5.3 Highway Policy, Route Location, Drainage, and Earthwork Slopes

5.3 Highway Policy, Route Location, Drainage, and Earthwork Slopes

The AMSTHC TOS area "Highway Engineering" has five competencies:

1. Recall highway policies and administration.
2. Review geometric design standards.
3. Determine route location.
4. Design and construct pavements.
5. Design drainage, foundations, embankments and cut slopes.

Section 5.1 covers geometric design and Section 5.2 covers pavements. This section covers the other three.

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Highway Policy and Administration

Planning and programming cycle. A typical cycle runs through the following stages:

1. Identify needs from traffic counts, road condition surveys and network gaps.
2. Prepare pre-feasibility and feasibility studies, including economic analysis and benefit-cost ratios.
3. Prepare detailed engineering design.
4. Acquire the right-of-way.
5. Procure and construct.
6. Maintain the road throughout its life.

Maintenance categories.

• Routine maintenance includes vegetation control, ditch cleaning and pothole patching.
• Periodic maintenance includes resealing and overlays.
• Rehabilitation and reconstruction restore a pavement that has failed structurally.

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Route Location

Route location balances construction cost, user cost, safety and environmental impact. Surveys move from broad to precise in three stages:

1. Reconnaissance. Study maps, satellite imagery and the terrain to identify feasible corridors and controls. Controls include fixed points such as river crossings and passes, and areas to avoid such as protected areas, unstable slopes and dense settlements.
2. Preliminary survey. Run traverses and levels along the most promising corridors, prepare strip topographic maps, and compare alternatives. Comparison criteria include length, ruling grade, earthwork, number of bridges and cost.
3. Location survey. Stake the final centerline with curves and stationing. Set slope stakes, reference points and benchmarks for construction.

Location principles:

• Keep grades within limits. The ruling grade is the maximum grade adopted for design. Long steep grades slow trucks and may need climbing lanes.
• Balance cut and fill. Mass-haul analysis, covered in Section 4.2, keeps haul distances within the limit of economic haul.
• Prefer good ground. Avoid swamps, landslide-prone slopes and repeated stream crossings. Cross rivers at right angles where the channel is narrow and stable.
• Coordinate horizontal and vertical alignment. Avoid placing sharp horizontal curves at the crests of vertical curves.

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Highway Drainage

Water is the main enemy of pavements: saturated subgrades lose strength, and surface water causes hydroplaning.

Surface runoff by the rational method

For small catchments, typically under about 80 hectares:

$Q = \frac{C\,i\,A}{360}$

• $Q$ = peak runoff ($\text{m}^3/\text{s}$)
• $C$ = runoff coefficient: roughly $0.70$ to $0.95$ for pavement, lower for grass and forest
• $i$ = rainfall intensity ($\text{mm/h}$) for a duration equal to the time of concentration $t_c$
• $A$ = drainage area (hectares)

For mixed surfaces, use the area-weighted coefficient $C = \sum C_j A_j / \sum A_j$.

Example. A $12\text{ ha}$ catchment has $4\text{ ha}$ of pavement ($C = 0.90$) and $8\text{ ha}$ of grass ($C = 0.30$). The design intensity at $t_c$ is $150\text{ mm/h}$.

• Weighted $C = (3.6 + 2.4)/12 = 0.50$.
• $Q = 0.50(150)(12)/360 = 2.50\text{ m}^3/\text{s}$.

Drainage elements

• Cross slope (crown) sheds water to the edges: about 1.5% to 2% for paved lanes and steeper for gravel shoulders.
• Side ditches are sized with Manning's equation (Section 8.4) for the design flow. They are lined where velocities would cause erosion.
• Culverts (pipe or box) carry streams under the road. Their capacity depends on inlet or outlet control, and the headwater must stay below the road edge.
• Subsurface drains (perforated pipes in filter material) lower the water table beneath the pavement. They are critical in cuts and on flat, wet ground.
• Energy dissipators and riprap are placed at culvert outlets to prevent scour.

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Foundations, Embankments and Cut Slopes

Embankment construction

1. Clear and grub the site, and strip topsoil and organic material from the embankment footprint.
2. On sloping ground steeper than about 1V:4H, cut benches so new fill keys into the existing slope.
3. Place fill in horizontal layers, typically about 150 to 200 mm compacted thickness. Compact each layer to the specified relative compaction before placing the next.
4. Do not use unsuitable materials, such as peat, highly plastic clay and organic soil, in the embankment.
5. On soft foundations, consider preloading with vertical drains, staged construction, or lightweight fill. Monitor settlement before paving.

Cut slopes and slope protection

Stable slope angles depend on material and water. Compact granular soils and sound rock stand steeper, while clays, weathered rock and wet ground need flatter slopes or benches.

For the slope-stability calculations themselves (infinite slopes, the Swedish circle, slices), see Section 10.4. For retaining walls, see Section 10.3.