Which tool should I use?
Use the SPT calculator when you have Standard Penetration Test data (depth and N-value), typical of onshore site investigations. Because an N-value alone cannot identify soil type, you supply clay or sand for each layer, usually read from the borehole log.
Use the CPT calculator when you have Cone Penetration Test data (cone resistance, sleeve friction and pore pressure), common offshore. The CPT measures several quantities at once, so both strength parameters are solved together and the soil behaviour type is classified automatically.
The parameters
Both tools return the four parameters most often needed for foundation and earth-structure design: cohesion (c, kPa), internal friction angle (φ, °), unit weight (γ, kN/m³) and elastic modulus (E, kPa), reported per layer and as a depth profile. The results are reference estimates that support the engineer's judgment; final responsibility rests with the engineer.
Methods
The SPT tool uses an energy-based inversion consistent with the SPT penetration resistance. The CPT tool follows the analytical bearing-capacity approach of Motaghedi and Eslami (2013), solving cohesion and friction angle simultaneously, and classifies soil behaviour type with Robertson's (2010) index. See each tool's page for details.