Page 367 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

ROADWAYS 8.29

If trimming is done with a scraper, one rear wheel should be on the bottom, the other on the
slope. If it is steep, the tailgate should be carried well forward so that loosened dirt will slide
downhill rather than enter the bowl.
The cut should not be deepened so far between trimmings that the grader cannot reach all the
steps. This is particularly important when the slope is so steep that it cannot be worked by machinery
later.
As the cut deepens, new slope stakes are placed. They are often set from the originals with a
string level, rule, and plumb bob. If they are driven in flush and marked with light sticks, a good
grader operator can trim the bank without knocking them out.

Finishing Subgrade. The bulk of a deep cut can be made without staking except for the slopes.
As it approaches bottom, however, grade stakes should be set, and digging done with sufficient
care to avoid overcutting and the resulting need for patch fills.
Good scraper operators can hit a grade within a fraction of an inch if the soil is smooth, but it
is often more economical to have them run a rough grade and go on to other work while a grader
finishes up. The grader will probably be required to cut and shape gutters, in any event.
The road, or the shoulders, is sometimes overcut to allow space for spoil from ditches. If this
is not done, ditch cuttings may be windrowed on the road for later removal.
Any areas that are cut and refilled in this manner must be thoroughly compacted. A few patches
may be rolled by trucks or scrapers, but regular rolling equipment should be brought in for any
extensive areas. Failure to compact may lead to local settlement and pavement failure.

Selected Base. A layer of porous soil with high bearing strength is usually placed between the
native soil and the pavement, in both cuts and fills. If it is obtained from nearby pits, it will probably
be economical to bring it in and spread it with scrapers.
The pit wall or floor may be shaped for direct scraper digging. If this is not possible, hauling
may be done with either scrapers or trucks loaded by excavators or other equipment. The scrapers
offer the advantage of doing their own spreading.
Self-loading scrapers are good in this work, where quantities are relatively small and pit shape
is irregular, as they do not have to be part of a balanced spread which may be difficult to organize.

PUSHER

The pusher is a separate tractor that pushes the scraper while it is loading. It is almost always
required for efficient full loading of single-engine self-powered scrapers, and is usually desirable
with two-engine scrapers and with crawler-drawn scrapers that are oversized for the tractor or that
are in hard digging. See Fig. 8.22.
Types. Pushers are usually crawlers with tractor weights of 20 tons or more.
There are also four-wheel-drive pushers with similar or greater total weight. These have the
advantage of much greater speed. They can often get a scraper through the cut much more rapidly
and with less scraper strain and wheel spin than a crawler of similar weight can, and they can
make fast moves from one scraper to the next, and between jobs. However, they do not have near-
ly as much push at stall speeds; they may lose their speed advantage by requiring shallower and
longer cuts. Tires lose traction badly on wet, slippery surfaces.
Crawler tractors with torque converters have maximum pushing power and traction, and have
some speed flexibility.
Several thousand dollars may be saved by buying a pusher with a fixed push plate instead of a
center-reinforced dozer blade. However, the fixed plate will not permit the tractor to do cleanup
or backripping work in the pit, and it is likely to find little use outside of it. It may cause difficulty
by not meeting the scraper at an efficient angle, or by losing contact on rough ground.
A good push plate needs little maintenance, but even the most heavily reinforced dozer blades
are likely to cave in when used for scraper pushing.