- Open Access
Deformation behaviors of peat with influence of organic matter
© The Author(s). 2016
- Received: 23 November 2015
- Accepted: 26 April 2016
- Published: 10 May 2016
Peat is a kind of special material rich in organic matter. Because of the high content of organic matter, it shows different deformation behaviors from conventional geotechnical materials. Peat grain has a non-negligible compressibility due to the presence of organic matter. Biogas can generate from peat and can be trapped in form of gas bubbles. Considering the natural properties of peat, a special three-phase composition of peat is described which indicates the existence of organic matter and gas bubbles in peat. A stress–strain–time model is proposed for the compression of organic matter, and the surface tension effect is considered in the compression model of gas bubbles. Finally, a mathematical model has been developed to simulate the deformation behavior of peat considering the compressibility of organic matter and entrapped gas bubbles. The deformation process is the coupling of volume variation of organic matter, gas bubbles and water drainage. The proposed model is used to simulate a series of peat laboratory oedometer tests, and the model can well capture the test results with reasonable model parameters. Effects of model parameters on deformation of peat are also analyzed.
- Compressibility of organic matter
- Entrapped gas bubbles
- Consolidation model
Peat is a kind of engineering material rich in organic matter. It has a widely distribution around the world and shows unique compression properties. Because of the complicated physical composition of peat, it has been recognized that the deformation of this material is extremely complex. Peat may undergo an axial strain as large as 50 % due to the highly compressible property of natural deposits (Berry and Poskitt 1972). The textures of peat natural deposits and the high content of organic matter have significant effects on the deformation behavior of peat.
Under appropriate climatic and topographic conditions, organic matter in peat is derived from vegetation that has been chemically changed and fossilized (Dhowian and Edil 1980). Minerals or solid phase are usually considered incompressible in soil, but it may not be appropriate for peat with high organic matter content. The organic matter phase or peat grains could be compressible, which could be an important factor effecting deformation properties of peat. Although some researchers have noticed this (Bery and Vickers 1975; Robinson 2003), no similar studies have been done to consider this point of view in peat. It’s necessary to understand how organic matter affects the deformation process of peat.
With the high content of organic matter, another feature of peat is that biogas (e.g., methane) can generate from its natural deposits. During generation and migration, biogas can be trapped in the micro voids of peat as small gas bubbles. For materials with gas bubble entrapment, the deformation behaviors and other mechanical properties are different from traditional unsaturated conditions where gas phase is assumed connected (Wheeler 1988; Sills et al. 1991). Materials containing gas bubbles are considered as a special type of engineering materials and are usually considered as “quasi-saturated” (Faybishenko 1995). In these materials, gas phase could be present as isolated bubbles once the water saturation degree is larger than 85 % (Sparks 1963). Studies have shown that gas bubbles present in offshore soils due to the decomposition of sedimentary organic matter (Whelan et al. 1975). With the present of gas bubbles, immediate undrained compressions have been found in gassy soils (Nageswaran 1983). Thus, the existence of entrapped gas bubbles within peat can exert a significant influence on the properties and deformation behaviors of peat.
Terzaghi’s one-dimensional consolidation theory has been widely used in the deformation problem for porous materials. Some researchers have extended the consolidation theory by considering the compressibility of solid phase and the existence of gas phase (Skempton 1961; Fredlund and Hasan 1979; Lade and De Boer 1997). In most cases, the consolidation theory is used for mineral materials like soil. The deformation behavior of peat may not be well-characterized by the traditional one dimensional consolidation theory due to the high organic matter content and gas bubble entrapment.
Some unique consolidation behaviors (e.g., large deformation, immediate settlement after loading and low permeability) have been observed in peat (Berry and Poskitt 1972; Long and Boylan 2013; Lee et al. 2015). It is thus important to propose an appropriate consolidation model to describe the mechanical characteristics and the deformation behavior of peat. In this paper, a consolidation model is proposed for peat under the three phase composition of this material. The model considers the compressibility of organic matter and entrapped gas bubbles in peat. The deformation of organic matter is described by a stress and time dependent empirical model. The mechanical properties of entrapped gas bubbles are studied as ideal gas. The proposed consolidation model is applied on a set of peat oedometer tests and the model can well describe the one dimensional consolidation behavior of peat.
