STEEL-WHEELED ROLLERS

A steel-wheeled roller, as shown in figure 11-44, is used for compaction and finish operations on base coarse materials and asphalt. This roller produces a smooth, solid surface under favorable conditions, but may fail to compact areas narrower than the roll, and do not compact deeply in proportion to their weight. The steel- wheeled roller does not change shape to bring suitable support for itself. Rather it sinks until enough bearing area has come in contact with the roll to support the roller weight.

The drive wheel is ahead of the tiller wheel in the direction of travel. The tiller wheel functions as the steering axle. As shown in figure 11-45, there is a downward vertical force caused by the weight or the wheel. The arrows, concentric with the steel wheel, represent the rotational force on the wheel. This force is transmitted to the base of the wheel, as the roller is propelled. This concentric force tends to move the material under the wheel, rather than to push it away. These forces result in a more direct vertical force than those of the forces under the tiller wheel.

ROLLING TECHNIQUES

Roller techniques arc basically the same with any type of roller. Some things you must consider are steering, changing direction and speed, and rolling sequence.

Steering

Steering sharply causes scuffing and damage to the surface; therefore, turns should be made slowly and gradually. You may have to back up several times to complete a turn.

Changing Direction and Speed

Starting and stopping should be done gradually to avoid scuffing the surface. Start stopping well ahead of the point where you want to stop. Engage the direction control slowly to avoid any wheel spin.

Rolling speed is 1 1/2 to 3 miles an hour. You must develop a rolling sequence to ensure the compaction is uniform throughout the fill.

Rolling Sequence

Overlapping is part of the rolling sequence. When rolling deep, loose fills, you should overlap at least half the drum width. Gradual extension of the rolled material into the unrolled area makes possible greater concentration of weight on local ridges and high spots.

In rolling a graded area with a side slope, as a crowned or banked road, you should work from the bottom to the top. The lower edges of the rolls have a tendency to push downhill, which can be best resisted by compacted material. In working uphill, the creep of soil away from the upper edge helps to preserve the slope.

A crowned road is rolled according to the pattern shown in figure 11-46, starting at one edge and working to the center line. Then move diagonally to the opposite side and work to the center line from that side. Each rerolling is done in the same manner.

It is efficient to roll in sections as long as you can overlap the sections, as shown in figure 11-47.

Banked or sloped elevated curves are rolled in the direction of travel, from the bottom (low side) to the top, as shown in figure 11-48. The rolling transition from the road crown to the bank curve is made by a diagonal from the center of the crown to the low side of the bank. The rolling transition from bank to crown is made straight to the adjoining low side of the road crown.

Rolling should be continued until no compaction advantage is noted on the fill from successive passes. Too much water in the fill material may make compaction impossible. This may require scarifying and windrowing the fill to aerate the material. A rubbery, or spongy, rolling action of the fill that springs back into nearly its original condition when the rollers have passed may indicate trapped water below the surface. The robbery, or spongy, area may require stabilization by other means, such as excavating the area and

Figure 11-46.-Crowned road rolling sequence.

Figure 11-47.-Overlapping sections.

Figure _11-48.-Banked curve rolling sequence.

placement of riprap, soil cement, asphalt stabilization, and so forth.