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The consolidation of peat: a literature review
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NATIoNAL RESEARCH COUNCIL CANADA
DIVISION OT' BUILDING RESEARCH
THE CONSOLIDATION OT' PEAT A LITERATURE REVIEW b y I. C. MacFarlane
Ar'i,ql"Yr.ID
T e c h n i c a l P a p e r N o . 1 9 5 of theDivision of Building Research
OTTAWA M a r c h 1 9 6 5
THE CONSOLIDATION OF PEAT
A LITERATURE REVIEW
by
I. C. MacFarlane
An irnportant factor in the over-all stability of any structure is the arnount of cornpression of the subsoil that will occur when the weight of the structure is applied. For clays, a rational systern has been developed for estimating the rnagnitude of the total and differential settlernents as well as for the time reguired for a given arnount of settlernent to occuri this systern is based on the Terzaghi theory of consolidation (Taylor, L94Zl.
A characteristic feature of highly organic soils (or peats) is their extrernely cornpressible nature; large settlernents can be
produced even under srnall loads. Consequently, peat is not a desirable soil type on which to build roads or other structures" Until recently, construction on peat has been lirnited and the chief interest in peat bogs or rnuskeg areas has been with regard to their potential for reclarnation for agricultural or forestry purposes, or in the rnining of the peat for fuel or as rrpeat rnoss. rt within the past l0 years or so, however, interest in peat as an engineering rnaterial has been substantially i n c r e a s e d a s a r e s u l t o f t w o r n a j o r factors:
( I ) T h e r a p i d l y a c c e l e r a t e d p a c e in Canada has required an increasing arnount
northern developrnent construction in regions of
)
L
-of alrnost continuous rnuskeg cover. In the construction of roads in such regions it is seldorn possible to avoid muskeg, and econornic considerations often rnilitate against excavation of the peat and back-filling with a rnore suitable rnaterial. The road, therefore, rnust be I'floatedtr on top of the organic terrain. Consequently, the compression characteristics of the underlying peat becorne a rnatter of sorne irnportance.
(Z) The second factor is related to rnore southerly regions where urban expansion requires the extensive utilization of forrnerly m a r g i n a l l a n d ( s u c h a s p e a t b o g s ) f o r a i r f i e l d s , s h o p p i n g c e n t r e s , f a c t o r i e s , o r w a r e h o u s e s . H e r e a l s o t h e q u e s t i o n o f t h e c o r n p r e s s i o n characteristics of peat becornes a rnatter of great irnportance.
Although the classical consolidation test has proven useful in analysing settlernent characteristics of clays, the wide variation
between the fundarnental nature of clays and peats is sufficient to reguire an independent evaluation of its use in assessing the cornpression
characteristics of peats. Several investigators in various countries have cornpared settlernent predictions based on laboratory consolidation tests on peats with observed behaviour in the field. In view of the unusual nature of the rnaterial it is not surprising that the conclusions of sorne investigators have been at rather rnarked variance with the conclusions of others. One obstacle to the validity of cornparing results of various investigators has been the lack of a rational (and international) systern of classifying peats. A11 systerns of classification in current
3
-use (including the Radforth systern, which is -used alrnost exclusively
in Ganada>i<) are qualitative rather than quantitative, and this is a serious
lirnitation to.investigations involving engineering characteristics of peat.
A particular area of controversy in the early literature was
centred around the relative contribution to the totaL settlernent of that
phase of consolidation known as I'prirnary consolidation" (associated
with dissipation of excess pore water pressure) and that known as I'secondary colrrpressiont' (the plastic deforrnation phase). This
controversy developed largely as a result of the difficulty in estirnating
when the prirnary phase was cornpleted and when the secondary phase
began. The relevant literature on laboratory compression tests and
field settlernent observations of peat extends over the past 25 years.
About two thirds of the papers exarnined that refer to consolidation and
settlernent characteristics of peat, however, have been written in the
past 5 years. A trend towards sorne unanirnity of opinion is only now
being e stablished"
One of the earliest investigators to study the cornpression
c h a r a c t e r i s t i c s o f p e a t , B u i s r n a n ( 1 9 3 6 ) c o n s i d e r e d c o n s o l i d a t i o n o f
p e a t t o i n c l u d e a l a r g e r r s e c o n d a r y r r o r t t s e c u l a r r t effect. He observed
that for laboratory peat sarnples of 2 crn (0.25 in. ) thickness the
settlernent - - log tirne curve becarne a straight line about one rninute
rrGuide to a Field Description of Muskeg (based on the Radforth
C l a s s i f i c a t i o n S y s t e r n ) " . N a t i o n a l R e s e a r c h Council, Assoc. Cornrn. on Soil and Snow Mechanics, Tech. Merno. 44, Ottawa, 1958.
4
-or a few rninutes after loading and rernained a straight line throughout
the test. The slope of this line increased for higher ternperatures.
Bui.srnan also noted that the settlernent -- log tirne diagrarn for a road
ernbank-rnent (observed over a ?-year period) was a straight line. He concluded
that laboratory consolidation is cornposed of a cornbination of rrdirect load
effectsrr and rrsecular tirne effects,rr the rnagnitude of the latter depending
u p o n t h e t e s t p r o c e d u r e .
