Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Drilling Rock and Earth

DRILLING ROCK & EARTH

DRILLING ROCK When work on the Hoosac Tunnel first began in the 1850’s hand drills were still the primary method used to create holes for loading explosives in hard rock. Charles Burleigh of Fitchburg, Massachusetts developed the first truly successful pneumatic drill and in 1866 it was first used in the Hoosac.

BURLEIGH DRILL Courtesy of the North Adams Public Library Working in the Hoosac Tunnel.

DRILLING ROCK

CONSTRUCTION DRILLING Blast holes for removal of rock, in a construction excavation or for quarrying.

Rock anchor/bolts in excavations and tunnels CONSTRUCTION DRILLING

Tunnels

CONSTRUCTION DRILLING Foundation grouting

DRILLING PRODUCTION ESTIMATE To begin a drilling production estimate it is first necessary to make an assumption about the type of equipment that will be used. Tables 12.7 & provide information to guide that first decision.

DRILLING PRODUCTION ESTIMATE The final equipment decision should only be made after test drilling the formation. Test drilling should help to quantify: Penetration rate Drilling method Bit size / Bit type

PENETRATION RATE Penetration rate is a function of: The rock The drilling method The size & type of bit

THE ROCK The rock properties which effect penetration rate are: Hardness Texture Tenacity Formation

HARDNESS A scientific definition of hardness is a measure of a material's resistance to localized plastic deformation. It is measured by the MOH scale (Friedrich Mohs).

HARDNESS The Moh hardness classifications are based on the resistance of a smooth surface to abrasion  the ability of one mineral to scratch another.

HARDNESS Moh’s scale for rock hardness

HARDNESS Hardness affects drilling speed.

TEXTURE Texture is the grain structure of the rock. A loose grained structure (porous, cavities) drills fast. Grains large enough to be seen individually (granite) will drill medium. Fine-grained rocks drill slow.

TENACITY Describes how the rock breaks when struck.

TENACITY Shatters - into small pieces from a light blow Brittle - breaks easily with a light blow Shaving - when shaved off in pieces they break easily Strong - resists breaking when hit hard Malleable - flattens instead of breaking

TENACITY Characteristic Drills Shatters - fast Brittle - fast to medium Shaving - medium Strong - slow to medium Malleable - slow

FORMATION Formation describes how the rock mass is structured. Solid mass - drills fast Horizontal strata (layers) - drills fast to medium Dipping planes - drill slow to medium

DRILLING METHOD ROTARY Rotary drilling uses high push-down pressure on the bit and rotation to grind the rock. Compressed air, water, or drilling mud carry the cutting out of the hole.

DRILLING METHOD ROTARY Feed pressure and rotation rate control drilling speed. Soft rock - use lower feed pressure and faster rotation. Hard rock - use high feed pressure and slower rotation.

ROTARY DRILL These drills are suitable for drilling soft to medium rock, such as hard dolomite and limestone, but are not suitable for drilling the harder igneous rocks.

ROTARY DRILLING Hardness Schist5.0 Granite 4.0 Dolomite 3.5 Limestone3.0 Galena2.5 Potash2.0 Gypsum1.5

DRILLING METHOD ROTARY- PERCUSSION The piston provides striking energy to the rock through the drill steel. There is rotation so the bit strikes fresh rock with each blow.

Drill steel ROTARY PERCUSSION

DRILLING METHOD ROTARY-PERCUSSION Compressed air or water is used to flush the drill cuttings from the hole. Drilling penetration (speed) decreases with depth.

PERCUSSION DRILLING Hardness Quartzite7.0 Trap Rock6.0 Schist5.0 Granite 4.0 Dolomite 3.5 Limestone3.0 Galena2.5

Flush the cuttings from the hole. CUTTINGS

DRILLING METHOD DOWN HOLE (DH) The DH drill provides striking energy directly to the bit. There is rotation so the bit strikes fresh rock with each blow.

DRILLING METHOD Down Hole Drill maintains constant penetration rate at all depths. Hammer mechanism

DRILLING METHOD DOWN HOLE Compressed air conducted through the drill steel is used to flush the drill cuttings from the hole. Performance will not decrease as depth increases.

