Real Boundaries – Choosing the Right Type

It is sometimes difficult to know the type of boundary condition to apply to a particular surface on the body being modeled. For example, in modeling a laboratory triaxial test, should the load applied by the platen be regarded as a stress boundary, or should the platen be treated as a rigid displacement boundary? Of course, the whole testing machine, including the platen, could be modeled, but that might be very time-consuming. Remember that 3DEC takes a long time to converge if there is a large contrast in stiffnesses. In general, if the object applying the load is very stiff compared with the sample (say, more than 20 times stiffer), then it may be treated as a rigid boundary. If it is soft compared with the sample (say, 20 times softer), then it may be modeled as a stress-controlled boundary. Clearly, a fluid pressure acting on the surface of a body is in the latter category. Footings on jointed rock can often be represented as rigid boundaries that move with constant velocity, for the purposes of finding the collapse load of the rock. This approach has another advantage: it is much easier to control the test and obtain a good load/displacement graph. It is well-known that stiff testing machines are more stable than soft testing machines.