NONLINEAR WATER FLOW CHARACTERISTICS DESCRIBING ORGANIC SOIL CONSOLIDATION

Edyta Malinowska¹, Wojciech Sas², Alojzy Szymański^1
1 Department of Geotechnical Engineering, Warsaw University of Life Sciences - SGGW, Poland
² Laboratory - Water Centre, Warsaw University of Life Sciences - SGGW, Poland

ABSTRACT

Constructions of embankments on soft organic soils cause many problems. Because of their specific properties the organic soils, such as peats, are characterized by a large porosity, small shear strength and a large initial permeability which decrease during consolidation. The main task of an engineer is to predict and calculate the real amount of vertical and horizontal deformations which are relatively large in soft soils. In the deformation process of soil skeleton under loading the porosity decreases and causes the changes of permeability characteristics. Therefore the water flow characteristics are very important to be considered in predicting and calculating the subsoil deformations under load. Also it is found in many papers that the relationship between the water flow velocity and the hydraulic gradient is non linear in the specific range of void ratio depending on the soil properties. In this paper the results of water flow characteristics obtained in laboratory tests are presented.

Key words: nonlinear water flow characteristic, consolidation process, deformations, organic soils, peat.

INTRODUCTION

Embankment dams, particularly tailings dams, are nowadays usually localized in swampy areas where the organic soils, such as peats, are often located. Because of their specific properties construction of engineering structures on soft organic soils causes some problems. One of them is large vertical and horizontal deformations which occur during and after the construction period [10,19]. A small initial shear strength often causes difficulties to achieve the embankment stability. That is why the load should be applied by stages or on the consolidated subsoil. As in other building projects the choice of the construction method is a matter of finding an optimum solution also for economic reasons [3].

During the deformation process under loading the shear strength increases which influences on the embankment stability. The consolidation process depends on distribution of water pore pressure which is connected to permeability [17].

That is why the design, construction and maintenance engineering constructions on organic soils should be preceded by analysis of value and course subsoil deformations and the water pore pressure [9].

Most of the consolidation theories are based on Terzaghi assumptions. The consolidation process described by many authors is supported by assumptions that the water flow characteristic and stress-strain relationships are linear. Nevertheless the previous works have shown that the linear Darcy’s law of the outflow of water from the soil can not be always accepted [7]. There are some limits in using this assumption.

Nevertheless some of the authors like Hansbo [4,5,6], Macioszczyk and Szestakov [10], Kany and Herman [8], Bartholomeeusen in. [2], Malinowska et al. [12,13] indicated that water flow characteristics in soft subsoils are non-linear in different stress values (Fig. 1-2). Having conducted extensive permeability tests on natural soft clays, Tavenas et al [20] found that Darcy’s law is valid in soft clays for hydraulic gradient ranging from 0.1 to 50.0 and the development of primary and secondary consolidation in clay made it almost impossible to verify the validity of Darcy’s law at very small gradients.

Therefore consolidation process depends on non linear relationship between stress – strain [16,17,18] as well as on nonlinear water flow characteristics [14].

METHODS OF LABORATORY INVESTIGATIONS

Different methods can be used to measure the water flow in saturated soils in the laboratory. They are divided into direct and indirect methods.

It is very important to choose a proper method because of the reliability of the test results, repeatability and reproducibility of the test results, the reconstruction of reflection in-situ conditions, difficulties and costs of the tests. In order to take into consideration these factors it is recommended to use direct laboratory methods to eliminate additional calculation errors. Reliable calculations of deformation process depend on the precise description of water flow characteristics in porous medium. Generally there are two laboratory methods: constant-head method and falling-head one.

Nevertheless in the last years with the technology evaluation the flow-pump technique was considered, initiated by Olsen [15]. The comparison between the flow-pump and constant head techniques was also presented by Aiban and Znidarcic [1], Malinowska and Hyb [12].

Different methods of permeability tests are shown in Fig. 3.

The main advantage of a direct, flow-pump technique is the time, in which the small, in-situ hydraulic gradients can be applied. Therefore this method has been used to characterize the water flow in peats under loading (Photo 1).

LABORATORY TEST RESULTS

Consolidation deformations are the largest in soft organic peats. Therefore it is very important to describe the dependence of consolidation process on the nonlinear water flow in peats under loading.

The test performed on peats samples have shown that low hydraulic gradient creates the nonlinear relationship between the flow velocity and gradient.

The permeability tests have been carried out on peats samples taken from the “Campus SGGW” (Warsaw University of Life Sciences Campus) deposit. The physical properties of tested soil are presented in Table 1.

The permeability tests contain: saturation, consolidation under different stress conditions, measurement of void ratio, and measurement of hydraulic gradient with the control of back pressure.

The tests were made on peats samples (NNS) taken from “Campus SGGW” test site. There were 221 water flow measurements made on 14 different saturated soil samples with 6 different consolidations stress conditions.

Most of the organic soils do not have a significant stress history. Therefore the effective in situ stress is relatively low because of a high ground water level and a small bulk density. Nevertheless for the comprehensive analysis of peats behaviour under loading, in this paper, the relationship between flow velocity, hydraulic gradient and void ratio is presented in the consolidation range of 10.0 kPa to 80.0kPa (Fig. 4-10).

The laboratory test results made by the flow-pump technique have shown that the relationships between the flow velocity and the hydraulic gradient for different void ratios are nonlinear (Fig. 11). According to that, the consolidation process which depends on the flow velocity in porous media should be characterized by nonlinear flow characteristics.

