EC7 fundamental issues and its implications on users.pdf

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Keynote lecture  Eurocode 7 – fundamental issues and some implications for users

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Eurocode 7 – fundamental issues and some implications for users

B. Simpson

ABSTRACT

As Eurocode 7 becomes more widely used, questions raised by designers hae highlighted issues that require !urther debate and clari!ication. "ost o! these hae e#isted, in one !orm or another, !or many years, but the adent o! a new code, proiding a common language, has brought them into sharper !ocus. Some o! these issues will be considered in this paper$ the selection o! characteristic and design alues o! soil parameters, design in situations dominated by water pressures, the releance o! the E%U limit state and the use o! numerical analysis !or U&S design.

'he selection o! parameter alues !or calculations !requently leads to debates among geotechnical designers. Eurocode 7 attempts, in an rather qualitatie way, to point towards a target reliability !or characteristic alues, while proiding a !ramewor( in which the precious e#pertise o! indiidual engineers can be !ully e#ploited. )roblems o! water pressures and the E%U limit state hae a lot in common$ how to ma(e proisions !or sa!ety in situations where !orces largely balance one another and material strength plays a small, but o!ten ital, part. *umerical models are now widely used to study sericeability, but their use in chec(ing ultimate limit states has been questioned+ how are partial sa!ety !actors to be applied, at what point in staged calculations, and can they be used with adanced nonlinear models o! soil

Each o! these issues is discussed and some practical solutions suggested.

Keywords: Codes of practice & standards; Design; Strength and testing of materials; Keywords: Codes of practice & standards; Design; Strength and testing of materials; Groundwater; Numerical modelling.

Groundwater; Numerical modelling.

1 ITR!"#CTI!

T$is paper considers topics t$at are currently under de%ate in relation to t$e application of Eurocode 7 &art 1 'E1((71 )**+, referred to $ere as EC7-. T$e issues raised are fundamental to eotec$nical enineerin, not artefacts of t$e ne/ code, t$ou$ t$ey may $a0e %een %rou$t to a $ead %y attempts to systematise eotec$nical procedures. T$e paper /ill pro0ide a re0ie/ of opinions on t$e issues discussed, and, /$ere possi%le t$e aut$or /ill i0e $is o/n opinion /it$ ustification.

T$e paper refers to pre0ious pu%lications in /$ic$ more detail, and in some cases a more riorous account may %e found2 Sc$uppener et al ')**(-, Simpson and 3ocom%e ')*1*- and Simpson et al ')*11-.

References to specific pararap$s in EC7 /ill %e s$o/n t$us2 4...5.

) SA6ET 6!R8AT !6 EC7

T$e safety format in EC7 uses a limit state approac$. 9imit states are states of a construction %eyond /$ic$ t$e %e$a0iour is considered unaccepta%le. T$e aim of analyses is t$erefore to s$o/ t$at t$ese states /ill not %e e:ceeded.

Simpson, B (2012) Eurocode 7 – fundamental issues and some implications for users. Keynote Lecture, Proc Nordic Geotechnical Meeting 2012. DGF Bulletin 27.

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) EC7 re;uires t$at %ot$ ultimate limit states '#9S- and ser0icea%ility limit states 'S9S- %e considered. 8ost of its te:t refers to #9S, for /$ic$ t$e main approac$ is %ased on use of partial factors. !pinions in Europe differ a%out /$ere and $o/ t$ese s$ould %e applied, and t$is is left to national c$oice< t$e 0alues to %e adopted for partial factors may also %e 0aried nationally. T$ree alternati0e ="esin Approac$es> $a0e %een de0eloped, com%inin partial factors in different /ays< t$e factor 0alues proposed in t$e European document are s$o/n in Ta%le 1, modified as noted %elo/ for "A1 Com%ination 1.

In "esin Approac$ 1 '"A1-, t/o =com%inations> of partial factors are specified, and t$e desin must %e s$o/n to

accommodate %ot$ com%inations.

Essentially, t$ey are used in t$e same /ay as load com%inations, %ut t$e concept is e:tended to include material strent$s and resistances. &artial factors are enerally applied to eit$er loads '%efore com%ination-or round strent$s '%efcom%ination-ore calculation of resistances-, t$ou$ /it$ some e:ceptions. In countries t$at use "A1, t$e factors on round materials and strent$s are enerally set to 1.* in Com%ination 1, as s$o/n in Ta%le 1. 6or desin of piles and anc$ors, factors are

applied to resistances rat$er t$an to material strent$s. T$ere are some situations in /$ic$ factorin loads at source leads to unreasona%le situations, especially in t$e desin of retainin structures. 6or t$ese, EC7 allo/s t$e factors to %e applied to t$e effects of t$e loads, and t$is is used /$ere appropriate in =Com%ination 1> of "A1.

In "A), partial factors are applied to loads and to round resistances. In a 0ariant of "A), "A)?, t$e e;uili%rium calculation

is carried out usin unfactored

'=representati0e>- loads, and t$e factors are applied to deri0ed load effects. It $as %een found t$at "A) and "A)? are unsuita%le for slope sta%ility pro%lems and for use of numerical met$ods, so most countries /$ic$ $a0e adopted "A) use "A@ for slope sta%ility and for numerical met$ods.

In "A@, factors are applied to material strent$ and to loads simultaneously, in contrast to t$e t/ocom%ination approac$ of "A1 in /$ic$ t$ey are applied to t$e t/o separately and t$e results compared. A fe/ countries propose to use "A@ for all types of desins, /it$ factors ;uite different from t$ose s$o/n in Ta%le 1 in most cases. 'able . /actors proposed by 0E* !or the three 1esign Approaches.

DA1

DA1 DA2 DA2 DA3DA3

Comb

Comb 1 1 Comb Comb 2 2 PilesPiles Actions

Actions Permanent unfav 1,35 1,35 1,35

fav Variable unfav 1,5 1,3 1,3 1,5 1,5/1,3* Soil Soil tanϕ' 1,25 1,25 Effective cohesion 1,25 1,25 Undrained strength 1, 1, Unconfined strength 1, 1, !eight densit" Spread Spread #earing 1, footings footings $liding 1,1 Driven Driven #ase 1,3 1,1 piles piles $haft %com&ression 1,3 1,1 (otal/combined %com&ression 1,3 1,1 $haft in tension 1,25 1,) 1,15 1,1

ote2 alues of all ot$er factors are 1.*. 6urt$er resistance factors are pro0ided for ot$er types of piles, anc$ors etc. ? 1. for structural loads< 1.@ for loads deri0ed from t$e round.

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@ @ C3ARACTERISTIC A" "ESI

A9#ES

2. Bac(ground in design practice

8any codes and te:t %ooDs tell t$e enineer $o/ to analyse results of specific, indi0idual tests and deri0e parameters t$at define t$e round, particularly its strent$ and deformation c$aracteristics, so t$at t$ese can %e used in calculations< some of t$is information is repeated in EC7 &art) 'E1((7)-. In practice, t$e eotec$nical process is more complicated t$an t$is, $o/e0er. A0aila%le information is often sparse in ;uantity, 0aria%le in ;uality and relia%ility, some of it from precise measurement at t$e rele0ant location in t$e round on t$e construction site, some inferred from a eneral understandin of t$e eoloy, some taDen from te:t %ooDs, papers or lecture notes, /$ere it may $a0e %een deri0ed %y %acD analysis of anot$er e0ent in a similar, %ut not identical situation some distance a/ay, and so on. T$ese sources may complement one anot$er, %ut t$ey may also %e found to %e inconsistent and contradictory. 6urt$ermore, e0en /$en rele0ant parameters can %e measured directly, per$aps %y an in situ test, t$eir 0alues may %e c$aned %y t$e construction process itself, or %y some future e0ent suc$ as loadin or e:ca0ation.

In structural desin, it is commonly t$e case t$at drafters of codes of practice $a0e more Dno/lede a%out t$e parameters of strent$ and loads rele0ant to a particular desin, and t$eir 0aria%ility, t$an does t$e desiner. 6or e:ample, code drafters may %e more Dno/ledea%le a%out /ind loadin, floor loadin, 0ariations in dimension of cast in situ concrete, or seismic loadin t$an is t$e desiner, and t$e same applies to t$e 0aria%ility of steel and concrete. 3o/e0er, in eotec$nical desin, t$e desiner Dno/s t$e location of t$e site, somet$in of its eoloy and round /ater conditions and t$e results, or paucity of results, of t$e round in0estiation, toet$er /it$ t$eir liDely relia%ility. T$is information 0aries considera%ly from one desin to anot$er and could not possi%ly %e Dno/n %y t$e code drafter. Because of t$is, t$ere are considera%le differences %et/een approac$es

to determinin c$aracteristic properties of materials in structural and eotec$nical desin.

