Estimation of soil strength in fine-grained soils by instrumented free-fall sphere tests

Abstract

The dynamic response of a sphere in soft clay is considered through field tests in which a 0•25 m dia. steel sphere was allowed to free-fall in water and dynamically penetrate the underlying soft soil. The test data, collected in a lake and a sea environment, relate to sphere velocities of up to 8 m/s, reaching sphere invert embedments close to ten diameters. An inertial measurement unit located within the sphere measured the motion response of the sphere during free-fall and penetration in soil. The resulting acceleration data were used within a simple framework that accounts for both geotechnical shearing resistance and fluid mechanics drag resistance, but cast in terms of a single capacity factor that can be expressed in terms of the non-Newtonian Reynolds number. The merit of the framework is demonstrated by using it as a forward model in a series of inverse analyses that calculate the undrained shear strength profile from acceleration data measured in free-fall sphere tests. The good match between these profiles and those obtained from 'push-in' piezoball penetrometer tests points to the potential for an instrumented free-fall sphere to be used as a tool for characterising the near-surface strength of soft seabeds.

An extended interpretation of the free-fall piezocone test in clay

“…Strategies for deducing the undrained shear strength, su, from FFP measurements depend on the FPP geometry and the extent and type of instrumentation included on the FPP. Where the instrumentation is limited to accelerometers, or for spherical FFPs (Morton et al 2015), su is derived indirectly from acceleration measurements via the equation of motion. For the more typical slender conical-tipped FFP geometry shown in Fig.…”

Centrifuge modelling of an instrumented free-fall sphere for measurement of undrained strength in fine-grained soils

“…In soft fine-grained soils, as an alternative to a free-falling CPT, a spherical free-fall penetrometer can be used, of the form shown in Figure 2 (Morton and O'Loughlin 2012;Morton et al 2015). A spherical free-fall penetrometer is attractive as it benefits from the D r a f t advantages of full-flow penetrometers, which have more tightly bracketed bearing capacity factors for deriving the undrained shear strength, s u , from the net penetration resistance (Chung and Randolph 2004).…”

In Situ Measurement of the Dynamic Penetration of Free-Fall Projectiles in Soft Soils Using a Low-Cost Inertial Measurement Unit

“…The sudden reduction in the accelerations along the z-axis during penetration in soil (evident in both the body frame and the inertial frame accelerations) is considered to be because of changes in the soil flow regime. This influences the magnitude of the drag resistance that dominates at these very shallow embedment depths in very soft soil and at high penetration velocities (Morton et al 2015). Fig.…”

“…For the projectiles considered in the previous section, understanding the soil-structure interaction at such high strain rates is crucial for predictive tools that calculate the final embedment depth of the anchors (DEPLA and DPA; e.g., O'Loughlin et al 2013b) or estimate the undrained shear strength based on the interpreted inertial frame accelerations (IFFS; O'Loughlin et al 2014; Morton et al 2015). This is because those strain rates are up to seven orders of magnitude higher than used for strength determination in a standard laboratory element test.…”