sher strength incresing from 15 to 60 kp nd sensitivity of pproximtely 22 Leroueil et l The Onsøy, Norwy test site hs thick deposit of highly uniform, high plsticity, low OCR mrine cly incresing form 10 to 25 kp nd sensitivity between 4 nd 8 Lunne et l Evlution of Penetrtion Resistnce Evlution of in situ full-flow penetrtion resistnce requires djustment for overburden stress s well s understnding of the influence of penetrometer geometry. Correction of Mesured Penetrtion Resistnce Adjustment of the mesured penetrtion resistnce to net penetrtion ws pplied to ll dt using the hydrosttic pressure u 0 for the pore pressure vlue in Eq. 1. This correction is importnt s smll chnges in resistnce cn influence the vlues of strength fctors nd other empiricl reltionships since they re often bck clculted ginst reference vlues. Moreover, fter correction to net penetrtion resistnce the extrction rtio, q ext /q in, should be constnt in homogenous soft cly deposits Rndolph Cyclic dt were djusted by n offset vlue to crete smoothed cyclic degrdtion curve. The offset of mesured penetrtion resistnce is greter thn for net penetrtion resistnce from q m to q net in Fig. 2 b. A smooth degrdtion curve results from djustment for the remining offset q net to q cyc. The mgnitude of the offsets in q net in this study ws generlly less thn 15 kp nd consistent within ech profile Tble 2. Though ll regressions presented were developed from dt djusted for the JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 17

5 Tble 2. Summry of Site Conditions nd Dt Mesured from Lbortory nd In Situ Tests Property Test method Units Amherst, Mss. United Sttes Burswood, Austrli Test sites Gloucester, Cnd Louiseville, Cnd Onsøy, Norwy Depth m vo kp u o kp V s m/s G mx MP OCR w L % w P % PI % Strength prmeters s u FVT kp s u CAUC kp s ur FVT kp s ur FC kp S T FVT/FVT S T FVT/FC CPTu q t kp q tnet kp N kt FVT N kt CAUC Full-flow penetrometers q in T-br kp Bll ,b q ext T-br kp Bll q rem T-br kp Bll ,b q ext /q in T-br Bll q Bll /q T-br Initil Remolded Offset in q in T-br kp Bll N su FVT T-br Bll N su CAUC T-br Bll N sur FVT T-br Bll N sur FC T-br Bll Mesured penetrtion resistnce djusted with Eq. 1. b Extrction resistnce, q ext djusted for cyclic offset. cyclic offset for consistency, their ppliction directly to q net dt herein is pplicble given the smll offset mgnitudes observed 15 kp. Influence of Penetrometer Shpe The bll penetrometer consistently mesures lrger penetrtion resistnce thn the T-br Tble 2. Bll penetrtion resistnce ws on verge between 5 nd 16% greter thn T-br penetrtion resistnce during initil penetrtion nd 3 to 24% greter in the remolded condition. The xisymmetric flow round the bll penetrometer results in soil flow converging towrd the push rod, wheres plne strin flow only directs soil ner the center of the T-br towrd the push rod. This, in ddition to the bll hving pproximtely 22% greter surfce re, my ccount for the observed differences in penetrtion resistnce. The CPT typiclly mesures greter penetrtion resistnce 18 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

6 Fig. 3. Rnge of mesured strength fctors for CPTu, bll, nd T-br. The FVT or CAUC in prentheses indictes the reference strength used for evluting N. Fig. 4. Reltionship between net penetrtion resistnce nd undrined sher strength t Onsøy. The slope of the line is equl to best fit N fctor for the dt set. thn the bll nd T-br becuse it is full displcement probe. The net CPTu penetrtion resistnce ws on verge between 27 nd 69% greter thn net bll resistnce nd 41 88% greter thn T-br resistnce. The only exception ws the Burswood site where the net CPTu resistnce is nerly equl to the T-br penetrtion resistnce. Determintion of Undrined Strength Using Site-Specific Strength Dt The improved resolution nd reltive independence from overburden stress of full-flow penetrometer resistnce enbles stright forwrd determintion of undrined strength. Two seprte nlyzes