Even so, these economic advantages can be quantifiable. We are told in the UK that open cut work still accounts for estimated losses, in terms of disruption and environmental impact, of $5 billion per year. My own civil engineering experience, using both traditional and modern techniques in the UK and Australia, suggests that a significant proportion of these losses could be avoided, using microtunnelling methods. For this article, the focus is purely with guided and unguided auger boring and, while I am aware of its real cost benefits, I continually face the general opinion that trenching is cheaper and therefore the method of choice.

The German Experience

In contrast,, in Germany, the open cut approach is becoming a “last resort” for the utilities, This is particularly true of urban areas, where the potential disruption is highest and it is certainly already the case in Berlin.

I recently visited Aachen, near Dusseldorf, to see the latest developments of auger bore manufacturers, Bohrtec. The company manufacture a complete range of guided and unguided auger boring machines that can install sizes DN100 to DN1400 working from pits or manhole sizes of 1.5 m – 6 m. While the most popular machines are the BM400 due to there compact size combined with power, the newest in the Bohrtec range is the BM800. This rig is capable of auger boring diameters of 1.4 m, with a jacking force of 300 tonnes and rotational torque of 50,000N/m. I was told this product was designed mainly for use in the USA but that it could also have potential for applications in countries like Australia and New Zealand. It is important not to confuse these rigs with cheaper under-powered alternatives available elsewhere.

The fact that the machinery is so highly developed there is a reflection of the success of auger boring in Germany. In the mid-80’s a civil engineer called Knut Moereng started to see the potential of Trenchless Technology to avoid traffic hold-ups, costly surface reparation and delays. As head of the authority responsible for Berlin’s sewer network, Moereng pushed for this system to be instated as the preferred method of sewer and storm water installation and rehabilitation. After a short period of time, guided auger boring became the method stipulated at contract tender stage.

The process now accounts for 90 per cent of the gravity sewer installed in Berlin today and information gathered by Bohrtec bears this out:

2004 - Total length of sewers auger bored 21.5 km, approximately 53 per cent with Bohrtec machines with nominal diameters DN 200, 250, 300, 400 and 500. This represents about 57 per cent of the total length of microtunnelling in Berlin in 2004.

2005 – (January-April) Total length of sewers auger bored 10.5 km, approximately 45 per cent with Bohrtec machines. This represents about 55 per cent of the total length of microtunnelling in Berlin in early 2005.

There are at least 40 of this manufacturer’s machines currently operating in the city with a further 40 or so worldwide. So what are the reasons for such a predominance of guided auger boring as a technique?

An important factor is the ground on which Berlin stands. It is mostly composed of sandy and clay soils, which allow this completely dry system to be used very efficiently. This allows for long, accurate drives using optical guidance, without the need for very specialist expertise. It may be useful at this point to review the basic technique:

Guided Auger Boring System

Guided auger boring is a method of installing gravity sewer, steel duct or concrete pipe to accurate line and level. It is based on a simple three-step process:

Step 1: Pilot stage

The machine is set up in the launch shaft, usually a 2.1 m caisson shaft or 2 m square pit for DN150 – DN500 and 4m pits for DN600 – DN800. Using plumb lines set up over the shaft by an engineer. The machine is also set up to the required gradient using a special spirit level.

A theodolite guidance system is then set up at the back of the machine, this shines down the centre of the machine onto a target located in the steering head. The required gradient is then input to the theodolite. A CCTV monitor is positioned in the shaft with the operator, displaying the image of the target. As the pilot rods are put into the ground, the direction and gradient of the string of rods are constantly checked using the theodolite camera and monitor and any deviations are counter-steered and corrected.

Step 2: Auger Stage

Once the pilot rods are through into the reception pit and the line and level has been checked, the auger stage can begin. The lead casing and auger drill are directly connected to the last pilot rod and pushed into the ground, following exactly the same path as the pilot rods. These casings are exactly the same size as the final pipe required and as they are pushed into the ground the auger drills are rotated to excavate the material down the casing. The excavated material is brought out through the auger drills into the launch shaft and collected in a specially designed skip located between the hydraulic rams of the machine.

Once the skip is full, it is lifted out of the shaft and emptied, reducing the amount of manual handling involved. While the augers and casings are being pushed into the ground from the launch shaft, the pilot rods are being pushed out into the reception pit. The rods are disconnected and lifted out of the pit.

Step 3: Product Pipe

With the auger stage complete, the product pipe can be installed. This process simply involves connecting the first pushing pipe onto the last auger and casing and pushing it through the ground behind it. As the product pipes are being pushed, the augers and casings are pushed into the reception pit, disconnected and lifted out of the pit in the same way as the pilot rods. This process is repeated until all the augers and casings are removed, leaving the fully installed product pipe.

Keeping it Simple

While special measures, such as double pilot rods and bentonite injection, can be used to assist in hard ground conditions, softer sands and soils simplify the procedure as a purely dry process, this allows contractors to undertake their work without having to excavate 10 m launch pits and erect large recycling units.

It is true that local legislation in Berlin has helped auger boring dominate this field of engineering in Germany. However, with this equipment now available in Australia, guided auger boring systems like this can pave the way to abolishing open cut work and ridding the city of unnecessary traffic delays, pedestrian access restrictions, backfill subsidence and tipping costs.