The SHANSEP MC model (Stress History and Normalized Soil Engineering Properties) constitutes a soil model implemented in PLAXIS, intended for undrained soil loading conditions. It is based on the linear elastic perfectly-plastic Mohr-Coulomb model, but modified such that it is able to simulate potential changes of the undrained shear strength s_u (c_u) based on the effective stress state of the soil. It takes into account the effects of stress history and stress path in characterizing soil strength and in predicting field behaviour.

Section 4. Failure mechanism of the Shansep MC model

Section 4. Failure mechanism of the Mohr Coulomb - Undrained B model

In order to obtain the model, send your request to sales@plaxis.com. Support on the use of the SHANSEP MC soil model is only provided for this DLL under the conditions of the PLAXIS VIP support service and Article 10 of the End-User Licence Agreement.

Fixed-End (FE) anchors are single point spring elements that can be used to model anchors, struts and other types of ‘flexible’ supports. PLAXIS 2D and 3D allow for only one FE anchor per geometry point. However, in some situations, it may be required to fix a structure in different directions using spring supports, such as indicated in the figure below.

This would require two or more FE anchors to be applied at the same geometry point, which is currently not possible.

The way to overcome this limitation is to use None-to-Node (N2N) anchors instead of FE anchors. N2N anchors form a spring connection between two geometry points. However, the second point may also be a fixed point at the model (bottom) boundary, which makes that the N2N anchor basically works as an FE anchor.

In the above example case, the vertical FE anchor could be replaced by a vertical N2N anchor with its lowest point connected to the bottom boundary.

Now, there are three issues to consider:

1. The direction of an FE anchor can be defined as an anchor property, whereas the direction of a N2N anchor is just its orientation in the geometry model. The spring support is in this direction. Considering that the bottom boundary is used for the fixed point, the replacement of an FE anchor by an N2N anchor works best for a (primarily) vertical support.
2. The equivalent length of an FE anchor can be defined as an anchor property, whereas the equivalent length of an N2N anchor is the distance between the two geometry points to which the anchor is connected. Since the distance between the point to be supported and the model boundary may be different than the desired equivalent length, a ‘scaling factor’ needs to be applied on the anchor stiffness in the corresponding material data set. For example, if the equivalent length is supposed to be 5 m and the length of the N2N anchor to the bottom of the model is 10 m, then the axial stiffness EA in the material data set must be set twice as high as the actual stiffness. Alternatively, the anchor spacing may be decreased by a factor 2, which has the same scaling effect.
3. Make sure that the boundary to which the N2N anchor is connected is active in all calculation phases in which the anchor is supposed to be active. For the bottom boundary this is generally the case, but note that if for some reason, the part of the boundary where the fixed point is attached is de-activated, the N2N anchor will automatically be de-activated as well.

Vacuum consolidation is a technique to apply preloading on a construction site by creating an 'under-pressure' in the ground and thus using the external atmospheric pressure as preloading. In this way, the stability of the sub-soil is increased and settlements during and after the construction are reduced. This technique is usually applied on near-saturated soils with a high water table. This article explains the details of modelling vacuum consolidation in PLAXIS.

There are various methods of vacuum consolidation in the real world, but they are all modelled in a similar way in PLAXIS. Most methods in reality are using vertical drains, which are somehow connected at the top to an air pump that reduces the air pressure in the drains until a near-vacuum exists. In practice, a complete vacuum (100 kN/m2 pressure) is not achievable, but an effective under-pressure of 60 - 90 kN/m².

Since PLAXIS does not take air pressure into account (atmospheric pressure is assumed to be the zero reference pressure level), a reduction of the groundwater head is used instead to simulate vacuum consolidation. This means that the way vacuum consolidation is modelled leads to negative pore stresses (suction), which are not there in reality.

1 Vacuum consolidation in a one-dimensional soil column

In the simplified case of a one-dimensional soil column, vacuum consolidation can be modelled by performing a groundwater flow calculation or a fully coupled flow-deformation analysis with hydraulic conditions at the model boundaries such that in the vacuum area the groundwater head is prescribed at a level that is 10 m (or less) lower than the vertical coordinate of the global phreatic level. A reduction of the groundwater head of 10 m is equivalent to an under-pressure of 100 kN/m² (i.e. complete vacuum).

2 Vacuum consolidation in a 2D or 3D model

In a 2D or 3D numerical model of a realistic project, vacuum consolidation can be modelled by performing a groundwater flow calculation or a fully coupled flow-deformation analysis with vacuum drains in which the head specified in those drains is 10 m (or less) lower than the vertical coordinate of the global phreatic level. A reduction of the groundwater head of 10 m is equivalent to an under-pressure of 100 kN/m². The distance between the vacuum drains in the model is arbitrary, but should be selected such that the difference in groundwater head in the vacuum area is limited. In general, a distance between the drains less than a quarter of the drain length seems appropriate (i.e. complete vacuum).

3 Other requirements

A reduction of the groundwater head implies that the soil in the vacuum area becomes unsaturated, whilst this soil volume is supposed to be fully saturated. The user must arrange additionally that saturated conditions apply to this volume. This requires the following settings to be made in the corresponding material data sets:

• The unsaturated unit weight, γ_unsat (General tabsheet of the Material data set), must be set equal to the saturated unit weight, γ_sat.
• The hydraulic model must be set to Saturated after selecting User-defined as hydraulic data set (Model group in Groundwater tabsheet).

If these settings are not made, the unit weight of the soil will change from saturated to unsaturated as soon as the phreatic level drops as a result of the reduction of the groundwater head in the vacuum drains. Moreover, the soil permeability will reduce according to the reduced relative permeability in the unsaturated zone, depending on the selected hydraulic data set (by default Fine material). Both effects are not realistic and can be overcome by making the aforementioned changes in the corresponding material data sets.

4 Calculation options

Vacuum consolidation (using reduced groundwater head boundary conditions or reduced heads in vacuum drains) can be applied in the following calculation types:

• Plastic (select Steady-state groundwater flow as Pore pressure calculation type);
• Consolidation (select Steady-state groundwater flow as Pore pressure calculation type);
• Fully-coupled flow-deformation analysis.

This means that all input requirements for a groundwater flow calculation have to be met, i.e:

• All material data sets must have non-zero permeabilities;
• Hydraulic boundary conditions (groundwater head and closed flow boundaries, if applicable) must have been specified.

Moreover, it is required to de-select the Ignore suction option in the Deformation control parameters section of the Phases window.

Note that only vacuum drains allow a groundwater head to be specified below the actual drain level, which leads to tensile pore stresses (suction). Normal drains do not allow for suction. Also note that, if vacuum drains are used in a Consolidation calculation whilst the pore pressure calculation type is set to Phreatic, the drains will work as normal drains rather than vacuum drains. This means that they only affect the consolidation of excess pore pressures, whilst the steady-state pore pressure is fully determined by the global water level and local cluster settings.

5 Switching-off vacuum

If the vacuum is to be 'switched-off' in subsequent calculations while the drains are still supposed to be active for consolidation purposes, the corresponding head in the drains needs to be changed from the reduced head level to the original global water level. This leads to the situation that the new pressure head in the drain is higher than the groundwater head in the area around the drain. As a result, one might expect that water
will be flowing from the drain into the ground, which is an artefact of the numerical modelling of vacuum consolidation.

In order to avoid such unrealistic behaviour, PLAXIS prevents at all times water to flow from a drain into the surrounding soil, since drains are meant to drain water out of the ground rather than bring water into the ground. Hence, the aforementioned artefact will not occur in PLAXIS.

This text is taken from the PLAXIS 2D 2016 Reference Manual