Concrete Roads
and Their Construction

132 CONCRETE ROADS wheels are of the open type, i.e., without an axle, and are arranged to obtain the maximum depth of trench with the minimum diameter of wheel. Ladder Excavators are much more powerful and are capable of digging to a much greater depth than wheel excavators. The ladder excavator usually consists of two endless chains running over an adjustable boom, the chains carrying a series of toothful cutters and buckets. As the chain revolves these cutters penetrate the ground slightly, and the material thus excavated is carried upwards to be discharged as the bucket reaches the top of the boom on to a chute or conveyor, which in turn transports the excavated material to either side of the machine. It can be dumped into a pile for backfilling or direct into tip wagons or carts. Ladder excavators should have ample strength, since excavating deep, wide trenches for sewer and similar projects subjects them to constant heavy strains. Ladder excavators of this type are manufactured by Pawling & Harnisehfeger, and sold through Gastons, Ltd. ; the Parsons trench excavators sold through the Allied Machinery Co., Ltd.; the Bucyrus sold through Messrs. George F. West & Co.; the Austin sold by the Austin Machinery Company. Backfillers.—Where openings are made in roadways it is always necessary to re-fill the trendies. This process—sometimes termed backfilling—is often done in an unsystematic and inefficient manner, and, consequently, is unnecessarily costly. Re-filling trenches can be accomplished economically by a mechanical backfiller such as is shown in Fig. 88, which consists of a scraper attached to a light motor-driven crane with a supplementary winding drum. The scraper is lifted and carried behind the pile of earth or other material ; it is then lowered and dragged across the hollow portion or ditch into which it discharges its contents, and is then ready to be lifted back preparatory to repeating the operation. Such a machine will re-fill a trench as rapidly as twenty-five to fifty men moving the material a distance of 15 ft. to 25 ft. The machine only requires one num for its operation. In the backfilling of trenches it is of groat importance that the material should he thoroughly rammed or lamped, and although this operation lias up till now been chiefly performed by hand, a machine for so tamping and trenching has been designed in America and is actually in use in this country.