[Solved - Intended Behavior] Support beams not supporting?

[ChunkHunter 73]

Have you read, understood, and followed all of the rules listed in large text at the top of the support forum? (Yes/No): Yes
 
TFC Version #: 0.79.24
Forge Version #:  10.13.4.1448
 

SSP/SMP (SinglePlayer/MultiPlayer):  SSP
 
Have you deleted your config files or are using default configs and are still able to reproduce this bug? (Yes/No): No, using default configs. 
 
Do you have any mods other than Forge and TFC installed? (Yes/No):  Yes
If yes, which mods?

WAILA

Smart Moving 15.2

JourneyMap 5.1.0

FastCraft 1.21

OptiFine 1.7.10_HD_U_B7
 
If you have Optifine or Cauldron installed, can you still reproduce the bug after uninstalling them? (Yes/No): Not tried - difficult to reproduce.
 
Pastebin.com link of the Crash Report: n/a

Description: 
 

I was tunneling in some basalt, and put down some support beams (to prevent caveins).

I had a simple 3x3 tunnel, and was digging out a block so I could prepare to put in my next support beam and I heard the sound of a cavein.

 

As far as I am aware (I may be wrong!) a support beam will support blocks on the same level as the horizontal part up to and including 3 blocks from the actual beam.

(See post #10 here http://terrafirmacraft.com/f/topic/8187-7917-getting-too-punishing/?p=106742#10)

 

 

An image showing what was happening.  At the top left of the image you can see the corner of my support beam.  The empty block top right is the one which caused the cavein sound.

It can easily be seen that there are only two intervening blocks beteween this and the support beams.  Is there some rounding error happening here?

Edited 29 Sep 2015 by ChunkHunter

[Darmo 303]

Do you know if there's nearby caves?  I'll leave the details to Kitty, but probably the cave-in is starting in nearby unsupported caves, and propagating back into your mine.

[Bunsan 323]

Mining raw stone triggers cave ins. The area that it can trigger is bigger than the area protected by support beams. So if there is any raw stone or ore that is unsupported within the area checked that is not being supported by the beams it can start a cave in. So air pockets and caves. Be aware a cave in can spread to a supported area and take out support beams and cave in the area.

[ChunkHunter 73]

I misunderstood that it was only mining raw stone that wasn't considered supported that could trigger a cavein - my apologies - you learn something new every day.

 

I guess if there were a deep cave underneath you the floor could easily give way - taking you to your death... hmmmmmmm....If I read Kitty's post correctly, it shouldn't result in your support beam being taken out unless there is a gap above it - because the horizontal plane immediately above the support beams is supported so won't pass rule #7 on her list.