Download Stress-strain behaviour of agricultural soils

Stress-strain behaviour of agricultural soils: towards new approaches (StressSoil)
Synopsis: Soil compaction is a serious problem in modern agriculture. State-of-the-art soil
compaction models derive from foundation engineering and consider soils as continuous and
homogeneous media. However, agricultural soils are structured at different scales. Modelling
mechanical strength as a probability-distribution function will account for anisotropy and
scale-dependency of soil physical properties. Discrete elements models can integrate this
information. This project will evaluate and implement the new approach, and hence
contribute to more precise prediction, i.e. prevention of soil compaction.
Fig. 1. Soil mechanical properties are, like soil
structure, scale dependent and their distribution in
the soil profile follows a probability distribution.
This can be included in discrete elements models
(DEM), in contrast to state-of-the-art compaction
models, which consider soil as an elastic or elastoplastic continuous material. The different concepts
are illustrated here by the probability, P, of the soil
tensile strength τ, as an example.
1. Objectives
Prevention of soil compaction requires accurate predictions of stress propagation in the soil
profile to be compared to reliable soil failure criteria (i.e. soil strength quantification). Up to
date, soil compaction models consider agricultural soils as elastic or elasto-plastic continuous
materials when calculating stress propagation, and soil failure is evaluated from
measurements of bulk soil compressive strength in the laboratory. However, agricultural soils
are aggregated at various scales (Fig. 1). Hence, Soil resistance to deformation is as
stochastic and scale-dependent as the structure. Discrete element models (DEM) offer a
framework that can be used to understand the effect of stochastic structural features on stress
propagation and deformation behaviour of aggregated agricultural soils (Fig. 1). We postulate
that soil mechanical properties are scale dependent and that their distribution in the soil
profile follows a probability distribution that can be described by discrete elements models
(DEM). Our approach centres on the followings hypotheses: H1: Inter-particle bonds are the
relevant soil mechanical parameters for the definition of the soil failure criterion (WP1, WP2,
WP4, WP6) (“particle” refers to textural particles and structural units). H2: Inter-particle
bonds follow a continuous probability distribution (e.g. Weibull distribution) integrating their
1
scale dependency (WP1). H3: The parameters of the probability distribution describing
mainly cohesion and friction will enhance the prediction of soil compaction through DEM
simulations (WP1, WP3, WP4, WP5, WP6, WP7). Based on these hypotheses, the main
project objective is to implement an alternative model for predicting soil compaction ready
for use in decision support tools created by the project consortium in other running projects.
2. State-of-the-art
Soil compaction is a major threat to a sustained soil quality in modern, mechanized
agriculture. A range of important ecological functions are affected when soil is compressed:
increase in surface runoff, water erosion, loss of nutrients and pesticides to the groundwater,
increase in the production of greenhouse gases, reduction in crop yields (e.g. van Ouwerkerk
and Soane, 1995). Soil compaction is defined as a reduction in volume of a soil element
subjected to mechanical stresses. On arable soil, these mechanical stresses originate mostly
from traffic by agricultural vehicles.
At present, the prediction of sustainable traffic on heterogeneous agricultural soils is
based on: (i) calculations of stress transmission where soils are considered as perfectly elastic
or elasto-plastic materials with isotropic mechanical properties; (ii) methodological
approaches for assessment of soil strength that were developed for foundation engineering
where large static loads are applied on homogeneous soils. State-of-the-art soil compaction
models suffer from drawbacks such as insufficient knowledge about the effects of soil
conditions (i.e. soil type, structure, moisture, density, etc.). The continued use of these
models is despite even early work identified influences of soil structural heterogeneities on
stress propagation (Taylor and Burt, 1987; Dexter et al., 1988). Recent studies further
documented that the analytical model based on the elasticity theory could not describe the
stress propagation in arable soils when deformations occurred (Lamandé et al., 2007; Keller
and Lamandé, 2010; Lamandé and Schjønning, 2011). Even more important is the
inadequacy of classical tests for soil compressive strength in predicting soil failure during
wheel traffic in agricultural soils (e.g. Trautner, 2003; Arvidsson and Keller, 2004; Keller et
al., 2004; Mosaddeghi et al., 2007; Keller et al., 2011). Three main reasons for this
inadequacy can be identified: (i) during traffic, stress application is thousands times shorter
than during the laboratory test (Keller & Lamandé, 2010), and the longer stresses are applied
the larger the final strain (Or and Ghezzehei, 2002); (ii) the direction of major principal stress
is not constant during traffic, which means that shear as well as compressive strength should
be taken into account (Kirby, 1991; Lamandé et al., 2007); (iii) neither soil structure nor soil
strength are isotropic or equally distributed at all scales (Peng and Horn, 2008). Soil
2
mechanical properties, needed to calculate stress-strain behaviour, depend on the soil type, on
the scale and are anisotropic. Probability-distribution functions (e.g. Weibull, 1951) have
successfully been used to describe particle size distribution (Rosin and Rammler, 1933;
Keller and Håkansson, 2010; Keller et al., 2011), aggregate size distribution (Perfect et al.,
1993; Keller et al., 2011) or tensile strength (e.g., Munkholm et al., 2002).