Another early investigator, Ringeling (1936) investigated
the settlernent characteristics of a peat layer 2 to 3 rnetres $-t to l0ft)
thick beneath a projected road. He determined both the rate of cornpression
under the fill load and the tirne at which consolidation would be cornplete.
Laboratory consolidation tests were carried out on several peat sarnples
and an experirnental road section was constructed to check actual
settlement against that predicted frorn test results. Settlernents of the
test fill did not agree with cornputed settlernents, being less for the
lower loads and greater for the higher loads than those predicted.
Ringeling also rneasured pore water pressures beneath the test fill and
noted that irnrnediately following loading of the peat, about 75 per cent
of the load is taken by the pore water. He pointed out that despite the
Iow perrneability of the peat, pore water pressures decreased practically
irnrnediately.
Van Mierlo and Den Breeje (1948) also indicated that the
5
-total settlernent of. peat. This secular effect had to be taken into account in calculating the rate of settlernent of a sand fill on peat and clay in the Spangen Polder. Laboratory compression tests on the peat showed a straight line relationship between settlernent and log tirne.
The average coefficient of perrneability of the peat tested. was I x t0-5"rrrr/"... Ug$-(1948) reported on an investigation into an ernbankrnent slip failure on peat. The water content of the peat varied frorn 800 to 1 0 0 0 p e r c e n t , t h e s p e c i f i c gravity, 1.2 to 1.5. Laboratory consolidation t e s t s o n 2 - i n . ( 5 . 0 8 c r n ) t h i c k sarnples were carried out; the samples never fully consolidated although all loads were left on for a week. The
predicted settlernent -- log tirne curve assurned a straight line, but the values were on the low side. Actual settlernent of the peat under the embankrnent was 4ft (I. ?Z rnl in Z years, about double the predicted value.
k l l g y ( 1 9 5 0 ) c a r r i e d o u t o n e - d i r n e n s i o n a l (4. 25 in. (I0.8 crn) diarneter sarnple) as well as triaxial consolidation tests on peat, which had an average moisture content of. 7zB per cent, an average organic c o n t e n t o f 8 3 p e r c e n t , a n d an initial void ratio range of.4.5 to t0.3. The triaxial apparatus was arranged with a scale -size sand drain installed in the centre to observe its effect on the rate of consolidation. Colley reported that the sand drain increased the rate of consolidation by 12 tirnes, in contrast to the observations of various other investigators who found that vertical sand drains do not increase the rate of consolidation
. 6
-in peat. col1ey measured the perrneability of the peat, which averaged o . 3 f t / d a y ( 1 . 0 5 9 x I 0 - 4 c r n / s e c l , i n t h e v e r t i c a l d i r e c t i o n and in the h o r i z o n t a l d i r e c t i o n i t r a n g e d f r o r n 0 . 2 t o 1 . 3 t t / d , a y ( 7 . 0 6 x l 0 - 5 t o
- 4
4 . 5 9 x I O - c r n / s e c ) .
Thornpson and Palrner (1951) reported on a fairly cornprehensive series of consolidation tests on a black fibrous peat ranging in natural
rnoisture content frorn 239 to 341 per cent, dry weight range of. 14.5 to Z 0 . 8 I b / c u t t ( Z 3 Z . 3 t o 3 3 ? . . ? k g / c : u r n l , s p e c i f i c g r a v i t y r a n g e o f 1 . 8 0 t o 2.04, and initial void ratio range of 5. I to 6.7. Consolidation specirnens w e r e 2 i n . ( 5 . 0 8 c r n ) i n d i a r n e t e r a n d , l/ 2 a n d I i n . ( l . 2 7 a n d . Z . 5 4 c r n l
thick. It was not possible to draw an e-1og p curve for these specirnens to deterrnine the preconsolidation load. The authors could see no line of dernarcation between prirnary and secondary consolidation in laboratory tests and postulated that the prirnary phase, if it exists, is apparently completed in less than one rninute. 'W'hen observed settlernents were plotted against 1og tirne, an approxirnately straight line was produced. Laboratory specirnens also showed no tendency to deviate frorn a straight Line" This line extended frorn one rninute to approxirnately one day, changing to another and steeper straight line frorn one day until the end of the test" Thompson and Palrner observed that the rate of consolidation of peat is independent of the distance of flow of the pore water and
consequently concluded that it is futile to atternpt to stabilize peat with v e r t i c a l s a n d d r a i n s .
7
-In his pioneer work on the engineering properties of peat, Hanrahan (l9SZ, 19541 carried out various physical and rnechanical tests on peat with the following general characteristics: water content r a n g e z B 0 t o r 4 5 o p e r c e n t , s p e c i f i c g r a v i t y r . z t o 1 . 7 , d r y d e n s i t y -4 . 0 t o 7 . S I b / c u f t ( 6 4 . 1 t o l z o . l k d c u r n ) a n d i n i t i a l v o i d r a t i o - 9 t o 2 5 . Hanrahan perforrned a nurnber of srnall - and large-scale consolidation tests and observed that the curve of settlernent plotted against tirne bore little resernblance to the theoretical curve. He attributed the discrepancy to various phenornena, including the abnorrnally large decrease in
perrneability that accornpanied the application of load, the d.ecreasing coefficient of cornpressibility, and thixotropy. High surface activity
and adsorption properties of peat also contributed to appreciable secondary consolidation. The cornpressibility of peat was observed to be temperature s e n s i t i v e , a g r e e i n g w i t h B u i s r n a n t s ( 1 9 3 6 ) observations. A s a r e s u l t o f cornparative srnall-and large-scale consolidation tests, Flanrahan concluded that accurate forecasts for settlernent can be rnade by assurning that the rnagnitude of settlernent is directly proportional to thickness and that the tirne of settlernent varies as the square of the thickness, for a considerable period after the cornpletion of the excess pore water pressure phase.
