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Mitigating foundation distress
- 1. Sanjay Gupta Ravi Sundaram CENGRS GEOTECHNICA PVT. LTD.CENGRS GEOTECHNICA PVT. LTD. Sanjay Gupta Ravi Sundaram CENGRS GEOTECHNICA PVT. LTD.CENGRS GEOTECHNICA PVT. LTD.
- 3. FOUNDATION STRUCTURAL DISTRESS GEO PROFESSIONAL To act as a Doctor with healing touch & not as Policeman
- 5. Foundation Distress MAN MADE NATURAL DISASTER / CASUALTIES MAY OR MAY NOT LEAD TO Well / pond Artesian Ignorance of soil / Loads / additional load
- 6. Variation in Loads on Foundations • Additional floors • Change in space utilization • Change in structural / architectural layout
- 7. Soil Conditions • Unusual soil conditions – loose soils, unmapped tunnels, localized problems • Strata not behaving as envisaged • Inadequate soil data - misinterpretation and / or wrong analysis •• DESIGN BASISDESIGN BASIS –– WITHOUTWITHOUT REFERENCE TO SITE CONDITIONSREFERENCE TO SITE CONDITIONS (MISTRY APPROACH)(MISTRY APPROACH)
- 8. Underground Tunnel / Cavity
- 10. Settlement Crack in Wall
- 11. Building Damage Due to Earthquake
- 12. TOTALTOTAL FOUNDATION DISTRESSFOUNDATION DISTRESS SETTLEMENTSSETTLEMENTS SHEAR FAILURESHEAR FAILURE SERVICEABILITYSERVICEABILITY FAILUREFAILURE DIFFERENTIALDIFFERENTIAL LOWER FACTORLOWER FACTOR OF SAFETYOF SAFETY Approach same as FORENSICApproach same as FORENSIC BUT without court of LAWBUT without court of LAW MITIGATING
- 13. Mitigating Distress • Steps taken before disaster strikes or on noticing first signs of distress • Preventive action to avoid failure • Experienced engineer can identify telltale signs
- 14. Mitigating Foundation Distress • Ensure desired factor of safety • Meet Serviceability criterion • Restrict further settlements • Allow additional load -vertical expansion • Facilitate adjoining constructions
- 16. 4 W’ S - What Went Wrong & Why Brain StormingBrain Storming InteractiveInteractive
- 17. The 4 W’s • Identify the problem • Investigate causes, assess risk • Review structural details & geotechnical data • Assess foundation behavior, perform back- analysis of data
- 18. 4W’s - Approach Mitigating distress Selection of Investigation Techniques Geology of Area & soil data Foundation detail Assessment of Reasons & nature of distress Design Profile Appropriate Analysis 4 W
- 19. Implementation • Develop technically feasible solutions • Review merits & demerits • Select final solution • Monitoring during Implementation
- 20. ProjectProject ProblemProblem Inadequate foundation / excessive settlement Solution ConceptsSolution Concepts Geotechnical DataGeotechnical Data ReviewReview Merits/DemeritsMerits/Demerits Structural DetailsStructural Details Analysis Prediction ofPrediction of Foundation BehaviorFoundation Behavior Final SchemeFinal Scheme Implementation
- 21. Failure - Distress • Actual Failure: – May be able to work back the soil parameters , by Back Analysis • Partial Failure / Distress – Serviceability Problem: May not be able to do back-analysis. Requires ingenuity of the engineer
- 22. Mitigating Distress -Approaches • Improve soil / strata strength properties – Engineer the supporting ground • Confinement of foundations – To restrict settlements – To increase shear resistance • Restrict lateral soil movement – Grouting – Contiguous piles / in combination with different techniques
- 23. Mitigating - Foundation Distress Projects executed - CENGRS
- 24. Janmabhoomi Temple, Mangarh Main Temple Garba Griha Platform Small Temples/ Corridor Small Temples/ Corridor
- 25. Mitigating Foundation Distress Janmabhoomi Temple , MangarhJanmabhoomi Temple , Mangarh • Distress – minor tilting of foundation block • Distress mitigated to achieve: – Desired factor of safety – Serviceability criterion – Stability – dome type structures
- 26. Temple at Birth Place of His Holiness KripaluHis Holiness Kripalu MaharajMaharaj a highly respected saint at MangarhMangarh, about 35 km from Allahabad towards Lucknow
- 27. Janmabhoomi Temple, Mangarh • Massive structure – traditional ancient Hindu temple architecture • Granite blocks – upto platform level • Carved granite blocks - temple structure • Architecture on same lines as – Ancient Somnath Temples – Modern Akshardham
