The following is a basic technique to mark the proposed grade line, also called the P-Line, in relatively easy topography that has uniform cross slopes, and rolling topography, where a benched side-cut road formation is the likely construction method. On steep broken country, or areas requiring more complex roading such as block cuts or switch backs, the grade line or P-Line needs to be more carefully laid out so that the final road location reflects the actual plan. The grade line is then augmented with more detailed survey information to create a corridor within which the road will be designed (see section 3.10).
As per the paper plan, normally more than one trial grade would need to be run to coincide with the control points to identify the best alternative. Grade lines can be set with an Abney level or clinometer. Clinometers are the most common instrument, but they can be difficult to sight with in low light levels. It is essential to check the calibration of the instrument in case it has been damaged, and especially the units used; most clinometers will show the angle in both percent and degrees.
Marking a proposed grade line in the field is done by placing flagging, also called marking tape, on trees or bushes at eye level approximately 20 m apart. Flagging is preferred to paint as it is easier to see and adjust if an alternative line is decided upon. The distance between stations generally depends on the line of vision. In thicker scrub, flagging may only be 10 m apart, or foliage may need to be removed with a slasher.
Marking a grade line with two people
Source: Walbridge, 1997Marking out the grade line is more accurate, faster and safer if done by a two-person team (see the adjacent illustration). With a two-person team, the person with Abney level starts by setting a ribbon at eye height on a tree or bush, sending the second person ahead about 20 m in-line. The Abney holder directs the second person to stand next to a tree which is nearly ‘on grade’. This person holds out the ribbon at the eye height of the Abney holder and, as directed, moves up or down the side slope until ‘on grade’. A ribbon is attached to a nearby tree or bush at this elevation. This procedure is repeated until the start and finish control points have been joined.
If only one person is available, grade lines can be set by pacing ahead of a grade mark, turning around, and then moving up and down the side slope until the grade is met. At this point, another grade mark is set and the procedure is repeated until the desired control point is reached.
Difficulties in maintaining a gradient can occur when:
- Adjusting the grade line to accommodate intermediate control point(s)
- Crossing a steep side gully
- Going around a long sharp ridge or requiring to block cut through it
- Continuing around a switchback.
Roadline salvage operations, where the trees are removed before road construction, are likely to remove or disturb the marking tape and grade lines may need to be re-established.