2.3 1e!initions in E* 445

E 1((* contains t$e concept t$at material properties, or resistances, are first entered into calculations as characteristic alues 6 ( , to /$ic$ prescri%ed partial factors

are applied to o%taindesign alues 6 d . T$is

relations$ip is pro0ided in E;uation .@ of E 1((*2

X d =η X k / γ m

'1-/$ere η is a con0ersion factor relatin 0alues measured in tests to actual 0alues in t$e real construction, and γ m is a partial

factor for t$e material. E1((* notes t$at η may %e incorporated into γ m 'i0in γ " - or

into t$e c$aracteristic 0alue.

T$us t$e c$aracteristic 0alues are used in t$e deri0ation of desin 0alues, /$ic$ incorporate all t$e safety elements re;uired %y t$e Eurocodes. To maDe t$is process /it$ prescri%ed partial factors useful, it is necessary t$at c$aracteristic 0alues are defined as clearly as possi%le.

E 1((* 4+.)5 says t$at t$e c$aracteristic 0alue of a material parameter /ill enerally %e a F fractile 0alue 'ie of test results-, unless ot$er/ise stated in t$e ot$er Eurocodes rele0ant to particular materials. 2.2 1e!inition in E07

T$e prime definition of c$aracteristic 0alue in EC7 is2 =T$e c$aracteristic 0alue of a eotec$nical parameter s$all %e selected as a cautious estimate of t$e 0alue affectin t$e occurrence of t$e limit state> 4).+..)')-5. EC7 re;uires t$at data from la%oratory and field tests s$ould %e =complemented %y /ellesta%lis$ed e:perience> 4).+..)'1-5.

T$ese pararap$s maDe it clear t$at t$e c$aracteristic 0alues re;uired %y EC7 are to %e estimated, re;uirin a deree of $uman udement, and t$ey are to %e cautious, not simply =%est estimates>, =most pro%a%le> or statistically mean 0alues. T$ey are to %e cautious estimates of =t$e 0alue affectin t$e occurrence of t$e limit state>, t$at is, t$e

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+ 0alue actually operati0e in t$e round, not simply t$e 0alues measured in tests, and t$ey are to taDe into account /ellesta%lis$ed e:perience as /ell as test results made for t$e particular proect.

Referrin to E;uation '1- a%o0e, taDen from E1((*, t$e re;uirement in EC7 for =t$e 0alue affectin t$e occurrence of t$e limit state> is e;ui0alent to incorporatin t$e con0ersion factor η , in t$is case relatin soil test results to real round %e$a0iour, into t$e c$aracteristic 0alue, as allo/ed %y E 1((*. &ararap$ 4).+.@'+-5 notes t$at assessment of round properties s$ould taDe account of =t$e effect of construction acti0ities on t$e properties of t$e round>.

T$e =e:perience> to %e considered in estimatin t$e c$aracteristic 0alue is noted in 4).+..)'+-52 eoloical and ot$er %acDround information, suc$ as data from pre0ious proects. T$is pararap$ also lists t$e follo/in items as rele0ant to t$e re;uired estimate2

• t$e 0aria%ility of t$e measured property

0alues and ot$er rele0ant information, e.. from e:istin Dno/lede<

• t$e e:tent of t$e field and la%oratory_{in0estiation<}

• t$e type and num%er of samples< • t$e e:tent of t$e Gone of round

o0ernin t$e %e$a0iour of t$e eotec$nical structure at t$e limit state %ein considered<

• t$e a%ility of t$e eotec$nical structure to transfer loads from /eaD to stron Gones in t$e round.

/igure . Building on estuarine beds.

Alt$ou$ t$e c$aracteristic 0alue is defined to %e =cautious>, not a statistical mean 0alue, it is noted t$at t$e round $as t$e a%ility to a0erae out some of t$e

0ariation seen in tests on small specimens. In t$ese cases, t$e c$aracteristic 0alue s$ould %e a cautious estimate of t$e mean 0alue for t$e Gone of round o0ernin t$e %e$a0iour of a eotec$nical structure at a limit state 4).+..)'7-5. T$is is illustrated in 6iure 1, /$ic$ s$o/s a %uildin to %e desined at t$e top of a slope formed in =estuarine %eds>, consistin larely of sands %ut /it$ some /eaDer clay inclusions. Considerin t$e o0erall slope sta%ility, any failure /ould $a0e to pass t$rou$ a maority of t$e sands, and could a0erae out t$e effects of t$e /eaDer clay Gones< so in t$is case t$e rele0ant c$aracteristic parameter, possi%lyϕH, /ould %e a cautious estimate of t$e mean for t$e slip surface. 3o/e0er, in considerin t$e indi0idual pad foundations for t$e %uildin, it could %e possi%le t$at a pad /ould %e located almost e:clusi0ely on clay, so t$e c$aracteristic 0alue for t$e foundation desin /ould %e %ased on t$e strent$ of t$e /eaDer clay. Anot$er possi%ility is t$at t$e desiner c$ooses to $a0e round %eneat$ eac$ pad pro%ed to c$ecD for clay, and t$is is to %e du out if it is found. In t$at case, t$e c$aracteristic 0alue could %e a cautious estimate of t$e strent$ of parameters of t$e stroner sand. It can %e seen, t$erefore, t$at t$e c$aracteristic 0alue depends on t$e failure mode, t$e e:tent of t$e Gone of round affected, and t$e /ay it $as %een in0estiated.

2. Use o! statistics

Alt$ou$ t$e definition of c$aracteristic 0alue in EC7 is not %asically statistical, statistical met$ods could %e useful in its assessment, and $elp to define t$e term =cautious>. &ararap$ ).+..)'11- says =If statistical met$ods are used, t$e c$aracteristic 0alue s$ould %e deri0ed suc$ t$at t$e calculated pro%a%ility of a /orse 0alue o0ernin t$e occurrence of t$e limit state under consideration is not reater t$an F.>. Aain, attention is dra/n to t$e real, o0erall %e$a0iour in t$e round – =o0ernin t$e occurrence of t$e limit state> – so t$e precedin pararap$s a%out mean

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 T$e re;uirement to consider information from all a0aila%le sources also still applies. &ararap$ ).+..)'1*- says t$at if statistical met$ods are employed t$ey s$ould allo/ t$e use of a priori Dno/lede of compara%le round properties, and differentiate %et/een local and reional samplin.

T$is re;uires ;uite ad0anced use of statistics. EC7 certainly does not encourae t$e replacement of /ell esta%lis$ed e:perience and /ellresearc$ed information %y simple statistical analysis of t$e

immediately a0aila%le test results.

e0ert$eless, statistical analysis of test results may pro0ide one source of useful information, to %e considered alonside ot$er a0aila%le sources. arious aut$ors $a0e considered statistical approac$es to a =cautious estimate of t$e mean 0alue for t$e Gone of round o0ernin t$e %e$a0iour of a eotec$nical structure at a limit state>. Sc$neider '1((7- suested t$at a 0alue taDen to %e *. standard de0iations from t$e mean of directly rele0ant test results could %e used. It can %e seen in 6iure ) t$at t$is is ;uite different from a F fractile of t$e test results, %ein muc$ closer to t$e mean of t$e results. In t$e conte:t of ort$ American practice, "a$l%er and Ronold '1((@- and BecDer '1((- for more eneral use proposed t$e use of a =conser0ati0ely assessed mean> 'CA8- as t$e c$aracteristic 0alue, suc$ t$at for a normal distri%ution 7F of t$e measured 0alues /ould %e e:pected to e:ceed t$is 0alue. T$is re;uires an offset of *.( standard de0iations from t$e mean, for a normal distri%ution, as s$o/n in 6iure ). 6oye et al ')**- taDe up t$e same idea proposin to use a CA8 /it$ *F e:ceedance, e;ui0alent to *.+ standard de0iations %elo/ t$e mean of t$e test results for a normal distri%ution. 8ore recently, Tiete et al ')*11- $a0e discussed $o/ c$aracteristic 0alues can %e deri0ed for slope sta%ility pro%lems, taDin account of t$e coefficient of 0ariation of test results and t$eir spatial correlation.

In t$e aut$orJs 0ie/, t$ese statistical approac$es are useful aids, %ut t$ey must ne0er %e allo/ed to replace or o0errule t$e use all information from all rele0ant sources, e0en /$en t$e sources are not easily

com%ined %y statistics. T$e desiner must %e con0inced t$at a =cautious estimate of t$e 0alue affectin t$e occurrence of t$e limit state> is %ein adopted.

/igure 3. 1eriations o! 8characteristic alues9

/igure 2. Undrained shear strengths !rom borehole samples on site.

