One of the latest technologies to have come to the fore and which is making great strides in modernising the way ground is excavated with increased safety and minimal interference with the surrounding and the daily lives of those living and working nearby is what has become known as vacuum or suction excavation.
In areas where there are existing buried pipes ducts and cables, traditional excavation by mechanical means is often seen as unacceptable due to the risk of damage to services and more importantly the potential for injurious or fatal cable strikes to the workforce. With the increasingly crowded underground environment seen today, with the likes of fibre networks, gas mains, water pipes, sewers, road drains etc. all competing for the same underground space, the likelihood of worker/machine/service impact increases accordingly, unless the best of ground investigations is completed before excavation starts.
Using traditional hand dig techniques, as they are seen as less aggressive than mechanical means, requires operatives to dig down with spades and shovels to expose the buried plant and usually requires them the actually get into the excavation. This in turn may have implications for confined space working requirements and other safety aspects such as ground support and ground water handling. This potential for damage and the danger of injury has led to the growth in the use of suction/vacuum excavation.
So what is suction/vacuum excavation? In short it is the use of a machine that is designed to move high volumes of air through a self-supporting hose at air speeds that allows any loose ground to be drawn into the hose by the moving air stream. The air/spoil mix is the passed into a storage tank where the main particles of ground are deposited either for disposal or recycling. The air stream is the 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.
Most systems 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. There are various sizes of equipment available for this type of work that is selected depending on the type of work being undertaken.
At the smaller end of the market there are the lower power, smaller capacity units (typically 0.25 to 1 m3 capacity) that 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 up to 150 mm diameter which is 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.
Moving up from the smaller units there are the Chassis Mounted systems. Typically on 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.
Whilst there are increasing numbers of companies manufacturing these machines they do tend to fall into recognisable categories and whilst designs vary they do tend to offer some very similar options and ‘extras’.
In the 7 to 10 tonne chassis class, typically units 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 standard telescopic hose booms (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.
What are known as ‘City Rigs’ are often found mounted on a 16 to 18 tonne chassis. Usually these operate with a Twin fan system whilst having a spoil capacity of between 2.5 to 4 m3. The fans are typically from 700 mm to 800 mm diameter offering a high performance air movement and depressions (suction) again typically offering between 30 to 36,000 m3/hour at negative pressures of 30,000 to 38,000 Pa. These units are built for use in urban areas and difficult access locations. 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 being possible. 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 being fitted with Telescopic and/or full power hose booms operated with remote control. The full power option also means that there is no the requirement for any additional input from the operator for final placement of the hose at the excavation site.
Moving up again there is the 26 and 32 tonne chassis class which again utilise mainly Twin 900 mm fans systems and spoil capacity of between 4.5 to 12 m3. The high performance fans produce 36 to 40,000 m3/hour air flow and up to 44 000 Pa 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.
Designed particularly for busy urban environments the next set of machines is again based on twin 800 city rigs but are mounted on a low entry chassis. These rigs are ideal for larger works in city areas offering greatly improved road user safety provided by the low entry, high visibility cabs that ensure that the drivers are fully aware of what is around them in terms of pedestrians, cyclists and other road users.
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, there is also available 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. 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, hand-balling etc. They are available from various manufacturers with capacities up to and in excess of 44,000 m3 air flow and depressions (negative pressures) up to 50,000 and 55,000 Pa.
SPECIALIST EXCAVATION
Further to the use of suction/vacuum excavation in more recent years there as been developed a technique known as keyhole excavation. The 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 designed and utilised predominately by the gas industry to date, keyhole rigs enable gas companies to access existing mains 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 the horizon 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 pretty much in its original condition. The major advantage for this type of excavation is that typically the works can be completed in just a few hours, which is a massive reduction in the 4/5 days that traditional techniques would be expected to take as well as eliminating the need for major 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 of 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 pretty much self-contained being 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.
