DEFORMATION BEHAVIOUR OF ORGANIC SUBSOIL ON THE BASIS OF FIELD AND LABORATORY TESTS

Alojzy Szymański¹, Wojciech Sas², Anna Niesiołowska¹, Edyta Malinowska^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

The paper presents the results of laboratory and field investigations carried out on two kinds of soft soils in north-western Poland. Comprehensive investigations which included the routine and oedometer tests as well as triaxial tests have been conducted in order to study the behaviour of consolidation process in soft subsoils. Analyses of factors which determine the assessment of the deformation process of soft subsoils are presented. The main emphasis has been placed on obtaining the non-linear deformation – stress characteristics, which are being used for modelling of the deformation of soft subsoils under the earth structure.

Key words: soft soils, deformation, field investigations, laboratory test.

INTRODUCTION

Construction on soft organic soils faces several difficulties. The settlements at loaded subsoil usually appear rather quickly but may also continue for long time due to creep. The low strength causes stability problems, and consequently the load has to be placed in stages or, alternatively, the soil must be improved through prior treatment [2].

As in other building projects the choice of construction method is a matter of finding an optimum solution. Economic as well as technical aspects must be considered. Such solutions will of course vary according to demands on the standard, as well as available construction time and geotechnical conditions [3].

Embankment dams, particularly tailings dams, are nowadays usually localized in barren (swampy) areas in order to save fertile lands and to protect environment. In such kind of terrain organic soils e.g. peat, are often encountered. Very special properties of organic soils make the improvement of soil foundation particularly important. One of the improvement methods is a consolidation by preloading or construction by stages. In order to accelerate the consolidation process vertical drains may be installed [5,11].

Comprehensive investigations comprising observations at test embankments as well as laboratory investigation were carried out to study the influence of stress path on the deformation characteristics [4,6].

This paper presents an analysis of factors which determine the consolidation course in organic soils loaded by earth structures.

The analysis is focused on non-linear deformation-stress characteristics used in calculation of subsoil consolidation under structures.

DESCRIPTION OF THE TEST AREA

The laboratory investigations were performed on soft soils taken from test sites located on organic soils.

The two test sites were located in north-western Poland in the Notec River valley with the first near the village of Antoniny (test site No. 1) and the second near the village of Mielimaka (test site No. 2). The distance between the two sites is approximately 20 km. The river valley is about 10 km wide and the area is relatively flat, seasonally flooded, and covered with grass vegetation. The upper soft soils in the area consist of a layer of amorphous peat on top of a layer of fine-grained calcareous soil, namely gyttja. Gyttja is organic soil that originates from the remains of plants and animals rich in fats and proteins in contrast with peat which is formed from the remains of plants rich in carbohydrates. These soft organic soils were underlain by a dense sand.

The geotechnical conditions before the test embankments that were constructed are summarized below. The physical properties of organic soils at the test sites are presented in Table 1 and Table 2.

Table 1. Physical properties of organic soils at test site No. 1

Properties	Peat	Calcareous soil
Water content w [%]	310	110
Plastic limit wp [%]	190	55
Liquid limit wl  [%]	315	110
Density of solid particles ρs [tm-3]	> 1.6	>2.55
Bulk density ρ [tm-3]	1.15	1.42
Dry density ρd [tm-3]	0.28	0.68
Organic matter content IOM  [%]	~ 80	~ 35
Degree of humification R [%]	~ 60	-

Table 2. Physical properties of organic soils at test site No. 2

Properties	Peat	Calcareous soil
Water content w [%]	380	115
Plastic limit wp [%]	140	50
Liquid limit wl  [%]	340	120
Density of solid particles ρs [tm-3]	> 1.6	>2.58
Bulk density ρ [tm-3]	1.05	1.42
Dry density ρd [tm-3]	0.26	0.67
Organic matter content IOM [%]	~ 80	~ 27
Degree of humification R [%]	~ 60	-

At the test site No. 1 the organic subsoil consists of a 3.1-m-thick peat layer and a 4.7-m-thick gyttja layer. At the test site No. 2 the 7.5-m-thick organic subsoil consists of a 6.7-m-thick peat layer and 0.8-m-thick gyttja layer. The groundwater table is present in the peat layer at a depth of 0.5-0.8 m below the surface. The GWT in the gyttja and sand layers is 0.6-1.6 m higher than that of the upper peat stratum because of artesian pressure in the sand layer.