S h e a ( I 9 5 5 ) r e p o r t e d t h a t t h e u s e o f c o n s o l i d a t i o n t e s t s f o r the prediction of settlernent of levees constructed on peat had been only partially successful" The arnount of settlernent could be deterrnined, with fair accuracy, but the predicted length of tirne required for it to take
8
-place was not even approxirnately correct. Consolidation tests indicated I5 to 20 years for 90 per cent settlernent, whereas the actual settlernent took place in a few days or weeks. Frorn settlernent plates installed beneath the levees on peat, Shea observed that the total settlernent carne very close to the predicted 50 per cent of peat thickness, but that
practically 100 per cent of this settlernent occurred during construction. This latter observation is at variance with the findings of rnost other inve stigators.
Cook (19561 presented a cornpilation of test data to show that a relationship exists between the rnoisture content and the |tcoefficient of cornpressibilityrr of a wide range of soils frorn pure peat to organic clay silts in the Vancouver area. He also shows that there is a relation-ship between the rnoisture content and the specific gravity, initial void ratio, and submerged weight of these soils. He suggests that these sirnple relationships perrnit the calculation of.expected settlernents without the trouble of lengthy consolidation tests.
Lewis (1956) concluded frorn his consolidation tests on peat (1 in. (2.54 crn) thick sarnples, average water content 580 per cent) that 100 per cent prirnary consolidation was about cornplete within the first I0 rninutes. Much of the consolidation was observed to be of a secondary nature , a fact that was attributed to plastic deforrnation, or creep, with the rate of rnovernent a function of rnanv factors such as the nature of the rnaterial, the rnagnitude of the applied stress and the ternperature.
9
-Although it was not possible to calculate rates of settlement, frorn results of Iong-terrn consolidation tests Lewis estirnated the ultirnate value of settlernent to be expected in the field" This agreed closely w i t h s u b s e q u e n t o b s e r v a t i o n s .
B r a w n e r ( 1 9 5 8 ) n o t e d t h a t f o r f i e l d o b s e r v a t i o n s o f a f i l l on rnuskegr graphs of settlernent versus log tirne (in days) were essentially straight lines for the period following construction. He concluded that the settlernent was principally secondary over this period. The rate w a s v e r y u n i f o r r n , r a n g i n g f r o r n r . 2 t o 7 . 4 f . t p e r l o g c y c l e o f t i r n e . T h e rate and rnagnitude of settlernent were observed to be aknost identical for sections of the road with and without sand drains, so that the sand drains did not accelerate the rate of the settlerrlent to any great extent. It was concluded frorn this that the rate of consolidation of the peat was independent of the length of the drainage path. Inlater reports, however, Brawner (19S9, 1961) suggested that the consolidation of peat rnay be largely prirnary rather than secondary. Piezometers installed beneath a roadway fill on DEI rnuskeg (average water content of the peat = 1396 per cent, specific gravity = 1.61, initial void ratio = 24.8) indicated that
prirnary consolidation of peat extends well into what is norrnally considered to be the secondary phase of consolidation, with the transition taking
place at about ?5 days. Brawner considered that the rnagnitude of the settlernent varies as the thickness of the peat layer, and that the rate of settlernent varies as the square of the thickness. Subsequent investi-gations, however, necessitated a revision of these conclusions.
I 0
-Hillis and Brawner (1p61) pointed out that sorne of the rnisunderstanding
regarding the process of settlernent within a peat layer had been caused
by the fact that significant pore pressures had been observed in peat for
a long tirne after settlernent had becorne directly proportional to 1og tirne.
They expressed doubt as to the validity of these pore pressure rneasurernents
because of inadequacies of the piezorneters used; and noted that for both
field and laboratory observations the settlernent of rnost peats continues
as a straight line on a serrri-Iog plot after a relatively short tirne. The
initial process is cornpared to the usual prirnary consolidation associated
with inorganic clays where the rate of settlernent is a function of the decrease
in pore water pressure. For peat this is qualified, however, by heterogeneityo
the presence of gas in the peat, and the rernarkable decrease in perrneability
w i t h i n c r e a s e i n s t r e s s " A J t e r t h e p o r e p r e s s u r e s h a v e d i s s i p a t e d , a n d
probably for sorne tirne before, the secondary colrlpression phase of
settlernent occurs, The rate is independent of drainage conditions and
thus theoretically independent of the thickness of the peat layer. Hillis
and Brawner suggested that the prirnary phase of consolidation in the field
can be calculated frorn the laboratory void ratio-log pressure curve, and
recornrrr.ended that each load incrernent be perrnitted to rernain in the sarnple
for only about 30 rninutes. Long-terrn tests can be run to define the
secondary stage" The secondary consolidation is deterrnined frorn the
f o r r n u l a : S - C _ - _ " H . l o g t - / t , w h e r e C i s t h e c o e f f i c i e n t o f s e c o n d a r y
S C C S C C Z ' I S C C
l t
-H is the thickness of the layer (after prirnary settlernent is cornpleted),
t, the tir4e of prirnary settlernent, and tZ - tt the tirne over which it is
desired to calculate secondary settlernent. The total settlernent at a
given tirne can be obtained by adding the settlernents for the prirnary and
secondary phases. Hillis and Brawner stated that sand drains do not have
a beneficial effect in peats.