2.: A &ondon e#ample

6iure @ s$o/s t$e results of a series of undrained s$ear strent$ measurements in 9ondon Clay. T$e measurements /ere made usin unconsolidated undrained tria:ial tests. A statistical mean line $as %een dra/n t$rou$ t$e data and it is clear t$at undrained strent$ increases /it$ dept$. A c$aracteristic line is re;uired, and t$is s$ould depend on $o/ t$e c$aracteristic 0alues /ill %e used  /$at is t$e limit mode %ein considered 6or e:ample, if t$e undrained strent$ is needed for calculation

Bored Bored pile pile

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 of t$e s$aft resistance of a pile, a 0alue suc$ as t$e Lcautious 'a0erae-J 0alue s$o/n on t$e fiure could %e used. 3o/e0er, for a mec$anism t$at mi$t taDe place in a small Gone of soil, suc$ as at t$e %ase of a pile, a more cautious 0alue  t$e Lcautious 'local-J 0alue  s$ould %e adopted.

6rom t$ese %ore$oles, results from standard penetration tests /ere also a0aila%le, as s$o/n in 6iure +. In 9ondon Clay, t$ere is usually a constant factor %et/een standard penetration and undrained s$ear strent$ results< t$e factor is a%out +. to . 3o/e0er, if t$e mean line from t$e S&T results is transferred onto t$e undrained strent$ plot, as in 6iure , it appears t$at t$e normal correlation does not /orD. In fact, t$e measured undrained strent$s are remarDa%ly $i$2 t$ey are consistent /it$ 0ery lo/ /ater contents, /$ic$ /ere measured, %ut t$is mi$t simply mean t$at t$e samples $ad dried out on t$e /ay to t$e la%oratory, t$ou$ t$ere /as no reason to suspect t$is. 6iure  also s$o/s lines representin mean 0alues t$rou$ data from ot$er near%y sites, %ot$ for undrained s$ear strent$ and S&T results. T$e usual close correlation applies to t$ese, and it is clear t$at t$e undrained strent$s for t$e ne/ site are remarDa%ly $i$.

!n t$e %asis of t$ese inconsistent data sets, /$at 0alue s$ould %e used as t$e c$aracteristic undrained strent$ T$e 0alues measured in t$e tria:ial tests s$ould not %e inored, %ut t$e S&T results and t$e data from adacent sites s$ould also affect t$e decision. T$e c$aracteristic 0alue proposed for t$ese data is s$o/n on 6iure . T$is is less t$an t$e initial assessments in 6iure @, /$ic$ /ere %ased on t$e tria:ial results only, and is closer to a lo/er %ound of t$is particular set of tria:ial results.

/igure ;. 0hosen characteristic alues. /igure . S)' results !rom boreholes on site.

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7 Enineers often need to follo/ t$is sort of process /$en tryin to interpret real data. It may %e t$at statistical met$ods could trace a similar loical se;uence. 3o/e0er, t$is /ould re;uire ;uite ad0anced met$ods and any statistical approac$ /$ic$ failed to taDe account of t$e di0erse array of data, typically a0aila%le, /ould %e $armful to t$e desin process.

2.; <hich alue o!ϕ = as characteristic? T$e ;uestion $as %een asDed2 =M$ic$ 0alue of ϕ = is t$e c$aracteristic 0alue> It is sometimes necessary to c$ose from one of t$e follo/in, dependin on circumstances2

• peaD, critical state or residual s$ear strent$

• ultimate strent$ or a =mo%ilised> 0alue

• strent$ of intact material or strent$ on oints

• strent$ at first loadin or after repeated loadin

• stiffness of intact rocD or of t$e ointed

material

• stiffness on first loadin, or on unload reload

In all cases, t$e ans/er of Eurocode 7 is2 =T$e one t$at is rele0ant to t$e pre0ention of t$e limit state under consideration.> EC7 does not differ in t$is respect from normal practice. 6or some particular situations, t$e code is a%le to specify /$ic$ of t$ese 0alues is rele0ant. 6or e:ample, /$ere concrete is to %e cast aainst round, /$ic$ mi$t t$erefore %e distur%ed, t$e critical state 0alue for t$e anle of s$earin resistance is re;uired 4..@'1*-, (..1'-5 .

T$is ans/er to t$e ;uestion is not t$e same as2 =T$e one /$ic$ /ould %ecome rele0ant if t$e limit state /as not pre0ented.> 6or e:ample, in most plastic clays, if a slip occurred, t$e anle of s$earin resistance /ould e0entually fall to t$e residual 0alue. e0ert$eless, it is not necessary to desin for residual strent$ in clays /$ic$ $a0e not pre0iously slipped. Similarly, it may %e unnecessary to desin for critical state 0alues, t$ou$ %rittleness and ductility must %e considered, as noted in 4).+.1'1@-5 and 4).+.@'+-5.

enerally t$e strent$ to %e used in Eurocode 7 is t$e ma:imum a0aila%le to pre0ent collapse, not a 0alue mo%ilised in a

/orDin state.

2.7 1esign alues o!ϕ =

M$ereas t$e selection of c$aracteristic 0alues is common to all t$e "esin Approac$es of EC7, only approac$es "A1 and "A@ re;uire deri0ation of desin 0alues of material properties. T$is section is rele0ant to t$em.

Eurocode 7 allo/s t/o alternati0e means of deri0in desin 0alues of material properties2

a- %y application of a partial factor γ " , as

discussed a%o0e.

%- %y =direct assessment>, in /$ic$ case =t$e 0alues of t$e partial factors recommended ... s$ould %e used as a uide to t$e re;uired le0el of safety>. It is often asDed /$ere t$e use of a critical state anle of friction fits into t$is sc$eme. T$e suestion is made %y some t$at as t$e anle of friction cannot fall any lo/er t$is 0alue could %e used as a directly assessed desin 0alue, re;uirin no furt$er

γ " . T$is /ould mean t$at no marin of

safety /ould %e applied to t$e strent$ of soil %elie0ed to %e in an initially loose state.

It is true t$at part of t$e uncertainty a%out ϕ = is t$e state of t$e soil, /$ic$ ele0ates its 0alue a%o0e t$e critical state 0alue. 3o/e0er, in t$e aut$orJs e:perience, round in0estiation is often inade;uate to i0e t$e desiners complete confidence in t$e actual nature of t$e soil, suc$ as its radin, apart from its state of compaction. !n t$is %asis, t$e desin 0alue of ϕ = for loose soil s$ould %e less t$an its anticipated critical state 0alue.

T$e critical state 0alue is directly rele0ant to t$e strent$s of interfaces %et/een t$e round and concrete cast aainst t$e round 4..@'1*-, (..1'-5. In t$e aut$orJs 0ie/ it is arua%le t$at t$e reduction factor γ ϕ

applied to t$is could %e less t$an t$e 0alue used enerally in t$e %ody of t$e soil, per$aps 1.1 instead of 1.), pro0ided its desin 0alue is still less t$an t$e desin 0alue for t$e %ody of t$e soil.

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 6or soil in a denser state, t$e desin 0alue in t$e %ody of t$e soil /ill normally %e o%tained from t$e c$aracteristic 0alue %y application of a partial factor γ ϕ . !ften, t$is

/ill mean t$at t$e desin ϕ = /ill %e less t$an a cautious estimate of t$e critical state 0alue, ie ϕ = d > ϕ = ( ? γ ϕ  ϕ = crit,( . In t$e aut$orJs

opinion, t$is is a 0ery useful additional safety c$ecD 'ie ϕ = d ϕ = crit,( -, and it /ould %e ood

if it /ere added, at least as an application rule, to EC7.

2. /urther deelopment by S07

CENTC)*NSC7 $as set up E0olution roup 11 on =c$aracterisation>, c$aired %y 9o0isa 8oritG of S/eden. !ne of t$e tasDs of t$is roup is to consider t$e deri0ation of c$aracteristic 0alues.

+ "ESIS "!8IATE" B MATER &RESS#RE

. ntroduction

In sur0eys of 0ie/s on EC7, reater clarity of re;uirements for safety pro0isions in relation to /ater pressures is a fre;uent re;uest. As in t$e case of deri0ation of c$aracteristic 0alues, t$e aut$or %elie0es t$is re;uest reflects a pro%lem t$at predates EC7. CENTC)*NSC7 $as set up E0olution roup ( on /ater pressures, c$aired %y &rofessor or%ert ot of ermany.

Mater pressures raise t/o particular pro%lems2

a- T$ey sometimes constitute lare forces t$at are critical to desin and $a0e /ell defined ma:imum 0alues, /it$ little real uncertainty.

