Terebro Case Studies

Auger Boring – Why It's Big In Berlin

Trenchless pipeline installation and rehabilitation is, in one sense, the Water Industry's equivalent of 'stealth' technology. When compared with open cut, it is very unobtrusive, often operating from one shaft, or manhole, to another and the economic advantages of trenchless working are largely intangible.

Even so, these economic advantages are quantifiable. We are told in the UK that open cut work still accounts for estimated losses, in terms of disruption and environmental impact, of £2 billion per year. My own civil engineering experience, using both traditional and modern techniques, suggests that a significant proportion of these losses could be avoided, using microtunnelling methods. These days, my concern is purely with 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

Contrastingly, in Germany, the open cut approach is becoming a last resort for the Utilties, 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 at auger bore manufacturers, Bohrtec. While the machines we typically hire out at Terebro are BM400 (ie nominal diameter DN150 - 500), the newest in the Bohrtec range is the BM800. This rig is capable of auger boring diameters of 1.4m, with a jacking force of 300 tonnes and rotational torque of 50,000 N/m. I was told this product was designed mainly for use in the USA but that it could also have applications in Europe.

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 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% 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 % with Bohrtec machines with nominal diameters DN 200, 250, 300, 400 and 500. This represents about 57% of the total length of microtunnelling in Berlin in 2004.

2005 – (January-April) Total length of sewers auger bored 10,5 km, approximately 45 % with Bohrtec machines. This represents about 55 % 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. So what are the reasons for such a predominance of auger boring?

An important factor is the ground on which Berlin stands. It is mostly composed of sandy and clay soils, which reduce the risk of the auger boring system encountering an obstruction. 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 2.1m caisson shaft or 2.0m square pit 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 rodding and bentonite injection, can be used to assist in hard ground conditions, the softer soils in Berlin simplify the procedure, allowing contractors to operate without subcontracting the auger boring. If they prefer not to tie up capital and maintenance costs in auger boring machinery, the rigs can be hired individually, along with an operator. Terebro finds this approach is also favoured in the UK, as it preserves profit and limits risk for the groundwork contractor.

What about other trenchless techniques? Directional drilling is an alternative to auger boring but requires a fall of at least 1 in 100 to ensure that a gravity sewer will function properly. This is because the drill tends to waiver off-line if it hits a stone or rock, introducing sections with a shallower gradient that may need periodic swabbing out in service. The problem is, a fault like this can only be detected by cctv after the sewer has been installed.

It is true that local legislation in Berlin has helped auger boring dominate this field of engineering. However, the availability of an accurate, guided auger boring system has paved the way to abolishing open cut work and ridding the city of unnecessary traffic delays, pedestrian access restrictions, backfill subsidence and tipping costs.

The Author
Cory Higgins is a founding director of Terebro Trenchless Solutions Ltd, Scunthorpe.

Submitted by: Cory Higgins - 04/10/2005