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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 ? ? ? ? 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)