GENEraL CONtrOLS FOr VErtICaL aLIGNMENt

7.2.1 Sight distance

Adequate sight distance is the most important characteristic of a road. The vertical alignment must be such that at least stopping sight distance is available at every point along the road.

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This has particular reference to crest curves. It also applies to sag curves where these occur on roads without the benefit of lighting. Desirably, crest curves should be designed to pro- vide passing sight distance but, in most case, this would probably prove to be unaffordable. The selection of values of curvature to ensure adequate sight distance is discussed in Section 7.5.

7.2.2 topography

Although the topography has an impact on the horizontal alignment of the road, this is largely as a result of an unfavourable effect on the vertical alignment and the need to avoid excessive gradients. The effect on the vertical alignment is even more marked than that on the horizontal alignment. In general, the topography is defined in three categories: flat, roll- ing and mountainous. The question most frequently asked is: How does one differentiate between flat, rolling or mountainous terrain? And the answer is to be found in truck per- formance. If a truck can maintain a fairly steady speed, the topography is described as flat. If truck speed reductions by 15 to 20 km/h are frequent or of extended duration, the topog- raphy is described as rolling. Trucks operating at crawl speed either at frequent intervals or over extended distances cause the topography to qualify for the appellation ‘mountainous’.

This is a somewhat circular definition because it is related to the speed at which trucks can move along the road and not directly to the topography itself. Given an unlimited bud- get, a road could be constructed to be dead flat regardless of the mountainous nature of the topography being traversed. The principal control is what constitutes a reasonable outlay for construction and maintenance cost plus road user cost.

7.2.3 Earthworks quantities and the mass-haul diagram

Construction cost is directly affected by the vertical alignment in relation to the natural topography as this defines the height of fills and the depth of cuts and hence the quantities of material that have to be moved. The calculation of quantities is invariably undertaken using commercially available software. Historically, it was a laborious process of plot- ting of cross-sections at some or other fixed interval and measuring of average end areas by planimeter. This, understandably, was not popular with young engineers. Designers should be aware of the intrinsic limits of accuracy due to variations of the terrain between surveyed cross-sections and variations from a truly prismoidal shape. The calculated vol- ume may vary from the actual volume by 5 per cent to 10 per cent from these factors alone (Austroads, 2009a).

The mass-haul diagram shows the volume of material, either cut or fill, available at any point along the road. The standard notation is that cut represents a ‘gain’ of material for use, that is, positive, and fill a ‘loss’ of material, that is, negative. The mass-haul diagram shows the transition from net cut to net fill and hence the direction in which material needs to be moved indicating also the lead or haulage distance involved. Shortfalls in material indicate where additional material has to be sought and hence the possible location of borrow pits. These locations are required because it does not follow that suitable borrow material will always be available where desired. The haul distance from the borrow pits to the road will have to be added to the mass-haul diagram. Excess material requires that spoil areas be located. In rural areas, spoil material could perhaps be used in the construction of farm dams or other beneficial environmental features as compared to merely being waste tips.

A typical mass haul diagram is illustrated in Figure 7.1. It show the gradeline and, below it, the volumes of material to be moved and the direction of movement.

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Construction of the earthworks requires the provision of a stable base for the fill and the pavement in cuts. In the calculation of quantities, it is thus necessary to establish the quantities of material such as topsoil and other material not suitable for construction. This material should be removed as part of the site clearing exercise and carefully stockpiled for later use such as the trimming of cut slopes and the like. It will obviously not feature in the derivation of the mass-haul diagram. Factors that do affect the earthwork quantities include

• The compaction factor, that is, the ratio in volume between one cubic metre of in situ material and the same material after placement and compaction. Cohesive materials commonly occupy less volume after compaction, whereas rock may occupy more vol- ume after excavation and placement.

• Material that is unsuitable for embankment construction including • Large boulders

• Excavated hard rock which may be uneconomical to crush to a size that can be compacted

• Any unstable or expansive material to be carted to waste

• Flattening of embankment batter slopes outside stability limits to provide for safety and maintenance requirements.