%- Besides constitutin forces, /ater pressures reduce t$e strent$ of frictional soils. T$us t$ey $a0e a dou%le effect in soil mec$anics, and t$is is also true of any partial factors applied to t$em. T$e issues raised %y desins dominated %y /ater pressure /ere considered %y Simpson , ot and 0an Seters ')*11- in a paper

re;uested %y t$e EC7 committee

CENTC)*NSC7. Reference to t$at paper is recommended for full details of t$e /orD, includin mat$ematics. A less mat$ematical

o0er0ie/ /ill %e pro0ided $ere in an attempt to $elp t$e reader understand t$e conclusions and remainin de%ate. Simpson et al discussed fi0e =simple> pro%lems intended to $i$li$t particular issues, toet$er /it$ t$ree more practical desins.

T$is /orD is limited to considerin conditions of $ydrostatic /ater pressures or steady state seepae, in /$ic$ /ater pressures are specified in calculations, independent of t$e loadin and stressstrain %e$a0iour of t$e round. Situations in0ol0in t$e timedependent response of t$e round are not discussed.

.3 Cequirements o! E07

EC7 reconises fi0e types of ultimate limit states2

EO#2 loss of e;uili%rium in /$ic$ t$e strent$s of materials is insinificant STR2 failure of structural elements E!2 failure in t$e round

#&92 failure due to uplift %y /ater pressure

'%uoyancy-3"2 $ydraulic $ea0e

T$e particular pro%lems of 3" /ill %e discussed in +.( %elo/._{STR and E! may occur toet$er and} t$ey are c$ecDed usin load factors of 1.@ and 1. on unfa0oura%le permanent and 0aria%le loads, respecti0ely, /it$ 1.* and *.* on t$e e;ui0alent loads /$en actin in a fa0oura%le manner. T$e load factors used for #&9 are enerally lo/er2 1.1 for unfa0oura%le actions, enerally /ater pressure, and *.( for fa0oura%le, enerally /ei$t of a potentially %uoyant structure. T$e /ay in /$ic$ t$e STRNE! factors are to %e applied /$en /ater pressure is a leadin action is a particular point of de%ate.

EC7 says2 =M$en dealin /it$ round /ater pressures for limit states /it$ se0ere conse;uences 'enerally ultimate limit states-, desin 0alues s$all represent t$e most unfa0oura%le 0alues t$at could occur durin t$e desin lifetime of t$e structure> 4).+..1'-&5. otin t$at =desin 0alues> are 0alues t$at already incorporate safety, re;uirin no furt$er partial factors, t$is pararap$ indicates direct assessment of #9S desin 0alues on t$e %asis of t$eir p$ysical limits in e:treme, %ut credi%le

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( situations. 6or ser0icea%ility limit states, t$e desin 0alues are to %e less se0ere, correspondin to =normal circumstances>.

E:pandin on t$is t/o pararap$s later, EC7 pro0ides an application rule2 ="esin 0alues of round/ater pressures may %e deri0ed eit$er %y applyin partial factors to c$aracteristic /ater pressures or %y applyin a safety marin to t$e c$aracteristic /ater le0el> 4).+..1'-5. It t$erefore appears t$at t$e desiner can c$oose %et/een direct assessment, factorin /ater pressures or adustin /ater le0els to deri0e #9S desin /ater pressures.

An earlier pararap$ on actions is also important2 =Actions in /$ic$ round and free/ater forces predominate s$all %e identified for special consideration /it$ reard to deformations, fissurin, 0aria%le permea%ility and erosion> 4).+.)'(-&5. T$is $as attac$ed a sinificant note, outlinin t$e single source principle2 =#nfa0oura%le 'or de and fa0oura%le 'or permanent actions may in some situations %e considered as comin from a sinle source. If t$ey are considered so, a sinle partial factor may %e applied to t$e sum of t$ese actions or to t$e sum of t$eir effects.>

.2 Cobustness D allow !or secondary

actions and action e!!ects

E0en in cases /$ere t$e manitudes of t$e primary actions are fi:ed /it$ no possi%ility of unfa0oura%le 0ariations, desins s$ould %e sufficiently ro%ust to accommodate unDno/n and unpredicta%le secondary actions. 6urt$ermore, e0en /$ere t$e manitudes of actions are fi:ed, t$e 0alues of resultin action effects /it$in a structure may $a0e some uncertainty< t$at is, t$ere is uncertainty in t$e loadin model.

In t$e cases considered $ere, t$e primary unfa0oura%le actions are deri0ed from /ater pressure, /$ic$ in some cases may $a0e 0ery clear limits. Secondary actions could include, for e:ample, sedimentation around a structure in /ater, e:ca0ation of t$e round a%o0e a structure relyin on t$e /ei$t of round, minor 0e$icle or s$ip impacts, considered too small to include in calculations, or 0andalism of 0arious Dinds.

If t$ese =secondary> actions or action effects are lare, failure could occur %ut t$e fault may %e seen to rest /it$ t$e o/ners or maintainers of t$e structure, or t$e 0andals< alternati0ely, t$e desiner s$ould $a0e foreseen t$em and /as /ron to omit t$em from t$e primary actions for /$ic$ t$e structure /as desined.

3o/e0er, if t$e secondary actions or action effects are small, t$e o/ner /ould reasona%ly e:pect t$e structure to %e sufficiently ro%ust to /it$stand t$em. In t$is conte:t, =lare> and =small> effects $a0e to %e uded in relation to t$e manitude of t$e primary actions.

It follo/s t$at e0en /$ere t$ere is no real possi%ility of unfa0oura%le 0ariation of t$e primary actions, it may %e necessary to include some 0ariation of t$em in desin in order to accommodate t$e possi%le secondary actions t$at are not ot$er/ise included. T$e uncertainty of t$e /ay t$e actions produce effects /it$in a structure also $as to %e accommodated. T$e 0ariations could %e applied eit$er to t$e actions t$emsel0es, in deri0in desin 0alues, or to t$e action effects.

. E#plicitly accommodate the worst water

pressures that could reasonably occur As noted in +.) a%o0e, EC7 re;uires for #9S desin t$at t$e desin /ater pressures desin 0alues s$all represent t$e most unfa0oura%le 0alues t$at could occur durin t$e desin lifetime of t$e structure.

/igure 7. raity wall retaining !ree water. 6iure 7 s$o/s a /all supportin /ater pressure. A drain is pro0ided, /it$ t$e intention t$at t$e dept$ of /ater %e limited to @m. 3o/e0er, if t$e drain s$ould %ecome %locDed and t$e /ater dept$ increases to +m,

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1* t$e %endin moment in t$e /all is increased %y a factor of ).. Clearly, a desin t$at tooD t$e e:pected /ater dept$, @m, and applied a partial factor of 1.@ to t$e /ater pressure or %endin moment /ould %e inade;uate if t$e

+m dept$ occurred.

T$is e:ample illustrates /$y it is essential t$at desins e:plicitly accommodate t$e /orst /ater pressures t$at could reasona%ly occur.

.: 'he single source principle

6iure  s$o/s an anc$or %locD, for /$ic$ t$e total /ei$t< is a permanent sta%ilisin 'fa0oura%le- force and t$e anc$or force / is a 0aria%le desta%ilisin 'unfa0oura%le- force. T$e c$aracteristic total density of t$e %locD is γ c and t$at of t$e /ater γ w. T$e /ater forces

are taDen to %e permanent.

T$e strent$ of t$e round or structure are not at issue, so t$e only ultimate limit state to %e considered for t$e anc$or %locD is uplift, #&9. 6or t$is, EC7 pro0ides t/o factors for permanent actions, a%%re0iated $ere as γ +dst

'enerally P 1- for t$e desta%ilisin force and γ +stb_{'enerally Q 1- for t$e sta%ilisin force<}

t$e factor for t$e 0aria%le desta%ilisin force is γ %+dst 'P 1-.

It is clear t$at t$e c$aracteristic /ei$t of t$e %locD,< ( , /ill %e multiplied %y γ +stb to

deri0e t$e desin 0alue for #&9, and t$e c$aracteristic anc$or force, / ( , /ill %e

multiplied %y γ %+dst . 6our possi%le met$ods of

applyin partial factors to t$e /ater pressures could %e considered, as listed in 6iure  and

illustrated in 6iure (. 8et$ods @ and + %ot$ follo/ t$e sinle source principle, noted in +.) a%o0e, %ut 8et$ods 1 and ) do not.

/igure 4. /actored water pressures on anchor bloc(. FaG 0haracteristic Fand "ethod G, FbG "ethod , FcG "ethod 3, FdG "ethod 2.

In 6iure 1*, t$e allo/a%le c$aracteristic anc$or force, / ( , is plotted aainst t$e

="ensity ratio> γ c ? γ w< / ( is normalised %y

di0idin %y < ( . 6or t$e purpose of t$is

fiure, t$e 0alues of partial factors $a0e %een taDen from t$e #K ational Anne:2

γ +dst  1.1, γ +stb  *.(, γ %+dst  1..