- 28. A View of the Constructed Temple Garba GrihaGarba Griha MandapMandap Small TemplesSmall Temples CorridorCorridor PlatformPlatform
- 29. Structural Details of Temple • Foundation depth 3 m below G.L. • Platform constructed upto 2.4 m above G.L. • Total 5.4 m thick massive granite structure – bearing pressure at foundation level equal to 14-15 T/m2 for platform (foundation block) only • Main block surrounded by 1.2 m wide peripheral granite stone walls for small temples/corridors
- 30. Geotechnical Considerations • Total settlements : no major problem • Tilting : not permissible • Differential settlement : to be minimized • Dome type structure : Cannot withstand tilt • Different loadings: – Platform : 15 ∼ 16.0 T/m2 – Main temple : 25 ∼ 27 T/m2 – Small temples : 20 ∼ 21 T/m2
- 31. Problem • Cracks noticed when constructed upto +2.4 m level (at loading of approx. 15.5 T/m2) even before construction of superstructure began • Engineers opined differential settlements / tilt critical under varied loading conditions vs. Soil characteristics especially in view of Dome type construction
- 32. Limitations to Solutions • MYTH – No steel should touch temple structures – No steel to be used in Mandap area • Limited work space for additional foundation system • No RCC for taking tensile / bending stresses • Basic structure not to be altered
- 33. Soil Profile – Mangarh
- 34. SPT Results – Mangarh Foundation Level
- 35. SCPT Results – Mangarh Foundation Level
- 36. Engineering Solution • Create confinement to main temple • Restrict outward flow of soil , in MANDAP • Improve soils around small temples / corridor
- 38. Confinement of Main Temple • To create cut-off-wall – Pressure distribution more uniform – Increase bearing capacity safety factor – Restrict lateral soil movement – Restrict differential settlement • Ensures total settlement more uniform in more controlled manner
- 39. Piles for Confinement • All around main temple block 500 ∼ 800 mm wide space available • RCC pile 400 mm dia, 12 m depth below G.L. (i.e. 9 m below foundn level) • Piles cast 700 mm below Platform level • Interconnecting beam in annular space • Top of beam 300 mm below platform level
- 40. Confinement Piles – Mangarh
- 41. Myth, Concept – Mangarh • RCC of beam and stone masonry separated by Shailtex board • Open space covered by granite slabs maintaining air gap between RCC beam and stone masonry
- 42. Details of Piles (for Confinement)
- 43. Mandap Area • Octagonal opening in centre • Big dome supported over 8 columns • Cannot tolerate differential settlement
- 44. Mandap Area CONCERNCONCERN • Soil plug may squeeze & may move up due to lateral soil pressure • No R.C.C. piles could be used - MYTH DECISIONDECISION • Concrete piles to confine Mandap area with concrete slab
- 45. Piles Layout – Mandap, Mangarh • Dome – Supported by 8 columns – Sensitive to tilting / differential settlement
- 46. Pile Details – Mandap, Mangarh
- 47. Small Temples / Corridor • Strip footing 1.2 m wide • Loading 21 T/m2 • Soils weak shear failure / punching failure • Settlement excessive under loading • Differential settlement due to variance in rigidity /stiffness of structure – main temple, small temples & platform
- 48. Small Temples - Improvement • By granular piles / stone columns – φ of composite material : 35 ∼ 45° – Avoid punching / shear failure • Compact soils around strip footing – improve bearing capacity – reduce settlements under loading of 21 T/m2 • Greater compatibility with main block reducing differential settlements
- 49. Ground Improvement • Rammed stone columns / granular piles 500 mm dia : 5 m below foundation level • To provide upto 1.5 m above foundation level or upto 1.5 m below ground level • Sand filling compacted above stone columns up to platform level
- 51. Small Temples – A View
- 52. Foundation Stabilized Effectively • Construction done at slow rate due to elaborate onsite carving – Advantageous from the geotechnical point of view – Controlled and uniform settlement THE TEMPLE IS PLANNED TOTHE TEMPLE IS PLANNED TO SURVIVE FOR CENTURIES!!SURVIVE FOR CENTURIES!!
