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, γunsat (General tabsheet of the Material data set), must be set equal to the saturated unit weight, γsat.
- 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