Pitfalls and fallacies in foundation engineering design. Bengt H. Fellenius. Amsterdam May 27, 2016

Transcription

1 Pitfalls and fallacies in foundation engineering design Bengt H. Fellenius Amsterdam May 27, 216 1

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4 The primary base for the design of a piled foundation is the pile capacity as determined in a static loading test. A routine static loading test provides the load-movement of the pile head... and the pile capacity? 4

5 CHIN and DECOURT 235 5

6 Some people consider the capacity to be the load applied to the pile head*) that caused a movement equal to 1 % of the pile head diameter. This is usually claimed to be recommended by Terzaghi. He knew better than that! Terzaghis's recommendation, and he did give one (Terzaghi 1942), was this: Do not try to estimate pile capacity unless the pile toe movement is equal to at least 1 % of the pile toe diameter. I think you agree that this is a very different recommendation. I have tried to find out where the misquote started and it seems to have been first put forward by E. DeBeer in the early 195's and then picked up from there by several of the "Old Masters". Terzaghi was referring to pile with diameters of about 3 mm. I have seen the misquoted definition applied to piles with a diameter larger than 1.5 m! At times even to footings! The pile diameter, let alone the pile head diameter, has nothing to do with a pile capacity. It is sad that a few codes and standards have designated this silly and ignorant definition as the one to use, notably the EuroCode. *) EuroCode has now (216) changed to refer the 1-% movement to the pile toe diameter. I do not think this is an improvement. 6

7 Araquari Prediction Event, IFCEE 2, Brazil. Test results Actual Load-Time Schedule of the Araquari Prediction Event Actual pile-head Load-Movement of the Araquari Prediction Event 1, APPLIED LOAD (kn) 12, LOAD (kn) 1, Unplanned unloading 8, 6, 4, 2, 8,?? 6, 4, 2, MOVEMENT (mm) TIME (h) Actual pile-head Load-Movement of the Araquari Prediction Event The participants in the prediction event were requested to provide the pile-head load-movement curve from start to 1 mm movement, i.e., the pile "capacity" of the 1, mm diameter pile, as defined by the Brazilian organizers. The pile was instrumented with strain-gages at several levels. However, the uneven load-holding durations and unequal load increments combined with the (unintended, as it were) unloading/reloading will make the evaluation of the straingage records ambiguous and unconvincing. 7

8 Cnt. Araquari Prediction Event As Predicted and As Compared to Actual Results Pile-Head Load-Movements Load Distributions 8

9 Araquari Prediction Event, IFCEE 2, Brazil. Assessments of Capacity I was one of the participants in the prediction event. On receiving the actual test results after the conference, I wrote to all predictors and asked them to tell me what capacity they would asses from the actual load-movement curve. Note, that assessment is no longer a prediction, but an assessment of fact applying the participants usual method for determining capacity as achieved by a static loading test. Twenty-nine of the participants replied giving me their capacity value. Actual test curve of the Araquari prediction event with capacity assessements by 29 participants 12, LOAD (kn) 1, 3 values 7 values 6 values 8, 6, 4, 2, MOVEMENT (mm) 9

10 Edmonton, Alberta, 211 Prediction of load-movement and capacity of a 4-mm diameter, 18 m long, augercast pile constructed in transported and re-deposited glacial till. 4, E = 2 GPa E = 35 GPa LOAD (KN) 3, 2, 1, 1 capacity predictions are at movements > 5 mm TEST RESULTS mm (4 mm + b/12 mm) MOVEMENT (mm) 1

11 Prediction Event in Bolivia April mm diameter, 17.5 m long, bored pile in fine sand Compilation of predicted load-movement curves and capacities, TP1 4,5 COMPILATION OF ALL PREDICTIONS 4, 26 3,5 11 LOAD (KN) 3, 2,5 BHF 126 2, 1,5 1, Geometric Mean 5 Capacity Predicted MOVEMENT (mm) 11

