Roadway Excavation Archives - Civil Engineering Tips

Earth Works – Drilling and Blasting

As the cost of drilling is a significant proportion of the total drilling and blasting operation the use of as large a drill hole as possible will result in greater hole spacings and reduced footage. However, the choice of hole size and drilling pattern is frequently restricted because of the depth of face possible and the degree of fragmentation required.

Material should be produced in a manageable size without the need for additional treatment. In a cut to fill operation, the material size is restricted by the maximum thickness of rock fill that can be compacted by the rollers available. This then places a limit on the drill hole size and spacing. Alternatively, it may be more economical to hire larger rollers capable of compacting deeper layers.

Deep faces (where possible) offer economy in the use of explosives but, for safety reasons, when using front end loaders for loading of the broken rock, a maximum face of 10 meters is recommended.

A decision to drill and blast need not slow down production. The blasting operation must be matched to the capabilities of the earth moving fleet and the total operation should be planned so that intermittent unscheduled blasting does not interfere with the project’s continuity.

Productively can be maintained by working several faces and arranging blasting to be undertaken at times when there is the least likelihood of interruption to other operations.

A blast should be designed to be as large as possible within circumstantial restraints, to ensure that the overall percentage of “Low productivity” material is kept to a minimum and that a minimum of productive time is lost in preparing and moving equipment.

In addition to producing the necessary quantities, the designed blast must achieve a degree of fragmentation which still not adversely effect the cost of handling during succeeding phases of earth moving. A well designed blast should allow loading to be undertaken without any dozer stockpiling being required.

When blasting adjacent to batters, holes should be more closely spaced and more lightly charged. Unless per-splitting is adopted, all batter holes should be shot on the last delay or shot separately after the main charge had been set off (perimeter blasting). Shooting should always be to a near vertical free face parallel to the holes; and the blast should be designed so that back break and damage to the sub graded and batters is minimized. To avoid such damage, “pin cushion” blasting (no free face) in conjunction with ripping should not be used

Earthmoving Operations-Ripping

The rippability of any rock is materially affected by the nature and composition of the rock. Sedimentary rocks generally offer the best opportunity to rip, whilst metamorphic rocks present
the least opportunity. However decomposed and weathered igneous and metamorphic rocks can often be ripped economically.

Rock with frequent bedding planes and cracks can usually also be ripped economically whereas massive (thickly bedded) rock formations generally must be drilled and blasted.

Some physical characteristics which favor ripping are :-

• Fractures, faults and planes of weakness of any kind.

• Weathering resulting from temperature and moisture changes.

• Brittleness and crystalline nature.

• Frequent cracks and bedding planes.

• Large grain size.

• Moisture permeated clay, shale and rock formations.

• Low compressive strength.

Ripping will be difficult if the rock formation is :-

• Massive and homogenous (i.e. lacks bedding planes and cracks).

• Non crystalline and therefore not brittle.

• Without planes of weakness.

• Fine-grained with a solid cementing agent.

• Of clay origin where moisture may impede ripping because it makes the material plastic.

The following are some further practical matters to be considered when ripping –

(1) When working a cutting always keep the outside deeper than the center. If the center is kept low, all plant tends to work away from the toe of the batter and it is expensive and difficult to then excavate the toe. In hard material, perimeter blasting or pre-splitting of batters even when the rest of the cutting is not blasted helps to position the batter toe and tends to stabilize the batter face.

(2) Rip in straight lines, never turn with the ripper buried as this can result in damage to the tyne tracks and frame.

(3) Do not leave blasting until all rippable material is removed unless there is over 3 m of material still to be excavated. Shallow holes require an excessive footage of drilling compared to the volume loosened and as such are uneconomical.

(4) To obtain penetration the tractor should not be jerked or tracks spun as this causes damage to the unit. Raising the tyne with the hydraulic cylinder will sometimes break out the rock.

(5) When working on side drains and in the vicinity of batter toes the “radical arc” or “adjustable parallelogram” rippers are preferred as the angle of penetration is variable, thus enabling material to be both ripped and lifted out.