Advances in Field Testing Methods and Code of Practice

Transcription

1 Advances in Field Testing Methods and Code of Practice Prof. V.S.Raju (Formerly: Director, IIT Delhi & Professor and Dean, IIT Madras) 1 My background as a Foundation Consultant over 45 years Started in 1965, as a doctoral student in University of Karlsruhe Germany. The Consultancy services involved a variety of projects all over the country and abroad. Fertilizer plants, Power Stations, Harbour structures involving all major ports in the country, Airports, Residential Towers and Industrial Structures. Consultant to Larsen & Toubro, ECC for over 35 years. With APGENCO, for all their Power Stations for 30 years Expert Consultant to Hyderabad Metro on Foundations. Over the last 3 years, to 30 Gated Communities in the NCR Region, Delhi. 2 1

2 GEOTECHNICAL DESIGN Extract from Eurocode 7, BS EN :2004 The provisions of this standard are based on the assumptions given below: 1.Data required for design are collected, recorded and interpreted by appropriately qualified personnel; 2.Structures are designed by appropriately qualified and experienced personnel; 3 Continued. 3. Adequate continuity and communication exist between the personnel involved in data collection, design and construction; 4. Adequate supervision and quality control are provided in factories, in plants, and on site; 5. Execution is carried out according to the relevant standards and specifications by personnel having the appropriate skill and experience; 6. construction materials and products are used as specified in this standard or in the relevant material or product specifications; 4 2

3 Continued. 7. The structure will be adequately maintained to ensure its safety and serviceability for the designed service life; 8. The structure will be used for the purpose defined for the design. 9. These assumptions need to be considered both by the designer and the client. To prevent uncertainty, compliance with them should be documented, E.g. in the geotechnical design report. 5 Eurocode 7 is the latest and best code of practise. It helps in evolving Optimum Designs Safety and Economy Ensuring this standard will benefit India significantly 6 3

4 Planning Subsoil Investigation Number of Boreholes One for each important structure Information for every 50m of loaded area Borehole for medium size structure only if variations Few boreholes for future expansion Depth of Boreholes 1.5 to 2 times times the width of foundation Few up to hard strata( N > 100 ) or rock 5 to 10m beyond foundation level on rock Selection of Agency Not by lowest quotation Short listing of Agencies based on equipments condition and laboratory facilities Qualified/Trained manpower Constant supervision at site 4

5 Methods of Investigation Open Pits Bore Holes Geophysical Investigation 5

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7 GEOTECHNICAL INVESTIGATIONS QUALITY OF INVESTIGATION High quality is the very basis for excellence in practice. In spite of detailed standard specifications (IS-codes), the range of practices vary very widely in the country. For field investigations, most primitive to fairly sophisticated equipments are in use. Unfortunately, in majority of cases the practice is nearer to the primitive. Similarly laboratory testing practices vary widely, with little or no standardization. 13 QUALITY OF INVESTIGATION India o Poor quality of the equipment. o Wash Boring mostly, o SPT Equipment - manual operation o Conventional static cone penetration equipment. World Standard o Highly sophisticated and mechanised equipment. o Continuous core sampling (in soils as well). o SPT equipment with automatic hammer release. o Static cone test with electric cone and piezocone 14 7

8 Pic.1.1. Non Standard Equipment No safety measures Pic. 1.2 International Standard 15 Pic. 1.3 SPT Hammer Dropped Manually (2013) Pic SPT with Auto Trip Hammer + Standard Rig 16 8

9 Pic.1.6 Static Cone Penetration - Reaction with Sand bags Pic. 1.7 Crawler Mounted SCPT Equipment 17 CPT Truck 9

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11 Pic. 1.7 Conventional Static Cone Penetrometer- Mechanical Cone Pic. 1.8 Electric Cone plus pore pressure sensor 21 11

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13 Marine cone penetration system Control room Dimensions of Platform: 18mx8m Counter weight = 10 ton Diameter of Counter weight = 3.4 m Counter Weight - Tower 13

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21 MAIN PROBLEM Insufficient Appreciation with regard to the Quality and Importance of Soil Investigation at all Levels. Responsibility Geotechnical community of the country; it is a major failure on our part. Owners, Structural Consultants Lack of appreciation. Whole hearted commitment towards excellence needed

22 Benefits of Good Quality Investigations Factor of Safety adopted in Pile Foundations Based on Initial Pile Load Tests Pile Capacity Minimum Factor of Safety IS:2911 Eurocode 7 (Simplified) Implication: For the same situation as per IS, We provide 33 % more number of piles. 43 Case Study Terminal T3 at Delhi International Airport Terminal T1 was on Pile Foundation T3 was originally planned with Pile Foundation. However later changed to a raft through reassessment. Additionally 2 footing load tests (1.5 m x 1.5 m) were conducted at Founding Level

