Staggered configuration with thin interburden

Crack propagation, angle of break and angle of draw

Extracting the lower panel from under the upper extraction in the staggered

configuration with thin interburden heavily influences the angles of draw and

break at different edges of the upper and lower panels (Figure 4.17). The

asymmetric positioning of the upper and lower seams results in an asymmet-

ric pattern of deformation. As a result, the angle of draw on each side of the

zone is 33◦, which is greater than the angle of draw for the upper panel (18◦).

On the other hand, the angle of draw at the lower panel end (8◦) is smaller

than the single-seam angle of draw and significantly smaller than the upper

panel end (Figures 4.17b). It appears that the presence of the upper panel

gob and the caved and disturbed zones above the upper panel significantly

affect the strength of the overlying strata, strata movement and the extent

of the subsidence after the lower seam extraction.

The lower extraction also creates new fractures in the disturbed zone

above the upper panel end. These new cracks reduce the angle of break at

the upper panel’s edge at this side (Figure 4.17). The angle of break at the

lower panel end is approximately zero (i.e. almost vertical) and a deep surface

crack is observed above this area (Figure 4.17b).

In addition, the cavity growth mapping illustrates that the cracks and

bedding planes in the vicinity of the overlapping area are mostly closed after

the lower extraction (Figure 4.18). Whereas, the bedding separations and

fractures above the upper panel end are mostly unaffected. In other words,

the previously existing cracks above the undermining area (overlapping area)

are affected more than the upper panel end area.

Strata movement profile

After extracting the lower panel, the most significant vertical deformation

occurred predominantly in the overlapping area (Figure 4.19). The inter-

burden layers show less deformation than the area above the overlapping

(a)

(b)

Figure 4.17: Angle of break and draw for staggered configuration with thin interburden at different stages after (a) upper panel extraction and (b) lower panel extraction.

lated upper panel end is similar to the resultant deformation pattern after

the single-seam extraction. At the other side of the model, relatively small

vertical deformation is observed above the lower panel end.

Subsidence, tilt and horizontal displacement profile

The maximum incremental multi-seam subsidence, as a result of the lower

panel extraction, occurs above the overlapping area and is not located above

the middle of the newly extracted panel (Figure 4.20a). The magnitude of

this subsidence also is significantly increased in comparison with the single-

Upper panel, single-seam

Upper panel

Overlapping zone

Minimum cavity Maximum cavity

(a)

Upper panel, multi-seam

Upper panel

Overlapping zone

Minimum cavity Maximum cavity

(b)

Figure 4.18: Cavity growth mapping for staggered configuration with thin interburden above upper panel after (a) upper extraction and (b) lower ex- traction.

The profile of the tilt (Figure 4.20b) shows that the steepness of the

subsidence on the right and left side of the lower panel is rather different.

In the area above the upper panel end, the subsidence curve (Figure 4.20a)

starts with a smooth curve and then drops to the maximum point above the

middle of the overlapping area. Whereas, above the lower panel end, the

(a) (b)

Figure 4.19: Strata vertical deformation pattern by DIC technique for stag- gered configuration with thin interburden after (a) upper panel extraction and (b) lower panel extraction.

constant decline. The tilt profile better illustrates this different movement

profile on two sides of the model. It can be seen that the maximum tilt on

the lower panel end is located above the edge of the extracted panel while

the minimum tilt is located beyond the edge of the lower panel and above

the upper panel end (Figure 4.20b). This observation also indicates a wider

subsidence profile above the upper panel end in comparison with the lower

panel end.

In addition, the magnitude of the horizontal displacement on two halves

of the lower panel is different (Figure 4.20c). Close to the lower panel end, the

maximum horizontal displacement is almost twice as big as the overlapping

zone. Furthermore, after the lower panel extraction, in the area close to a

surface crack above the upper panel end, a smooth decline in the magnitude

of horizontal displacement, together with a wide limit angle are observed. In

contrast, above the lower panel end, a sharp increase in horizontal displace-

Upper panel

Lower panel Middle of lower panel Maximum incremental

subsidence Maximum subsidence

Middle of overlapping area

Uppr panel Lower panel Lower panel incremental

Location from middle of the model (cm)

-100 -80 -60 -40 -20 0 20 40 60 80 100 Subside nce ( cm ) 0 -0.5 -1 -1.5 -2 (a)

Location from middle of the model (cm)

-100 -80 -60 -40 -20 0 20 40 60 80 100 Lower panel Upper panel Upper panel Lower panel ×10-2 Tilt ( cm /c m) 15 10 5 0 -5 -10 -15 (b) Lower panel Upper panel Upper panel Lower panel

Location from middle of the model (cm)

-100 -80 -60 -40 -20 0 20 40 60 80 100 Hor iz ontal dis pla ce me nt ( cm) 0.5 0.25 0 -0.25 -0.5 (c)

Figure 4.20: (a) Subsidence, (b) tilt and (c) horizontal displacement profile for staggered configuration with thin interburden.