Carbonate and pumice sands: the soils that break correlations.

Why these soils are different

Carbonate sands (biogenic, calcareous deposits) and pumice sands are both fundamentally crushable:

• Grains are hollow, angular or vesicular rather than dense and rounded.
• They can show high friction angles and high CPT tip resistances — yet compress and crush under sustained load or densification energy.
• Strength and compressibility are stress-level and loading-path dependent in ways silica correlations don't capture.
• Standard SPT/CPT-to-design correlations derived from silica sands routinely over-predict stiffness and under-predict settlement.

The result: deceptively strong-looking ground that settles more than expected, or densifies in unexpected ways during vibrocompaction and dynamic compaction.

What we do

• Characterisation programmes — grain crushability testing, one-dimensional and cyclic testing, drained and undrained triaxial on intact and remoulded specimens.
• CPT interpretation for crushable soils — where qc-based correlations need adjustment or calibration against laboratory and field performance. See CPT and seismic CPT.
• Ground improvement design and verification — realistic acceptance criteria for vibrocompaction and dynamic compaction in crushable fills, based on behaviour rather than silica-derived targets. See ground improvement for reclaimed land.
• Settlement and bearing assessments that account for particle breakage rather than assuming silica behaviour.

Ground improvement and acceptance on crushable fills

Compaction of carbonate and pumice fills needs acceptance criteria calibrated to the material. Standard practice borrows silica-sand targets; the senior practice is to establish a shell correction factor for the specific fill — through trials, CPT verification and laboratory calibration — so that "compacted" means something real in that ground, not something borrowed from another geology. See ground improvement for reclaimed land.

Why it matters on a reclamation project

Carbonate and pumice sands are common across the tropics and in volcanic settings — exactly where much of the world's reclamation is happening. A project that treats them as "just sand" discovers the difference during compaction or in the first years of settlement. A project that characterises them properly builds to a verified specification.