In fixed-support three-point bending tests, the support friction tends to disturb the stress field around the crack tip, thereby introducing errors when calculating the mode I fracture parameters and evaluating the size effect of compacted clay. This study investigates the mode I fracture size effect of compacted clay using single-edge notched deep beam (NDB) and notched semicircular bending (NSCB) specimens, with explicit consideration of support friction. First, the finite element over-deterministic (FEOD) method is applied to calibrate the crack tip stress field Williams series expansion coefficients under different prefabricated crack lengths, support spans, and friction coefficients, and a friction-corrected crack tip stress field parameter database covering the dimensionless stress intensity factor, T-stress, and higher-order term coefficient is established. Multi-scale mode I fracture tests are then conducted to determine the apparent fracture toughness (KIC), while Digital image correlation (DIC) is used to characterize the evolution of the fracture process zone (FPZ) and measure the FPZ length lFPZ at peak load. On this basis, a friction-corrected modified maximum tangential stress (MMTS) size effect model is established to quantitatively evaluate the influence of support friction on the agreement between MMTS-calculated fracture parameters and experimental observations. The results indicate that support friction reduces the magnitudes of the singular and third-order Williams coefficients, thereby modifying the near-tip stress field. Neglecting support friction leads to larger MMTS-calculated critical distances rc and affects the correspondence between the theoretical critical-distance scale and the DIC-measured FPZ length lFPZ. Both parameters increase with the characteristic specimen size. With support friction included, the average relative deviations between the MMTS-calculated critical distance rc and the DIC-measured FPZ length lFPZ are 10.79% for NDB specimens and 12.71% for NSCB specimens. The research results provide a theoretical basis for accurately calculating the fracture parameters of compacted clay under fixed support conditions and informing the crack resistance design for compacted clay core walls.
