10.1 Straight Dozing, Slot Dozing & Production Pushing

Straight Dozing, Slot Dozing & Production Pushing

Mechanics of the Straight Dozing Cycle

Bulldozer earthmoving operations rely on the repetitive execution of a standardized production cycle designed to maximize cubic yards moved per hour while minimizing fuel consumption and track component wear. In straight dozing—the baseline method of pushing earth directly in line with the machine's travel path—the production cycle is divided into three distinct operational phases: the cut phase, the drift (or carry) phase, and the spread phase.

+-----------------------------------------------------------------------------------------+
|                                 STRAIGHT DOZING CYCLE                                   |
|                                                                                         |
|  1. CUT PHASE                2. DRIFT PHASE                   3. SPREAD PHASE           |
|  - Penetrate subgrade        - Raise blade to grade line      - Lift blade gradually    |
|  - First or second gear      - Push live rolling boil         - Deposit uniform lift    |
|  - Load in 1-2 tractor       - Balance draft via tilt         - Feather material or     |
|    lengths (15-30 ft)        - Conserve momentum & power        push over crest edge    |
+-----------------------------------------------------------------------------------------+

1. The Cut Phase (Loading the Blade)

The cut phase begins with the bulldozer positioned at the rear of the excavation cut. The operator shifts the transmission into first gear (or low hydrostatic range) and sets the engine hand throttle to high idle. The blade is lowered into the ground while applying downward hydraulic pressure to force the cutting edge through the topsoil, hardpan, or compacted fill. The primary objective is to penetrate to the maximum optimum cutting depth immediately and load the moldboard to full capacity within the shortest possible travel distance—ideally within one to two tractor lengths (15 to 30 feet).

As the cutting edge shears the soil, the earth is forced upward along the concave face of the moldboard. Skilled operators monitor the center of the blade: when the soil boils upward over the center moldboard and begins spilling across the top spill plate, the blade has reached maximum capacity. At this precise moment, the operator must terminate the cut phase.

2. The Drift Phase (Carrying the Payload)

Continuing to cut into undisturbed subgrade after the blade is fully loaded is a severe operational error. Deep cutting with a full blade forces the engine into excessive torque converter slip or hydrostatic relief, causing track shoes to break traction and spin wildly against the ground. This track slippage accelerates costly grouser, pin, and bushing wear without adding material to the payload.

To transition into the drift phase, the operator eases the hydraulic blade lift lever upward just enough to position the cutting edge directly at original ground elevation. Instead of carving new soil, the bulldozer now glides the accumulated heap of earth across the surface. Because the machine no longer expends tractive horsepower shearing undisturbed ground, all available engine power and track momentum are dedicated to overcoming the sliding and rolling friction of the dirt heap. During the drift phase, operators typically shift into second gear (if ground resistance permits) to increase forward travel speed, maintaining momentum until reaching the dump point.

3. The Spread Phase (Discharging the Material)

At the end of the push distance, the operator initiates the spread phase. Depending on job specifications, spreading occurs through one of two techniques:

• Feathered Spreading on Fill: When placing earth across an embankment or structural fill area, the operator raises the blade gradually while traveling forward, allowing the material to flow under the cutting edge in a smooth, continuous horizontal layer of specified loose thickness (typically 6 to 8 inches).
• Crest Dumping: When feeding a hopper, filling a ravine, or building an edge stockpile, the dozer pushes the full load to the edge of the slope, halts the machine with the tracks securely on solid ground, and allows the material to spill over the edge. The operator instantly reverses travel, selecting high reverse gear to minimize return cycle time.

Blade Load Control and Rolling Dynamics

Controlling the soil load in front of the moldboard requires continuous coordination between track traction, engine power, and hydraulic blade adjustment. The physical interaction between the steel moldboard and the soil dictates whether the load behaves as a "live load" or a "dead load":

• Live Rolling Load: Modern bulldozer moldboards feature a precisely curved radius designed to roll the soil forward and upward in a continuous, churning boil. As the earth rolls inward upon itself, internal soil friction is minimized, and the force required to propel the mass forward is significantly reduced. A live rolling load keeps the dirt active, allowing the tractor to drift larger payloads at higher speeds.
• Dead Load: If the soil is excessively sticky, if the cutting edge is dull, or if the operator drives too slowly without proper blade roll, the soil compacts against the moldboard face into a dead, inert mass. A dead load acts as a solid brake against the tractor, drastically increasing drawbar resistance, generating excessive heat in the drivetrain, and inducing track slippage.

   LIVE ROLLING LOAD vs. DEAD PACKED LOAD
   ---------------------------------------
   Live Rolling Load:              Dead Packed Load:
         (Boiling Roll)                  (Solid Wedge)
         __                             __
       /   \  <- Rolling boil          |  |  <- Compacted dead mass
      |  O  |                          |  |     drags on subgrade;
       \___/                           |__|     causes track slip
      (====) Moldboard                (====) Moldboard
      /____\                          /____\

Balancing Draft Loads with Hydraulic Blade Tilt

During both the cut and drift phases, soil density and ground hardness rarely remain uniform across the full width of the blade. When one corner of the blade bites into a hard clay seam, compacted boulder, or tree root, that side encounters substantially higher mechanical resistance. This uneven resistance induces "draft steer" or yaw, pulling the front of the bulldozer toward the heavy side.