23 Settlement (mm) Results of Footing Load Tests for Delhi International Airport 1 st Footing load test: Soil type = Sandy silt SPT N = 50 Size of footing = 1.5 m x 1.5 m For q = 40 t/m 2 = 4 Kg/cm 2, Settlement (S) = 4 mm Load Vs Settlement Curve for DIAL Load Intensity (t/m 2 ) E s = * = 1119 Kg/cm 2 As per Shultze and Muhs for SPT N = 50 Es = 800 Kg/cm 2 Ratio = 1119/800 = In one of the sites in 2013 Pic. 1.5 Bent SPT Sampler Project: 10 Towers, 15 storeys each 46 23

24 Settlement (mm) Source of Disturbance During drilling operations Improper cleaning of the borehole bottom During driving of the sampler tube Stress release due to removal of overburden During ejecting the sample from sampling tube During storage of sample During preparation of sample for testing 2 nd Footing load test: Type of soil = Silt with fine sand SPT N = 22 Size of footing = 1.5 m x 1.5 m q = 38 t/m 2 = 3.8 Kg/cm 2 Settlement (S) = 5.4 mm Load Vs Settlement Curve for DIAL Load Intensity (t/m 2 ) E s = * = 787 Kg/cm 2 As per Shultze and Muhs for SPT N = 22 Es = 530 Kg/cm 2 Ratio = 787/530 =

25 Comparison of Soil Modulus (Es) values in kg/cm 2 from SPT correlations and from load test results: S. No SPT N Es in kg/cm 2 As per Bowles book As per IS Code As per Schultze and Muhs From Footing Load test at DIAL *** E s values calculated from Bowles book are for corrected N values. ***However, for the same N value at a depth of 8 m, the E s value would be only 400 kg/cm 2 as per IS. +++ However, for the same N value at a depth of 8 m, the E s value would be only 320 kg/cm 2 as per IS. 49 Delhi Airport T3 Based on these results, Raft Foundation was found fully satisfactory and adopted. 6 months of Time Saving A huge advantage, on total investment of 12,000 crores. Direct cost savings as well

26 PILED RAFT As compared to a full pile solution, pile assisted raft has a major advantage of substantial reduction in number of piles, which in turn results in savings of cost and time. It also to some extent removes uncertainties associated with bored piles and driven cast in situ piles. Extensively used world wide. 51 Design Philosophy of Piled Rafts Conventional Pile Design Method Disregards the capacity of Pile caps/rafts Increased number of piles or length of piles Very small allowable settlement Pile factor of safety (FS 2) Piled Raft Design Method Raft is the main bearing element Design for full utilization of pile capacity (FS 1) Piles are Settlement reducers Consideration of the optimal location of piles to decrease the differential settlement and bending moment of raft

27 Load (t) Concept of Piled Raft : A very good reference The Piled Raft Foundation for The Burj Dubai Design and Performance, IGS-Ferroco Terzaghi Oration 2008, by Prof.Harry Poulos. 53 Raft Piles Settlement (mm) Fig. 1 Load settlement curves for piles and Raft on cohesion-less soils. Rafts have large settlements before failure (sands & non plastic silts) 54 27

28 Schematic Model of Burj Kalifa on Piled Raft World s tallest building. 160 storey high rise tower. 55 Permissible settlements: As per IS 1904, 1986 (Reaffirmed1995): For spread foundation resting on Sand and Hard clay Maximum settlement (mm) Isolated Raft Type of structure foundation foundation For reinforced concrete structures

29 Settlement Observations from Literature Details of piled raft foundation of the buildings in Germany (Katzenbach,et.al., 2000) Structure Messe Torhaus Messeturm DG-Bank (Westend Strasse 1) American Express Max Height above ground surface (m) Basement floors Foundation area (m 2 ) 2 x Foundation level below GL (m) / Raft thickness (m) Number of piles 2 x Observed pile load (MN) Observed Max. settlements (mm) Contd 57 Structure Forum(Kasto Taunusto r and pollux) r Japan- Centre Congress centre Messe Frankfurt Main Tower Eurotheum Max Height above ground surface (m) / Basement floors Foundation area (m 2 ) Foundation level below GL (m) Raft thickness (m) Number of piles 25 26/ Observed pile load (MN) Observed Max. settlements (mm) /

30 Settlement of the piled raft foundation of the Burj Kalifa: 160 storey high rise tower Founded on 3.7 m thick raft supported on bored piles (1.5 m diameter, 50 m long). Estimated total settlement is 45 mm to 75 mm. Settlements under dead load (February 2008) 43 mm. Extrapolated to full load is 55 mm to 60 mm Predicted final settlement is 70 to 75 mm 59 Adoption of Piled Rafts in National Capital Region (NCR) Starting in 2009, we have consulted to 32 projects in the NCR. Except in 2 cases, foundation is either a raft or a pile assisted raft including a 50 storey tower. In about 6 cases we got a fresh soil investigation done. A proper analysis has lead to a very economical foundation designs. The details are given in the following slides 60 30

