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Home , Pore water pressure, Soil compaction, Water content

INTERNATIONAL SOCIETY FOR MECHANICS AND

This paper was downloaded from the Online Library of the International Society for and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library

This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. XIIIICSM FE, 1994. New Delhi. India / XIII CIMSTF, 1994. New Delhi. Inde S r i ? BUI THE DEVELOPMENT OF PORE- IN A COMPACTED SOIL L’AUGMENTATION DE LA PRESSION DE L’EAU INTERSTITIELLE DANS UN SOL COMPACTE

Amaro Henrique Pessoa Uns1 Sandro Salvador Sandroni2

'Associate Professor, Federal University of Pernambuco, Recife, Brazil 2Director, GEOPROJETOS, Rio de Janeiro, Brazil

SYNOPSIS

This paper deals with the development of positive pore-water pressure in an unsaturated compacted soil submitted to isotropic compression at constant . Results of two series of tests, with of the pore air and with no air drainage, are used to obtain correlations between the conditions and the air content and confining all round pressure when the pore-water pressure becomes positive. This confining pressure is denominated "Positivation Pressure".

INTRODUCTION development of positive pore-water pressure during constant water content tests is investigated. This pressure will be defined as "positivation pressure". Results of tests with free The air in the pores of an unsaturated soil may be present in drainage of the pore air and tests with no air drainage are two different forms : Firstly, the air may exist in continous presented. passages through the soil, in which case it is generally at atmospheric pressure (OPEN STATE); Secondly, the pore air may exist in isolated pockets in the fill, and both the pore air THEORETICAL MODEL and pore water may be positive (CLOSED STATE).

Constructional pore pressures during construction of earth A simple model for representing the development of pore-air embankments can seriously affect stability during and and pore-water pressure (U. and Uw) during compression at immediately after construction. In most theoretical methods constant water content tests is shown schematically in Fig. 1. for predicting the development of pore-water pressure during Let us consider that, at the beginning of the test, the air in construction of earth embankments it is assumed that the the pores is interconected and subjected to atmospheric initial pore-water pressure and pore-air pressure are equal pressure. As the sample compresses the air escapes from the to atmospheric pressure. Moreover, it is supposed that the voids and saturation increases, causing a reduction in increment in the pore-air and pore-water pressures due the capillarity tension and a consequent increase in pore-water increment in total pressure are equal. pressure. When occlusion occurs, any further compression results in an increase in pore-air pressure according to The water content at which the is constructed Boyles's law. The pore-water pressure may be positive or has the largest influence on the magnitude of the pore negative, depending on the relative magnitud of capillarity pressures which develops. A high earth constructed of tension and pore air pressure, according to capillarity almost any type of soil with an average water content near equation, Standard Proctor Optimum water content will develop construction pore water pressures. However, in a number of large constructed with low water contents, the influence U. - U , = 2 i/r (1) of surface tension was strong enough to produce negative construction pore-water pressure in the impervious section (Sherard et al. ,1967). x - Surface tension r - Curvature radius.

Negative pore-water pressure in compacted earthfill during Fig.l shows that with increasing total stress a point is construction do not represent any danger to the slope reached where the increase in U. and Uw become essentially stability. On the other hand, positive pore-water pressure equal. As saturation is approached, the increase in U. and U„ constitute an instability factor and its effect on the stability tend to be equal to the increment in applied total s tr e s s . analysis must be considered. So, it would be of great importance to the geotechnical engineer to establish the conditions at which positive pore-water pressure are DESCRIPTION OF APPARATUS expected to occur.

In this paper, the correlation between the soil compaction A double walled triaxial cell, similar to one developed by conditions and the confining pressure required to cause the Wheeler (1986), was used to perform the tests. A schematic

177 SOIL PROPERTIES AND SAMPLE PREPARATION

The tests reported here were conducted on samples of a residual soil from Rio de Janeiro- Brazil. The sample has a fraction of 62%, 4% of and 34% of , LL=78%, PL=32%. The Proctor compaction curve is shown in Fig. 3 .a . The degree of aeration, defined as the ratio between the volume of air in the voids to the total sample volum e, is plotted against water content, corresponding to the compaction curve, in Fig.3.b.

Partly saturated compacted samples were prepared by sta tic compaction into a steel mould 5.08 cm diameter, in six layers Fi g. 1. Theor et i cal model ( f r om Sandr oni and Bar bosa da of 1.67 cm of thickness each. A thin layer of silicon e grea se Sil va, 1989) was used to reduce the between the mould and sample. After compaction, samples were left at rest for 15 days, sealed in plastic bags, at constant moisture and constant temperature conditions. Four series of sam ples w ere prepared at +2.4%, -0.3%,-3.5% and -4.5% of optimum w ater content, with dry densities corresponding to the Proctor compaction curve. It was intended to obtain samples containing air in the open and closed states. Details of the compaction conditions of samples are given in Table 1.

