CONSOLIDATION OF PEAT SOILS

N. N. Morareskul and V. N. Bronin UDC 624.131.374- 78 : 624.131.276

Expansion of construction in swampy regions of the USSR has increased interest in problems of con- solidation of peat soils. At the present time, however, there is no single opinion concerning the nature of deformation in peat with time. Some authors [1-5] have noted inconsistencies in the views of filtrational consolidation with their investigations, and some [6-7] reject the possibility of using this concept for peat soils.

In this connection there is in te res t in the labora tory investigations ca r r i ed out at the Leningrad Con- s t ruct ion Engineer ing Institute to examine the ro le of pore wa t e r and of the soil f ramework in consolidation of peat and to obtain standard cha rac t e r i s t i c s neces sa ry in checking the f i l t ra t ion theory.

Lowland sedge-sphagnum peat was studied: of undisturbed s t ructure , density of 1.63 g / c m 3 degree of decomposi t ion of 20%, ash content of 5-7%, natural mois ture content of 800-1200~, and a porosi ty of 16- 21%.

The permeabi l i ty of peat samples 3 m high to wate r was de termined in a f i l t r a t ion-compress ion de- vice, designed at the S. Ya. Zhuk All-Union Design, Investigation, and Sc ien t i f ic -Research Institute, fo r con- solidation p r e s s u r e s of 0, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 k g / c m 2 and with two reg imes of applying hydraulic gradient: low and high (0-900). The normal method was used in the f i r s t case [8]. In the second, a special f i l t r a t ion -compress ion device was developed.

Determinat ion of permeabi l i ty at high gradients has been hampered by the difficulty of using ordin- a ry methods at p r e s s u r e s g r e a t e r than 1 k g / c m 2 and by the tendency of the gradient applied to the sample to approach the p r e s s u r e gradient ar is ing in the peat samples during their consolidation in the laboratory. Thus, with a consolidation p r e s s u r e of 2 k g / c m 2 and an initial pore p r e s su re of/3 o = p w / p = 0 . 6 at the center of a sample 3 cm high, the hydraulic gradient that a r i s e s is equal to

j _ ~ 0 p 0 , 6 . 2 . 1 0 a - - = 800. 0.5 ~wh0 0 . 5 . 1 . 3

The re la t ion between f i l t ra t ion ra te and p r e s s u r e gradient proved to be l inear over a considerable zone (Fig. 1), except fo r the init ial segment. The permeabi l i ty obtained during the f i r s t r eg ime of tests there fore proved to be lower than during the second, the deviation increasing with increase in consolidation load (Fig. 2).

Statistical treatment of the experimental results has shown great variation in the filtrational proper- ties of peat. The average value of the coefficient of variation for the coefficient of permeability was 59%.

The pore pressure during consolidation of peat samples of undisturbed structure by different loads was measured by means of hydro-aerostatic manometers by the method of [9]. Averaged curves of pore- pressure change with time and curves of the initial pore pressure ~0 and of time of conditional dissipation of pore pressure tcd versus load are shown in Figs. 3 and 4. For ted we use the time interval during which the pore pressure dissipates to a value amounting to 5% of the applied loading step.

Compression tests were carried out in an open scheme with water-saturated samples of peat of in- tact structure, with heights of 2, 3, and 5 cm, in consolidation presses (oedometers) designed at the S. ¥a. Zhuk All-Union Design, Investigation, and Scientific-Research Institute, having a piston area of 39.91 cm 2. Consolidation pressures were 0.I, 0.4, 0.8, and 1.6 kg/em 2.

Leningrad Construction Engineering Institute, Leningrad. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. i, pp. 31-33, January-February, 1974.

© ]974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 t e s t /7th Street, ~ke~ York, .\'. Y. IOOt]. No part of E~is publication may be reproduced, stored in a retrieval system, or transmitted, in any form or b) any means, electronic, mechanical, p[~otocopying, microfilming, recording or otherwise, without written permission of tl~e publisher. /! cop)" of this article is available from the publisher for $15,00.

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q,m/day

10

Z2

i

/

! ,-/ 4~

L 4

20~ 400 ~r

k, m/day

0,O6

t

O, Oz .11

o o,, ¢8 1,z l,; 40

Fig. 1 Fig. 2

I

G4P, kg/cm2

Fig. i. Relation of filtration rate to pressure gradient at different loads, i) At p= 0, g =20.9; 2) atp=0.2 kg/cm 2, g =15.71; 3) at p=0.4 kg/cm 2, e =1.16; 4) at p=0.8 kg/em 2, g =8.59; 5) at p=l.6 kg/em 2, g =5.89; 6) at p=3.2 kg/cm 2, g =4.09.

Fig. 2. Relation of coefficient of permeability to load. i) At high pressure gradient; 2) at low pressure gradient.

o,8

/,~._.4/I L

icd'

~o .,

÷0

g,8 I,Z p, kg/cm 2

rain

0 o,~ 0,4 o,6

pw, 0'8

kg/cm 2

~8 • ~ ..... i._~:

lO N z ~ N+ •5 N e t, sec

Fig. 3 Fig. 4

Fig. 3. Relations of load. I) Coefficient of initial pore pressure; 2) time of conditional dissipa- tion of pore pressure, tcd.

