Parameters are specified in the PLAXIS input in the following order:
Parameters:
Parameter 1  critical state friction angle
Parameter 2   shift of the mean stress due to cohesion. The effective stress
used in the model formulation is replaced by
.
Nonzero value of is needed to overcome problems with stressfree state.
If , it will be replaced by a default value of 1 kPa. Any other value can be input by user (for basic hypoplasticity, set to very low number, e.g. ).
Parameters 35  parameters of the basic hypoplastic model for clays , , .
Parameters 6  Parameters of the basic hypoplastic model for clays (in place of the parameter of the Mašín (2005) [3] model, calibrated using the same procedure).
Parameters 7  , ratio of horisontal and vertical shear moduli.
Parameters 810  parameters of the model for clays with metastable structure (, and ).
Parameters 1415  Very small strain shear stiffness parameters and . If the intergranular strain concept is switched off and the problem is simulated using the basic hypoplastic model.
Parameter 17  bulk modulus of water for undrained analysis using the penalty approach with userdefined value of .
In drained analysis, consolidation analysis, and undrained analysis using PLAXIS option undrained should be set to 0.
Parameter 18  Specifies vertical direction (set to 2 in PLAXIS2D, 3 in PLAXIS3D, 2 in SoilTest independently whether run from PX2D or PX3D).
Parameter 19  Parameter (ratio of horizontal and vertical Young moduli). If set to zero, than calculated automatically from empirical formulation
.
Parameter 20  Parameter
(ratio of horizontal and vertical Poisson ratios). If set to zero, than calculated automatically from empirical formulation
.
Parameter 21  Parameter . Additional control of nonlinearity inside state boundary surface. If set to zero, default value calculated from model parameters is used.
Parameter 22  initial void ratio or overconsolidation ratio . If
, then .
If
, then
.
Parameters 2328  initial values of the intergranular strain tensor
in Voigt notation (
,
,
,
,
,
).
Parameter 29  initial value of sensitivity (model for clays with metastable structure). If , the basic model is used.
State variables:
The routine uses 16 state variables:
State v. 16  intergranular strain tensor
in Voigt notation (
,
,
,
,
,
).
State v. 7  void ratio .
State v. 8  Excess pore pressure for undrained analysis using userdefined value of . In undrained analysis using PLAXIS
option undrained this variable is equal to 0 end excess pore pressure may be found in standard PLAXIS menu.
State v. 9  Effective mean stress.
State v. 10  Number of evaluation of the constitutive model in one global time step (for postprocessing only).
State v. 11  Mobilised friction angle
in degrees (for postprocessing only).
State v. 12  Normalised length of the intergranular strain tensor
(for postprocessing only).
State v. 13  Suggested size of the first time substep (for calculation controll).
State v. 14  sensitivity (for model with metastable structure).
State v. 15  overconsolidation ratio, defined as .
State v. 16  0 means model interation goes without problem. 1 means state boundary surface cutoff has been activated.
The hypoplastic model for granular materials is implemented via user defined subroutine usermod. To use the model in PLAXIS, copy the files UDSM_HPS.dll and UDSM_HPS64.dll into the PLAXIS installation directory. Then, select "userdefined model" from the Material model combo box in the General tab sheet (Fig. 4.1). After selecting the userdefined model, correct userdefined dynamic library (typically UDSM_HPS.dll) needs to be selected in the "Available DLL's" combo box under "Parameters" tab sheet. In the "Models in DLL" combo box, model with ID 2 (Hypoplas.  clay) must be selected. The parameters can then be input into the parameter table (Fig. 4.3).
Figure:
Selecting clay hypoplasticity model in the "Parameters" tab sheet.
Parameters of the clay hypoplastic model for different soils have been evaluated by Mašín and coworkers. They are given in Table 4.3. Parameters of the model for clays with metastable structure are in Tab. 4.4 and typical parameters of the intergranular strain concept for finegrained soils are in Tab. 4.5.
Table 4.3:Typical parameters of the hypoplastic model for clays.
Brno clay
22
0.128
0.015
1.51
0.33
1.35
London clay
21.9
0.095
0.015
1.19
0.1
2
Kaolin
27.5
0.065
0.01
0.918
0.35

Dortmund clay
27.9
0.057
0.008
0.749
0.38

Weald clay
24
0.059
0.018
0.8
0.3

Koper silt
33
0.103
0.015
1.31
0.28

Fujinomori clay
34
0.045
0.011
0.887
0.36

Pisa clay
21.9
0.14
0.01
1.56
0.31

Beaucaire clay
33
0.06
0.01
0.85
0.21

Trmice clay
18.7
0.09
0.01
1.09
0.09

For estimation of , see database from [7], here
for free download in the form of PDF preprint.
Table 4.4:Typical parameters of the model for clays with metastable structure (from [4])
Pisa clay
0.4
0.1
1
Bothkennar clay
0.35
0.5
1
Table 4.5:Typical parameters of the intergranular strain concept for clays