At the lieu-dit La Barrière on the RD990, Carlat (Cantal, France), thirteen years of successive repairs failed to stabilise a road pavement affected by recurring differential settlement on soft compressible ground.
In 2022, the Conseil Départemental du Cantal chose a Nidaplast RA500 cellular lightweight fill solution — the only approach capable of permanently eliminating the source of the problem without prior soil treatment or micropiles.
2022
year of construction
800 m3
of lightweight fill
3 layers
of RA500 cellular blocks
RD990, Carlat: Timeline of a Road Subsidence Failure on Compressible Soft Ground
The first signs of distress appeared in 2009 with minor cracking on the RD990 carriageway.
A first drainage intervention and surface regrading was carried out in 2012.
Cracks reappeared in 2015 and worsened in 2016. Two further regrading operations were performed in 2017 and 2020 — each time, the problems returned and deteriorated rapidly.
After thirteen years of unsuccessful repair attempts, the diagnosis was unambiguous: differential settlement of the slope on soft compressible ground was causing the pavement deformation, and surface regrading could not address the underlying cause.
In 2022, the Conseil Départemental du Cantal (client), working with design engineer Geofit Expert and contractor Matière SAS, undertook a definitive rehabilitation of the site.
Among the solutions considered, Nidaplast RA500 cellular lightweight fill was selected for its proven effectiveness in eliminating differential settlement without soil treatment.
Why Nidaplast RA500 Lightweight Fill Was Chosen for This Road Rehabilitation
Conventional granular fill, by reconstituting a heavy load on soft ground, merely perpetuates settlement.
Micropiles represent significant cost and programme impact for a regional road.
Nidaplast RA500 offers a third option: by substituting the existing fill mass with 40 kg/m³ cellular blocks, the load applied to the compressible ground is reduced by the same factor, eliminating the cause of differential settlement at source.
- Dry unit weight RA500: 40 kg/m³ vs 1,700–1,900 kg/m³ for standard granular fill.
- Short-term compressive strength (520 mm block): 400 kPa.
- Long-term allowable bearing pressure (safety factor ×2, 50-year design life): 65 kPa — maximum 3.30 m of overlying fill.
- 95% void ratio: water circulates freely through the blocks — no buoyancy force if the water table rises.
- Manually handled blocks: no mechanical connections, rapid installation on difficult sloping terrain.
Nidaplast RA
Nidaplast RA lightweight embankments are extremely light and resistant. These honeycomb structures are the recommended solution for constructing landscape embankments, road embankments, or building embankments.
View product sheetNidaplast RA500 Lightweight Fill Installation at RD990 Carlat: Construction Sequence
1. Excavation and formation level:
The slope was cut and excavated to allow the cellular blocks to be placed in 3 layers.
A bed of crushed stone (20/40 mm) was laid at the base of the excavation to seat the blocks.
2. Anti-contamination geotextile:
A geotextile separator was placed on the prepared formation, wrapping the entire lightweight fill mass.
3. Cross-jointed block laying:
Nidaplast RA500 blocks were installed with staggered joints in both longitudinal and transverse directions to maximise stability.
Blocks are manually handled and require no mechanical connection.
4. Geotextile wrapping:
The geotextile was closed over the top and along the lateral faces of the fill mass.
On the slope face, the geotextile follows the stepped profile formed by the block layers.
5. Covering structure:
A 10 cm reinforced concrete slab was cast directly on the blocks, followed by an unbound granular sub-base (GNT 0/31.5), then the road pavement structure: 2 × 7 cm bituminous base course + 6 cm wearing course (BBSG).
Results: Permanent Road Pavement Repair on Soft Compressible Ground
- Differential settlement eliminated: the root cause of the distress is addressed, not masked by surface patching.
- Durable rehabilitation: full pavement structure reinstated above the lightweight fill mass.
- Controlled programme: rapid installation using manually handled blocks, no lifting equipment required.
- Cost-effective: no micropiles, no soil treatment, no preloading by surcharge.
Working on a road rehabilitation project affected by subsidence or differential settlement on soft ground?
Contact a Nidaplast engineer
RD990, lieu-dit La Barrière, Carlat (Cantal, France): permanent rehabilitation of a regional road pavement affected by recurring differential settlement on soft compressible ground since 2009. Client: Conseil Départemental du Cantal. Design engineer: Geofit Expert. Contractor: Matière SAS. Volume: ~800 m³ of Nidaplast RA500. Date: autumn 2022.
Solution: Nidaplast RA500 cellular lightweight fill (SAUL — ultra-lightweight cellular structure) in polypropylene (PP), 40 kg/m³ dry unit weight, 95% void ratio, installed in 3 cross-jointed layers. Base: crushed stone 20/40 mm bed. Anti-contamination geotextile wrapping the entire fill mass. Covering structure: 10 cm RC slab + unbound granular sub-base (0/31.5) + 2 × 7 cm bituminous base course + 6 cm wearing course. Long-term allowable bearing pressure RA500: 65 kPa (safety factor ×2, 50-year design life). No micropiles, no soil treatment, no preloading.
Outcome: differential settlement permanently eliminated after 13 years of unsuccessful repair attempts (2009–2022). 100% French manufacturing. Website: www.nidaplast.com
