Application – what it does/is used for [or chart showing suitable for/not suitable for]
For the installation of smaller diameter utilities, exposure of existing buried utilities and services and the minimum surface disruption when accessing existing utilities and services.
Minimal excavation technologies have modernised the way ground is excavated with increased safety and minimal interference with the surroundings and persons living and working nearby.
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.
This technique is designed to provide a means of accessing buried services for requirements such as repairs, service connections etc. without the need to excavate large access pits, particularly in busy traffic lanes or pedestrian areas where the disruption would be intense and costly both for the contractor and for local businesses.
Whilst developed and utilised initially by the gas industry, keyhole rigs enable companies owning and operating buried networks to access existing cables, pipelines and ducts for repairs, inspection, camera works etc. This is achieved by first using a special drilling device that cuts and removes a ‘key’ in the surface, whether this is the road surface or pedestrian walkway. Typically the key is between 500 and 600 mm diameter. Once the key has been cut and removed a suction/vacuum excavation unit, normally mounted on the same chassis as the drilling arm, is used to excavate the soils beneath down to the level of the service in question. The spoil is stored in the suction/vacuum excavation unit spoil tank for later reuse.
Special tooling then allows the required works to be completed on the now exposed service via this small access point. The suction/vacuum excavator enables a fast and safe dig down without the need for operatives to enter the pit. Once the service work has been completed the excavated spoil is reused in the backfill of the hole and the surface core is repositioned and bonded back into the road/pavement surface leaving the road as close as possible to its original condition. The major advantage of this type of excavation is that the works can be completed in just a few hours, rather than several days that traditional techniques may be expected to take. In addition there is no need for longer term traffic control or diversions that might otherwise have been required.
Typically the keyhole units consist of a single or twin fan, 10 to 16 tonne truck-mounted unit with something like a 1.5 t spoil tank. The fact that excavated material is reused on site means there is no requirement for transport and safe disposal of waste materials. The hose boom doubles as a crane to lift the coring equipment on and off the chassis. The units are normally self-contained and able to core, suction/vacuum excavate, complete the repair/maintenance works, reinstate and reset the core all from one single operating unit with a crew trained to manage and affect the repair works that are necessary.
The terms Suction or Vacuum excavation are to many interchangeable and the use depends largely on what term the equipment manufacturer uses to describe its product(s). The systems rely on large fans moving high volumes of air through an excavation pipe which is fast moving to the point that it will carry with it quantities of broken soil which are then deposited in a collection tank for disposal or re-use as the case may be. The air stream is then filtered to remove any other particles that are of a size that does not drop easily out of the moving air stream before expelling the cleaned air back to atmosphere.
What may be termed open circuit suction systems differ from close system Vacuum systems only in the way this volume of air movement is generated. Potential users should investigate thoroughly which system would be most beneficial in the operations on which the equipment would be used.
In areas where there are existing buried utilities and services, traditional mechanical excavation is now seen as unacceptable due to the risk of damage to these services and more importantly the potential for injury or fatal cable strikes to the workforce. The increasingly crowded underground environment with fibre networks, gas mains, water pipes, sewers, road drains etc. means the likelihood of worker/machine/service impact increases significantly. This is why techniques covered elsewhere on this website should be utilised to complete effective ground investigations before any excavation is undertaken (see the Location, Detection and Inspection subsection of this website).
These techniques also minimise the need to have on site a workforce qualified in confined space working and other safety aspects of more traditional mechanical excavation such as ground support and ground water handling.
Most systems also require or utilise additional air-powered tooling that has the ability to cut soils in to manageable sized pieces if the suction/vacuum system cannot itself be used to cut the soil.
In many cases where suction/vacuum excavation is used and the soil to be removed requires it, the initial opening of the excavation or soil breakdown at the excavation site is achieved using an air lance (also known as an air spade). This device is a nozzled tool that expresses high pressure compressed air through a nozzle directed at the soil surface to break down the soil structure, or it can be used on larger broken soil pieces to break them down, to a size that can be removed by the accompanying suction/vacuum excavation machine hosing being utilised. Given the nature of the high pressure air jet that breaks down the soil, a high level of caution is required by the operator and the correct PPE should be worn at all times as well as maintaining a safe distance between passers-by or worksite colleagues as the air lance may inadvertently throw excavated soil some distance.
There are various sizes of suction/vacuum excavation equipment available for this type of work that is selected depending on the type of work being undertaken.
The smaller capacity units, typically 0.25 to 1 m3 capacity are suitable for small excavation works and trial holing with production rates of probably no more than 5 excavations per day. These units are usually skid, trailer or crawler mounted and come with smaller diameter suction hoses from 75 mm to 150 mm diameter attached to a very basic hose support boom that is either manually controlled or utilises a basic remote control system. Often these smaller units require a separate compressor to enable the associated air tools (air knives/spades) to be used effectively. These units are often designed to be towed on a trailer or directly behind a service vehicle for transport and to access site.
Larger truck mounted systems typically comprise trucks from 7.5 to 32 tonne in weight. These machines use turbines to produce the airflow required to provide the volume of air movement that will suck up the excavated material.
There is an increasing number of companies manufacturing these machines they fall into recognisable categories and whilst designs vary they do tend to offer some very similar options and extras. The most suitable machine for the type of work expected should be investigated fully by the potential end-user with the range of manufacturers available.
In the 7 to 10 tonne chassis class, units typically operate with a single fan system that provides the air movement. The units have a spoil capacity of around 1 to 2 m3. Typically used for smaller service pits and trial holing, the units have a lower performance and so are better suited to easier ground with usually around 5/10 pits per day being an average production rate. Suction hoses are typically in the 100 to 180 mm diameter range with a standard telescopic hose boom (fitted with a floppy hose) that is hydraulically operated, and may use a remote control system with the operator holding the suction end of hose over the excavation.
Larger 16 to 18 tonne chassis-mounted machines usually operate with a twin fan system and have a spoil capacity of 2.5 to 4 m3. The fans provide high performance air movement and depression (suction) typically between 30 to 36,000 m3/hour at negative pressures of 30 to 38 kPa. These units are built for use in urban areas and difficult access locations where higher operational capacity is required. Typically used for multiple utility service pits they can perform in excess of 25 excavations per day at depths of up to 10 to 15 m. They can also be operated at distances of up to 25 to 30 m away from the main unit. Hose sizes are usually 200 or 250 mm diameter with the units fitted with telescopic and/or full power hose booms operated with remote control. The full power option also means that there is no requirement for any additional input from the operator for final placement of the hose at the excavation site as the boom is fully remotely controlled.
The larger 26 and 32 tonne chassis class again utilises mainly twin 900 mm fan systems and has spoil capacity of 4.5 to 12 m3. The high performance fans produce 36 to 40,000 m3/hour air flow and up to 44 kPa negative pressure (suction). These units are suitable for a wide range of applications and ground conditions including heavy clays. Excavations that can be undertaken range from small service pits through to large excavations on civils works. Material removal works at depths up to 20 m and over remote distances from the main unit of 50 m or more.
If depth from surface and distance from the main rig to the excavation site is a problem for the more typical suction/vacuum excavation units, a range of triple and quad 900 mm fan units with the capacity to work at depths up to 50 m and distances of 150 to 200 m from the main rig depending on the materials being handled is available. These units are usually utilised to remove materials over a distance in construction, tunnelling works, site clean-up, industrial maintenance etc. reducing the need for conveyor systems, handballing etc. Various manufacturers offer these systems with capacities up to and in excess of 44,000 m3/hour air flow and depressions (negative pressures) up to 50 and 55 kPa.