GRUNDODRILL 18ACS masters difficult drilling job in the Swabian Alps
Heavy rainfall is a constant source of worry for the water supply cooperative “Kornberggruppe Eislingen” in Germany. When the rain is heay the seventy-year-old cast iron potable water transport pipeline, running through a canyon called “Teufelsloch” (Devil’s hole) in the neighbourhood of the highway A 8 near Bad Boll and installed close to the surface, might well be jetted free by torrential downpour and torn apart at any time. Several times it was already necessary to labouriously repair the pipeline running through an inaccessible nature reserve.
In an attempt to protect the water pipeline and to counteract erosion caused by heavy downpour, so-called ravine dams with massive boulders were installed in the ravines of the valley (narrow, notched valley cuts) to regulate the steeply sloped effluent of the surface water and prevent further soil degradation.
However, due to the great age of the cast iron pipes, the drinking water pipeline in a total length of 1800 m was pending for new installation. This also involved the “Devil’s hole”. The water cooperative entrusted the renowned engineering office Fritz GmbH from Bad Urach with the planning of the measure.
It was obvious from the beginning that narrow confines were set for a new installation in the “Teufelsloch” due to the geological conditions and site preservation, Thus, installation applying a fluid-assisted HDD method was considered for the first time.
The soil in the outlier zone of the Swabian Alps consists of sedimentary marine deposits in great layers of clay, limestone and marl being a great technical challenge for the drill rig. These rock formations were generated during the lower and medium Jurassic at the bottom of a sea which covered great parts of Europe app. 200 to 150 million years ago. Jurassic rock formations are classified into Black, Brown and White Jurassic. The rocks of the Black Jurassic (Lower Jurassic) consist mainly of clay and clay marl. The Brown Jurassic (dogger) is a mixed sequence of sandy-clayey marl, sandstones, clays, stone marl and iron ore seams. The White Jurassic (Late Jurassic) consists mainly of limestone, dolomite, marl and lime marl.
In the “Devil’s hole”, Brown Jurassic rock sequences can be found; the layers alternate continuously between sandstone, sandy-clayey marl (so-called sand-fibre layers), clay marl and iron ore. It is precisely this alternation in the rock on site which presents an enormous challenge for HDD bores because the drilling tools must adapt to the varying hardness. After all, we must not forget that a bore hole diameter of 450 mm is required after the final reaming stage.
After a course of discussions and careful weighing up of all arguments, the cooperative decided to invite tenders for the fluid-assisted HDD method. Decisive factors were not the geology alone but also environmental considerations like limiting intervention in nature and landscape to the absolutely inevitable. To be on the safe side, the assessed installation time was three months, but the drilling crew was able to clear the jobsite after only eight weeks.
The responsible nature conservation authority defined the environmental specifications very precisely: temporal observance of the growing season, animal protection, forgoing to cut down trees or shrubs without permit and avoiding pollution by using bio-degradable fuels.
The new bore path was planned to run parallel with the old one. After an approximately 100 m long, relatively flat course, the bore path drops away abruptly from 585 m in height to 500 m. Then, after overcoming the hollow, it rises again to 570 m. Decline or ascent sometimes reaches up to 50 %. The complete bore length is app. 860 m.
The bore path was split into three sections: on the right and left side of the valley 200 m and 210 m respectively, with a bore length of 450 m in-between. Planning was supported by special planning software which is able to determine the optimal drilling radius in relation to the pipe intended for pulling in and the drill rods.
Specially selected as a new pipe for the “Devil’s hole” was the SLM pipe OD 288 from Egeplast. The multilayer pipe consists of an extremely abrasion-resistant, mineral-filled polypropylene. This hard casing holds off direct loads from the pressure-bearing pipe and evenly distributes the additional tension caused by sharp-edged rocks. The PE pipes ND 200, SDR 11 were delivered as off-the-rack goods, each one 12 m long, and welded together to form a pipe string on the site of a forest road. The maximum possible tensile load of the pipe is 160 kN and must not be exceeded.
The tender was granted to the company WMZ from Wittislingen by the Danube. The company WMZ exclusively uses TRACTO-TECHNIK drill rigs and has completed many spectacular drilling projects with great know how and experience during the 20 years of its existence. “This is, among others, because we accompany our projects on site personally, from the beginning to the end,” Erwin Zintz, one of the two managerial shareholders tells us. “We are extremely dedicated. The drilling business is our passion.”
The project commenced in the 2nd half of July 2013. The rock drill rig GRUNDODRILL 18ACS, with difficult geologies being its strongest point, was employed for the complicated job.
First of all, the two sections on the right and left side of the valley were installed. Then the 450 m long connecting bore could be started. It was an all-time record for the GRUNDODRILL 18ACS, not only because of the bore length but also because of the bore diameter, the extreme location and the soil conditions, with ascending and downward slopes partially up to 50 %.
The pilot bore with a diameter of 160 mm caused no problems whatsoever. The maximum drilling depth was 17 m. After that, the time-consuming reaming operations upszing to 12“, 14“, 16“ and 18“ started, demanding enormous patience; sometimes, reaming one drill stem length took up to 40 minutes, sometimes only 4 minutes, according to Michael Miklikowski, co-owner of WMZ. The drilling fluid was provided by the mixing system positioned on the associated truck. Due to the high clay share in the soil, a relatively small amount of Bentonite without chemical additives was required. A tractor with water tank arrived frequently with the fresh water supply.
Early in September, the great day had finally arrived. The concluding act was about to be tackled: pulling in the pipe.
Laying out the pipe string and inserting it in the bore hole had to be conducted very gently to preserve flora and fauna, thus it was more time-consuming than the pipe pulling itself. Thes process also flowed friction-freely in the truest sense of the word. Although the pipe had a dead weight of 15 t, the maximum pulling force required was only 60 – 80 kN. The tensile load was not allowed to exceed 160 kN, the complete process was monitored and logged using the GRUNDOLOG tensile load measuring system. This way the crucial requirements were fulfilled.
All participants engaged in the project were relieved and satisfied at the end. The favourable ending was owing to the accurate planning of the engineering office Fritz and the professional performance of the company WMZ. After all welding and connecting works to integrate the final line section in the already installed sections, the access road, specially laid out for the project before starting with the drill works, was partially worked back to allow rapid renaturisation after the minimally invasive intervention in nature.