CEEN 641 - Lecture 5B - BONUS Content on Using Dynamic Methods to Obtain Elastic Properties of Soil

How to Measure Dynamic Soil Properties

Introduction to Dynamic Soil Properties

• The lecture introduces the topic of measuring dynamic soil properties, focusing on wave propagation through different materials and vibratory motion.

• Dynamic properties include P-wave and S-wave velocities, damping characteristics, and shear and constrained moduli of soils.

Methods for Measuring Dynamic Soil Properties

• Measurement methods are categorized into two groups: field methods (in-situ) and laboratory methods. Each has its advantages and disadvantages.

• Field methods involve low strain tests (inducing shear strains < 0.001%) or high strain tests (> 0.001%).

Field Methods Overview

Geophysical Testing

• Geophysical testing is a specialized field requiring expertise; this lecture aims to provide foundational knowledge for better communication with experts.

Low Strain Field Testing

• Low strain geophysical testing uses geophones arranged across a site to measure wave velocities induced by seismic sources like hammer blows or explosives.

Seismic Reflection Test

• A basic test that measures wave propagation velocity and layer thickness in a two-layer soil system, assuming the second layer is denser.

• The reflection time can be computed using an equation involving the thickness of the upper layer, distance from source to receiver, and propagation velocity.

Seismic Refraction Test

• More complex than reflection tests; it involves multiple receivers measuring travel times of first arrival waves to determine velocities in both layers.

• This method allows for non-horizontal boundaries between layers, accommodating various subsurface shapes.

Critical Angle of Incidence

• The critical angle is where transmitted waves follow the boundary between layers, continuously reflecting back up to the surface.

Comparison Between Reflection and Refraction Tests

• Both methods are effective for mapping boundaries between high contrast soils but may yield inaccurate results if soil density does not consistently increase with depth.

Surface Wave Methods

Introduction

• Surface wave methods have gained popularity over recent years due to their ability to analyze Rayleigh waves for dynamic property assessment.

Techniques Used

• These methods involve spectral analysis using Fourier spectra recorded from geophones, allowing interpretation of soil layering without drilling holes.

Reliability Factors

• The reliability of these analyses depends heavily on the qualifications of the analyst performing them; experience is crucial for accurate interpretations.

Borehole Methods

Uphole Test

• Involves placing a source down a borehole while measuring wave propagation velocity at known distances from the borehole's wall.

Down Hole Test

• Similar setup as Uphole but with receivers placed down in boreholes while sources remain above ground.

Cross Hole Test

• Utilizes two boreholes with one containing a source and another containing a receiver to measure direct propagation velocity between them.

In-Situ Testing Methods for Soil Properties

Cross Hole Testing and Seismic CPT

• Cross hole testing measures the propagation velocity of seismic waves through a drilled shaft, indicating the integrity and quality of the concrete foundation.

• The seismic cone penetration test (CPT) integrates a geophone into traditional CPT to measure soil properties while inducing seismic waves at various depths.

Advantages and Disadvantages of Borehole Methods

• Borehole methods provide reliable data with minimal subjectivity, allowing for in-situ soil sampling during drilling or CPT.

• However, these methods are expensive, limited by borehole depth, and can create environmental challenges due to messiness during drilling.

High-Strain vs. Low-Strain Testing

• High-strain tests induce plastic strains in soil rather than measuring elastic properties directly; correlations with other tests must be understood carefully.

• While high-strain field methods offer insights into dynamic properties, they rely on correlations that may exhibit significant scatter.

Laboratory Test Methods for Soil Properties

• Obtaining undisturbed soil samples is crucial for laboratory testing; minimizing disturbance during retrieval is essential for accurate results.

• Bender element tests use electrodes to apply small shear pulses in soil samples, allowing precise strain measurements despite challenges in inducing small strains.

Advanced Laboratory Techniques

• The resonant column test applies torsional shear strains cyclically to determine dynamic properties and damping characteristics of soils based on resonance frequency.

• High-strain laboratory methods like cyclic triaxial tests simulate real-world loading conditions but struggle with accurately modeling shear wave behavior due to vertical loading mechanisms.

Cyclic Direct Simple Shear Test Insights

• The cyclic direct simple shear test allows more realistic deformation patterns under confinement compared to traditional direct shear tests, simulating earthquake-like conditions effectively.

• Despite its advantages in simulating side-to-side stresses during earthquakes, this method has limitations regarding stress state measurement and is costly due to specialized equipment requirements.

Conclusion on Dynamic Soil Properties Testing

• Understanding both low-strain and high-strain testing methodologies provides a comprehensive view of soil dynamics; however, practical challenges remain in sample collection and testing execution.