2026 bands by treatment class, per kilometre of 7-metre carriageway: routine grading and shaping of gravelled roads — the grader-and-water-bowser pass that restores camber and clears silt — at KES 80,000–200,000 per pass, typically two to four passes a year on active rural roads. Murram restoration — regravelling thin, rutted surfaces with 50–100mm of fresh material — at KES 300,000–800,000 per km depending on material haul distance, which dominates the arithmetic. Pothole repair programmes on paved roads — cut, clean, tack, fill, compact — at KES 150,000–400,000 per km for scattered repairs; continuous distressed lengths re-price as resurfacing. Resealing/surface dressing — the preventive coat that doubles asphalt life — at KES 400,000–900,000 per km. The economics that decide the budget: preventive maintenance costs a tenth of reconstruction — a road resealed on schedule never needs the KES 8M–15M per km rebuild that a starved one demands within a decade. The calculator prices any of these against your network.

The formation chain is always the same; the speed varies with traffic and rain. It starts beneath the surface: water enters through unsealed cracks — every unsealed crack is a funnel. The water softens the base and subgrade, or pumps them into suspension under passing wheels. Traffic loads then flex the weakened surface until it fatigue-cracks; the cracks widen, more water enters, and freeze-thaw is irrelevant here — Kenya's wet-season thermal cycling and millions of wheel passes do the same work. Chunks work loose under tyres — the pothole is born — and it grows geometrically: each passing wheel hammers the edges, ejects material, deepens the cavity. The aggravators that speed the chain: poor original compaction leaving a soft base; missing or blocked drainage letting water stand; overloaded trucks flexing surfaces beyond design; and deferred maintenance letting the first crack become the first crater. The engineering corollary: potholes are a drainage and maintenance failure before they are a surface failure — which is why the cheapest pothole programme is the one that keeps water off the road and seals the cracks while they are still cracks.

The permanent repair sequence — the one that outlasts the next rains — in six steps. Cut: saw or jackhammer the pothole into a rectangular geometry with vertical edges, extending into sound material around the perimeter — feathered patches fail at their feather edge. Clean: remove all loose material, dust and water — compressed air and wire brush; a dirty, wet hole guarantees the patch peels. Tack: coat the faces and base with bituminous tack coat — the bond between old and new that stops water migrating along the interface. Fill: hot asphalt mix placed and compacted in lifts if deeper than about 75mm — cold mix serves for small emergency repairs but is the temporary answer by design. Compact: plate or roller compaction to refusal — density is the repair; under-compacted patches compact under traffic and re-sink. Seal: the perimeter sealed flush so no water re-enters. The throw-and-go method — cold mix dumped and driven over — costs half and lasts weeks; the six-step method costs double and lasts years. County programmes that batch potholes by ward and run gangs systematically beat reactive one-at-a-time repairs on every metric: cost per hole, lifespan per patch, and complaints per kilometre.

Murram restoration re-lays the wearing surface of gravelled roads: fresh lateritic gravel spread, shaped to camber, watered and compacted over the existing formation. It is needed when the surface has worn or been lost beyond what grading can recover — ruts deeper than the grader can cut and fill, corrugation that returns within days of grading, gravel displaced to the shoulders and drains, or a surface so thin the subgrade shows. The method: grade and shape the existing surface first, correcting camber and clearing silted drains; then place 50–100mm of fresh material in compacted lifts — material specification matters, a plasticity index below about 12% and a proper gradation, because bad murram turns to grease in the rains; water to optimum moisture and compact to 95%+ Proctor; finish with a tight, slightly proud surface that traffic beds in. Frequency on active rural roads: every two to four years, with grading between. Costed 2026: KES 300,000–800,000 per km, haul distance the dominant variable — a county pit twenty kilometres away can double the material line. The alternative arithmetic: a murram road maintained on schedule costs a fraction of the paved equivalent per service year, which is why the murram-versus-asphalt decision remains Kenya's standing infrastructure debate.

By traffic and climate, not the calendar alone — but the calendar disciplines the programme. Active rural roads carrying daily traffic: grading every two to four months — the grader pass that restores camber, cuts corrugation, re-shoulders and clears silted drains. Low-traffic ranch and access roads: once or twice a year. After every significant storm event: an inspection pass, because blocked drains and scoured shoulders are the pothole seeds of next season. The grading method that makes each pass count: scarify first where the surface has set; cut high spots and fill lows across the full width, not just the wheel tracks — wheel-track-only grading creates a bathtub that ponds water and destroys the road faster than no grading; restore camber to the 2.5–4% crossfall; re-compact the loosened material; and clear every drain and culvert on the run. The fleet is modest — a motor grader, water bowser and roller — and the economics decisive: a KES 150,000 grading pass every quarter protects a KES 3M-per-km asset; the same road starved of grading needs full murram restoration at five times the annual cost. Maintenance is the cheapest construction Kenya never budgets for.