of undrined strength fctors Eq. 3 were performed. The first only used dt from depths where cyclic testing ws performed, which llowed for correltions with remolded resistnce nd strength Tble 2. This set of dt is generlly preferred since the mgnitude of cyclic offset is known. The second nlysis used dt from ll depths where FVT undrined strength ws vilble; enbling the nlysis of lrger dt set of strength fctors. All probbility distributions presented re from the ltter nlysis. The men strength fctors N from cyclic depths for ech profile bsed on FVT nd nisotropiclly consolidted undrined trixil compression test CAUC reference strength mesurements re presented in Fig. 3 nd Tble 2. Bll N fctors rnge from 5.9 to 14.8 with the FVT strength s the reference nd 7.2 to 10.4 with CAUC strength s the reference. T-br N fctors rnge from 5.9 to 14.1 nd 6.5 to 10.0 for the FVT nd CAUC reference strengths. The N fctors for the CPTu re greter thn tht for the T-br nd bll; with the FVT strength s reference nd with the CAUC test strength s reference. A site-specific N fctor cn be estimted from liner-fitted slope of s u versus q net Fig. 4 for ll sites where the FVT mesured strengths which serve s the reference strengths increse linerly with depth. Generlly, the strength fctors obtined bsed on this method were within +/ 0.2 of the men N obtined bsed on dt from only the cyclic depths Fig. 3. The vlidity of fitting liner trend line to the dt sets nd subsequently compring the men nd stndrd devition of strength vlues requires tht no trend in the residuls the difference between the mesured dt nd the strength fctor predicted by the liner trend line exist between the N fctors with respect to depth, mesured undrined sher strength, nd net penetrtion resistnce. As exemplified with the Onsoy dt set Fig. 5 with FVT s reference strength, no dependent trends in the residuls were evident. Furthermore, the distribution of N fctors could be resonbly represented with norml distribution function Fig. 6. Stisfying these requirements, the men N fctors t ech site re smllest for the T-br nd gretest for the CPTu from ll depths where FVT undrined sher strength ws vilble Tble 2. This is expected since the CPTu penetrtion resistnce is greter nd the T-br the lest. An exmple of the vrition of N versus depth with +/ one nd two stndrd devitions indicted for ech penetrometer type t the Onsøy test site is presented on Fig. 7. At ll sites, the stndrd devition of N is smllest for the T-br nd generlly lrgest for the CPTu. The expected distribution of N for ll probes t site is then represented resonbly with norml probbility distributions Fig. 6 b. From Fig. 6 b, slightly lrger distribution of the CPTu strength fctor is evident s compred to the bll nd T-br. A more pproprite comprison of the rnge of undrined sher strength tht would be predicted with ech penetrometer is the normliztion of the N fctors by the men N for ech specific site nd penetrometer Fig. 8. The distributions for ll penetrometers in Fig. 8 re similr nd the probbility density function for ech site is generlly the most concentrted for the T-br. This clerly demonstrtes tht full-flow probes provide comprble predictions of undrined sher strength compred to the CPTu. This is further evidenced in the coefficient of vrition of N fctors, which re within +/ 0.03 for ll probes nd test site except for the Amherst, Mss. test site, indicting no prcticl difference in the rnge of undrined sher strength estimted with ny penetrometer. Anlysis of the N fctor distributions indictes tht full-flow penetrometers provide comprble estimtes of undrined strength to the CPTu. Becuse the estimtion of undrined sher strength with the CPTu requires more ccurte pore pressure nd penetrtion resistnce dt to compute net penetrtion resistnce, the reduction in dependency of N for full-flow penetrometers on these prmeters mkes them potentilly more relible tool for estimting undrined strength. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 19

