Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2023.45.0053 Acta Polytechnica CTU Proceedings 45:53–58, 2023 © 2023 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague EVALUATION OF DEFORMATION AND STRENGTH PARAMETERS OF ORGANIC SOILS FOR THE DESIGN OF GEOTECHNICAL STRUCTURES Zbigniew Lechowicz∗, Katarzyna Goławska Warsaw University of Life Sciences, Institute of Civil Engineering, Department of Geotechnical Engineering, 166 Nowoursynowska Str., 02-787 Warsaw, Poland ∗ corresponding author: zbigniew_lechowicz@sggw.edu.pl Abstract. The evaluation of deformation and strength parameters of Holocene organic soils obtained from DMT dilatometer tests based on empirical relationships and ANN Artificial Neural Networks is presented. The evaluation of undrained shear strength τfu, deformation modulus E0.1% and initial shear modulus G0 of Eemian organic soils obtained from DMT dilatometer tests and SDMT seismic dilatometer tests is given, and then its comparison with the values obtained from triaxial tests for the design of underground structures supported by diaphragm walls of the stations of the underground line II in Warsaw, is shown. Keywords: Deformation modulus, eemian organic soils, holocene organic soils, initial shear modulus, undrained shear strength. 1. Introduction Earth structures such as dam embankments, levees, dykes, and road and railway embankments are of- ten constructed in difficult geotechnical conditions [1–6]. In general geotechnical practice organic soils are considered as difficult subsoils due to their high compressibility with creep effects, low undrained shear strength and nonlinear variability of material charac- teristics [7–13]. Holocene organic soils are classified as very soft soils with the consistency index IC < 0.5 and undrained shear strength τfu often not exceeding 25 kPa. Organic soils formed during the Eemian Inter- glacial of the Pleistocene Period reveal slightly better index properties, and higher stiffness and strength than Holocene organic soils. However, due to the composition of the soil skeleton with significant or- ganic matter and calcium carbonate contents, they also show up non-linear mechanical characteristics and a time-dependent response to loading. For the design of embankments on organic subsoil, the cru- cial problems to be solved include the assessment of embankment stability, and large vertical and hori- zontal deformations of the subsoil during and after construction [1, 2]. Embankment construction on or- ganic subsoil is possible using a staged construction involving ground improvement by consolidation or various ground improvement methods, e.g. inclusions of different rigidity. Nowadays, the construction and modernization of facilities and infrastructure in urban agglomerations is often performed in complex geotechnical conditions. Subsoil studies and tests with regard to the importance of the structure and complexity of the foundation con- ditions are based on the combined analysis of field and laboratory test results. In recent practice, among pos- sible in situ tests, the SDMT seismic dilatometer test is increasingly used also for heavily preconsolidated cohesive soils and organic soils. When modelling the behaviour of deep excavation casings by diaphragm walls using a linear elastic-perfectly plastic model, it is necessary to apply deformation parameters deter- mined in the range of small strains. For the design of diaphragm walls in organic soils, it is advisable to use a more advanced soil model, e.g. a non-linear elastic-plastic model with creep [14–18]. Practical cases of evaluating deformation and strength parameters of organic soils loaded by em- bankments are shown in the paper. The evaluation of deformation and strength parameters of Holocene organic soils from the Antoniny test site located in north-western Poland in the Noteć River valley and obtained from DMT dilatometer tests is here presented. The constrained modulus Moc for the preconsolidated state and the constrained modulus Mnc for the nor- mally consolidated state, as well as the recompression index Cr and the compression index Cc for the normally consolidated state of Holocene peat and gyttja, are pre- sented. The undrained shear strength τfu of Holocene