Fig. 4. Change of relative strains and pore pressure with time during consolidation of peat sam- ples with height h0=3 cm and a pressure of: I) 1.6; II) 0.8; III) 0.4; IV) 0.i kg/em 2. I) Experi- mental curves of relative strains; 2 and 3) curves computed from filtration theory at maximum Cvmax and minimum C%,mi n values of consolidation coefficient, respectively.

56

F

~o

i//31 !

iO ~ iO ~ iO ÷ IO s t, sec

Fig. 5. Change with time of relative de- formation of peat samples of different heights at the following loads: I) i. 6, If) 0.8, iII) 0.4, and IV) 0.i kg/em 2. The short-dash, long-short-dash, and long- dash lines represent experimental values of deformation in samples 2, 3, and 5 cm high respectively; 1-3) calculated values of deformation in samples 2, 3, and 5 em high respectively.

For each sample height, at least three samples were investigated for all stages of loading; with increasing diver- gence the number of samples tested increased. The tests lasted for 2-3 days for samples 2-cm and 5-cm high and for 120 days for samples 3-cm high. Since the effect of sample size on time of their deformation, according to filtration theory, is manifested only during the period of compression of pore water, duration of tests of samples of different ~heights may be restricted to this period. In Figs. 4 and 5 we have shown on semilog paper the relative strains in peat of dif- ferent heights and at different lengths of testing.

In investigating the structural strength of peat, ten stages of loading were applied successively to the sarnples, of 0.03 kg/em 2 each, then two of 0.i kg/cm 2, and a last stage of 0.5 kg/cm 2. A break was observed on the compression curve at a pressure of 0.183 kg/cm 2, attesting to structural strength in the investigated peat. It should be noted that even at pressures less than the structural strength the peat sam- ples had considerable deformation, but the inflection point on the compression curve separated only zones of deforma- tion differing in intensity.

On the basis of the experimentally obtained characteristics of permeability and compressibility of peat, computations were made on deformation of peat samples with time according to filtration theory.*

In order to determine the possible application of the theory of nonlinear filtrational consolidation to peat, deformation was plotted for all experimental curves for two computed values of the consolidation coef- ficient: maximum Cvmax and minimum Cvmin. For the calculations we used tl~ coefficient of permeability computed at pressure gradients equal to those that arose during consolidation of the peat. The coefficients of relative compressibility were determined from the results of 120-day compression tests on samples 3 cm high.

In these calcalations we used the final settlement caused by each stage of loading, taken directly from the experiment. In Fig. 5 we have shown a curve for the change with time of relative strain in peat samples of different heights, obtained experimentally and by calculation according to the maximum value of the con- solidation coefficient Cvmax.

The effect of changes in permeability and compressibility on the deformation of peat, according to fil- tration theory-, may be judged from the computed curves 2 and 3 in Fig. 4.

The experiments and calculations have shown that the deformation of peat with time is affected both by the force of filtration resistance and by creep of the framework, the role of which varies according to the value and rate of applied loading. At small loads (up to 0.2 kg/cm 2) a decisive role is played by the rheological properties of the framework, and here it is advisable to use "pure" theories of creep. At large loads, it is necessary to use in the calculations theories that take into account both filtration resistance and creep of the peat framework.

I°

2.

3.

LITERATURE CITED

N. Ya. Denisov, "Principles of effective stresses and strength of clay soils," Osnovaniya, Fundamenty i Mekhan. Gruntov, No. 2 (1963). D. Schroeder and N. E. Wilson, "Analysis of secondary consolidation of peat," Canada, Nat. Res. Council, Assoc. Committee on Snow and Soil Mechanics, Tech. Memo 74, pp. 130-144, October (1962). J. R. Lake, Pore-Pressure and Settlement Measurements during Small-Scale and Laboratory Experi- ments to Determine the Effectiveness ofiVertical Sand Drains in Peat, Road Research I~boratory, Department of Scientific and Industrial Research.

* The following students of the Leningrad Construction Engineering Institute participated in the calculations: V. Purtova, L. Izboishskaya, L. Myastenkova, L. Stolyarova, and S. Dorofeeva.

57

4o

5.

6.

7.

8.

9.

L. Casagrande, Erfahrungen beim Bau yon Strassend~mmen anweichen Bodenschicten in den U° S. Ao, Strasse u Autobahn, 16, No. 1, 1-8 (1965). G. I. Pokrovskii, '~Physical premises in calculating the time scale in soil deformation," Tekhniche- skaya Fizika, No. 15 (1938). A. A. Tkachet~ko, "Application of the theory of fittrational consolidation to peat," Osnovaniya, Funda- menty i Mekhan. Gruntov, No. 2 (1963). N. E. Wilson, "Consolidation flow character is t ics of Peat," Canada, Nat. Res. Council, Assoc. Com- mittee on Snow and Soil Mechanics, Tech. Memo, Ottawa (1962). L. S. Amaryan, Methods of Determining Propert ies of Peat Soils [in Russian], Kalinin PolyteehnieaI Institute (Kalininskii Politekhnicheskii Institut) (1970). N. P. Kovalenko, V. D. Sokolov, and Ya. Yu. Marko, Study of Changes in Pore Pressure during Con- solidation of Water-Saturated Soil [in Russian], Trudy ALTIg. Argangel' ska.

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