Ground Risk: why take the chance? The application of Advanced Continuous Surface Wave data to managing ground risk

Transcription

1 Ground Risk: why take the chance? The application of Advanced Continuous Surface Wave data to managing ground risk Ground Risk: Why Take the Chance? The application of ACSW data to managing ground risk Chris Milne, Director, Ground Stiffness Surveys Limited

2 Continuing challenges for GI Risk unforeseen ground conditions, services, hazardous or sensitive environments Cost expenditure due early in project life Time access restrictions; need for rapid results Conservatism wider industry slow to accept new approaches Lack of understanding of value of properly scoped GI (still!) for some projects

3 New challenges for GI Focus on SLS need for accurate strain softened stiffness values Recognition of limitations of traditional investigation approaches for design e.g. SPT stiffness relationships Advances in design software increasing use of FEA Digital data requirements

4 Ground Risk: Why Take the Chance? The application of ACSW data to managing ground risk Surface wave methods Testing measures speed of surface (Rayleigh) waves travelling through the ground over a range of frequencies Profile depth based on wavelength (velocity/frequency) Known velocity relationships between Rayleigh wave velocity (V r ), shear wave velocity (V s ) and stiffness (G 0 ) see Tomlinson! Known stiffness relationships between G & E for design strain-softening Obvious advantages rapid, economical, non-intrusive, in-situ, bulk stiffness measurement - why not commonly used? Surface wave testing has been the next big thing for the last 20 years

5 Ground Risk: Why Take the Chance? The application of ACSW data to managing ground risk What is ACSW? Advanced Continuous Surface Wave (ACSW) developed to address perceived issues with surface wave testing: Speed & cost Practicality in UK construction environment Standardisation as an engineering test Repeatability& quality control Flexibility & accessibility of outputs

6 Single operator vehicle lift ACSW test setup Warning signs prohibiting unauthorised access to test area Generator & test equipment operated within vehicle Armoured shaker power cable (230v 3-phase) Test location as selected by client G0 3 m G1 G2 G3 G4 G5 1 m Data collection within vehicle Data cable from geophones Numbered spike geophones (typical 0.6m spacing) 6 m Shaker seated on 25mm sand bed in hand excavated approx.. 400Lx400Wx50D mm excavation (if required) See GSS Test Layout Drawing GSSDWG001 Adapted 4WD testing vehicle

7 Rapid and low impact test methodology Robust all-weather system Automated by easy-to-use software Rapid (20 mins per test) Low intensity single technician, small working area Not sensitive to site noise working around other operations possible Low cost about that of a PLT Low risk non-intrusive, low vibration ACSW construction testing system capable of 12 to 20 tests per shift

8 C-DAS control & analysis software Modelling Test control Field user interfaces Test: Test notes: CSW_091P Date: 28 February 2018 First test of night in Up 4 foot at southern end of site. Good model fit (10 layers - minimum 2m, maximum 3m thickness). Flexible reporting Default values of density and Poisson's Ratio in the highlighted columns may be adjusted to known values Strain level of softened value of Young's Modulus using the Rollins equation can be adjusted in the cell below. See the GSS report ref GSS060-CSW-05 for conditions of use of data Strain level softened to: 0.1 % Vs Thickness Depth Density Go ν Eo E at x% strain DON'T (m/s) (m) (m) (kg/m 3 ) (MPa) (MPa) (MPa) CHANGE

9 Typical ACSW field testing arrangement Wheelbarrow cradle Vehicle lift Safe one person operation from modified 4WD vehicle Shaker Geophone array

10 Site operation

11 Dispersion curve V r against frequency Simple inversion approximate average stiffness with depth C-DAS field output

12 C-DAS reporting output Editable spreadsheet output giving strain softened E Modelled layered stiffness profile

13 Ground Risk: Why Take the Chance? The application of ACSW data to managing ground risk ACSW risk reduction applications Rapid, low risk, low intensity, low cost: 1. supplementary GI e.g. sensitive, difficult to investigate or high-risk sites 2. high quality stiffness data 3. validation & control testing e.g. ground improvement 4. hazard assessment e.g. shaft investigations 5. Rayleigh wave investigation e.g. HS2 Easy to understand immediate on-site outputs for targeted investigation

14 Supplementary GI: Ground profile comparisons Boundaries indicated in simple inversion data

15 Supplementary GI: Ground profile comparisons Increasing near surface stiffness with increasing depth of Oxford Clay cutting G 0 Stiff MG and granular Alluvium over very soft Clay and Peat

16 Supplementary GI: problematic ground How to investigate intrusively? 2 nd Forth Crossing Approach Embankment

17 Stiffness data: ACSW vs High Pressure Dilatometer HPD G ur data points HPD G ur data points HPD G i data points Simple inversion HPD G i data points Advanced inversion

18 Conservative empirical relationships between CPT & SPT & stiffness Sensitive rock flour silt Stiffness data: Settlement analysis case study - 1 Heymann, G., Rigby-Jones, J., Milne, C.A. (2017) The application of Continuous Surface Wave testing for settlement analysis with reference to a full scale load test for a bridge at Pont Mein Rûg, Wales. SAICE journal.

19 Stiffness data: Settlement analysis case study - 2

20 Validation & control testing: ground improvement Reinforced soil block Soil beyond reinforcement Soil mixed columns in domestic waste 3D modelling of backfilled workings Untreated ground profile

21 Possible shaft location Highway construction Possible shaft location Hazard assessment: Shaft investigations

22 Evaluating impacts of design mitigations Higher mode & effective dispersion curve modelling Consistency of approach: C-DAS used for modelling & field data evaluation Rayleigh wave investigation: High Speed Rail modelling & mitigation design Modelling using derived BH Vs profiles (direct import)

23 Discussion ACSW can offer a rapid, cost-effective means for geotechnical risk reduction ACSW is another tool in the GI armoury it doesn't replace the need for a comprehensive GI and other techniques may sometimes be more appropriate As with all testing, ACSW needs to be undertaken and reviewed by a competent professional in the light of all available geotechnical data Research and development is ongoing expect to see more published ACSW applications A significant advancement in development of a non-intrusive in situ testing that has the potential to change Site Investigation work 2018 Ground Engineering Award Judges