12 So, when designing foundations for capacity, is applying a factor of safety or a resistance factor correct, safe, and economical, or do we by this approach open ourselves for litigation and demands on our liability insurance? 12

13 What really do we learn from unloading/reloading and what does unloading/reloading do to the gage records? 13

14 Does unloading/reloading add anything of value to a test? A test on a 2.5 m diameter, 8 m long bored pile 3 3 Repeat test plotted in AcceptanceCriterion Criterion Acceptance Criterion Repeat Acceptance test sequence of testing LOAD LOAD (MN) MOVEMENT (mm) MOVEMENT (mm) 14

15 Plotting the repeat test in proper sequence 3 Repeat test plotted in sequence of testing Acceptance Criterion 25 LOAD (MN) MOVEMENT (mm)

16 Also the best field work can get messed up if the analysis and conclusion effort loses sight of the history of the data 2, LOAD (KN) (KN) LOAD 1,5 DYNAMIC TEST DYNAMIC Repeated TEST in STATIC a DYNAMIC series TEST oftest blows STATIC TEST 1, MOVEMENT MOVEMENT (mm) (mm) 3 3 The dynamic test (CAPWAP) was performed after the static test. The redriving (ten blows) forced the pile down additionally about 45 mm. 16

17 The Pile Toe is Really a Footing: The Bearing Capacity Formula ru c ' N c q ' N q.5b ' N where ru = ultimate unit resistance of the footing c = effective cohesion intercept B = footing width q = overburden effective stress at the foundation level = average effective unit weight of the soil below the foundation N c, N q, N = non-dimensional bearing capacity factors Factor of Safety, Fs Fs = ru/q 17 17

18 rt q ' N q Nq was determined in tests model-scale tests Min to max Nq ratio is up to 2 for the same φ! Nq The log-scale plot is necessary to show all curves with some degree of resolution. Why is it that nobody has realized that something must be wrong with the theory for the main factor, the Nq, to vary this much? Let s compare to the reality? 18 18

19 Results of static loading tests on.25 m to.75 m square footings in well graded sand (Data from Ismael, 1985) 7 2, 1. m 1,8.5 m 1,6.25 m 5 L O A D ( KN ).75 m 4 Normalized 3 2 S T R E S S ( KPa ) 6 1,4 1,2 1, m m.5 m 2.25 m SETTLEMENT (mm) MOVEMENT/W IDTH (%) MOVEMENT 19 19

20 Texas A&M Settlement Prediction Seminar As before, we divide the load with the footing area (to get stress) and divide the movement with the footing width, as follows. Load-Movement of Four Footings on Sand Texas A&M University Experimental Site J-L Briaud and R.M. Gibbens 1994, ASCE GSP 41 2, 12, 3. m 8, 2.5 m 6, Normalized 4, 1.5 m 2, 1. m S T R E S S ( KPa ) ( KN ) 1, LOAD 1,8 3. m 1,6 1,4 1,2 1, MOVEMENT ( mm ) MOVEMENT / WIDTH (%) 2

21 To repeat, when designing foundations based on capacity are we not basing our design on an illusion? 21

22 Group Effect Single Pile 8 9-pile Group c/c = 1 b 4-pile Group LOAD PER PILE (KN) 2 m Average of 4 Piles Single Pile 6 Average of 9 Piles 4 2 c/c = 1 b PILE HEAD MOVEMENT (mm) O Neill et al. (1982) 22

23 Shaft Resistance and t-z and q-z functions 14 Ratio, θ =.2 12 Hyperbolic (r = 12 %) rtrg SHAFT SHEAR (% of rult) Strain-hardening Exponential, b = Elastic-plastic Hansen 8 % 8 δtrg Strain-softening Zhang, a = RELATIVE MOVEMENT BETWEEN PILE AND SOIL ELEMENT (mm) The t-z and q-z functions are fundamental to the analysis of pile response 23 23

