Pressing All The Right Buttons: The Demolition Blast
Listed inDrilling & Blasting
Principal structural engineer for Sandwell Metropolitan Borough Council (SMBC) David Rowe laughs when he hears the comments on the simplicity of quarrying. If quarrying is just creating and filling holes then demolition is simply making a mess.
“Yes we get a similar thing. When it comes to demolition most people think we are simply blowing things up and creating rubble. But the number of skills required in your average blow-down are huge,” he says.
And if any one should know it is Rowe. He is a structural engineering lifer. Making his way into the trade at 19, he has 25 years in private practice, eight years at SMBC, and for the past 17 years has been an examiner for the Institution of Structural Engineers.
When it comes to attracting younger people to the trade, his concerns are an echo of Smallshaw’s at Cloud Hill. Structural engineering just isn’t sexy. One bonus of not having new blood coming through, however, is that salaries are going up.
“Young people don’t want to work through the maths and physics needed for the job. And as the money is not that good many leave. It seems odd but many go into the City because they can think logically.
“The money side is changing though because of supply and demand. The new people just aren’t there and with people leaving as well salaries, are growing. It is a sign of the problems we have,” he told MQR.
As with Smallshaw, Rowe is a man who cannot imagine doing another job. Engage him on the subject of designing and executing a blow-down and this becomes clear. The Hamilton House demolition project is a good example.
Hamilton House sits, or at least used to sit, on 0.5hectares of land in Grove Lane, Smethwick in the West Midlands. Hurled up in the early 1960s, the 24storey Bison construction monolith, like many tower blocks, failed to live up to the hype.
With a history of vandalism, shootings, arson and drug dealing, the tower block saw eight police officers take up residence on the ground floor in 2001 in a bid to introduce stop checks and help clear up its crime infestation.
A couple of years ago the land around Hamilton House was earmarked as part of a possible site for a new “super” hospital sitting on the Birmingham and Sandwell border. So housing strategy in SMBC gave Rowe a ring and asked him to do a feasibility study to find out the costs involved in tearing it down.
“The feasibility study is a general costing with caveats for a demolition project,” he says. “Sometimes it is done to weigh the costs for both keeping and removing a building. It includes asbestos and waste removal, demolition, services, and a range of other costs.”
Feasibility is a process governed by the Construction Design and Management (CDM) Regulations, which have now been revised. First brought in 1994, the CDM regs seek to make C&D sites safer. The updated version went live on 7 April.
This revised version tightens up the law in terms of ensuring safety is the main choice between feasibility options and not cost. An example of these options is whether to use high-reach excavators or explosives on a job.
Reclaiming a building by high-reach is the cheaper option but not the safest, says Rowe. He is happy to use high-reach excavators for up to 12-14storey projects but anything over that is just too risky.
This made the Hamilton House decision easy. But if there is any doubt Rowe chooses the side of caution.
He recently brought a 14storey building down using explosive even though many were telling him do it by high-reach.
“Some are happy to build a one or two storey ramp for the high-reach. The job would have needed a ramp at about 30º angle for the excavator to mount in order for it to reach the top. I wasn’t sure the wall could take it; these are high tonnage machines.
“It also has to stand out further from the building to reach up high, in a situation where balance is everything. There is the matter of un-compacted rubble building up under the excavator on the ramp. I have never seen one tip over but we all know they can.
“Safety needs to be the overriding concern. As with all H&S it depends how far you take it. But if you end up in court and they ask if there was another way to do the job safer, and the answer is yes, then under CDM it means prison,” he explains.
Rowe also prefers explosives for tighter areas. A controlled blast can be accurate with the higher levels of a building trapping the lower ones. This stops material from blowing around and endangering people which he sees as a risk on high-reach jobs.
Then there is the dust suppression issue. Non-explosive demolition needs constant watering down as it is a long process. You can attach a pipe to the high-reach but then you need to ensure the water is safe to go down the drains, he says.