6iure 1*a s$o/s t$at for 8et$od 1 t$e allo/a%le anc$or force depends on t$e /ater dept$ 'normalised %y di0idin %y t$e $ei$t of t$e %locD-. T$is occurs %ecause different factors are applied to t$e desta%ilisin and sta%ilisin /ater forces. T$is is considered to %e p$ysically unreasona%le, e:cept, per$aps, in 0ery rare circumstances for /igure . Submerged anchor bloc(.

*  * *

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11 /$ic$ t$e pressures a%o0e and %elo/ t$e %locD are independent %ecause t$ey are not from a =sinle source>. As t$e /ater %ecomes deeper, t$e allo/a%le anc$or force reduces for t$e same %locD, and ford?H  no force can %e taDen unless t$e density of t$e %locD is more t$an t/ice t$at of /ater.

T$e results for 8et$ods ) to +, s$o/n in 6iure 1*%, are independent of t$e /ater dept$. 6or 8et$od ), t$e allo/a%le / ( tends

to/ards t$e unfactored 0alue for lo/ density ratios. 6iure 1*c is similar, e:cept t$at it is assumed t$at t$e anc$or force is permanent, rat$er t$an 0aria%le 'ie γ +dst $as %een applied

to 6 in place of γ %+dst -. In t$is case, 8et$od )

pro0ides 0ery little safety for lo/ density ratios. A furt$er important o%ection to 8et$od ) is t$at it applies a reduction factor

'γ +stbQ1- to t$e %uoyancy effect of t$e /ater,

/$ic$ is clearly a desta%ilisin effect. 8et$ods @ and + %ot$ follo/ t$e sinle source principle, and so a0oid t$e need to distinuis$ %et/een sta%ilisin and desta%ilisin actions of /ater pressures. 8et$od @ pro0ides apparently reasona%le results, t$ou$ in effect t$e density of /ater is factored, /$ic$ could lead to difficulties in more comple: situations /$ere t$e strent$ of soil is affected %y /ater pressures. T$is difficulty mi$t %e a0oided if all actions of connected /ater are com%ined to find a resultant desta%iliGin uplift force, /$ic$ is t$en factored %y γ ,dst . T$is met$od clearly

s$o/s /$ere safety on /ater pressures is applied, %y considerin t$e %locD /ei$t and /ater uplift separately.

8et$od +, /it$ no factors on t$e /ater forces, also pro0ides reasona%le results,

indicatin t$at for t$is pro%lem it may not %e necessary to apply factors to /ater pressure, eit$er directly or indirectly. T$e resultant of /ater actions, /$ic$ is desta%ilisin, is not increased, so t$e o0erall factor of safety is lo/er t$an o%tained /it$ 8et$od @.

It is concluded t$at met$ods /$ic$ follo/ t$e sinle source principle '8et$ods @ and +- are to %e preferred. 8et$od +, in /$ic$ /ater pressure is not factored, appears to %e ade;uate for t$is e:ample.

.; )artial !actors on the density o! water-Referrin to t$e e:ample s$o/n in 6iure 11, Simpson et al ')*11- discussed /$et$er t$e density of /ater s$ould %e factored in order to pro0ide a safety marin. Alt$ou$ t$ey areed t$at t$at s$ould %e a0oided, t$ey noted t$at some safety formats /ould factor t$e /ater pressures deri0ed from t$e unfactored density, and ot$ers /ould factor t$e forces deri0ed from unfactored pressures.

/igure . raity construction retaining water.

EC7 $as no =middle t$ird rule> for eccentrically loaded spread foundations, t$ou$ it says t$at special care s$ould %e e:ercised if t$e resultant force does not lie /igure 5 Submerged anchor bloc( D allowable anchor !orce in relation to density o! bloc(.

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1) /it$in t$e middle t/ot$irds, as illustrated in 6iure 11. Simpson et al ')*11- suested t$at t$is limit s$ould %e applied for situations dominated %y /ater pressure.

.7 Use o! an o!!set in water

leel-6iure 1) s$o/s a deep %asement e:tendin %elo/ t$e /ater ta%le. o drainae is pro0ided %eneat$ t$e %ase sla%, so $ydrostatic /ater pressures are e:pected. Some unplanned 0ariation in t$e /ater le0el is possi%le, for e:ample due to leaDae from a /ater main. T$e total /ei$t of t$e structure, /$ic$ could include superstructure %uilt on t$e %asement, is < and its area in plan is A. If needed, tension piles are to %e pro0ided to pre0ent uplift.

/igure 3. 1eep basement subIect to upli!t. T$e uplift force %eneat$ t$e %asement is i0en %yU  γ w Ah, /$ere γ w is t$e /ei$t

density of /ater.

If t$e c$aracteristic uplift force U approac$es or e:ceeds t$e c$aracteristic /ei$t< , t$e tension force' in t$e piles $as to %e deri0ed. 6or #9S #&9 /e find2

U k · γ G,dst = W k · γ G,stb +T d

')-/$ere γ G,dst and γ G,stb are partial factors on t$e

permanent distur%in and sta%ilisin actions. T$is means t$at

T d = U k · γ G,dst– W k · γ G,stb

'@-It is also possi%le to consider t$e pro%lem as #9S STRNE!. T$en /e et2

T d = U k · γ G – W k · γ G,inf

'+-M$ere γ G and γ G,inf are partial factors on t$e

unfa0oura%le and fa0oura%le actions. 6or all cases2

T d ≤ R d = R k / γ P,t

'-/$ere γ P,t is a resistance factor on pile

capacity in tension.

In situations /$ereU reatly e:ceeds< , t$e precise se;uence of calculation in /$ic$ t$e factors are applied and t$e 0alue of t$e partial factors may 0ary accordin to

national practice, %ut t$e outcome is muc$ t$e same. T$e case of< reatly e:ceedin U , /$ic$ /ould re;uire compression piles if t$e sla% is suspended, is not considered $ere. T$e pro%lem is more de%ata%le /$en t$e c$aracteristic 'unfactored- 0alues of < and U are close, especially in formats t$at use γ +in!  1.*, /$ic$ is common. If< ( U ( and

γ P1 is applied to /ater pressure, tension

piles are needed, %ut if /ater pressure is not factored or adusted in some ot$er /ay no piles are needed, e0en if a factor is applied to t$e resultant 'U ( −< ( -, /$ic$ in t$is case

e;uals Gero.

To illustrate t$is pro%lem, suppose n piles are to %e pro0ided eac$ /it$ a c$aracteristic resistance in tension C( . 6or t$e purpose of

plottin results of calculations, it is con0enient to define < wγ w A1< t$is is not

t$e %uoyancy force, /$ic$ is U ( γ w Ah.

M$enU ( < ( ,h?1 < ( ?< w. In 6iure 1@ t$e

num%er of piles re;uired, n, represented %y nC( ?< w, is plotted aainsth?1 for a typical

case in /$ic$ < ( ?< w*.). T$e 0alues of

partial factors used $ere are adopted for illustration only, and may not represent any particular national practice. Some countries prefer to 0ie/ tension piles as pro0idin a fa0oura%le action, /$ic$ /ould also lead to adoption of different factors. In 6iure 1@'a-, t$e critical area of t$e rap$ is s$o/n as an enlared detail as 6iure 1@'%-.

In t$e unfactored case, piles only %ecome necessary /$enh?1 P< ( ?< w  *.) in t$is

e:ample. If factors are applied to t$e unfactored resultant force in t$e piles, toet$er /it$ pile resistance factors, a line suc$ as line '%- is o%tained, for /$ic$ γ C1.7

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1@ T$e radient of t$is line depends on t$e 0alues of t$e factors, %ut /$enh?1 < ( ?< w

 *.) no tension piles /ill %e pro0ided and t$ere is no reser0e of safety for de0iation from t$e c$aracteristic 0alues of /ater pressure and /ei$t. T$is is considered to represent an unaccepta%le situation. 3o/e0er, for a $i$ /ater ta%le 'h?1 approac$in 1-, t$is case is taDen as a reasona%le uide to t$e num%er of piles needed.

If t$e /ater pressure %eneat$ t$e %ase is multiplied %y a partial factor γ 1.@, a line

suc$ as line 'c- in 6iure 1@ is o%tained< in plottin t$is line a lo/er 0alue of pile resistance factor γ C1.@ $as %een adopted, in

acDno/ledement of t$e increased 0alue of γ . In t$is case, a reser0e of safety is

pro0ided /$enh?1 < ( ?< w, re;uirin some

tension piles. 3o/e0er, t$e num%er of piles mi$t %e rearded as e:cessi0e for t$e case of a $i$ /ater ta%le, h?1 approac$in 1, /$ere t$e /ater pressure %eneat$ t$e %ase %ecomes p$ysically unreasona%le.

