Experience with Testing Box Core Samples of Soft Clays Using Miniature Full-Flow Penetrometers-Paper

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The paper evaluates miniature full-flow penetrometer testing of box-core samples to characterize very soft seabed clays in the upper 0.5 m, a zone important to offshore pipeline construction and operation. Conventional sampling and testing can inadequately capture these low-strength sediments.<br /><br />The authors describe a portable, computer-controlled system used on approximately 80 box-core samples from six offshore investigations. It performs T-bar and ball penetration profiles and cyclic tests, alongside motorized laboratory vane measurements. Testing is conducted soon after sample recovery, with corrections applied for push-rod friction and overburden effects. Vane results provide reference strengths for deriving empirical penetrometer bearing-capacity factors, or N factors.<br /><br />Across the sites, the T-bar generally measured greater resistance than the ball. Observations of cavities and changing soil-flow behavior around the ball suggest that it did not consistently develop a full-flow mechanism. The T-bar therefore provided the more reliable measurement for the probe dimensions and soils studied. Compared with motorized vane strengths, average factors were 11.6 for intact T-bar measurements and 12.3 for remolded T-bar measurements. The corresponding ball factor for intact strength averaged 7.7.<br /><br />The paper recommends prioritizing the T-bar, spacing test locations about 100 mm apart, and using a penetration rate of 20 mm/s unless otherwise specified. A rod profile should be performed first to identify hard layers and help select a suitable load cell. The protocol then includes a full-depth T-bar profile, cyclic testing at selected depths, and a rod test to estimate friction. Motorized vane testing can be added for site-specific strength correlations.<br /><br />Overall, box-core testing with miniature penetrometers provides high-resolution strength profiles and helps assess the transition from intact to remolded soil, supporting improved pipeline–seabed design.

Keywords

miniature penetrometer

box-core sampling

soft seabed clays

T-bar penetration

ball penetration

motorized vane test

offshore pipeline

bearing-capacity factor

full-flow mechanism

remolded strength