organic soils evaluated from DMT dilatometer tests based on empirical relationships and Artificial Neural Networks is also shown. The evaluation of deformation and strength parameters of Eemian organic soils in the design of underground structures such as deep excava- tions supported by diaphragm walls for the stations of the underground line II in Warsaw is shown. The values of undrained shear strength τfu, deformation modulus E0.1% at vertical strain ε1 of 0.1 % and initial shear modulus G0 for Eemian gyttja evaluated from DMT dilatometer tests and SDMT seismic dilatome- ter tests are compared with the values obtained from triaxial tests. 53 https://doi.org/10.14311/APP.2023.45.0053 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en Zbigniew Lechowicz, Katarzyna Goławska Acta Polytechnica CTU Proceedings Soil type Organic content Iom [%] CaCO3 content [%] Water content wn [%] Liquid limit wL [%] Density unit ρ [t m−3] specific ρs [t m−3] Peat 65–75 10–15 310–340 305–450 1.05–1.10 1.45–1.50 Gyttja 5–20 65–90 105–140 80–110 1.25–1.40 2.2–2.3 Table 1. Physical properties of Holocene organic soils. Soil type Organic content Iom [%] CaCO3 content [%] Water content wn [%] Liquid limit wL [%] Density unit ρ [t m−3] specific ρs [t m−3] Organic mud 8–12 – 32–34 50–55 1.60–1.65 2.4–2.5 Gyttja low-carbonate mineral 7–9 29–38 60–80 75–85 1.60–1.65 2.40–2.45 Gyttja high-carbonate mineral-organic 17–25 54–77 80–120 120–130 1.55–1.60 2.30–2.35 Table 2. Physical properties of Eemian organic soils. 2. Characteristics of the organic soils 2.1. Holocene organic soils The physical properties of typical Holocene organic soils: organic mud, high-carbonate gyttja and amor- phous peat are shown in Table 1. In order to correctly identify organic soils within the basic physical proper- ties, in addition to natural moisture, liquidity limit, bulk density and specific density, it is necessary to determine the organic content Iom, by burning the soil at a maximum temperature of +440 ◦C, and to determine the calcium carbonate CaCO3 content. 2.2. Eemian organic soils Older organic soils formed during the Eemian Inter- glacial of the Pleistocene Period show slightly better physical and mechanical properties. Table 2 shows the physical properties of typical organic soils from the Eemian Interglacial: organic mud, low-carbonate mineral gyttja and high-carbonate mineral-organic gyttja. 3. Deformation and strength parameters 3.1. Holocene organic soils Experience from organic soils indicates that the value of constrained modulus M is not constant but is an effective stress dependency. For staged construction of embankments on organic soils, when effective vertical stress several times exceeds the initial preconsolidation pressure, for calculation of subsoil settlement not only the constrained modulus Moc for the preconsolidated state but also the constrained modulus Mnc for the normally consolidated state should be evaluated. Analysis of test results for Holocene organic soils indicates that based on DMT dilatometer tests the constrained modulus for the preconsolidated state Moc and for the normally consolidated state Mnc can be calculated using factors Roc M and Rnc M from the following relations [19]: Moc = Roc M · ED, (1) Mnc = Rnc M · ED, (2) for peat: Roc M = 0.20 + 1.60 · log KD, (3) Rnc M = 0.90 + 0.60 · log ID, (4) and for gyttja: Roc M = 0.12 + 2.10 · log KD, (5) Rnc M = 0.95 + 0.55 · log ID, (6) where: ED – dilatometer modulus, KD – horizontal stress index, ID – material index. The profiles of the constrained module for the organic subsoil at the Antoniny site evaluated from DMT dilatometer tests based on the Equations (1)-(6) are shown in Figure 1a and compared with the values obtained from the oedometer tests. A comparison of dilatometer data with the results of oedometer tests indicates that the relationship between the compression index Cc for normally 54 vol. 45/2023 Organic Soils for Geotechnical Structures Figure 1. Compression moduli and compression indexes for Holocene organic subsoil at the Antoniny site based on DMT dilatometer tests: a – moduli Moc and Mnc, b – compression indexes Cr and Cc. consolidated Holocene organic soils and material index ID can be expressed in the form [19]: Cc = a(ID)m. (7) The obtained values of empirical coefficients a and m for peat and gyttja are equal to 1.8, 0.7 and −0.30, −0.20, respectively. Analysis of test results indicates that the recompres- sion index Cr can be calculated from the relationship between the ratio Cr/Cc and the lateral stress index KD in the form [19]: Cc Cr = b(KD)n. (8) For Holocene organic soils the values of empirical coefficients are b = 0.27 and n = 1.9. The profiles of the recompression index Cr and the compression index Cc for the normally consolidated state for organic subsoil at the Antoniny site evaluated on the basis of the Equation (7) and (8) using the values of empirical coefficients mentioned above are shown in Figure 1b and are compared with the values obtained from the oedometer tests. Analysis of DMT and triaxial test results carried out by Lechowicz indicates that, particularly for peat and gyttja, the relationship between the normalized undrained shear strength and the horizontal stress index KD differs from that proposed by Marchetti [20, 21] and can be modified as follows [22, 23]: τfu σ′ v0 = S, (9) where S = (τfu/σ′ v0)nc is the normalized undrained shear strength for normally consolidated soil. The value of parameter S for peat is equal to 0.5, but for calcareous gyttja and calcareous-organic gyttja, it is at 0.40 and 0.45, respectively. A multi-factor relationship was proposed by Rabar- ijoely [25] to evaluate the undrained shear strength τfu of organic soils from DMT dilatometer tests based on the net value of the corrected first pressure read- ing (p0 − u0), the net value of the corrected second pressure reading (p1 − u0), and the effective vertical stress σ′ v0; and it is as follows: τfu = α0 · (σ′ v0)α 1 · (p0 − u0)α 2 · (p1 − u0)α 3 , (10) where α0, α1, α2, α3 are empirical coefficients, σ′ v0 is the in situ effective vertical stress, p0 and p1 are corrected pressures from DMT tests, and u0 is the in situ pore water pressure. The empirical coefficients in Equation (10) for organic soils can be evaluated as functions of the void ratio e and empirical coefficients Ci and Di shown in Table 3 according to the following formula [25]: αi = Ci · e + Di, (11) where subscript i = 0, 1, 2, 3. 1.1.1 Coefficients αi = Ci · e + Di i = 0, 1, 2, 3 i = 0 i = 1 i = 2 i = 3 Ci 0.149 −0.0233 0.0065 0.0114 Di 1.003 0.3406 0.1104 0.1847 Table 3. Values of empirical coefficients Ci and Di for Equation (11). The profiles of the undrained shear strength of Holocene organic soils at the Antoniny site before 55 Zbigniew Lechowicz, Katarzyna Goławska Acta Polytechnica CTU Proceedings Figure 2. Undrained shear strength of Holocene organic soils at the Antoniny site based on DMT dilatometer tests. Figure 3. Architecture of the two-layer artificial neural network 5–4–1 for the evaluation of undrained shear strength of Holocene organic soils based on DMT dilatometer tests [24]. embankment construction and after the 3rd embank- ment stage based on DMT dilatometer tests, using the Equations (9) and (10), are shown in Figure 2. Obtained values of undrained shear strength are com- pared with the values of corrected undrained shear strength from the field vane tests and the values from CKoU triaxial tests. The method of evaluating the undrained shear strength of organic soils from DMT dilatometer tests using the Artificial Neural Network was presented by Lechowicz et al. [24] based on the normalized net value of the corrected first pressure reading (p0 − u0)/σ′ v and the normalized net value of the corrected second pressure reading (p1 − u0)/σ′ v from dilatometer tests and organic soil properties such as the organic content Iom, void ratio e, and a stress history indicator (oc/nc). The architecture of the two-layer artificial neural network 5–4–1 for the evaluation of undrained shear strength of Holocene organic soils based on DMT dilatometer tests is shown in Figure 3. 