High in organic matter content
Gas bubble entrapment
The temperature is considered constant during tests, the surface tension q is dependent on the temperature, therefore q is constant as well. And the diminution of surface tension q with increasing air pressure can be neglected as discussed by Schuurman (1966), so a constant value (7.4 × 10−3 N/m) of q is used in the paper.
Entrapped gas could be exist as the form of small bubbles compared with average particle size or large gas voids. Wheeler (1988) and Pietruszczak and Pande (1996) discussed the difference between the two kinds of gassy soils. When the gas bubbles are small compared with peat particle size, the bubbles fit within the normal void spaces and the radius of curvature of gas–water interface is equal to the radius r of the bubble. At the opposite extreme, gas bubbles are much larger than peat particle size, which generates a large gas-filled void. Then the gas–water interfaces are formed by lots of small menisci which bridge the gaps between the particles. The radius of curvature of these menisci is not necessarily equal to the radius r of the bubble. As a simplification, the size of small gas bubbles is assumed to be trapped within the voids of peat grains.
Electron microscope scanning tests of peat from different places have been carried out by some researchers. Lv et al. (2011) obtained the results that the average void diameter is about 10 μm and the large void is up to 25 μm diameter for peat samples from northern east China. Xiong (2005) and Liu et al. (2014b) got the average void diameter is about 13.65 μm for peat samples from Kunming. A void diameter range of 3–20 μm is obtained by Wang (2013)and their tested peat samples are from Hangzhou, east China. Considering the void size of peat, the radius r of gas bubbles can be determined and it should be smaller than void sizes. An average initial radius r 0 of gas bubbles are used in the following case studies.
Three phase composition of peat
Usually the basic geotechnical indexes of peat are known quantities including density of peat (ρ), water content (ω), organic matter content (ω m ) and unit weight of peat solid phase (γ p ), then above defined parameters can be calculated with mass and volume conservation for a certain peat sample.
Based on above assumptions and parameter definitions, a mathematical model for peat deformation is derived with considering the compression of organic matter and entrapped gas bubbles.
Based on above constitutive relations, the finial mathematical equation can be obtained. But the three parts dV c/dt, dV m/dt and dV g/dt in the right side of Eq. (11) need to be determined respectively.
In Eq. (17), the calculation of ∂a/∂σ ′ is similar to ∂e/∂σ ′ (compression coefficient c) of Terzaghi’s theory. For a certain peat sample, the value is calculated by the total increment Δa/Δσ ′ in the whole consolidation process.
The other two parts dV m/dt and dV g/dt in Eq. (11) represent the compression behaviors of organic matter and entrapped gas bubbles respectively, which can be determined based on the discussions in above sections.
One-dimensional finite difference form of Eq. (24) is employed to solve this equation in Excel. By fitting the numerical and experimental consolidation curves, the compressibility of organic matter and gas content can be determined. The model is applied on a set of historical consolidation data of peat in the following sections. The calculation results agree well with test results in each case study.
Case 1: Peat from northern east of China
Properties of peat samples in case 1
γ p (kN/m3)
ω m (%)
Model parameters for peat samples in case 1
E m (MPa)
S g (%)
r 0 (μm)
1.2 × 10−10
1.5 × 10−10
1.7 × 10−10
2.0 × 10−10
4.0 × 10−10
Table 2 shows that the gas content (S g ) and the parameters of organic matter compression (E m , λ) obtained from the model are different in the five samples. In general, the gas content S g increases and the compressive modulus E m decreases with the increasing of organic matter content as shown in Tables 1 and 2. The time factor λ has a slight diminution with the increasing of organic matter content, but it keeps in a range of 0.21–0.25. The compression modulus of the organic matter E m ranges from 1.0 to 5.0 MPa. The values are slightly higher than the initial constrained modulus E 0 of Szczecin peat found by Meyer (1997). This may be due to the fact that the initial constrained modulus considers the compressibility of both peat and gas in peat samples. Gas content is within the range of 2–5 %, and the value increases with organic content and void ratio as shown in Table 2, but the conclusion on this can not be made based on this result.