1. BURETTE 4. VALVE 2. AIR-W ATER INTERFACE 5. COMPRESSED AIR SYSTEM 3. NITROGEN BOTTLE 6. PRESSURE TRANSDUCER WATER CONTENT (%) F ig . 2. Lay out of equi pment lay out of the triaxial apparatus is ilustrated in Fig. 2. The confining pressures into the inner and outer cells are applied simultaneously by the same pressure source, in such a way that no volume change of inner cell occurs. Small correction s for volume changes in the inner cell are necessary to account for the compression of the perspex cylinder and water in the chamber, and the water absorption by perspex. Calibrations tests showed a maximum volume change in the inner cell o f 1.5 cm3 and a rate of absorption of 0.045 cm3/h when a p r e ssu re of 1200 kPa was applied. Measurement of overall volume changes of sample was made recording the flow of water into or out the inner cell. A twin tube burette with paraffin/water interface was used to measure the volume changes. The p o re- water pressure was measured at the base of the sample u sin g WATER CONTENT (%) eletric pressure transducers. A fine grained porous d isc having an air-entry value about 500 kPa was sealed into the Fi g. 3. (a) Compact i on cur ve, ( b) Degr ee of aer at i on- wat er specimen pedestal with an epoxi resin. In the tests w ith free cont ent drainage of the air, an air-pressure was applied to the top of the sample. A thin layer of a sinthetic membrane ("G oltex") was used as an interface between sample and the air lin e, to TESTING PROCEDURE avoid leaks of water from sample if the water-pressu re into the sample equaled applied air-pressure. The sinthetic membrane proved to be impermeable when submitted to w ater Two series of isotropic constant water content tests w ere pressures up to 400 kPa, for periods of 2 hours. performed in the triaxial cell. In the first series , no air drainage was allowed during compression (PH tests); in the The isotropic compression tests were carried out on 5 .0 8 cm second one, the air was free to drain throu'gh the air line diameter by 10 cm high samples, sealed in a standard la ttex conected to the top of sample, provided the air was rubber membrane. Ambient pressure was applied to th e cell interconected in the void spaces (PHO tests). water using a nitrogen bottle conected to an air-oil-w a ter interface. The pressure was adjusted periodically to keep the Before assembling the cell, the pore-water pressure fluctuations below ± 5 kPa. measuring system was checked for full saturation of porous

178 Table 1. Compaction details PH TESTS Test No. Wc S e (Va/Vt) PHO TESTS "-"op u.a (%) (KN/m 3) (%) 16

PH5-1 27.5 14.47 • 0.85 0.89 7.3 PH5-2 27.5 14.49 0.85 0.89 7 .3 PH 05-1 27.5 14.50 0.85 0.89 7 .2 PH3-1 30.2 13.88 0.85 0.97 7 .6 PH3-2 30.2 13.89 0.84 0.97 7 .6 200 400 600 800 1000 1200 14Ò0 PH3-3 30.2 13.88 0.84 0.97 7 .7 CONFINING PRESSURE (kPa) PH 03-2 30.2 13.89 0.82 0.97 8.4 25.4 14.21 0.75 0.93 12.1 PH2-1 PH TESTS PH2-2 25.4 14.22 0.75 0.92 12.1 PH 02-1 25.4 14.25 0.75 0.93 12.0 PH 02-2 25.4 14.21 0.75 0.93 12.1 PH6-1 24.3 14.11 0.71 0.94 14.2 PH6-2 24.3 14.12 0.71 0.94 14.1 PH 06-1 24.3 14.13 0.71 0.94 14.1 PH 06-2 24.3 14.12 0.71 0.94 14.1 PH4-1 23.3 13.84 0.65 0.98 17.3 PH4-2 23.3 13.83 0.65 0.98 17.4 200 400 600 800 1000 1200 1400 PH 04-1 23.3 13.89 0.65 0.98 17.2 PH 04-2 23.2 13.90 0.65 0.97 16.9 CONFINING PRESSURE (kPo) (a) stone and the time response under applied water pre ssu re % was measured. It was observed that time responses of less than 1 min. for 98% of equilibrium were reached. Having checked the system, the cell was drained, the sample was put Ì on the pedestal and sealed in j rubber membrane. The ä assemblage of the cell was made submerged to avoid o entrapment of air bubbles. u

In PH tests, the confining pressure was applied in st e p s , and readings of volume and pore-water pressure were tak en , at regular intervals, until equilibrium was obtained. In PHO CONFINING PRESSURE (kPo) tests, the measurement of residual pore-water pressu re was made before the loading stage. The axis translation tech niq ue (Hilf,1956) was used in order to bring the initial n egative ------PH TESTS pore-water pressure to the positive range. The cell p r essu re ------PHO TESTS and air pressure inside the sample were applied simultaneously by a compressed air system. The differen ces between cell pressure and air pressure was maintained below 5 kPa. For the rest of the test, the air pressure was kept constant and cell pressure was increased in steps. R eadings of volume and pore-water pressure were taken for period s of 15 m in.. At a maximum confining pressure about 1200 kPa the ■I------cell pressure was decreased in steps to 10 kPa. 600 800 1000 1200 1400 CONFINING PRESSURE (kPo) (b ) TEST RESULTS Fig. 4. Results of PH and PHO tests Figs. 4.a and b show the results of two PH and two PHO te s ts in two samples compacted at optimum and dry of optimum. The most striking feature of these data are that PH and PHO te s ts Fig. 6 presents "positivation pressure" plotted against on samples at optimum gave similar results, while PHO te s ts degree of aeration of samples as compacted . The degree of in dry samples presented larger volume changes and lower aeration seems to be a better index to be related to pore-water pressure than in PH tests. This is due to the "positivation pressure" once it represents the influence of effect of air drainage in the dry samples containing free air both water content and dry density. It is apparent the in the void spaces. In samples compacted at optimum the air increase of "positivation pressure" with incresing d egree of was occluded and air drainage was unlikely to have o ccu rred . a era tio n .