From the results of constant rate of strain (CRS) oedometer tests carried out with different rates of strain it is clear that the peat and gyttja are preconsolidated with overconsolidation ratio (OCR) of 3 to 4 and 1 to 2 respectively.

FIELD INVESTIGATIONS

At the test sites two embankments were built in stages to reach the final height of 4.0 m the embankment construction had to be divided into three phases [8]. The subsoil behaviour was monitored by means of piezometers, various types of settlement gauges, and inclinometers that allowed measurements of vertical and horizontal displacements and pore pressures.

The measured deformation and pore pressure response at both experimental sites provided the basis for the estimation of the influence of vertical drains on organic subsoil deformation. Observation of vertical displacements in the subsoil was performed by means of settlement gauges of 4 types: hose, plate, screw and magnetic. Settlement plates and screw plates were installed at the ground surface and at the interface between peat-gyttja and the calcareous soil to determine the settlement distribution in the main layers.

Flexible tube of hose settlement gauge used to obtain continuous settlement distributions across the embankment was installed in subsoil before the construction started. In order to obtain more detailed information on the distribution of settlements with depth, a number of magnetic screw settlement gauges were installed. The pore pressures in the compressible layers were measured at different levels and locations using the BAT system.

The vertical gauges and piezometers were installed at the centre of the embankment, at the middle of the slopes, at the toes of the slopes and outside the embankments. Plastic tubes for measurement with the hose settlement gauge were installed in shallow ditches across the centre parts of the embankments. The magnitude of subsoil deformation during each construction stages are presented in Fig. 1.

Fig. 1. Vertical displacements of embankment subsoil at test site No. 1

The observation of deformation performance indicated that the consolidation process consist of two phases (Fig. 2 and Fig. 3).

• Primary settlement (immediate and consolidation)

• Secondary and tertiary settlement (creep)

• Primary settlement is the result of:

• Immediate (initial) undrained elastic deformation of the subsoil under aplied load.

• Consolidation of the soil connected with settlement and the expulsion of excess of pore from the soil under applied load.

Fig. 2. Course of vertical strain of peat during subsoil consolidation from field investigation at Antoniny site

Fig. 3. Course of vertical strain of calcareous soil during subsoil consolidation from field investigation at Antoniny site

Secondary and tertiary settlement is the result of creep of soil skeleton under the effective stress. It depends on rheological properties of soil and it's signifficantly depend on time. The calculation of initial settlement are performed on the base of elastic theory using undrained Young's modulus E_u and Poisson's Ratio u = 0.5. To evaluate the deformation characteristics triaxial undrained tests are performed. Parameters for calculation the settlements of the consolidation stage are delivered from compression tests, mainly from oedometer IL (incremental loading) test or oedometer test with continous loading CL as well as triaxial tests.

For calculations the secondary compression (creep settlements) mainly coefficient of secondary consolidation c_α is applied. The coefficient is evalueted for each load of step during oedometer IL test. This parameter is a function of stress history and deformation and the variation within the particular range of stresses and deformations to be applied in the field should be determined. The coefficient of secondary compression may be expressed as C_α = de/dlog t or as C_αε = dε/dlogt, where C_α = C_αε(1+e_o). Conventional creep settlements are regarded as being approximately linear in log time and are described as secondary settlement. In long – term tests there is sometimes a downwards curvature of the log time-settlement curve in the secondary compression phase. This phenomenon is sometimes called tertiary compression.

LABORATORY TEST RESULTS

Laboratory tests presented in the paper were performed on peat and calcareous soil samples taken from organic subsoil. These laboratory investigations consist of routine test, oedometer and triaxial tests as well as permeability tests using flow-pump techniques. Triaxial tests were performed to evaluate the deformation and strength characteristics for overconsolidated and normally consolidated stress states, which are required for estimating the displacement of organics subsoil. In order to determine the deformation parameters for undrained and fully drained conditions, triaxial tests were carried out [10]. The results obtained in laboratory tests for undrained conditions are presented in Fig. 4 and Fig. 5 and for fully drained conditions are shown in Fig. 6 and Fig. 7 [1].

