The effective friction angle (ϕ') is one of the most sought soil properties in geotechnical analysis and design that controls the fundamental soil strength behaviour. In this paper, A recently compiled database on intermediate grain size soils that were tested by variable rate piezocone penetration tests (VRCPTU) is used to examine an effective stress limit plasticity solution using piezocone data developed by NTH (Norwegian Institute of Technology) towards prediction of the geomaterials’ effective stress friction angle. It is shown that penetration rate during probe advancement influences the CPTU measured data and the drainage conditions can be indicated by corresponding normalized cone resistance and normalized porewater pressure measurement. Further analysis revealed that an input parameter termed the angle of plastification (β) from the CPTU solution correlates with soil contraction and dilation tendency and can be used to evaluate the ϕ' of investigated soils, together with other normalized CPTU measurements. The tentative suggested formula of β for intermediate soils undergone VRCPTU testing is given based on the investigated database. A comprehensive testing program on the Yellow River Delta (YRD) silts in China and a silty geomaterial from northern Italy in Europe including benchmark laboratory triaxial tests on high quality samples, field and calibration chamber variable rate CPTU are showcased to demonstrate the verification of the NTH solution for evaluating ϕ' of intermediate grain size soils.