John A. Howie

A laboratory apparatus for streaming potential and resistivity measurements on soil samples

We describe an apparatus designed to perform streaming potential and resistivity measurements on unconsolidated soil samples. The apparatus enables the use of both unidirectional and oscillatory flow methods to measure the streaming potential coupling coefficient (C); the direct current resistivity method is used to measure the bulk resistivity (rho) of the soil sample. Measuring both of these parameters on the same sample under the same conditions enables us to properly characterize the streaming current cross-coupling coefficient (L). The apparatus is designed to test reconstituted saturated soil samples up to a maximum grain size of 9.5 mm, and hydraulic gradients from less than 0.1 up to a maximum of 4 m of H(2)Om in flow-through experiments. Excellent agreement between C values measured using the unidirectional and oscillatory flow methods validates the oscillatory flow method for unconsolidated samples.

Combined Time and Frequency Domain Approach to the Interpretation of Bender-Element Tests on Sand

Shear wave velocities are obtained from bender-element tests on laboratory specimens by analyzing the trigger and response signals. The response signal is a highly distorted translation of the trigger signal, obscuring the identification of the shear wave arrival. This has led to the publication of many criteria to guide subjective decisions on the selection of trigger waveform and frequency and of applicable interpretation methods. Current methods of interpretation result in determination of either the group or phase velocity. As soil and bender-element responses are dispersive, and as the group velocity is only valid in non-dispersive systems, the phase velocity should be measured. A combined time and frequency domain method is presented to allow interpretation of the phase velocity, minimizing subjective input to the interpretation. The method is first demonstrated using simplified synthetic signals and is then applied to laboratory test data. The reproducibility of the results is demonstrated from measurements on ten triaxial specimens of saturated Fraser River sand. The group velocities are shown to be very sensitive to dispersion, not reproducible, and contingent on the selected frequency window, whereas the phase velocities are considerably more repeatable. The combined time and frequency domain method results in the interpretation of a phase velocity using only measured parameters.

Specific Depth Cone Resistivity Measurements to Determine Soil Engineering Properties

The University of British Columbia (UBC) began performing piezocone penetration tests (CPTU) with electrical resistivity measurements (RCPTU) in 1989. Since then, RCPTU research at UBC has focused on obtaining geo-environmental parameters such as fluid resistivity and soil engineering properties such as porosity and degree of saturation from measurements of bulk soil electrical resistivity using the empirical relationship proposed by Archie (1942). Within this framework, the paper illustrates and discusses important design and calibration issues for resistivity modules such as the use of isolated circuitry to achieve linear calibrations over large ranges of resistivity. The suitability of RCPTU measurements for determination of geo-environmental and geotechnical parameters are assessed using typical ranges of soil and groundwater properties and methods of isolating individual factors for study are discussed. Illustrative examples of RCPTU research efforts including the environmental-characterization of mi...

Interpretation of Resistivity Piezocone Tests in a Contaminated Municipal Solid Waste Disposal Site

A resistivity piezocone (RCPTU–resistivity cone penetration test with pore pressure measurement) and groundwater and soil samplers were used to detect contamination from a landfill for urban solid waste located in the state of Sao Paulo in southeastern Brazil. Background resistivity values were obtained in the laboratory using undisturbed soil samples. The strong influence of clay minerals common in tropical soils made it difficult to interpret the tests and to differentiate potentially contaminated zones from changes in soil type. A local correlation between fines content and the soil behavior index (Ic) of the various collected soils allowed the RCPTU tests to be interpreted to identify the contaminated regions of the aquifer. These results showed excellent repeatability and allowed for a detailed stratigraphic analysis of the highly heterogeneous profiles. Electrical resistivity measurements have proven to be an interesting resource to help detect contaminated soils, thus improving the quality and efficiency of geoenvironmental site investigations using integrated direct and indirect techniques. The interpretation of resistivity piezocone tests for the study site is not straightforward as it is in sedimentary sands since soil genesis affects soil behavior and soil and water sampling is required to support interpretation.