Another key safety concern is asbestos. Unearthing the carcinogenic insulating fibre is not easy. There are three levels of asbestos survey that can be done numbered one to three and ranging in thoroughness. At feasibility stage it is mostly guesswork, or level one.
Rowe will walk around looking for signs of asbestos in a building as well as consulting old records of the building. Asbestos can account for up to 20-30% of the costs of demolition.
A level two survey includes drilling test holes through plaster or cutting out sections but this will not give a good idea of removal needs and costs. Only a level three survey will do this and Rowe demands at least a floor is free of people before doing one.
“It is Housing Strategy’s job to remove tenants from blocks set for demolition and I tell them I want a floor totally free of people before proceeding because asbestos is an emotive issue.
“The survey includes cutting into ducts, floors and ceilings and placing asbestos stickers in relevant areas. It’s a very intrusive survey. When people realise what you are doing they get worried so we need to be careful,” he says.
When all the people have been moved out he orders the service disconnections: cable, BT, gas, water and electricity. Water and gas cause him the most problems as they still use old cast iron pipes which are susceptible to vibration from a blow-down. Then there is privatisation, Rowe explains.
“It used to be a case of phoning up and getting either the gas or electricity board to cut them off and take the meter out. Now you have entire tower blocks all getting their services from different firms and we have no right to ask where they get their supply from. It is a bit of nightmare.
“For gas I have a private firm working for me connected to the gas industry. It has software giving the last meter reading of all properties. It contacts all the gas companies and gets them to remove the meters. National Grid won’t cut the gas off until all meters have been removed,” he says.
He has also worked out a similar system with Energy Watch for electricity disconnections. Both systems are going well at the moment, he says, but can be hit and miss. Water, telecoms and cable are more straightforward, at least in a tower block.
Once the services have been disconnected the soft strip and asbestos removal begins. This includes the removal of items such as baths, kitchen units, curtain rails, pipes and even tiles as they may contain asbestos. This ensures just a concrete shell with brickwork remains.
Rowe is looking for a 95% recycling rate on the blow-down and the soft strip helps to achieve this with less work. “You could blow it all down together but then you would have to remove all the intermingled wood and metal making it more difficult,” he says.
Just after the feasibility report has been accepted by Housing Strategy, Rowe will work on the pre-weakening design of the building. With only the shell now remaining this will be implemented. But as he explains, the term is not a true reflection of the work.
“It sounds as if we are making the building weaker but we aren’t. When an explosion goes off on the lower levels there should be no column or wall left so it all collapses. The best way to do this is to put explosive in all the elements you want to remove.
“But if you have a 6m long element you hit a problem; you don’t get 6m drills and you need to drill to insert the explosive. So I cut an opening of, say, 2.5m in the wall leaving 3.5m. I can then drill two 1.75m holes. The opening is the pre-weakening.
“So you are not weakening the building you are simply cutting out holes to ensure you can makes holes with the sizes of drill you have,” he explains.
But it still needs to be exact. Top and bottom floors in a tower block carry wind load while the bottom floors carry the weight of the building. In short, too much taken from the bottom and the building can collapse; too much from the top and it can blow over.
At Hamilton House this was made increasingly tricky as the wall panels were generally without reinforcing bars short of a few perimeter steels. Also the floor was constructed of ribbed, unprestressed, pre-cast panels. It was essentially hollow: 50mm concrete on top, a 150mm void and 50mm on the bottom.
The floor panels sat on the concrete panel walls and as these were only 150mm thick the floor panel could only sit on them at most by 75mm. So Rowe had un-reinforced walls which couldn’t act as a lintel with slabs on top which couldn’t distribute the load.
To counter the problem he put in structural steel sections to act as support for the slabs. So if anything moved after pre-weakening the floor could sit on the support.