24 "Capacity"? HEAD 3 mm diameter, 3 m long concrete pile 2, 2, E = 35 GPa; ß =.3 at δ = 5 mm; Strain-softening: Zhang with δ = 5 mm and a =.125 TOE LOAD vs. Head movement (kn) (kn) LOAD atload PILE HEAD MOVEMENTS AT GAGE LOCATIONS 1,5 1,5 SHORTENING SHORTENING (Telltale measurement) HEAD HEAD 1, 1, SHAFT 5 TOE LOAD vs. TOE MOVEMENT 3 3mm mmdiameter, diameter,3 3 m m long longconcrete concretepile pile E = 35 GPa; Strain-softening RRss MOVEMENT MOVEMENT (mm) (mm) TOE TOE First shown important by Bram van Weele back in the 196s 24

25 If we want to know the load distribution, we can measure it. But, what we measure is the increase of axial load in the pile due to the load applied to the pile head. What about the axial force in the pile that was there before we started the test? That is, the Residual Force 25

26 Normalized Applied Load Load distributions in static loading tests on four instrumented piles in clay D E P T H 26

27 Example from Gregersen et al., 1973 LOAD (KN) (KN) LOAD LOAD (KN) (KN) LOAD Residual Residual Pile DA Pile DA DEPTH (m) 4 DEPTH (m) 4 DEPTH (m) DEPTH (m) 2 True True A. Pile BC, Pile BC, Tapered Tapered True True minusminus Residual Residual Distribution of residual force in DA and BC B. Load and resistance in DA before start of the loading test for the maximum test load 27

28 FHWA tests on.9 m diameter bored piles One in sand and one in clay (Baker et al., 199 and Briaud et al., 2) Cone Stress and SPT N-Index (MPa and bl/.3 m ) Silty Sand Silty Clay 4 4 DEPTH (m ) DEPTH (m ) Sand 1 Cone Stress (MPa) qc 6 Clay 6 N 8 8 Sand 1 1 Pile 4 12 Pile

29 ANALYSIS RESULTS: Load-transfer curves LOAD (KN) LOAD (KN).. 1, 1, 2, 2, 3, 3, LOAD (KN) 4, 4, 5, 5, Residual Load 4. DEPTH (m) DEPTH DEPTH (m) (m) Measured Measured Distribution Distribution Residual Load 2, 3, 4, 5, True Distribution 1, Measured Distribution True Distribution PILE 4 PILE 4 SAND SAND 1. PILE 7 CLAY

30 Determining True Resistance from Measured Resistance ( False Resistance ) LOAD (KN) 5 1, 1,5 2, 2,5 5 Static Loading Test at Pend Oreille, 1 Sandpoint, Idaho, for the Clay DEPTH (m) realignment of US m diameter, 45 m long, closed-toe pipe pile driven in soft clay Fellenius et al. (24) m

31 A -5 LOAD (KN) 5 1, 1,5 2, ß =.6 AS MEASURED, i.e. "FALSE RES." 5 ß =.9 1 ß =.6 DEPTH (m) 2 25 RESIDUAL LOAD 3 "TRUE RES." AFTER 1st UNLOADING ß =.6 5 Test on a strain-gage instrumented, 46 mm diameter, 45 m long pile driven in soft clay in Sandpoint, Idaho 31

32 Presence of residual force is not just of academic interest 7, 7, Shortening Head Offset Limit 6, Load for 3 mm toe movement 5, 5, Shaft ("False") 4, LOAD (kn) LOAD (kn) 6, 3, 2, Shortening Offset Limit 4, Head Shaft 3, Toe 2, Toe ("False") 1, Load for 3 mm toe movement 1, MOVEMENT (mm) Results from a test on a m long, 6 mm diameter, jacked-in concrete pile (Fellenius 214). The manner of testing built-in considerable residual force in the pile MOVEMENT (mm) Same pile assumed tested without residual force being present. (The test results were first fitted to a UniPile t-z/q-z analysis, whereafter the analysis was converted to results for an identical pile and soil, but with no residual force present). 32

