Burland and Burbidge- SPT footing - 1985

Burland and Burbidge- SPT footing - 1985 - mm Ci». Engrs,...

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Unformatted text preview: mm Ci». Engrs, Part 1. 1985. 78. Dee. 1325—1381 GROUND ENGINEERING GROUP ’ Settlement of foundations on sand and gravel 3. BURLAND, PhD, DSdEng), FEng. FICE, MlStrnctE M. C. BURBIDGE, BSc, use. me, Post ‘ he Paper describes the analysis of over 200 records of settlement ol’ foundations, tanks and .‘nhnltntents‘ on sands and grade. A remarkably simple picture has emerged relating the . to the bearing pressure the, breadth of loaded area and the average SPT blow f or conexresistanee over the depth of The influence of a number of factors is depth efrl’mmd‘ation, depth of water table, grain sizeiand time have been i The Paper first briefly the application of the results to the prediction . i with particular on the limits of accuracy. Paragraphs 6-24 are self and may be used on their own for design purposes. The Paper follows this with a , ; Hfiletlzeeount‘ of the analysis of them records. ’ Notation ‘ ot'loadedrarea “inflation subgradeeompressibility (Am/A43; mrn/(kN/m’) '35 Fibbabk value Of a, Width-oilde area. In depthoffdtmditlg level efl‘ectitte Young’s modulus Winn factor for thickness of sand layer ' actor'ngr term”. “91"” met/5°?) fitebtincreagegetYenng’s modulus with depth ' ofinfl‘u'enee , Howe-aunt alert/m3 ' r _ (expressed as a proportion nip.) occurring during first 3 mm {14) w“ Netting; $.30‘pm 25 February 1986. Written discussion closes l4 March 1986. H Mauser-Mitt 3 I'm ' ’ W39 ofsm " i lou- “oCOMIflfiEE-‘Enfineering Geologist. BURLAND AND BURBiDGE 1 time in years 2, depth of influence of loaded area v’ efl'ective Poisson‘s ratio p, final measurement of settlement _ p. settlement at the end ofconstruction or completion of loading p, settlement at timer after completion of loading a standard deviation oz, maximum previous ell'ective overburden pressure. kN/m’ Introduction Numerous methods of predicting settlement of foundations on sands and grants have been published—many more methods than for clays. The reason lies in the extreme difficulty of obtaining undisturbed samples for the laboratory determi- V nation of compressibility under appropriate conditions of stress and stress history. Hence resort has been made to the interpretation of field in situ tests such as the standard penetration test (SPF), cone penetration test and plate loading test. and much of the literature has been devoted to such interpretations. This extensive literature will not be reviewed here as it has been adequately covered by Suther- land,‘ Simons and Menzies“ and Nixon.3 I 2. The practical importance of the problem was perhaps putin perspective by Terzagh‘i‘ when he stated that all buildings. resting on sand which were known to him had settled less than 75 mm (3 in) whereas the settlement of buildings an clay foundations quite often exceeded 500mm (20 in). This statement provided the impetus for the study described in the present Paper in which a large number of case of settlement on sands and gravels‘ have been aSsenibled by Burbidge’. _ i 3. The essential details of most of these records are tabulated in lol’the present Paper and the associated aregiven in 21hr reeo’rd numbering used by Burbidge has been retained for ease of 4. prime of the study was to Whether the shore statement of Termghi’s still held true and reference to Appendix I shows that, with a few it does for buildings. Hooter/er, ~settlem¢nts well in excess of 751nm have recorded for tanks and on loose sands. in oi’the small settlements usually experienced with sands and gravels the second objective of the study was to analyse the data on actual observations ol‘settlement in: a minimum, of interpretation to if a slinple' and useful A p " - 1 study of this type was undertaken by Burland er al.‘ and a but I , ere approach is here. 5. that has emerged from the statistical analysis clover Mensa is simple and gives a range of settlements which is generally less than the range of predictibns adored by the current cominonly accepted methods.‘ A description of the method and its application is given first. followed by a account of the analysis of the settleni'entreoords. 