Practical criteria for assessment of horizontal borehole instability in saturated clay

Horizontal borehole instability raises concern due to negative impacts on success of drilling, reaming and pipe installation during Horizontal Directional Drilling. Shear failure (blow-out) and tensile failure (hydraulic fracture) are the two main failure mechanisms controlling borehole instability in saturated clay. Criteria for categorizing shear failure versus tensile failure are given which are functions of the ratio of undrained shear strength to vertical effective stress Su/σv′ and the Lateral Earth Pressure Coefficient at Rest K0′. Given that Su/σv′ and K0′ can be expressed as a function of soil parameters like friction angle, Over Consolidation Ratio (OCR) and Liquidity Index (LI), new forms of these criteria are developed for situations where Su/σv′ and K0′ cannot be obtained directly. It is confirmed that tensile failure generally controls in brittle clays like heavily overconsolidated and/or low liquidity index materials. Cases from the laboratory and the field as reported in the literature are used to examine the effectiveness of the new criteria, with inconsistency between theory and the physical evidence explained.