8.3 Groundwater lowering

sensitive existing structure or even to maintain water

capable of both accurate work and high outputs when

handled by a skilled operator. This machine is manu­

levels for environmental or ecological reasons. If the

factured in this format over a large range of sizes, the

natural phraetic gradient between the dewatered area

largest still being the preferred option in open cast coal

and the recharged area cannot be achieved a full or

extraction.

partial cut-off, in the form of sheet piling or a

diaphragm wall, has to be inserted into the intervening

8.4.2 Backacter

corridor. Recharge is likely to be an important con-

.sideration whenever a new power station is being

The backacter bucket is mounted on a hinged arm, and

constructed adjacent to an operating power station or

as its name implies it digs straight back towards the

other major structure, especially if that station is

machine and below track level. It can operate to fine

nuclear.

tolerances in ground conditions ranging from soft to

226 ‘

rock ripper, which is a hydraulically-operated claw

Rigged as a crane, this machine has multiple lifting uses

driving forward

large shuttering

or a stockpile, thereby churning up the surface they are

closely confined excavation work such as tunnels.

placement piles when fitted with further suitable

but are most likely to be seen on power station sites in

the 0.5 m3 to 3 m3 range. The size and number required

large bucket or bowl mounted on rubber tyres and its

is dependent on both the site and the programme for

motive power is provided either by a crawler-mounted

completion, and can vary appreciably. Excavating pro­

tractor which tows the bucket unit or else it is an

duction capability is also influenced by the transport

wheeled machine with engine and cab at

capability to the dumping grounds, which may be

the front end. Digging is achieved by lowering a wide

situated up to several kilometres from site. Off-site

cutting edge into the ground while travelling forward;

dumping is generally performed by road tipper larries,

the depth of cut being a number of centimetres

commonly of 6 m3 to 15 m3 capacity. On-site dumping

depending on the nature of the ground and the work.

often enables much larger capacity dumpers or tractor­

Such machines are commonly used for topsoil strip­

drawn dump trucks to be utilised. Vehicle sizes and

ping, site levelling, shallow excavations over large areas

numbers have to be related to the size and capability of

and constructing earth embankments. Tractor-drawn

the excavator buckets, as it is important to ensure that

scrapers have a limited economic distance over which

enough transport is available to accept the continuous

they can travel to discharge as they are relatively slow

output from the very expensive unit priced excavating

and have high powered engines to boost the thrust

required for scraping loading. By contrast the all­

As well as the variants of the basic machine reviewed

wheeled articulated self-propelled scrapers have less

in the foregoing, modern practice is increasingly

excavating power but offer high travelling speeds and

favouring highly mobile excavating plant based on large

hence can accept longer economic haul distances. If

tractor units, ‘dozer’ type machines and motorised

this version of scraper has deficiencies in self-loading

scrapers. Equipment mounted on lar^e tractors is often

(which can be the case in hard ground conditions) the

highly specialised and does not merit further descrip­

problem is usually overcome by stationing a high

tion here. But the dozer range of excavating plant and

powered crawler tractor at the excavation area to assist

the motorised scrapers contribute greatly to general

the loading phase by pushing the scraper at the rear.

Bucket sizes can be very large, 30 m3 being relatively

common. Such plant docs have operating limitations,

being unable to perform successfully on either very

hard or soft and/or waterlogged ground. In the range of

suitable ground conditions between these extremes, a

The original bulldozer was a tracked vehicle with a

substantial part (and in a few cases the whole) of bulk

cutting/trimming blade at the front. Developments to

excavation for power station foundations have been

this concept happened quickly due to its high degree of dug using scraper equipment.

manoeuvrability and adaptability and its common suc­

cessor now has a hydraulically-operated bucket on the

front and may be mounted on a rubber-tyred or tracked 8.5

Construction by diaphragm

chassis. Sometimes the bucket is hinged in two parts sowalling techniques

that it can additionally act as a grab. These machines

undertake a wide range of work, sometimes operating

This technique involves the excavation of an unsup­

in an attendance capacity on major excavating plant.

ported trench, stabilised by being full of bentonite

used for spreading and levelling, as could the original

• bulldozer. Bucket sizes range from 0.5 m3 upwards, but

most are of at least one cubic metre capacity. The

smaller ones sometimes have a hydraulically-operated

backactor arm mounted at the rear and others carry a

228“..,.