Soils are structured (voids and compound particles of different size and shape) and
fractured (e.g. drying cracks) (e.g. Dexter, 1988), which implies that soil may be better
characterized as a granular (where the elements represent the soil structural features) than a
continuum material. Discrete element models (DEM; e.g. Cundall & Strack, 1979) offer a
framework that can be used to understand particle-particle contact properties, stress
propagation (Radjai, 1998) and displacement behaviour of granular media. Recent studies in
granular material science focused on the influence of various local properties such as particle
size distribution (Voivret et al., 2007), frictional sliding and rolling (Estrada et al., 2008),
particle shape (Azema et al, 2009), and cohesion (Delenne et al., 2004) on stress-strain
behaviour of geomaterials. To our knowledge, DEM has not been used to simulate soil
compaction due to agricultural field traffic, but it may provide a promising method for better
understanding soil deformation and stress transmission at different scales (Van Baars, 1996;
Delenne et al., 2004; Zhang and Li, 2006; Lamandé et al., 2010).
3. Project organisation
The project will be lead by Per Schjønning, who has extensive experience in leading research
projects and supervising PhD students. A leader for each of the seven workpackages has been
appointed (please see Section 4), reflecting the complementary scientific experience of the
research group. A PhD study, funded by the Dept. of Agroecology and Environment (Aarhus
University), including both experimental and modelling work, will be conducted in the
project and supervised by project participants. It will focus on the definition of scaledependent soil strength expressions and their implementation in DEM (WP1, WP3, WP5).
4. Research plan and methodology
The project is composed of three themes. Theme 1 focuses the soil mechanical properties on
a discrete element basis. In theme 2 we will perform experiments on the soil as a system. And
in theme 3 we put the knowledge together in models.
3
Theme 1. Soil mechanical properties
WP
1.
Scale-dependent
soil
strength
expressions (Mathieu Lamandé)
Objective and approach: To quantify
mechanical strength as a probabilitydistribution function. Shear parameters
(cohesion and friction) will be measured
using the direct shear box method on soil
cores sampled systematically on a grid in
soil profiles and on samples of different
sizes. This sampling will be repeated for
several soils presenting contrasted texture
and structure, and different soil water
suction for each soil type. A probabilitydistribution function (e.g. Weibull, 1951)
will be fitted to the experimental data, and
the adjusted values for the parameters of
this function will be related to intrinsic soil
properties.
WP 2. Soil failure and soil functions (Per Schjønning)
Objective and approach: To quantify soil failure effects on soil pore functioning for different
stress fields, i.e. for different combinations of the major and minor principal stress. Soil
samples are collected at different coordinates in the profile of a soil subjected to controlled
stress application. Air permeability at controlled water suction is used as a structural
fingerprint reflecting the effect of compressive and shear strains.
Theme 2. System experimentation
WP 3. Test for an idealized discrete element soil (Mathieu Lamandé)
Objective and approach: To characterise stress-strain behaviour of beads of soil aggregates
under loading. Soil aggregates will be sampled in the field. Tensile strength and compressive
strength of each aggregate type/size, and cohesion between aggregates will be measured, at
controlled water suction, in the laboratory. Soil aggregates will be rearranged in a device
equipped with stress sensors in the laboratory. The walls of the device will be transparent to
allow the visualisation in 2D of aggregate movement and deformation during loading.
WP 4. Tests for real soil (Mathieu Lamandé)
Objective and approach: To quantify vertical and horizontal stresses in situ during loading
(Lamandé et al., 2006), as well as deformation in 3D using very sensitive micro4
accelerometers (MicroStrain®). Stochastic variations will be covered by the measurements
program to be able to relate to the activities in WP 1. Strains will be related to soil pore
functions measured in WP2.
Theme 3. Discrete Element Modelling
WP 5. Stress-strain behaviour simulated with DEM (Jean-Yves Delenne)
Objectives and approach: (i) To implement the mechanical strength as a probabilitydistribution function in DEM, and (ii) to simulate traffic on the idealized discrete elements
soil. Experiments of WP3 and WP4 will be used to parameterize DEM simulations:
compressive and tensile strength of aggregates will be used as input values to the model.
Simulations outputs will be evaluated by the stress-strain measurements performed in WP3
and WP4. A sensitivity analysis of the effect of the probability-distribution function
parameters on the stress propagation will be conducted.
WP 6. Soil failure criteria (Thomas Keller)
Objective and approach: To develop new soil failure criteria from the scale-dependent soil
strength expressions (WP1) and soil pore functions (WP2). The new failure criteria will
combine the probability distribution for soil strength with characteristics of the stress field.
WP 7. Synthesis: DEM for decision support (Mathieu Lamandé)
Objective and approach: To develop a pedo-transfer function for the parameters of the
probability-distribution function. Interpretation of data from WP1 and meta-analysis of a
comprehensive data set collected in a previous project (Schjønning, 1991, 1999) will provide
the basis for correlations between intrinsic soil properties (e.g. soil texture) and soil water
suction on one side and the probability distribution for soil strength on the other side. An
alternative model for predicting soil compaction ready for use in the TERRANIMO decision
support tool (www.soilcompaction.eu) decision support tool will be implemented.
5. Dissemination
The www.soilcompaction.eu web platform will be used by the project group and added with a
component describing achievements of the StresSoil project to the public. The project results
will be integrated in the TERRANIMO decision support tool available to the public at the
same platform. The wheel load carrying capacity maps of Europe produced by the ICT-AGRI
project “PredICTor” as a deliverable to the EU Joint Research Centre will similarly be
updated with the discrete element approach. The StressSoil project aims at eight peerreviewed publications covering the basic soil mechanical approach as well as the impacts of
the results on decision support tools.
5
References
Arvidsson, J. & Keller, T. 2004. Soil precompression stress. I. A survey of Swedish arable
soils. Soil Tillage Res. 77, 85-95.
Azéma, E., Radjai, F., Saussine, G., 2009. Quasistatic rheology, force transmission and fabric
properties of a packing of irregular polyhedral particles. Mechanics of Materials 41, 729741.
Cundall, P.A., Strack, O.D.L., 1979. A discrete numerical model for granular assemblies.
Géotechnique 29, 47-65.
Delenne, J.-Y., El Youssoufi, M.S., Cherblanc, F., Bénet, J.-C., 2004. Mechanical behaviour
and failure of cohesive granular materials. Int. J. Numerical Analytical Methods
Geomechanics 28, 1577-1594.
Dexter, A.R., 1988. Advances in characterization of soil structure. Soil Tillage Res. 11, 199–
238.
Dexter, A.R., Horn, R., Holloway, R., Jakobsen, B.F., 1988. Pressure transmission beneath
wheels in soils on the Eyre peninsula of South Australia. J. Terramechanics 25, 135-147.
Estrada, N., Taboada, A., Radjai, F., 2008. Shear strength and force transmission in granular
media with rolling resistance. Phys. Rev. E 78, 021301.
Keller, T., Håkansson, I., 2010. Estimation of reference bulk density from soil particle size
distribution and soil organic matter content. Geoderma 154, 398-406.
Keller T., Lamandé M., 2010. Challenges in the development of analytical soil compaction
models. Soil Tillage Res. 111, 54-64.
Keller, T., 2004. Soil Compaction and Soil tillage – Studies in Agricultural Soil Mechanics.
Acta Universitatis Agriculturea Sueciae, Agraria 489. Ph.D.-thesis. 75pp.
Keller, T., Arvidsson, J., Dawidowski, J.B., Koolen, A.J. 2004. Soil precompression stress. II.
A comparison of different compaction tests and stress-displacement behaviour of the soil
during wheeling. Soil Tillage Res. 77, 97-108.
Keller T., Lamandé M., Schjønning P., Dexter A.R., 2011. Analysis of soil compression
curves from uniaxial confined compression tests. Geoderma, In press (doi:
10.1016/j.geoderma.2011.02.006).
Kirby, J.M., 1991. Strength and deformation of agricultural soil: measurement and practical
significance. Soil Use Manage. 7, 223-229.