6. The outcome of the analysis of the large number of settlement records summarized in Appendix 1 is presented first. in the form of a simple direct method of settlement prediction. Paragraphs 6—24 are self contained and can be used on their own for design purposes. However, frequent cross—references are made to the work described later, so that the basis of the various assumptions can be studied. - --’ SETTLEMENT OF FOUNDATEQNS SAND AND GRAVEL Determination of the foundation subgrade c‘ompressibilr‘t y 7. The nub of the method is the empirical relationship which has been estab- irlted betWeen the slope of the pressure-settlement relationship for the foundation (Ame the breadth of the foundation B and the average SPT blow count N over the depth of influence of the foundation. The quantity ApJAq’ is the foundation nibgrade compressibility. denoted by an and the units are mm/(kN/m’). The relationship is shown in Fig. l. where a,/B°"’ is plotted against N on double log axes. The quantity rig/8°" is denoted as Ic , the compressibility index. The full line in Fig, 1 has been derived from a regression of Over 200 settlement records on sand and gravel. The chain dotted lines approximate to two standard devi- ations above and below the mean line. mathematically the regression line is given by 1'71 ‘ I‘ with of correlation atom. & The-following features should be noted about Fig. l. (a) a, is the subgrade compressibility for a normally sand or gravel. ln §§ 64—72- it is shown that the relationship bearing and settlement is approximately: linear for normally consoli- dated granular ma? i " for of safety against failure of 3- or the’mate’r‘ia‘lis or [at mom or an excavation, the vetoes era; Ic are by a factor of 3 for pressure below the ell‘eetive preconsolidation pressure ‘61.. .. (b) blow count is for en‘eetiVe and the"; horizontal »axis,isfthereiore not-strictly a of relative I; anew to : “ ‘fbilit‘y‘ grades” . eorrelan'. j 'on N and - is given-in Tablet and. in “Fig. l. the ‘ j j ’ proved ' _ "' ' 11y - his in (1:) Although are not for it is to matte j" 1'? other It is in Section 558.5 thatxior silty, sands-below the water the by and Peek" iinproved results, Le. N‘ is yeaterthani ‘15- M=15+o5(1v—15) (2) whereN’ is the value of N. When the of it is shown in§§ 103—106 that a should N’ = NS x N (3) (d) The results Tot cone penetration tests may be converted to eqniyalent N ‘ V values using Fig. 2, where (MN is related to grain size‘iq‘is in MN/tn’). (e) The results of plate loading tests may be related to compressibility grade using the methods in § 47—50. Care must be used in the application of equation (I) in conjunction with plate loading test results BURLAND AND BURBIDGE 100 N y: I ~7- (a,/B°'7)x102 $ A ‘7 7/ 6., gz '. ..e::_,;%_mmxmm 100 SP‘FN Fig. 1-. compressibility (1,) and mean SPT blow count (fl) of dotted lines show upper and lower limits (see Figs 2.’ ofcampressibiiity ofmally andyrave’ls with SPT blow count Uncorrected for overburden pressure. 1328 ,, SETTLEMENT DE FOUNDATHJNS ON SAND AND GRAVEL- (ac/N (MN/m9) x to Fig. 2. Relationship between qJN and grain size. Values ofN are not corrected for wheeler: pressure since as 8 increases the value of N Will often increase as well due to the associated increase in the depth of influence. Depth ofiMuence and the derivation of}? _ 9. An important feature of the method is the assessment of the depth of influ- z, of the foundation. This is discussed in detaii in=§§51~63 wheteit is shown whim N increases with deptmthe reiative of influence (2/8), decreases significantly as the breadth of the foundation Although the depth or depends on many factors. for present purposes. it is to be given the full tine in Figs 3' for where oris constant with depth. N shows aconsismnt decrease With. depth thed‘epth is taken as not the bottom orth‘esoft layer, whichever is The value of N for in Fig; l or equation (Dis given by the arithmetic of tire measured N values over thedepthofinfiuenee. Caieuiation ofsettlemeru' to. For a normally consolidated sand the immediate average settlement p, at ofeonst'r‘nctiom corresponding to theaverage e‘fi'ective foundation pressure is: even by pg=4XB°.7Xle p, is in mm, q’ in Hi]!!!2 and B in metres Values of 1., corresponding to the estimate and the upper and lower limits, are obtained from. Fig. 1. 11. For an over eonsoiidated sand, or foriioading at-th'ebase of an excavation, for which the maximum previous effective overburden pressure is ac. , the average and of construction settlement p: corresponding to the average gross efi'ective ptessure q’ (where q’ > 03°) is made up of two components as follows ‘An‘ sysfisun AND Bugsmgg tOO 1 1 V 10 I 100 Fig. 3.. Relationship between breadth of loaded area B and depth of influence 1. (within which 75% ofthe settlement takes place) I p‘=o’,,x 8°77x§5+(q'~a’,‘)x8"‘7 x1¢mm =(9'—§¢§JB°'7XI¢M (5|! Whenq’isless than 0;, thesbove expression becomes p,=q’xB°"’x%mm (5H correctiousfor depth-of founding. depth of water table, shape and thickness oflayer 12 In 91—106 a statistical. analysis of the influence of the above foetus it to: (condemns with, ra’tibs 0/!) < 3. It is shown that. within the j, the e ‘ then 1 is no obvious correlationbetween DID ' " V " agrees with the results of D‘Appolonia et al.