Forest Road Engineering Guidebook - Ministry of Forests
Forest Road Engineering Guidebook - Ministry of Forests
Forest Road Engineering Guidebook - Ministry of Forests
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<strong>Forest</strong> <strong>Road</strong> <strong>Engineering</strong> <strong>Guidebook</strong><br />
108<br />
The convention for specifying the opening size (inside measurements) <strong>of</strong> a<br />
log culvert is height (vertical distance between the deepest point along the<br />
channel floor and the s<strong>of</strong>fit <strong>of</strong> the stringers) followed by width (horizontal<br />
distance measured at right angles between the inside face <strong>of</strong> the sill logs).<br />
Log culvert length<br />
Culvert length, as measured in the direction <strong>of</strong> the stream or watercourse, is<br />
determined by considering the following:<br />
• road width<br />
• depth <strong>of</strong> road fill over the log culvert, and fill slope angles<br />
• type <strong>of</strong> fill over the log culvert<br />
• inlet and outlet treatments<br />
• culvert gradient<br />
• culvert skew.<br />
<strong>Road</strong> width<br />
If the culvert is located within a horizontal curve, extra road width to accommodate<br />
side tracking <strong>of</strong> logging trucks and hence additional culvert length is<br />
required. The required width can be found in Table 2 in Chapter 1, “<strong>Road</strong><br />
Layout and Design.”<br />
Depth <strong>of</strong> road fill<br />
Use <strong>of</strong> deep road fills on top <strong>of</strong> the log stringers is not recommended. Should<br />
a failure occur, there is a potential for large volumes <strong>of</strong> sediment-producing<br />
materials to enter the stream channel. Measures to reduce road fill depth<br />
include:<br />
• use <strong>of</strong> a longer span culvert, or a bridge for V-shaped channels<br />
• increasing the culvert height, by use <strong>of</strong> log cribs<br />
• relocation <strong>of</strong> the road to a more suitable crossing.<br />
Log culverts requiring road fill depths greater than 2 m should be reviewed<br />
by a pr<strong>of</strong>essional engineer, and deactivation plans should address fill removal<br />
measures that prevent material from entering the stream’s wetted perimeter.<br />
Log culverts should be sufficiently long to contain the fill and prevent material<br />
from entering the stream. As the fill height increases for a given roadway<br />
width, so should the culvert length.<br />
<strong>Road</strong> fill containment measures should be incorporated into the design. This<br />
can be achieved by making the culvert extra long (at least 1 m per side<br />
beyond the toe <strong>of</strong> the road fill), and securing a large containment log (at least<br />
400 mm diameter) at the toe. See Figure 20.
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