Numerical and experimental investigation on hydraulic fracturing for offshore wind turbine rock-socketed pile foundation construction

Rock-socketed pile is one of the important foundation forms for offshore wind turbines, however, the construction of rock-socketed piles is very difficult. In this work, we propose to use hydraulic fracturing to pre-crack the rock foundation seabed around the piles to assist the construction of rock-socketed piles. Understanding the propagation mechanism of hydraulic fractures is the theoretical basis for hydraulic fracturing of rock seabed. Using mortar specimens containing single internal cracks and corresponding numerical simulations, hydraulic fracturing tests were carried out to study the influences of pre-existing crack characteristics and in situ stress on hydraulic fracture propagation behavior. The mortar specimens showed a tensile-shear failure mode under confining stress in hydraulic fracturing tests, and a macro-fracture penetrated the upper and lower ends of the specimens. Quantitative analysis of hydraulic fracture propagation behavior shows that under the comprehensive influences of the pre-existing crack angle and the stress, it is relatively easy to obtain good engineering results by selecting the pre-existing crack dip angle of 45 degrees similar to 60 degrees for directional hydraulic fracturing, and the increase of in-situ stress will improve the stability and deflection capacity of the cracks in the seabed.