Short answer
On an urban foundation excavation that meets rock, blasting often never even comes up: neighbouring structures, the permit process and vibration limits close that option from the start. Mechanical excavation is what remains, and the real question there is not which attachment to use but in what order the excavation advances. In mechanical excavation progress comes from creating a free face; an excavation started in the middle of a mass does not advance, one started at its edge does. So the plan is a cutting and benching plan before it is an attachment list. The attachment is the tool of that plan.
Why the order matters more than the attachment
In blasting the energy is released inside the mass and the rock cracks from within. In mechanical excavation the energy comes from outside, and the rock only breaks where there is a void it can expand into. That void is called a free face.
With no free face the breaker strikes, the energy turns into heat and nothing advances. A significant share of the situations described on site as "this rock will not break" are about the absence of a free face rather than the hardness of the rock.
So mechanical excavation advances in benches: you start at an edge, open a narrow channel, that channel becomes the free face and the excavation widens from it. Bench height and width are then set by machine reach.
The excavation plan
- 1
Establish the neighbouring structures and the vibration limit
The limit constrains the attachment choice directly. Which measurement and which limit apply depends on local regulation and the project specification; we print no figures here.
- 2
Decide where the free face will be opened
The most accessible edge of the excavation is chosen and the first channel opened there. That channel is what all later progress rests on.
- 3
Set bench height against the machine
The height at which the machine can set its tool square determines the bench height. Working at an angle above that reach means lateral load and slow progress.
- 4
Open the first channel with the most controlled attachment
Opening the channel is the stage with the heaviest vibration, because there is no free face yet. Where there is a vibration limit, using a drum cutter at this stage is both faster and safer than forcing a breaker.
- 5
Speed up once the free face exists
Once the channel is open the mass can break towards it; a breaker speeds up markedly here and changing attachment makes sense.
- 6
Plan the formation separately
The level and flatness of the foundation formation are defined in the specification. A formation opened with a breaker is uneven; where level precision is required, the last layer is taken with a different attachment.
Attachment choice and vibration
- Breaker: the fastest once a free face exists. It also produces the most vibration; an enclosed housing lowers noise but does not solve vibration.
- Drum cutter: advances by cutting, with markedly lower vibration and a clean surface. It is the only attachment that can advance without a free face; the price is speed.
- Vertical drum cutter: takes the lead in narrow, deep sections, particularly at the channel opening stage.
- Ripper: where the rock is layered it can tear without needing a free face; in massive rock it is no use.
- Crusher bucket: for using the excavated material on site; haulage is among the most expensive items in an urban setting.
Vibration measurement is a record question, not an attachment question
On urban excavation a complaint from a neighbour is nearly inevitable, and the argument usually runs without any measurement. Recording the existing condition (including any existing cracks) before excavation starts, and measuring vibration through the work, decides the ground that argument stands on. We covered this separately on the noise and vibration page.
Timing: how much slower is mechanical excavation?
Mechanical excavation is slower than blasting, but comparing only excavation hours misleads. Blasting has its own time: permits, notifying neighbours, drilling, charging, the exclusion zone and clearing up afterwards.
In an urban setting those additional times grow, and in some cases the total catches up with mechanical excavation. On top of that sits the risk of the permit not being granted; a refused permit invalidates the entire planned duration.
So the decision is made on "which is predictable" rather than "which is faster". Mechanical excavation is slow but its duration can be calculated from the start; blasting is fast but in a city its duration depends on a permit.
Do and don’t
Do
- Draw up the free face plan before excavation starts; the attachment decision follows it.
- Set bench height against the height at which the machine can set the tool square.
- Where there is a vibration limit, consider a drum cutter for the channel opening stage.
- Have the existing condition of neighbouring structures recorded before excavation.
- Plan to use the excavated rock on site; haulage is expensive in a city.
Don't
- Do not start the excavation in the middle of the mass; without a free face there is no progress.
- Do not persist with a breaker where there is no free face; the energy turns to heat and the tool mushrooms.
- Do not set bench height above the machine’s reach; working at an angle is lateral load.
- Do not finish the formation with a breaker; where level precision matters the last layer is taken with another attachment.
- Do not enter a vibration argument without measurement.