Inexperienced operators attempt to correct this deviation by riding the steering levers or foot brakes, which disengages power or applies friction braking to the track, wasting up to 50 percent of tractive power. Professional operators correct draft steer dynamically using the hydraulic blade tilt function:

• If the right side of the blade bites deep and pulls the machine to the right, the operator tilts the right corner upward while lowering the left corner slightly.
• Equalizing the cutting depth across the blade rebalances draft resistance across both tracks, allowing the tractor to track in a perfectly straight line with full engine power delivered to both track chains.

Track Slippage Management

Maximum drawbar pull occurs when crawler tracks experience between 7 and 15 percent track slippage. Slippage below 5 percent indicates the bulldozer is underloaded and operating below peak capacity. However, when track slippage exceeds 15 percent, the steel grousers begin grinding, churning, and spinning in place, polishing the subgrade into a slick trench. Operating with spinning tracks destroys track pins and bushings, reduces hourly production to near zero, and wastes diesel fuel. When the operator hears engine RPM drop and detects track spin, the correct response is to momentarily raise the blade slightly to shed a fraction of the load and restore tractive grip, rather than trying to force the push.

The Slot Dozing Method: Principles and Production Gains

In conventional flat-ground straight dozing, loose material continuously spills around the left and right outer wings of the blade. This side spillage—known as end drift or windrowing—progressively sheds material along the push path. As a result, a blade that was loaded to 100 percent capacity at the 30-foot mark may retain only 60 to 70 percent of its payload after drifting 150 feet.

Slot dozing is an advanced earthmoving technique engineered specifically to eliminate side spillage and maximize retained payload over medium-to-long push distances (typically 75 to 250 feet).

+-----------------------------------------------------------------------------------------+
|                                 SLOT DOZING CROSS-SECTION                               |
|                                                                                         |
|  Windrow Wall                Active Slot Trench               Windrow Wall              |
|  (2 to 3 ft high)           (Blade Width Clearance)           (2 to 3 ft high)          |
|      /\                  _____________________________                  /\              |
|     /  \                |                             |                /  \             |
|    /    \_______________|   DOZER BLADE CONFINED HERE |_______________/    \            |
|   /      Loose Spillage |                               | Loose Spillage    \           |
|  /       Forms Berm     |_____________________________| Forms Berm          \          |
|                          Undisturbed Cut Floor Base                                     |
+-----------------------------------------------------------------------------------------+

The Slot Dozing Procedure

1. Excavating the Initial Slot: The operator makes repetitive cutting and drifting passes along the exact same path. As soil spills off the blade edges during the first few passes, it builds up into parallel, consolidated ridges or windrows on both sides of the trench, typically 2 to 3 feet high. Once the slot is established, the blade operates inside a recessed trench where the trench sidewalls act as physical sideboards.
2. Confining the Dirt Mass: With the sidewalls preventing lateral spillage, material that would otherwise escape off the wings is trapped in front of the moldboard. The soil boils upward against the face and piles higher than the top spill plate, allowing the bulldozer to carry noticeably more than its flat-ground load; Caterpillar's production estimating uses a factor of about 1.2 (20 percent) for slot dozing, and the Army's earthmoving manual (FM 5-434) says slot dozing can increase output by as much as 20 percent.
3. Preserving Slot Integrity: The operator maintains smooth slot sidewalls and a uniform floor grade. Steering maneuvers inside the slot must be avoided, as turning causes the blade corners to gouge into the sidewalls, knocking loose dirt into the trench and disrupting the slot boundary.
4. Excavating Adjacent Slots and Removing Sliver Berms: Once the first slot is excavated to design depth, the operator shifts over, leaving a narrow, unexcavated ridge of earth—called a sliver berm—approximately 1 to 2 feet wide between the slots. The operator digs a second parallel slot identical to the first. After multiple parallel slots are cut across the excavation area, the dozer operator makes cleanup passes along the sliver berms. Because the sliver berms are narrow, unsupported, and bounded by open slots on both sides, the bulldozer easily shears them away at high speed, pushing massive combined volumes to the fill zone.

Downhill Dozing and Gravity Production Dynamics

Gravity exerts a profound influence on heavy earthmoving equipment. When a bulldozer operates on a slope, the combined operating weight of the tractor and the retained soil mass is subjected to gravitational forces acting parallel to the grade:

$\text{Gravitational Assistance Force} = (W_{\text{machine}} + W_{\text{payload}}) \times \sin(\theta)$

Where $W_{\text{machine}}$ is the operating weight of the tractor, $W_{\text{payload}}$ is the weight of the soil load in front of the blade, and $\theta$ is the slope angle of the terrain.

