We just back-analyzed 18 mm differential over 9 m on a light slab where two borings read uniform CL (PI=18, SPT N=9–12) yet a follow-up CPT found a about 0.6 m loose sand lens at 3.8 m that aligned with the settlement trough. If you were scoping the initial investigation, would you have tightened spacing or mixed in CPT/SCPT to improve resolution on thin layers, and do you rely on sleeve friction to flag these in low fines content soils?
On sites like this, I’ve had better luck adding a single SCPTu transect at about 5 m spacing across the load path and keying on a qc dip with a u2 spike — the ‘0.6 m’ lens at about 3.8 m would usually show up that way, and it’s typically cheaper than another boring… Caveat: in stiff crusts the signal can smear, so I’ll backstop with one targeted dissipation at the suspect depth.
I got burned by a “0.6 m lens at about 3.8 m” once, so I now build a trigger into the scope: run one pilot CPT at the first boring and, if there’s a qc trough/u spike around 3.5–4 m, immediately add two CPTs at 6–8 m spacing along the load path — usually a half‑day and under ~$2k. If CPT access is tight, I have the driller log MWD (torque/advance) and take continuous 0.3 m spoons through that interval to catch thin sand ribbons.