3.2. Eemian organic soils For design of deep excavations supported by di- aphragm walls, the evaluation of the undrained shear strength τfu, the deformation modulus E0.1% at ver- tical strain ε1 of 0.1 % and the initial shear modulus G0, are needed. The deformation modulus E0.1% of Eemian organic soils from SDMT seismic dilatometer tests using the dilatometer modulus ED and the empirical coefficient RE can be evaluated based on Equation 12 [26]: E0.1% = RE · ED, (12) for Eemian organic mud: RE = 2.4 + 2.36 · log KD, (13) and for Eemian gyttja: RE = 2.15 + 2.10 · log KD. (14) The initial shear modulus G0 from SDMT seismic dilatometer tests using soil density ρ and shear wave 56 vol. 45/2023 Organic Soils for Geotechnical Structures Figure 4. Profiles of the deformation modulus E0.1% and shear modulus G0 from SDMT seismic dilatometer tests and the modulus values obtained from triaxial tests CD with measurements of shear wave velocity Vs for the Eemian gyttja from the Płocka underground station. Figure 5. Undrained shear strength profile from DMT dilatometer tests and τfu values from triaxial tests for the Eemian gyttja from the Płocka underground station. velocity Vs can be evaluated from the following equation: G0 = ρ · V 2 s . (15) Figure 4 shows the profiles of the deformation modulus E0.1% and the initial shear modulus G0 from SDMT tests and values obtained from the isotropically consolidated drained CD triaxial tests with shear wave velocity measurements Vs for the Eemian gyttja from the Płocka underground station. Figure 5 shows the profile of undrained shear strength from anisotropically consolidated undrained CK0U triaxial tests and from the SDMT test based on the empirical Equation (10). Empirical coefficients for Eemian gytjja are: α0 = 1.25, α1 = 0.30, α2 = 0.12, α3 = 0.30, and for the Eemian organic mud are: α0 = 1.12, α1 = 0.13, α2 = 0.10, α3 = 0.44. 4. Conclusions The paper presents the problem of evaluating the deformation and strength parameters of Holocene and Eemian organic soils obtained from DMT and SDMT dilatometer tests. Empirical relationships used to evaluate the constrained modulus Moc for the preconsolidated state and the constrained modulus Mnc for the normally consolidated state, as well as the recompression index Cr and the compression index Cc of Holocene peat and gyttja, are presented. The evaluation of undrained shear strength τfu of Holocene peat and gyttja from DMT dilatometer tests based on empirical relationships and the Artificial Neural Network is also shown. The evaluation of undrained shear strength τfu, deformation modulus E0.1% and initial shear modulus G0 of Eemian organic soils from DMT dilatometer tests and SDMT seismic 57 Zbigniew Lechowicz, Katarzyna Goławska Acta Polytechnica CTU Proceedings dilatometer tests is then presented. A comparison between the evaluated and obtained values from oedometer tests and triaxial tests shows a good agreement. References [1] J. Hartlén, W. Wolski. Embankments on organic soils. Elsevier, 1996. [2] J. M. Duncan, S. G. Wright, T. L. Brandon. Soil strength and slope stability. John Wiley & Sons, 2014. [3] W. Wolski, A. Szymanski, J. Mirecki, et al. Two stage-constructed embankments on organic soils. Field and laboratory investigations-Instrumentation-Prediction and observation of behaviour. Statens geotekniska institut, 1988. [4] W. Wolski, A. Szymanski, Z. Lechowicz, et al. Full-scale failure test on stage-constructed test fill on organic soil. Statens geotekniska institut, 1989. [5] M. Mirjalili, S. Kimoto, F. Oka, T. Hattori. Long-term consolidation analysis of a large-scale embankment construction on soft clay deposits using an elasto-viscoplastic model. Soils and Foundations 52(1):18–37, 2012. https://doi.org/10.1016/j.sandf.2012.01.010 [6] C. Zwanenburg, R. Jardine. Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat. Géotechnique 65(4):309–326, 2015. https://doi.org/10.1680/geot.14.P.257 [7] Z. Lechowicz, A. Szymanski. Creep behaviour of organic soils. Annals of Warsaw Agricultural University Land Reclamation 24:99–106, 1988. [8] R. Larsson. Behaviour of organic clay and gyttja. Rapport-Statens geotekniska institut 38, 1990. [9] Z. Lechowicz. An evaluation of the increase in shear strength of organic soils. In Advances in Understanding and Modelling The Mechanical Behaviour of Peat: Proceedings of the International Workshop, 16-18 June 1993, Delft, Netherlands, pp. 167–179 1994. [10] J. Desrues, R. Chambon, M. Mokni, F. Mazerolle. Void ratio evolution inside shear bands in triaxial sand specimens studied by computed tomography. Géotechnique 46(3):529–546, 1996. https://doi.org/10.1680/geot.1996.46.1.1 [11] J. Ching, K.