Strength Properties of Subsea Rockfills at Low Stress Level-Paper

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The paper reviews available laboratory data and design experience to estimate the strength properties of subsea rockfills installed by the fall-pipe method. These rockfills are generally loosely placed and operate at low effective stresses—typically about 1–40 kPa—where direct test data are limited.<br /><br />The assembled results show that peak friction angle (φ′p) increases as effective stress decreases, with the stress effect becoming less pronounced below roughly 10 kPa. The authors recommend representing this relationship with a hyperbolic model and provide lower- and higher-estimate design curves. Critical-state friction angle (φ′cs), by contrast, is treated as largely independent of stress and initial porosity; the proposed lower and higher estimates are 33° and 39°. Maximum dilation angle (ψmax) is also stress-dependent and can be estimated from φ′p and φ′cs using a simplified stress–dilatancy relationship adapted from Bolton’s model for sands. A typical submerged unit weight of 8.5–11 kN/m³ is suggested, depending on rock density and porosity.<br /><br />A design example illustrates how the proposed parameters can be applied to a rock berm. The authors recommend accounting explicitly for stress-dependent strength in design and, where possible, using constitutive models that also capture strain-dependent behavior and progressive failure.<br /><br />The recommendations are based on public-domain data and are most applicable to typical subsea rockfills. Rock-specific large-scale testing may be needed for unusual materials or where greater confidence is required. Care is also advised when continuum-based numerical models are used for structures whose dimensions are not large relative to the rock particles.

Keywords

subsea rockfills

fall-pipe method

peak friction angle

critical-state friction angle

stress-dependent strength

hyperbolic model

maximum dilation angle

submerged unit weight

rock berm design

constitutive models