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Note that you might find it easier to define your data in "Elevation Mode" rather than "Offset Mode". Select Input | Project Options and select "Elevation" for the field ELEVATION TYPE. All of your offset data will automatically be converted to the appropriate elevation.
Note that your pipe link outlet can be at whatever elevation (or offset) you want. Generally, however, it is at the junction invert.
If the hydraulic gradeline can rise above the ground surface at an access hole site, such as in a sanitary force main or siphon, special consideration must be given to the design of the access hole frame and cover. The cover must be secured so that it remains in place during peak flooding periods, avoiding an access hole "blowout." A "blowout" is caused when the hydraulic gradeline rises in elevation higher than the access hole cover and forces the lid to explode off. The difference between specified SURCHARGE ELEVATION and the MAX/RIM ELEVATION should correspond to the maximum or design pressure for the access hole frame and cover. If "blowout" conditions are possible, access hole covers should be bolted or secured in place with a locking mechanism.
A foot of surcharge on a 3 ft diameter manhole cover exerts an upward force of about 441 pounds. It would not take much surcharge to cause the manhole cover securing bolts to fail. In addition, generally a weaker part of the manhole is the connection between the frame and the manhole, between manhole rings, and between manhole riser sections. At some point, the surcharge pressure may even cause unrestrained pipe joints to fail. In practice, it could be easy to envision any of these joints separating enough to relieve the surcharge pressure and then settling back down, leaving little evidence that the surcharge event occurred.
If you continue to have questions, I recommend that you call into our technical support team and they can remotely jump on to your computer and review your data and provide you some suggestions in defining the data to meet what you are attempting to model.
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