Lea (1958) pointed out that for peat soils, where the
coefficient of consolidation is not a constant but rnay vary as rnuch as
100 tirnes in the comrnon range of pressures, sorne rnodification rnust be
rnade to the theoretical analysis of the non-steady flow condition. He
suggested that the basic principles of larninar flow would be applicable
and that studies of prirnary consolidation of peat should be based on these
general principles rather than on the rnathernatics of the Terzaglni theory.
Lea considered that primary consolidation is a rnuch more irnportant factor
in peat consolidation than is generally believed. The rate of volurne change
of peat under load is affected by a variation of strength with tirne and a
gradual crushing of cells and slipping of fibres. Lea and Brawner (Lg59I
carried.'.this thinking a step further and advocated a fresh approach to the
analysis of peat consolidation: to assurne that consolidation is prirnary and
that Darcyts Law applies" They suggested that calculations of settlernent
in peat can be deterrnined by direct proportionality between laboratory
tests and field predictions. They used the square rule to estirnate the
t z
-rates of settlernent in a test section and laboratory tests.
I n a r n o r e r e c e n t p a p e r L e a a n d B r a w n e r ( 1 9 6 3 ) r e p o r t , however, that additional inforrnation does not support the view that the tirne to 100 per cent prirnary consolidation is proportional to the square of the thickness; rather than the exponential rrirt is closer to the power 1"5 than it is to ? as for the classical theory. They report that both Iabotatory and field tirne curves usually show the characteristic ttStr shape on serni-log paper and indicate that it is usually not difficult to establish 100 per cent prirnary consolidation. Secondary settlernent follows a straight line on a
serni-log plot of tirne settlernent. Records are assembled to show that laboratory tests give a reliable estirnate of rnagnitude of settlernent. The rnagnitude of secondary cornpression (t"".) was colrrputed frorn the forrnula a l r e a d y r e f e r r e d t o ( H i l l i s a n d B r a w n e r , 1 9 6 1 ) : t " " " = C " . . . H . 1 o g t Z / t l The rate of secondary cornpression was found to be rnuch greater in the field than in the laboratory by a factor of up to 5. Values ot C"u. were observed by the authors to depend to a substantial extent upon the load history of the deposit and the rnagnitude of the load. Lea and Brawner also point out that rebound can be quite a significant rnatter in the rernoval of surcr'-rge - frorn l5 to 35 per cent of the settlement upon full rernoval of load. They found also that, in general, the field rebound is about double the laboratory rebound. They suggest that this rnay be a result of elastic rebound plus gas expanding and corning out of solution.
1 3
-In their cornprehensive report on a study of vertical sand drains, Moran et al.(1958) considered in sorne detail the consolidation characteristics of organic soils, including peat. They indicated that the effects on consolidation of gas in the pore spaces (caused by decornposition of organic rnatter) are quite significant: (l) when load is applied, cornpression of the gas rnay cause a delay in the response of pore pressures to a change in boundary water pressures; (2) when
loading has been cornpleted, expansion of the gas will decrease the rate of consolidation. The effect of gas on tirne-settlernent curves is sirnilar, therefore, to the effect of secondary compression, and care must be exercised to distinguish between these two effects" In the authors! wiew, alrnost all of the prirnary consolidation under a particular load is cornpleted before an appreciable arnount of secondary cornpression takes place, so that practically no excess pore pressures exist during
secondary cornpression. The rnagnitude of secondary cornpression, cornpared to the rnagnitude of prirnary consolidation, varies with soil type, state of stress and ternperature and is rnaxirnurn for highly
organic soils. Relative proportions of the two phases of settlernent are said by the authors to be approxirnately independent of the rnagnitude of effective stress for stresses in excess of the preconsolidation value. The rnagnitude of settlernents resulting frorn secondary cornpression was calculated frorn the forrnula: 0"""" = Co.Hr' log ,r""/tn, which i s e s s e n t i a 1 l y t h e s a r n e a s t h a t u s e d b y @ ( l g o t } a n d
1 4
-and is equal to the amount of secondary compression per unit thickness of soil occurring in one cycle incrernent of tirne. The rate of secondary compression is independent of the perrneabitity of the soil and of the coefficient of consolidation as is the rate per unit thickness at which secondary compression occurs also independent of thickness. Laboratory consolidation tests to deterrnine the effect of preloading on second.ary compression indicated that although preloading rnay not cornpletely elirninate settlernents due to secondary corrrpression, they reduce thern to very srnall values.
