Reinforced Concrete (before)

Reinforced Concrete (after)

Experienced contractors worldwide know that it is a challenging task to drill through the reinforcing grid, which is designed to support the high Mpa concrete, which clings onto the reinforcing steel bars as the design intended.

The task is further complicated by the fact that drilling is a hit-and-miss affair determining a pattern, not by choice, unless diamond core drills are used at very high cost, which cut and penetrate the steel reinforcing bars as well as the concrete.

It’s a fact that cutting the reinforcing makes the task easier. The gas pressure then forces the concrete off the free face end where it was cut, easily. It is seldom possible to gain access and do that, however, so plan B is usually adhered to.

Drill and charge as best possible, crack the concrete with Nonex, then use mechanical means such as an machine mounted hydraulic hammer or a heavy duty, pneumatic pavement breaker to remove the cracked concrete; an almost impossible task unless the concrete is first cracked.

Finally, professional demolition contractors will use non-explosive technology as one of their options. It is not intended to be an exclusive demolition solution, would not be considered for hi-rise building or stack implosions and would feature along with other mechanical demolition tools of the trade.

Trenching

Whatever process or technology is used, experienced contractors agree that trenching is one of the most challenging applications. The reason for this is that a trench is normally deeper than it is wide, it is therefore very confined and the rock on surface is usually weathered and fractured, is often porous like sandstone can be, and leaks gas.

The basic formula applicable is as follows:

• break out a trench row by row towards a free face, which if necessary must be created by vee-ing it out.
• Use an excavator, under your control, to dig it out row by row continuously
• drilling pattern triangular, two holes on each edge’ one in the middle further back, same spacing, like an isosceles triangle .
• therefore 5/6 holes per linear metre are required; (work it out on graph paper)
• there must be a foolproof sign off criterion for final level, preferably the client’s responsible person’s signature, otherwise you will be called back to remove ‘bumps’ and similar imperfections at your cost!
• Do not try to drill the trench upfront, then initiate; it seldom works
• Don’t make any other philanthropic assumptions; trenching is a tough challenge, the reason it is often sub-contracted.
• Doing it this recommended way, it is possible to change burdens and charge rates as circumstances relating to rock condition in nature varies
• This method statement is failsafe effective; all other methodologies are lotteries of imperfection and hope!

Variations on the Trenching Theme

Large trenches for big pipelines usually go deeper, like six metres, and change the ground rules, such as decking more than one cartridge per hole, with intermediate stemming. Consult with us about major pipelines.

Mass Breaking - Sinking Shafts

Nonex can be used to do mass breaking including shaft sinking through the application of a specific methodology

Guiding Principle

The pressure of expanding gasses breaks rock in tensile > the free face burden. A mass break is created when multiple holes are drilled vertically and charged with a large number of heavily charged cartridges such as 500g x 60mm diameter Nonex 50060 (drilled using a rig). The free face becomes the surface cap above the charged holes. Depending upon the nature of the rock, it is feasible to drill to a depth of 2m with spacing of 1m, charging up to 300 holes, on a bench 40m x 7,5m..300 x 500g cartridges initiated simultaneously produce 150,000 litres of gas, which must go somewhere. This combined volume of gas takes off the cap, shattering it into small particle sizes. Noise and fly rock is controlled by a layer of conveyor belting mats over the charged bench and up to 2m of earth cover on top of the mats.

Practical Hints

• A mass initiation of this size requires serious planning and attention to detail; the circuit resistance for 300 cartridges will be in excess of 150 ohms, measured on an ohmmeter, requires a high capacity shot firer like AECE 1224 (otherwise there will be misfires)
• Mass rock breaking requires the support of an excavator, preferably 30t +capacity, to dig out the broken rock, as well as to cover the charged bench from the edge. (reach 7,5m, unless covering from both sides)
• The covering process risks breaking the wiring. Use an ohmmeter with an alert charge hand in control. His job is to note exactly where and when continuity was broken, call a stop and point to the recovery spot; otherwise it is an almost impossible time-consuming task, like looking for a needle in a haystack.
• In shaft sinking however, the shaft diameter only permits access to small excavators with a small capacity bucket,, or to a grapple handled by a tower crane, so the drilling depth must be reduced to say 1,5m maximum and the spacing to about 0,5m in order to create smaller fragmentation for loading
• As the shaft gets deeper, the use of earth cover becomes expensive and time consuming. The solution is to construct a steel deflector shaped like a saucer, designed to be held in place over the mouth of the shaft on initiation in order to deflect fly rock back into the shaft bottom
• As the shaft descends, ground water will enter from the sides, requiring water-resistant cartridges. Waterproof connections in the wiring-up process are essential. High capacity shot firers deliver more than 1000 volts, which will short out due wet connections and cause massive misfires.
• Insulation tape with bitumen works but is dependant on the charge hands finger dexterity ; the best solution is to use 3M UY2 connectors designed for telecommunications;. a crimping tool designed to regulate pressure is essential. A very strong, waterproof connection is provided at relatively low cost. Locating a break in a shaft containing water is challenging. The wires to be reconnected must first be dried to avoid shorting and resultant misfires
• Stemming materials and stemming techniques are crucial, otherwise the gas escapes through the drill holes by attacking the stemming. -5mm crusher dust is a benchmark in dry holes, a mix with 13mm aggregate works best in wet holes. Tamping with broken drill steels adds weight and speeds up compaction (is safe with Nonex cartridges) Pneumatic , stainless steel stemming loaders are available to blow crusher dust into horizontal holes; compacts automatically due speed of entry under pressure.