I pulled together a 1‑page crib based on Robertson (2010) and Kulhawy & Mayne (1990) to go from qc to φ’, γ, and su, then straight into bearing and Schmertmann settlement for spread footings; it’s been reliable on recent mat designs over loose Holocene sands in Galveston where qc about 6–10 MPa at 3–8 m… If you want the sheet (Excel + PDF), I can share — also keen on better correlations for carbonate sands or cemented layers that tend to overpredict φ’.
I’ve gotten closer settlement predictions on loose coastal sands by capping Schmertmann’s modulus at Es ≈ 2.5·qc for mats, since shell fragments tend to inflate qc and make the profile look stiffer than it behaves. In your “qc about 6–10 MPa” band that kept footing movements within about 10–15% of monitoring. If Fr is consistently <0.5% you can push higher, but I’d sanity-check against Robertson’s guide: https://greggdrilling.com/wp-content/uploads/2016/08/Guide-to-Cone-Penetration-Testing-2015.pdf.
@OP quick tip: in those Galveston loose sands (qc about 6–10 MPa at 3–8 m), using qt and normalizing to Qtn for φ′/Es tightened my Schmertmann predictions with the shallow GWT — raw qc was making mats too stiff. I cap γ_sat ≈ 19 kN/m³ and shave φ′ by 1–2° when Ic creeps past about 1.8. Handy ref: https://www.cptbased.com/.
I’ve found that factoring in the local groundwater trends can significantly impact your predictions for those shallow sites. In my recent work in coastal areas, I’ve adjusted settlement models based on GWT fluctuations, which has refined our estimates. It might be worth considering as you tweak your sheet.