Subsidence model offers insights

RECENT work by Seedsman Geotechnics has produced a simple subsidence model that provides insights into the possible mechanisms of down-slope movement, valley closure, upsidence, and the depression of shallow groundwater tables.
Subsidence model offers insights Subsidence model offers insights Subsidence model offers insights Subsidence model offers insights Subsidence model offers insights

Ross Seedsman, Seedsman Geotechnics

Staff Reporter

The model is based on the translation of blocks on a smoothly deforming surface and, according to principal consultant Ross Seedsman, the mechanics of the model have been checked using the UDEC code for jointed rock.

“This new model is much more powerful than the current explanation for valley closure and upsidence that invokes horizontal stresses, as it offers explanations for the variability seen in the survey data,” Seedsman said.

He said a range of disordered movements are overprinted on smooth systematic subsidence profiles and for supercritical layouts these disordered components can be readily related to rock breakage extending to the surface.

But for subcritical extraction, an alternative explanation is required.

“In the model, the near-surface is considered to consist of a series of joint-bounded blocks resting on an interface located at lowest point of the topography [for example, at the base of a river valley],” Seedsman said.

“Valleys are represented by the removal of blocks. If the interface is deformed in a sagging mode, there is a tendency for the top of the blocks to interact and, in the proximity of a free face provided by the valley, lateral translation of the blocks can occur.”

He said it is this lateral translation of the blocks that causes valley closure and down-slope movements. No lateral movements are induced in the hogging mode, but it is possible that the sagging-induced movements can be transferred to a valley above a hogging phase.

Seedsman said the model can also explain valley upsidence (the deforming interface is located just below the base of the river), and increased vertical permeability (the lateral movements are not recoverable as the subsidence wave passes and hence joints are opened).

The model, along with other subsidence impacts, will be examined in the Subsidence Short Course prior to COAL 2006, held on July 5 to 7.

Registration for the short course closes June 16, 2006. For further details, visit