Presentation on theme: "ANALYSIS OF THE LIGHT WEIGHT DEFLECTOMETER IN-SITU STRESS AND STRAIN"— Presentation transcript:
1 ANALYSIS OF THE LIGHT WEIGHT DEFLECTOMETER IN-SITU STRESS AND STRAIN Patrick K. MillerBSCE Tufts UniversityMS Colorado School of MinesProject Engineer – Olson Engineering
2 LWD Background Purpose To measure in-situ elastic modulus of soils QC/QA deviceOperation1 person to operate1-3 minutes per testWeighs approximately 20 kgCommon DevicesPrima 100Zorn ZFG 2000Prima 100 Zorn ZFG 2000
3 Current Analysis Technique Based upon Boussinesq’s theoretical solution to a static load applied through a rigid circular plate on an elastic half-space.
4 Previous In-situ Stress and Strain Research Fleming (2000)Used in-situ stress sensors to measure stress induced by the LWDDid not explore drop height, plate diameter and soil type effectsDid not measure in-situ strainSeveral Researchers have used stress sensors to measure in-situ stress levels from various loading conditions and devices.Few Researchers have used potentiometers, LVDT’s, or accelerometers to measure in-situ displacement and/or strain produced by various devices.
5 Main Research Objectives Employ In-situ Sensors to Measure LWD Induced Stress and Strain LevelsCharacterize stress and strain state under LWD loadingDetermine how stress and strain vary with loading plate diameter and drop height (applied force)Compare Secant Modulus from in-situ stress and strain data to modulus value given by the current analysis methodCharacterize “Influence Depth” of the LWD
7 Sensor Calibration EPC Calibration LVDT Calibration EPC’s calibrated in a laboratory calibration device at UMN.Potential Issues include: stress concentrations, shadowing effects, variable temperature effects, etc.LVDT CalibrationFactory calibrationNo known calibration issues
8 Sensor Placement Procedure EPC PlacementPlaced by hand in lightly compacted new liftEncased in a pocket of the calibration sandLVDT Placement
9 Soil Profiles Tested 4 Locations tested for each profile (2 EPC, 2 LVDT)0.14 m0.42 m0.68 mBuried EPCs
10 In-situ Stress Results Key PointsMagnitude and duration of the stress pulse is greater in the sand than in the clayAt the deepest layer, the homogeneous profile has a greater magnitude and duration than the layered profile
11 Contact Stress Distribution Terzaghi (1943) theorized that arigid circular plate produces a:Inverse Parabolic Distributionon cohesive soilsParabolic Distributionon non-cohesive soilsUniform Distributionon soils having mixedcharacteristicsTherefore: Uniform and Parabolic loadings produce E’s of127 and 170 % of the Inverse Parabolic loading
12 In-situ Stress Results Employing Static Theory of ElasticityThe increase in stress at depth z due to a surface loading is given by:Experimental data verifies Terzaghi’s theory of soil dependent contact stressSuggests that the LWD analysis should reflect the soil type tested
13 In-situ Stress Results Terzaghi also theorized that the contact stress between a rigid plate and soil is dependent upon the level of loadingA cohesive material exhibits an inverse parabolic distribution at low levels of loading and trends toward a uniform distribution at loads producing failureThe experimental data also appears to confirm this theoryTherefore understanding the level of loading due to the LWD may also be important in the data analysis
14 Plate Diameter and Drop Height Effects Key PointsStress magnitude of 200 mm load plate is greater near the surface but not at depthThe stress magnitude at each layer is proportional to the applied force (drop height)
15 In-situ Strain Results Employing Static Theory of ElasticityThe increase in strain at depth z is given by:Where:Using a constant modulus the in-situ strain data was fitThe strain decreased much more rapidly with depth than the stressNote that only the 200 mm plate and largest drop height produced measurable strain at the second layer of sensors
16 In-situ Strain Results An elastic modulus which increased with depth was utilized to fit the strain dataIt is well know that E increases with a decrease in deviator stress and an increase in confining stress, both cases exist hereThe exponentially increasing E provided the best fitThe deviator and confining stress dependent E equation provided a much better fit than the constant EMore data is needed to validate these findings
17 Stress/Strain Results The secant modulus of the vertical in-situ stress and strain data was calculated and deemed ErEr and ELWD values were significantly different, and displayed different trendsEr vs. ELWD Values
18 ConclusionsContact stress between the soil and LWD is dependent on the soil type and level of loadingCohesive soil ~ inverse parabolic distributionNon-cohesive soil ~ parabolic distributionMixed characteristic soil ~ uniform distributionStrain decreased much more rapidly than stress with depthA modulus profile which increased with depth more closely matched the experimental strain data.The secant modulus values calculated from the in-situ stress and strain data did not compare well with values obtained from the LWDContinuing ResearchMore data needed from all soil types, focusing near the surfaceTactile sensors – to measure pressure distributionRefinement/Laboratory calibration of strain sensors
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