Fig. 4. Variability of undrained modulus Eu obtained in triaxial tests CU for peat

Fig. 5. Variability of undrained modulus Eu obtained in triaxial tests CU for calcareous soil

Fig. 6. Relationship between Young's modulus E and effective stress component σ'3 obtained in triaxial tests CD for peat

Fig. 7. Relationship between Young's modulus E and effective component σ'3 obtained in triaxial tests CD for calcareous soil

The relationship between the Young modulus E_u for undrained conditions versus deviatory stress q and consolidation stress σ_c can be shown as follows:

E = β₀ · q^β₁ · σ_c^β₂,              (1)

where β₀, β₁, β₂ – empirical coefficients.
The analysis of test results gives the following values of empirical coefficients to Equation (1) for peat: β₀ = 17.5, β₁ = -0.86, β₂ = 1.70 and for calcareous soil β₀ = 3.51, β₁ = -0.78, β₂ = 2.11. The relationship between the Young modulus E for fully drained conditions versus effective stress components σ'₁ and σ'₃ can be shown as:

E = α₀ · σ₁'^α₁ · σ₃'^α₂              (2)

where α₀, α₁, α₂ – empirical coefficients.
For organic soils from the Antoniny site the following values of empirical coefficients to Equation (2) for peat are obtained α₀ = 2770, α₁ = -1.95, α₂ = 2.16 and for calcareous soil α₀ = 947, α₁ = -1.12, α₂ = 1.53.

Considerable secondary deformations which depend on time occur in organic soils [7]. Creep tests were performed in standard triaxial cells for peat and calcareous soils. For each soil two series of tests were performed: first on unconsolidated samples and second on samples consolidated under the effective stress of about 35 kPa. Test series were done under differrent deviatoric stresses. Creep tests were performed to evaluate the stress-strain, stress-rate of strain and the time dependent characteristics (Fig. 8).

Fig. 8. Strain versus log time for consolidated peat

Results of laboratory tests indicated that the parameters describing secondary compression depend on considerable extent on the effective stress level. Conventional creep settlements are regarded as being approximately linear in log time and are described as secondary settlements. In the long-term tests there is sometimes an upwards curvature of the log time-settlement curve in the secondary compression phase. This phenomenon is called tertiary compression [9]. The analysis of the development of vertical and horizontal strains in organic soils during the deformation process indicates significant creep of the soil skeleton.

It is important that the rate of strain can increase or decrease during the creep phase and depends on the level of deviatoric stress (Fig. 9). The rate of strain decreases when the applied deviatoric stress is lower than deviatoric stress at failure. On the other hand when deviatoric stress is higher the rate of strain initially decreases and than continuously increases until creep failure. Consolidation of subsoil significantly caused a decrease of the strain rate.

Fig. 9. Rate of strain versus log time for in-situ calcareous soil

DISCUSSION OF THE RESULTS

Observations of the consolidation process in organic soils demonstrate large values and a non-linear character of deformation. Therefore, the prediction of consolidation performance in organic subsoil should be carried out by methods which take into account the variation of soil parameters and large strains analysis. Laboratory tests presented in this paper indicate that the strength parameters c' and φ' depend on the stress range, therefore the different values should be used for overconsolidated and normally consolidated stress state. It is important to note that deformation parameters E and E_u depend not only on the stress range but also on stress level and stress history.

Analysis of the development of vertical and horizontal strains in organic soils during the deformation process indicates significant creep of the soil skeleton. The part of strain can be described by ε_s for a given consolidation stress depending on time and stress level. Observations of the consolidation process in organic soils demonstrate large values and a non-linear character of deformation. The use of consolidation theory for the prediction of soil displacements under embankments requires taking into consideration the variable soil parameters which depend on the effective stress level and preconsolidation phenomena. This fact should be taken into consideration in the modeling process of consolidation performance.

CONCLUSIONS

Observations of the consolidation process in organic soils demonstrate large values and a non-linear character of deformation. The use of consolidation theory for the prediction of soil displacements under embankments requires taking into consideration the variable soil parameters which depend on the effective stress level and preconsolidation phenomena.

The results of laboratory and field tests of organic soils indicate a different character of variation within the parameters. The passage from the overconsolidated stage to the normally consolidated state produces important changes in the values of strength parameters c' and φ', as well as in the deformation parameters E and E_u.. The proper values of modulus numbers and exponents, which are valid for overconsolidated or normally consolidated state, should be applied. Moreover, the secondary compression described by ε_s should be predicted using parameters which depend on time and effective stress components.

