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UNDRAINED STIlENGTH CHARACTERISTICS OF COHESION LESS SOILS

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INTRODUCTION

In most practical cases where saturated couestontess soils are involved In slablllly problems, Ihe rat e of applicallon of dl:Hur\)ing tor c es Is so slow that any chauges In pore water pressure Induced by Ihe loads an' di ss ipated as u.e loading progresses. Thus the strength characteristics of lnt e reat a r e Ihose developed under drained condtttona In the laboratory . In cas es Where rapld loading occurs, however, even cohesi on less soils may nol have time to drain during the loading process and the undrained strength must be determined. Such cases Involving one -directton loading might Include (t) the effects of btast loading on soli deposits, (2) Ihe effects In the luunedlat e epl ceut ral region of a small local ea rthquak e, for which ground accel erations may ue very high but of very short duration, and (3) Ihe eff ects In v e ry c los e proximity to me causallve fault of a strong eartnquakc.atthoug}, Ihis may depend on t he r elattve position of the earlh srructure involved and Ihe zone of fault displacement. Under more general ea rthquak e condlu ous, conestontess soils Illay he undrained during loading but tne srr e s s or sj r aln a ppl tc a t to ns will ue cyclic In uatur e , C yc li c loading effecls a re quite dlffcl'ent from those In whl eh the S\l'ess o r strain Is Increased progressiv ely unlll fail­ ure occurs. Conventional sf r ength det er mlnattons are only made fur Ihis laller - - -I\O~- l li s e u s STo-;-;oil" n unlll- .\prlli-:t9li8. To eXlendlllt: -;;i~SI~1l II.I-;;-on" IlIunl~ \\'1'111 1:1\ 1'!!'1ue SI must lie lII!!d \\'1111 Ihe Lxceullvc S!!Crcl.ry, ,\SCE. This Ilap"r is pari of lhe copyr illhlcd Journul 01 Ihe Soli ,\Icehanlcs allil 1'0ulld"IIo,>IIS 11I\'IslulI. I'rucccdlllils ul Ihe ,\ Il\e d e,," ~dely 01 Civil 1I11l11l"crs. \'01. !l3. :-': 0 . ~ ~ l ti . :-'; u\'c," loc r , I~I; ' . ~luIIII- . scripl wns sull,"IHc,1 for review for poasuue (lIIlIlIcall')1I UII ~11I1' ~ Ii. 1!lli" ,..c '" I Prof. of Clv. ElIl(rll., IJnlv. of Cullfom ill. IIcl'kl:lcr. Calif. , 2 ,\Ssl. Pr of. of ElIl;rt:,. ('"Iv , of Calilurnla. Los ..\IIIl"I"s . Calli .

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Sin.,(, lhe ('arlr da~sical expr-rjuu-ut s of nl'rnohl~,' ougtnecrs have 1'1'1:01:' nlH'd that H"II~" ~all"~ 1"llrl III ,lilal" wlU:iiSiI'~ . By filling a rubhur LJag with llah1l'alo',1 ~allli and !hell(ll'fol'lnlllj( II, Ir eynokts ueruonsu-nt ed ljualilallvl,l\, · Iha l 1111 Ul\D1IAIi"t::D TESTS

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If It is as sunu-d Ih al IIIl' r elutroustup between crilical confining pres sure ruut void rnt io Is 11IIT,'''S'''' III "11/"1' (;.-cllh'j' Thun o'crll 'I.-If the confininl: pr essurc Is tuc rcust-d 10 0""'11 -; Il,,-ril -I,lh(' st r ess cundit lons In a sample will hI' tI,IISI' illusl r alf'(1 in fig, 5( 10). a~ain wit h a ll the increased I:onlininl{ pres sure, 0 ...... 11 - 0,,,,. hl'ing ca rrh-rl IIy the po 1'1' wate r and the effective conf iniug pressure remaining al till' va lue 0" .. If now t he deviator sl r e s s is increased Ihe sample will a",lin lelld lu dilute (0,,­ 0""ltl".".. \'01. ;. 195;. flfI. 4:l-~5 . "Iluwe. p, II"~B Illlutuncy lIelullon lor ~llIlIe E'lulIllJrhln, uf all .-\~s~lIlhly ul Pn ,'C1cles In Conlacl" I'rocee'lIog~. lIoyal ~oclety. Series ,\ . 1'01. 209. \leI.. I%2, (II', [)1)0-521 .

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STIl ENGTIl CIIAIlACTEIlISTI C:i

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range 01 valu es 01 c rit lca l confining pressure 101' this saud 101' samples varying Irum lons e III ur -us e , Ihl' va riuus cu ve lopes shown In Fig , 10 arc in r easonably c los e ag rccruent . The te st d at a lor samples showlnl: no volum e chalice al fall­ u re is som ewhat uu ermed tate helween Ihe other Iwo curves and might there­ 101'1.' he co ns i dc red 10 prov id e a reas on able value of Ihe angl e of fri cti on IIn(l"r e ons ta nt volum e coun tu ons . From nu s envelope, lhe ang le of fr iclion und e r constaut volume cc nutu ons Is foullu 10 be c los e 10 :11 '. fr om th ese dat a, values of th e undrained str engtn of lh c sanu may " ea,lily hc p redtcr ed . Fur exampl e, Ihe predicted envelope of Iaiture lor 10lal str es s es Ior a s eries of uucousolluateu-uudr atn ed t est s on sampl es of m edium dense s and Initially c onso t l da t ed und er a confining p r es sure of 3 kg per sq CIlI hi shown in fig. II , The position of uus envelope was del e r m ined a s foll ows . The Initial void ratt o of thc m edium dense sand was 1' / " 0 .11. from fig . 6, thc void rau o after consotruau ou under a confining IlI 'l'SSlIl' C of :J kg per sq