33 From Recent experiences with static pile load testing on real job sites and General Report Design methods based on static pile load tests. From Proceedings of ETC3, Symposium on Design of Piles in Europe Leuven, Belgium, April 216 Load-distribution from a 58 mm diameter driven cast-in-place pile installed to m depth. Load-distribution from a 62 mm diameter, 16.6 m long, screw pile installed to m depth. Figure 1 on Page 74 Figure 5a on Page 9 The authors mention various reasons for the lack of shaft resistance along the lower length, but do not address the main reason, which is that the piles have significant residual force, causing the shaft resistance along the upper length to appear too large and that along the lower length to appear too small or non-existent. 33

34 Load-movement curves from a 56 mm diameter "displacement screw pile" (CFA pile). Pile length and soil type not mentioned. Residual toe force can be assumed small. Load-movement curves from a 58 mm diameter driven cast-in-place pile. Pile length and soil type not mentioned. (Not the same pile as that used to show load distribution). Residual toe force is usually considerable for driven piles. Quote from the Proceedings "The design based on the results of pile load tests is based on the results of the ultimate or pile capacity at 1% *) diameter pile base (toe) displacement". Toe and shaft movements are now shown after a moderate correction for residual force Figure 3 on Page 68 Proceedings of ETC3 Symposium on Design of Piles in Europe Leuven, Belgium, April 216 Figure 2 on Page 67 Same as Figure 2 on Page 67 with presumed "true" shaft and toe curves *) Triple mentioning of "base" will not negate the fact that the "1-%" definition of capacity is applied to a displacement that can be up to 1 % wrong! 34

35 Residual Force Head-down loading test on a 6 mm diameter, 54 m long cylinder pile in Busan, Korea, 26. (Kim, Chung, and Fellenius, 211) Answer to the question in the graph: No, there's always residual force (axial) in a test pile. 35

36 Gages were read after they had been installed in the pile ( = zero condition) and then 9 days later (= green line) after the pile had been concreted and most of the hydration effect had developed. 36

37 STRAIN (µε) Strains measured during the following additional 29-day wait-period d d 5 23d 1 3d 2 39d 25 49d 3 59d 35 DEPTH (m) 4 82d 45 99d 5 122d 55 6 At an E- modulus of 3 GPa, this strain change corresponds to a load change of 3,2 KN 218d Day of Test 37

38 The bi-directional test The difficulty associated with wanting to know the pile-toe load-movement response, but only knowing the pile-head load-movement response, is overcome in the bidirectional test, which incorporates one or more sacrificial hydraulic jacks placed at or near the toe (base) of the pile to be tested (be it a driven pile, augercast pile, drilled-shaft pile, precast pile, pipe pile, H-pile, or a barrette). Early bidirectional testing was performed by Gibson and Devenny (1973), Horvath et al. (1983), and Amir (1983). About the same time, an independent development took place in Brazil (Elisio 1983; 1986), which led to an industrial production offered commercially by Arcos Egenharia Ltda., Brazil, to the piling industry. In the 198s, Dr. Jorj Osterberg also saw the need for and use of a test employing a hydraulic jack arrangement placed at or near the pile toe (Osterberg 1989) and established a US corporation called Loadtest Inc. to pursue the bi-directional technique. On Dr. Osterberg's in 1988 learning about the existence and availability of the Brazilian device, initially, the US and Brazilian companies collaborated. Somewhat unmerited, outside Brazil, the bidirectional test is now called the Osterberg Cell test or the O-cell test (Osterberg 1998). During the about 3 years of commercial application, Loadtest Inc. has developed a practice of strain-gage instrumentation in conjunction with the bidirectional test, which has vastly contributed to the knowledge and state-ofthe-art of pile response to load. 38

