The error message “Contact Overlap Too Great” can occur during cycling if one block penetrates too far into another. The maximum amount of overlap allowed by the code is one-half the rounding length. If such an error occurs, it is usually necessary to restart the problem from an earlier state. However, before restarting, it is important to identify the cause of the error and correct it.
Useful information concerning the contact location(s) where overlap occurred is given preceding the contact overlap message. Also, use the PLOT overlap command to identify the blocks involved. The following possible causes for contact overlap can be identified.
Joint Normal Stiffness Too Low – If the joint normal stiffness is unrealistically low for the loads applied, the blocks will penetrate too far into each other. This cause can often be identified by (a) plotting a close-up of the affected area; or (b) printing contacts in the same area. See note 4, above, for a remedy to this problem.
Numerical Instability – Numerical instabilities, characterized by increasing amplitude of oscillations, result from timesteps that are too large. History plots that show wild fluctuations are indicative of a numerical instability. The only way to correct a numerical instability without changing other problem parameters is to reduce the timestep by using the FRACTION command. Increasing mass damping parameters can often hide instability but will not likely eliminate it. UDEC automatically determines a timestep that is stable for most cases. However, situations may arise when this timestep is too large. Several situations have been identified as causing numerical instability:

Contact Overlap Too Great

1.节理法向刚度太低

2.数值不稳定性

(1) use of large values of stiffness proportional damping at high frequencies (see Section 4.2 in Special Features); (2) use of high values of joint dilation; (3) use of problem geometries in which one block contacts many (more than 3) other blocks on one side; and (4) use of nonreflecting (i.e., viscous) boundaries in which the bounding material is significantly stiffer than the material in the problem domain.
If the cause of the contact overlap cannot be identified, it may be necessary to use the SET cscan command. This command causes the location of the center of rounded corners to be updated more frequently, resulting in more accurate calculation of contacts and overlap.

1. 在高频下采用了较大的刚度阻尼成分(与质量阻尼对应的阻尼) <见理论与背景的第4.2节>；
2. 采用了较高的节理剪胀角；
3. 釆用的几何模型中有一个块体在一个边与很多（多于3个）其他块体接触；
4. 采用了非反射 (即粘滞)边界，且边界材料的刚度远远高于模型内部材料的刚度。

Another procedure for eliminating this error is to increase the overlap tolerance at the start of the problem. Use the command SET ovtol. This procedure is useful if the original rounding length was very small and/or the problem geometry involved blocks with very acute angles. However, this approach can also be very dangerous if not used thoughtfully, because erroneous results can occur if ovtol is too large.