It is pushed out into the sea on the end of a flat, narrow strip of land, and is suggestive of a "gob
" of mud on the end of a shingle.
"We sit around and gaily pound, And bear no acrimony Because our ob--ject is a gob
Of sizzling abalone."
Goaf with caving were modelled as porous medium with permeability depending on destratification resistance of rocks forming these gobs. Numerical tests were performed using ANSYS Fluent software.
The main aim of studies carried out was to determine impact of type roof rocks forming goaf with caving on the physical parameters of air stream flowing through the longwall heading and these gobs. Utilization of results of performed tests in real conditions enabled to partial validation of obtained results from numerical analysis.
The coefficient value of gobs permeability depends on rocks types forming caving.
where: k(x) is permeability coefficient of gob ([m.sup.2]), [[mu].sub.g] is the coefficient of dynamic viscosity ([Nsm.sup.-2]), l is the total length of the longitudinal longwalls (m), [r.sub.0], a are empirical factor depending on the mining-geological conditions gobs.
A borehole camera detection method  was used to detect aquifers above the abandoned gobs in the Yongan mine.
Adopting this type of wellbore configuration can improve the efficiency of draining the abandoned gobs and reduce construction costs of the surface vertical wells.
The six wells were closed and sealed after drilling was completed, and then pressure gauges (Figure 4) were installed at the upper part of each well to monitor the residual methane pressure in the abandoned mine gobs, as shown in Figure 5.
Due the excavation of upper Jurassic coal strata at the early stage and large gobs of the lower carbonic coal strata, during the production process of lower extra-thick coal strata, the perturbation and excavation of the working faces may once again affect the stability of roof structure of the gobs of overlying Jurassic excavated coal strata.
According to the difference of the direction of layout of the working faces of Jurassic coal strata at Tongxin coal mine and the location of coal pillars in the gobs (as shown in Figure 1), the combined natural arch structure of the roof of the gobs of close distance coal strata can be categorised into aligned and misaligned types, as shown in Figure 2.
It is evident from Figures 2 and 3 that the self-weight of overlying rocks after the excavation of close distance coal strata is applied ultimately on the lower coal stratum flooring via coal pillars and the waste rocks in the gobs. Therefore, the stress distribution in the combined natural arch structure of rock-overlaid roofing of close distance coal strata is evaluated in this study to provide basis for the analysis of the effect of concentrated stress on the coal pillars in the gobs of Jurassic coal strata.