Continuous Monitoring of Bender Element Shear Wave Velocities During Triaxial Testing

Bender element testing is used to measure shear wave velocities (VS) across soil specimens in the laboratory. Conventional bender element testing is carried out at a few discrete points during an experiment. This paper presents a method which uses bender elements to monitor VS continuously throughout a triaxial test. The method is based on monitoring the change in phase angle between a continuous trigger signal and a received signal. It allows the phase velocity at multiple selected frequencies to be monitored throughout a test. The method is described in detail and its use is illustrated for triaxial tests on loose specimens of Fraser River sand prepared by water pluviation. The variation of shear wave velocity is demonstrated during consolidation, ageing, and shearing to failure along a conventional stress path. Potential difficulties of interpretation are presented and discussed. In addition, the potential for continuous excitation of the specimen to alter the test results is considered and dismissed based on a comparison of samples tested without benders and with continuous benders at a range of excitation voltages. The proposed method for continuously monitoring bender element shear wave velocities does not use special or unique equipment. It results in a continuous VS from a trigger and receiver element installed on opposite ends of a triaxial specimen. This method provides a measure of VS during dynamic phases of an experiment that have not previously been observed. For example, this paper includes measures of VS at the onset of creep, the onset of shearing, and over the phase transformation from contractive to dilative behaviour. The relationship between fundamental soil behaviour and VS can now be more easily explored.

DEVELOPMENT OF A SPREADSHEET FOR MODELING SPT STRESS WAVE DATA

The advantages of measuring and correcting for variations of Standard Penetration Test (SPT) stress wave energy are well documented. Despite this fact, geotechnical engineers are often hesitant to measure SPT energy, due to high cost and uncertainty about data quality or the reliability of energy calculation methods. Two spreadsheets that model the propagation of stress waves through simple and safety hammers (and attached rod strings) were developed to address the issue of data quality. This paper describes the development of the spreadsheets, including critical aspects of hammer-anvil interaction that are not well documented in the geotechnical literature. Spreadsheet output is verified by comparison to stress wave data collected under controlled laboratory conditions. The safety hammer spreadsheet is then used to assess the quality of stress wave data collected during an actual field investigation. In all cases, the modeled and measured data are in good-to-excellent agreement.

Comparing Frequency and Time Domain Interpretations of Bender Element Shear Wave Velocities

Bender element testing is the most common means to measure the shear wave velocity across soil specimens in the laboratory. There are two classes of shear wave velocity interpretations: time domain and frequency domain, and different methods of interpretation tend to give different answers. Frequency domain approaches provide a means to account for the systematic effect of frequency on the interpreted velocity. Various proposed frequency domain methods have been published, but have so far obtained values that disagreed with more common time domain interpretations. This paper demonstrates that the errors in bender element interpretations of the shear wave velocity are predominantly systematic. The source of these systematic errors is traced to the frequency content of the selected trigger signal waveform and the interpretation method. The systematic errors are demonstrated using experimental data from bender element testing on a triaxial specimen of loose saturated Fraser River Sand. The results from a suite of bender element triggers and interpretation methods found that the shear wave velocity had a range of 30.9 m/s. It is demonstrated that this range is a systematic effect of the bender element trigger signal and interpretation method, not random error. The consequence of the systematic error is that there is no scientifically justifiable reason to take the average of a suite of trigger signals and interpretation methods. The average would not be an unbiased estimate of the mean velocity due to the unaccounted for effects of trigger signal type, frequency, and interpretation method.

Specific Depth Cone Resistivity Measurements To Determine Soil Engineering Properties

The University of British Columbia (UBC) began performing piezocone penetration tests (CPTU) with electrical resistivity measurements (RCPTU) in 1989. Since then, RCPTU research at UBC has been focused on obtaining geo-environmental parameters such as fluid resistivity and soil engineering properties such as porosity and degree of saturation using the empirical relationship proposed by Archie (1942). The relationships between these three factors and the bulk soil electrical resistivity are concisely reviewed within the framework of Archie’s relationship and methods of isolating individual factors for study are discussed. Key design issues for resistivity modules such as the use of isolated circuitry to achieve linear calibrations over large ranges of resistivity are described. The suitability of RCPTU for monitoring individual factors is assessed using typical ranges of soil and groundwater properties. Practical aspects of resistivity module calibration including calibration chamber dimensions and temperature considerations are discussed. Illustrative examples of RCPTU research efforts including the environmental characterization of mine tailings, delineation of saline water intrusions in fresh water aquifers and the quality control of geotechnical ground densification are presented throughout the text.