Drilling in Alstäket, Sweden
The company BABRörtryckning AB from Sweden was facing a very challenging drilling job inÅlstäket in Sweden in December 2011.
For a new pressuremain line from an existing pump station in direction of a lake a steel pipe 609mm x 10 mm had to be installed by trenchless technology over a length of 95 mand below a traffic circle in order to finally pull in a PE pipe DN 500 intothis steel pipe. The PE line should be laid with a down-grade of 1.2 percent. Theowner was the municipality of VÄRMDÖ.
The soil on sitemainly consisted of sand with a small amount of cohesive parts. Howeverboulders or large stones with predicted edge lengths of up to 60/80 cm wereexpected in this ground.
Two similar drillingsnearby this site were performed unguided but had ended with considerabledeviations due to the challenging geology. To avoid these unacceptabledeviations on this long jacking length BAB Rörtryckning AB was looking now fora guided technology.
In December 2011,theycontacted Bohrtec to use the guided Front Steer system for this challengingjob.
All parties wereaware from the start that a guided drilling in the described geology was verychallenging as eventually occurring stones or boulders were very big inrelation to the steel pipe diameter (609 mm/24 in.) and could have negativeinfluence on the steering behavior.
Discussing all of the pro and cons, BAB Rörtryckning ABdecided to use a long frame machine of Bohrtec of the type BM 400 LS with aFront Steer 609 system and electronic inclinometer, electronic hydrostaticwater level and horizontal direction control.
The machine, as wellas the whole equipment, was delivered in January 2012. The BM 400 LS wasequipped with a 12-m long frame extension as it was planned to work with 12-mlong steel pipes. The Front Steer 609 mm was equipped with a down-the-hole-hammerwith a drilling head diameter of Ø 650 mm.
BAB Rörtryckning ABprovided an appropriate compressor with a performance of 27 m³/min (250ft³/min) at 25 bars.
Due to the elevationprofile on site, it was not possible to use a conventional starting shaft. Infact the machine had to be placed at-grade and in order to lead in the jackingforce an “above ground” pressure skewback was created. Additionally, themachine had to be placed further backward because in front of the machinesupply lines avoided a placement more near to the street.
At the end of January,the drilling work began. On the first 30 m (98 ft) of the drilling,no larger stones/boulders were encountered and a daily performance of approximately15m/day (50 ft/day) was reached.
After approximately55 m (180 ft), boulders and a consolidated rock horizon were crossed for thefirst time. Although in this area the machine had to be driven significantlyslower, the system could still be steered without problems and was driven onthe nominal axis.
After crossing thispure stone formation, in the following 20 m (65 ft) of mixed soilsmany stones were encountered during the drive.
In this challengingsoil/rock conditions the excellent steering behavior and performance of theFront Steer system could be proven again.
The adaptation of adown-the-hole-hammer to the Front Steer broadens the working range for theguided auger boring systems of Bohrtec into the fields of hard rock anddifficult soil conditions. The guiding components consisting out of andelectronic inclinometer, electronic hydrostatic water level and horizontaldirection control guarantee an exact measuring, which is the precondition forhighly accurate laying of any kind of product or steel pipes.