Elastoplastic Soil Models
In this section:
The Modified Cam-Clay Soil Model
The Cam-clay material model was developed in the 1970s at the University of Cambridge, and since then it has experienced many modifications. The Modified Cam-Clay (MCC) model is the most commonly used model due to its smooth yield surface, and it is the one implemented in COMSOL Multiphysics.
The modified Cam-clay model is a so-called critical state model, where the loading and unloading of the material follow different paths in stress space. The model also features hardening and softening of clays. Different formulations can be found in textbooks, see for instance Ref. 15, Ref. 16, and Ref. 17.
The yield function is written in terms of the stress invariants
Following the Structural Mechanics Module sign convention (the pressure is positive in compression), the yield function reads:
This is an ellipse in the pq-plane, with a cross section independent of the Lode angle. Note that p, q, and pc are positive variables.
The parameter M > 0 defines the slope of the critical state line in the pq-plane. This parameter can be matched to the angle of internal friction ϕ in the Mohr–Coulomb criterion as
(3-112)
Figure 3-26: Modified Cam-clay ellipse in the pq-plane. The ellipse circumscribes a nonlinear elastic region.
The slope of the critical state line M can either be a material property or it can be matched The Mohr–Coulomb Criterion and derived from the angle of internal friction ϕ. It can also be matched to The Matsuoka–Nakai Criterion; see Equation 3-121.
The soil response to isotropic compression is described by the curve of the void ratio (or specific volume or volumetric strain) versus the logarithm of pressure as shown in Figure 3-27.
The void ratio e is the ratio between the pore space and solid volume. It can be written in terms of the porosity ε as e = ε/(1 − ε).
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Figure 3-27: Slopes of the virgin isotropic consolidation line and swelling line in the e versus plane.
The compression index λ is the slope of the virgin isotropic consolidation line, and the swelling index κ is the slope of the swelling line (also called unloading-reloading line) in the e versus ln(p) plane. The reference void ratio eref is measured at the reference pressure pref and initial void ratio e0 is measured at the initial mean stress pi. The starting value of initial mean stress pi is the reference pressure pref. The equation for the virgin isotropic consolidation line is written as
The void ratio at the initial consolidation pressure pc0 is given by
The equation for the swelling line is
where e0 is the initial void ratio calculated as
The initial void ratio e0 can be directly entered as user input. In this case, the void ratio at initial consolidation pressure, ec0, is not needed as intermediate variable.
Assuming small deformations and adopting the sign convention of Structural Mechanics, the volumetric strain is related to the void ratio as
For elastic response, the incremental volumetric elastic strain is written as
(3-113)
Integrating Equation 3-113, the equation is reformulated as
Then the nonlinear relation between pressure and volumetric strain is obtained by
(3-114)
where
The contribution to initial mean stress pi also comes from the first invariant of the initial or external stress tensor of Initial Stress and Strain or External Stress feature. The reference pressure is generally a unit pressure in the used unit system, in the literature the value varies from 1 kPa to 100 kPa. In COMSOL Multiphysics the default value is 100 kPa.
At zero volumetric elastic strain and with zero contribution from Initial Stress and Strain or External Stress features the initial mean stress is nonzero and equal to the reference pressure pref. The reference pressure acts as an in situ stress.
(3-115)
The tangent bulk modulus Kt is written as
The secant bulk modulus Ks (or bulk modulus K) is written as
The total volumetric strain increment in written as
The evolution of void ratio is then written as
(3-116) or
(3-117)
Equation 3-116 is used to compute the void ratio which matches exactly with Equation 3-117 which may be used for verification purposes.
In the modified Cam-clay model, hardening is controlled by the consolidation pressure pc, which depends on the volumetric plastic strain εp,vol as
(3-118)
The elastic and plastic volumetric strains are available in the variable solid.eelvol and solid.epvol, respectively. The consolidation pressure is available in the variable item.pc.
The evolution of the consolidation pressure depends on the values for the initial void ratio e0, the swelling index κ, and the compression index λ, which are positive parameters that fulfill
and
If an Initial Stress and Strain node is added to the Cam-clay material, the initial consolidation pressure pc0 must be equal or larger than one third of minus the trace of the initial stress tensor, otherwise the initial stress state is outside the Cam-clay ellipse.