Root (1958) considered that settlernent analyses of peat by the conventional rnethods developed for clays are not always reliable. He questioned whether prirnary consoLidation can be identified in the consolidation tests of peats. lt there is prirnary consolidation he believed that it takes place during the very early stage of the test and is cornpleted in I to 10 rninutes. The initial perrneability of peat before loading is relatively high but dirninishes rapidly as the peat is compressed. Root suggested that at some tirne subsequent to cornpletion of the initial rrprirnary consolidationtr the perrne ability of peat rnay
decrease to a point where t},e rate of consolidation is influenced. or controlled by the dissipation of pore pressure. In the field, the rate of settlernent greatly dirninishes a short tirne after loading of the fill is discontinued, thereafter following a straight line on a serni-log plot. Root reports that the slope of the line varies rnarkedly even between points where fill loads are cornparable and the rnoisture content and
I 5
-thickness of the peat layer appear to be the sarne. Root also pointed
out that the rate of settlernent is not proportional to the thickness of
the peat layer. Long-terrn consolidation, which is proportional to
log tirne, he considered due to possible plastic deforrnation or slow
structural failure of peat fibres. It was found that sand drains did
not accelerate settlernent in peat, but did effect an increase in its shear strength during construction.
Tressider (1958) pointed out that the first phase in the
consolidation process in peat is governed by the rate at which the water
can escape frorn and through it" This is predorninantly a hydrodynarnic
phase and the tirne of consolidation varies as the sguare of the length
of the drainage path. The tirne involved for this first stage varies, but
he noted that it is often very lengthy and rnay continue for rnany years
in deep peats. Tressider stated that the second phase continues at a
rate independent of the drainage process and thickness of the peat
layer" This occurs sirnultaneously with prirnary consolidation, but
continues long after prirnary consolidation has ceased. Laboratory
sarnples were observed to cornpress slowly for years under load. He
pointed out that the perrneability of peat decreases significantly during
the consolidation process; and also stated that there is very little
rebound in peat when consolidation is complete and the load removed.
Arrderson and Hernstock (1959) observed that Ior a test fill on FI rnuskeg (with peat ranging frorn non-woody fine fibrous to
r 6
-amorphous-granular) initial or prirnary consolidation takes place within a rnafter of days. This was indicated by the observed excess pressures being dissipated at an 8-ft depth in 3 to 4 days under a load of 0, Ztott/sqft (0. t9S kg/"g crn). Settlernent, in addition to this initial arnount, continued at a rate proportional to log tirne, as was confirrned by laboratory
consolidation tests. Anderson and Hernstock considered that this secondary compression is independent of drainage, but influenced by the soil type and by the state of stress. Settlernent for the first several weeks under the loading conditions noted above was 0.IIft per ft of depth per log
cycle of tirne. On the other hand, field observations of settlernent gauges under oil tanks on ABI rnuskeg indicated a rate of secondary consolidation o f 0 . 0 I ? f t p e r f t o f d . e p t h p e r l o g cycle of tirne, for a load of 0.15 tons/ftz
. ) ( 0 . 1 5 k g / c r n ' ) .
Zegarra (I959) reported on consolidation tests carried. out on a very fibrous tropical peat (water content range T19 - 795 per cent, specific gravity 2.r, organic rnatter ?7.8 per cent). He pointed out that unless the vegetal fibres were broken during loading, the peat would regain its original volurne after rernoval of cornpressive load if no
restraint were placed on the rebound" This was not too ewident frorn the e - log P curves; the discrepancy was attributed to side friction.
Settlement -- log tirne curves did not produce the usual straight-line portion (for tirnes up to 1000 rnin) but all the curves are slightly convex down for all load incrernents except the final one of 3.5 tons/ttz(E.+zt<g/crnzl
- 1 7 _
when the curve has a slight tenddncy to flatten.
Keene (1960) found that for Connecticut upland and tidal peats settlernent estirnates based on consolidation tests agreed quite well with field observations. Preconsolidation load obtained by the Casagrande rnethod gave close agreement with the caleulated overburden Ioad for norrnally consolidated peat, and settlernent calculations frorn the virgin cornpression portion of the e-1og P curve were confirrned by
observed settlements. Rates of consolidation presented a nrore difficult problern. Keene noted frorn laboratory and field observations that when prirnary consolidation is alrnost cornplete, secondary consolidation
proceeds as a straight line on a settlernent -- log.tirne graph. The slope of the secondary line gives a settlernent in one cycle of tirne of about 10 to 2O per cent of the prirnary settlernent. Keene pointed out that it is often difficult to deterrnine frorn the data at what point the prirnary settlement is about cornpleted and the secondary settlernent has begun.
Carrying out field and laboratory tests on Scottish peats (average water content = 800 per cent), @!s (1960, 196I) found that vertical sand drains did not effect a significant increase in rate of settlernent, although they did increase the rate of excess pofe water pressure dissipation. Laboratory tests also indicated that the rate of dissipation of excess pore water pressure was affected by the length of the drainage path, conforrning to the Terzaghi square law. He noted that excePt for the first incrernent of load, the rates of settlernent of the
I 8
-laboratory sarnples one rninute after loading were largely independent of the length of the drainage path. Lake observed that for both
srnall-scale field tests and laboratory tests the settlenrent -- log tirne curves were not linear - contrary to the observations of rnost other investigators.
