The expansion of a massive Serbian coal mine requires moving a river.
By Svetomir Prokic and Ronald K. Frobel
The Kolubara open pit mine is the largest and most critical supplier of lignite coal in the Republic of Serbia and a crucial component in that country’s emerging economy. The mine encompasses more than 600km2 and is located approximately 50km southwest of Serbia’s capital, Belgrade.
This coal mine produces more than 26 million tons of lignite coal per year. In operation since the 1890s and converted to open pit operations in 1950, the mine is continuously expanding into new areas of high-quality coal. By 2000, a decision was reached that would allow a major expansion of mining operations.
That decision? Move a river.
The Kolubara River meanders northward through the middle of the Kolubara coal basin in central Serbia. It has a maximum flood of Q = 650m3/s. The design of the river relocation project was undertaken by the Jaroslav Cerni Institute in Belgrade and includes three distinct phases.
In addition to the obvious detailed hydraulic studies and subsequent designs, it was discovered that potentially unstable soil conditions in large open pit fill areas would require special treatment for long-term seepage control along approximately 5km of the river. Combinations of compacted clay liner (CCL) and highly extensible rubber EPDM geomembrane with geotextile protection were selected to satisfy design requirements.
At the completion of Phase I, more than 100,000m2 of geomembrane and 200,000m2 of protection geotextiles were installed along with more than 265,000m3 of CCL to provide the requisite hydraulic seepage barrier. The geomembrane installation was completed in March 2007 and the newly realigned river is expected to be flowing by August 2007. Without the use of geosynthetics in the design, the relocation of the Kolubara River could not have been constructed as planned, and a critical and timely expansion of the Kolubara coal mine would have been in jeopardy.
Serbia and the Kolubara mine
The Republic of Serbia is a land-locked nation bordered by the countries of Hungary, Romania, Bulgaria, Macedonia, Albania, Montenegro, Bosnia/Herzegovina, and Croatia. Serbia is currently on a fast track in rebuilding its economy and infrastructure, following years of warfare in the Balkans. The ultimate transition to a market-based economic system from primarily a state-controlled and state-run system will include deregulating and liberalization of the domestic market and foreign trade. This will enable Serbia to join the European Union (EU) within perhaps 5-8 years, according to EU sources.
The Electric Power Industry of Serbia (EPS) was founded in 1947 and is a crucial component of the economic restructuring process, responsible for the supply of electric power to all regions of Serbia and Montenegro. More than 62% of electric power is generated by coal-fired power plants that use coal from Serbia, and 33% is supplied by hydroelectric generation.
The largest coal mine in Serbia is the state-run MD Kolubara, which is located 50km southwest of Belgrade. This mine is a crucial coal supplier, providing lignite coal to 6 major power plants and supplying more than 85% of total coal production for Serbia and Montenegro.
Yearly production of primarily lignite coal is more than 26 million tons, extracted from 4 active open pits by giant excavators. The ratio of coal to soil averages 1:2.5 and coal strata is generally located 10-20m below the surface. Ten giant rotor excavators (Figure 1), most from German manufacturers, are used for coal extraction. These excavators are more than 30m tall and are designed to extract coal from the thick coal seams for transport to conveyors.
The Kolubara coal basin is approximately 600km2 comprising the regions of Lazarevac, Lajkovac, Ub, Koceljeva, and Arandjelovac. The coal basin is separated by the Kolubara River into an eastern section and a western section. The eastern part of the basin includes about 20% of the mine’s total surface area, 120km2. The coal reserves in this area have long since been exhausted. The western section, however, includes more than 480km2 and remains largely untapped. This region will supply coal to Serbia and Montenegro for the next 50 years. However, to expand mining operations into this western region required moving the channel of the Kolubara River.
The central part of the Kolubara lignite basin has a well-developed hydrographic network. The Kolubara River is the main watercourse in this area. The Pestan and Lukavica rivers join the Kolubara in the “south field”—a future open pit mine area— while the Vranicina River joins the Kolubara on the western side of this field. All of these watercourses are located within the coal deposit area for future open pit operations. To prepare for future mining in the western field of the Kolubara basin, the watercourses and in particular the Kolubara River had to be relocated in a timely manner (with phased relocation construction) that would coincide with the start of the open pit expansion.
