Highway geometric design,
made visual.

Study → consultant designs (survey + ERA standards) → contractor builds → as-built survey checks what was built.

• Every road job has two sides: the consultant (design and supervision) and the contractor (construction).
• A road starts with a study of whether it pays: transport needs, goods movement, economic benefit. Example from class: the old two-lane Addis Ababa–Debre Zeit road could not carry growing traffic, so a new road was needed.
• ERA tenders the design to consultants. Design can take 5–6 years before the job is awarded to a contractor.
• Survey first: GPS control points are monumented in concrete and protected; then benchmarks (BM1, BM2 …) are set, and ground points and cross-section points are picked along the route.
• The designer’s goal: minimum excavation and earthwork — that is what saves the government budget, especially in mountainous northern Ethiopia.

Horizontal alignment = WHERE (location). Vertical alignment = HOW HIGH (elevation). Cross-section = WHAT is built at each station, every 20 m.

• Horizontal alignment (plan view): the route on the map. It tells you the location (easting, northing).
• Vertical alignment (profile): the road’s elevation along its length. It tells you the elevation at every point.
• Cross-section: a slice across the road at a station. It shows the pavement layers and the earthwork.
• A station (chainage) is written km + metres: 0+120 means 120 m from the start.

AADT = Annual Average Daily Traffic. It puts the road in a design class (DC1 … DC8), and the ERA manual then gives the standards for that class.

• How it is counted: a person stands at the road with a form and ticks every vehicle that passes (small car, big truck …).
• What AADT decides: the pavement thickness (together with axle load), the number of lanes (2 or 4), whether the road is paved (asphalt) or gravel, and the design speed.
• Design speed also depends on terrain: flat, rolling, mountainous, escarpment (and urban).
• Axle load is calculated and used to decide the thickness of the road layers.
• A new road with no traffic yet? The government or the Ministry of Transport decides it, e.g. because many goods will move on it and it pays off economically.
• Money back: an expressway repays itself through tolls and can cover its own maintenance. A fuel levy — about 10% when fuel is imported — goes to the Road Fund for maintenance.

Carriageway, shoulder, median, side drain, back slope (cut, goes UP), side slope (fill, goes DOWN) and berm.

Shoulder — why it shrinks in the mountains

• Purposes: room for pedestrians, protection from water (its main benefit is a longer road life), and road users’ comfort.
• ERA standard for DC8: 3 m on flat, 3 m on rolling, only 0.5–2.5 m in mountainous terrain.
• Why: in mountains there are back slopes and side slopes on both sides. The wider you make the shoulder, the more you must dig, and the budget rises fast. So shoulders are reduced in mountainous and escarpment terrain.

Median — think about the future

• Outside towns, medians are made wide (10–15 m). Compensation is paid once for the land (right of way). When traffic grows, a lane is added inside the median without new compensation.
• Right of way is a big problem on road jobs: cemeteries, churches and other places that cannot be touched force the road to change direction.
• In the city, sidewalks are now about 6–7 m (because of the corridor project), for pedestrians and green area.

Get rain off the road fast (crown → ditch). Wet expansive soil swells and shrinks → potholes. Compact at the Optimum Moisture Content (OMC) to 95–99% of lab density.

Expansive soil

Expansive (black-cotton type) clay expands when it gets water and shrinks when it dries. That repeated movement under traffic creates potholes. The fix is drainage: make the water run quickly into the side drain.

Compaction and OMC

• Asphalt roads have a base course and a sub-base. Each layer is compacted at the Optimum Moisture Content (OMC).
• The lecturer’s analogy: it’s like beso (roasted-barley flour) — it only holds together with the right amount of water.
• Target: 95% – 99% of the laboratory (maximum) density.
• If water gets in later, the layer soaks, shrinks when it dries, and sinks under load. Materials engineers use Atterberg limits and the traffic to decide the pavement thickness.

Water on slopes and landslides

• Water density is 1,000 kg/m³. Soil that soaks up water gets heavier, and with load on top it starts to slide (e.g. the landslide that cut the road near Shashemene).
• So a cut-off (“furrow”) ditch is dug on the hill above the cut, to turn the water back before it reaches the slope.

Avoid closely spaced contours. Max gradient in towns is 8% (sometimes 6%). Trucks can’t climb more than about 12%, so steep hills need switchbacks.

• Closely spaced contours = steep ground. Try to avoid them when choosing the alignment.
• If there is a big hill over a short distance: max gradient in town 8%, sometimes 6%.
• Otherwise use switchback curves (S-curves), like the road up Entoto. The lecturer’s example: to climb 1 m you travel 20 m (5%), because vehicles can’t climb more than about 12% safely.