The 3 Percent per 1 Percent Rule

The Caterpillar Performance Handbook gives a standard rule of thumb for dozer grade performance: for every 1 percent of favorable grade (pushing downhill), production increases by about 3 percent, and for every 1 percent of adverse grade (pushing uphill), production decreases by about 2 percent, for grades up to 10 percent (steeper grades use the handbook's grade-correction chart).

When pushing down a 10 percent favorable slope, the gravitational component delivers an effective 30 percent boost in hourly production ($10 \times 3\% = 30\%$). Gravity pulls both the machine and the soil mass downward, drastically reducing the drawbar horsepower needed to drift the load. This allows the dozer to carry significantly larger blade loads in higher transmission gears without inducing track slippage or overheating powertrain fluids.

Whenever site topography permits, civil earthwork excavations should be engineered and staged to push material downhill. Starting at the highest elevation and cutting downward utilizes gravity to accelerate production, shorten cycle times, and conserve thousands of gallons of diesel fuel over the project lifecycle.

Dozing Techniques & Productivity Factors Comparison

Field Operational Scenario: Overburden Stripping Production Calculation

On a major highway bypass project, an earthwork contractor must strip and drift 60,000 loose cubic yards (LCY) of dense silty clay overburden from a roadway cut to an adjacent embankment fill. The site supervisor is tasked with establishing equipment requirements, cycle times, and operating procedures for a fleet of 45,000-pound production bulldozers equipped with 6.0 LCY Semi-Universal (SU) blades.

Baseline Production on Flat Terrain

Initial geotechnical testing and site layout establish the following baseline parameters for flat-ground straight dozing over an average push distance of 150 feet:

• Cut time (first gear, 30 ft): 0.20 minutes
• Drift time (second gear, 120 ft): 0.45 minutes
• Spread time (feathering load): 0.10 minutes
• Reverse travel time (third gear reverse, 150 ft): 0.40 minutes
• Gear shift and directional pause: 0.10 minutes
• Total Cycle Time: $0.20 + 0.45 + 0.10 + 0.40 + 0.10 = 1.25\text{ minutes}$
• Cycles per 50-Minute Operating Hour: $\frac{50}{1.25} = 40\text{ cycles/hr}$
• Baseline Hourly Production: $40\text{ cycles/hr} \times 6.0\text{ LCY} = 240\text{ LCY/hr}$

At this baseline rate, moving the 60,000 LCY requires: $\text{Total Required Hours} = \frac{60{,}000\text{ LCY}}{240\text{ LCY/hr}} = 250.0\text{ machine operating hours}$

Optimized Production: Implementing Downhill Slot Dozing

To accelerate the project schedule, the project engineer reconfigures the cut geometry. The excavation is staged from the top of the hill downward, creating an 8 percent favorable downgrade toward the fill. The operators are instructed to implement slot dozing, cutting parallel trenches with 2.5-foot sidewalls and leaving 1.5-foot sliver berms.

The engineering production factors are calculated as follows:

1. Slot Dozing Factor: Using the handbook's slot-dozing factor of 1.20, the effective blade payload increases from 6.0 LCY to $6.0 \times 1.20 = 7.2\text{ LCY}$.
2. Grade Factor: The 8 percent favorable downgrade is within the rule's 10 percent limit and provides a production increase of 3 percent per 1 percent slope. Grade multiplier = $1.0 + (8 \times 0.03) = 1.24$.
3. Combined Productivity Multiplier: Combining the slot payload gain and gravitational grade assistance yields an overall production factor: $\text{Combined Factor} = 1.20 \times 1.24 = 1.488\text{ (about a 49\% increase in hourly output)}$
4. Optimized Hourly Production Rate: $\text{Production}_{\text{optimized}} = 240\text{ LCY/hr} \times 1.488 = 357.1\text{ LCY/hr}$

Operational Outcome and Savings

With the optimized downhill slot dozing strategy, the total operating hours required to move the 60,000 LCY are reduced to: $\text{Optimized Total Hours} = \frac{60{,}000\text{ LCY}}{357.1\text{ LCY/hr}} \approx 168.0\text{ machine operating hours}$

By reconfiguring the cut geometry, the contractor eliminates about 82.0 machine operating hours (a 32.8 percent reduction in total machine time). At an operating cost of $185 per hour (factoring fuel, operator wages, track wear, and depreciation), this procedural shift saves the contractor about $15,170 while finishing the phase ahead of schedule.

To prevent water pooling in the slots in the event of an unpredicted thunderstorm, the site supervisor establishes two mandatory operational rules:

• At the end of each shift, the operators must bulldoze through the downhill end of each slot to ensure positive drainage into perimeter sediment basins.
• Slivers between active slots must be shaved down at the end of each work day to avoid creating steep trench walls that could collapse during overnight rains.