The Tamnava-Eastern Excavation Field will be exhausted in 2007 and so it was essential that the Kolubara River be dislocated and rerouted to open the new mine fields to the west. Figure 2 shows the Kolubara Lignite Basin (central portion), the position and borders of the planned open pit mines, and the hydrographic network. Figure 3 is a photo of the current Kolubara River.
This river dislocation scheme is a very complex project to say the least. The scope and complexity largely overcome classic hydrotechnical tasks related to the minor redirection of watercourses for the following reasons:
- Due to the excavation of large areas of the open pit mines to depths of more than 200m, the dislocation of the watercourse is drastic from its original route.
- The dislocated riverbed will be in very close proximity to future open pit mines, and thus, they must be designed to prevent seepage into the open pit area.
- The watercourse must be dislocated along the open pit mine contours and, in this regard, the riverbed section and structures must be geotechnically stable.
- The watercourse must be sized for maximum flood and protect riparian lands from flooding.
- The displaced river must be designed to be stable as regards bed and bank erosion during maximum flood of 650m3/s.
- The displaced watercourse must be designed to flow over waste soil deposit areas, as well as through variable geologic strata and sedimentary layers, some of which are highly permeable.
The Kolubara River dislocation general design was completed in 2000 by the Jaroslav Cerni Institute for the Development of Water Resources and included a uniquely complex dislocation in three phases, as shown in Figure 2.
The three phases as shown in Figure 2 are:
Phase I (blue, Figure 2)—Redirection of the Kolubara River along the inside of the waste soil deposit—OPM Tamnava-East Field— for release of OPM Crljeni coal extraction.
Phase II (green, Figure 2))—Redirection of the Pestan and Kolubara rivers for partial release of the OPM South Field coal extraction.
Phase III (red, Figure 2)—Redirection of the Kolubara for full release of the OPM South Field coal extraction.
The Phase I design included an experimental reach inside the dump of the OPM Tamnava Field to determine the hydrogeological parameters of the waste dump soil, riverbed permeability at full head, and engineering properties of the waste dump soil for possible design of a clay or clay/geomembrane barrier.
Phase I design considerations: riverbed and riverbanks
Based on extensive geotechnical investigations of the OPM Tamnava-East Field waste soil and excavations through sedimentary soil layers as well as the experimental reach, the design of the river bed and slopes was completed and included an extensive seepage control barrier to prevent water seepage into the future open pit mine areas OPM V. Crljeni.
It was determined that not only must seepage into the open pits be prevented, but the potential forsoil instability must also be prevented in order to protect against a catastrophic collapse of the new river channel. In addition to an effective seepage barrier, specific reaches of the Phase I redirection would be subjected to differential settlements over time, thus stressing the critical seepage barrier.
The final design included three types of typical sections for approximately 5km of the new Kolubara watercourse Phase I. The design section required for a given reach was based on specific requirements, including maximum control of seepage and control of seepage plus potential for differential settlements.
Included in Phase I was a small lake (120,000km2) that serves as a recreational and ecological feature to help improve the area devastated by mining. The lake was constructed over an old coal-fired power plant ash waste disposal pit and thus was not considered a critical element for seepage control. Nonetheless, the entire lake bottom was lined with 1m-thick CCL up to an elevation of 84m.
The upper slopes of the lake section were cut through sedimentary layers of gravel with relatively high permeability. The steeper (2.5H:1.0V) upper slopes were lined with a geomembrane with protection geotextiles and overlain with crushed stone (300<d<500) armor protection mechanically placed over the top protection geotextile. Figure 4 illustrates the typical lake bank section with anchor bench CCL/EPDM tie-in at the 84m elevation.
For Phase I, there are generally three types of channel cross sections for the Kolubara relocation. Canal Type I is designed for reaches which are excavated through original ground containing highly porous gravel layers that require effective seepage control for both the bottom and slopes. This section requires crushed stone armor (300<d<500 mm) protection directly over the upper protection geotextile on the 3H:1V side slopes and a 3-layer erosion protection on the bottom (including a 0.3m-thick fine gravel layer protection for equipment travel on the bottom). Figure 5 illustrates a typical Canal Type 1 section.
