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In-situ testing to verify ground improvement

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In-situ testing to verify ground improvement

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In-situ testing to verify ground improvement

  1. 1. RaviRavi SundaramSundaram & Sanjay Gupta& Sanjay Gupta Cengrs Geotechnica Pvt. Ltd. Noida A-100 Sector 63, Noida Ph: (O): 0120-42067111 (M): 9810538095
  2. 2. Ground Improvement Process Modification of soil properties to achieveModification of soil properties to achieve improvementimprovement Densification of loose soils Mitigation of liquefaction potential Strengthen soft clays Increase safe bearing capacity Reduce foundation settlement Ensure that foundation behavior is within the acceptable limits
  3. 3. Success of Ground Improvement Process Identify target soil properties to beIdentify target soil properties to be achieved after improvement, whatachieved after improvement, what minimum value is acceptableminimum value is acceptable InIn--situ testing is essential to ensuresitu testing is essential to ensure that the desired improvement isthat the desired improvement is achievedachieved The soil characteristics afterThe soil characteristics after improvement should be comparedimprovement should be compared with the target soil propertieswith the target soil properties
  4. 4. This presentation covers Two case studiesTwo case studies Ground improvement for a Gas Based Power Plant inGround improvement for a Gas Based Power Plant in North Delhi byNorth Delhi by VibroVibro--ReplacementReplacement In-situ tests: dynamic cone penetration tests, and Load test on the stone columns Ground Improvement for a university at GreaterGround Improvement for a university at Greater NoidaNoida by Dynamic Compactionby Dynamic Compaction In-situ tests: Boreholes with SPT Static Cone Penetration tests
  5. 5. VibroVibro--Replacement for a PowerReplacement for a Power Plant in DelhiPlant in Delhi
  6. 6. Case Study-1: Vibro-Compaction 108 MW Gas Based Power Plant in north Delhi Facilities planned include STG, GTG, Steam Turbine, Boiler, Chimney, Cooling Water System, Switchyard, etc. Alluvial Plains of River Yamuna Earthquake Zone IV as per IS 1893-2002 Loose sands to 8 m depth prone to liquefaction during major earthquakes
  7. 7. Site Layout Plan 15 boreholes – 30 m depth 6 static cone penetration tests(SCPT) Spectral Analysis of Surface Waves (SASW) tests along 8 lines 3 cross-hole seismic tests (CHST)
  8. 8. Typical Borehole Data Loose surficial fill to 0.5-2 m depth Natural deposits primarily fine sand / silty sand with intermediate minor layers of sandy silt Groundwater at 5.2 – 6.4 m depth
  9. 9. Static Cone Penetration Test
  10. 10. SASW and CHST
  11. 11. Design Profile
  12. 12. Liquefaction Assessment Seed & Idris (1971) method – NEERI Summary Report Project area in Earthquake Zone IV Maximum Credible Earthquake Design Earthquake Magnitude: 6.7 on Richter scale Peak Ground Acceleration: 0.24g Cyclic Resistance Ratio (CRR) determined from SPT & SCPT
  13. 13. Liquefaction analysis results A critical factor of safety of 1.2 was considered for the analysis Soils to a depth of 8.0 m are be susceptible to liquefaction during earthquakes
  14. 14. Foundation System Open foundations bearing on natural soils - Not feasible Critical or heavily loaded plant facilities – STG, GTG, Steam Turbine, Boiler, Chimney 600600 mm diameter bored castmm diameter bored cast--inin--situsitu piles extending well below thepiles extending well below the liquefiable zoneliquefiable zone
  15. 15. Foundation System Medium loaded plant facilities – Cooling Tower,Cooling Tower, ClariflocculatorsClariflocculators & other facilities of the& other facilities of the Water Treatment system,Water Treatment system, SwitchyardSwitchyard Ground improvement doneGround improvement done byby vibrovibro--replacement methodreplacement method
  16. 16. Vibro-Replacement Method Dry Vibro Stone columns installed by bottom- feed method 500 mm dia extending to 10 m depth Centre-to-centre spacing: 1.5 m Design Net Bearing Pressure: 160 kPa
  17. 17. In-Situ Tests Dynamic Cone Penetration Tests (DCPT) Evaluates extent of improvement achieved with depth Load Test on Stone Columns Evaluates load-settlement behavior of improved ground
  18. 18. Dynamic Cone Penetration Tests After CompactionAfter Compaction Blow Counts exceed 15 below 2 m depth Substantial improvement in penetration resistance Medium dense to 4-5 m depth Dense below 5 m depth Improved soils not likely to liquefy during earthquake
  19. 19. Load Test on Vibro-Columns
  20. 20. Plate size 1.51.5 m xm x 1.51.5 mm, square, 30 mm thick, Loading intensity: 1st cycle – 240 kPa 2nd cycle – 500 kPa Safe bearing pressure > 160 kPa Loading Intensity vs. Settlement
