A guide to help understanding the behaviour of the initial degree of saturation for soil activated during staged construction in the Advanced Calculation Mode.

Introduction

In practice, when for instance an embankment is made, the water content of the soil used to create the embankment is not necessarily equal to the water content of the in situ soil. This can be due to:

• embankment being constructed with soil from another location, therefore it is dryer or wetter than the subsoil
• embankment being constructed with a different soil type than the subsoil, so that it will probably have a rather different degree of saturation even when stored under the same atmospheric conditions.

In PLAXIS, when soil is activated in a staged construction phase, users have the possibility to specify an initial degree of saturation. PLAXIS uses this saturation in the newly activated soil which defines the suction value through the retention curve. The retention curve that links suction and degree of saturation is defined in the material set parameters for that soil.

Please note that whenever the expressions initial degree of saturation and initial pore pressure are used in this article it refers to the degree of saturation and pore pressure at the start of the calculation phase, hence the initial situation for that calculation phase.  It does not refer to the degree of saturation or pore pressure of the initial phase.

Pore pressures vs. degree of saturation

Users can directly specify the saturation value. The behaviour of the unsaturated soil in PLAXIS is driven by pore pressures whereas degree of saturation is the result of the pore pressures. As mentioned in the introduction above, when soil is activated in a staged construction phase, it will by default follow the General Phreatic Level. This can be changed to Head, Custom Pheatic Level, Dry, Interpolate, User-defined pore pressure or Unsaturated. Hereafter, the effects of these water conditions on soil that is activated in the current phase are discussed.

Global phreatic level (GPL)

If newly activated soil has the GPL, the initial pore pressure distribution will be according to the GPL. This implies that if the newly activated soil is

• below the GPL it will have pore pressures according to the GPL and a 100% degree of saturation;
• above the GPL it will have suctions linearly increasing from the GPL upwards and a degree of saturation according to the retention curve used.

Custom phreatic level (CPL)

If newly activated soil has its own CPL, the initial pore pressure distribution will be according to this CPL. This implies that if the newly activated soil is

• below its CPL it will have pore pressures according to the CPL and a 100% degree of saturation
• above its CPL it will have suctions linearly increasing from the CPL upwards and a degree of saturation according to the retention curve used.

User-defined pore pressure (UDPP)

Here we have to distinguish between a UDPP of zero (p_ref = p_inc = 0) or a UDPP not equal to zero.

If the UDPP is zero, the degree of saturation is 100%. This is based on the reverse logic and definition of a phreatic level: as the phreatic level is defined as the level where the pore pressures are zero (no pressures and no suctions), therefore it must be that where the pore pressures are zero, that is where the phreatic level will be located. Since at the phreatic level the degree of saturation is 100%, this implies that when pore pressures are zero, the degree of saturation is 100%.

If the UDPP is not equal to zero, the degree of saturation is 100% when the UDPP are negative pore pressures. 

Cluster dry

When a cluster is set to dry, the degree of saturation is assumed to be 0%.

Interpolate

A cluster that has been set to the Interpolate option gets the same suctions and degree of saturation as if it was set to General Phreatic Level.

Initial water condition and pore pressure generation method

Pore pressures can be generated by phreatic levels, steady-state groundwater flow and transient groundwater flow. The above mentioned options to define the initial pore pressures when activating a soil cluster in a staged construction phase behave somewhat different depending on the chosen method of pore pressure generation.

Pore pressure generation by phreatic levels

The pore pressures and degree of saturation generated according to any of the options mentioned in the previous paragraph (hence, general phreatic level, cluster phreatic level, etc.) remain constant during the calculation phase. For instance, a newly to be constructed part of an embankment that is given a zero pore pressure (and therefore a 100% degree of saturation) has a 100% degree of saturation in the whole calculation phase.

Pore pressure generation by steady-state groundwater flow

Any of the options mentioned in the previous paragraph is ignored; the calculation will always start from a distribution of suction and degree of saturation for the newly activated soil according to the initial general phreatic line.
A steady-state flow field is the flow field at infinite time. If one would deposit for instance very wet soil, the water from that soil will seep into the subsoil. Since no more water is added after activating the soil at some point all water that was initially in the newly activated soil will have sunk in the subsoil so that at infinite time the newly activated soil will have a smooth suction distribution connected to the subsoil. This is the steady-state situation. Therefore any initial situation for the newly activated soil will lead to the same steady-state solution and it is therefore unnecessary to specify any of the options mentioned in the previous paragraph and the user can simply use the default setting.

Transient groundwater flow

In the time-dependent analysis of pore water pressures, the initial degree of saturation is set as the initial condition of the newly activated soil layer. As time progresses in the calculation phase, saturation will change according the groundwater flow boundary conditions and material retention curves.    

Summary

It’s possible to implicitly influence the initial degree of saturation for soils that are being activated in a staged construction phase. When doing so, some guidelines should be taken into account:

1. The initial degree of saturation can be set by setting a saturation value. 
2. If a newly activated soil is dry, the option “Cluster dry” should be used. It is not possible to use a user-defined pore pressure that equals zero as a zero pore pressure is interpreted as a 100% degree of saturation.
3. For a steady-state groundwater flow analysis setting an initial degree of saturation has no effect on the final flow field and is therefore unnecessary.
4. For a transient groundwater flow analysis, the initial degree of saturation set for a newly activated soil is taken as the initial condition. The degree of saturation will change in that phase as time progresses depending on the hydraulic boundary conditions and the material retention curve. 

Example

An embankment is constructed from soil with a degree of saturation of 70%. In order to specify this initial degree of saturation in the newly activated part of the embankment for every construction phase the saturation can defined directly by clicking on that soil cluster and then selecting Unsaturated from the Selection explorer and then specify the saturation value: