Numerical simulation of long-term peat settlement under the sand embankment

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1 Available online at ScienceDirect Procedia Engineering 00 (2016) st International Conference on the Material Point Method, MPM 2017 Numerical simulation of long-term peat settlement under the sand embankment Dmitry A. Tyurin a, *, Alexander L. Nevzorov a a Northern (Arctic) Federal University named after M.V. Lomonosov, Northen Dvina emb., 17, Arkhangelsk, , Russian Federation Abstract Long-term surface settlement in the cities, which were founded in swamp lands, is a reason of road pavement, underground pipelines, building damage. One of such cities is Arkhangelsk, which was founded on the swamp with peat thickness of 4-6 meters and even 8 meters. In such difficult geological conditions, the prediction of long-term settlements of a peat layer under an embankment is the significant task. One of the contemporary and effective ways of its solution is numerical simulation. The numerical simulation was performed using the software PLAXIS. The model of Soft Soil Creep was used. The objective of this project was comparing the long-term peat settlement values by way of numerical simulation, using the results of peat compression tests as initial data, with the data of monitoring. Thus, we can say about the model verification. On the first step of the research input parameters λ*, μ* were received from the laboratory tests data according to the basic methods. After that numerical simulation of oedometer tests was done. The small disarrangement between calculated settlementin-time plots and the oedometer test data showed that that the SSC model gives an adequate representation of the physical processes in the peat specimen. On the second step, we simulated the experimental field, which settlement data has being collected more than 23 years. During this period, peat settlement was m, depending on the thickness of the sand embankment. Input parameters λ*, μ* from the oedometer test were used. The comparison showed the low level of convergence of the settlement-in-time plots with the field data, so the model parameters calibration was required. The correction factors were to be added to the parameters λ* and μ* calculated by the results of oedometer tests The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of the 1 st International Conference on the Material Point Method. Keywords: numerical simulation; peat; settlement; primary consolidation; secondary consolidation *Corresponding author. address: d.a.tyurin@yandex.ru The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of the 1 st International Conference on the Material Point Method.

2 2 D.A. Tyurin & A.L. Nevzorov / Procedia Engineering 00 (2016) Introduction For historical reasons the city of Arkhangelsk founded in the end of XVI century was built in extremely unfavorable geotechnical conditions of the swamp plain where the peat layer thickness was about from 4 to 6 meters and even 8 meters. As it is impossible to remove completely the peat from such a vast area the sites for construction were prepared and still are prepared by sand filling of 2-4 meters thick layer. The complete peat excavation was performed only under the city mains. Due to the peat in the bedding, all buildings and underground pipelines are built on pile foundations. The results of monitoring indicate that peat settlement doesn t stop for several decades after the filling of embankment causing quite a serious problem for the city. Irregularities sometimes achieving from 20 to 30 cm arise on road and sidewalk pavements over the pipelines and wells supported by piles (Fig.1). Fig. 1. Some examples of deformations: (a) around a well of the drain system; (b) over a pipeline on pile foundation. In 1992, an experimental site 190x280 m within an undrained raised bog not far from Arkhangelsk was arranged to research the process of peat deformation [1]. The thickness of the peat layer varied from 3.8 to 6.7 m. Lacustrineglacial clay underlays the peat. The ground water table initially was nearby the bog surface, but during the period of monitoring, it subsided to m. In 1993, a sand embankment of 0.9 to 3.7 m thickness was filled at the bog surface. The nineteen settlement plates with steel pipes were installed at the peat surface before sand filling. At the beginning of monitoring, the settlement was measured monthly with accuracy 5 mm and later two or three times per year with the accuracy 2 mm. The objective of this research was comparing the long-term peat settlement values by way of numerical simulation, using the results of peat compression tests as initial data, with the data of monitoring. 2. Laboratory test results The peat initial properties were as follows: degree of decay 22%, density g/cm 3, specific gravity g/cm 3, void ratio , degree of saturation [2]. The laboratory tests for peat consolidation were carried out without using bottom porous stones so that the pore water drainage direction was the same as for the field experiment. The test specimens were 8.7 cm in diameter and 5 cm in height. They were loaded in one step so that the total consolidation stress was equal to 12.5, 25, 50 and 100 kpa. The experiments lasted for 3-4 weeks up to the rate of deformation of 0.01 mm/day. The numerical simulation of the laboratory tests was performed using the software PLAXIS 2D The model of Soft Soil Creep (SSC) was used, because it is characterized by a correct consideration of significant soil deformations and has a creep parameter among the initial parameters [7]. The stiffness parameters for this model are as follows: modified compression index λ*, modified swelling index κ*, modified creep index μ*. Strength parameters: effective cohesion с, friction angle φ, dilatancy angle ψ. The plots of relative strain (ε) versus logarithm of pressure (ln p) and relative strain (ε) versus logarithm of time (ln t) were built [3, 4] (Fig.2 and Fig.3).

3 D.A. Tyurin & A.L. Nevzorov / Procedia Engineering 00 (2016) Fig. 2. Determination of λ* as per oedometer test data: (a) by the moment of primary consolidation; (b) by the moment of settlement stabilization. Fig. 3. Determination of μ* as per oedometer test data. The calculated values of λ* and μ* are shown in Table 1. It should be noted that λ* values appeared to be almost the same as for the moment of primary consolidation end as for the moment of achieving the secondary consolidation stabilization. Table 1. Peat stiffness values determined by oedometer tests. Consolidation stress, kpa λ* μ* As an alternative, the following factors may be used: C c =8.28, C r =0.83, C α = After that, the numerical simulation of the oedometer tests using virtual laboratory instruments was carried out. The model parameters were fit by the backward analysis procedure so that the calculated settlement-time plots would match the experimental data to the best. In this research, the Mohr Coulomb parameters of peat were set with constants: effective cohesion с=10 kpa and friction angle φ=16 [5, 6]. As the specimen unloading wasn t considered, the impact of the parameters related to this mechanism was considered to be negligible, accepting in accordance with the PLAXIS user manual [3]: the modified swelling index κ* equal to λ*/5 and the Poisson's ratio for unloading-reloading equal to ν ur Table 2 shows the results of numerical simulation.