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em may he s een 10 be l'r = 0.70 a nd from Fig. 9, the cl'lll cal connn tng pres­ co rresponding 10 this void r allo Is o,crll ; 9.3 kg per sq em. Thus for the constant volume angle o( friction, 0' ; 3'1", the eCCecllve major principal stress at failure In any noncavtt attng test, for which the e((ecllve milia" prin­ c ipal stress at failure will be equal 10 ~ cl'll' may be determin ed Irom Eq . -I to be : oil; 9.3 tall ' 145 + (3'11/2) ; 37.2 kg per s q cm. The cor r cspoudlng valu e of Ihe deviator st ress a t failur e In noncavllatlng tests wlllb c oj,. - 0ij 3'/ .2 - 9 .3 ; 21.9 kll pe l" s q Clll and Ihls condition will occur in all unconsolidated-undrained tests conducted wllh an applied confining pr-essure grea te r Ihan o,c"11 - I ; 6,3 kg per sq em , For tests conducted at confi ning p res s u res less than this value, cavllallon will Occur bef or e failure and the enve lope of failu re for total strcsaes wilithere(ore be parallel to Ihe d ratned strengu: envel ope hUI ulspta ceu about I kg per s q ern 10 ui c lefl. Thus the

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co mp le te ('I\vel" pl' nf Ia ilure In t e rms Ilf tuta l stressus, will be thai :;III ",'n in fl~ , II. Mnhr e i r c Ics 1"1' lolal s la'eSlies at Ia iture lOr a number of uu cou sol id ated­ uudr aiued Ies ts conducted on s a ruplcs pr epared and eousultdated as outl iued ahove are shown In fl/{. 12, for compa rison the pr-edicted envelope, shown in fiR. II, is also plOIll'11 with lhe rest dara . The agr-eement between predi ct ed alld ('xllI'rinlcillal rcsulrs Is exeelleul. Sl ml la r ag r eernent was onta tne u in

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The measured str e ugt hs are slightly higher Ihan Ihe pr edict ed strengths in th is case, Similar data (o r sea 'ies 01 C- U tests on Initially dense samples ol sand are s hown In fig . 14. for these samples th e predict ed and m easur ed st r eugths are In excell enl agreement. Two oth er series '01 C-U leSIS were pertorrn ed on samples of the same

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FU' . 1:l.-COMPAIlI:>OI..: OF OUSt: II \'1:: 0 ANO I'll t:IlICT EIl FAIL Cllt: E /\ \'E I.O I'I:::> 1/\ C O ~ :> O Ll O ,\ TI:: U- LJ N I Jl I A I NE O TESTS

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COMPARISON Of PREDICTED AND ODSERVED RESULTS O F

CONSOl.IDATED-UNDRAINEO TESTS ON SATURATI::D SAND

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lnv csugaunns Similar 10 Ihal desc ribed above were conuuc t ed 10 compa rc' Ihe strengths 01 s a tura ted sampl es of sand In cou sotldat ed-undra iued l eSls wilh tnos e predtct ed Ir om a knowl edge nl Ihe cdll cal co nfini ng pres sures , The pr-edicted aurt measured positions 01 the total stress failure envelopes lor a ser ies 01 C- U rest s Ilel'l orm ed "n Inlli a ll y loose sand a r e shown In Fig. 13,

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By far the highest value of the critical confining pressure uet e rnuued was Ihe value of 110 kg per sq cm for uenae samples of Ottawa s aud . The next highest value was 58 kg per sq cm for dense samples of glass beans, Both uf these materials have well-rounded and very sound particles . As mlghl be expecled In view of Ihe grealer ease of crushing, values of the critical confining pressure 101" dense sands composed ,of subangular or an­ gul a I ' l)a r ll c les are substanllally lower lhan Ihose for sound, rounned pa rucles . "'11I5hol' • .\ . W.. " A t.n rge s hc llr 1I0K lor Testln!:

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:";e l he r lllnTIII::NGTII AND VOIIl Il"TIO AT r"llUIl£ .1' NONCAVITATING CONSOI.ID" n :o- UNIIIlAIN 1::11 TESTS

drained str ength at any void rallo Is apparently Independent 01 whelher the sample Is normally or over-consolldaled. ENVELOPE Of FAILURE IN TERMS OF EFfECTIVE STRESSES

FOR UNDRAINED TE,STS AT CONSTANT VOLUME

To del ermine Ihe undrained sf rength 01 a saturated sand in a noncavllallng t est II Is necessary 10 know Ihe angle of Irlctlon or envelope of lallure under cons ta nt volume co nd ili ons . Several ways 01 delerminlng Ihls Ir oru drained t est data have already been des cr tbed . The rest dala for noncavllallng undrained tests provides means 101' deler­

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