Case 2: Middleton peat from Wisconsin, USA
Properties of peat samples in case 2
γ p (kN/m3)
ω m (%)
Model parameters for peat samples in case 2
E m (MPa)
S g (%)
r 0 (μm)
6.0 × 10−10
6.0 × 10−10
6.0 × 10−10
6.0 × 10−10
6.0 × 10−10
The compression modulus E m of the organic matter obtained for the peat samples is relatively consistent comparing with that of the peat samples in case 1. This may due to the fact that the peat samples used in Mesri et al. (1997) are from a 2.5 m by 2.5 m test pit, which suggests that the variation of the properties in the peat samples could be less comparing to the samples used by Lv et al. (2011) where the peat samples are from a relatively larger area. The peat samples used in Mesri et al. (1997) have the same organic matter content around 92.7 %, on the contrary the 5 peat samples used by Lv et al. (2011) have an organic matter content range of 36.34–85.36 %. The time factor λ in this case has similar values with peat samples in case 1. What need to be noted in Table 4 is that the values of E m and λ of samples T10 and T13 are a little larger than others. This may be due to the different loading levels applied on the samples. The vertical stress on samples T10 and T13 (30–41 kPa) are smaller than others (90–96 kPa). Apart from that, the parameters of the organic matter compression from the two cases are in a similar range.
In above 2 cases, the theoretical predictions of deformation or strain for peat samples show very close agreement with test results. The figures show that the proposed model is suitable for peat. For the deformation or strain curves of peat, an obvious initial deformation appears in a relative short time during the initial loading period. Then the strain rate tends to be slow with gradually completion of primary consolidation. But a significant deformation still develops during the following consolidation process. Plausible explanations of these phenomena are offered and mathematical treatments have been given in our model.
As mentioned, the author has done some preliminary studies on Victorian brown coal from Latrobe Valley, Australia. The compression modulus E m of peat organic matters obtained from the calculation is much lower than the values of coal (about 30 MPa). That is because peat samples are normally consolidated but brown coal in the Latrobe Valley is highly over consolidated with overconsolidation ratio of 10 or above at the depth where the samples been taken. Therefore, the fibrous structure in peat is much more compressive comparing to coal grains. The time factor λ for peat obtained from the model is also higher than that of brown coal (0.042). This may due to the fact that the hollow structure in the fibers of peat samples are still well maintained as shown in Mesri et al. (1997), and the deformation of the hollow fibers may contribute to the creep of the organic particles which results in higher λ values. Although similar properties have been found in both peat and brown coal, different model values are obtained for the two materials. This may mainly due to the geological history, material structure and different basic indexes. Brown coal is usually fossilized from peat after a long time of coalification process. Peat has a lower density and higher void ratio and water content than brown coal.
Volume ratio of gas e g
A value range of 2–5 % for S g is obtained in the model calculation. Literature reviewing shows that peat from field usually has a gas volume content of around 5–11 % (Hobbs 1986; Mesri et al. 1997). Considering peat samples are usually water saturated for a short time before testing, so a value range of 2–5 % for S g is reasonable in the calculation.
Volume ratio of organic matter em
Time factor λ
Parameter λ is a time factor in proposed compression model of organic matter. A relatively stable value range of 0.20–0.25 for λ is obtained in model calculation. The value is acceptable with slight variation, which reflects the deformation properties of organic matter with time t. The slight variation of λ could be caused by reasons like different organic matter content of peat, geological history and applied stress levels.
Initial constrained modulus Em
In above sections, a numerical model is established to study the consolidation behavior of peat containing organic matter and gas bubbles. In the model, the compression modulus E m and time factor λ which describe the compression properties of organic matters, gas content S g and initial gas bubble radius r 0 are used. To study the effect of each parameter on the consolidation behavior of peat, sensitivity analyses are carried out in this section. In the analysis, only one of the 4 parameters is considered changing to calculate the consolidation curves. Taking peat sample 3 in case study 1 as an example in the following studies. What need to be noted is that the calculated curves are not all real for sample 3. The work mainly wants to show how the tendency of consolidation curves effected by a certain parameter.
Effect of organic compression modulus
Effect of time factor
Effect of gas content
Effect of initial gas bubble radius
Peat has special natural characteristics and engineering properties due to the high content of organic matter. The composition and structure of peat are complicated, and organic matter in peat can be compressible. Biogas generates from the natural deposits of peat and some will be trapped as small gas bubbles. Peat has special three phase composition containing gas bubbles, water, compressible organic matters and incompressible minerals. A consolidation model is proposed to study the deformation behavior of peat. The deformation of peat is considered as a coupling process of the volume change of gas bubbles, compression of organic matter and the drainage of water. Then the application of the model is carried out on some historical test data of peat by fitting the experimental results with the modelled consolidation curves.
