Application – what it does/is used for [or chart showing suitable for/not suitable for]
For the installation of smaller diameter utilities and services with the minimum of surface disruption using minimal trenching techniques that do not involved significant soil removal and/or use of excavated soils as backfill.
Benefits
The main benefits of these techniques are that excavation which is absolutely necessary is kept to a minimum thus reducing the necessity for disposal of large quantities of soil which may be classed as contaminated. They also enable excavations to be undertaken with minimum disruption to the locality and people and businesses present nearby. In many cases the soils/ground excavated using these techniques can be reused as backfill once the operation is completed thereby minimising the need to transport new backfill to site improving environmental advantages by reducing the need to excavate virgin fill for reinstatement and reducing the transport requirements and therefore the carbon emissions levels of the operations significantly.
Technology Description – what it is
Ploughing
Ploughing or Mole Ploughing (USA: plowing) is a form of trenching that effectively opens the ground to the width and depth required, allows the product pipe/duct or cable to be installed in the trench created and then backfills the trench with the excavated soils in one single operations using a specially design mobile unit.
The machines are specially designed to carry on a reel the utility pipe or cable that is to be placed underground. The machine then has a plough attachment that can be pushed into the ground to the required depth and pulled forward as the machine advances. This excavates the soil just ahead of the pipe/duct or cable which is fed from the reel into the excavation. Then once the service is in the ground and as the machine passes over the area the excavated soil is pushed back into the trench using another attachment on the unit and the operation to bury the service is complete.
The machines come in a variety of sizes and designs and usually are available as simple plough systems or as vibratory ploughs.
In essence a plough system uses a specially designed ploughshare, blade or lamella which is pulled through the ground using a strong wire cable attached to a towing unit or a wheeled/tracked towing unit, or which is attached directly to a tractor unit or specially designed carriage, depending on the manufacturer.
Some machine options allow the system to be used to install trench bedding material if required and the design of the system normally allows the weight of the ploughing unit to be utilised to compact the base of the trench as the furrow is opened.
This technique is recognised by the International Society for Trenchless Technology (ISTT) as a trenchless technique as it offers so little impact on the immediate surroundings and those living and working there.
The technique is not generally applicable to the tight urban spaces often associated with trenchless techniques because it cannot break open road and similar hard surfaces. It is better suited to open, cross-country situations. However, where grass or open soil verges of some length exist in urban areas it may be suitable provided that other services are not likely to be disturbed by the plough.
The main advantage of this system is that great distances can be achieved in a single working shift with up to 5,000 m being completed in a single day in the right circumstances. This is well in excess of the tens of metres per day achieved using more traditional open cut techniques and even outstrips significantly the output of the more recognised trenchless methods such as HDD.
Technology Description – what it is
Microtrenching
Microtrenching is highly applicable in the world of modern communications where the need for easily and quickly installed fibre optic cable is increasing. Whilst there are trenchless systems that will assist in this endeavour, there are also available very small trenching systems commonly known as microtrenchers that are available for this type of work.
Microtrenchers are generally operated to produce trenches of between 9 and 38 mm wide to depths of between 230 and 400 mm deep which in many locations runs above the general horizon for other buried services. They comprise a cutting disc (very similar to a diamond saw but large enough to meet the depth requirements for the fibre installation) mounted on a walk-behind or ride-on frame depending on the machine selected. The microtrench can be cut quickly and effectively. It is important that a thorough ground investigation is carried out however as the cut occurs very quickly and there would be no warning of any potential interaction with other buried services.
Many fibre optic systems are designed to run in protective cases that slot neatly into these sizes of trench which can be backfilled with materials once the fibre cable is in place.
However, users should be aware that the route of such a trench needs to be highlighted effectively with all other utilities and buried service providers. If not there is every possibility that someone with a bigger machine will come and carve through the often barely visible fibre route and not even notice!
Narrow Trenching
Where the trench involved requires a limited size there are more mechanised trenching options among which is the use of the narrow trencher or utility trencher. A technique developed in the USA, this involves the use of equipment specifically designed to cut a small width trench to a set depth using a toothed chain saw attachment.
These trencher units come in a variety types and sizes depending on the manufacturer and may have a selection of optional attachments that enable the one unit to be utilised for a variety of operations.
In the main the units fall into specific categories and these in turn relate the type of use/trench that is being dug and include:
Walk-Behind trenchers – the operator controls the machine from a standing position behind the unit, walking forward with the unit as the trench is excavated. These machines tend to be used for the smaller dimension trenches and can usually provide a cut of around 150 mm (6 in) wide to depths of between 600 mm to 1,200 mm (2 ft to 4 ft). Usually the cutting chain excavates the soil and deposits it to the side of the excavation using a spoil blade. This enables the machine to cut a clean trench whilst limiting the space taken to deposit the excavated soil. The cutting blade/chain is usually fitted with carbide teeth to break down the soils being excavated into a fine spoil that is easy to handle.
Ride-On trenchers – the operator controls the machine from a seat on the unit. Again the operational capacity of these machines depends on the manufacturer and unit being operated. However, an indication of the range of capabilities is that they can excavate trenches from around 1 m (3 ft) to 2.5 m (8 ft) deep with widths of between 100 mm to 300 mm. There are also special machines with tracks instead of tyres for use on less stable or wet ground conditions or rougher terrain. As well as the cutting chain there are machines that utilise a large diameter cutting wheel which is again generally fitted with carbide insert teeth to provide the cutting capability. The diameter of the wheel fitted to the machine dictates the depth to which the trench can be cut and it is often down to individual preference as to which option, chain or wheel machine, will be used or purchased by any one contractor. Furthermore the type of operation that can be undertaken by these machines can be varied depending on the ground type as they are usually available with alternative tooling such as plough attachments and even rock cutting equipment; but care must be taken to select the right tooling to fit the circumstances.