BIT SIZE

BIT TYPE Insert bit Button bit

CARBIDE INSERT BITS Four grades are usually available. Increasing Hardness

CARBIDE INSERT BITS Susceptibility to breakage increases with hardness. However, abrasion resistance also increases. If excess insert breakage occurs, a softer grade should be tried.

BUTTON BITS Button bits can yield faster penetration rates in a wide range of drilling applications. Fewer bit changes are required. Most button bits are run to destruction and never reconditioned.

PRODUCTION ESTIMATE

STEP 1 DEPTH OF HOLE When drilling for blasting it is often necessary to subdrill the hole below the planned final grade elevation. SUBDRILLING FIG 12.3, page 339

STEP 1 DEPTH OF HOLE DRILL DEPTH (ft) = Height of face + subdrilling

STEP 2 PENETRATION RATE The penetration rate will be an average rate (ft/min) from the field drilling tests based of specific equipment, bit type and bit size. Do not use an instantaneous rate.

STEP 2 PENETRATION RATE If no project specific data is available Table 12.7 provides order-of- magnitude guidance.

STEP 2 PENETRATION RATE EXAMPLE What would be the order- of magnitude direct penetration rate in granite for a 6 ¼ in. rotary bit with 30,000 lb pulldown.

STEP 2 PENETRATION RATE EXAMPLE Table ¼ in. rotary bit with 30,000 lb pulldown Not recommended, should consider a different bit

STEP 3 DRILLING TIME DRILLING TIME (min) =

STEP 4 CHANGE STEEL Steel

STEP 4 CHANGE STEEL Shank (Striking Bar) Bit Coupling Steel

STEP 4 CHANGE STEEL Steel, approximate weights:

Steel The length of steel for rotary drills varies considerably, in the range of 20 to 60 ft. These rigs have mechanized steel handling. STEP 4 CHANGE STEEL

Steel The time to change steel is approximately constant for all diameters, but varies with length.

STEP 5 BLOW HOLE After completing the drilling it is necessary to clean out all cutting from the hole (see discussion page 353 and Example 12.1). The time to clean out will be dependent on the depth of hole.

STEP 6 MOVE TO NEXT HOLE Travel time depends on distance, terrain, and the type of drill mount. A discussion of travel speed is given on page 353.

STEP 6 MOVE May have to lower the mast

STEP 6 MOVE TO NEXT HOLE If drilling for blasting operations distance will be set by the blasting pattern. An 8  10 pattern means 8 ft between rows and a 10 ft spacing between holes. Therefore, the travel distance moving along the row is only 10 ft.

The travel time may not be controlled by the drill. STEP 6 MOVE TO NEXT HOLE

STEP 7 ALIGN STEEL Once over the hole location it is necessary to position the mast or steel for the proper angle of attack. This is usually vertically but not always.

STEP 7 ALIGN STEEL Time to align is discussed on page 353. Outrigger for leveling

STEP 8 CHANGE BIT Bits, shanks, couplings and steel are all high wear items that must be replaced frequently.

STEP 8 CHANGE BIT The time allowance for replacement is a factor of both the actual time to remove and replace, and the frequency of such changes. Table provides frequency information.

STEP 8 CHANGE BIT CHANGE BIT TIME (min) = If you do not have any company and rock specific data, average life of bit data is provided in Table

STEP 9 TOTAL TIME Total time (min) = Drill (min) + Change steel (min) + Blow hole (min) + Move (min) + Align Steel (min) + Change bit (min)

STEP 10 OPERATING RATE OPERATING RATE =

STEP 11 EFFICIENCY Experienced drillers working under good conditions should have a 50 min-hr efficiency on a production job. Under sporadic drilling conditions efficiency may go down to 40 min-hr.

STEP 11 EFFICIENCY Terrain is a critical factor impacting efficiency.

STEP 12 PRODUCTION HOURLY PRODUCTION (ft/hr) = Efficiency (min/hr)  Operating rate (ft/min)