The results show that the proposed model can well capture the unique consolidation behavior of peat. The gas content and the compression parameters of organic matter can be obtained using the model. Based on the experimental and numerical modelling results, conclusions can be obtained:
1: The obvious initial settlement observed in peat samples is due to the existence of gas bubbles and the compressibility of organic matters. The proposed model can be used to simulate this process, and as a result the gas content and compression parameters of the organic matter can be obtained by fitting the experimental and modeled results.
2: Based on the modeled results of the samples, the compression modulus of organic matter in peat is in a range of 0.8–5.0 MPa. The large range variation of this value is probably due to the organic matter content. The creep effect observed in peat at the late stage of the consolidation tests can be modelled with the stress–strain–time model of organic matter by introducing a time factor. The late stage deformation in peat is relatively large due to the presence of organic matter, which has been reflected by a large value of time factor in around 0.20–0.25 for the peat samples.
3: The mechanism of gas is very complicated in practical situation. In the proposed consolidation model, gas bubbles are simply assumed to follow Boyle’s law with considering the surface tension effect. Increasing gas content can cause larger settlements of peat and the surface tension effect can not be neglected when considering small gas bubble sizes.
The proposed model could be used in analyzing the consolidation behavior of peat, which contains both gas bubbles and compressible organic matters. It has the potential to be used for modelling compression behavior of similar engineering materials, for example brown coal.
KL established the main model, collected the data, performed analysis on the data, wrote first draft of the manuscript; MY conceived and suggested the main idea of the study, edited the manuscript. All authors have read and approved the final manuscript.
The research is supported by the National Natural Science Foundation of China under Grant No. 41572258. Part of the work in this paper was completed in Monash University, Australia where the first author worked as a visiting student. The authors acknowledge the support received from the organizations.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Berry PL, Poskitt TJ (1972) The consolidation of peat. Géotechnique 22(1):27–52View ArticleGoogle Scholar
- Bery PL, Vickers B (1975) Consolidation of fibrous peat. J Geotech Eng ASCE 101(8):741–753Google Scholar
- Choo H, Bate B, Burns SE (2015) Effects of organic matter on stiffness of overconsolidated state and anisotropy of engineered organoclays at small strain. Eng Geol 184:19–28. doi:10.1016/j.enggeo.2014.10.022 View ArticleGoogle Scholar
- Dhowian AW, Edil TB (1980) Consolidation behavior of peats. ASTM Geotech Test J 3(3):105–114View ArticleGoogle Scholar
- Faybishenko BA (1995) Hydraulic behavior of quasi-saturated soils in the presence of entrapped air: laboratory experiments. Water Resour Res 31(10):2421–2435. doi:10.1029/95WR01654 View ArticleGoogle Scholar
- Fredlund DG, Hasan JU (1979) One-dimensional consolidation theory: unsaturated soils. Can Geotech J 16(3):521–531. doi:10.1139/t79-058 View ArticleGoogle Scholar
- Hayashi JI, Li CZ (2004) Advances in the science of Victorian brown coal: structure and properties of Victorian brown coal. Elsevier, MelbourneGoogle Scholar
- Hobbs NB (1986) Mire morphology and the properties and behavior of some British and foreign peats. Q J Eng Geol Hydrogeol 19(1):7–80. doi:10.1144/GSL.QJEG.1986.019.01.02 View ArticleGoogle Scholar
- Lade PV, De Boer R (1997) The concept of effective stress for soil, concrete and rock. Géotechnique 47(1):61–78. doi:10.1680/geot.1918.104.22.168 View ArticleGoogle Scholar
- Landva AO, Pheeney PE (1980) Peat fabric and structure. Can Geotech J 17(3):416–435. doi:10.1139/t80-048 View ArticleGoogle Scholar