4 4 D.A. Tyurin & A.L. Nevzorov / Procedia Engineering 00 (2016) Table 2. Parameters of the peat stiffness calculated by the virtual laboratory. Consolidation stress, kpa λ* μ* As it is seen from Table 2, the values of the factors λ* and μ* calculated by the virtual laboratory are quite close to the data shown in Table 1 making to conclude that the SSC model gives an adequate representation of the physical processes in the peat specimen. 3. In-situ monitoring results The in-situ monitoring showed that in the first year after loading the settlement increased most intensively. Then its rate became slower and five years later, it was equal to mm/year. Ten years later, it decreased up to mm/year and by 2015 it reached 5-10 mm/year. It should be noted that the load on peat gradually reduced because of embankment submerging below the ground water table. In the presented calculations, the embankment loads were accepted as per finite value considering the submerged unit weight of sand. The pore water was drained only through the top boundary of peat. The values λ* and μ* are obtained by building the plots ε = f(ln p) and ε = f(ln t) (Fig.4 and Fig.5) and they are shown in Table 3. Fig. 4. Determination of λ* by in-situ monitoring: (a) by the moment of primary consolidation end; (b) by the moment of the last measuring.

5 D.A. Tyurin & A.L. Nevzorov / Procedia Engineering 00 (2016) Fig. 5. Determination of μ* by in-situ monitoring data. Table 3. Peat stiffness values determined by long-term field monitoring. Consolidation stress, kpa λ* μ* As an alternative, the following factors may be used: C c =12, C r =1.5, C α = Comparing the data shown in Tables 1, 2 and 3, we may see that the factor λ* for peat changes from 0.20 to 0.29 depending on the method for its determination. The deviations in the values of the factor μ* are more significant up to 10 times, and by the field test data, as opposed to the oedometer tests, the modified creep factor μ* is a variable value and it increases with the peat load increase. The numerical simulation of long-term peat deformations under the embankment was performed using the SSC model in axisymmetric system. More than node triangle finite elements were used. The boundary conditions were as follows: top free, bottom fully fixed, right border - horizontally fixed, vertically free. Water level coincided with the peat surface. More than node triangle finite elements were used. The procedure for processing the monitoring results to get the input data as well as the assumed values of c, φ, κ*, ν ur were taken the same as for the laboratory tests. Over consolidation ratio (OCR) was taken to be [8]. The numerical simulation of the peat settlement under the embankment using the SSC model with stiffness values form Tables 1 and 2 revealed significant discrepancies between settlement-in-time curves and the monitoring results. As the model was verified by processing oedometer test data in the virtual laboratory, the specified discrepancies are caused not by the model errors but by the physical phenomena taking place in the peat layer for 23 years. The final step was correction of the model λ* and μ* parameters up to the complete convergence of the calculated settlement-in-time curves with the monitoring data. The results are shown in Table 4 and in Fig. 6. Table 4. Model parameters determined by oedometer tests and by numerical simulation of peat layer under embankment. Consolidation stress, kpa λ*oed λ*sim λ*sim/ λ*oed μ*oed μ*sim μ*sim/ μ*oed

6 6 D.A. Tyurin & A.L. Nevzorov / Procedia Engineering 00 (2016) Conclusions Fig. 6. Dependences of model parameter values. The virtual laboratory software PLAXIS may be well applied for simulation of peat oedometer tests. There is a strong correlation between modified creep index μ* and consolidation stress in-situ monitoring results. When performing numerical simulation for peat using the Soft Soil Creep model, correction factors are to be added to the parameters λ* and μ* calculated by the results of oedometer tests. The correction factor for λ* is a linear function of consolidation stress, and the correction factor for μ* is exponential function of consolidation stress. The cause of the discrepancy between the parameters μ* and λ* calculated by the results of oedometer tests and in-situ monitoring should become the subject for studying the long-term processes taking place in the peat. References [1] A.L. Nevzorov, The long-term peat settlement under the sand embankment, Proceedings of the 5th International Geotechnical Symposium. University of Incheon, Republic of Korea (2013) [2] A.L. Nevzorov, A.V. Nikitin, A.V. Zaruchevnih, Gorod na bolote, NArFU, Arkhangelsk, 2012 [3] R.B.J. Brinkgreve, PLAXIS 2D Manual, Delft, [4] R.W. Day, Foundation engineering handbook. Design and Construction with the 2009 International Building Code, second ed., The McGraw- Hill Companies Inc., San Diego, 2010 [5] Jon W. Koloski, Sigmund D. Schwarz, And Donald W. Tubbs, Geotechnical properties of geologic materials, Engineering Geology in Washington, Volume 1, Washington Division of Geology and Earth Resources Bulletin 78, 1989 [6] Haider Al-Ani, Erwin Oh, Engineering properties of peat in estuarine environment, Foundation and Soft Ground Engineering Conference Thu Dau Mot University ICTDMU-1, Binh Duong, June 5 6, 2013 [7] Djamalddine Boumezerane, Gustav Grimstad, A rheological model for peat that accounts for creep, Deformation characteristics of geomaterials V.A. Rinaldi et al. (Eds.) IOS Press, 2015 [8] C. Zwanenburg, and R.J. Jardine, Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat, Géotechnique, Volume 65 Issue 4, April 2015, pp