39 The bi-directional test Arcos Egenharia Bidirectional test at Rio Negro Ponte Manaus Iranduba, Brazil 39

40 Schematics of the bidirectional test Typical Test Results (Meyer and Schade 1995) (Data from Eliso 1983) 1 Pile PC-1 8 June 3, mm 11. m 4 2 Upward Downward E 2. m MOVEMENT (mm) Test at Banco Europeu Elisio (1983) LOAD (tonne) 4

41 Inchon, Korea (Fugro Loadtest) 41

42 Sao Paolo, Brazil (Arcos Egenharia) 42

43 The bidirectional cell can also be installed in a driven pile. Here in a 6 mm cylinder pile (spun pile) with a 4 mm central void (installed after the driving). 43

44 Test on a 1,25 mm diameter, 4 m long, bored pile at US82 Bridge across Mississippi River installed into dense sand 12 MOVEMENT (mm) 1 8 Shaft 6 4 UPPER PLATE UPWARD MVMNT 2 LOWER PLATE DOWNWARD MVMNT -2 Weight of Shaft -4-6 Residual Residual Force Load Toe -8 2, 4, 6, 8, 1, LOAD (KN) Bidirectional load-movement curves (The bidirectional jack is placed at the pile toe) Test data from Fugro Loadtest 22 Bidirectional load-distribution as-measured and as- flipped over to show the equivalent head-down distribution. (The red curve is the curve fitted in a UniPile calculation applying effective stress analysis with the target betacoefficients indicated to the right). 44

45 The Equivalent Head-down Load-movement Curve Measured upward and downward curves From the upward and downward results, one can produce the equivalent head-down load-movement curve, the curve that one would have obtained in a routine Head-Down Test. The curve needs to be corrected for the increased pile compression in the head-down test. Construction of the Direct Equivalent Curve It also needs to be corrected for the fact that the bidirectional test engages the stiffer soil layers first, while the head-down test engages them last (addressed in the next two slides). Reference: Appendix to regular reports by Fugro Loadtest Inc. 45

46 Cnt. Test on a 1,25 mm diameter, 4 m long, bored pile at US82 Bridge across Mississippi River Unit shaft resistance vs. movement as evaluated from strain-gage records and telltale-measured movements. The bidirectional load-movement curves (blue lines and square dots) simulated and fitted using UniPile (red lines). LOAD (kn) 2, 4, 6, 8, 1, 12 MOVEMENT (mm) 1 Simulated curve UPWARD 2 DOWNWARD Weight of Shaft Residual Force Simulated curve -8 Once the simulation of the test is completed, the analysis can produce the equivalent headdown load-movement curves, applying the t-z and q-z functions resulting from the fit to the measured bidirectional curves. Fellenius (2) 46

47 The difference between the measured upward cell movement and the simulated Equivalent Headdown load-movement for the pile length above the bidirectional cell is due to the fact that the upward cell engages the lower soil first, whereas the head-down test engages the upper soils first, which are less stiff than the lower soils. The left graph shows the measured and simulated upward and downward curves (same as on previous slide) The right graph shows the upward curve as measured and as simulated (in a 1st quarter plot) and compared to an Equivalent Head-down test (shaft resistance only) on the pile using the t-z curves fitted to the measured test. LOAD (kn) 2, 4, 6, 1, 8, 1, Upward Test Measured and simulated 12 8, ( ) Simulated curve 8 6 As in a Head-down Test 6, 4 UPWARD 2 LOAD (kn) MOVEMENT (mm) 1 Measured Upward 4, -4-6 Shaft resistance only DOWNWARD -2 Weight of Shaft 2, Residual Force Simulated curve MOVEMENT (mm) 47

