Download Use of the Light Falling Weight Deflectometer (LFWD) as a

Use of the Light Falling Weight Deflectometer (LFWD) as a
site investigation tool for residual soils and weak rock
Dr. David Lacey, Dr. Burt Look & Dave Marks
ISC'5 (2016)
Summary of Presentation
• Residual Soils / Weak Rock profile - Definition
• Prevalence of Residual Soil / Weak Rock profiles within South East Queensland
Introduction • Issues with current site investigation techniques
Data
Insights
• Light Falling Weight Deflectometer (LFWD) Test - Methodology
• Details of SEQ Study
• Results of Investigation
• Demonstration of LFWD Suitability to investigation of Residual Soils / Weak Rock Profile
• Comparison with other common test techniques
• Comparison between various LFWD equipment
ISC'5 (2016)
Residual Soil / Weak Rock Profile - Definition
Profile of a residual soil and rock
weathering profile (after Little, 1969).
Residual vs transported soil formation (after Wesley, 2010).
ISC'5 (2016)
Residual Soil / Weak Rock Profile - Definition
ISC'5 (2016)
Distribution of residual soils in SEQ
Profile Type / Parent Rock
Material
Alluvial Soils
Cohesive Residual Soils
(from Claystone / Mudstone /
Coal / Shale)
Expansive Clays
(from Basalt)
Sandy Clay / Clayey Sand –
Various Parent Rock Types
(Igneous  Sedimentary)
Approx. Distribution of Materials
within SEQ area
(km2)
% of study area
4,650
16.0%
6,025
20.7%
4,500
15.5%
13,890
47.8%
Total Residual Soil / Weak Rock Profile > 80%
ISC'5 (2016)
Issues with Current Site Investigation Techniques
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Soil Drilling:
Soil:
Rotary Drilling /
• No significant Rock materials present
Penetration Tests
Auger Bit Refusal TRANSFORMATION ZONE: Extremely Weathered
(XW) Rock / Highly Weathered (HW)
• Rock and Disintegrated Rock Materials
• Original Rock Structure Exists
Sig. Core Loss
Diamond Bit
Drilling:
Full Rock Core
Recovery
ISC'5 (2016)
Competent Rock Material:
• Continuous Framework / Rock Material
Light Falling Weight Deflectometer (LFWD)
Directly Measures Insitu Young’s Modulus (E)
ELFWD = [A x P x R x (1 – u2)] / d0
Where:
A = Plate rigidity factor (/2 for rigid plate)
PMAX = Maximum Contact Pressure
R = Radius of plate
d0 = peak deflection
u = Poisson’s Ratio
ISC'5 (2016)
Light Falling Weight Deflectometer (LFWD)
• ‘Small Strain’ Instrument – Test Strain increases as plate diameter is reduced
ISC'5 (2016)
Light Falling Weight Deflectometer (LFWD)
• Modulus is stress dependent
Expected Stress
Dependent Response
Granular
Fine Grained /
Cohesive
ISC'5 (2016)
Light Falling Weight Deflectometer (LFWD)
• Modulus is stress dependent – Standardisation is required to allow direct comparison of results
ELFWD-100kPa
LFWD Modulus @
100kPa Test Stress
ISC'5 (2016)
Details of Site Investigation
• 14 Test Pits
• Up to 1.2m depth
• Non-reactive materials
• Investigation covered
Residual Soil to HW Rock
material profile
• Site Investigation Tests:
• LFWD
• DCP
• Density
• Laboratory Classification
ISC'5 (2016)
Results of Site Investigation
• General (as expected) trends of investigated residual / weak rock profile:
ISC'5 (2016)
Results of Site Investigation
• LFWD Derived Modulus – by rock weathering unit
ISC'5 (2016)
Results of Site Investigation
• LFWD Derived Modulus:
Weathering State
Average LFWD Modulus at 100kPa
Stress (ELFWD-100kPa)
Percentage Increase above Residual Soil
Modulus
Residual Soil
38.2 MPa
-
XW Rock
69.8 MPa
+82%
XW / HW Rock
85.1 MPa
+123%
HW Rock
134.3 MPa
+250%
• All attempted LFWD tests successful
• Gradational (near-linear) increase of modulus associated with weathering profile was quantified
• Weak rock material deformation response fully within the LFWD equipment’s calibrated range
LFWD Suitable for use in site investigation of Residual soil / Weak Rock
ISC'5 (2016)
Correlation with other insitu tests
• With DCP:
• DCP test continually refused in XW/HW Rock materials
• Variability (CoV) of test results of DCP >> LFWD
• DCP results then processed via generic correlations to produce modulus (E)
ISC'5 (2016)
Correlation with other insitu tests
• With Soaked CBR:
ISC'5 (2016)
A final word of Warning – Not all LFWD are equal
• Variations in LFWD type result in different E values:
ISC'5 (2016)
Conclusions
This study has demonstrated:
The gradational change between Residual Soil to Weak Rock has associated gradational change
in stiffness parameter
ISC'5 (2016)
Conclusions
This study has demonstrated:
The gradational change between Residual Soil to Weak Rock has associated gradational change
in stiffness parameter
The LFWD is suitable for use in site investigation of Residual Soil – Weak Rock materials
(bridges the gap across ‘soil’ and ‘rock’ testing techniques)
ISC'5 (2016)
Conclusions
This study has demonstrated:
The gradational change between Residual Soil to Weak Rock has associated gradational change
in stiffness parameter
The LFWD is suitable for use in site investigation of Residual Soil – Weak Rock materials
(bridges the gap across ‘soil’ and ‘rock’ testing techniques)
Direct measurement of Modulus via LFWD has significant advantages over indirect estimation
from penetration tests
ISC'5 (2016)
Conclusions
This study has demonstrated:
The gradational change between Residual Soil to Weak Rock has associated gradational change
in stiffness parameter
The LFWD is suitable for use in site investigation of Residual Soil – Weak Rock materials
(bridges the gap across ‘soil’ and ‘rock’ testing techniques)
Direct measurement of Modulus via LFWD has significant advantages over indirect estimation
from penetration tests
Laboratory determined CBR values do not reflect field conditions within weathered rock
materials
ISC'5 (2016)
Conclusions
This study has demonstrated:
The gradational change between Residual Soil to Weak Rock has associated gradational change
in stiffness parameter
The LFWD is suitable for use in site investigation of Residual Soil – Weak Rock materials
(bridges the gap across ‘soil’ and ‘rock’ testing techniques)
Direct measurement of Modulus via LFWD has significant advantages over indirect estimation
from penetration tests
Laboratory determined CBR values do not reflect field conditions within weathered rock
materials
Depth of measurement of LFWD is re-affirmed to be approx. 1.3 x Plate Diameter
ISC'5 (2016)
Questions
ISC'5 (2016)