Initial Situation
In hard coal mining, high production rates and the concentration of extraction activities in only a few mining districts meant that direct and continuous transport of coal from extraction to the coal preparation plants was often no longer possible. For this reason, underground main or secondary bunkers were installed as intermediate storage systems.
The bunkering of raw coal, which was mainly transported by conveyor belt systems, had to be carried out as gently as possible. For this purpose, spiral chutes had been used for decades to guide the material into the bunker in a controlled manner.
Planning and Original Construction
At the beginning of the 1980s, the construction of a raw coal bunker on the 6th level became necessary at the Heinrich Robert mine. The contract for planning and construction was awarded in 1979. The construction period was approximately 18 months.
The bunker was excavated conventionally using a large-diameter borehole method. During sinking, the bunker wall was secured section by section with cast-in-place concrete. The final concrete block lining was later installed from bottom to top. The originally installed double spiral chute consisted of precast reinforced concrete elements.
The bunker outlet was then constructed. The steel and base frame received two outlet openings to discharge the bunker contents via vibrating feeders onto the roadway conveyor. After installation of the base frame, the outlet slopes and bunker saddle were formed and concreted. Finally, the spiral inlets and outlet slopes were lined with cold metal plates, while the spiral chute itself was lined with fused basalt plates.
During the previous operating period, approximately 30 million tonnes of raw coal passed through the bunker. The expected remaining service life was still around five to six years.
Damage Pattern
In the final years of operation, increasing damage occurred inside the bunker. This damage was mainly caused by mining-induced ground movements. Rock pressure and ground movement partially destroyed the bunker wall and both bunker inlet tracks.
The precast reinforced concrete elements of the spiral chute were also displaced vertically and horizontally by ground pressure and were partly cracked. The destruction of the bunker inlet tracks and the normal spiral chute also led to damage of the bunker outlet slopes, because the conveyed material impacted these areas in free fall.
As a result, the existing cold metal lining was extensively destroyed or worn away. The underlying concrete substrate was locally broken out to depths of up to 1 m. This caused constant material build-up in the bunker outlet. Removal of these deposits was required almost every week. Despite these cleaning measures, there was a constant risk of operational disruption due to blockage of the bunker outlets or vibrating feeders.
Rehabilitation Concept
To prevent a possible interruption of coal transport, the mine management decided to carry out a comprehensive rehabilitation of the bunker.
Based on a detailed repair cost analysis, it was decided to abandon the original bunker design with double spiral chute function. Instead, the bunker was to be converted into a drop bunker.
The following work steps were required:
Rehabilitation of the bunker wall
Renewal of the bunker outlet by installing a new wear-resistant lining
Installation of steel chutes to redirect the material flow from the spiral chute into free fall
Since no interruption of coal transport was permitted during the repair works, all activities had to be carried out during the available conveyor-free periods, particularly on consecutive weekends.
Rehabilitation of the Bunker Wall
For safety reasons, the rehabilitation of the bunker wall had to be carried out first. Loose concrete blocks and spiral chute stones were removed. In the next step, expansion sleeve anchors and bonded anchors were installed. Reinforcement mats were then tensioned using dome-shaped anchor plates.
A 5 to 10 cm thick shotcrete shell made of load-bearing or high-strength material was then applied. M24 anchors with a length of 1.5 m were used. Reinforcement was mainly provided by welded mesh reinforcement, in some areas in two layers.
Consolidation mortar CM 25 and early high-strength spray mortar FH 3 S were used to produce the shotcrete shell. Both mortars were applied using the dry spraying method. A rotor spraying machine positioned at the bunker head was used for this purpose. The materials were delivered in bags and transported by the mine to the point of use.
After completion of the safety and securing works on the bunker wall, work could begin on the significantly more difficult and extensive repair and renewal of the bunker outlet.
Renewal of the Bunker Outlet
A particularly wear-resistant mineral mortar was used for the renewal of the bunker outlet. The material WPE-DK Mine 14x was applied as a highly wear-resistant material with a steel fibre content of approximately 3% by weight.
