How to Perform Dewatering in PLAXIS

Richard Witasse, Principal Product Manager, Geotechnical

The problem of changing water conditions is omnipresent in geotechnical engineering. Groundwater is systematically found beneath the Earth’s surface in soil pore spaces and rock formation fractures. The flow of groundwater is a fundamental aspect that controls the strength and compressibility of soil, impacting soil’s ability to sustain structural loads. The study of groundwater is essential for geotechnical engineers who construct building foundations, tunnels, water conveyance channels, dams, mines, and other structures.

In this context, PLAXIS 2D and PLAXIS 3D are fully equipped with a complete set of tools to model changes of pore water pressure and, more specifically, dewatering, along with its influence on soil mechanical behavior.

Terzaghi principle of effective stress

The Terzaghi principle is a fundamental concept used in geo-mechanics in general, and in PLAXIS in particular, to understand how a change of water pressure induces change of soil stresses and associated soil deformations. The Terzaghi principle states (in a very symbolic way) the following:

Total stress σ = Effective stress σ’+ Pore water pressure u

Figure 1: Terzaghi effective stress principle

The total stress σ is balanced by the applied load (soil weight, infrastructure weight, service loads), whereas all strength and stress-strain characteristics of the soil are linked to changes in effective stress σ’.

Lowering the groundwater will reduce the pore water pressure u and hence will cause an increase and redistribution of soil effective stresses, which will then lead to the settlement and evolution of active and passive pressure in surrounding buried structures.

Water levels and hydrostatic pore pressure distribution

By default, PLAXIS computes pore water pressure fields in the model based on a hydrostatic distribution (phreatic method). In such situations, water is implicitly assumed to be at rest such that pore pressure depends on depth only: u = γ_w.depth.

Figure 2: Steady-state pore pressure definition on a 3D model

The definition of water levels in PLAXIS can be performed during soil stratigraphy definition when defining boreholes or within Flow mode. The Flow mode enables the definition of additional water levels different than the initial one. Additional water levels might then be referenced (globally, or locally in some limited numbers of soil clusters in the model) to simulate the process of dewatering.

(a) PLAXIS Input definition

(b) Considered pore water pressure distribution in PLAXIS Output

Figure 3: Dewatering by means of the phreatic method

Groundwater flow analysis

The FE method can also be used in PLAXIS for groundwater flow modeling to analyze hydrogeological problems, such as dewatering, in multilayer soil profiles. Such types of analysis can be run in either a steady-state model or in a time-dependent model. Sources and wells can be placed in various aquifers inside the PLAXIS model. Also, PLAXIS enables the calculation of the groundwater flow around and the leakage through watertight structures, such as sheet pile walls or cut-off walls.

Such groundwater analyses require the definition of additional parameters for the constitutive materials, such as:

Permeability values
Porosity
Unsaturated zone behavior eventually

Along with specific flow boundary conditions, including:

Prescribed head of water flow
Closed flow
Seepage flow boundary
Drains
Wells (both infiltration and extraction)
Precipitations

For the particular case of watering, steady-state groundwater flow analyses are usually carried out for relatively permeable situations for the sake of finding the updated pre-water pressure field, along with the resulting water level and the water flow distribution at a steady state regime, as shown in Figure 4.