Lamandé, M., Schjønning, P., Drøscher, P., Steffensen, F.S.F., Rasmussen, S.T.,
Koppelgaard, M. 2006. Measuring vertical stress and displacement in two dimensions in
soil during a wheeling event. Proceedings, 17th International Conference of the
International Soil Tillage Research Organization, Kiel, Germany, 27. August – 1.
September 2006, 293-297.
Lamandé, M., Schjønning, P., Tøgensen, F.A., 2007. Mechanical behaviour of an undisturbed
soil subjected to loadings: effects of load and contact area. Soil Tillage Res. 97, 91-106.
Lamandé, M., Keller, T., Berli, M., Carizzoni, M., Delenne, J.Y., Markgraf, W., Or, D., Peth,
S., Rabbel, W., Rajchenbach, J., Selvadurai, P., 2010.Towards new approaches for
modelling soil mechanical behaviour. 1st International Conference and Exploratory
Workshop on Soil Architecture and Physico-Chemical Functions "CESAR". Research
Centre Foulum, Tjele, Aarhus University pp. 189-194.
Lamandé, M., Schjønning, P., 2011. Traffic-induced stress transmission in an undisturbed
Stagnic Luvisol. Part II: Effect of tyre size, inflation pressure and wheel load. Soil Tillage
Res., In press (doi:10.1016/j.still.2010.08.011).
Mosaddeghi, M.R., Koolen, A.J., Hajabbasi, M.A., Hemmat, A., Keller, T. 2007. Suitability
of pre-compression stress as the real critical stress of unsaturated agricultural soils.
Biosystems Engineering 98, 90-101.
6
Munkholm, L.J., Schjønning, P., Kay, B.D. 2002. Tensile strength of soil cores in relation to
aggregate strength, soil fragmentation and pore characteristics. Soil Tillage Res. 64, 125135.
Or, D., Ghezzehei, T.A., 2002. Modeling post-tillage soil structural dynamics: a review. Soil
Tillage Res. 64, 41-59.
Peng, X., Horn, R., 2008. Time-dependent, anisotropic pore structure and soil strength in a
10-year period after intensive tractor wheeling under conservation and conventional
tillage. J. Plant Nutr. Soil Sci. 171, 936–944
Perfect, E., Kay, B.D., Ferguson, J.A., da Silva, A.P., Denholm, K.A., 1993. Comparison
functions for characterizing the dry aggregate size distribution of tilled soil. Soil Tillage
Res. 28, 123–139.
Radjai, F., Wolf, D., Jean, M., Moreau, J.J., 1998. Bimodal character of stress transmission in
granular packings. Phys. Rev. Lett. 90, p.61.
Rosin, P., Rammler, E., 1933. Laws governing the fineness of powdered coal. J. Inst. Fuel 7,
29–36.
Schjønning, P., 1991. Soil mechanical properties of seven Danish soils. Report No. S2176.
The Danish Institute of Plant and Soil Science, Copenhagen, 33 pp. (in Danish).
Schjønning, P., 1999. Mechanical properties of Danish soils—a review of existing knowledge
with special emphasis on soil spatial variability. In: Van den Akker, J.J.H., Arvidson, J.,
Horn, R. (Eds.), Experiences with the Impact and Prevention of Subsoil Compaction in
the European Community. Proceedings of the First Workshop of the Concerted Action
‘Experiences with the Impact of Subsoil Compaction on Soil, Crop Growth and
Environment and Ways to Prevent Subsoil Compaction’, 28–30 May, 1998, Wageningen,
The Netherlands, pp. 290–303.
Taylor, J.H., Burt, E.C., 1987. Total axle load effects on soil compaction. J. Terramechanics
24, 179-186.
Trautner, A., 2003. On soil behaviour during field traffic. Acta Universitatis Agriculturae
Sueciae, Agraria 372. Ph.D.-thesis. 55 pp.
Van Baars, S., 1996. Discrete element modelling of granular materials. Heron 41, 139-157.
Van Ouwerkerk, C. and Soane, B.D. (eds) (1995) Soil compaction and the environment.
Special Issue, Soil and Tillage Research 35, 1-113.
Voivret, C. , Radjai, F. , Delenne, J.-Y., El Youssoufi,M.S., 2007. Space-filling properties of
polydisperse granular media. Phys. Rev. E 76, 021301.
Weibull, W., 1951. A statistical distribution function of wide applicability. J. Appl. Mech.Trans. ASME 18, 293–297.
Zhang, R., Li, J., 2006. Simulation on mechanical behaviour of cohesive soils by Distinct
Element Method. J. Terramechanics 43, 303-316.
7