CAVE-INS AND COLLAPSE
DETERMINGING SUBSIDENCE
Subsidence is defined by the cross-sectional width of a cave divided by its height below surface.
100% Subsidence: Width of Cave/Cover depth=1.1-1.4*
10% Subsidence: Width of Cave/Cover Depth=0.1-0.5*
*This range depends on the rock type. The high thresholds are more likely granite, midway is sandstone and limestone, while the low end of the scale is dirt/gravel.
That means that a cave with a 50' cross section sitting below 20' of rock has a value of 2.5 (well in excess of 100% Subsidence): Such a cave began to transfer the effects of subsidence to the surface back at 10% so the ground will be sagging and unless this chamber is propped up with columns it will collapse like a sinkhole.
Likewise a 10' wide tunnel below 20' of rock has a value of 0.5 (at the upper limit of 10% subsidence). Here the stress of failure has reached the surface - Fine cracks that will probably not be visible at the surface due to topsoil.
REDUCING SUBSIDENCE TO 10%
Columns are rarely introduced after the effect. Standard Practice requires a column to be cut from the existing rock strata and left behind undisturbed by mining and excavation.
Column Width= 0.12 x cover depth
Goaf (space between columns) width = 0.6 x cover depth.
What does it mean? Our 50' cross-section cave above can be reduced to a subsidence of 10% by including at time of construction columns to support the ceiling.
Column Width: 0.12 x 20' = 2.4'
Column Spacing: 0.6 x 20' = 12'
A chamber supported by 2.4' wide columns spaced 12' apart. Inflict some damage to those columns (beyond the damage inflicted by the existing subsidence) and the entire Dungeon will have to be tested (%percentage roll for each chamber and corridor) as to whether it has collapsed with the mountain coming down on the Adventurers.
SUBSIDENCE AT THE SURFACE
When Subsidence exceeds 10% it begins to show at the surface. To determine the Surface Area Affected by Subsidence, you must take a 35 degree angle from the bottom of the workings edge. This means that even Rock that doesn’t overlap a collapsed/failing Cave will be showing failure. The Actual depth of collapse at surface is 80% of actual cave height into which the rock may collapse.
Subsidence= 0.8 x cave height
Cave-edge collapse radius = SIN 35 degrees x depth of cave bottom
So our 50' cross section cave with its 15' high chamber will if and when it collapses create a 12' deep sinkhole with signs of subsidence out to (SIN 35 degrees x 35') = 20' beyond the area of the cave (effectively the 50' cave left a hole 90' across - slopping down into a 12' deep, 50' wide pit).
SURFACE SIGNS OF SUBSIDENCE
Signs of Failure: The failure of a cave is inevitable. The signs of failure begin at 10% subsidence where this initial stress is visible at the surface as cracking. This progresses toward 100% as Depression/subsidence bowls, Sink holes, Fissures, Chimneys, Block Collapse and Terracing occur. At 60% failure Cracks allow for transfer of water downward. The effects of failure extend beyond the cave area projected upward at surface (R=Sin 35 degrees x h). Failure appears at the surface as a collapse distance of 0.8 x m (where surface cover h> cave height m).
10%<------------------------>100%
Cracks* - Sagging – Depression Bowl
Cracks* – Fissures – Chimneys
Cracks* - Block separation – Terracing
Cracks* - Sagging - Sinkholes
*Cracks are not always visible at the surface due to topsoil
NATURAL SUBSIDENCE OVER TIME AND MAGICAL TUNNELLING
As a mine is increased through mining it becomes increasingly unstable due to the natural failure over time. The rate of failure over time is determined by a curve associated with the rate at which mining occurred.
A tunnel cut slowly by Dwarves might progress toward a natural failure over years or even centuries. Whereas the use of Disintegrate to dig your way through is considered an instantaneous advance which means the natural progression toward failure is instantaneous.
The use of Disintegrate to tunnel instantly pushes the rock structure to 100%.
Our 50’ cross-section cave might progress to 100% subsidence over eight years but the instant someone uses a Disintegrate spell to tunnel through a wall, the new section may collapse as a block or separate from the old cave as a stress boundary fissure or chimney.
APPLYING CAVE SUBSIDENCE TO PITS
It means that any unreinforced Pit wall will fail out to 35 degrees from bottom of any pit with 11′ of width per 10′ of depth burying anyone who dares enough to dig below 5′ depth x physical height.
If we apply these rules to an adventure like H1 Keep on the Shadowfell we find that the Entry stairs descending 20' from surface represents about 10' of cover (h=10') which puts a 10' wide corridor below 10' of cover equates to w/h=1.0 (71% Subsidence) This means the cracks in the roof are sufficient to transfer rainwater which means the floor is wet.
Any rooms of 20' wide or greater on this 'first level' will have fallen in (20/10=2). Lower floors (those down another 20' of staircase - without being level 2 of Shadow Keep) will have various levels of subsidence.
WIth exception the Primary room with the columns supporting the roof. as stated, Columns are desugned to reduce the subsidence of the roof to below 10%. However, despite this knowledge - the map designer doesnt know the rules. column separation (goaf distance)=0.6 x 10' =6'.
The Columns are required to be in greater population in this room. They need to be at least 1.2' in diameter and as close as 6'. So even this room is above 10% (the distances between column tell us it is at above 85% between the columns (12/10=1.2).
But you have to remember bring it to much to life and well you kill the game, after all its a fantasy.
If they dont think to dig through thirty feet of rock into the crypts and level 2 (or clear out the rubble of the collapse)- they will only find Kobolds hold up in an uncollapsed section of the Dungeon level 1.
No Cult of Orcus here - Drinks back at Winterhaven...Meanwhile Karleah completes the ritual deep beneath the surface.
A month from now Karleah will be building a Monastary in the Ruins.