Curve types (know all four)

SSD = distance to see a hazard, react and stop. PSD = distance needed to overtake safely. Both are limited in the ERA manual according to design speed and friction.

• Stopping Sight Distance (SSD): the distance a driver needs to stop, counting the reaction time.
• Passing Sight Distance (PSD): the distance needed to overtake safely (much longer than SSD).
• Both depend on design speed and the coefficient of friction between tyre and road. Friction differs for asphalt vs gravel.
• Both are found in Chapter 7 of the ERA manual.

Crest and sag vertical curves

• Crest (hill top): the curve must be long enough that a driver can see a car coming over the hill in time.
• Sag (valley): at night, headlight light hits the road ahead. The curve is designed with headlight sight distance, so the beam lights enough road and isn’t reflected back into the eyes.

Bigger radius = flatter (closer to a straight line); smaller radius = sharper. Sharp curves need widening and superelevation (tilt) against centrifugal force.

Curve widening

• On a tight curve, a long vehicle leaves its lane and enters the other lane, so the road is widened on curves.
• R > 250 m → no widening needed. R 120–150 m → widen 0.6 m.
• Fill widening: where the fill is higher than 3 m, widen about 30 cm to the side, for psychological reasons — drivers fear the drop.

Transition (spiral) curve

A transition curve (spiral) between the straight (tangent) and the circular curve lets the car enter the bend gradually, not with a sudden turn of the steering wheel.

Superelevation

• Entering a curve, centrifugal force pushes the vehicle outward. Superelevation tilts the road (outer edge raised) to balance it.
• Normal crown slope = 2.5% (both sides falling away from the centre) — for drainage.
• Maximum superelevation = 8% (rural, in Ethiopia) and 4% in towns.
• The length over which the outer edge goes from −2.5% to +8% is the run-off length (L).

The main cost of road building is earthwork, not pavement. Balancing cut and fill is the designer’s biggest job.

• Even 8% superelevation, which looks small, makes the side slope much longer on a high fill. That increases both fill and cut quantities.
• Fill is placed and compacted in 20 cm layers, each tested. So 3 m of fill = 15 tests.
• Quantities come from cross-sections (every 20 m). End-area method: V = (A₁ + A₂) / 2 × distance.

Use a retaining wall when a slope would run too far or space is tight. Masonry up to 8–10 m, concrete above. Weep holes are a MUST.

• When: e.g. in the Abay gorge one side drops away. A normal 3:1 fill slope would never reach the ground, so a retaining wall is chosen.
• Masonry (stone) walls up to about 8–10 m. Higher → concrete shear wall.
• In the city there is no space for side slopes, so retaining walls save space (and look good).
• Cut bench: in high cuts in a mountain, a bench with a berm is made about every 10 m.
• Fill on sloping ground: new soil won’t bond to sloping ground, and vehicles add a surcharge load that pushes it off. So steps (benches) are dug about every 3 m, and the roller compacts step by step.

Flexible = asphalt (layers, flexes, expands/shrinks). Rigid = concrete slab (heavy loads, but joints every ~5 m make the ride bumpy).

The hydrologist uses Q = C·I·A and Manning’s formula, then decides culvert, bridge or ford. The materials engineer samples every 500 m (CBR, Atterberg limits, PI).

Hydrology

• Rational formula Q = C · I · A — C = runoff coefficient (surface type), I = rainfall intensity, A = catchment area.
• Manning’s formula is then used to set the ditch width and depth.
• The hydrologist decides whether a culvert, bridge or ford is needed.

Materials

• The materials engineer takes a sample about every 500 m and tests CBR (California Bearing Ratio), Atterberg limits and the Plasticity Index.
• If the soil is weak, capping material is added, or about 1 m is excavated and replaced.

Power, telecom and ICT go under the sidewalk/shoulder. The sewer is the exception: deep (3–4 m) and under the carriageway.

Clean the data in Excel → import the points → create the surface (max triangle length 70) → contours → polyline from CL points → alignment → apply the ERA design criteria → fix the warnings by increasing the radius.

