P. A. de Alba

Relationship Between Acoustic and Mechanical Properties of Two Marine Clays

Marine clay samples from two geographic locations were tested in a triaxial-acoustic testing chamber. This chamber was equipped with piezoelectric benders and thickness expanding disks in the end caps for the generation and reception of shear and compressional waves respectively. The chamber design facilitated both triaxial strength and acoustic measurements to be made on a particular sample without disturbing the test set-up. The acoustic data was digitized and processed for time delay assessment for velocity determination. Staged triaxial tests were used to maximize information from each sample. Measurements were made both under increasing consolidation pressure and during rebound. Preliminary analysis indicate that the relationship between undrained shear strength and shear wave velocity is both material and stress history dependent.

Residual Strength of Liquefied Sand Measured in a Ring Shear Device

Earthquake-induced liquefaction flow slides have resulted in loss of life and major damage at many sites around the world. In order to better understand the mechanics of such slides, it is necessary to quantify the residual shearing strength of the liquefied soil. Small-scale stress-controlled experiments suggest that this residual strength is not a constant, but that liquefied sand can be modeled as a highly viscous stress-thinning fluid, whose resistance varies with the velocity of flow. We present results obtained with a ring shear device designed specifically to measure the large-displacement post-liquefaction residual strength of sands under strain-controlled conditions. Residual strength of a fine uniform sand was measured for a range of relative densities (Dr) from 19 % to 36 % at four different shear-strain rates, varying from 11 to 44 s−1 representative of flow slide velocities. Measurements show that the strain-rate-dependent Herschel–Bulkley model for stress-thinning fluids applies to the liquefied sand, with resistance increasing as strain rate increases, but suggest that at relative densities higher than perhaps 50 %, relative density dominates, and residual strength can be approximated as a constant.

Design and Testing of a Debris Flow ‘Smart Rock’

This paper describes the development of a “smart rock,” an instrumented device for the study of debris flows, which is often triggered by earthquakes, heavy rain events, and rising groundwater conditions. Debris flows are very destructive forms of landslide consisting of a mixture of rocks, saturated soil, and debris typically flowing at high rates of speed and over long distances. In an effort to better understand the mechanics of debris flows, the smart rock was developed with a sensor package to be used in U.S. Geological Survey experiments at their flume facility in the Willamette National Forest, OR. The instrumented rock contains an inertial measurement unit (IMU) to measure acceleration and rotation rate about three body fixed axes, and two pressure sensors to measure pore water pressure. The sensors provide information about the movement of the rock and pore water pressures within a debris flow. One of the objectives of the sensor package is to use this information to track the position of a particle in the flow with an accuracy of 1 m over the course of 10 s. Calculation of position using the IMU requires the use of strapdown inertial navigation equations. Unfortunately, noise and bias in the rotation rate sensor introduce significant error in the position calculation. The results from one of the USGS debris flow experiments using the smart rock show that an ad hoc filtering method on the IMU data provides a rough estimate of the rock position in the flume, but far from the desired level of accuracy. Pressure and velocity recorded by the smart rock, while comparable to those measured by the USGS during the debris flow test, cannot be verified. Position accuracy can only be improved by using a better IMU and obtaining known rock positions versus time during the debris flow. Based on the results of this work, it is hoped that improved technology will result in a smart rock that can successfully provide useful and insightful debris flow data.