Road Construction Roadway Excavation

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

Road Construction Roadway Excavation

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.