The Canal Type 2 section is designed for reaches constructed through the dump waste soil that will be susceptible to differential settlement. This section is also a composite lining with the geomembrane placed over the 1.0m-thick CCL base layer. The zone 5 is a ballast layer over the geosynthetics, designed to prevent uplift as well as protection. To prevent scour of the soil ballast layer at high flood conditions, crushed stone erosion control “belts” are placed fully across the canal section at 100m intervals (see detail B of Figure 6 ).
Canal Section Type 3 is designed as a 1.2m-thick CCL layer only for reaches that are not susceptible to potential settlements. The requisite ballast layer of 1.3m thickness is still required to protect the CCL as well as provide erosion control. Figure 7 shows a completed section of Canal Type 3 CCL.
Lining material requirements for special channel sections
As discussed in previous sections, there were geotechnical and hyrotechnical requirements to provide critical containment for specific sections or reaches of the dislocation. In general, there were 3 lining systems: compacted clay liner (CCL), geomembrane with protection geotextiles, and CCL in combination with a geomembrane (composite liner).
Compacted clay liner (CCL)
The CCL is designed for bottom lake lining as well as for specific reaches of the Kolubara dislocation channel. These reaches will not be susceptible to differential settlements and are not connected to structures. The lake lining CCL is designed with a special transition zone for a watertight connection/transition to the geomembrane-lined banks of the lake. The design requirements and mechanical properties for the CCL were:
- Compacted clay dry density: 1.65 kN/m2
- Permeability coefficient: K < 10E-07 cm/s
- Grain size distribution uniformity coefficient: Cu > 9
- Flammable and organic mattercontent: < 6 %
- Clay liner thickness: d > 1.2 m
Geomembrane and geomembrane/CCL composite
Specific reaches of the channel sections are considered critical in terms of the design for long-term seepage control. The following are design considerations for using techniques other than just a CCL with erosion protection:
- Seepage control to prevent subsoil instability
- Seepage control to prevent water leakage into open pits
- Seepage control over highly porous sedimentary layers in channel cut
- Differential settlements and long-term maintenance of the hydraulic barrier
- Positive seal to concrete bridge structure
- Second defense to clay alone (composite barrier)
- Positive barrier under armor protection areas (lake shoreline)
To effectively address these design requirements and geotechnical concerns, a geomembrane lining system was investigated during the design process. The design team at the Jaroslav Cerni Institute for the Development of Water Resources considered several types of polymeric geomembranes and investigated the properties and relative cost of each. After consideration of the design requirements and properties, an EPDM geomembrane was selected for the lining of special canal sections and the upper bank lake lining, based on the following selection criteria:
- Highly extensible for differential settlements (> 300% strain with no yield point)
- Proven historical usage and longevity in canals and large water projects(> 50 years)
- Thermally weldable fabrication for on-site installation and effective quality control procedures
- Full cross channel width sections with large coverage area
- Proven quality assurance system (QAS) procedures and methodology
- High conformance to subsoilirregularities
- Construction stress resistance (i.e.,ballast placement with 300<d<500mm stone)
- Construction during cold winter months (< 0° C)
- Fast and efficient installation due to project time constraints
Large-scale site performance testing
The design requirements for the protection of the lining system from wave action and erosion due to high channel flows necessitated the placement of various size large stone on the geomembrane system. This required the use of protection geotextiles. Nevertheless, the method of placing the large stone with heavy equipment and equipment travel necessitated the use of a field-loading test using the materials and equipment specified.
The Jaroslav Cerni Institute initiated and supervised a large-scale demonstration section on the lake bank prior to approving the methods for stone placement. The test section was 20m x 30m and allowed full-scale placement of the stone layers, equipment travel, and movement (equipment turning). The test section included the 1.0mm geomembrane and 350gm/sm protection geotextiles top and bottom of the geomembrane. After placement of all stone layers with contractor equipment proposed for use, all stone was removed and the layers inspected. No geotextile damage or puncturing of the membrane was noted in the trial.
Fabrication and installation
Due to the size of the project and time constraints for installation, large prefabricated panels, custom-sized for specific design sections, were required. A large prefabrication facility located near Belgrade and about 25km from the work site was used to assemble the various panel sizes. The manufactured roll goods (1.7m x 125m) were shipped from Sweden to the fabrication facility and thermally welded into large panels. The large panel size greatly reduced the field seaming requirements and covered large areas of prepared soil.