Volumetric Elastic Deformation
The stress-strain relation beyond the elastic range is of great importance in soil mechanics. For additive decomposition of strains, Cauchy’s stress tensor is written as
with
Here, σ is the stress tensor, ε is the total strain tensor, εel is the elastic strain tensor, σ0 is the initial or external stress tensor, and G is the shear modulus.
At zero volumetric strain, and with zero contribution from Initial Stress and Strain or External Stress features, the pressure in the MCC model is equal to the reference pressure (see Equation 3-114 and Equation 3-115). This is needed as the MCC model do not have stiffness at zero stress. The reference pressure appears as an additional term in the variational formulation (weak equation).
As opposed to the Linear Elastic Material, the MCC model introduces a nonlinear relation for the hydrostatic pressure as a function of the volumetric elastic strain given by Equation 3-114.
Hardening and Softening
The yield surface for the modified Cam-clay model reads
The plastic strain tensor εp is computed from the flow rule
where λp is the plastic multiplier and the derivatives of the plastic potential Qp with respect to the stress tensor σ. An associated flow rule is used such that Qp = Fy.
See also Plastic Flow for Additive Decomposition and Numerical Solution of the Elastoplastic Conditions. If a multiplicative decomposition of strains is used, the flow rule is reformulated according to Plastic Flow for Multiplicative Decomposition.
The plastic strain rate tensor includes both deviatoric and isotropic parts. Note that
and
These relations can be used for writing the plastic flow as
since the associated flow rule implies a plastic potential such as
the plastic flow rule simplifies to
The trace of the plastic strain rate tensor (the volumetric plastic strain rate ) then reads
This relation explains why there is isotropic hardening when the pressure is p > pc/2 and isotropic softening when p < pc/2. As opposed to what happens in J2 plasticity, in the modified Cam-clay soil model the volumetric plastic strain can either increase or decrease as plastic deformation occurs.
In the MCC model, hardening is controlled by the consolidation pressure pc as a function of volumetric plastic strain, as described in Equation 3-118.
Hardening introduces changes in the shape of the Cam-clay ellipse, since its major semiaxis depends on the value of the consolidation pressure pc. The initial consolidation pressure pc0 defines the size of the ellipse before plastic deformation occurs.
Including Pore Pressure
When an external pore pressure pf is added to the MCC material, the yield function is shifted along the p axis, and the yield function reads:
The quantity p − pf is normally regarded as the effective pressure, or effective stress, which should not be confused with the equivalent von Mises stress. To add the effect of a fluid pressure in the pores pf to the modified Cam-clay material, add an External Stress feature.
See also the description of the Elastoplastic Soil Material materials in the Solid Mechanics interface documentation.
The Modified Structured Cam-Clay Soil Model
The Structured Cam-Clay (SCC) model was developed (Ref. 22, Ref. 23) to circumvent the limitations of the Cam-clay model when applied to structured soils and clays. The SCC model, however, does not consider the influence of the soil structure neither on strength characteristics (especially cohesion) nor in the softening behavior, and it is also not suitable to model cemented clays.
The Modified Structured Cam-Clay (MSCC) model was further developed to model destructured, naturally structured and artificially structured clays (Ref. 22), and it is the one implemented in the Geomechanics Module.
In the MSCC model, the reduction of mean effective stress due to structure degradation, pb, depends on the shear plastic strain εp,dev. The destructuring mechanism is the process of reducing structure strength due to the degradation and crushing of the structure. The structure degradation is given by
(3-119)
Here, pbi is the initial structural strength, pbf the failure structural strength, εp,devc is the equivalent plastic strain at failure, and ds is the destructuring index due to shear deformation.
The equivalent deviatoric plastic strain at which the crushing of the structure begins, εp,devc, has a typical value between 0.15 and 0.3 for most clays.
Structured clays show a higher void ratio than destructured clays at the same effective mean stress. The virgin compression behavior during the destructuring process is expressed by
where is e void ratio of the structured clay, Δe is additional void ratio, e* is void ratio of the destructured clay at the same stress state. The void ratio can be found by using Equation 3-117. Further, Δei is the additional void ratio at consolidation pressure, and dv is the destructuring index due to volumetric deformation.
The yield surface for the modified structured Cam-clay model reads
The nonassociated plastic potential reads
Here, ς is a parameter to smooth the shape of the plastic potential. The slope of the critical state line, M can be found by using Equation 3-112.
