Groundworks specialist Abrahams Brothers has achieved major cost savings by casting aside the traditional breaker in favour of milling attachments.Rodney Byles reports
GROUNDWORKS contractor Abrahams Brothers has achieved both time and cost savings by using excavator-mounted milling attachments in place of conventional breaking on a major retail project in South Elmsall, Yorkshire.
The company is installing water and sewerage services and foundations for the warehouse and is using the Terex-Schaeff WS-60 and WS-90 cutting units for excavation of deep, vertically-sided pipeline trenches, sewer tanks and for stanchion foundation bases.
The firm has also found the units quieter and more environmentally acceptable than traditional trenching with breakers.The cutters have been able to reduce the amount of material that would have to be excavated in a battered trench with the material excavated and pulverised also able to be recycled as trench backfill.This contributes towards savings on materials, transportation and lorry movements as the trenching technique avoids the need to cart away spoil and import new backfill.
Abrahams Brothers, based in Dewsbury, West Yorkshire, specialises in groundworks contracting and pipeline trenching. It has mainly used excavators and breakers to form trenches but in the summer of 2003 a friend suggested using the Terex-Schaeff Cutting Units.
Terex-Schaeff, based at Tuxford, Nottinghamshire, has five models of the rotating cutter drums with tungsten carbide picks for use with excavators from 1.5 tonnes to 50 tonnes.
The units go on the end of the excavators' dipper arms in place of the host machines'standard buckets, with the cutter drum driven from the excavators'auxiliary hydraulic circuit.The rotating milling cutter is simply moved back and forth along the trench line, gradually increasing depth.
The opportunity to try the units came in March 2004 with the award of a 46-week groundworks subcontract for a retail distribution warehouse. As part of the contract, working for main contractor Marshall Construction, the firm had to excavate about 800 m of trenches up to 5 m deep and 2 m wide, along with interceptor tanks and manholes in yellow magnesium limestone. It also had to excavate around 280 footings for mass concrete foundation bases.
'We initially hired a WS-90 to see if it could cope with the limestone and deep trenches, ' says John Abrahams.'We were very pleased with it and it was a big success. It was able to cut a very neat and accurate trench and straight down without any overbreak.We were also able to recycle and reuse the excavated spoil as backfill. If we had dug and broken it out in the normal way the trench would have been battered and much wider at the top and the excavated spoil would be no good as backfill.'
The milling technique also proved its worth on the concrete bases. 'We were able to neatly cut through C35 concrete ground beams without damage to surroundings. It reduces the amount of shoring needed and was quieter than the breaker, with less vibration and less wear and tear on the excavator.'
The initial trial was so successful that Abrahams bought the WS-90 and the smaller WS-60.They also hired in an extra WS-90 for the start of this job.
The cutting units accurately form a straight sided, flat-bottomed trench and after pulverising the material a second excavator follows on digging out the graded spoil to complete a section of the trench.
The sections of pipe are placed on a 150 mm-thick bedding layer of 10 mm or 20 mm aggregate, which is also used as an initial 150 mm-thick covering, prior to the pulverised spoil being placed back on top as backfill.
The technique differs slightly for the excavation of the interceptor tanks, manhole excavations and footings for the building's stanchion foundations.
'Using the cutting units we form a clean slot round the perimeter of these excavations to full depth and then use a breaker to remove the remaining infill, 'he explains.'We then trim the bottom of the holes with the cutting units to form very neat and accurate flat bottomed excavations. And, because there is no over-break or any damage or shattering of the surrounding rock, which we would get if we just used the breakers, we are able to make considerable savings on concrete.'
The firm is averaging about 30 cu m a day excavating the limestone with the smaller machine, while the larger one is averaging between 30 cu m and 50 cu m.'This is probably about the same speed had we used breakers, but we are getting a much better finish and making savings on materials.We bought 50 extra picks with the machines and on average only change one or two picks a week. I'm very impressed with the units and they have contributed to knocking a week off our original programme in the first two months on the job.'