‘ who [and of results on one site that only a 12% reduction in “ii ‘Tff’fihen BIB I.1701!) 3-5 to H). 7 '13.. His the he! of the water table beneath the founding lets! not, have s-statis’tieatty influence on the setttcment. This mot to Meyerhol’s’ that the street of the water table is reflectedie the blow count. Thus water table changes subsequent to the denud- nation of I? may have some influence on settlement. .14. The statistical analysis indicates that there is a significant Correlation o Gila mwzmvmsmzzmxmw "at s w x: “Nd? m , ‘ ($43» ’ SETTLEMENT OF FOUNDATIONS ON SAND AND GRAVBL \ between settlement and L/B (the length-to-bre’adth ratio of the foundation). The correction factor is quite small and can be expressed as __ 1'25L/B 2 f' " [(143) + 0-25] ‘6) where “(L/B > 1) =1; x p‘(L/B = 1). It can be seen that 1; tends to l-56 as L/B tends to infinity. 15. There were insufficient data to study the influence of the thickness of the sand or gravel layer beneath the foundation (H) but it is recommended that when ll, is less than 2. (the depth of influence) a correction f. should be applied such that [FE-3(9- -3;'—‘) m 3| -‘vt ‘ Timedependent settlement V 16. The case records referred to in §§ 107-415 indicate quite clearly that foun- dations on sands and gravels exhibit time—dependent settlement. However, no pattern emerges. In some the time-dependent process appears to be more or less continuous, with the settlement following an approximately linear log time relationship (after an initial transition period). In other cases the process appears to be stepwise with quiescent of up to 3 years interspersed with of significant rates of settlement 2 17. records show very clearly that foundations subject to fluctuating loads such as tall chimneys, bridges, silos and turbines exhibit much larger time- dependent settlements than those subject only to static loads. l8. The results suggest that the time correction factor for the settlement (1),) at any time t, when t is 3 years or more after the end ofconstruetion, is given by L=£l=(l +R,+R;log£) (8) P; _ 3 f, is the correction factor for time. :2 3 years. R, is the time-dependent settlement (expressed as a proportion of )1.) that takes place during the 3 ' construction and R. is the settlement (expressed as a propor- tion of gather takes place each log cycle of time after 3 years. ' its. _ For static loads conservative values or it, and R. are 0-3 and M reSpe‘c- at t a: 30 yearsifl 2: 1'5. For fluctuating loads conservative values of ll,and [anew and 08 respectide so that at t a: 303mm,}; = 2-5. 'Smmry 19‘. In summary the average settlement of a foundation at the end of construc- then at any time t, 3 or more years after the end of construction may be ' ‘ ‘ ' by the following equations; ps=f. x1. x [tr-tat.) x 3°" x 1,1 mm on and pr =f. x p. (9b) where q' is the average gross effective applied pressure (kN/m’), 0;, is the s 1% ’3 t s a Q g I t u i i a mailmtzwm/We sunrise AND guanine: maximum previous efl‘ective overburden pressure (RN/ml), B is the breadth in metres, lc is the compressibility index obtained from Fig. l or equation (1),)“. is a shape correction factor given by equation (6), f, is a correction factor for the thickness of the sand layer given by equation (7) and j; is a time factor given by equation (8). 21. The probable limits of accuracy of equation (9a) can be assessed from the upper and lower limits of Ic given in Fig. l and it may be necessary to talte these into account in the design. , 22 It must be that the factor of safety against bearing capacity failure should always be in addition to the settlement. If the factor of I safety is less than about 3 the pressure settlement curve may be non-linear and the method will underestimate thesettlement. - p the method has been besed on case studies with quartzitie sand and gravel Sites where coral (calcite) or other mineralogically are encountered should not be analysed by this unless deformation of deposits can be demonstrated to to a 24. is suited for routine purposes. However, it is suggested that, for or those where the proposed structure has strict total settlements, other well-established methods otesti- the used as'a such projects it may prove valuable to; refer listed in 2 in which similar or ground- are involved. In it unlikely-that the of improved unless resortis‘ made to the direct determi- o‘tinitu ‘1 ‘bility. 