7 Fig. 5. Strength fctor versus depth, undrined sher strength, nd net penetrtion resistnce from the Onsøy site Fig. 6. Distribution N fctors for the Onsøy test site: norml distribution of N T-br with histogrm; b probbility density function PDF for N T-br, N Bll, nd N kt Fig. 7. Strength fctors versus depth for the Onsøy test site with the first nd second stndrd devitions indicted; b strength fctors versus depth, undrined sher strength, nd net penetrtion resistnce 20 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

8 Fig. 8. PDF for the T-br, bll, nd CPTu t Amherst, Mss.; b Burswood, Austrli; c Gloucester, Cnd; d Louiseville, Cnd; nd e Onsøy, Norwy Estimtion of Undrined Strength nd Sensitivity Using Full-Flow Dt During erly stges of site investigtion high qulity reference strength dt my not be vilble. In these situtions, it is desirble to leverge ll informtion contined within the full-flow dt to estimte the N fctor nd obtin preliminry estimte of strength. Full-flow penetrometer dt, nd the cycling dt in prticulr, inherently contin site-specific informtion regrding soil strength initil penetrtion resistnce, rte of strin softening rtio of initil extrction to penetrtion resistnce, nd soil sensitivity defined conventionlly s the rtio of the undrined strength to the remolded strength s stted in Eq. 4 Mitchell nd Sog 2005 nd therefore relted to the rtio of the initil penetrtion resistnce to remolded penetrtion resistnce vi Eq. 3. These dt re used to develop correltions for N fctors to estimte undrined sher strength on site-specific bsis. Regressions were developed with only dt from depths t which cyclic tests were performed, which is the most relible due to the mesure- Fig. 9. Reltionship between N nd sensitivity for cyclic full-flow dt corrected for cyclic offset; b bll; nd c T-br dt not djusted for cyclic offset; nd d bll; nd e T-br dt djusted for the verge cyclic offset of the profile. Eqs. 5 nd 6 were developed on cyclic dt djusted for cyclic offset. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 21

9 ment of remolded penetrtion resistnce nd the bility to ssess the cyclic offset mgnitude. Sensitivity is the soil property tht primrily influences the N fctors for the dtbse developed nd exmined herein. Strength fctors computed for the T-br in lower sensitivity soils included in this study S T 8 generlly fll within the rnge reported by other studies. N decreses with incresing sensitivity nd generlly stbilizes t bout 5.1 nd 5.3 for the T-br nd bll in highly sensitive soils S T 8 Tble 2. This is shown in Fig. 9 using dt from cyclic depths tht re corrected with the cyclic offset. Considering the experimentl dt nd the theoreticl N fctors for soils with sensitivity of unity, the following equtions were developed: N Bll = 13.2 N T-br = S 3 T S 3 T 10 It is noted tht these reltionships compliment those developed by Yfrte et l for estimting the remolded strength N fctor for the bll nd T-br bsed on sensitivity. The vlidity of these reltionships is demonstrted in Figs. 9 b e t ll depths where FVT strength is vilble. Strength fctors computed from net penetrtion resistnce re presented in Figs. 9 b nd c for the bll nd T-br while strength fctors computed from net penetrtion resistnce djusted the verge cyclic offset re presented in Figs. 9 d nd e for the bll nd T-br. As evident, though greter sctter exists, Eqs. 5 nd 6 fit resonbly well for soil with sensitivities less thn bout 20 Figs. 9 b nd c, but under predicts N fctors t higher sensitivities predominntly for the Gloucester, Cnd test site. The similrity in the extent of dt sctter indictes tht the bove equtions cn be resonbly used with or without correction for the verge cyclic offset provided the mgnitude of the cyclic offset is reltively smll. The use of Eqs. 5 nd 6 require knowledge of the sensitivity bsed on FVT dt. This requires FVT dt from the site, which my not be vilble erly in site