Breakouts in Soft Sediment: Applications to IODP Expedition 314

IODP/ODP boreholes show enlargements in shallow sediments interpreted as breakouts. These features begin at depths less than 200m in sediments with porosities > 50% (e.g. Sites C0001, C0004). From a theoretical perspective borehole failure in sediments should be treated using critical state soil mechanics, rather than the Coulomb failure criterion, applicable to rocks. Borehole failures in ODP/IODP holes in the Nankai Trough have been defined by resistivity imaging recorded about 3 m above the bit. Given typical penetration rates of 25 to 30 m/hr the borehole failure is then imaged < 10 minutes after the passage of the bit. Cutting the borehole provides a nearly instantaneous reduction in lateral confining stress. This reduction in confining stress results in a strain path moving away from the consolidation line towards the critical state line and consequent failure. In fine-grained sediments subject to deformation and failure in < 10 minutes, the stress path is undrained and constrained to a plane of no volume change. This stress path approximates Coulomb failure. Laboratory experiments with ODP/IODP samples from Nankai Trough holes with breakouts show angles of internal friction of 20-28 deg. and discrete surfaces of shear failure deviating 30 to 35 deg. from sigma 1, consistent with Coulomb failure. Most borehole failures are imaged as low resistivity zones, due to spalling that increases the amount of conductive fluid between the RAB sensors and the formation. But a Gulf of Mexico IODP Site (U1322) shows high resistivity RAB tracks down the borehole. This image may represent an incipient state of ductile failure where sediment is bulging into the borehole in the Sh min direction, prior to spalling and forming a breakout. Laboratory experiments support such ductile failure in samples from very shallow depths, whereas the Coulomb-style failure occurs at higher degrees of consolidation. The absence of these high resistivity borehole failures at convergent margins (observed to date) may reflect superimposed tectonic consolidation.