Broken shaft

A shaft breach is the term used to describe a mostly sudden loss of the shaft backfilling when day shafts are thrown off . After a shaft has been broken, the contour of the former shaft is very often visible on the surface or a funnel-shaped fracture is formed. In order to prevent manholes from breaking in the manholes that are thrown out today, they are filled with special building materials according to the recognized rules of technology.

Basics

Discarded shafts used to be inadequately filled after mining stopped. Unsuitable backfill or collapse masses were often used, which were then poured onto platforms that were attached to the shaft extensions in the shaft . In some cases, the shaft openings were only insufficiently secured with cover plates that were placed on insufficiently dimensioned foundations. When the extension failed, the offset column slipped or the cover plate collapsed. In contrast to the break in daylight, a pre-marked chimney is formed when a manhole is broken. The reason for this is the different geomechanical fracture processes in the two events. In Germany alone there are several hundred unsecured manholes, the fill level and exact location of which are mostly unknown.

Fracture processes

The formation of shaft cracks can have different causes depending on the shaft closure used. With the previously used partial backfilling of the shaft with loose material, over the years it can happen that the loose material filling column settles and the soil slips into the cavity created as a result. Another possibility is that the platform on which the filling column rests becomes rotten and breaks after a while. Due to the frictional forces in the carbonic solid rock, the filling column is initially held in place in the shaft if it is long enough. If the filling column is now eroded, the cavity is widened to the side. As a result, the entire filling column plunges into the depths and the shaft collapse occurs immediately. In both cases, the dimensions of the hole are not significantly larger than the shaft diameter.

In the case of unfilled manholes with manhole plates, after a certain time the manhole lining is destroyed. As a result, the shaft wall loses its load-bearing effect in the area of the moist, loose rock. Then the loose rock breaks out and falls into the shaft. This gradually undermines the load-bearing edges of the manhole cover. The size of the breakout depends on the size of the manhole cover. Due to the forces that occur, the plate can either break or, depending on its size, fall into the shaft.

Area of action

There is a certain area of action around a shaft in which the loose material filling column has slipped . Based on an evaluation of all known damaging events carried out in the 1970s, a calculation method was created to assess the area of impact of shaft breaches. The impact area consists in seigeren shafts composed of a dilapidated, a reduction at risk and a reduction atmospheres. For the calculation of the area of action at the surface ( EB ), the thickness of the shaft lining ( A ), the depth of the rock line ( ), also known as the depth relevant to the action, and the shaft diameter ( ) must be known. The clear width of the manhole is used to calculate the area of action for sloping manholes and a safety distance ( S ) of 1.5 meters is taken into account. Assuming that an angle of action of 50 gons is taken into account for both the filling column and the overburden, the following formula applies to the calculation of the area of action for sloping manholes: ${\ displaystyle T_ {F}}$ ${\ displaystyle D_ {S}}$

${\ displaystyle EB = D _ {\ text {S}} \ +2 \ cdot \ left (A + S + T_ {F} \ right)}$

In the case of shafts with a length of tonnes, the area affected has different dimensions for the direction of incidence and strike . Due to the spatial position of the shaft, the area of action in the case of shafts with a length of tonnage takes on the shape of a trapezoid, in which the base is perpendicular to the direction of incidence. The trapezoidal axis lies congruently above the shaft axis in the direction of incidence. In order to calculate the area of action in shafts with a length of tonnes, the depth of the action and the shaft diameter in the direction of incidence or strike must be known in addition to the thickness of the shaft lining. The depth relevant to the action is determined for tonnage shafts using a nomogram drawn up by Hollmann and Nürenberg. Taking into account the width of the daybreak-endangered zone at the level of the rock line ( ), the area of action is calculated using the formula: ${\ displaystyle B_ {TZ}}$

${\ displaystyle EB = D _ {\ text {S}} \ + B _ {\ text {TZ}} \ +2 \ cdot \ left (A + T_ {F} \ right)}$

Within the area of action of a barrel-length shaft, the surface in the direction of strike and incidence of the shaft is at risk of subsidence, collapse and even collapse.

Safeguards

In the event of a shaft breach, there are three types of safety measures to combat damage:

Initial backup
Permanent backup
Safekeeping

The initial protection, also provisional protection, serves to immediately protect the damaged area and thus to avert danger to life, limb and property. Barriers, warning tapes and signage are used here.

With permanent security, the damaging event is not fundamentally changed or eliminated, but the threat to public safety is considerably reduced by more durable measures over a limited period of time. Solid fences, moldings, earth walls, walls or covers made of reinforced concrete are used for this. The measures are checked regularly.

Maintenance -free and effective measures are taken to remedy the damage during storage . These include concrete seals, reinforced concrete slabs or hardening offset. These measures are designed in such a way that they are effective for at least 100 years.

Individual evidence

^ Günter Meier: Modification of daybreak forecasts . Online (PDF; 1.3 MB).
↑ Securing road areas at risk of breaking in old mining and subrosion areas . Online ( Memento from June 11, 2007 in the Internet Archive ) (PDF; 681 kB).
↑ Mark Mainz: Geotechnical models for the assessment of hazard areas of the old mining and shaft protection areas in the Aachen coal mining area . (Dissertation) Online (PDF; 4.0 MB).
↑ Michael Clostermann: Old mining and land-use planning: Dealing with the legacies In the past - today - tomorrow ? Geokinematic Day Freiberg 2009.
^ Günter Meier: principles of safekeeping in old mining . Online (PDF; 143 kB).
↑ Michael Clostermann, Peter Hogrebe: Schacht Meyer - small break in day with big consequences .

[Quelle_1-1] Günter Meier: Modification of daybreak forecasts . Online (PDF; 1.3 MB).

[Quelle_2-2] Securing road areas at risk of breaking in old mining and subrosion areas . Online ( Memento from June 11, 2007 in the Internet Archive ) (PDF; 681 kB).

[Quelle_3-3] Mark Mainz: Geotechnical models for the assessment of hazard areas of the old mining and shaft protection areas in the Aachen coal mining area . (Dissertation) Online (PDF; 4.0 MB).

[Quelle_4-4] Michael Clostermann: Old mining and land-use planning: Dealing with the legacies In the past - today - tomorrow ? Geokinematic Day Freiberg 2009.

[Quelle_5-5] Günter Meier: principles of safekeeping in old mining . Online (PDF; 143 kB).

[Quelle_6-6] Michael Clostermann, Peter Hogrebe: Schacht Meyer - small break in day with big consequences .