Precise trenching in Magnesium Limestone. Note the reuse of cut material for pipe bedding and trench backfill. (WS90 unit mounted on Komatsu PC-240)

Excavation of footings for concrete foundation in magnesium limestone.
Note the cutting precision.
(WS60 unit mounted on Daewoo 220LC-V)

Precise trenching in Magnesium Limestone.
(WS90 unit mounted on Komatsu PC-240)

Abrahams Brothers has also found the Terex-Schaeff cutting units to be considerably quieter and more versatile, efficient and environmentally acceptable than traditional trenching with noisy breakers. The company has established that the cutters are able to considerably reduce the amount of material that would have to be excavated in a battered trench with the traditional excavator mounted breakers. The material excavated and pulverised by the Terex-Schaeff cutters can also be recycled as trench backfill. This considerable additional benefit contributes towards savings on materials, transportation and lorry movements as the trenching technique, with the Terex-Schaeff cutting units, eliminates the need to cart away spoil and import new suitable backfill.

The company based in Dewsbury, West Yorkshire and founded in the late 1980s by brothers John and Phil Abrahams, specialises in ground works contracting and pipeline trenching. Abrahams Brothers has mainly used a combination of excavators and excavator mounted breakers to form trenches. However, the company is always on the look out for new and alternative methods to improve the efficiency of its operations. In the summer of 2003 a friend of Abrahams director John Abrahams suggested using the Terex-Schaeff Cutting Units as an alternative trench excavating technique.

Abrahams Brothers needed an opportunity to try the Terex-Schaeff Cutting Units and that came in March 2004 with the award of a substantial 46 week ground works subcontract for a new retail distribution warehouse. Abrahams Brothers, working for main contractor Marshall Construction (West Yorkshire), had, as part of its contract, to excavate about 800m of 1m to 5m deep trenches up to 2m wide and interceptor tanks and manholes in 24.5 MPa compressive strength yellow magnesium limestone for foul sewers and storm water drainage. In addition the specialist contractor had to excavate around 280 footings for mass concrete foundation bases to support the huge warehouse’s steel stanchions.

“We initially hired a WS-90 Cutting Unit from Terex-Schaeff 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 5m deep, 1m wide trench and straight down without any over break. We were also able to recycle and reuse the excavated spoil, pulverised by the cutting unit, as trench backfill. This was a major advantage as it contributed towards savings on carting away and bringing in new backfill. The trench would have been battered and much wider at the top and the excavated spoil unsuitable as backfill if we had dug and broken it out in the normal way with a breaker. The cutting unit produced a much neater and accurate job with less damage to surrounding areas. We were also able to neatly cut through C35 concrete ground beams without damage to surroundings. It reduces the amount of shoring needed, was also considerably quieter than the breaker and with much less vibration has much less wear and tear on the excavator.”

The initial trial was so successful Abrahams Brothers bought the WS-90. “We only had it about a week and were so pleased with it we bought it and ordered a smaller WS-60 at the same time and hired in a second WS-90 for the initial phase of this job,” says John Abrahams. “We mounted our WS-60 and WS-90 on our own 22t Daewoo 220LC-V excavators and the hired WS-90 on a hired 24t Komatsu PC-240.” The Terex-Schaeff Cutting Units are mounted on quick hitch attachments, which are fitted on the end of the excavators’ dipper arms in place of the host machines’ standard buckets. The Terex-Schaeff Cutting Units, with their rotating tungsten carbide tipped cutter drums, are driven from the excavators’ auxiliary hydraulic circuit and pushed into the limestone. The rotating milling cutter is simply moved back and forth along the trench line, gradually increasing depth.

The WS-60 and WS-90 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 excavation. The sections of pipe are placed on a 150mm thick bedding layer of 10mm or 20mm aggregate, which is also used as an initial 150mm thick covering, prior to the pulverised spoil being placed back on top as backfill and compacted.

Abrahams Brothers is also using the Terex-Schaeff Cutting Units to excavate the footings for the 5m x 5m x 3m deep pipeline interceptor tanks, 20m3 manhole excavations and 1.2m x 1.2m x 1m deep 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,” explains John Abrahams. “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.”

Abrahams Brothers is averaging about 30m3/day excavating the yellow magnesium limestone with the smaller Terex-Schaeff WS-60, while the larger WS-90 is averaging between 30m3 and 50m3/day. “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 1 to 2 picks a week. I’m very impressed with the Terex-Schaeff Cutting Units and in the first two months on the job they have so far contributed to knocking a week off our original programme.”