Pressing All The Right Buttons

In many aggregate depots today you will find 20mm virgin quarried material sat next to 20mm recycled demolition waste. Focusing on a quarry blast at Ennstone’s Cloud Hill Quarry in Leicestershire we investigate the boom and bust involved in both types of aggregate production.

For Ennstone Group chairman Vaughan McLeod the attraction of quarrying is…well, simple. “You get some land, get some permission, dig aggregates out, process them, put them into products and sell them. It’s all very simple,” he says. But then, he is very good at making things look easy.

At the rockface – the Carboniferous dolomitic limestone rockface of McLeod’s Cloud Hill Quarry in Leicestershire to be exact – things are not quite as uncomplicated. Today is a blasting day and while the sun shines down on a relaxed chairman’s office in Breedon Hall, a mile down the road the same early spring rays are radiating across a hive of industry.

Julian Smallshaw, is the new man in charge at the quarry. Up until three months ago he was managing an operation in Ras Al Khaimah, a 9million tonnes a year business under-managed by the Crown Prince of the emirate which makes up one of seven forming the United Arab Emirates (UAE).

He is one of the people who chose to come back to the UK from the UAE – a tax-free environment can be a big attraction. However, his plan was to do two to three years and then return. So when his former boss at Hanson, Karl Ashurst, gave him the nod that he was moving on from Cloud Hill, Smallshaw took the opportunity.

And the former operations manager of Hanson’s Chipping Sodbury and Tytherington quarries is happy to be back. A Leicestershire lad by birth he has been hooked on quarrying ever since he gained an interest in geology and mineralogy as a child, a route into the industry which he feels is not being fully exploited today.

“I was in an organization that visited quarries when I was a child and Julian Cleeton, who was later at Doncaster University, was assistant manager at Croft Hill Quarry. He gave me the contacts to go out and visit companies and push for work. He inspired me to take my hobby forward and work towards becoming a quarry manager.

“I don’t feel the industry is targeting young people enough and there are no routes for them to take. The QPA should be doing more; the industry should be doing more. We are running out of people. The industry isn’t that big so you don’t need a large quantity but you do need a pool to draw from in the first place,” he tells MQR from his open plan operational hub at the entrance to the quarry site.

Then the radio crackles. Assistant quarry manager Ken Harrison gives him an update on preparations for the 1.00pm blast. Harrison’s about three years from retirement and when he comes into the office he nods in agreement at Smallshaw’s views on industry succession planning. The radio crackles again and it’s time to don PPE.

While getting kitted out with reflective gear, boots and helmet, Smallshaw stresses his 28 years in the industry haven’t dampened his appetite for it. It is quite the opposite, in fact. The job of quarry manager has enough to keep anyone on their toes, he says.

“It is all encompassing. You have to have a handle on health and safety, know about production, the product, the technical side, geology and operations and then there is blasting, explosives, plant, engineering, accountancy…you can go on and on,” he says.

Fifteen minutes later and we are standing with Harrison on the north side of the quarry. Worked since around 1900, Cloud Hill is now 95m deep and stretches for around 1km in front of us to the opposite, south side where the seven-hole blast is set.

Smallshaw disappears to warn the neighbours of the blast. Regulations dictate regular vibrotech readings in the nearby Worthington village. Smallshaw, however, contacts local people before each blast offering to take readings of noise levels and ground vibration. “Good relations are paramount,” he says.

Another radio crackle. “They’re about to sound it,” says Harrison. A claxon echoes for 20 seconds. Shortly after it ceases a bright flash can be seen pooling out around the surface of the blast site. Moments later and the north side of the quarry shudders. Last to catch up is the sound.

The blast holes have a burden of 5m. This is the material between the 16m high quarry face and the blast hole. The resulting explosion causes a 40m wide section of the quarry to come crashing down. “It’s a good ’un,” says Harrison.

When the dust settles we go down to take a look. It is a very mixed bag with various sized blocks of 354million year old Carboniferous limestone along with younger Triassic red clays and mud overburden, which will go to landfill.

It all runs so smoothly you begin to think that, for the uninitiated, McLeod is on to something when he talks about the simplicity of quarrying. After all, what can really be involved? Well, quite a lot, as it turns out. And it is a mixture of science and art.

Firstly you have to have a design to the site and know what you are after. There are two distinct areas to Cloud Hill, the main production face producing the high value stone we are after for the article, and another primary operation producing type-1 or fill.

To secure a steady supply of higher value stone they are always planning ahead. Local contractor Terry Inskip has been carrying out the drilling and he needs to be working a week ahead so the holes are ready for the blast.

Before drilling, the face is profiled not only in front of the blast hole area but also in areas either side of the primary profile using a PMS Impulse 100LR laser. This extra profiling identifies weaknesses that are not directly in front of the blast hole.

The information is then sent to accompanying Blast Designer software, which uses these data to generate burdens in front and to the side of the blast hole. The holes are then marked out and drilled with the driller keeping a log, an important information source.

Smallshaw explains: “The log is a record of whether the rock is hard or soft, or whether there is any clay in the rock, for example. This is done by judging the penetration weight of the drill and examining the chippings.

