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

Abstract

Six degree-of-freedom motion data from projectiles free-falling through water and embedding in soft soil are measured using a low-cost inertial measurement unit, consisting of a tri-axis accelerometer and a three-component gyroscope. A comprehensive framework for interpreting the measured data is described and the merit of this framework is demonstrated by considering sample test data for free-falling projectiles that gain velocity as they fall through water and self-embed in the underlying soft clay. The paper shows the importance of considering such motion data from an appropriate reference frame by showing good agreement in embedment depth data derived from the motion data with independent direct measurements. Motion data derived from the inertial measurement unit are used to calibrate a predictive model for calculating the final embedment depth of a dynamically installed anchor.

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

“…This may then be translated into a profile of su with depth, z, using double numerical integration of the acceleration measurements to determine z. It may be necessary to pre-process the acceleration data due to any non-verticality of the device, as described in Blake et al (2016).…”

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

“…The MEMS accelerometer measures both linear and gravitational acceleration (depending on the sphere orientation) in three orthogonal body frame axes that are common to both the IMU and the sphere. In order to distinguish the sphere's linear acceleration component from the acceleration detected by the sensorwhich will differ if the sphere rotates it is important to transform the body frame acceleration measurements to accelerations that are coincident with the Earth-fixed inertial frame using rotation matrices, described in detail by Blake et al (2016). Linear accelerations corresponding to the inertial frame z-axis (i.e.…”

“…In the 15 Clyde tests, the sphere release height was varied between 1 and 20 m, which resulted in impact velocities between 4•0 and 6•3 m/s and sphere invert embedments of up to 0•782 m ($3•13 sphere diameters). Embedment depths were established from the IMU data as described in brief in the previous section and in detail in Blake et al (2016), with direct measurements based on mudline observations of markings on the retrieval line using a drop camera. These direct measurements were made to confirm that the analyses had not produced any gross error, as the resolution in the mudline observations was much lower than was possible from the IMU data.…”

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

“…In equivalent field experiments reported by O'Loughlin (2012), O'Loughlin et al (2014) andMorton et al (2015), an inertial measurement unit (IMU) measured acceleration along three orthogonal axes and rotation rates about the same three axes. Interpretation of the IMU data(Blake et al 2015) required that the measurements were considered within a fixed frame of reference to establish the projectile velocity and displacement along the projectile's direction of motion. This approach becomes complex in a centrifuge environment as the gyroscope-measured rotation rates include a component of the centrifuge rotation rate.…”