25. In conclusion it is appropriate to bear in mind the following remarks by Sutherland‘ , fBeforeadesigner entangled in the details of predicting settlement (in ‘fjhe sahsfy‘ i ' ‘ himself”: , WW 1 ' ‘ra real problem crusts» ' and ' ‘ ' and cocoa, I f can mutt from refinem‘ ‘ cats in " ' .. _ :otumemeni; :2 *' ,t '* ‘xahdizl‘fivels . get the in? section of the Paper was to assemble‘ ‘ j With: “Tbility of {ground within the: o! _ : of- ma,-depth of water table, time , t slit to the above three: and: be separately alter the (trends had been 5 j record the quantities p, q' and B are defined. Thus. in ’ . et al.‘ to correlate pig, with «B. The com " 1’ f 'es otltlteground are much difficult to define and Burlan‘d er at. only distinguished between three categories of granular material: dense and dense. In the present study the Same basic approach is adopted but a more refined method of classifying the compressibility of sands and wtthxmeta“swarmNewWeownw'5.fimeWmum’fiwmmNWfiWfiWWWW ,, mwmwww m-awwr‘x ' I SETTLEMENT OF FOUNDATIONS ON SAND AND GRAVEL gravel: has been found to be justified. Moreover. it has proved necessary to consider in some detail the depth within which the compressibility significantly the settlement (Le. the depth of influence‘za andalso the validity of the assumption of a linear pressure—settlement relationship. These matters are dis— cussed in the following paragraphs as a preliminary to the presentation of the of the case records. Tiestandard penetration test (SPT) as a measure of compressibility ‘ 28. Fur the majority of the case records assembled for this study the ground were investigated using the standard penetration test (SPT). For this and his a test which is widely used, it was decided to use the SPT Howcount as a measure of the compressibility of granular 'soils. Nevertheless, it is ef'th'e utmost importance to appreciate the limitations both of the test itself and the-correlation of its results with compress'ihility.‘o [29. The. standard penetration test. At present the two most widely used stan- a’re BS 1377: 1975“ and ASTM D1586—67.” The testing procedures are , similar, and outside the UK and the USA one of these two standards is ’ " y imponant exception to the general SPT procedure is in Brazil Mohr-Geoteenicasampler is extensively used. 30.0 There are numerous details of the test and its operation which are not d.” For example, there are considerable differences in the dimensions and of drilling rod used in the test. A _, the driving technique can vary . ' ' ‘- " 3y. The British and European standards specify use of a trip hammer ‘ American practice is to operate the driving weight manually using a " ,- factors which can influence the N value are the of easing. of the driving shoe, the type of boring rig and the method of cleaning the borehole. According to Schmertmann" almost all samplers used in have enlarged internal diameters to hold a liner. However. they are without a liner. which to a significant reduction in the N Over and‘abo‘ve all these factors the crucial -. importance of maintaining an of water in the borehole must of course be emphasiZed. [ill It ‘ been that the SM is an empirical test. It is a test to become completely standardized if its use as a yardstick for V properties, such as compressibilityfis to be The need for ' titan emphasised by Nixon’ who calls for international _ 197:7 iISSMI-‘B ‘ of the Sub-Committee on Penetration Test for Win Europe’.” future 0r standardization in the test that do take ' not deviate from present procedures. so that experience from the testis not lost. of grain size. effects of grain properties, such as angularity on SPT resistance have not adequately studied. and, :D‘Appolonia“ suggest that the is influenced by the angularity soil. Gibbsa‘nd Holtz" found that the grain influence. dry loos: showed that the N value for coarse marginally 1 for fine and, at the same relative density and overburden pressure. for sand there was no appreciable difference between fine and sands. D’Appo‘lonia and D’Appolonia" concluded that the particle size does'not appear to have a major influence provided gravel sizes are not present. 33. Influence of submergence. Sehultze and Menzenbach' 9 and» eraa’” have that the SPT resistance for coarse sand and gravel is not affected by i, g i -...,«.e.».«.;:wnTwamwm...when».ammw‘w‘. ...
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Burland and Burbidge- SPT footing - 1985 - mm Ci». Engrs,...

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