The subgrade beneath a new warehouse in Riverlands Industrial Estate behaves completely differently to the Wither Hills silts up on Maxwell Road. On the plain you are dealing with recent alluvial deposits—sandy silts and soft clays that lose significant strength when saturated. Up on the terraces the weathered loess can stand up well in a cutting but still needs a soaked CBR to confirm its resilience under wet conditions. We run laboratory CBR tests in Blenheim on remoulded and undisturbed samples so the pavement engineer gets a design value that reflects the actual moisture sensitivity of the material, not a generic number pulled from a table. The NZS 4402:2016 procedures are non‑negotiable in our lab—four‑day soak, surcharge weights matching the design pavement depth, and a penetration rate of 1.27 mm per minute—because shortcuts here translate into rutting and edge‑break within two summers. For projects where fill is being imported, we often pair the CBR with a Proctor compaction test to verify that the specified density is achievable with local borrow, and when the pavement structural number needs refinement we combine soaked CBR with grain‑size analysis to confirm the drainage characteristics of the subgrade.
A Wairau silt can lose half its bearing strength after four days of soaking—if your pavement design uses an unsoaked CBR, you are designing for a condition that never exists after the first winter.
