Bored piles in clay and Codes of Practice. Malcolm Bolton

Transcription

1 Bored piles in clay and Codes of Practice Malcolm Bolton

2 Background: design and decision-making Why is design not yet evidence-based? Reliance on past practice, rather than validation. Safety factors have dominated calculations. Serviceability has been given lip-service. So what goes wrong? Excessive deformations. And why is UK infrastructure so expensive? weaknesses in procurement, subcontracts? attitudes to risk? clinging to outdated practice? 52 nd Rankine Lecture Malcolm Bolton 2

3 Example: Bored piles in London clay Design against ULS using Eurocode 7 MSD of straight-shafted bored piles Selecting appropriate value of b, γ M=2 Formula for pile head settlement Comparison with Dinesh Patel s 1992 database So what should we design for: SLS, ULS? 52 nd Rankine Lecture Malcolm Bolton 3

4 Design Codes: progress, or not? 52nd Rankine Lecture Malcolm Bolton 4

5 Code design of bored piles Vardanega et al (2012a) show that Codes typically require a lumped safety factor of 2.5. Design to EC7-UKNA: DA1-2 requires:... G, V are estimated permanent and variable loads; Q s, Q b are estimated shaft and base capacities; 1.3 is a load factor; 1.6 and 2.0 are material factors; and 1.4 is a model factor to take account of the range of uncertainty in the results of the method of analysis. Where do these factors come from? 52 nd Rankine Lecture Vardanega, Kolody, Pennington, Morrison & Simpson (2012a) Geotechnical Engineering 5

6 Shaft resistance Q s : α-method Maximum shaft resistance τ s = αc u where c u relates to a mean design line through scattered data of 100mm diameter triaxial tests or equivalent SPT correlation. Main reason for α: brittle fall to critical state strength. Following Patel (1992) for London clay: in CRP ~ 60 mm/h α 0.60; in ML ~ 10-1 mm/h α 0.45; Main reason for reduced strength in slower tests: creep/relaxation ~ 12% per x10 on strain rate If so, for 1mm/year ~ 10-4 mm/h, α 0.3 which falls below the conventional value of 0.5 by factor nd Rankine Lecture Vardanega, Williamson & Bolton (2012) Geotechnical Engineering 6

7 Shaft resistance Q s : β-method Shaft resistance τ s = βσ v where β = K s tanδ. Although σ v is reliable if soil density and WT are known, there is some uncertainty over K s and δ. Although the soil may start at K 0, casting the concrete should send the lateral total contact stress to γ conc z. Although driving a pile in clay reduces δ to φ res, there is evidence for bored piles that δ φ crit. So with a water table at z w below ground surface, we can estimate τ s at depth z: τ s = [γ conc z γ w (z z w )] tanφ crit Vardanega, Williamson & Bolton (2012) 52 nd Rankine Lecture Geotechnical Engineering 7

8 Q s : α versus β for London clay Take a typical London clay profile with: c u = z kpa (following Patel, 1992) z w = 3 m, γ conc = 23.5 kn/m 3, φ crit = 21 Calculate τ s at z = 10 m (e.g. mid-depth of a pile) Using α = 0.5, τ s = 0.5 x 125 = 63 kpa Using tanφ crit = 0.38, τ s = 0.38 x 166 = 63 kpa Apparently, there need be little uncertainty in τ s! Does the partial safety factor of 1.6 on Q s really control settlements? Do we need a further model factor of 1.4? See Vardanega et al (2012). Vardanega, Williamson & Bolton (2012) 52 nd Rankine Lecture Geotechnical Engineering 8

9 Analysis of shearing around a long pile shaft r 0 τ 0 τ r = But clay stress-strain satisfies: If shaft safety factor is F, = For vertical equilibrium of concentric cylinders For a rigid pile, take soil at mid-point, find τ 0, deduce τ at r, find γ at r and then integrate to get pile settlement w. Vardanega, Williamson & Bolton (2012) 52 nd Rankine Lecture Geotechnical Engineering 9

10 Settlement of bored piles in London clay Typical soil properties: c u = z kpa (following Patel, 1992) γ M=2 = log 10 z % b = 0.6 Pile settlement (using properties at z = 0.5L): /.. / Vardanega, Williamson & Bolton (2012) 52 nd Rankine Lecture Geotechnical Engineering 10

11 Comparing MSD with pile test database Patel (1992) Piling Europe 1/1.6 1/(1.6 x 1.4) Patel{ MSD { <10 mm / 1 m so OK! Vardanega, Williamson & Bolton (2012) 52 nd Rankine Lecture Geotechnical Engineering 11

12 Lessons on bored piles MSD of bored piles in London clay appears robust, both for safety and settlement. A partial factor of 1.6 on shaft resistance and 1.2 on load should suffice. No need in London to apply an additional model factor of 1.4, saving 40% of piles and their carbon emissions. The concrete pressure approach to design by the β-method makes no allowance for poor workmanship, or for effective stress reductions on the shaft due to radial consolidation, especially of nc clays. But these are liable to give negative skin friction anyway 52 nd Rankine Lecture Malcolm Bolton 12

13 Eurocode safety factors lack objectivity Partial factors fixed arbitrarily on characteristic soil strengths, loads, and calculation models. By code calibration, the lumped safety factor product was preserved at 1960 values. This entailed further spurious factors and also muddled the difference between ULS and SLS. Partial factors have created a gulf between design calculations and actual performance. 52 nd Rankine Lecture Malcolm Bolton 13

14 Performance-based design Use databases: of soil behaviour, geo-structural system behaviour, live loads (wind, snow, earthquakes), and structural durability. Satisfy performance requirements of strength, deformation and durability in a set of design situations that define worst credible conditions. Where the consequences of failure would be unacceptable, design assumptions should be verifiable by monitoring, and design calculations should be verified by FEA. 52 nd Rankine Lecture Malcolm Bolton 14

15 Comparing EC7 and MSD Education for EC7: words, facts and flowcharts; evaluation by testing, but have people noticed? Design to EC7: formulaic and ultimately wasteful. Education for MSD: engineers are taught how geotechnical structures actually work in the field; the best possible foundation for their careers. Design to MSD: ultimately cost-saving; linked to soil stiffness data, construction monitoring and back-analysis, all generating high-tech jobs. 52 nd Rankine Lecture Malcolm Bolton 15

16 But is it safe to go beyond current practice? Is it better to rely on the opinion of those who have been properly apprenticed in the design methods and safety factors of current practice? Or to trust in science? Evidence-based medicine has now supplanted traditional remedies. When you are ill, would you prefer a herbalist to treat you, or a doctor who has been taught physiology and pharmacology? 52 nd Rankine Lecture Malcolm Bolton 16

17 Have we forgotten anything? Do we need a Rumsfeld factor of ignorance? Engineers can make mistakes, but randomly, so we must eliminate them not anticipate them. A code can and should set an envelope of worst credible design situations. Engineers can and should be taught to use inherently safe parameters such as φ crit, to derive worst credible groundwater pressures, and to estimate and verify values of ground stiffness. 52 nd Rankine Lecture Malcolm Bolton 17