investigtion progrm. A direct 1:1 reltionship between sensitivity nd full-flow mesurements does not exist since the soil filure mechnism induced by FVT test which consists of the formtion of defined shering plnes on cylindricl surfce differs from full-flow penetrometers which consists of flow nd destructuring of the soil fbric for ll soil engged in the flow mechnism. However, since both tests provide mesures of the soil strength s the fbric degrdes, the soil sensitivity cn be estimted from the extrction rtio rtio of q in /q ext Yfrte et l S T = q 3.7 in q ext 7 A similr reltionship tht utilizes the remolded resistnce rtio q in /q rem is lso presented in Yfrte et l Both of these equtions enble the estimtion of strength fctor Eqs. 5 nd 6 without direct mesurement of sensitivity. The strength fctor N then increses with incresing q in /q ext. This enbles the bove equtions to be directly recst in terms of the extrction rtio 5 6 N Bll = 13.2 N T-br = q 20 in/q ext q 20 in/q ext 1.8 It is noted tht these reltionships complement those developed by Yfrte et l for estimting the remolded strength N fctor for the bll nd T-br bsed on the extrction rtio. The bove reltionships re shown in Fig. 10 for dt obtined from depths where cyclic testing ws performed nd with the correction of the cyclic offset. Incresed sctter exists when dt from ll depths is pplied, s shown in Figs. 10 b nd d for the bll penetrometer with nd without the correction for the verge cyclic offset vlue nd in Figs. 10 c nd e for the T-br penetrometer. The bility to obtin initil estimtes of N using only penetrtion nd extrction resistnce does not require dditionl site dt or reference strength mesurements. Exmple of Undrined Strength Anlysis Using Full-Flow Penetrometer Dt In situ testing results from thick soft deposit of Sn Frncisco By Mud in Novto, Cliforni locted on the former Hmilton Air Force Bse Bonprte nd Mitchell 1979 is presented s n exmple ppliction of the regressions presented herein. The site hs FVT undrined sher strengths of kp nd sensitivities of between 6 nd 8. Full flow, piezocone, nd FVT testing ws performed t the site. Eqs. 5, 6, 8, nd 9 re used to estimte strength fctor, undrined sher strength nd sensitivity, with dt mesured from bll nd T-br tests nd presented longside vlues FVT vlues for comprison. The sequence of nlysis presented would likely be similr to tht in site investigtion progrm where full-flow penetrometer dt re vilble before relible reference strength dt from FVT or lbortory tests. This sitution would require strength estimtion vi correltions using only full-flow penetrometer dt first, followed by direct N fctor clibrtion ginst site-specific strength dt. A cler correltion between soil sensitivity nd the strength fctor hs been estblished Eqs. 5 nd 6, Fig. 9. Soil sensitivity cn either be known from FVT test dt or cn be estimted using correltions bsed only on full-flow penetrometer dt q in /q ext or q in /q rem. The use of q in /q rem s detiled in Yfrte et l cn only be pplied t depths where cyclic testing hs been performed, but is rgubly more relible since the q in /q rem rtio is directly kin to soil sensitivity. It cn lso be used in conjunction with Eqs. 5 nd 6. The lterntive method to estimte soil sensitivity, bsed on q in /q ext Eq. 7, is dvntgeous since N fctors cn be estimted for the entire full-flow penetrometer sounding. At depths where cyclic testing ws performed, the two methods slightly overestimte sensitivity by between 0.1 nd 2.5, providing resonble vlues for initil design. This is shown for the bll in Fig. 11 with the T-br results being very similr in this nd subsequent plots. The strength fctor required to compute s u Eq. 3 cn be estimted using four methods depending on the informtion known. If sensitivity is known from site-specific FVT dt, Eqs. 5 nd 6 cn be used to estimte the strength fctor directly. If / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