An alternati0e approac$ could %e to a0oid factorin /ater pressure %ut to re;uire an increase in t$e /ater $ead $. 6or e:ample, line 'd- in 6iure 1@ s$o/s t$e results /$en t$e free $ei$t a%o0e t$e /ater ta%leF1− hG is reduced %y 1*F. T$is $as an ad0antae in t$e case /$ere $ is lare 'eh?11- t$at it does not en$ance t$e /ater pressures unreasona%ly, re;uirin too many piles. T$e amount %y /$ic$ t$e /ater $ead s$ould %e raised is difficult to specify for eneral

application in a code of practice, $o/e0er. If t$is approac$ is preferred, it may %e necessary to rely more $ea0ily on t$e e:pertise of t$e desiner to decide /$at marin is appropriate. T$is is consistent /it$ t$e approac$ of EC7 4).+..1'-&5 usin direct assessment of desin 0alues2 =M$en dealin /it$ round/ater pressures for limit states /it$ se0ere conse;uences 'enerally ultimate limit states-, desin 0alues s$all represent t$e most unfa0oura%le 0alues t$at could occur durin t$e desin lifetime of t$e structure.>

In relation to EC7, t$e discussion a%o0e relates to t$e =STRNE!> re;uirements normally used for findin t$e num%er and re;uired resistances of piles. EC7 $as anot$er re;uirement for uplift cases, #&9, /$ic$ is normally understood to re;uire a factor γ +dst P 1 applied to upliftin /ater

pressure and a factor γ +stb Q 1 applied to

sta%ilisin total /ei$t. 9ine 'e- in 6iure 1@ is plotted for typical 0alues γ +dst  1.1,

γ +stb  *.(, /it$ t$e resistance factor for t$e

piles γ C1.7. T$is re;uirement can produce

sensi%le results pro0ided t$at 'a- it is areed t$at piles are to %e desined usin loadin deri0ed from #&9 and '%- an appropriate system and 0alues of factors is adopted in applyin t$ese loads to pile desin. As /it$ ot$er sc$emes in0ol0in factors on /ater pressure, it %ecomes unreasona%le /$en t$e /ater ta%le approac$es round le0el 'h?1 1-and may dem1-and more piles t$an are really needed.

/igure 2. *umber o! piles required FnormalisedG. FaG un!actored, FbG pile resistance !actored, FcGγ  >

.2: on water pressure, FdG water table adIusted, FeG U)&, F!Gγ +!a > 5. on weight.

0 1 2 0 0.2 0.4 0.6 0.8 1 n n . . R R k k / / W W w w h/D h/D (a) (b) (c) (d) (e) (f) 0 0.2 0.4 0.1 0.2 0.3 0.4 n n . . R R k k / / W W w w h/D h/D (a) (b) (c) (d) (e) (f) h/D (a) (b) (c) _(d)(e) (f) h/D 0.2 0.3 0.4 0 0.1 0.2 0.4 0 1 2 n · R k / W w n · R k / W w 0 0.2 0.4 0.6 0.8 1 (a) (b)

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1+ In t$is pro%lem, it is necessary to c$ane t$e /ater pressure or t$e %uildin /ei$t from t$eir c$aracteristic 0alues in order to increase safety /$en U ( is close to < ( . A

possi%le alternati0e, not considered %y Eurocode 7 %ut recommended for furt$er consideration, /ould %e to apply a reduction factor to t$e /ei$t of t$e %uildin, say *., /$ile lea0in t$e /ater pressure unfactored. T$is is s$o/n as line 'f- in 6iure 1@, plotted /it$ γ C1.7. T$is pro0ides safety /$enh?1

 < ( ?< w, %ut it a0oids factorin /ater

pressure and $as a smaller effect t$an some of t$e alternati0es, suc$ as #&9, /$en h?11.

!f all t$e approac$es presented $ere, t$e aut$or recommends adoption of eit$er 'd- an increase in t$e /ater $ead $ or 'f- a reduction factor applied to t$e /ei$t of t$e %uildin of a%out *..

. 'he 8star9 approach D 1A3J  1AJ

-In any sc$eme of load factorin, it can %e arued t$at e;uili%rium $as to %e %roDen at some point in t$e c$ain from density to desin action effect. T$e present aut$orJs 0ie/ is t$at eotec$nical calculations can %est %e carried out %y preser0in e;uili%rium /it$ unfactored /aster pressures up to t$e point of deri0in structural action effects. T$is is consistent /it$ t$e note in EC7 4).+.)'(-5, discussed in @.) a%o0e, =a sinle partial factor may %e applied to t$e sum of t$ese actions or to t$e sum of t$eir effects>.

T$is sc$eme could %e applied to EC7Js "esin Approac$ 1, /$ere it is only rele0ant to Com%ination 1. It could %e termed approac$ "A1?, %y analoy /it$ "A)? in /$ic$ load factors are al/ays applied at a late stae in t$e calculation. An important difference is t$at "A1?, as intended $ere, is only used for pro%lems dominated %y /ater pressures, in /$ic$ t$e uncertainty of t$e action effects of t$e /ater, toet$er /it$ ot$er =secondary> actions, are pro%a%ly more important t$an any ad0erse 0ariation of t$e /ater pressures t$emsel0es t$at $a0e not already %een incorporated.

.4 H1 D E07 Equation 3.4

"iscussion of EC7 E;uation ).( /as not considered %y Simpson et al ')*11-, %ut it is

included $ere %ecause it $as %een a furt$er point of contro0ersy. T$is e;uation relates to states of $ydraulic $ea0e in t$e round caused %y $ydraulic radient, as illustrated in 6iure 1+.

/igure . LH1M in E07

EC7 $as an e;uation for c$ecDin $ydraulic $ea0e /$ic$ appears in total stress and effecti0e stress forms – E;uations ).(a and ).(%. It also i0es t$e 0alue of partial safety factors, %ut it is unclear a%out $o/ t$ese factors s$ould %e applied. T$is $as led to muc$ confusion, /it$ de%ate a%out /$et$er ).(a or ).(% is correct.

EC7 4).+.7.'1-&5 states2 =M$en considerin a limit state of failure due to $ea0e %y seepae of /ater in t$e round '3", see 1*.@-, it s$all %e 0erified, for e0ery rele0ant soil column, t$at t$e desin 0alue of t$e desta%ilisin total pore /ater pressure 'udst+d - at t$e %ottom of t$e column,

or t$e desin 0alue of t$e seepae force

'S dst+d - in t$e column is less t$an or e;ual to

t$e sta%ilisin total 0ertical stress 'σ stb+d - at

t$e %ottom of t$e column, or t$e su%mered /ei$t ' stb+d - of t$e same column2

udst+d ≤ σ stb+d

').(a-S dst+d ≤ N stb+d ').(%->

Anne: A of EC7 pro0ides 0alues for partial factors to %e used for 3", γ +dst 

1.@ and γ +stb  *.(. But t$e code does not

state /$at ;uantities are to %e factored. !rr ')**- reported calculations for t$e situation of potential $ydraulic $ea0e s$o/n in 6iure 1. 3e found t$at t$e calculated allo/a%le

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1 $ei$t of /ater 3 could 0ary from ).7m to .+m due to application of t$e same factors, taDen from EC7, at different points in t$e calculation.

TaDen at face 0alue, E;uations ).(a and ).(% are alternati0e representations of t$e same p$ysical re;uirement. T$ey are e:pressed in terms of parameters t$at are interdependent, and desin 0alues are used. In t$e a%sence of prescription in EC7, !rr ')**-, and ot$ers $a0e interpreted t$ese e;uations to mean

γ +dst _udst+( _≤_γ+stb_σ stb+(

and

γ +dst S dst+(≤ γ +stb N stb+(

/$ere t$e su%script D indicates c$aracteristic, unfactored 0alues for t$e parameters.

T$e t/o re;uirements e:pressed in t$is /ay $a0e sinificantly different effects, as illustrated %y !rr, %ecause t$e factors are applied to different ;uantities. If t$e factors are applied to t$e same ;uantities, E;uations ).(a and ).(% are alternati0e statements of t$e same re;uirement.

As in e:amples presented a%o0e, t$e main pro%lem arises $ere if a partial factor is applied to c$aracteristic /ater pressureudst+( .

A factor can %e applied to e#cess /ater pressure or e:cess $ead, and t$is is e;ui0alent to factorin t$e seepae force

S dst+( . !n t$e ri$t $and side of t$e

re;uirement, t$e issue is /$et$er to factor total density or %uoyant density of t$e round. Since t$e proposed factor, *.(, is close to unity, t$is is less important< in 0ery li$t /ei$t soils factorin %uoyant density $as no effect, /$ic$ suests t$at it is safer to factor total density.