Typical results of U„ versus confining pressure for PH and Figs. 7.a and b show the path followed by the degree of PHO tests are shown in Figs. 5.a and b. The curves o f PH aeration in PH and PHO tests. It is remarkable that all tests show that cavitation must have occurred in the pore- samples reached the "positivation" at an unique valu e, water pressure measuring system in the initial loading independent of initial degree of aeration. The degree of stages. The use of axis translation technique in PHO te s ts aeration at "positivation" in PH tests is near the degree of enabled the measurement of actual pore-water pressu re s for aeration at the point of optimum of the compaction cu rv e, as the complete range of confining pressure. An increase may be can be seen in Fig. 3.b. Additional tests should be done, observed, in both series of tests, of "positivation p ressu re" usiQg different and stress systems to check if similar with decreasing water content. results would be obtained.

179 600- O AT BEGINING OF TEST • AT POSITIVATION „ 400- O 1 5 - - o O Ï 200- ¡5 K O uj < 10+- ( a ) W o p - 4 . 5 %

4 - UJ - , Ï oc 5 - - 200 400 600 800 1000 1200 1400 O CONFINING PRESSURE (kPa) g OC 800- UJ h- 22 24 26 28 30 32 I COMPACTION WATER CONTENT (%) I 20 LU CE o O AT BEGINING OF TEST CL Z • AT POSITIVATION 0 1 5 O I O Ld 10 200 400 600 800 1000 1200 1400 < (b ) CONFINING PRESSURE (kPa) fe O Fig. 5. Pore-water pressure-Confining pressure (a)P H , (b ) CC 5 P H O te s ts o i a

22 24 26 28 30 32 COMPACTION WATER CONTENT (%)

Fig. 7. Path followed by degree of aeration in (a) P H a n d (b ) P H O te sts

ACKNOWLEDGEMENTS

The research described in this paper was carried out in the Laboratory of Soil Mechanics of the Department of Engineering Science, of the University of Oxford, headed b y DEGREE OF AERATION (% ) Dr. Gilliane C. Sills. The former author was supported b y CAPES and Federal University of Pernambuco, Brazil. Fig. 6. Positivation pressure - degree of aeration

REFERENCES

The degree of aeration at "positivation" in PHO tests is lower Bishop, A.W. (1969). Pore Pressure measurements in the than in PH tests. It means that conventional tests w ith no air field and in the laboratory. Proc. 7th ICOSMFE, drainage lead to higher estimates of pore-water pre ssu re s Vol.3,pp.427-444. than do the tests where air drainage is allowed. It has a Hilf,J.W. (1956). An investigation of pore water pr e ssu r e in practical importance on the estimates of pore-water p r e ssu re compacted cohesive soils. Tec. Mem. 654 ,U .S.B .R .,U SA . during construction of embankments compacted in the d ry Lins,A.H .P.(1991). Strength and Pore Pressure develop ed side of compaction curve, where the air is intercon ected to in an unsaturated compacted soil in laboratory. D.S c . atmosphere. Thesis, COPPE/UFRJ. Pacheco Silva,F. (1972). Pore-pressures and settlem en ts in Earth Dams. 8th Brazilian Symposium on Large CONCLUSIONS Dams, Brazil. Sandroni,S.S. and Barbosa da Silva,S.R. (1989). Estim ates of construction pore-pressures in clayey embankments: Based on the results herein presented it can be sug g ested The PN open tests. Symposium on New concepts in that positive pore-water pressure in compacted unsatu rated laboratory and field tests in geotechnics, Rio de Janeiro, soils, during compression at constant water content, are Brazil,Vol.1, pp.279-292. unlikely to occurr for soils presenting a degree of aeration S h e r a r d , J.L., Woodward,R. J. Gizienski,S.F. and below the corresponding value at optimum water conte n t. Clevenger, W.A. (1967). Earth and Earth- dams. John Wiley and Sons,Inc., New York, NY. The results of PH and PHO tests showed that tests perform ed Wheeler,S.J. (1986). The stress-strain behaviour of so ils completely undrained to air and water tend to overestim ate containing gas bubbles. Ph.D. Thesis, Dep.Eng. the pore-water pressure. Sci. .University of Oxford.

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