10 Fig. 10. Reltionship between extrction rtio q in /q ext nd strength fctor N for cyclic full-flow dt corrected for cyclic offset; b bll; nd c T-br dt not djusted for cyclic offset; nd d bll; nd e T-br dt djusted for the verge cyclic offset the profile. The regressions in Eqs. 8 nd 9 were developed on cyclic dt djusted for cyclic offset. sensitivity is unknown, Eqs. 5 nd 6 cn be used with sensitivity vlues estimted from the methods previously discussed. Using two correltions to estimte the strength fctor is not desirble, nd it is insted preferble to directly estimte the strength fctor from full-flow dt using Eqs. 8 nd 9. As evident in Tble 3, the estimted strength fctors re similr. The N fctors for the bll re over predicted by bout 0.1 to 1.9, nd the N fctors for the T-br between 2.5 nd 1.1 of directly computed N T-br fctors. The profile of N fctors with depth using these two pproches is presented in Fig. 11 b for the bll. The undrined sher strengths t depths where cyclic testing ws performed were predicted using the derived N fctors nd Eq. 3. The predicted undrined sher strengths generlly gree well with mesured vlues Tble 3. The difference of predicted Fig. 11. Predicted sensitivity; b strength fctor, N; nd c undrined strength for the Novto test site using proposed equtions for the bll penetrometer JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 23

11 Tble 3. Exmple Appliction t the Novto, Cliforni, Test Site Prmeter 5.5 m 8.5 m Reference predicted vrible Direct mesurement s u kp FVT q in kp Bll q ext kp Bll q rem kp Bll q in kp T-br q ext kp T-br q rem kp T-br Direct clcultion S T FVT N Bll FVT N T-br FVT Empiricl prediction S T q in /q rem Bll reltionship from Yfrte et l S T q in /q rem T-br reltionship from Yfrte et l S T Eq. 7 using q in /q ext Bll S T Eq. 7 using q in /q ext T-br N Bll Eq. 5 using S T mesured N Bll Eq. 5 using S T from Yfrte et l N Bll Eq. 5 using S T from Eq. 7 N Bll Eq. 8 using q in /q ext N T-br Eq. 6 using S T mesured N T-br Eq. 6 using S T from Yfrte et l N T-br Eq. 6 using S T from Eq. 7 N T-br Eq. 9 using q in /q ext s u,bll kp Eq. 5 using S T mesured s u,bll kp Eqs. 3 nd 5 using S T from Yfrte et l s u,bll kp Eqs. 3 nd 5 using S T from Eq. 7 s u,bll kp Eqs. 3 nd 8 using q in /q ext s u,t-br kp Eqs. 3 nd 6 using S T mesured s u,t-br kp Eqs. 3 nd 6 using S T from Yfrte et l s u,t-br kp Eqs. 3 nd 6 using S T from Eq. 7 s u,t-br kp Eqs. 3 nd 9 using q in /q ext nd mesured undrined sher strength is 2.9 nd 0.2 kp for the bll nd 2.3 nd 4.1 kp for the T-br. The greement of the entire predicted undrined strength profile using Eq. 3 in combintion with Eqs. 5, 6, 8, nd 9 reltive to the FVT mesured undrined strength is evident in Fig. 11 c for the bll. Implementtion of the methods proposed, both direct bsed on mesured FVT dt nd indirect through correltions of fullflow dt, ll provide resonbly ccurte estimtes of the undrined strength profile mesured t the Novto test site. Methods bsed on site-specific mesured strength dt provide greter ccurcy nd should be used when possible. The correltions for estimting N nd S T re prticulrly useful in the bsence of complimentry site-specific dt, but will likely result in reduced ccurcy. Conclusions Guidelines for the estimtion of the undrined sher strength profile using full-flow penetrometers in soft cly hve been presented. The genertion nd nlysis of full-flow dt requires creful probe design, testing procedures, nd dt correction. Once high qulity dt re obtined, the full-flow dt itself, nd the cycling dt in prticulr, inherently contin informtion regrding soil properties. Full-flow penetrometers were shown to provide estimtes of undrined sher strength with ccurcy nd relibility comprble to nd in some cses better thn the CPTu for given site. This ws estblished through sttisticl nlysis men nd coefficient of vrition of the N fctors obtined for the T-br, bll, nd CPTu when clibrted ginst site-specific FVT dt. Full-flow penetrometers my therefore