In the modified structured Cam-clay soil model, hardening is controlled by the consolidation pressure pc as a function of volumetric plastic strain, and is described as
Here, λ* is the compression index for destructured clay, κ is the swelling index, and η is ratio of shear stress to mean stress.
The MSCC and MCC models are equivalent when Δei = 0, ς = 2, and pb = 0.
The relationship between hydrostatic pressure and volumetric elastic strain is the same as the one outlined in The Modified Cam-Clay Soil Model,
(3-120) with
The stress tensor is then computed from
At zero volumetric strain, and with zero contribution from Initial Stress and Strain or External Stress features, the pressure in the MSCC model is equal to the reference pressure (see Equation 3-120 and Equation 3-115). This is needed as the MSCC model do not have stiffness at zero stress. The reference pressure appears as an additional term in the variational formulation (weak equation).
The Extended Barcelona Basic Soil Model
The Barcelona Basic Model (BBM) was developed to simulate the loading of unsaturated and partially saturated soils, by incorporating an extra state variable for the pore suction. The suction value depends on the amount of water in the soil, and it affects the flow in porous soils as well as the deformation and stress distribution.
The BBM model uses the concepts of plasticity theory, incorporating the critical state model (Ref. 24). This soil model matches the results obtained with the modified Cam-clay model in fully saturated soils.
The so-called Extended Barcelona Basic Model (BBMx) was further developed to overcome numerical limitations of the original BBM model (Ref. 25). The model implemented in COMSOL Multiphysics follows Ref. 25 with certain modifications described in this section.
The BBMx model presents a smooth yield surface with respect to both stress and suction
Here, p and q are stress invariants as defined in The Modified Cam-Clay Soil Model, pcs is the consolidation pressure at current suction, ps is the tensile strength due to current suction, s is the current suction, b is a dimensionless smoothing parameter, sy is the yield value at current suction, and pref is the reference pressure at which the reference void ratio eref was measured.
The tensile strength due to current suction, ps, is linearly related to the suction level as ps = ks, where k is the tension to suction ratio.
The consolidation pressure at current suction pcs is calculated from
where λ(s) is the compression index at current suction, λ0 is the compression index at saturation, and κ is the swelling index. The compression index at current suction, λ(s), is given by
where w and m are weighting and soil stiffness parameters.
The slope of the critical state line M can be computed from the Matsuoka–Nakai criterion, in which case it depends on both the Lode angle θ and the angle of internal friction ϕ
(3-121)
where
At the tensile or compressive meridians, where the Lode angle is θ = 0 or θ = π/3, the slope of the critical state line achieves the same expression as when matched to The Mohr–Coulomb Criterion, see Equation 3-112
It can also be matched to The Matsuoka–Nakai Criterion.
The associated plastic potential for the BBMx model reads
The plastic strain increments are computed from the derivatives of the plastic potential with respect to stress only.
As in The Modified Cam-Clay Soil Model, hardening is controlled by the evolution of the consolidation pressure pc, which depends on the volumetric plastic strain εp,vol.
(3-122)
The initial void ratio e0, the swelling index κ, and the compression index at saturation λ0, are positive parameters.
The evolution of the yield value at current suction, sy, is also governed by the volumetric plastic strain εpl,vol as
(3-123)
Here, λs is the compression index for changes in suction, κs is the swelling index for changes in suction, and patm is the atmospheric pressure, all positive parameters. Note that Equation 3-122 and Equation 3-123 are normally given with opposite sign. Here, however, the structural mechanics convention is used, so the increments in consolidation pressure and suction are positive in compression.
The evolution of void ratio is then written as
or
where
In the BBMx model, the total volumetric elastic response is combination of elastic response by pressure and suction,
where
Here K is the bulk modulus, and Kcs the stiffness to suction. Note that the pressure p in compression and suction s is positive variables, but the elastic volumetric strain εel,vol is negative in compression. The volumetric elastic response due to suction is given by
Here, s0 is the initial suction. The volumetric elastic response due to suction is accounted in the total strain tensor for BBMx model, so any changes in suction with keeping pressure constant can cause volumetric strains.
The relationship between hydrostatic pressure and volumetric elastic strain is the same as the one outlined in The Modified Cam-Clay Soil Model
(3-124) with
The stress tensor is then computed from
At zero volumetric strain, and with zero contribution from Initial Stress and Strain or External Stress features, the pressure in the BBMx model is equal to the reference pressure (see Equation 3-124 and Equation 3-115). This is needed as the BBMx model does not have stiffness at zero stress. The reference pressure appears as an additional term in the variational formulation (weak equation form).