If t$e factors are applied to e:cess pore pressure, or e:cess $ead, and to %uoyant /ei$t, t$e allo/a%le $ei$t of /ater 3 follo/in !rrJs calculations is .+m, usin eit$er form of t$e e;uation. If, instead, t$e factor γ +stb is applied to total density, t$e

allo/a%le $ei$t of /ater 3 is only sli$tly less at .1m, no/$ere near t$e unreasona%le 0alue of ).7m o%tained %y !rr on $is interpretation of E;uation ).(a. !nce it is decided to /$at parameters t$e factors are applied, t$e c$oice of E;uations ).(a or ).(% maDes no difference.

.50onclusions on water pressures

&rescription of safety in desin for pro%lems dominated %y /ater pressure is a

su%ect of onoin de%ate. 6or

completeness, t$e conclusions reac$ed %y Simpson et al ')*11- are listed $ere. Areement /as reac$ed on t$e follo/in points2

1. T$e effects of /ater pressures are 0ery important in eotec$nical desin. T$eir actual 0alues can $a0e sinificant uncertainties, and 0alues outside t$e rane anticipated in desin can cause maor failures.

). &artial factor desin applies factors to a small num%er of leadin, or =primary> actions. In real desin situations, secondary actions of relati0ely small %ut unpredicta%le nature and manitude s$ould also %e accommodated< t$at is, a deree of ro%ustness it re;uired. !ften, t$ese are accommodated %y increasin t$e partial factors applied to primary actions or action effects.

@. "esiners must e:plicitly accommodate t$e /orst /ater pressures t$at could reasona%ly occur. Reliance on factors of safety toet$er /it$ less e:treme /ater pressures or /ater le0els may i0e a

false sense of security.

+. Application of partial factors to t$e density of /ater s$ould enerally %e a0oided.

. !ne useful /ay to maintain a prescri%ed deree of safety is to re;uire an offset in /ater pressure, raisin or lo/erin t$e /ater surface or pieGometric le0el. . T$e sinle source concept s$ould %e

applied /$ene0er possi%le. /igure :. Hydraulic problem considered by

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1 7. T$e =star> approac$ '"A)? or "A1?,

introduced $ere- $as ad0antaes /$en dealin /it$ pro%lems dominated %y /ater pressures %ecause it a0oids t$e application of partial factors to t$e density of /ater or to /ater pressures. . In uplift pro%lems, it is necessary to 0ary

eit$er /ater pressures or t$e manitudes of fa0oura%le, sta%ilisin /ei$t, in order to ensure safety in 0ie/ of possi%le secondary actions. In order to a0oid factorin /ater pressures, t$e possi%ility of a reduced factor on fa0oura%le /ei$t, per$aps %et/een *. and *.( s$ould %e

considered.

(. To pre0ent topplin failure of structures loaded laterally %y /ater pressure, a =middle )N@rds> rule could %e considered, applied to unfactored actions.

1*.Alt$ou$ t$ere are o%0ious ad0antaes in maDin codes of practice as precise and prescripti0e as possi%le, t$e need for enineerin e:pertise and careful e0aluation of t$e full rane of credi%le scenarios cannot %e replaced. T$is is particularly true of situations in /$ic$

/ater pressure $as a dominatin role. T$e follo/in points /ere not areed and remain to %e de%ated and researc$ed furt$er. In some cases, appropriate conclusions may depend on ot$er features of t$e safety formats adopted, for e:ample t$e differin "esin Approac$es of Eurocode 7.

11.M$et$er it is desira%le to apply factors to /ater pressures. Se0eral approac$es t$at a0oid t$is $a0e %een discussed, %ut in some approac$es factors are applied to /ater pressures in some circumstances. 1).M$et$er it is reasona%le to apply partial

factors to forces 'action effects- directly deri0ed from /ater pressures. It is areed t$at t$is may raise pro%lems, /$ic$ /ere discussed, %ut t$e aut$ors could not aree t$at it can al/ays %e a0oided.

[email protected]$e use of t$e =star> approac$, factorin_{action effects, in cases /$ere it is directly} e;ui0alent to factorin /ater pressures, eit$er complyin /it$ t$e =sinle source> principle or not compliant. T$e pro%lem particularly relates to situations in /$ic$ e;uili%rium is not maintained t$rou$out t$e eotec$nical calculations of sta%ility, includin slidin, %earin, topplin and uplift. 9ess concern is felt a%out application of factors to action effects

internal to structures, suc$ as %endin moments in /alls and sla%s or forces in piles.

 T3E =EO#> &R!B9E8 :. Outline o! the issue

E 1((* defines a mode of failure termed t$e EO# limit state. It relates to situations in /$ic$ t$e effects of actions comin from a sinle source effecti0ely %alance, cancellin eac$ ot$er out in some respects, and little or no strent$ of material is apparently needed.

9imit state EO# is descri%ed in E 1((* and E 1((71 %y t/o sli$tly different definitions2

E 1((71, ).+.7.1'1-& i0es2 =9oss of e;uili%rium of t$e structure or t$e round, considered as a riid %ody, in /$ic$ t$e strent$s of structural materials and t$e round are insinificant in pro0idin resistance>

E 1((*, .+.1'1-& i0es2 =9oss of static e;uili%rium of t$e structure or any part of it considered as a riid %ody, /$ere2 • minor 0ariations in t$e 0alue or t$e

spatial distri%ution of actions from a sinle source are sinificant, and • t$e strent$s of construction materials

or round are enerally not o0ernin.>

9oad factors re;uired %y EO# differ from t$ose of t$e main ultimate limit states in0ol0in strent$ of materials, STR and E!, as s$o/n in Ta%le ).

'able 3. &oad !actors in limit states S'C, EO and E%U Fsimpli!iedG

STR/GE E!" STR/GE E!" Permanent unfavourable favourable 1+35 1+ 1+1 + Variable unfavourable favourable 1+5  1+5 

EO# is particularly rele0ant /$en t$e actions come from a sinle source, so, in accordance /it$ EC7 4).+.)'(-&5 noted in +.) a%o0e, no im%alance /ould %e created %y application of different factors. T$e classic situation of a %alanced cantile0er

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17 s$o/n in 6iure 1. If %ot$ t$e forces < come from a sinle source, t$en reular application of load factors /ould lead to calculation of no %endin moment in t$e column and e;ual compressi0e forces in t$e piles. 3o/e0er, t$is /ould %e an unsta%le e;uili%rium. T$e purpose of EO# is to ensure sta%ility /it$ an ade;uate marin of safety %y applyin different factors to t$e t/o forces M, e0en t$ou$ t$ey come from a sinle source.

/igure ;. Balanced structure on piled !oundation.

T$e pro%lem t$is creates is t$at a liDely desin response to any im%alance in EO# is to pro0ide some material resistance to assure sta%ility. 6or e:ample, in 6iure 1 if t$e factorin system creates a differential %et/een t$e t/o forces, a possi%le response is to pro0ide %endin resistance in t$e column and t$e a%ility to taDe larer compression, or possi%ly tension, in t$e piles. T$is, $o/e0er, is inconsistent /it$ t$e definition of EO#.

T$e pro%lem also affects structural desin and $as %een reconised %y ul0anessian et al ')**), section 7..1-.

:.3 Pariety o! iews

Treatment of EO# in t$e eotec$nical conte:t $as %een discussed %y Sc$uppener et al ')**(-, /$o noted t/o alternati0e concepts2

• Concept 1 proposes 0erifyin only EO# in t$ose cases /$ere loss of static e;uili%rium is p$ysically possi%le for t$e structure or part of it, considered as a riid %ody. Similarly Concept 1 proposes 0erifyin only STRNE! in situations /$ere t$e strent$ of material or round is sinificant in pro0idin resistance. • Concept ) proposes 0erifyin EO# in

all cases< it is interpreted as a load case. M$ere minor strent$ of material or round is in0ol0ed, t$e com%ined EO#NSTRNE! 0erification may %e used, if allo/ed %y t$e national anne:.

6or t$e situation s$o/n in 6iure 1, Concept 1 /ould re;uire t$at t$e column and piles must %e desined as t$ou$ t$e t/o loads M are independent, /it$ factors of 1.* and 1.@, e0en t$ou$ t$ey come from t$e same source, /$ic$ implies t$at t$e desiner does not e:pect t$em to %e une;ual. T$is i0es forces in t$e piles of < r '1.17*.@a?b-. 3o/e0er, Concept 1

states t$at t$is is not to %e applied if no material strent$ is in0ol0ed.

Concept ) re;uires t$at t$e piles %e desined for t$e smaller factors of EO#, i0in sinificantly smaller forces of < r '1*.)a?b-. Concept ) suests t$at EO#

is =ust anot$er load case> rat$er t$an a different limit state, and t$at all desins s$ould %e a%le to accept t$is load case.