CONCLUSIONS

To describe the permeability characteristics the prelineary and postlineary filtration should be considered. The prelineary filtration characterises the flow at very low hydraulic gradient which usually appears in the field. The postlinear filtration characterised the flow at high hydraulic gradient which can appear in rocks or during hydraulic perforation. Similarly phenomenon was observed during laboratory permeability tests under very high consolidation stress in soft organic peats. It causes high value of hydraulic gradient which do not occur in fields.

The test results show that relationship between flow velocity and hydraulic gradient is always nonlinear.

The results allowed describing the relationship between the flow velocity, the hydraulic gradient and the void ratio. The permeability characteristics were elaborated using statistic and the regression function (Fig.12).

In the description of the consolidation process of organic soils the nonlinear permeability characteristics describing the pore water outflow from consolidated soils should be taken into consideration using the following relationship:

V = 4.2 ∙ 10^-15 ∙ i^1.56 ∙ e^7.60

where: V – flow velocity [ms^-1], i – hydraulic gradient [-], e – void ratio [-].

The analysis of the consolidation process in organic soils indicates large values and a nonlinear character of deformation as well as nonlinear relationship between the flow velocity and the hydraulic gradient. The use of consolidation theory for the prediction of soil displacements under embankments requires taking into consideration the variable soil parameters and permeability depending on the effective stress level and variation in the void ratio during the deformation process.

REFERENCES

1. Aiban S.A., Znidarcic D., 1989. Evaluation of the flow-pump and constant head techniques for permeability measurements. Geotechnique 39, 4, 655-666.

2. Bartholomeeusen G., Znidarcic D., Hwang Ch., Sills G.C., 2001. Seepage Inducted Consolidation Test. University of Colorado, UK, www-civil.eng.ox.ac.uk/people/gb/FrameSet_sict.htm.

3. Garbulewski K. 2000. Dobór i badania gruntów uszczelnień składowisk odpadów komunalnych [Selection and investigation of liners for landfills of municipal wastes]. Wyd. SGGW – Warszawa, 169 [in Polish].

4. Hansbo S., 1960. Consolidation of clay with special reference to influence of vertical sand drains. Doctoral Thesis, Swedish Geotechnical Institute, Proceedings, 18, 160. Stockholm.

5. Hansbo S., 2001. Consolidation equation valid for both Darcian and non-Darcian flow. Geotechnique 51, 1, 51-54.

6. Hansbo S., 2003. Deviation from Darcy’s law observed in one-dimensional consolidation. Geotechnique 53, 6, 601-605.

7. Ing T.C, Xiaoyan N., 2002. Coupled consolidation theory with non-Darcian flow. Computers and Geotechnics 29, School of CSE, Nanyang Technological University, Singapore, 29, 169-210.

8. Kany M., Herrmann R., 1987. Water motion in soils based on a diffusion theory of mixtures (part 2). Proc. of the 9^th Europ. Conf. on Soil Mech. and Found. Eng. Dublin 1022.

9. Ladd C.C., Foott R., 1980. The behaviour of embankments on clay foundations. Canadian Geotechnical Journal, 17(2), 236-260.

10. Lechowicz Z., Szymański A. 1984. Prediction of consolidation of organic soil. Annals of Warsaw Agricultural University, 20, 55-59.

11. Macioszczyk T., Szestakow W.M., 1983. Prawo filtracji, hydrauliczne charakterystyki strumienia, filtracja ustalona, Dynamika wód podziemnych – metody obliczeń [The filtration law, the characteristics of hydraulic flow, steady filtration. Ground water dynamic – the method of calculation]. Wyd. Geologiczne, Warszawa [in Polish].

12. Malinowska E., Hyb M., 2004. Wyznaczanie współczynnika filtracji na podstawie badań laboratoryjnych [The determination of permeability coefficient in laboratory tests]. EU GeoEnvNet Seminar, Wyd. SGGW, Warszawa [in Polish].

13. Malinowska E., Szymański A., Sas W., 2005. Wyznaczanie charakterystyk przepływu wody w gruntach organicznych metodš flow-pump [Determination of water flow characteristics in organic soils by the flow-pump technique]. Przeglšd naukowy Inżynierii i Kształtowania Srodowiska, SGGW – Warszawa, 1(31), 114-121 [in Polish].

14. Malinowska E., 2005. Analiza odkształceń wybranych gruntów organicznych organicznych uwzględnieniem nieliniowych charakterystyk przepływu [The description of soft soils deformations by use of the nonlinear characteristic of water flow]. Doctor’s thesis, Warsaw Agricultural University [in Polish].

15. Olsen H.W., 1966. Darcy’s Law in Saturated Kaolinite. Water Resources Research 2, 6, 287-295.

16. Sas W., 2001. Modelowanie odkształceń gruntów organicznych z uwzględnieniem zmian własciwosci osrodka [The modelling of deformation process of organic silos including changes in porous media]. Doctor’s thesis, Warsaw Agricultural University [in Polish].

17. Szymański A., 1982. Charakterystyki procesu odkształcenia pod obciażeniem wybranych rodzajów torfów [The deformation characteristics of selected kind of peats under loading, doctor thesis]. Doctor’s thesis, Warsaw Agricultural University [in Polish].

18. Szymański A., Sas W., 2000. Modelowanie procesu odkształcenia gruntów organicznych [The modeling of deformation process in organic silos]. XII Krajowa Konferencja Mechaniki Gruntów i Fundamentowania, 285-297 [in Polish].

19. Szymański A., Sas W., Drożdż A., Malinowska E., 2004. Secondary compression in organic soils. Annals of Warsaw Agricultural University – SGGW. Land Reclamation, 35a, 221-228.

20. Tavenas F., Mieussens C., Bourrges F., 1979. Lateral displacements in clay foundations under embankments. Canadian Geotechnical Journal, 16(2), 287.

Accepted for print: 5.11.2007

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