Floor heave and "squeeze"
Definition and formation: Floor heave is the relative rise of a clay-rich mine floor after a passage (entry, roadway, etc.) has been cut underground (Wuest, 1992). Floor heave occurs when the load on the floor exceeds the bearing capacity of the floor. The resulting rise of the floor, sometimes to the top of the mined passage, is also termed “squeeze.” Failure of the floor may begin with arching in the center of a heading between pillars, or along the rib, but progresses with breaking and upward buckling/rising of the floor across the entry (Wuest, 1992). Floor heave can be slow or rapid. It can occur during mining, or long after a passage is mined through.
Rockaway and Stephenson (1980) described three types of failure mechanisms:
- General shear failure occurs when the floor is relatively competent, but pillar load is too great for the floor, resulting in the development of a shear surface in the floor, along which the floor extrudes or bulges between pillars.
- Pillar punching occurs when the floor is soft (or softens because of moisture) and pillars punch into the floor without a distinct shear surface. Cracks may form along the pillar, and ridges of displaced material may rise parallel to the rib.
- The third failure mechanism occurs in interbedded floors, where a thin hard layer occurs above softer strata. If the upper layer does not hold, a failure surface may develop in the softer layer and be displaced upward and outward until the floor buckles from tension.
Wuest (1992) summarized two mechanisms:
- Plastic flow of weak clay floors.
- Buckling of relatively competent floor by pillar punching or high in situ horizontal stress. Buckling can occur through a break along a rib or near the entry axis. This type of failure appears to partly overlap with interbedded floor mechanisms in which an upper rock layer detaches from a lower layer.
Progressive floor failure can result in pillar failure, roof falls, and squeezing or even sealing a mined passage.
Discontinuities and obstacles: Floor heave can obstruct or completely block mine passages, sometimes necessitating abandonment of equipment, sections of a mine, or reconfiguration of the mine plan to avoid squeezed passages. It may be relatively confined to a few passages or much broader, affecting multiple passages.
Potential roof-fall hazards: Roof strength is weakened by floor heave and pillar settling. Sags in the roof followed by falls are common. If heave or settling continues, zones of shear stress may be concentrated above rib lines and rib corners, whereas tensile and compressive stresses are concentrated above pillars (Wuest, 1992). Kink zones have been encountered in shale roofs above areas of floor heave; especially where influenced by regional stress fields. Roof falls can also accelerate floor heave. In a Springfield coal mine in western Kentucky, floor heave started slowly but accelerated after roof falls. Some roof falls in the mine were along north-south headings, so were likely related to the regional stress field. A thick sandstone roof may also have added to weight on the pillars.
Roof falls above areas of floor heave can occur long after mining, and have resulted in surface subsidence in Illinois (Chugh, 1986a, b; Bauer, 2013).
Trends: Floor heave is related to several factors, including clay thickness, moisture, clay chemistry, and mining methods. A summary of floor heave in the Illinois Basin by Kester and Chugh (1980) noted these common poor floor conditions:
- Underclays 3 to 5 feet thick; more common where clay is more than 5 feet thick.
- Soft clays (psi 100–300), and worse for very soft clays (less than 100 psi).
- Underclays with 8 to 10 percent moisture, and worse when moisture is more than 10 percent.
- Underclay contains montmorillonite (within 5 feet below first competent bed in floor); increasing montmorillonite content causes weaknesses.
Analytical data on clays and clay testing from Illinois Basin mines can be found in White (1954, 1956), Ganow (1975), Rockaway and Stephenson (1979, 1980), Speck, (1979), Chugh, (1986a, 1986b), and Gadde (2009). In general, each foot of increase in underclay thickness corresponds to a large decrease in the bearing capacity of the floor (Rockaway and Stephenson, 1980).
Floor heave can also be accentuated by the lateral stress field. Trends of floor heave along near north-south orientations or in association with north-south-oriented cutters and kink zones may be caused by horizontal stress (Molinda and Mark, 2010).
In Illinois, floor heave is also locally associated with channel margin conditions (Nelson, 1983). Strong roofs such as sandstones (associated with paleochannels) above areas of floor heave can exert pressure on adjacent pillars.Floor heave can also be accentuated beneath shallow cover and beneath Pleistocene valleys (Nelson, 1983).
If floor heave is not addressed, pillars and roof may fail, which will redistribute overburden load to surrounding pillars, which can cause failure to spread (Rockaway and Stephenson, 1980).
Floor heave is also influenced by nongeologic mining factors including mine type (room-and-pillar, longwall, retreat, etc.), pillar size, and entry size (Wuest, 1992), which need to be considered when trying to identify or interpret trends of occurrence.
Known Kentucky occurrences: Many Lower and lower Middle Pennsylvanian coals in Kentucky, including most of the most heavily mined coals in eastern Kentucky, are characterized by relatively thin underclays, so mines in them do not commonly encounter floor heave issues. In contrast, upper Middle Pennsylvanian coals such as the Springfield and Herrin in western Kentucky have very thick underclays; many mines have reported floor heave in these coal beds. Perry and others (2016) reported on 3-D modeling of floor heave in a Herrin coal mine in western Kentucky.
Planning and mitigation: Mapping underclay thickness can aid in planning prior to mining. Likewise, sampling and testing thick underclays encountered in cores prior to mining for their moisture content and other important engineering parameters can aid in planning. Chugh (1986a, b) and Gadde (2009) reviewed different types of analyses need to determine various engineering parameters for determining floor bearing-capacity. Gadde (2009) also reviewed equations, methods, and numerical modeling techniques to best predict floor-bearing capacity in mines so that mine designs can mitigate the influence of weak floors.
Wuest (1999) summarized mine maintenance, adjustment, and supplemental support techniques for floor heave. Mine maintenance in areas susceptible to floor heave includes (1) keeping floors dry (and continually monitoring in wet areas), (2) grading heaved roadways (which is a common practice, but in some cases may require sustained maintenance), (3) cutting vertical slots in the floor to relieve horizontal strain, and (4) blasting the floor for stress relief. Mine-plan-adjustment techniques are common for abatement and include (1) leaving a strong layer (in some cases, bottom coal) in the floor, (2) decreasing entry width, (3) increasing pillar size, (4) using yield pillars, and (5) reorienting entries so they are not parallel or perpendicular to the lateral stress field. Supplemental support techniques are really only used on floor heave where the entry has to remain open. Supplemental support techniques for the floor include (1) installing floor bolts and (2) installing concrete lining or injecting polyurethane; but these techniques have only been used in extreme cases where leaving a passage open was critical to the mining operation.
Roof support: Where roof falls occur above areas of floor heave, supplemental support methods generally are similar to those for weak conditions in the rock type or rock-stacking type in the roof (e.g., weak shale roofs, stackrock, etc.), depending on severity, including cribbing, as shown in the example above.