:.2 'he authorMs iew

T$e present aut$or considers t$at Concept ) is to %e preferred, for t$ree reasons.

'a- Concept 1 disreards t$e sinle source principle, /$ic$ /as set up in E 1((* specifically to a0oid creation of unnecessarily se0ere loadin conditions /$en loads come from a sinle source, t$at is, t$ey are clearly correlated.

'%- Concept 1 implies s$arp

discontinuities in desin re;uirements. 6or e:ample, in relation to 6iure 1, if a?b 

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1 +.(( %ot$ concepts c$ecD EO# and conclude t$at t$ere is no tension re;uirement for t$e piles. 6or 0ery sli$tly $i$er a?b  .*1, Concept 1 re;uires t$at t$e piles %e desined for a sinificant tension of *.< r . In

contrast, for t$is 0ery small c$ane in a?b Concept ) re;uires 0ery little tension capacity, *.**)< r .

'c- In some cases, Concept 1 re;uires t$at t$e foundations %e desined for more se0ere loadin /$en t$eir strent$ is limited t$an /$en t$eir strent$ is considered unlimited, implyin t$at t$ere is no strent$ re;uirement. Sc$uppener et al considered t$e retainin structure s$o/n in 6iure 17. In Concept 1 t$e pro%lem /as uded to %e one of o0erturnin if t$e round /as infinitely stron, for /$ic$ t$e load factors applied /ere t$ose of EO#. But if t$e round /as not infinitely stron in %earin, t$e larer load factors of E! $ad to %e considered. In t$e aut$orJs 0ie/, it is unreasona%le to c$ane t$e loadin in t$is /ay as a function of t$e strent$ of t$e round. T$is issue does not arise /it$ Concept ).

/igure 7. )otential oerturning o! retaining structure.

:. An alternatie !ormulation

E 1((* permits national anne:es to accept an alternati0e set of factors for EO# com%ined /it$ STRNE!, usin 1.1 on fa0oura%le permanent actions and 1.@ on unfa0oura%le permanent actions. Referrin to 6iure 1, t$is /ould i0e t$e same %endin moment in t$e column as t$e %asic E! com%ination of '1.1, *.(-, supportin t$e case for Concept ) discussed a%o0e. Sc$uppener et al ')**(- s$o/ t$at t$is formulation can alle0iate some of t$e eotec$nical pro%lems of EO#.

Mit$ t$e '1.1, *.(- com%ination, tension occurs for a?bP. 3o/e0er, for t$e '1.@, 1.1- com%ination, t$e forces in t$e piles are < r '1.1*.)a?b-, so t$ere is no tension until a?bP.7. T$is difference mi$t %e more sinificant t$an /as intended %y t$e code drafters. In t$e #K ational Anne:, t$e '1.@, 1.1- com%ination $as %een accepted for %uildin desin %ut not for %rides.

 #SE !6 6E8 6!R #9S

;. ntroduction

umerical met$ods, taDen $ere to refer to finite element and finite difference computations, are often used /it$ unfactored parameters to analyse ser0ice conditions, c$ecDin for ser0icea%ility limit states. Some results taDen from t$ese analyses may %e factored to pro0ide desin 0alues for ultimate limit state c$ecDs< t$is applies mainly to structural forces and %endin moments. 3o/e0er, t$e use of numerical met$ods for full #9S analyses, in /$ic$ t$e strent$ or resistance of t$e round is modified, is comparati0ely ne/ and still a su%ect of de%ate. To study t$is and related issues, CENTC)*NSC7 $as set up E0olution roup + on numerical met$ods, c$aired %y "r Andre/ 9ees of Cyprus.

#se of numerical met$ods /it$ EC7 raises t$e follo/in issues2

1. Can numerical met$ods %e used for #9S analysis in accordance /it$ all of EC7Js "esin Approac$es

). If t$e strent$ of round is to %e reduced, $o/ is t$is to %e applied in practical computations

@. "oes factorin round strent$ lead to t$e =/ron> failure mec$anism +. 6or staed construction or e:ca0ation,

at /$at stae s$ould factorin %e applied

. Can ad0anced soil models %e used /it$ partial factor met$ods

. 3o/ are factors to %e applied for

undrained %e$a0iour and time

dependent consolidation

T$ese issues are in current de%ate, /it$ no enerally areed ans/ers. T$ey /ill %e

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1( outlined %elo/ and t$e aut$or /ill offer opinions. 6ollo/in t$is, a practical e:ample of computations to "esin Approac$ 1 /ill %e presented.

;.3 0an numerical methods be used !or all 1esign

umerical met$ods can %e used relati0ely easily for #9S computations if t$is merely re;uires usin factored 0alues for t$e input to t$e proram, or simply factorin t$e structural action effects resultin from t$e eotec$nical proram. "esin Approac$ ) '"A)- re;uires factors to %e applied to ;uantities t$at are internal to t$e eotec$nical analysis suc$ as acti0e and passi0e forces or pressures, and %earin resistance for spread foundations. So it is enerally accepted t$at full numerical analyses of ultimate limit states cannot %e undertaDen for "A). 8ost countries t$at use "A) re;uire use of "A@ for numerical analysis.

"esin Approac$ 1 /as t$e only approac$ in t$e E 0ersion of EC7 pu%lis$ed in 1((. In its de0elopment, t$e possi%le use of numerical met$ods /as considered, so it can %e used relati0ely easily. T$e use of "A1 is_{not 0ery different from t$e com%ined use of} "A) and "A@, e:cept t$at "A1 re;uires c$ecDin of t/o calculations, /$ereas com%ined use of "A) and "A@ could imply acceptance of a desin t$at passes accordin to one "A %ut fails accordin to t$e ot$er. T$e leal implications of suc$ as situation mi$t %e de%ata%le.

;.2 How should strength !actors be

applied-"esin Approac$es 1 and @ re;uire t$e strent$ of t$e round to %e reduced %y partial factors. T$e aim is to s$o/ t$at no #9S occurs 'strictly, none is e:ceeded- /$en t$e round is assumed to $a0e its reduced design strent$.

T/o alternati0e met$ods are a0aila%le for application of strent$ reduction factors2

a- Reduce t$e round strent$s used as input to t$e numerical computation. T$is could %e done %y t$e user or automatically %y t$e proram.

%- Carry out t$e computation first /it$ unfactored strent$s, t$en proressi0ely reduce t$em until a #9S is reac$ed. An

e:ample of t$is approac$ is t$e =cϕ reduction> in &la:is.

In met$od 'a-, t$e aim is to s$o/ t$at no #9S occurs /it$ t$e reduced strent$s. T$e 0arious strent$ parameters in0ol0ed,cu,c= ,

ϕ = , ot$er soil parameters and structural strent$s, can all %e factored %y different amounts if re;uired. ormally t$e occurrence of a #9S /ill not actually %e modelled. T$is met$od can s$o/ t$at t$e desin complies /it$ code re;uirements, %ut it may not maDe clear $o/ muc$ additional reser0e t$e desin $as, or $o/ muc$ furt$er economy mi$t %e a0aila%le.

8et$od '%- models an actual #9S and t$e aim is to s$o/ t$at t$e reduction factors t$at apply to material strent$s at t$e #9S are reater t$an t$ose re;uired %y t$e code. T$e difference %et/een t$e code re;uirements and t$e actual reduction factors at failure may i0e some indication of t$e reser0e of safety and t$e furt$er economy a0aila%le. E0en so, it /ill not lead directly to an impro0ed desin< t$is $as to %e ac$ie0ed %y furt$er analysis of a re0ised eometry.

It is important t$at t$e appropriate partial factors are applied to eac$ strent$ parameter in0ol0ed, suc$ as cu, c= , ϕ = , and

possi%ly structural strent$s. T$is is easily ac$ie0ed /it$ met$od 'a-< facility for met$od '%- /ill %e dependent on t$e soft/are.

As an e:ample, 6iure 1 s$o/s t$e displacements computed for an m deep e:ca0ation /it$ a diap$ram /all and sinle prop. T$e desin moment of resistance of t$e /all is 1***DmNm. 6ollo/in met$od 'a-, /it$ a strent$ reduction factor γ ϕ of

1.) t$e mo%ilised %endin moment is 77)DmNm '6i. 1(-. o #9S occurs, so t$e desin is 0erified. 6iure )* s$o/s t$e de0elopment of a mec$anism as ϕH $as radually %een reduced to a 0alue e;ui0alent toγ ϕ  1.+, follo/in met$od '%-< for t$is analysis t$e computed %endin moment is 17)(DmNm, far a%o0e t$e moment of resistance of t$e /all. T$e met$od '%-analysis, /it$ an e:cessi0e reduction in t$e strent$ of t$e soil and an unaccepta%le %endin moment, yields no useful information, failin to clarify /$et$er t$e