-K. Phoon. Multivariate distribution for undrained shear strengths under various test procedures. Canadian Geotechnical Journal 50(9):907–923, 2013. https://doi.org/10.1139/cgj-2013-0002 [12] A. Madaschi, A. Gajo. One-dimensional response of peaty soils subjected to a wide range of oedometric conditions. Géotechnique 65(4):274–286, 2015. https://doi.org/10.1680/geot.14.P.144 [13] M. P. Acharya, M. T. Hendry, C. D. Martin. Creep behaviour of intact and remoulded fibrous peat. Acta Geotechnica 13:399–417, 2018. https://doi.org/10.1007/s11440-017-0545-1 [14] G. Grimstad, S. A. Degago, S. Nordal, M. Karstunen. Modeling creep and rate effects in structured anisotropic soft clays. Acta Geotechnica 5:69–81, 2010. https://doi.org/10.1007/s11440-010-0119-y [15] M. Karstunen, Z.-Y. Yin. Modelling time-dependent behaviour of murro test embankment. Géotechnique 60(10):735–749, 2010. https://doi.org/10.1680/geot.8.P.027 [16] Z.-Y. Yin, M. Karstunen. Modelling strain-rate-dependency of natural soft clays combined with anisotropy and destructuration. Acta Mechanica Solida Sinica 24(3):216–230, 2011. https://doi.org/10.1016/S0894-9166(11)60023-2 [17] Z.-Y. Yin, Q. Xu, C. Yu. Elastic-viscoplastic modeling for natural soft clays considering nonlinear creep. International Journal of Geomechanics 15(5):A6014001, 2015. https: //doi.org/10.1061/(ASCE)GM.1943-5622.0000284 [18] N. Sivasithamparam, M. Karstunen, P. Bonnier. Modelling creep behaviour of anisotropic soft soils. Computers and Geotechnics 69:46–57, 2015. https://doi.org/10.1016/j.compgeo.2015.04.015 [19] Z. Lechowicz, S. Rabarijoely. Evaluation of organic subsoil settlement from dilatometer test. In Problematic soils, pp. 115–118. 1998. [20] S. Marchetti. In situ tests by flat dilatometer. Journal of the geotechnical engineering division 106(3):299–321, 1980. https://doi.org/10.1061/AJGEB6.0000934 [21] S. Marchetti, P. Monaco, G. Totani, M. Calabrese. The flat dilatometer test (dmt) in soil investigations - A report by the ISSMGE Committee TC16. In Proceedings of the International Conference on In situ Measurement of Soil Properties and Case Histories, Bali, Indonesia, p. 41. 2001. [22] Z. Lechowicz. Undrained shear strength of organic soils from dilatometer test. Annals of Warsaw Agricultural University Land Reclamation 28:85–96, 1997. [23] Z. Lechowicz, S. Rabarijoely. Use of dilatometer test in evaluation of organic subsoil strengthening. In Proceedings of the Conference on Recent Advances in Soft Soil Engineering, vol. 1, pp. 185–196. 1997. [24] Z. Lechowicz, M. Fukue, S. Rabarijoely, M. J. Sulewska. Evaluation of the undrained shear strength of organic soils from a dilatometer test using artificial neural networks. Applied Sciences 8(8):1395, 2018. https://doi.org/10.3390/app8081395 [25] S. Rabarijoely. The use of dilatometer test for evaluation of organic soil parameters [in Polish: Wykorzystanie badań dylatometrycznych w wyznaczeniu parametrów gruntów organicznych obciążonych nasypem]. Ph.D. thesis, Warsaw Agricultural University-SGGW, Land Reclamation Warszawa, Poland, 2000. [26] Z. Lechowicz, M. Bajda, S. Rabarijoely, G. Wrzesiński. Use of SDMT for the evaluation of the geotechnical parameters of organic soils. In: CPTU and DMT in soft clays and organic soils by Z Młynarek & J Wierzbicki, Poznań, Wydawnictwo Exemplum pp. 107–118, 2014. 58 https://doi.org/10.1016/j.sandf.2012.01.010 https://doi.org/10.1680/geot.14.P.257 https://doi.org/10.1680/geot.1996.46.1.1 https://doi.org/10.1139/cgj-2013-0002 https://doi.org/10.1680/geot.14.P.144 https://doi.org/10.1007/s11440-017-0545-1 https://doi.org/10.1007/s11440-010-0119-y https://doi.org/10.1680/geot.8.P.027 https://doi.org/10.1016/S0894-9166(11)60023-2 https://doi.org/10.1061/(ASCE)GM.1943-5622.0000284 https://doi.org/10.1061/(ASCE)GM.1943-5622.0000284 https://doi.org/10.1016/j.compgeo.2015.04.015 https://doi.org/10.1061/AJGEB6.0000934 https://doi.org/10.3390/app8081395 Acta Polytechnica CTU Proceedings 45:1–6, 2023 1 Introduction 2 Characteristics of the organic soils 2.1 Holocene organic soils 2.2 Eemian organic soils 3 Deformation and strength parameters 3.1 Holocene organic soils 3.2 Eemian organic soils 4 Conclusions References