Excess Pore Pressure

The Excess Pore Pressure option in the Define Material Properties dialog, will only be available if the Excess Pore Pressure option is selected on the Groundwater page of the Project Settings dialog.

Excess pore pressure refers to a sudden increase in pore pressure within a soil due to rapidly applied loading conditions (undrained loading). Materials with low permeability such as clays may exhibit this behaviour. With the so-called "B-bar" method, the change in pore pressure is assumed to be directly proportional to the change in vertical stress. The excess pore pressure is given by:

where (B-bar) is the overall pore pressure coefficient for a material (Skempton (1954)). The change in vertical stress can be due to:

• the weight of added layers of material,
• external loads,
• seismic loads,
• or a combination of these factors,

as specified by the user. Only the vertical component of loads can affect the pore pressure.

For example, if an embankment is constructed over a clay foundation, this can create a temporary condition of "excess pore pressure" within the clay. Due to low permeability, the excess pore pressure within the clay cannot readily dissipate. A non-steady-state situation then exists, in which the elevated pore pressure can lead to lower safety factors, and possible failure. Note that the term "rapid" loading is relative to the ability of a material to dissipate pore pressure, and may refer to time periods of days or even weeks.

The final pore pressure which is used in the stability calculations, is equal to the Initial Pore Pressure + Excess Pore Pressure. The Initial Pore Pressure is specified by the Groundwater Method and the Water Parameters used for the initial pore pressure calculation (e.g. Water Table). The Excess Pore Pressure is calculated from the B-bar coefficient and the change in vertical stress.

B-bar

To calculate Excess Pore Pressure for a material, enter a B-bar coefficient greater than zero. The value of B-bar is usually between 0 and 1, but may be greater than 1 in some cases.

For materials which are free-draining (i.e. will not develop excess pore pressure in response to a change in vertical stress), use B-bar = 0.

In the case of rapid drawdown models using B-bar, a B-bar value of 0 does not represent free draining. It means that the pore pressure remains the same as the initial pore pressure with full ponding. Only in the case of positive changes in vertical stress, due to added embankments or loads, does B-bar=0 represent free draining. If you want your material to be free draining for a rapid drawdown model, do not check the undrained behavior checkbox.

The B-bar method of rapid drawdown is based on the paper Stability Charts for Earth Slopes During Rapid Drawdown by Morgenstern. For a simple example examining how B-bar affects pore pressure, download the B-bar Documentation files. The following example demonstrates a simple computation of effective vertical stress at the coordinate (180, 30) for both cases. Noting the depth of soil and depth of water at this point are 30ft and 40ft respectively and the unit weight of soil is 124.8 lb/ft3 and the unit weight of water is 62.4 lb/ft3:

Before Drawdown

sigv' = sigt - u = (30*124.8+40*62.4) - 70*62.4 = 1872 lb/ft2

After Drawdown

du = Bbar*dsigv = -Bbar*gammaw*hw = -1.0*62.4*40 = -2496 lb/ft2

u = uinitial + du = (70*62.4) - 2496 = 1872 lb/ft2

sigv' = sigt - u = (30*124.8) - 1872 = 1872 lb/ft2

where:

sigv' = effective vertical stress

sigt = total vertical stress

u = pore pressure

du = change in pore pressure due to drawdown (also termed the excess pore pressure in Slide2)

uninitial = initial pore pressure prior to drawdown

dsigv = change in vertical stress due to drawdown

Bbar = B-bar coeficient = 1 for the undrained material in this example

So in this case, the effective vertical stress does not change. Obviously this is not always the case and depends on the relative magnitude of the soil unit weight and the water unit weight, and the location within the slope. Looking at the two B-bar example files, you'll also notice the effective normal stress is quite similar between the two models.

Additionally, having a B-bar=1 is the same as defining a water surface along the surface of the slope for the undrained material after drawdown and will give the same FS result.

In the case of rapid drawdown models using B-bar, a B-bar value of 0 does not represent free draining. It means that the pore pressure remains the same as the initial pore pressure with full ponding. Only in the case of positive changes in vertical stress, due to added embankments or loads, does B-bar=0 represent free draining. If you want your material to be free draining for a rapid drawdown model, do not check the undrained behavior checkbox.

Material weight creates excess pore pressure

If the "Material weight creates excess pore pressure" checkbox is selected for a material, then the material weight will contribute to the "change in vertical stress" used to calculate the excess pore pressure in materials which lie beneath that material in the model.

For a tutorial which demonstrates the Excess Pore Pressure option, see Tutorial 12 on the Slide2 Tutorials page.