31 PROJECTS NEAR DELHI: Sector Area Number of storeys Ground Water Table (m) Liquefaction Potential Foundation Recommendations 103 Cosmocity Cosmocity Cosmocity 3 Gurgaon 1B + S B+S+30 2 Yes Raft Foundations Gurgaon 1B + S to 4.5 Yes Raft Foundations Gurgaon 1B + S Yes Pile Assisted Raft Foundations 68 Gurgaon 1B + G+ 13 to 25 2B + G+ 25 to No Raft Foundations 86 Gurgaon 1B + G+ 13 to No Raft Foundations 16 B Gurgaon 3B + G+ 17 to No Pile Assisted Raft, Raft Foundations 67 Gurgaon 2B + G+ 18 to No Raft Foundations 62 Gurgaon 3B + G+ 29 to 37 8 No Pile Assisted Raft Foundations Contd 61 Sector Area Number of storeys Ground Water Table (m) Liquefaction Potential Foundation Recommendatio ns 16 Noida G + 22 to No Pile Assisted Raft, Raft Foundations Gwalpahari Gurgaon 3B + G No Raft Foundations 48 Gurgaon 2B + G No Pile Assisted Raft Foundations 58 Gurgaon 2B + G+ 23 to 30 7 No Raft Foundations 58 Mixed Use Gurgaon 1B+G+25 to 29 8 to 13 No Raft Foundations, Pile Foundation Gwalpahari Gurgaon 2B + G+ 28 Not met No Raft Foundations 102 Gurgaon 1B + G Yes Pile Foundations 62 31

32 Sector Area Number of storeys Ground Water Table (m) Liquefaction Potential Foundation Recommendati ons 128 Noida 2B + G+ 35 to 38 8 No Pile Assisted Raft Foundations 60 Gurgaon 2B+G+8 25 No Raft Foundations 66 Gurgaon 2B+G No Raft Foundations 67 Gurgaon 2B+G No Raft Foundations River Heights Ghaziabad 3B+G No Raft Foundations Golf Links Ghaziabad 2B+G No Raft Foundations 63 Sector Area Number of storeys Ground Water Table (m) Liquefaction Potential Foundation Recommendat ions 88 A Center Court Solitairian City Kristal Court, Wish Town 5A Palwal 4A Bahadurgar h Gurgaon 2B+G No Raft Foundations Greater Noida NOIDA 1B+G No Pile Assisted Rafts & Raft Foundations 1B+G+20 t No Pile Assisted Raft Foundation Haryana G Yes Raft Foundation Haryana S to 2.7 Yes Raft Foundation on Vibro Stone Columns 64 32

33 Sector Area Number of storeys 77 Winter Hills 78 Monsoon Breeze Gurgaon 1B+G+15 to 17 Ground Water Table (m) Liquefaction Potential Foundation Recommendati ons 37 No Raft Foundation Gurgaon S+13 to No Raft Foundation Iconic Tower Kanpur 2B+G No Pile Assisted Raft Foundation DDA EWS Kalkaji S+14 Not Met No Strip Rafts 65 Importance of Settlement Observations The settlement information is most valuable, as these measurements will significantly contribute to improvements in design procedures, ensuring economy along with safety 66 33

34 Settlement Observations Method: Any precision leveling will be O.K Fix plates on the floor or to the columns ( basement is ideal), ground floor level is also adequate. It is adequate if the measurements are done from stilt level or ground floor level. This may be much easier than transferring the level from the reference mark on the ground to the basement level 67 Examples in Delhi Region Settlements of Rafts 1. DIAL Terminal 3 on Raft Settlements measured and reported as not significant. 2. Sector 58, Gurgaon Structure : Residential building 2B+30 floor on Raft. Frame completed Dead load settlements = 32 mm Estimated total settlement = 40 mm 68 34

35 Settlements observations are being carried out for 6 structures One structure in Noida is fully instrumented, where load on the piles, reactions of the base raft and the settlements are being measured. For one more structure in Noida, pile loads and settlements are planned to be observed. My Recommendation: Make Settlement Observations mandatory for all structures with more than 10 storeys 69 THE WAY FORWARD ON GEOTECHNICAL INVESTIGATIONS: Recognise the importance of a good Geotechnical Investigations and educate Geotechnical, Structural Consultants, Architects and the owners Soil Investigations have to be of International Standards and have to be supervised by an Independent qualified / trained persons. Prequalify the soil investigation agency based on standard norms, Highly desirable that the agency is suitably accredited. Involve a specialist Foundation Engineer / Consultant (Preferable in-house in case of a big consulting office) right from the start

36 Specialist Consultant has to decide the scope of investigations, in consultation with Structural Consultants who has to give inputs with regard to Structures to be designed / built. While engaging a geotechnical investigation agency, their scope could be given in 2 parts, viz Part A Field and Laboratory Investigations and Part B Foundation Analysis and Recommendations. Part B could be made optional depending on expertise of the agency, should be done only when all the Structural details are available. The Foundation and Structural Consultants should work very closely to arrive at a safe and optimum solutions. During execution, the Foundation Consultant has to visit the site to ensure that foundation work is being done as per specifications