A guide to help understanding the behaviour of the initial degree of saturation for soil activated during staged construction in the Advanced Calculation Mode.
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:
In PLAXIS, however, when soil is activated in a staged construction phase, the user does not have the possibility to specify an initial degree of saturation. PLAXIS uses default suctions in the newly activated soil according to the existing general phreatic level, which directly defines the default degree of saturation 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.
The behaviour of the unsaturated soil in PLAXIS is driven by pore pressures whereas degree of saturation is the result of the pore pressures. Hence, there is currently no possibility to specify a degree of saturation as input value, but by adequately specifying (user-defined) pore pressures an effect of initial degree of saturation can be obtained. 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 Cluster Pheatic Level, Dry, Interpolate or User-defined pore pressure. Hereafter, the effects of these water conditions on soil that is activated in the current phase are discussed.
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
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
Here we have to distinguish between a UDPP of zero (pref = pinc = 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), reversively it must be that where the pore pressures are zero is the phreatic level. 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 pressures and the degree of saturation is less than 100% when the UDPP are suctions, according to the retention curve used. This leads to an interesting trick: by applying a very high suction as UDPP one can force the cluster to have a degree of saturation equal to the residual degree of saturation (close to 0% for coarse high permeable soils and up to 60% for fine low permeable soils).
When a cluster is set to dry, the degree of saturation is assumed to be 0%.
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.
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.
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.
Any of the options mentioned in the previous paragraph is ignored; the calculation will alway 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 for time is infinite. 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 for time is infinite the newly activated soil has the same suction distribution as 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.
If the newly activated soil has one of the pore pressure distributions as mentioned above, it will keep this distribution throughout the entire phase where this has been defined. For example, we make a first layer for an embankment with very wet soil; 100% degree of saturation. During the consolidation the water in the embankment will flow into the subsoil, but the embankment remains with 100% degree of saturation as if the water is automatically filled up again. There is a practical numerical reason for this behaviour. In the phase where the embankment is activated the weight of the embankment is slowly applied. If in this phase for the embankment a true consolidation analysis would be performed, this would lead to unrealistic excess pore pressures due to the application of the embankments weight. This problem is overcome by assuming that the embankment is perfectly drained, so no excess pore pressures will develop. However, since no real flow analysis is being done for the newly activated embankment no change of degree of saturation can be calculated, hence the degree of saturation remains constant. Only in the phase after the phase in which the embankment is activated the water will flow out of the embankment and the degree of saturation in the embankment will drop as the limited amount of water that was in the embankment flows into the subsoil.
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:
An embankment is constructed from soil with a degree of saturation of 70%. In order to specify this initial degree of saturation during construction the suction has to be defined in the newly activated part of the embankment for every construction phase.