- 55. Over head water tank – 4 W’s • OHT of 30 m high • Has cyclic loading conditions – filling & emptying daily • During hydrotest , slight tilt was observed • Designs were checked & found O.K. • Investigations carried out • Inferred – soil strata is responsible Domain of GeoDomain of Geo –– ProfessionalProfessional
- 56. Stratigraphy Under Tank Loose Sandy SiltLoose Sandy Silt N = 8N = 8--1010 Sandy silt with gravels /hard strataSandy silt with gravels /hard strata • Geotechnical problem – understand what went wrong & why -- 4W’S – Differential settlement – Variation in compressible layer thickness – Under ground flow of water
- 57. Foundation Strengthening Scheme • Curtain wall to create confinement, restrict lateral soil movement and control migration of fines • A combination of closely spaced short piles seated on underlying hard strata and cement grouting of the loose soils
- 59. Foundation Strengthening Scheme PilesPiles Grout HolesGrout Holes Edge of RaftEdge of Raft FoundationFoundation
- 60. First Phase • 300 mm diameter auger piles extending to top of hard/refusal strata • A conduit pipe was placed in pile bores prior to concreting • After concreting, cement grout was pumped through the conduit pipes Conduit PipeConduit Pipe FromFrom Grout PumpGrout Pump 300mm GLGL Hard/Refusal StrataHard/Refusal Strata Concrete PileConcrete Pile ReinforcementReinforcement steelsteel
- 61. Second Phase • Grout holes of 75mm diameter were drilled in the space between the piles and raft foundation up to the refusal/hard layer • The grout holes were plugged at foundation level • Pressure grouting with cement slurry was done till refusal to further grout intake was met under a pressure of 2 to 2.5 kg/cm2 Conduit PipeConduit Pipe FromFrom Grout PumpGrout Pump 75mm GLGL Hard/Refusal StrataHard/Refusal Strata Founding LevelFounding Level
- 62. RESULT • Tank in operation on daily basis • Water filling / emptying daily • No further distress / Tilt noticed over last few years
- 63. Mitigating Foundation Distress Factor of Safety - Criterion
- 65. Residential Complex – 4 W’s • 4 storeyed residential blocks constructed • During excavations for services – Observed that foundations were extending just beyond wall widths & to shallower depths • Alarm soundedAlarm sounded – other wall foundations were checked – less foundation widths & depths were observed • Though structures were not showing any distress BUT FACTOR OF SAFETY WAS LOWER THAN THE REQUIRED
- 66. Typical Strata Structural Details • Load bearing walls with few columns • Spread footings • Foundation depth as 1.5 M , below GL • Designs - All. Loading intensity as 15 T / Sq.M • Weathered Quartzite bed rock
- 67. Evaluating the Foundations • At several locations, small pits were excavated to measure the actual foundation sizes • From the loading data, the actual bearing pressures were computed • Actual computed factor of safety ranged from 1.3 to 2.1 , i.e. less than desired F.O.S. of 2.5
- 68. SOLUTIONS • Various solutions were reviewed for supporting structural loads by additional foundation system w.r.t merits & demerits – By under pinning – By providing ISMB S , etc. FINAL CHOICE By providing PILESPILES –– NEEDLE BEAM SYSTEMNEEDLE BEAM SYSTEM
- 69. Piles – Needle Beam System • Providing piles on either side of wall • Connecting piles by Needle Beam over piles & through the wall • Concreting & grouting the gap between the beam & wall • Provide Re-bar in walls upto 1.2 m ht. Above plinth level to act as beam for distribution of loads
- 70. The Needle Beam Concept Brick WallBrick Wall Damp Proof Course (DPC)Damp Proof Course (DPC) Needle BeamNeedle Beam Existing BrickExisting Brick FootingFooting PilePile Hard StrataHard Strata Typical SectionTypical Section 750 mm750 mm750 mm750 mm 300300 300300
- 71. APPROACH • STEPS – Excavate pits for each wall foundation – Measure existing foundation sizes – Calculate load that can be transferred – Compare with structural loading – Compute deficiency in loading – Design spacing for pile group to meet deficiency
- 72. Installing the Needle Beam
- 73. Typical Layout 16 T/m2 16 T/m2 17 T/m2 17 T/m2 9.5 T/m2 11T/m2 9.5T/m2 9.5T/m2 11 T/m2 BED ROOMBED ROOM DRAWING CUMDRAWING CUM DINING ROOMDINING ROOM KITCHENKITCHEN BED ROOMBED ROOM CourtyardLineCourtyardLine
- 74. RESULT •Housing complex handed over after strengthening •All houses under occupation NO DISTRESS / CRACKSNO DISTRESS / CRACKS OBSERVEDOBSERVED Structures in use for over 6Structures in use for over 6--7 years7 years with Desired Factor of Safetywith Desired Factor of Safety
- 75. CONCLUDING REMARKS • MITIGATING FOUNDATION DISTRESS • A STEP PRIOR TO DISASTER / FAILURE • Approach similar to human body • Diagnostic approach - 4 W’s – History – Pathological Investigation • Treatment - check response • Consult your GEOGEO--PROFESSIONALPROFESSIONAL as you would consult your DOCTOR