The WPE-DK Mine 14x mortar was placed by pumping, using a mixing and pumping unit installed at the bunker base in the roadway of the 6th level. This system was specifically designed for processing high-strength, wear-resistant mortar systems.
The mixing and pumping unit mainly consisted of:
Turbomixer U 80
Pre-pressing device
Differential piston pump P 300/6 Duo
Due to the very low water addition, the high stickiness of the WPE-DK Mine 14x post-mix and the increased mixing energy required, the energy demand during mixing was significantly higher than for conventional concrete. The sticky consistency also made suction by a piston pump more difficult. Therefore, forced feeding of the mortar to the suction side of the pump by means of a pre-pressing device was necessary.
In addition, the bauxite aggregate contained in the material, combined with the high bulk density and the steel fibre content, considerably increased the pumping resistance. For this reason, a differential piston pump with dual drive was selected.
An additional challenge was that the mortar had to be pumped from bottom to top. A height difference of approximately 15 m and a pumping hose length of up to 50 m had to be overcome. The material was conveyed through a 50 mm diameter high-pressure hose line via one of the vibrating feeder openings in the bunker outlet to the respective installation location.
The average pumping capacity was approximately 3 t/h. This corresponds to around 0.9 m³/h and represented a very satisfactory performance under the given operating conditions.
Quality Control and Material Properties
For quality control, test specimens were produced on site. These specimens were removed after several weeks and tested by an officially recognised testing institute.
Compressive strengths of the WPE-DK Mine 14x mortar of up to 218 N/mm² were determined. The average compressive strength was 207 N/mm².
Strength Development and Wear Values of WPE-DK Mine 14x
|
Test age / test method |
Result |
|
Compressive strength after 12 hours |
60 N/mm² |
|
Compressive strength after 24 hours |
116 N/mm² |
|
Compressive strength after 7 days |
163 N/mm² |
|
Compressive strength after 28 days |
210 N/mm² |
|
Wear value according to DIN / Böhme |
1.5 cm³ / 50 cm² |
|
Wear value according to sandblasting method |
6.1 cm³/h |
Finally, the inlet chutes below the two conveyor belts were installed in order to guide the material centrally into the bunker.
Construction Time, Quantities and Costs
The extensive rehabilitation works were completed within eight weekends. No conveyor shutdown occurred during the works.
The detailed preparation work, installation of the steel structures and concreting works required a total of 638 man-shifts. The installed steel structures weighed approximately 22.5 t. The costs for these steel structures amounted to approximately 178,000 EUR.
The following material quantities were required for the rehabilitation works:
|
Material |
Quantity |
|
Consolidation mortar CM 25 |
22 t |
|
Early high-strength spray mortar FH 3 S |
82 t |
|
WPE-DK Mine 14x mortar |
80 t |
The material costs, including steel fibre costs, amounted to approximately 650,000 EUR.
Operating Result After Rehabilitation
After completion of the rehabilitation works, the bunker handled approximately 1.5 million tonnes of raw coal between October 1990 and February 1991. During this period, no material build-up was observed in the bunker outlets. As a result, the personnel costs previously required for regular cleaning and removal of deposits could be eliminated.
Final Assessment
Approximately five months after completion of the rehabilitation works, no relevant wear was detected in the bunker outlet. Only the WPE-DK layer on the rail heads had been worn away as expected.
The rehabilitation led to the following conclusions:
The rehabilitation measures carried out were successful.
The selected technical approach was appropriate.
The highly wear-resistant material WPE-DK Mine 14x showed no visible wear after five months of exposure to abrasion and impact loading.
The high rehabilitation costs were justified in relation to the weekly maintenance work that had previously been required.
The selected process and organisational measures enabled the rehabilitation to be carried out without affecting ongoing coal transport.
The project demonstrates that highly wear-resistant UHPC-based mortar systems such as WPE-DK Mine 14x can provide a technically effective solution for heavily stressed raw coal bunkers. In areas exposed to continuous material flow, severe abrasion, localised impact loading and difficult-to-access repair zones, a pumpable, high-strength wear protection mortar offers clear advantages over conventional protection systems.
Keine Kommentare:
Kommentar veröffentlichen