be preferble in soft cly due to the reduced uncertinty of ssessing net penetrtion resistnce nd incresed lod cell sensitivity s well s the dditionl informtion obtined from cyclic degrdtion testing. Furthermore, correltions using only full-flow dt nmely the q in /q ext hve been developed tht provide n estimte of the undrined sher strength profile when site-specific mesured undrined strength dt re not vilble. In combintion with previously presented regressions Yfrte et l. 2009, these reltionships cn be used to estimte ll necessry strength prmeters for initil geotechnicl design including undrined sher strength, remolded un- 24 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011

12 drined sher strength, nd sensitivity with only full-flow penetrometer dt. Using n dditionl site in Novto, Clif., the regressions for estimting sensitivity, strength fctor, nd undrined sher strength were shown to provide resonble ccurcy for initil design, without the use of dditionl dt. The use of full-flow penetrometers ws restricted to medium stiff nd softer soils in this study nd the conclusions nd regressions presented herein re not expected to pply ppropritely to soils where either higher stiffness or miniml overburden stress result in the formtion of n open cvity behind the penetrometer. As lwys, the estimtion of strength in soft cly requires incresed ttention to detil nd rigorous qulity control. It hs been shown tht full-flow penetrometers cn, in such circumstnces, produce results of t lest comprble qulity to the CPTu nd FVT while gthering dditionl useful informtion regrding soil strength. Acknowledgments Funding from the Ntionl Science Foundtion Grnt Nos. CMS# nd OISE# is pprecited. The collbortion with Tom Lunne nd Knut Anderson of the Norwegin Geotechnicl Institute nd with Prof. Mrk Rndolph t the University of Western Austrli, with respect to field testing nd dt nlysis, is prticulrly pprecited. Any opinions, findings, nd conclusions or recommendtions expressed in this mteril re those of the writers nd do not necessrily reflect the views of the Ntionl Science Foundtion. Finlly, the writers would like to thnk the individuls who provided thorough reviews of the mnuscript. References ASTM Stndrd test method for performing electronic friction cone nd piezocone penetrtion testing of soils. ASTM-D , West Conshohocken, P. Bemben, S. M., nd Myers, H. J The influence of rte of penetrtion on sttic cone resistnce in Connecticut River Vlley vrved cly. Proc., Europen Symp. on Penetrtion Testing (ESOPT), Ntionl Swedish Building Reserch, Stockholm, Sweden, Biscontin, G., nd Pestn, J. M Influence of peripherl velocity on vne strength of n rtificil cly. Geotech. Test. J., 24 4, Bonprte, R., nd Mitchell, J. K The properties of Sn Frncisco By mud t Hmilton Air Force Bse. Rep. Prepred for the Dept. of Civil Engineering, University of Cliforni, Berkeley, Berkeley, Clif. Chndler, R. J The in-situ mesurement of the undrined sher strength of clys using the field vne. Vne sher strength testing of soils: Field nd lbortory studies, A. F. Richrds, ed., ASTM, West Conshohocken, P., Chung, S. F Chrcteristion of soft soils for deep wter developments. Ph.D. thesis, Dept. of Civil nd Resource Engineering, The Univ. of Western Austrli, Nedlnds, Western Austrli. Chung, S. F., nd Rndolph, M. F Penetrtion resistnce in soft cly for different shped penetrometers. ISC-2: 2nd Int. Conf. on Site Chrcteriztion, V. d Fonsec nd P. Myne, eds., Millpress, Rotterdm, The Netherlnds, Chung, S. F., Rndolph, M. F., nd Schneider, J. A Effect of penetrtion rte on penetrometer resistnce in cly. J. Geotech. Geoenviron. Eng., 132 9, DeGroot, D. J., nd Lutenegger, A. J Geology nd engineering properties of Connecticut Vlley vrved cly. Chrcteriztion nd engineering properties of nturl soils, Vol. 1, T. S. Tn, K. K. Phoon, D. W. Hight, nd S. Leroueil, eds., Blkem, Rotterdm, The Netherlnds, DeJong, J. T., Yfrte, N. J., DeGroot, D. J., nd Jkubowski, J