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The New Interface Model in SIGMA/W

Several people have been asking how the new interface material model in SIGMA/W works. In general, you can assign any material model to an interface element that is automatically meshed along a line geometry object. However, if you want to model cohesion or friction based slippage, you must use the new slip surface C - Phi based interface model. This article demonstrates modeling a cohesional strength case called the “filling bucket” whereby the weight of the bucket increases until the strength between the bucket and surrounding walls is lost and the bucket falls.

There are also links to two other detailed example discussions with slightly different applications of the interface model. The Sliding Block example adds pore-water pressures into the system such that the normal stresses at the interface are effective stresses. That is, the actual normal stress that is controlling the frictional behavior is comprised of the total normal stress minus the pore-water pressure.

Problem definition:
Below is an image of the filling bucket example. The yellow colored material is the bucket with a unit weight of 5 KN/m3 assigned to it such that the weight is added on every load step. The volume of the bucket is 4 m3, therefore, on each load step a weight of 20 KN is added to the bucket.

There are two lateral normal stresses of 100 kPa applied to the sides of the bucket on the first load step. This normal stress remains constant for all load steps.

The cohesive strength of the slip surface material between the bucket and the walls is set at 100 kPa, which means that for the 2 m vertical length of the bucket, the maximum resisting shear forces are 200 KN on each side, or a total of 400 KN resistance for the system.

Figure 1 Filling bucket example

Solution:
Two different cases are analyzed in this example. In the first case, the bucket is filled by adding the self weight of the material (5 KN/m3) on each of 25 load steps. You can see that eventually the bucket gets heavy enough for the resisting shear forces to be met, at which point the bucket falls down.

Figure 2 Displacement of filled bucket at failure

The two images below show the total slip force in the y direction along the contact between the bucket and walls for all 25 load steps (left image) and for the first 19 load steps (right image). You can see on the left that at the exact point where the total weight of the bucket reaches the limiting value of 200 KN (for half the bucket) the y displacement becomes very large. In fact, at the point of limit equilibrium, the displacement equations lose their meaning. The image on the right shows the elastic displacement up to just before the point of failure. This elastic displacement is controlled by the shear modulus of the slip surface material. At the microscopic level, there is elastic deformation before full slippage occurs and the SIGMA/W slip model will take this into account.

Figure 3 Slippage forces and movement for filling bucket case

In the second case, the self weight is turned off and the vertical displacement of the bucket is fixed at 0.001 m on each of 25 load steps. Therefore after 20 load steps, the bucket will have moved 0.02m downwards. The image below shows that the elastic deformation stage of the loading is the same as the filling bucket case above. However, because we are now controlling the displacement, we can see more clearly that once the resisting shear forces are met (200 KN for half the model) then they are maintained at that value and the bucket slowly slides downwards.

Figure 4 Slippage forces and movement for displaced bucket case

For more information on the new Slip Surface model in SIGMA/W, please refer to the following resources:

Sliding block analysis (44 KB)
Sliding block analysis (1.30 MB)

Pile pull out test (92 KB)
Pile pull out test (3.03 MB)

Filling bucket (239 KB)

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