- Lee JS, Seo SY, Lee C (2015) Geotechnical and geophysical characteristics of muskeg samples from Alberta, Canada. Eng Geol 195:135–141. doi:10.1016/j.enggeo.2015.04.030 View ArticleGoogle Scholar
- Lin HD, Wang CC (1998) Stress–strain–time function of clay. J Geotech Geoenviron Eng 124(4):289–296View ArticleGoogle Scholar
- Liu K, Mackay R, Xue JF, Tolooiyan A (2014a) Experimental study of brown coal hydraulic behavior at low confining stress. In: Unsaturated soils: research and applications—proceedings of the 6th international conference on unsaturated soils, Sydney, pp 1125–1130Google Scholar
- Liu Y, Cao GZ, Meng YG, Liu MX (2014b) Study on the microstructure feature and strength mechanism of the Tien Lake peat soil. Adv Mater Res 864:2695–2702. doi:10.4028/www.scientific.net/AMR.864-867.2695 View ArticleGoogle Scholar
- Long M, Boylan N (2013) Predictions of settlement in peat soils. Q J Eng Geol Hydrogeol 46(3):303–322. doi:10.1144/qjegh2011-063 View ArticleGoogle Scholar
- Lv Y, Nie L, Xu Y, Liu F, Zheng M (2011) The mechanism of organic matter effect on physical and mechanical properties of turfy soil. Chin J Geotech Eng 33(4):655–660Google Scholar
- Mesri G, Febres CE, Shields DR (1981) Shear stress–strain–time behavior of clays. Géotechnique 31(4):537–552View ArticleGoogle Scholar
- Mesri G, Stark TD, Ajlouni MA, Chen CS (1997) Secondary compression of peat with or without surcharging. J Geotech Geoenviron 123(5):411–421. doi:10.1061/(ASCE)1090-0241(1997)123:5(411) View ArticleGoogle Scholar
- Meyer Z (1997) Consolidation model for organic soils. Proc ICE Gr Improv 1(4):239–248. doi:10.1680/grim.1922.214.171.124 Google Scholar
- Nageswaran S (1983) Effect of gas bubbles on the sea bed behaviour. Dissertation, Oxford UniversityGoogle Scholar
- Pichan SP, O’Kelly BC (2012) Effect of decomposition on the compressibility of fibrous peat. In: ASCE GeoCongress, pp 4329–4338. doi:10.1061/9780784412121.445
- Pietruszczak S, Pande GN (1996) Constitutive relations for partially saturated soils containing gas inclusions. J Geotech Eng ASCE 122(1):50–59. doi:10.1061/(ASCE)0733-9410(1996)122:1(50) View ArticleGoogle Scholar
- Robinson RG (2003) A study on the beginning of secondary compression of soils. J Test Eval 31(5):388–397Google Scholar
- Schuurman IE (1966) The compressibility of an air/water mixture and a theoretical relation between the air and water pressures. Géotechnique 16(4):269–281. doi:10.1680/geot.19126.96.36.1999 View ArticleGoogle Scholar
- Sills GC, Wheeler SJ, Thomas SD, Gardner TN (1991) Behaviour of offshore soils containing gas bubbles. Géotechnique 41(2):227–241. doi:10.1680/geot.19188.8.131.52 View ArticleGoogle Scholar
- Singh A, Mitchell JK (1968) General stress–strain–time function for soils. J Soil Mech Found Div 94(1):21–46Google Scholar
- Skempton AW (1961) Effective stress in soils, concrete and rocks. In: Proceedings of conference on pore pressure and suction in soils, Butterworths, London, pp 4–16Google Scholar
- Sparks ADW (1963) Theoretical considerations of stress equations for partly saturated soils. In: Proceedings of 3rd African conference on soil mechanics and foundation engineering, Salisbury, Rhodesia, pp 215–218Google Scholar
- Wang FZ (2013) Settlement observation of a pedestrian bridge and investigation of underlying West Lake peat soil behavior. Dissertation, Zhejiang UniversityGoogle Scholar
- Wheeler SJ (1988) A conceptual model for soils containing large gas bubbles. Géotechnique 38(3):389–397. doi:10.1680/geot.19184.108.40.2069 View ArticleGoogle Scholar
- Whelan TIII, Coleman JM, Suhayda JN (1975) The geochemistry of recent Mississippi River Delta sediments: gas concentration and sediment stability. In: Proceedings of the 7th offshore technology conference, Houston, vol 1, pp 71–83Google Scholar
- Xiong EL (2005) Research on physical properties and relationship between strain and stress of peat & peaty soil in Yunnan. Dissertation, Kunming University of Science & TechnologyGoogle Scholar