longwall mining







Chocks (also known as "Powered Supports", "Supports" or "Shields")

Roof support in early longwalls (in the days of hand mining) was by timber props and bars, withdrawn from the goaf side as the faces advanced and re-used if still intact. Eventually these were replaced by steel bars supported by yielding props (eg friction props where resistance to yield was provided by a wedge system or hydraulic props which were individual props filled with fluid which could be pumped with an internal hand pump and released using a valve).

In time the hydraulic props (now referred to as legs) were combined in pairs, mounted on a base and joined with a roof canopy, with adjacent pairs being connected by a frame containing a horizontal hydraulic cylinder. This enabled each "chock", as the 4 leg sets were called, to advance itself with one pair of legs, released from the roof, pushing against the 2nd pair which remained set. Such chocks were set along the length of the face forming a continuous line of "self-advancing supports", sometimes also referred to as "goal post supports or chocks".

Simplified sketch of "goalpost chock"

Further development saw the legs being mounted closer together on a single solid base with a solid, cantilevered roof canopy allowing the front line of legs to be a little further from the face while still providing adequate support close to the freshly exposed roof. The horizontal cylinder in these chocks attached the chock base to the face coal haulage system (an AFC – see later notes). The cylinders were used to push the AFC forward and then drag the chocks forward one at a time as the face advanced. The chocks were interconnected with hydraulic hoses and connected back to a pumping arrangement in the gate road by a hydraulic fluid reticulation system. The hydraulic fluid used was (and still is) mostly water with a low concentration of soluble oil, partly to assist in lubrication but mostly to inhibit corrosion. At times 6 leg chocks were used with 4 close together at the rear and 2 close to the AFC, leaving a travelling way between them.

Simplified sketch of early longwall chock

Over time the rear of the chocks was partially closed-in with flexible arrangements of steel plates, chains and timber to try and prevent broken material from flushing through from the goaf into the face area.

Similar support systems were developed where the 4 vertical legs were replaced by 2 larger legs set at the rear of the base and angled towards the face. These had a somewhat larger canopy with a rear section connected to the base with a "lemniscate" linkage which enabled the base and canopy connection to be fully covered, while the main canopy remained essentially parallel to the base at whatever set height was used. These supports were called "shields" instead of chocks.

Modern 2 leg shield Again, over a period, further developments combined the best aspects of chocks and shields into what were referred to as "chock-shields" but are now often referred to using either term by itself, but the terms "supports" is probably the most common term now used (and the term used in this document).

Most modern supports are two leg types, though four leg shields and chock-shields are also in use, all four legs mounted towards the rear of the base with the front pair angled towards the face and the rear pair towards the goaf.

The rear section of the chocks which contains most of the operating valve systems and the legs, is partially covered by side plates as well as being enclosed from above and behind. These side plates have a top cover and can slide sideways, pushed by small hydraulic cylinders, so that the chocks can stand skin-to-skin and provide continuous cover over the full length of face.

In a further development the front tip of the roof canopy is articulated and connected to another small cylinder allowing a greater load to be applied to the roof at this point to improve roof control.

For thick seams, where coal falling from the face can be a hazard supports can be fitted with an articulated plate attached to the support tip which can swing down and provide a horizontal support to the exposed face. Once again this is controlled hydraulically and obviously has to be lifted clear again before the next web of coal is cut.

Most modern supports are fitted with "base lifters" another hydraulic arrangement which allows the base to be lifted up, a very useful function in soft floor conditions where support bases may sink into the floor and limit the ready advance of the face.

Modern supports are very complex pieces of equipment, made even more complex by the primary method of operation being via remote control or automated, requiring electronic control and monitoring, with manual control also being fitted. Longwall faces are also now extensively illuminated and these functions require hydraulic and electrical connections from support to support and back to the maingate area, a typical face carrying many hoses and cables. The hoses and cables between supports need to be flexible and have sufficient length to allow for the distance any support will be advanced ahead of adjacent supports.

Because of the high setting pressures of modern supports a two stage setting system may be used, an initial "low pressure" set (of the order of 320 bar) being boosted to the final set by a high pressure supply (of the order of 420 bar) - yet another set of hoses to be included.

Supports are designed to operate through a range of heights to accommodate variations in working heights and possibly some degree of unplanned loss of roof or floor. It is also necessary that they can be closed down low enough to allow transport around the mine in whatever height is available. Support legs are often multi-stage legs to allow additional travel.

Support widths vary, mostly between 1.5 and 2m. Note that as a chock is made wider the load/metre run along the face which can be applied to the roof reduces for a given leg capacity. Also the wider a support, the heavier it becomes and more difficult to handle. As a support is made narrower it becomes less stable if subjected to uneven ground. Also the narrower a support, the more supports are required for a given face length and with each support requiring a set of control valves and interchock hoses, so the greater the cost. The ideal support width will be the best compromise between the conflicting aims.

An aspect of support design beginning to receive more attention is ergonomics. When a face is "closed-up", especially in lower height seams, travelling along the face can be very arduous. To increase the width of walkways involves extending the length of the supports which has ramifications on roof loading and strata control (as well as costs), so some degree of compromise is required.

There is a tendency to design for "average" size personnel which, by definition, means that half the workforce are likely to experience some difficulty or discomfort. It may be better, within reason, to design for the tallest and widest person likely to travel the face. Allowance has to be made for equipment being carried on a regular basis (cap lamps, self rescuers, etc).

The purpose of the roof supports on a longwall face is not to prevent roof movement but to control it so that the immediate roof remains essentially intact where the coal is cut and within the area of the face where personnel have to work. Once the work area has moved forward it is acceptable, indeed desirable, that the roof collapses or "caves" (a term frequently used). The ideal situation is that the roof caves immediately behind the supports as they are moved forward; if the collapse is delayed the roof strata will hang out into the goaf in a cantilever putting extra load on the supports.

This cantilever effect was largely responsible for early failures of longwall mining in Australia, circa 1970. Supports at that time had been developed in Europe where roof strata was generally weaker and laminated and caved readily. Support loading capacities of 100 tonnes or less were adequate to control the roof. In Australia more massive strata is common which breaks and falls less readily and the cantilever effect leads to very high chock capacities being required, sometimes over 1000 tonnes.

In order to prevent damage to the hydraulic legs, chocks are designed to yield (ie release the hydraulic pressure) at a set value, so the roof is allowed to lower in a controlled fashion.

For best roof control a high chock set pressure is required, as close as possible to the yield pressure – it is not practical to set at the yield pressure.

As the roof lowers, the strata above will begin to bend and then beds will fracture under tension from the higher levels extending down towards the immediate roof (these fractures being known as "goaf breaks"). Ideally these fractures should reach the roof as the rear of the supports passes that point, allowing immediate caving.

Ideal formation of roof fractures, reaching seam roof at rear of support

If it occurs ahead of this point then the roof above the chocks will break-up and this may give rise to problems on the face itself. Broken roof above the canopies is not usually a big problem as the broken strata will remain more or less in place as long as the chocks are not lowered away from the roof when they are moved. Broken roof ahead of the canopies can lead to total loss of control and major falls on the face itself.

Cavities or broken roof above the canopies can result in:

  • not being able to set the supports to the roof at the designed set pressure because the canopy has to remain reasonably level; if there are cavities one end of the canopy may push up into the cavity causing the other end to come away from the roof

    Sketch to show effect on support setting (if full pressure set used) of cavity in roof over chock

  • very high point loads at contact points between strata and canopy which have to take the share of load which cannot be transmitted through the cavity.