Also, on the blast floors the windows needed to be removed. However, the steel window frames acted as structural support with only a one meter high wall of clinker block and brickwork being left when they were removed. Again, further support was needed.
The drilled holes for the charges in the pre-weakened areas have diameter ranges of 30-35mm, just enough to fit two cables of PETN detonating cord, which is all that is needed to bring the building down to rubble.
Surprisingly there is no booster TNT or ANFO as in quarrying. Both get gravity to work for them although in different ways. Explosives expert for the Hamilton House blast John Turner explains.
“The idea is to blast away the supporting concrete to either expose the reinforcing bars or take out the column or wall – the latter in the case of Hamilton house. These then buckle under the weight of the building.
“There we have drilled 1,200 holes with 40g per meter cord being used for the ground floor and 20g+12g on the upper floors. It is great in terms of cost because it makes it cheap. It doesn’t make us very popular with the explosives companies though. I think we spend on explosives in a year what quarriers spend in a week,” he told MQR.
Before the building is wired to crumble a single element such as a column or a wall is test blasted to determine the charges required. The element is pre-weakened and the holes drilled. They then push in the charges of the correct weight.
In order to protect people in the test area two layers of chain link fencing are tightly wrapped around the element, which stops the large chunks of concrete from flying out. Then two or three layers of geotextile are layered around the fencing to block smaller debris. The layers also aid in maintaining blast pressure.
Holes are then made in the geotextile and the charges threaded in. Quick drying cement or dental plaster is used to fill the hole and then 1.5m x 1.5m white-painted plywood boards called “witness boards” are hung. They act as an indicator of blast debris quantity. If necessary the ceilings and floors are also covered in geotextile. Then one of three things can happen, as Rowe explains.
“After the test blast either the concrete is loose and so the charge is right or when you undo the geo-textile the concrete is just cracked and so you know you need to increase the charge.
“It may be that nothing has happened at all and so you then need to go through it all again a week later. When you get it right you apply it to the whole building.”
In Hamilton House, charges were laid on the ground and first floors and then on the fifth, tenth, fifteenth and twentieth floors. Each floor is cut up into four zones, which, from a rectangular plan view of the building equates to cutting it into four squares.
The zones run vertically through the building. Charges on the floors running through zone one are set on a delay of 0.3seconds, zone two at 0.5seconds, zone three at 0.7seconds and zone four at 0.9seconds.
No charges are set into the back wall of the building – except on the first floor to blow the base away – which means it acts as a hinge moving the building one way as it falls. And as the zones fire at different times it has the effect of collapsing the building in on itself.
Like quarriers Turner uses shock tubes. They are connected into bundles to time delayed detonators – which once initiated are said to be “cooking”. The 1,200 tubes in Hamilton House connected to the detonators in the building lead down to one initiator which acts like a prime mover in the domino effect with a 0.3second delay built in.
This delay allows a back-up circuit to kick in should there be any failures. Within just over a second the explosions on each floor leave five blocks of around four floors each acting independently with a 2.5m gap between each until they hit the rubble pile.
In a quarry it is the blast which creates the air overpressure, or energy transmitted from the blast site in the atmosphere in the form of a pressure wave. With the Hamilton House blast it is the progressive collapse. The effect of both can be the same – annoyed neighbours.
While Ennstone’s Smallshaw visits local residents, taking readings, Rowe employs independent consultants Vibrock to install air overpressure meters on the day of the blow-down in case anyone complains of broken windows.
He also includes in the feasibility study a security zone as well as the HSE requirement for an exclusion zone. While the latter will not allow people in up to and shortly after blasting, the former allows him to offer greater security to local people.
“People are moved from their houses on blasting day and this leaves them open to burglary. Sometimes I take houses not in the exclusion zone into the security zone to help protect them. We can also put plant and people in it, again improving protection.
“On the day we offer those moved out of their homes a rest centre for the morning with games for the kids and breakfast. Sometimes we offer quiet rooms in case people have been on nightshift,” he says.
