5.3 Marking clearing widths

Well-marked road clearing limitsThe clearing limits were not marked wide enoughGetting the right roadline clearing width is important. Too little, and the road construction team does not have enough room to establish the cut and fill slopes. This may lead to trees being buried or more expensive construction. If the clearing widths are too wide, it may lead to windthrow or increase the harvest setting’s logging rate. The clearing distance is the horizontal distance, so on steep slopes what appears to be a significant clearing width can ultimately be too narrow.

There are two ways of setting out the clearing width – an important first step in the road construction process. It sets the scene for the following operational steps, as it helps define the working area for the construction crew. The first and predominant way is to mark out the road centreline, or grade line, and specify a distance to each side to define the clearing width. The second is clearly defining the extent of the clearing width by pegging the outermost extent of it on either side at regular intervals.

The centreline marking approach is widely used in New Zealand. It relies on the principle of operator ‘knows best’. This method generally works well, and is best suited if you are constructing in easy to moderate country and have experienced crews and supervisors. The advantage is that planning and layout is quicker and easier.

Marking a grade lineSource: Walbridge, 1997Where unexpected soil material, drainage or stability issues are encountered, or construction problems arise, either improve operator skills through training or start pegging out. Use experienced earthworks operators in steep, challenging country where centreline and salvage limits are not marked. Ensure supervisors regularly visit the job. Sole reliance on a marked grade line can potentially lead to problems during construction. For example, the engineer could underestimate the location of the centreline in relation to the grade line, the road line salvage crew could take out too few or too many trees in the clearing width, and the extent of the earthworks could be under or overestimated.

The second approach – clearly defining the extent of the clearing width by pegging the outermost extent of it on either side at regular intervals – is carried out for roads that have a full design road plan (see Chapter 4), and the top of the cut and the bottom of the fill from the proposed road location can be identified. Another option, if using a modern mechanised felling plant, is to use the electronic tools available to set the boundary.

5.3.1 Determining clearing widths for non-surveyed roads

In order to determine the extent of the road, the road centreline needs to be marked. From the centerline, the road profile can be estimated or marked out; this defines the upper and lower limits of the roadway. As a rule of thumb, allow at least 3 m beyond the upper and lower limits for contingency. This becomes the clearing width. Major construction issues result if too few trees are marked out. Allow more downslope clearing limits as the terrain steepens, and where benching is required.

Re-establishing the grade line after road line salvage using taped branches as markersThere is no standard approach to marking clearing widths to help the roadline salvage crew remove the appropriate width of trees. Some companies mark the roadline width with the trees that are marked to remain, but some rely on the skill and experience of the crews, and the supervision of them, to do a good job.

Marking the boundary has several advantages – the planner determines the width; the contractor removes the minimum number of trees, and the operational control may not need to be as close. These advantages can reduce the cost of road line harvesting and road construction. However, marking out does take time and good crews quickly learn what is required if road line salvage limits are not marked. Another option, where available, is to use the electronic tools in the felling plant to set the boundary.

On gentle to rolling country, the centreline typically follows near to the grade line. On steep and bisected country, the grade line may not be representative of the road centreline at all. This is especially true in country with lots of small gullies and ridges due to the grade line not physically being able to follow the actual road alignment.

The grade line location is marked at the finished road level, and is not the same as the road centerline.

Estimating clearing widths for cut and fill cross sections

A typical cross-section layoutFor some sections of a non-surveyed road, it may be prudent to peg out the extent of the cut and fill slopes. This will enable the machinery operator to better visualise the finished road and use the marker pegs to more effectively construct the road segment. The adjacent diagrams show a typical cross-sectional layout, and the equation to calculate the width. Ideally, the engineer would use road design software to determine this, rather than the slow and less flexible manual approach and calculations.

Where:

A = Offset distance along natural ground on uphill or cut side in metres

B = Offset distance along natural ground on downhill or fill side in metres

U = Depth of cut from the top of batter in metres

T = Depth of fill from the toe of fill in metres

c = The cut slope in degrees. The appropriate value is dependent on the stable angle of repose for the cut slope material.

f = The fill slope in degrees. The appropriate value is dependent on the stable angle of repose for the fill slope material. The value selected for f cannot be less than or equal to the value for s, as the fill slope and side slope will not converge, creating an infinite fill slope

s = The side slope in degrees for the road cross section. This value is taken from survey data

W1 = The road formation width, in metres, that will be constructed on the cut bench. Forest roads constructed on steep side slopes (angles greater than 20%) should have the majority of the road formation constructed on the cut bench to limit the risk of fill collapse under heavy vehicle loads

W2 = The road formation width, in metres, that will be constructed on the fill slope.

The total road width is W1+ W2.

Note: These equations need to be converted from degrees into radians to use in Excel. For example, variables c, s and f and the 180⁰ constant all need to be converted to radians.

Large volumes of material need to be shifted when roads are constructed on steep side slopes. To limit the quantity of earthworks required in steep country, a steeper cut slope (C%) can be made. The following table shows the advantages and disadvantages of a steep cut slope.

Advantages and disadvantages of a steep cut slope

Setting out a road using pegsSource: Walbridge, 1997

Steep cut slopeA very high batter with a steep cut slope. While this slope may be susceptible to some slumping and raveling over time, the alternative of cutting the slope back would result in a much greater road prismThe top of the batter and the toe of fill can be located by measuring the ground slope in the field using a clinometer, and then measuring out the appropriate slope distance. Pegs are then used to mark these positions. See above.

Setting out a road section

The following examples show the setting out for a 6.6 m road cross section. The first is on the gentle side of a slope, the second is on a slope that requires full bench construction. A cut slope of 200% and a fill slope of 70% is used for these examples.

As the slope steepens and approaches the natural angle of repose for the fill, the road needs to be full bench constructed, as shown in the figure below.

The equations to calculate the offsets and cut and fill depth can be used, together with the road width and slope angle assumptions, to develop a table that shows appropriate set-out data. The lookup table can then be used when setting out the road formation in the field. A table for this example is shown below.

Note that the table is not appropriate for setting out block cuttings or all fill cross sections. These require different calculations, and road design software packages are a better option to determine these clearing widths.

Road cross section on a 40% slopeGeneral set out of a 6.6 m road cross section with side slope < 40% (Source: Walbridge, 1997)

Road cross section on a 60% slopeSet-out information for a 6.6 m road cross section with a side slope of 60% (Source: Walbridge, 1997)

5.3.2 Determining clearing widths for surveyed roads

Offset for formation width (W) 6.6m Cut slope = 200% Fill slope = 70Set-out data for a cut and fill formation width of 6.6 metresThe previous section provided tools that help estimate the extent of cuts and fills so that an acceptable clearing width could be calculated. For roads that have been surveyed and geometrically designed, the set-out widths and the extent of the earthworks have already been determined. This leaves pegging the road boundary at each of the survey points.

The logging crew should be instructed to remove all the trees contained within the pegs. After the trees have been removed the pegs should be checked to ensure they are still in place. The pegs determine the width for the bulldozer or excavator to clear all organic matter.

Clearing for earthworksLeft: Trees that are to be cleared for earthworks. Right: The width to be cleared of all organic material for earthworks (Source: Walbridge, 1997)