The Bulgarian Association for Trenchless Technology (BATT) and the Bulgarian Ministry of Emergency Situations jointly presented the paper, entitled Innovative solutions for prevention and reconstruction of critical infrastructure – Implementation of Trenchless Technology ‘rock drilling’ in water main rehabilitation at the conference, which was attended by Municipality Mayors and Managers of regional Water Boards, non-government organisations, pipeline construction contractors and experts from financial institutions.

Chairman of the Bulgarian Water Association Valery Nikolov addressed conference delegates as a keynote speaker, and many BATT members – representatives of the academic, design and business communities – presented their views on the implementation of Trenchless Technology in infrastructure construction. During the practical part of the Conference, a real-life example was presented.

Complex installation

The water main in question is a vital conduit – supplying drinking water to several villages in the Kardjali region and servicing a total population of approximately 11,000 people – and was first laid during the 1960s with 200 mm diameter steel pipes.

The ground covering the original network was gradually washed away, however, by the Varbitsa’s seasonally erratic currents and eventually the pipe was ruptured by the volatile flooding waters in 1996, at which point the first repair was made.

A total of 150 metres of pipe was replaced – including the damaged portion and adjoining sections – with a new HDPE pipe that was laid approximately two metres deeper than the original section.

Ten years later, the problem recurred; the Varbitsa River had once again washed away the covering earth, exposing the pipe at a length of more than 30 metres. At this point the pipeline was still serviceable, having withstood the currents, but its rupture was imminent.

The Kardjali Water Board, therefore, found itself facing the complex decision of how to deal with this issue. The traditional construction methods and process had proven very slow, labour-intensive, and above all ineffective – as the Varbitsa had already shown twice. The ideal solution would be to embed the pipe deeper underground in the rock bed underneath the river, but this rock only starts under a layer of sand and gravel of 4-5 metres, and 6.5-8 metres at the river banks.

The company Stroitelna Mehanizatsia AD – a BATT member and co-founder – found a solution in the form of its specialised horizontal directional rock drilling machine, the Ditch Witch JT 2720 All Terrain, which the company had purchased the previous summer as the first of its kind in Bulgaria.

With the JT 2720, the company drilled a total length of around 165 metres, of which approximately 100 metres was through the rock bed underneath the river and its banks. Drilling depth into the rock reached up to 13 metres below the surrounding terrain – or 3-5 metres below the sand deposits.

To ensure the faultless operation of the water main at a relatively high working pressure of 16 bar, a special double-ply pipe was used, which had been manufactured by the German company Gerodur. The outer ply is a protective layer that guards the working pipe during installation from damages that could lead to failure later on.

Two foreign consultants were recruited to assist the Stroitelna team: a mud drilling specialist from Germany and an experienced rock driller from the USA.

The works amounted to a total investment of approximately $US166,528, which was provided to the Djebel municipality by the Joint Committee for reconstruction and population relief after industrial accidents and natural disasters at the Council of Ministers. Work began in October 2008, and the water main returned to full operation soon after.

Due to the innovative implementation of modern and effective Trenchless Technology, the water main will no longer be susceptible to damage caused by weather conditions or the Varbitsa’s temperamental currents and long term, dependable water supply is guaranteed.