  21. 21. Liquefaction Mitigation Untreated ground (before compaction) is susceptible to liquefaction to 8 m depth After compaction, Factor of Safety against liquefaction > 1.2 Susceptibility to liquefactionSusceptibility to liquefaction successfully mitigatedsuccessfully mitigated
  22. 22. Dynamic Compaction for aDynamic Compaction for a University in GreaterUniversity in Greater NoidaNoida
  23. 23. Case StudyCase Study--2: Dynamic Compaction2: Dynamic Compaction A major university at Greater Noida, UP Covers an area of about 500 acres 84,000 m2 of constructed area, 30% green cover Site in the flood plains of the River Yamuna, about 2 km from river
  24. 24. Vicinity Map
  25. 25. 25 Site Conditions Loose alluvium - fine sand (Yamuna Sand) met in project area Site is in Earthquake Zone IV - IS 1893: 2002 Groundwater met at shallow depth Every structure individually assessed to evaluate liquefaction potential Sand to 8-12 m depth is prone to liquefaction during major earthquakes
  26. 26. Geotechnical Investigations Over 600 boreholes and 150 SCPT’s done all over the university area Each structure assessed to evaluate liquefaction potential THIS PRESENTATIONTHIS PRESENTATION covers geotechnical investigation before & after improvement for Boys Hostel 8.1KBoys Hostel 8.1K
  27. 27. Site Layout Plan – Boys Hostel No. 8.1KBoys Hostel No. 8.1K Before Improvement 4 BH – 15 m 1 SCPT – 15 m After Improvement 4 BH – 15 m 1 SCPT – 15 m
  28. 28. Typical Borehole Data The soils at the site classify primarily as sandy silt / clayey silt to 1.5~2 m depth, underlain by fine sand to 15 m depth Fines content:3- 10 % Groundwater at 3.5-4 m depth, may rise to GL
  29. 29. Liquefaction Assessment Seed & Idris (1971) method – NEERI Summary Report Project area in Earthquake Zone IV Design Earthquake Magnitude: 6.7 on Richter scale Peak Ground Acceleration: 0.24g Cyclic Resistance Ratio (CRR) determined from SPT & SCPT Fine sands to 8 m depth at Boy’s Hostel is susceptible to earthquake during the design earthquake
  30. 30. Foundation System Open foundations bearing on natural soils - Not feasible Pile foundations - high foundation cost and time of construction – Not preferred by client Solution - Ground improvementGround improvement by dynamic compactionby dynamic compaction
  31. 31. Dynamic Compaction Dropping a heavy weight can compact loose sands to substantial depth Done on a grid pattern Next cycle: Weight dropped at intermediate points Effective for sands only with little fines
  32. 32. Conceptual Illustration
  33. 33. Crane & Pounder Conventional Crane – TLC 955A 11.65 T pounder falling from height of 14 m Energy: 1600 kN-m Corresponding depth of improvement: 9 m
  34. 34. Compaction – 2 phases 1 week time lag in between – to allow pore pressures to dissipate 1st Phase 4 x 4 m grids 2nd Phase staggered 2 m No. of drops: 10 at each grid point Depth of the craters formed: 1.0-1.5 m approx
  35. 35. Ironing Phase Craters filled with GSB Grade II material Hammer weight: 11.65 T Height of fall: 6 m No. of drops: 5 Energy: 2114 kN-m Area graded with 10 passes of 10 T vibratory roller
  36. 36. In-Situ Tests Standard Penetration Tests (SPT) in boreholes Static Cone Penetration Test (SCPT) Compare SPT and qCompare SPT and qcc valuesvalues before and after compactionbefore and after compaction Assess Liquefaction PotentialAssess Liquefaction Potential after densificationafter densification
  37. 37. SPT & SCPT before & after improvement
  38. 38. Extent of Improvement Achieved After compaction, N>16After compaction, N>16--20, qc > 520, qc > 5 MPaMPa Peak improvement: between 1 and 5 m depth Improvement below 10-11 m depth is marginal
  39. 39. Liquefaction susceptibility analysis before and after compaction Before CompactionBefore Compaction -- Liquefaction to 12 m depthLiquefaction to 12 m depth After CompactionAfter Compaction -- No LiquefactionNo Liquefaction Susceptibility to liquefaction successfully mitigatedSusceptibility to liquefaction successfully mitigated
  40. 40. Liquefaction Mitigation Untreated ground (before compaction) is susceptible to liquefaction to 13 m depth After compaction, Factor of Safety againstFactor of Safety against liquefaction > 1liquefaction > 1 Foundation for the Boys Hostel building 8.1K: Isolated column footings withIsolated column footings with interinter--connecting plinth beamsconnecting plinth beams Net allowable bearing pressure:Net allowable bearing pressure: 175175 kPakPa
  41. 41. Concluding Remarks Conducting in-situ tests is essential to verify effectiveness of ground improvement For reliable and effective improvement, sufficient tests should be performed before and after improvement The testing should ensure that the target soil properties are achieved Mitigating liquefaction Densification of loose soils Desired Bearing Capacity
  42. 42. Thank You!

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