1. Excel: fix the survey data. Each point has Northing, Easting, Elevation, Description.
2. Import points into Civil 3D and create a surface. Set maximum triangle length = 70 m.
3. Contours: if they are too crowded, edit the style: 0.25 m minor, 1.25 m major.
4. Centreline: filter only the CL points. In Excel: =CONCATENATE(Easting, ",", Northing) → copy → in Civil 3D type PL (polyline) and paste.
5. The surveyor’s CL may come out zigzag. The designer looks at the contours and chooses the better path.
6. Alignment → Create Alignment from Objects → pick the polyline → name it “CL”.
7. Design Criteria tab → tick Use criteria-based design → design speed 50 km/h → load the ERA design criteria file (XML) → e max 8%, 2-lane → OK.
8. Yellow warning symbols appear = a curve is outside the design standard: the radius is too small, or the tangents overlap.
9. Fix: Alignment → Alignment Creation Tools → Alignment Grid View → increase the radius (e.g. 50 → 80 or 100) → the warning disappears.

Never triangulate raw data. Remove wrong codes (e.g. “RB”), save clean CSV, triangulate with a sensible max length (class: 70), and press Apply.

• The RB problem: the as-built file mixed wall-top points (≈2386 m) with points coded RB at a different level (wall bottom). Triangulated together, the triangles link the two levels and give in-between, wrong elevations. So filter RB out and keep it separately.
• A survey file must always come with a legend that explains its codes. Without one, you work out the codes from the values.
• An Excel filter only HIDES rows — they are still data. Delete them, or copy the clean rows to a new file.
• Save as CSV (comma delimited). A CSV keeps only the active sheet; an .xlsx workbook keeps all sheets.
• Maximum triangle length depends on the point spacing. Dense data: 10–30 (e.g. points every 5 m → about 20–30 for OGL, 15–20 for RL). The class used 70 for all four surfaces.
• Triangulate after all imports, and press Apply — nothing happens until you do. The triangle count that appears confirms it worked.

InRoads (Bentley) runs on MicroStation and saves every item as its own file. Open the .xin first; save separately, then bundle them in a .rwk project.

Make 4 surfaces → import CSV with the Text Import Wizard → triangulate → set symbology → horizontal alignment from graphics → two vertical alignments from surfaces (VL Top, VL Bottom).

The four surfaces ★

1. File → Open → civil.xin (if it isn’t listed, change “files of type” to Preferences / All Files).
2. File → New → Surface — create OGL, As-built, Rock top, RL (just the names, Apply each).
3. File → Text Import Wizard — import type must be Surface (the same wizard also imports alignments!). Browse with “…”, choose the start row (skip headers), comma delimited. Every column says Skip: right-click to set Northing, Easting, Elevation (description optional). On the last page pick the target surface and a Seed Name. A report shows the point count and min/max elevation.
4. Surface → Triangulate Surface — max length 70 → Apply. Check the triangle count.
5. Symbology (Surface Properties → More → Advanced): OGL → Existing Ground, As-built → As-built, Rock top → Rock top, RL → Reduced Level (red). Apply for each surface. Each surface then shows in its own colour on profiles and cross-sections.
6. File → Import → Geometry → From Graphics — type = Horizontal Alignment, name it (e.g. “Center” / “Masonry”) → Apply → pick the line at the end where 0+000 should be → accept (right-click). Check with Geometry → Review Horizontal.
7. File → Import → Geometry → Vertical from Surface (with the horizontal alignment selected): As-built → VL Top; RL → VL Bottom. “Successful completion” appears. View with Evaluation → Profile → Create Profile.
8. Save each surface (.dtm), the geometry (.alg), then the project (.rwk).

Components are closed shapes of KNOWN size (wall, concrete). End conditions SEARCH for a surface (earthwork). Constrain every point except the two that ride the alignments.

Building it (Modeler → Create Template)

1. Template library: right-click → New → Folder (e.g. “Training”/“Class”) → right-click → New → Template. Start fresh for each project.
2. Right-click → Add New Component → Constrained. Click the points, right-click → Finish (Close Shape). The click order doesn’t matter, but the first point has no constraint, so put it at the bottom.
3. Name and style FIRST — double-click the component: Name + Style (Lean Concrete, Cyclopean, Retaining Wall), with Closed Shape ticked.
4. Double-click each point → set Horizontal and Vertical constraints to a parent point (negative = left or down). A Slope constraint keeps the battered face — the software computes the %.
5. Set the two control points (top and bottom) to None / None so the alignments can drive them.
6. Right-click → Change Template Origin → click the top point (→ Set Dynamic Origin). For a new road the origin is at 0,0 on the centre line.
7. Right-click → Add New Component → End Condition, name and style “Cut”: segment 1 = 0.5 m horizontal, Check for Interception OFF; segment 2 = vertical, interception ON, End Condition is Infinite ✓. Mirror for the other side (−0.5).
8. Test → Draw a trial ground (e.g. a 10% slope) to see the end conditions work. Save the template library.

