Bulldozers and excavators are widely used in road construction in New Zealand. Up until about the mid-1980s, bulldozers were the machines of choice. There are many advantages of pairing up dozers with excavators as each machinery type has significant advantages over the other and jointly, they lead to a better-quality job. However, excavators are much more adaptable and useful.
The earthworks machinery needs to be matched with the intended job. Bigger is not always better. Sometimes it is best to forego operational efficiency and substitute a different, less efficient machine to achieve an improved outcome. For example, use an excavator to shift and carefully place material rather than use a dozer which has less control over the material.
There are strict requirements to machinery protective structures within forestry. Refer to WorkSafe’s Approved Code of Practice for Operator Protective Structures on Self-Propelled Mobile Mechanical Plant.
The operator is as critical to the job as the right machinery.
5.8.1 Excavators
Correct excavator specifications are important to obtain the best performance. Local conditions will determine the selection of the most appropriate boom and bucket configuration for the soil types and construction method. This will affect the production rate.
There are several classes of buckets. They range from extra heavy duty for rough rock and other severe applications, to heavy duty for use where rock is encountered occasionally, and medium and light duty buckets for soft materials. Buckets can be changed in a matter of minutes using a quick coupling system.
As a rule, the softer the material the wider the bucket that can be used, and vice versa. For excavating hard materials, a boom and a short tip radius bucket achieve the highest tooth forces. Using a short boom will reduce the reach but increase the digging force. A short tip radius bucket will increase the breakout force. In hard rock reduce the bucket width to apply a higher force. A wider bucket can shorten the boom life through the twisting action, which occurs when the bucket corner hits a ‘fixed’ object while digging.
It is widely thought that excavators compact the soil through track rolling. However, excavators typically are not very effective because their tracks are designed to minimise ground pressure. Also track rolling is not efficient due to an excavator’s slow travel speed and low ground pressure.
Excavators that work in the cutover as part of their normal operation need additional protection for forestry work. A protective plate under the upper structure is needed to protect against debris being pushed up from underneath.
5.8.2 Bulldozers
Bulldozers are commonly used to clear the right-of-way, to cut and fill, and to construct an even subgrade. Match the machine size with its intended use. The weight and horsepower of the machine determines its ability to push. Local conditions will determine the selection of the most appropriate size and configuration for the topography, geology and soil types. This will affect the production rate.
There are a range of bulldozer blades available, dependent on the individual machine. The blades and their intended use are:
‘U’ – Universal, large wings for moving large loads over long distances
‘S’ – Straight, most versatile ‘all-rounder’
‘A’ – Angling, can be positioned straight or angled 25° to either side for side casting
‘C’ – Cushion, designed for large tractors for push-loading scrapers, but may be used for general dozing jobs.
5.8.3 Advantages of excavators compared with bulldozers
As mentioned above, dozers and excavators have areas of road construction in which they excel. The following compares the advantages of these machines:
Excavators
- Are more precise at formation in difficult topography than a bulldozer
- Handle materials that may be inaccessible to a bulldozer
- Have better control over material, leading to a more consistent subgrade. Can select and borrow suitable rock and blend other formation materials
- Have lower track ground pressure, enabling them to work in wetter terrain
- Can work in a narrower construction corridor, leading to less soil disturbance. For example, a steep side hill cut is easier with an excavator because less material needs to be moved
- Are superior for back sloping and for working around rocks
- Can easily and effectively put in water control structures. They are ideal for culvert installation
- Great at upgrading existing roads, to load gravel, to extract hard fill, and to work in environmentally sensitive areas such as river crossings
- Are more general purpose than bulldozers.
Bulldozers
- Much more effective at moving material short distances
- More efficient stumping with some exceptions, for example wet areas
- More efficient at levelling the road.
5.8.4 Scrapers
Scrapers are employed where large volumes of subgrade materials have to be transported over longer distances than bulldozers can economically push. Scrapers require specially skilled operators to work efficiently and effectively.