F o r a 3 - i n . ( 7 . 6 z c r n ) d i a r n e t e r s a r n p l e , 0 . 5 i t t . ( 1 . 2 ? c r n ) thick, settlernents that took place after pore water pressures returned to norrnal varied frorn
39 per cent of the total after the first load incrernent to 8I per cent of the total after the final load increment.
Frorn his research on Japanese peats (water content range 300 to 1000 per cent; organic rnatter content range 50 to 95 per cent; dry d e n s i t y r a n g e 5 . 0 t o 1 5 . 5 l b s / c u f t ( 8 0 . 1 t o Z 4 8 . 3 k g l c u c r n ) , M i y a k a w a ( 1 9 6 0 ) found that frorn a fairly early stage settlernent becarne a straight line with relation to log tirne, both in laboratory tests and in the field. He suggested that the arnount of prirnary settlernent could be predicted frorn the ordinary consolidation test with a 24hour loading period, which contrasts with the 30 minute loading period recornrnended by Hillis an{B1qw4er (1961). Mayakawa calculated the rnagnitude of total settlernent frorn the equation:
6 = a * b log ttt/to), where ttarr is the settlernent of the prirnary phase, rtbrr is the rate of settlernent of the secondary phase for one cycle of log tirne, rrttr is the elapsed tirne, and rttott is the tirne for the prirnary phase. The coefficient of secondary consolidation, c", is equal tob/H, where ilI{il is the thickness of the cornpressible 1ayer. He found that C" increased with
1 9
-no direct relationship. Miyakawa also jrdicated that C" can be shown to vary directly with the rnoisture content of the peat. He doubted that sand drains would be effective in accelerating settlernent in peats except in the prirnary stage of consolidation.
Adarns (196r) conclud.ed frorn earry tests on woody, fine fibrous peats (water content range frorn 375 to 430 per cent; specific
g r a v i t y r a n g e f r o r n I . 6 2 to l. ?0; ash content range frorn IZ. Z to ZZ. 5 per cent) that the consolidation of peat is predorninantly due to the expulsion of water under excess hydrostatic pressure. A large proportion of the settlernent takes place very rapidly when the rnaterial is relatively pervious. with tirne the perrneability is reduced with a corresponding reduction in the rate of settlement. He reported that an increase in load resulted in a significant reduction in voids volurne and in perrneability, changing frorn approxirnately I x l o-r tt/rnin (5. r x r o-z
"^/sec) at zero road. to I x ' 1 0 - 6 t t / r n i n ( 5 . I x I O - 7 .o y ' " e c ) at Z.8J kg1lcrn}. Adarns considered that the consolidation occurring under excess pore pressure was extended to relatively long-terrn cornpression by this large red.uction in perrneability. He estineated that approxirnately 90 per cent of the consolidation was due to expulsion of water under excess hydrostatic pressure. rn a rnore recent paper, however, Adarns (19631 reported that for peat sarnples 8 i n . ( 2 0 . 3 2 c r n ) i n diarneter and 3.g in. (9"65 crn) thickthere was an initial settlernent of relatively large rnagnitude which occurred in a very short period of time (5 rnin ). The long-terrn settrernent occurred
20
-relatively slowly and was linear with tog time. Excess pore pressures were dissipated during the initial consolidation period, although a residual excess pressure of low rnagnitude rernained and showed only a slight reduction with tirne. On the basis of these laboratory tests and the results of field instrumentation, Adarns has concluded that the
consolidation of peat occurs in two separate stages: an initial or irnrnediate stage and a long-terrn stage, which continues indefiniteLy at a slow rate. Available data indicate that the rnagnitude of the initial settlernent and the rate of long-terrn consolidation bear a predictable relationship to the applied load and the thickness of the peat. Adams suggests further that the initial consolidation is due to the expulsion of the pore water in the peat rnass, and that the long-terrn consolidation is due to the expulsion of the water in the solid rnatter. This is essentially the same conclusion a s t h a t r e a c h e d b y E v g e n b v ( 1 9 6 1 ) .
Barber (tg6t), in an effort to deterrnine the cause of secondary consolidation, carried out a rather extensive series of
laboratory tests on various organic materials, including peat, using ?-in. ( 5 . 0 8 c r n ) d i a r n e t e r , 0.5-in. (I.27 crn) thick sarnples. Tests on desiccated, air dry and wet rnaterials indicated that the seat of secondary consolidation is in the hygroscopic rnoisture. Barber noted that for a specified loading incrernent for a peat sarnple, the typical S-shaped prirnary consolidation phase was essentially cornplete in 0.1 day. Linear extension of the
. z I
-the consolidation at I00 days. Barber suggested that secondary
consolidation rnay also produce an s-shaped curve sornewhat like that of pri:nary consolidation, but extending over rnore log cycles of tirne. He showed that an increase in load tends to accelerate the rate of
sepondary consolidation and to reduce the magnitude of the long-terrn settlernent.