The modelling of consolidation course in organic soils should take into account non-linear permeability characteristics describing the pore water outflow from consolidated soils. It is important that the rate of strain can increase or decrease during the creep phase and depends on the level of deviatoric stress. When applied deviatoric stress is lower than deviatoric stress at failure the rate of strain decreases on the other hand when deviatoric stress is higher the rate of strain initially decreases than continuously increases until creep failure. Consolidation of subsoil significantly caused the decrease of the strain rate.

REFERENCES

1. Drożdż A., 2006. Badania charakterystyk odkształceniowych gruntów słabonośnych. Przegląd Naukowy Inżynieria i Kształtowanie Środowiska [Investigations of deformation characteristics of soft soils]. Zeszyt 1 (33), Rocznik XV, Warszawa, 53-62 [in Polish].

2. Hartlen J., Wolski W., 1996. Embankments on organic soils. Elsevier, Amsterdam.

3. Larsson R., 1986. Consolidation of soft soils. Report No 29, Swedish Geotechnical Institute, Linköeping.

4. Lechowicz Z., 1994. An evaluation of the increase in shear strength of organic soils. Proceeding of the Workshop on Advances in Understanding and modelling the Mechanical behaviour of Peat. Balkema, Rotterdam, 167-179.

5. Lechowicz Z., Szymanski A., Wolski W., 1987. Effects of groundwater on embankment subsoil deformation. Proceedings 9^th European Conference on Soil Mechanics and Foundation Engineering. Dublin, 1, 451-454.

6. Szymanski A., 1994. The use of constitutive soil models in consolidation analysis of organic subsoil under embankment. Proceeding of the Workshop on Advances in Understanding and modelling the Mechanical behaviour of Peat. Balkema, Rotterdam, 231-240.

7. Szymanski A., Sas W., 2001. Deformation characteristics of organic soils. Ann. of Warsaw Agricult. Univ. – Land Recl. 32, 117-126.

8. Szymanski A., Sas W., Drozdz A., Malinowska E., 2005. Field and laboratory experience with the construction of embankments on organic soils. Proceedings of International Conference on Problematic Soils. Eastern Mediterranean University, Famagusta, N. Cyprus, 2, 747-755.

9. Szymanski A., Lechowicz Z., Drozdz A., Sas W., 2005. Geotechnical characteristics determining consolidation in organic soils. Proceedings of 16^th International Conference on Soil Mechanics and Geotechnical Engineering. Osaka, Japan, Vol.2 1b: Laboratory testing (II): Strenght, Large Deformation and Hydraulic Properties, 603-606.

10. Szymanski A., Sas W., Drozdz A., Malinowska E., 2006. Soft subsoil improvement and factors which determine the consolidation. Proceedings of XIII Danube-European Conference on Geotechnical Engineering. Vol. 2, Ljubljana, Slovenia, 131-136.

11. Wolski W., Szymanski A., Mirecki J., Lechowicz Z., Larsson R., Hartlen J., Garbulewski K., Bergdahl U., 1988. Behaviour of two test embankments on organic soils. Report No. 32. Swedish Geotechnical Institute, Linköeping.

 

Accepted for print: 26.01.2009

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Alojzy Szymański
Department of Geotechnical Engineering,
Warsaw University of Life Sciences - SGGW, Poland
Nowoursynowska Str. 159, 02-776 Warsaw, Poland
Phone: + 48 22 59 35401
email: alojzy_szymanski@sggw.pl

Wojciech Sas
Laboratory - Water Centre,
Warsaw University of Life Sciences - SGGW, Poland
Nowoursynowska Str. 159, 02-776 Warsaw, Poland
Phone: + 48 22 59 35401
email: wojciech_sas@sggw.pl

Anna Niesiołowska
Department of Geotechnical Engineering,
Warsaw University of Life Sciences - SGGW, Poland
Nowoursynowska Str. 159, 02-776 Warsaw, Poland
email: anna_niesiolowska@sggw.pl

Edyta Malinowska
Department of Geotechnical Engineering,
Warsaw University of Life Sciences - SGGW, Poland
Nowoursynowska Str. 159, 02-776 Warsaw, Poland
email: edyta_malinowska@sggw.pl

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