C-DRONE GUIDE · 5 SEPTEMBER 2026

Drone monitoring of coal spoil heaps: combustion, erosion and slope stability

A coal spoil heap ("terril") is never just a pile of rubble. In the Nord-Pas-de-Calais coalfield, fifty-one of them are among the assets inscribed since 2012 on the UNESCO World Heritage list, alongside pit headframes and mining towns. But beneath a silhouette that has become part of the landscape's identity, some spoil heaps are still burning slowly, sometimes for decades, through self-heating of their residual carbonaceous material. Others move: slope erosion, localised slides, and vegetation that either recolonises the surface or, on the contrary, signals ground still too hot to settle. Monitoring these structures falls under a precise post-mining framework overseen by the French State. Here is what it requires, what a drone survey adds, and what it does not replace.

Published on 5 September 2026, reviewed on 11 September 2026 — regulations in force as of September 2026.

A protected heritage, an unprotected risk

On 30 June 2012, UNESCO inscribed the Nord-Pas-de-Calais coalfield on the World Heritage list as an "evolving cultural landscape": 353 elements across 109 sites, including 51 spoil heaps, pit headframes, mining towns and former mineral railway tracks. That recognition creates a duty to preserve the landscape, which coexists with, but does not replace, the management of residual post-mining risk.

That second strand answers to a separate legal framework: the law n° 99-245 of 30 March 1999, France's "post-mining law", created Mining Risk Prevention Plans (PPRM), now codified under article L.174-5 of the mining code and linked to articles L.562-1 to L.562-7 of the environmental code. On the ground, GEODERIS — a public interest group formed by the State, BRGM and INERIS — carries out post-mining expertise on the State's behalf: monitoring structures, issuing technical opinions, and publishing notices on residual hazards linked to shafts and spoil heaps. Between 2009 and 2012, GEODERIS ran a national inventory of mining waste deposits under EU directive 2006/21/CE. For a municipality or an inter-municipal body in the coalfield, this double constraint — preserving a listed landscape while documenting a hazard that no protective listing shields against — directly shapes the choice of monitoring method: a non-intrusive tool that alters neither the ground nor the structure's silhouette.

Spontaneous combustion: what a thermal flight reveals

A spoil heap is not an inert mound of earth: it holds a variable share of residual carbonaceous material — unrecovered coal, colliery shale, traces of hydrocarbons — capable of spontaneous combustion through self-heating once the deposit's internal structure and ventilation allow it. In the Nord-Pas-de-Calais coalfield, surveys have measured temperatures above 250°C at the surface and reaching 800 to 1,000°C at depth on some actively burning heaps, releasing CO2, methane and toxic gases (CO, SOx, NOx, mercury vapour, radon). The phenomenon is far from marginal across the coalfield, and its thermal monitoring — by infrared thermography — long relied on conventional aerial means (fixed-wing aircraft, helicopter), costlier and less frequent than a drone flight.

Research has documented this drone contribution on comparable spoil heaps abroad. A Chinese team led by Xiaoyuan He published in 2020, in Scientific Reports, a method for identifying and delineating coal fire zones through drone-borne infrared thermography combined with ground survey, at the Huojitu mine (Shenmu, China) — an approach transferable to an actively burning spoil heap to prioritise where to intervene (see the study on Google Scholar). On a spoil heap proper, a Portuguese team led by Ana C. Teodoro published the same year, in the proceedings of the SPIE conference on earth resources remote sensing, a drone-imagery study of soil movement on the self-burning São Pedro da Cova heap (Portugal), comparing successive digital elevation models — a method that directly matches a French manager's dual need: mapping the hot spot and tracking its progression over time (see the study on Google Scholar). Our aerial thermography service details the radiometric measurement principle applied to this kind of structure.

Erosion, slope movement and vegetation change

Beyond combustion, a spoil heap remains an artificial slope several dozen metres high, exposed to run-off, frost and its own internal settling. A repeated photogrammetric survey — two to four flights a year depending on the site's sensitivity — produces at each pass a georeferenced digital surface model comparable to the previous one: a gully opening on a slope, a localised slide starting, or a slight creep of the crest all show up as a gap between two successive models, before any disorder becomes visible to the naked eye from the foot of the structure. Our guide to landslide monitoring by drone photogrammetry details this method of comparing successive models, directly applicable to a spoil-heap slope.

Vegetation, meanwhile, works as a free, indirect indicator: an area where grass or shrub cover fails to take hold, or dies back after establishing, often signals ground still abnormally hot at depth — a useful cue for targeting a follow-up thermal flight without covering the whole structure at high resolution. On a World Heritage site, this combination of orthophoto, 3D model and thermography has an additional benefit: it documents the structure's condition without installing fixed sensors or altering its appearance, a constraint that permanent instrumentation (visible extensometers, inclinometers) would not satisfy on a listed asset.

Who commissions this kind of mission, method and 2026 prices

These missions are commissioned by a range of actors: municipalities and inter-municipal bodies in the coalfield concerned about a spoil heap near housing or a hiking trail, the Mission Bassin Minier (the body running the UNESCO site), consultancies mandated as part of a post-mining risk notice, or public land agencies ahead of a redevelopment project on a neighbouring former colliery yard. A drone never substitutes for GEODERIS's regulatory expertise; it provides documentary support — a dated, georeferenced, repeatable record to attach to the monitoring file.

Ranges observed in France in 2026, excl. VAT:

These amounts cover the flight, the photogrammetric or radiometric processing and data delivery; they exclude the post-mining technical expertise itself, which remains the responsibility of GEODERIS and its mandated consultancies. Our guides to mining subsidence and sinkhole monitoring and brownfield mapping cover two neighbouring strands of the same post-mining picture: the underground and the redeveloped surface. For a local authority, a consultancy or a mining heritage manager, request a quote stating the spoil heap concerned, its known combustion history and the desired monitoring frequency.

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Put it into practice

• Drone security & surveillance: rates and cities covered from €600
• Security & surveillance in La Rochelle Nouvelle-Aquitaine
• Security & surveillance in La Seyne-sur-Mer Provence-Alpes-Côte d'Azur
• Security & surveillance in Niort Nouvelle-Aquitaine

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