Evgenlev (196i) concluded frorn an analysis of nurnerous consolidation tests that deforrnation of peat under load consists basically of two phases: (I) deforrnation due to squeezing out of water frorn the open macropores and (Z) deforrnation resulting frorn the squeezing out of water frorn the closed pores. In the first phase, the rnovement of water follows Darcyts Law and compression proceeds very quickly even und.er srnall loads. The arnount of consolidation varies considerably, depending uPon the loa<iing history of the peat and the disturbance of the
sarnple. Evgentev found that deforrnation in this phase arnounted to 75to !0 per cent of the total settlernent. The arnount of deforrnation in the second phase is believed to depend basically upon the degree of decornposition of the peat and varies within rather narrow lirnits for a given type. Evgenlev presented tabulated data for the arnount of deforrnation to be expected in the second phase for various peat types and loads.
He suggested that for calculations not requiring great accuracy the
second phase can be ignored and the settlernent of the first deterrnined by a relatively rapid rnethod involving application of load incrernents every
z z
-24 hours rather than at the end of consolidation. Evgenlev observed that when the load was rernoved frorn peat sarnples they rebounded alrnost exactly tp the deforrnation stage of the beginning of the second phase, frorn which he concluded that the second phase is cornpletely elastic deforrnation.
!!!g\}gbg53ggb. (1961), in describing the perforrnance of ernbankrnents constructed on rnuskeg, stated that the rnagnitudes of prirnary settlernent were found to be sornewhat less than had been predicted on the basis of laboratory consolidation tests. He reported that the trend on the tirne-settlernent curve for field conditions indicated that for the rnuskeg under consideration (depths up to 8ft (Z.44rn)) ttre rnajor portion of the secondary colrrpression would take place in the first 3 or 4 years after cornpletion of the ernbankrnent.
Rip1ev and Leonoff (I96I) reported significant differences between the observed behaviour of peat and the predicted behaviour
cornputed by conventional theoretical analyses based on field and laboratory test results (arnorphous granular peat with woody fine fibres; average water content = 1000 per cent). The data frorn long-terrn laboratory consolidation tests (both one dirnensional and triaxial) indicated that long-terrn settlernents occurring during the period norrnally attributed to secondary cornpression rnay arnount to 50 per cent of the total
settlernent. Their field settlernent rneasurernents revealed a high rate of continuing long-terrn settlernents. Settlernents of raft sections of
2 3
-an ernb-ankrnent where no fill load had been placed for 3 -l/Z years were observed to be continuing on a straight line relationship at a relatively steep slope.
Goodrnan and Lee (1962) considered that the classical consolidation theory predicts the rnagnitude of expected settlernents of peat rather well, although the rate of consolidation in theory and practice do not agree. Their conclusions were based on experience with peaty soils in the Syracuse, New York area (water content range 282 to 403 per c e n t ; i n i t i a l v o i d r a t i o r a r l g e 5 . 7 3 t o 5 . 4 8 ; s p e c i f i c g r a v i t y l . 5 l to l.62; organic content 42.5 to 75.2 per cent). In contrast with rnost other investigators, Goodrnan and Lee believed that relatively rninor effects would result frorn secondary corrrpression, since their observations indicated that most of the settlernent of structures on peat appeared to have taken place shortly after construction.
schroeder and wilson (1962) carried out an extensive series of consolidation tests on partly disturbed peat sarnples with initial void ratios varying frorn 40 to 10. The initial heights of the specimens r a n g e d f r o r n 0 . 5 i n . t o 2 5 i n . ( 1 . 2 T to 63.5 crn), and were consolidated u n d e r l o a d s o f 0 . 5 to 20 psi (0.035 to r.406xg/.rnzl. T h e y o b s e r v e d . that the settlement -- Iog tirne curves for peat had characteristics
sirnilar to clay consolidation curves for the first incrernent only. For succeeding load incrernents the tirne graphs were either straight lines or slightly curved downwards at the beginning. Points of zero pore
2 4
-pressure occurred at various positions on the straight-line portion of the tirne-settlernent curves owing to differences in drainage paths, but were observed to have no effect on the cuaves thernselves. Schroeder and Wilson concluded frorn their test results that gecondary consolidation and, consequently, plastic or viscous tirne relationships are rnore
irnportant for peats than for clays, at least for a load incrernent ratio of unity. A second series of tests was carried out using sarnples taken frorn one large undisturbed specirnen and all sarnples having the same drainage path. These sarnples were exarnined and analysed on the basis of rheological principles. From this they concluded that
consolidation characteristics of peat are governed by: (I) variation in the rnaterial; (21 drainage conditions; (3) load incrernent ratios; (4) range of void ratios; (5) stress and strain history; and (6) viscous or plastic resistance to flow. In a more recent report, l4rilson (1963) described a s e r i e s o f l a r g e - s c a l e c o n s o l i d a t i o n t e s t s ( 7 5 . * ( 9 . 8 i n . ) d i a r n e t e r b y 75 crn(29.5 in,) high) carried out on disturbed arrorphous granular peat to rneasure the pore water pressures during consolidation. These tests showed that the pore water pressure, irnmediately after loading was considerably less than the theoretical value, and that it then increased to a value slightly less than the theoretical value during the early part
of the test. Wilson cornpared. the theoretical and experirnental U/T curve and reported that the plastic structural deforrnations took place at a rapid
rate in the early stages of consolidation as a result of a decrease in Pore water pressure with consequent increase in effective strees.