Once the dust has cleared local residents are let back into the area half an hour before the public. Then it is the turn of the rubble to be sorted. In come Coleman’s Liebherr excavators and they start shifting and sorting the former tower block into a pile. They also semi-process the material by breaking rebar from the concrete.
The semi-processed and oversize concrete, brick and clinker block remains of Hamilton House are then taken by tipper to Coleman’s Meriden Quarry near the NEC. From here the route they take depends on the location of plant at the time.
On the day we visit, the first smaller arisings to arrive at the site are being crushed not by the static Brown and Lennox crusher but by a range of Dig-a-crusher bucket crushers as part of an open day to promote the product.
Oversize material is dealt with by a monster Hitachi Zaxis 850 with a pulveriser. This takes the concrete down to 100-150mm. This then enters the Brown and Lennox to produce a -75mm or type-1 material.
A range of screens also operate at the quarry to further process the material including the UK’s only tracked Powerscreen 725 trommel – a design which was the brainchild of David Coleman and Powerscreen. Also on site are a Powergrid 800 and a Chieftain.
While Meriden deals with recycled top soil, bulk fill and roadstones, Coleman’s also has its 27% WRAP-funded Shady Lane recycling site offering high-end washed and graded 40mm, 20mm, and 10mm aggregates.
Material from the quarry can be taken to the site for processing, cleaning and stockpiling. It is too early to say how much of Hamilton House will be taken here but if any material does make the journey the first port of call will be a Powerscreen Commander 1400.
It is a dry screen with a vibrating head comprising 40mm top deck screen and 5mm piano wires for retrieving fines – an overband magnet removes ferrous metals. Anything over 40mm is removed and can be crushed using a Tesab or Pegson X400 mobile crusher.
Mid-sized material between 5mm and 40mm is conveyed to a Powerscreen Powerscrub 120 log washer. Here contaminants are floated out leaving the clean product to be conveyed to a Powerscreen 12 x 5 static screen with three-way split to produce the clean 10mm, 20mm and 40mm end products.
All materials under 5mm go to a CDE sand plant where the clean sand is stockpiled. The slurry gets flocculants added and drops to a sludge bed. The solids work their way to a Diemme filter press while the clarified water re-enters the aggregate washing cycle. And this brings us to the end of the process.
So we have an urban quarry and a traditional quarry. Two end products created by two separate process but having more in common than most people would think.
Both are heavily regulated by HSE, both use explosives to varying degrees and both share the same types of plant – whether it be yellow or blue machinery. Also, both need to be very wary of the local environment and those who live in it. And most importantly both need to have a tight handle on health and safety.
“One thing I always think my job shares with a quarry manager is the emphasis on safety,” explains Rowe. “He has to get aggregates out of the ground in the safest way possible while I have to get buildings down in the safest way possible while thinking about the recycling rate.”
There are, of course, a load of differences in the process other than the people involved in the chain and the quantity of explosives each uses.
Quarry managers rarely have to worry about asbestos or applying for permission months in advance to shut down main city centre roads when they want to do a blast. They also don’t have to worry about taking out local residents when a blast goes wrong in a tight area. And quarrying processing tends to be done on the same site in which the explosion took place.
It is in the process where most similarity is found. Both need careful planning and preparation, both require the careful use of explosives, both use similar plant, and most importantly, both are selling into the same market.
And it is on this latter point that Rowe has a tale of caution about the need for quarrying and new permissions.
While WRAP suggests recycling will hit its peak in 2011, Rowe raises a point about the quality of future recycled products and its interrelationship with traditional quarrying.
“Recycled material is good currently because we are recycling products that were made with a lot of rock. But modern construction materials are not like that. They are increasingly containing less and less primary material. The clinker block in Hamilton House is a good example of this.
“There could eventually come a time when it will not be worth recycling,” he says.
Dave Rowe: 0121 569 4561