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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 624 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, Widthoilde 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
, Howeaunt
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 MauserMitt
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 pressuresettlement 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.
& Thefollowing 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 notstrictly a of relative I; anew to : “ ‘fbilit‘y‘ grades” . eorrelan'. j 'on N and  is givenin 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, sandsbelow 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 atth'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 depthof 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!) 35 to H). 7 '13.. His the he! of the water table beneath the founding
lets! not, have sstatis’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 lengthtobre’adth ratio of the foundation). The
correction factor is quite small and can be expressed as __ 1'25L/B 2
f' " [(143) + 025] ‘6)
where “(L/B > 1) =1; x p‘(L/B = 1). It can be seen that 1; tends to l56 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 [FE3(9 3;'—‘) m
3 ‘vt ‘ Timedependent settlement V 16. The case records referred to in §§ 107415 indicate quite clearly that foun
dations on sands and gravels exhibit time—dependent settlement. However, no pattern emerges. In some the timedependent 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 timedependent 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 03 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,}; = 25. '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 [trtat.) 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 nonlinear 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 wellestablished 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 unlikelythat 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 mawwr‘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 thecorrelation 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 MohrGeoteenicasampler 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 SubCommittee 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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This note was uploaded on 03/24/2011 for the course ECI 173 taught by Professor Dejong during the Spring '11 term at UC Davis.
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