Evlution of the undrined sher strength profile in soft lyered cly using full-flow probes. Proc., ISC-2: 2nd Int. Conf. on Site Chrcteriztion, V. d Fonsec nd P. Myne, eds., Millpress, Rotterdm, The Netherlnds, DeJong, J. T., Yfrte, N. J., DeGroot, D. J., Low, H. E., nd Rndolph, M. F Recommended prctice for full-flow penetrometer testing nd nlysis. Geotech. Test. J., 33 2, Einv, I., nd Rndolph, M. F Combining upper bound nd strin pth method for evluting penetrtion resistnce. Int. J. Numer. Methods Eng., 63 14, Kulhwy, F. H., nd Myne, P. W Mnul for estimting soil properties for foundtion design, Electricl Power Reserch Institute, Plo Alto, Clif. Leroueil, S., Hmouche, K., Rvensi, F., Boudli, M., Loct, J., nd Virely, D Geotechnicl chrcteriztion nd properties of sensitive cly from Quebec. Chrcteriztion nd engineering properties of nturl soils, Vol. 1, T. S. Tn, K. K. Phoon, D. W. Hight, nd S. Leroueil, eds., Blkem, Rotterdm, The Netherlnds, Lo, K. Y., Bozozuk, M., nd Lw, K. T Settlement nlysis of the Gloucester test fill. Cn. Geotech. J., 13 4, Lunne, T., Long, M., nd Forsber, C. F Chrcteriztion nd engineering properties of Onsøy cly. Chrcteriztion nd engineering properties of nturl soils, Vol. 1, T. S. Tn, K. K. Phoon, D. W. Hight, nd S. Leroueil, eds., Blkem, Rotterdm, The Netherlnds, Lunne, T., Rndolph, M. F., Chung, S. F., Andersen, K. H., nd Sjursen, M Comprison of cone nd T-br fctors in two onshore nd one offshore cly sediments. Proc., ISFOG 2005, S. Gourvenec nd M. Cssidy, eds., Tylor & Frncis, London, Lunne, T., Robertson, P.K., nd Powell, J. J. M Cone penetrtion testing in geotechnicl prctice, Chpmn & Hll, London. Mitchell, J. K., nd Sog, K Fundmentls of soil behvior, Wiley, Hoboken, N.J. Peuchen, J., nd Myne, P Rte effects in vne sher testing. Proc., 6th Int. Offshore Site Investigtion nd Geotechnics Conf., Society for Underwter Technology, London, Rndolph, M. F Chrcteristion of soft sediments for offshore pplictions. Proc., ISC-2: 2nd Int. Conf. on Site Chrcteriztion, V. d Fonsec nd P. Myne, eds., Millpress, Rotterdm, The Netherlnds, Rndolph, M. F., nd Andersen, K. H Numericl nlysis T-br penetrtion in soft cly. Int. J. Geomech., 6 6, Stewrt, D. P., nd Rndolph, M. F A new site investigtion tool for the centrifuge. Proc. Centrifuge, 91, Stewrt, D. P., nd Rndolph, M. F T-br penetrtion testing in soft cly. J. Geotech. Engrg., , Weemees, L., Howie, J., Woeller, D., Shrp, J., Crgill, E., nd Greig, J Improved techniques for the in-situ determintion of undrined sher strength of soft clys. Se to sky géotechnique, Cndin Geotechnicl Society, Richmond, Cnd, Yfrte, N. J., nd DeJong, J. T Considertions in evluting the remoulded undrined sher strength from full-flow penetrometer cycling. Proc., Int. Symp. on Frontiers in Offshore Geotecnics (ISFOG), S. Gourvenec nd M. Cssidy, eds., Tylor & Frncis, Rotterdm, The Netherlnds, Yfrte, N. J., nd DeJong, J. T Influence of penetrtion rte on mesured resistnce with full-flow penetrometers in soft cly. Proc., Advnces in Mesuring nd Modeling of Soil Behvior, GeoDenver 2007, Geotechnicl Specil Publiction 173, ASCE, Reston, V. Yfrte, N. J., DeJong, J. T., nd DeGroot, D. J The influence of full-flow penetrometer re rtio on undrined strength mesurements. Proc., 6th Int. Conf. on Offshore Site Investigtion nd Geo- JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011 / 25

13 technics, Society for Underwter Technology SUT, London, Yfrte, N. J., DeJong, J. T., DeGroot, D. J., nd Rndolph, M. F Evlution of remolded sher strength nd sensitivity of soft cly using full-flow penetrometers. J. Geotech. Geoenviron. Eng., 135 9, Zhou, H., nd Rndolph, M. F Resistnce of full-flow penetrometers in rte-dependent nd strin-softening cly. Geotechnique, 59 2, Zhou, H., Rndolph, M. F., nd Andersen, K. H Numericl investigtions into cycling of full-flow penetrometers in soft cly. Geotechnique, 59 10, / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / JANUARY 2011