“The shotfirer will use this information as part of the overall assessment when designing the blast. The more information available the better,” he says.

Once the hole has been drilled a probe is dropped down it, as quarry shotfirer David Winfield explains.

“This gives us the hole’s depth, direction and angle. The face is also profiled again and then all these data go to the computer,” he told MQR.

It is then time to design the blast which is part art, part science. Cues are taken from the driller’s log. If there appears to be a lot of clay, for example, then Winfield will cut back on the charge. This is the art.

Meanwhile, the data from the profiling and the probe take care of the science. The software advises on type and amount of primer, connector and explosive. It even gives Winfield an idea of how much it will all cost.

He adjusts the shot depending on the fragmentation desired. The south side face is for granular sub-base (GSB) so he does not want any large lumps. Our blast holes were around 18m deep and as it is for GSB a lot of explosive is needed to fragment the rock.

“About 7tonnes of rock to every kilo of explosive is needed to create material for GSB,” says Winfield. “If you wanted block it would be 12tonne of rock to a kilo. With GSB we don’t want anything above 3-4 inches to go through the crusher as we need fast processing to keep production ticking over.

“At the same time we don’t want to pulverise the chipping stone because there would be a lot of waste. And I’ll get moaned at. Can’t have that,” he says with a joking roll of the eyes.

For our 20mm end product the burden measures about 3m with 4.5m spacing between each hole. The explosive used is the heavier Orica gold 85mm ammonium nitrate prills to kick the base away and a lighter charge to release the face (see box page 22 for more on explosives).

In short, the hole is part charge, part chippings, which fill in the gaps. The blast hole is topped off with a 4m stemming of inert material. A connector block housing comprising a detonator and a delay sits on top of the blast hole.

It is connected to two shock tubes, or non-electrical initiators. These are lined inside with a thin film of explosive. The tube in turn is connected to two detonators further down the hole – one is a back-up, an HSE requirement. These connect to a 500T booster. It is a small TNT primer that initiates the main charges.

The top electrically powered detonator ignites the shock tubes. The explosive lining in the tubes ignites, rushing through the tube at speeds of around 6,000metres per second, sending shock waves into the base detonator, which ignites the PTEN booster and BOOM.

Well…not quite BOOM. It is more boom. A delay of milliseconds across the charges is necessary due to the close proximity of Worthington village. There is a firing order to minimise noise and ground vibration, as Smallshaw explains.

“With this blast we are going for a maximum instantaneous charge (MIC) of 100-110kgs, so effect of the blast is no more than 100-110kgs. Having two detonations in the blast hole halves this figure to 50-55kgs even though the blast is over a tonne.”
Which brings us back to the rock face in our pursuit of 20mm. Once the blast has reduced the face a Caterpillar 988G loading shovel picks up the shot rock and feeds it into a 125tonne Kleeman+Reiner Mobicat MC122Z mobile jaw crusher.

Capable of processing between 400-600tph comfortably, its jaws open to 1,250 x 1,000mm. It is set to produce -150mm and has a hydraulic breaker attached to deal with troublesome blockages in the feed hopper.

The material drops onto wheeled Telestack link conveyors connecting the crusher to a range of 42m old coal board conveyors (see picture left) stretching to the top of the quarry and capable of shifting anything between 200 and 600tph of –150mm depending who you talk to!

A fleet of four 50tonne dumpers used to move the material up the one in eight gradient from the crusher to the processing plant at the top of the hill. Harrison admits he was sceptical when he heard they were to be replaced by belt conveyors.

“I did wonder about it as it was such an unknown. No-one had done it before and I thought the belt would be forever ripping. But I was wrong. It is reliable and it helps us move more material without the dumper truck emissions. It also saves on tyres, labour and fuel,” he says.

When the material reaches the top it discharges on to a primary surge pile delivery conveyor before the –150mm enters a secondary crusher, a 77tonne Kleemann+Reiner Mobirex MRB152Z.

A double-deck pre-screen on the Mobirex takes care of scalpings, which are stockpiled by a Telestack TS-525 radial conveyor – although changes are underway to deal with the scalpings at another part of the quarry. Meanwhile the rest of the material is conveyed to the top of the screen house up a 100m inclined belt.

The crushed limestone is split into two streams, one for 10mm to dust and the other for a range of products. However, demand is for –20mm products so material over 20mm feeds back for tertiary crushing by a Hazemag APSM 1020 tertiary impactor.

Then the 6mm, 10mm, 14mm and 20mm products are either loaded directly on to delivery vehicles through loading bays or are stockpiled in one of four 25,000tonne capacity concrete storage bays. Where it goes from here is another story.

McLeod is right in one way. Compared with the high-tec industries the process of quarrying is simple. You dig a hole and then fill it in. But then such a view would also deny a distinction between laying a sewer pipe and the construction of the channel tunnel.

The end product may be simple in concept but the processes involved in creating it are not. And similar thing can be said for recycled aggregates, especially when they start life as a 24storey tower block in the West Midlands, as we are about to find out...

Cloud Hill: 01332 862254