The Medard – Libik quarry, located west of Sokolov Town in the Czech Republic, was once an important source of coal in the region. However, due to the diversification of energy sources and strict environmental requirements, demand for coal declined in the mid-1990s, and the Czech Government decided to cease mining in the area.

There remained the question of how to redevelop the now inactive quarry. The aim was to eliminate the effects of mining activities and facilitate a return to human activity, as the quarry had devastated the surrounding landscape, destroying the settlement, industrial enterprises, agricultural land, roads, streams and historical and cultural monuments.

Various alternatives were assessed, but it was ultimately decided to flood the residual shaft of the quarry with water from the Ohe River. This would create the Medard – Libík lake, which would raise the aesthetic and environmental value of the area and create a facility for sporting, recreation, bathing and game fishing activities. It was intended to build cycling lanes, walking tracks and other sporting facilities on the edge of lake, with the hope that this would attract entrepreneurs who would build guesthouses, cafes and restaurants.

To transport the water from the Ohe River to the quarry, it was necessary to install an underground water pipe. The intake facility is situated on the Ohe River bank, close to the road that runs between Citice Village and Sokolov Town, and the route was designed to create the shortest possible line between the quarry edge and the Ohe River. Given that the pipe route traverses towns, roads and railway lines, open cut installation was not an option, and instead pipejacking will be used.

The pipejacking will be done by Vodohospodáské stavby Teplice (VHS Teplice), which has many years of experience in trenchless works throughout the Czech Republic. The firm has been subcontracted by Sdružení SVATAVA; the general supplier of the conduit, operating under EUROVIA, a.s. The conduit investor is Sokolovská uhelná spolenost.

The jacking will install 56 m of double Hobas DN 1,200 mm pipe under the Sokolov – Citice road and Sokolov – Cheb railway. Two parallel jackings of DN 1,400 mm steel pipes will be done for the above pipe. The outlet facility is connected to the jacking and situated at the dump in the southern part of the mine. The slip is connected to the open channel and proceeds to the adapted bottom of the mine.

The VHS Teplice experts have already constructed similar underground structures when flooding the Ležáky mine and when dewatering the SM Mstišov quarry. On these projects there was a hazard of a slide of the opencast quarry slopes, and this will also present a potential challenge at Medard – Libik

The success of the project is based upon the high quality starting shaft, operated by the AD 20 tonne mobile crane. The shaft has to comply with the regulations of the Czech Mining Authority – the state supervisory authority for jacking technology.

The starting shaft buttress wall has to be perpendicular to the jacking axis and must allow the pressure for the whole borehole to be spread into the surrounding natural ground. The slope of sidewalls is 1:0.5 for a cohesive soil; for a frictional soil, it is necessary to encase the starting shaft according to the approved design. The slope of the starting shaft hard bottom shall be equal to the slope of the jacked sleeve. The directional and height tolerance is 1 cm per 1 running metre at maximum.

When jacking a steel pipe, the steel pipe is jacked while removing earth inside the pipe. The muck extracted at the jacking face is loaded onto a trolley, transported to the starting shaft in the trolley and then to the surface using hoisting equipment. When the whole length of the pipe has been jacked, another pipe is brought to the starting shaft using hoisting equipment, which is welded together with the already jacked pipe. This procedure is repeated until the whole jacking process has been completed.

Depending on the geological conditions, a steel ring may be welded onto the outside surface of the pipe to reduce friction. A ring welded on the inside wall of the pipe improves the workability in the case it is a cohesive soil. A linear hydro motor, located in the rear part of the starting shaft, produces the force necessary to jack the pipe face into the soil and overcome the friction of the outer skin and the soil. It pushes, through the adjustable adapter pieces and transition spacer, onto the rear face of the pipe and jacks it into the soil. Extraction of muck from the starting shaft is mostly done using a light derrick.

Steel pipes may be jacked in the soils and rocks with workability class 1 to 4 (SN 733050). When employing small blasting operations, the smallest sleeve diameter is DN 1,200 mm. A pre-condition for defining the jacking technology and method statement is the geological and hydro-geological survey made in the location of the starting and end shafts.

Engineers working on the project were faced with the technical challenge of transporting water from the river to the quarry, and also ensuring that the water in the lake was kept at a constant level. When the water level in the lake drops, the water will be supplied from the Ohe River, and when the water level in the lake rises during heavy rains, the water will be brought through an overflow into the Ohe River. The design of the facility allows it to be fully closed by flood-gates designed for a two-directional pressure.

The water quality in the lake will be a sensitive issue. One of the measures will be limiting the water supply through the water-collecting facility after the lake has been filled so that further water supply in the lake just replaces the evaporation from a free water surface. This, apart from the unwanted water level fluctuation in the lake, will also minimise the supply of nutrients in the supply water.

The water-collecting facility construction commenced in 2009, and it is intended that filling the lake with water will commence in late 2010. The filling could be completed, at the average hydrological conditions (flow-rate in the Ohe River) in 2011 to 2012.

Once the quarry has been flooded, the Medard – Libik lake will once again become a beautiful part of the North-West Bohemia, and Trenchless Technology will have played a vital role within this revitalisation.