Corridor = horizontal + vertical + template. Active surface = OGL. Drop the template every 5 m, tie the bottom point to VL Bottom, create the design surface, then cut cross-sections.

1. Modeler → Roadway Designer — it combines the three separate pieces: horizontal, vertical, typical section.
2. Corridor → Corridor Management — name the corridor, horizontal = your alignment, vertical = VL Top (because the origin is at the top) → Add → Close.
3. Active Surface = OGL — not the as-built. The as-built only gives the wall top; volumes are measured from the OGL.
4. Corridor → Template Drops — interval 5 m (the wall is about 50 m long; 10, 20 or 25 m are other options), pick the template → Add. If an end condition doesn’t appear, it found no OGL on that side.
5. Corridor → Point Controls — point = the bottom (lean-concrete) point, mode = Vertical, controlled by vertical alignment VL Bottom → Add. The lecturer: “the main purpose we came for”.
6. Template drop shows red? → Synchronize with Library.
7. Corridor → Create Surface — name “design”, tick Empty Design Surface and Triangulate → Apply. Save the roadway (.ird) and template library (.itl) separately.
8. Evaluation → Cross Section → Create Cross Section — set each surface’s style (design → “use features”, OGL → Existing Ground, As-built, Rock top, RL), interval 5 m, and left/right offsets wide enough (too small and the section is clipped).

Answer, read the explanation, move on. Use Retry wrong ones at the end to drill your mistakes.

AADT

Annual Average Daily Traffic — average vehicles per day; decides the design class.

Design class (DC1–DC8)

ERA traffic-based road class; DC8 is the highest (10,000–15,000 AADT).

Carriageway

The part of the road vehicles drive on.

Shoulder

Strip beside the carriageway; pedestrians, stopped vehicles, edge protection.

Median

Strip separating opposite directions; wide outside towns for future lanes.

Back slope / side slope

Cut slope going up to ground / fill slope going down to ground.

Berm / bench

Flat step on a high slope; breaks the slope, diverts water.

OMC

Optimum Moisture Content — water content giving maximum compacted density.

Expansive soil

Clay that swells when wet and shrinks when dry; causes potholes.

SSD / PSD

Stopping / Passing Sight Distance.

Crest / sag curve

Vertical curve over a hill top / through a valley.

Superelevation

Banking of the road on a curve; max 8% rural, 4% urban.

Run-off length (L)

Length over which the outer edge goes from −2.5% to full superelevation.

Transition (spiral) curve

Gradual curve between tangent and circular curve.

Curve widening

Extra width on tight curves (none if R > 250 m).

PI / PC / PT

Point of Intersection of tangents / start of curve / end of curve.

PVI

Point of Vertical Intersection (vertical alignment).

Station (chainage)

Distance along the alignment, e.g. 0+120.

Benchmark (BM)

Surveyed reference point of known elevation.

OGL / OG

Original Ground Level — natural surface before works.

As-built

Survey of what was actually constructed.

RL

Reduced Level — here the bottom / founding level surface.

TIN / triangulate

Surface made by joining survey points into triangles.

Seed name

Name given to an imported batch of points so it can be filtered or deleted later.

Symbology

Colour / line style of a surface or element in plan, profile, section.

Template

Typical cross-section applied along an alignment.

Component

Closed shape in a template with known size (wall, concrete, pavement).

End condition

Open template part that seeks a target surface (earthwork).

Check for interception

Lets an end-condition segment stop where it meets the surface.

Template origin

Point that rides the horizontal + vertical alignment.

Corridor

3D model = alignment + template (+ controls) for one site or segment.

Template drop

Placing the template along the corridor at an interval (5 m in class).

Point control

Forces a template point to follow another alignment (bottom → VL Bottom).

Active surface

The surface end conditions target — OGL.

End-area method

Volume = (A₁ + A₂) / 2 × distance between sections.

CBR

California Bearing Ratio — strength test of soil.

Atterberg limits / PI

Moisture limits of clay behaviour / Plasticity Index.

Capping material

Better-quality layer placed over weak subgrade.

Weep hole

Drain hole through a retaining wall that releases water pressure.

Surcharge load

Extra load on top of soil (e.g. traffic) that pushes on slopes and walls.

Lean / cyclopean concrete

Thin blinding layer / concrete with large stones (wall footing).