Prefabricated panel sizes were determined by the location and cross section of the river banks. Layout drawings of each section were first made and approved so that each panel number was located in its precise location on the river section. For riverbanks and sections (Canal Types 1 and 2), the size of the panels varied between 12m-15m wide x 49m-67m long. The length was determined by the full canal section width so that only longitudinal cross-channel field seams were used (no horizontal seam allowed on the slopes).
This also allowed for full double-fusion field welds and QC air channel testing across the full channel section for each field seam. Maximum river section panel sizes were 1,000m2. The lake bank section required panels 20m-35m wide x 50m long, with a maximum panel size of 1,750m2.
The prime contractor scheduled incremental canal sections for preparation of earthworks, lining installation, and cover soils placement on a 1-week sequential basis. Thus, all prefabricated panels were transported to the site just prior to installation. This allowed for minimal on-site storage or delays, and allowed for close scheduling of prefabrication and installation. This essentially avoided rehandling of panels and prevented damage due to storage, vandalism, traffic, and weather. Additionally, only that soil area to be immediately lined was prepared so that damage to the subgrade was avoided and dessication of the CCL surface was minimized.
Immediately after positioning and overlapping with the previous panel, the cross-canal seams were made by dual-track thermal fusion bonding. In this regard, the sheet is laminated with a thin layer of thermoplastic olefin (TPO) that has many of the same characteristics of vulcanized rubber but is capable of being thermally welded. This allows the EPDM geomembrane to be spliced or welded using conventional hot wedge equipment in the form of two parallel welds with an air channel between. The air channel allows for the installer to perform continuous 100% nondestructive testing on all field seams with air pressure in accordance with ASTM D 5820.
In addition, all subgrade preparation, panel placement procedures, welding, testing, and reporting was accomplished in accordance with the manufacturer’s quality-assurance system (QAS) and under full third-party CQA observation. Figures 8, 9, and 10 illustrate typical channel preparation, panel placement, and thermal-fusion welding. Immediately after panel placement and acceptance, the top protection geotextile was placed, followed by the requisite ballast soil cover material and aggregate on the upper slope areas. Figure 11 shows a completed section of the canal relocation and the recreational lake.
Construction QC and QA
A fully designed and implemented quality assurance system (QAS) was made a part of the contract documents and utilized to the full extent on the Kolubara project. The QAS was comprehensive and included quality procedures from original roll goods manufacture through cover soil or aggregate placement. The QAS included the following:
- QC manufacture—polymer
- QC manufacture—roll goods
- QC fabrication—panels
- QC—CCL material and placement
- QC—subgrade preparation for liner placement
- QC—prefabricated panel placement
- QC—thermal-fusion field seaming
- QC—cover soils and aggregate placement
Implementation of the QAS in the field and at the fabrication plant was carried out by a third-party CQA consulting organization, Institute for Transportation CIP, Belgrade. All procedures from panel fabrication through installation, thermal welding, and testing was fully observed and documented.
The Kolubara Open Pit Coal Mine is a critical supplier of lignite coal for the production of electrical energy in Serbia and Montenegro and supplies more than 85% of that demand. To continue the current production of more than 26 million tons of coal per year, the mine within the coal basin had to be expanded from the eastern region to the western region. In order for the first phase to be accomplished in 2007, the Kolubara River and its tributaries had to be redirected for more than 5km. In addition to the major accomplishment of design for river relocation, it was determined that the new river channel must be designed with an impervious lining system to prevent infiltration of river water into the open pit mines and to prevent a potentially catastrophic collapse of the river channel during high flow or flood conditions.
In addition to the use of a CCL over stable excavation areas, it was determined that a combination of CCL and highly extensible geomembrane would be required in areas that would be subjected to differential settlement. The 1.0mm-thick EPDM geomembrane was protected during construction and during potential movements by placing 350g/sm nonwoven geotextiles top and bottom of the geomembrane system.
A detailed QAS was specified and implemented to ensure quality of the critical lining system from manufacture through protective cover soils and aggregate placement. The additional requirements for large and custom-made panels and rapid deployment during staged construction of the new river channel was also critical.
For the owner, timely planning and implementation for the mine’s largest expansion and redirecting a major river was crucial, and the use of a geomembrane system was instrumental in achieving this goal.