_ 2 5 _
Tests conducted with identical loads on sarnples of different heights showed that the tirnesfor the dissipation of excess Pore water pressures conforrned to the Terzaghi theory relatjng length of drainage path to tirne. Due to structgral deforrnations, however, the rate of settlernent or change in void ratio did not conJorrn to the theory.
Wahls (I962 ) reported on an experimental study of the prirnary and secondary consolidation characteristics of a calcareous organic silt, the results of which rnay be applied generally to sorne peats. Both undisturbed and rernoulded sarnples wele used. Frorn his experirnental results he concluded that the coefficient of secondary
compression C^, is dependent on the void ratio (and, consequently, on the total pressure) and independent of the magnitude of the pressure incrernent and pressure-incrernent ratio. The coefficient of secondary cornpression, Co, represents the rnaxirnurn rate, with respect to log tirne, at which secondary compression occurs during a pressure
incrernent. During prirnary consolidation, secondary colnpression is .' occurring at a rate tlnat is not linear with respect to the logarithrn of tirne. The tirne required for the rate of secondary colnpression to approach Co is directly related to the tirne required for cornpletion of prirnary consolidation.
As the pressure-incrernent ratio is reduced, the shape of the corrlpression-logarithrn of tirne curve differs rnore radically from the theoretical curve for prirnary consolidation.
2 6
-For sufficiently srnall pressure-incrernent ratios (np/po < t/! tne tirne at which prirnary consolidation is completed cannot be found by the conventional Casagrande rnethod. In such instances, the rna-:<irnurn
slope of the curve during prirnary consolidation is less than the final secondary slope, and thus there is no inflection point on the curve. As the pressure-increment ratio is reduced, the tirne required for the cornpletion of prirnary consolidation, as rneasured by the coefficient of consolidation C.r, increases. 'Wahls concluded, therefore, that: (a) investigations of secondary compression effects are sirnplified
if srnall pressure-incrernent ratios are used, and
( b ) the use of laboratory tests with Ae/eo = 1.0 in the analysis of practical problerns for which the field pressure-incrernent ratio is rnuch less than one rnay lead to unconservative
conclusions. In such cases the secondary effects will have greater significance in the field than in the laboratory.
As the observed behaviour of a soil exhibiting considerable secondary colrrpression cannot be explained by classical consolidation theory, W'ahls proposed a new rnathernatical rnodel for the consolidation process. This rnodel is used to explain the effect of pressure-incrernent ratio and to analyse tirne-cornpression curves when secondary cotrrpression predorninate s.
SUMMARY
( 2 1
? 7
-be drawn on which there is fairly general (although not necessarily universal) agreernent. These are as follows:
( l ) In general, field and laboratory tirne-settlernent curves for peat on a serni-log plot show initially the characteristic |tSrt shaped curve, although it is not as clearly pronounced as for inorganic soirs. rt is usually diJficult to define any crear dernarcation between the end of the prirnary consolidation phase and the beginning of the secondary colnpression phase. In the laboratory prirnary consolidation occurs very rapidly, generarly within a few rninutes, and rnay constitute as little as 50 per cent of the total settlernent.
The rnagnitude of settlernent in the prirnary consoridation
phase varies directly as the thickness of the peat; conseqluently, this phase of field settlernent can be predicted frorn laboratory t e s t s .
The tirne rate of the prirnary consolidation phase of peat
compression is also proportional to the thickness. It is variously reported to be directly proportional to the thickness, proportional to the thiclqress to a power of 1.5, and proportional to the thick-ness to a power of 2. This variation rnay be a function of the peat type; further investigation of this possibility is required. (4)
( 3 )
cornplete consolidation of peat is reached only after a 1ong period of tirne several rnonths even for srnall specirnens -( 5 )
( 6 )
( 8 ) ( 7 )
z B
-due to secondary effects.
Secondary corrrpression is considered to be of a viscolls or plastic nature, its rnagnitude affected by ternperature, the nature of the peat and the state of stress. It is independent of drainage conditions. Secondary cornpression is unusually high for peats and rnay account for as rnuch as one half of the total settlernent. The order of rnagnitude of field settle-rnents due to secondary compression can be calculated frorn long -terrn consolidation tests.
Secondary colnpression follows a straight line on a serni-1og plot of tirne-settlernent for both laboratory and field curves. The slope of this line, the coefficient of secondary cornpres-sion, depends upon the load history of the peat and is influenced by the rnagnitude of the applied load"
The coefficient of perrneability of norrnally consolidated peat is initially very high, but it dirninishes rapidly as the peat is cornpressed and is dependent upon the rnagnitude and duration of the load. The coefficient of perrneability is
greater in the horizontal plane than in the vertical plane, with an anisotropy ratio of about 3 to 6.
If the load is fully rernoved during or following the secondary corrpression phase, the rebound rnay be a very significant f a c t o r t o c o n s i d e r .
( l 0 )
_ zg_
Gas (caused by decornposition of organic rnatter) in the pores rnay be a factor co'tributing to the difficulty in interpreting tirne -settlerne nt characteristics for peat.
( l l ) Vertical sand drains do not they rnay effect an increase during construction.
accelerate settlernent of peati in the shear strength, however,
3 0
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-3e*
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_ 3 3 _
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_ 3 4 _
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