Note that the material property λ0 is the compression index at saturation, which does not depend on the suction. The variable λ(s), which is a function of the current suction, is the compression index (slope) in the void ratio versus logarithm of the mean stress plot. The material property λs is the compression index (slope) in the void ratio versus logarithm of the matrix suction plot, which does not depend on the mean stress.
The Hardening Soil Model
The Hardening Soil model is an elastoplastic material model with a stress and stress path-dependent stiffness. It is a so-called double stiffness model, meaning that the soil stiffness is different during primary loading and unloading/reloading load paths (Ref. 26).
The yield surface for the hardening soil model is a combination of a conical surface and an elliptic cap surface in stress space.
In the original hardening soil model, failure in shear occurs according to The Mohr–Coulomb Criterion. In the so-called Hardening Soil Smooth model (Ref. 26), failure in shear occurs according to the The Matsuoka–Nakai Criterion. In Ref. 28, the so-called Panteghini and Lagioia version of Matsuoka–Nakai criterion is used.
The 50% failure stiffness for primary loading, E50, and the stiffness for unloading and reloading, Eur, are used to define the plastic potential and failure criterion. These are given by
and
Here is the reference failure stiffness for primary loading at reference pressure, is the reference stiffness for unloading and reloading at reference pressure, c is the cohesion, ϕ is the angle of internal friction, and m is the stress exponent.
The ultimate deviatoric stress qf and the stress to failure qa are derived from The Mohr–Coulomb Criterion
and
where Rf is the failure ratio. Consider the stress invariants and , the yield function and plastic potential for the shear hardening are given by
where γp is the accumulated plastic shear strain and ψm is the mobilized dilatancy angle.
The definition of the shear strain γp in the original hardening soil model is not compatible with pure volumetric loading as it does not vanish during pure volumetric straining. Therefore, the shear strain measure γp is defined as , see Nonlinear Elastic Materials.
The yield function for the elliptic cap, and the associated plastic potential, are also defined in terms of stress invariants, and given by
Here, Rc is the ellipse aspect ratio, and it can directly entered, or given in terms of the coefficient of earth pressure at rest
The coefficient of earth pressure at rest is computed from the angle of internal friction ϕ
The modified deviatoric stress is defined as
where
The internal variables pc and γp depend on the volumetric plastic strain εpl,vol and the plastic strain invariant J2 pl), and their evolution is defined as
where H is the hardening modulus which depends on the bulk modulus in compression Kc and the bulk modulus in swelling Ks
where
The dilatancy cutoff is implemented by setting the mobilized dilatancy angle ψm equal to zero when the void ratio reaches the critical void ratio emax.
Cauchy’s stress tensor is then written as
where C is a function of the stiffness modulus Eur and Poisson’s ratio ν.
Mobilized Dilatancy Angle
The mobilized dilatancy angle ψm is explicitly used in the definition for the plastic potential Qp. Different authors define this variable using different expressions. The original expression for the mobilized dilatancy derived by Rowe (Ref. 26) is
(3-125)
here, ϕc is the critical friction angle and ϕm is the mobilized friction angle, and these are derived from
where ψ is the dilatancy angle, σi are the principal stresses, c is the cohesion, and ϕ is the angle of internal friction.
Rowe’s original model for the mobilized dilatancy angle is highly contractive at low angles, so in the original hardening soil model it is modified to
(3-126)
however, this modification also gives too little plastic volumetric contraction.
Other expressions (Ref. 26) for the mobilized dilatancy angle include the Wehnert correction
(3-127)
the Soreide correction
(3-128)
and the correction by Li and Dafalias
(3-129)
The Rowe–Li–Dafalias model uses Equation 3-126 to define the mobilized dilatancy angle when , and Equation 3-129 otherwise.
The Hardening Soil Small Strain Model
The Hardening Soil Small Strain model is a combination of Small Strain Overlay model and The Hardening Soil Model.
The initial shear modulus is computed from other material parameters, as done in the hardening soil model
Here is the reference initial shear modulus for primary loading at reference pressure, c is the cohesion, ϕ is the angle of internal friction, and m is the stress exponent.
The critical shear strain, defined as
is used to switch between the shear stiffness derived from the small strain overlay and the hardening soil models
here, Gur is the shear modulus for unloading/reloading, as defined in the The Hardening Soil Model.