• Photogrammetric bias; it is preferable that ground survey be used for detailed design, in order to eliminate this factor and improve accuracy.

The quantities of cut include considerations of material type being hard rock, which requires blasting, soft rock, which can be ripped, and soft material, which can be picked up by a front-end loader without prior processing. The nature of the material has an obvious impact on the costs of excavation and subsequent compaction. As stated previously, when in

situ material is disturbed and loaded, it bulks and, when placed in the fill, it is compacted.

Bulking and compaction factors have to be estimated and allowed for in the achieving of a

0+00 1+00 2+00 3+00 4+00 5+00 6+00 7+00 8+00 9+00 Profile Balance point Bo rr ow Bo rr ow Proposed road 200° Grade point Cut Fill Grade point Cut Fill 2000 1500 1000 500 Existi ng su rface

Free haul distance as measured from grade point –500 –1000 –1500 –2000 112 108 104 100 92 88 0 Wa st e Wa st e Wa st e

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balance between cut and fill (Queensland Department of Transport and Main Roads, 2002). This information is presented on the mass-haul diagram.

Balancing the earthworks is often understood to be a volumetric balance. This is not entirely correct or even desirable. Drilling and blasting to achieve a hypothetical balance with adjacent fill volumes is significantly more expensive than raising the gradeline over the rock layer and opening a borrow pit where the shortfall in material occurs. On the other hand, it may be found that the rock layer is ideally suited for use as a crusher-run base course, chippings for bituminous surfacing or concrete aggregate. The gradeline is then adjusted accordingly but the additional material so gained is not included in the calculation of the mass haul diagram, which is exclusively related to the construction of the bulk cut and fill of the earthworks.

Side cut, where part of the cross-section is in cut (as opposed to a box cut) and part in fill, can lead to distortions of the mass-haul diagram. This is because it is theoretically, but only theoretically, possible to move material directly across the cross-section from the cut side to the fill side. About the only device that can achieve this operation is a drag-line scoop. This type of plant tends to be very large and confined to opencast mining. It is not likely to be found on a normal road construction site. A bulldozer acquires and spreads its load by mov- ing approximately longitudinally down the road. At the end of its run, it has to turn around and repeat the operation in the reverse direction. This is for two reasons:

• The elimination of dead haul when the bulldozer is travelling without a load • To even the wear on the steering clutches of the tractor

7.2.4 the critical length of grade

The speed reductions forced on trucks by the gradient of the road have safety implications as it has been found that the crash rate increases exponentially with increases in speed dif- ferentials. The safety slogan ‘Speed kills’ is not entirely correct; it is the speed differential that kills. It has been found that, with a speed reduction of 15 km/h or less, the crash rate is between 1 and 5 crashes per million kilometres of travel, whereas with a doubling of the speed differential, the crash rate increases to about 21 crashes per million kilometres.

As a general rule, a speed reduction of 15 km/h is sufficient to warrant the addition of a climbing lane to the cross-section. Reference is made to the critical length of grade, which is the distance that a truck will travel before its speed has reduced by 15 km/h. Climbing lanes are discussed in Chapter 8.

The relationship between speed and gradient is illustrated in Figure 7.2. This figure assumes a starting speed of 100 km/h. If the upgrade is preceded by a downgrade, truck drivers will accelerate to get the benefit of a higher entry speed. The vertical axis of the graph could thus be moved down to illustrate a higher entry speed, which should not, in any event, exceed about an additional 10 to 15 km/h. These speeds are hypothesized on a loaded truck with a mass to power ratio of 120 kg/kW, which may be considered to be the design vehicle for performance on a gradient.

7.2.5 aesthetics

The profile cannot be designed in isolation. It has to be coordinated with the horizontal alignment to form a harmonious whole. In Chapter 9, reference is made to internal and external harmony. The coordination of the horizontal alignment and the vertical alignment forms part of the internal harmony, the abstract ribbon in space concept, which is sought. This is dealt with in detail in Chapter 9.

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