48 Analysis of the results of a bidirectional test on a 21m long bored pile in Sao Paolo, Brazil compact SAND CLAY A bidirectional test was performed on a 5-mm diameter, 21 m long, bored pile constructed through compact to dense sand by driving a steel-pipe to full depth, cleaning out the pipe, while keeping the pipe filled with betonite slurry, withdrawing the pipe, and, finally, tremie-replacing the slurry with concrete. The bidirectional cell (BDC) was attached to the reinforcing cage inserted into the fresh concrete. The BDC was placed at m depth below the ground surface. compact SAND dense SAND The sand becomes very dense at about 25 m depth 48

49 The soil profile determined by CPTU and SPT Sleeve Friction, fs (kpa) Cone Stress, qt (MPa) Friction Ratio, fr (%) Pore Pressure (kpa) N (blows/.3m) DEPTH (m) 1 DEPTH (m) 1 DEPTH (m) 1 DEPTH (m) DEPTH (m) compact SAND 1 CLAY compact SAND dense SAND 25 49

50 The final fit of simulated curves to the measured 5 4 Pile Head UPWARD. m m -1-2 BDC DOWNWARD , 1,2 14 Ratio LOAD (kn) 12 Hyperbolic (ru = 12 %) SHAFT SHEAR (% of rult) MOVEMENT (mm) 3 Simulation performed by effective stress back-calculation with input of t-z and q-z curves to the UniPile software (see Slide 23) r1 or ru 1 Exponential 8 Hansen 8 % δult Zhang RELATIVE MOVEMENT BETWEEN PILE AND SOIL ELEMENT (mm) 25 5

51 The test pile was not instrumented. Had it been, the load distribution of the bidirectional test as determined from the gage records, would have served to further detail the evaluation results. Note the below adjustment of the BDC load for the buoyant weight (upward) of the pile and the added water force (downward). N (blows/.3m) LOAD (kn) , 5 5 compact SAND 1 BDC load 2 Less buoyant weight DOWNWARD DEPTH (m) DEPTH (m) UPWARD 1 With water force The analysis and results of the simulation appear to suggest that the pile is affected by a filter cake along the shaft. It has probably also a reduced toe resistance due to debris having collected at the pile toe between final cleaning and the placing of the concrete. CLAY compact SAND 2 dense SAND

52 The final fit establishes the soil response and allows the equivalent head-down loading- test to be calculated 2,5 Equivalent Head-Down test 2, LOAD (kn) HEAD 1,5 Pile head movement for 3 mm pile toe movement 1, Pile head movement for 5 mm pile toe movement 5 TOE MOVEMENT (mm) When there is no obvious point on the pile-head load-movement curve, the capacity of the pile has to be determined by one definition or other there are dozens of such around. I prefer to define it as the pile-head load that resulted in a 3-mm pile toe movement. As to what safe working load to assign to a test, it often fits quite well to the pile head load that resulted in a 5-mm toe movement. In this case, that happens to be at Q = 75 kn. The most important aspect for a safe design is not the capacity found from the test data, but what the settlement of the structure supported by the pile(s) might be. How to calculate the settlement of a piled foundation is not addressed here, however. 62

53 A CASE HISTORY Bidirectional tests performed at a site in Brazil on two Omega Piles (Drilled Displacement Piles, DDP, also called Full Displacement Piles, FDP) both with 7 mm diameter and embedment 11.5 m. Pile PCE-2 was provided with a bidirectional cell level at 7.3 m depth and Pile PCE-7 at 8.5 m depth If unloaded after 1 min ß =.4 to 2.5 m δ = 5 mm ϴ =.24 ß =.6 to 6. m δ = 5 mm ϴ =.24 ß =.9 to cell δ = 5 mm ϴ = E MOVEMENT (mm) MOVEMENT (mm) 2 5 ß =.5 to 2.5 m δ = 5 mm ϴ =.3 ß =.7 to 6. m δ = 5 mm ϴ =.3 ß =.8 to cell δ = 5 mm ϴ = ß =.6 to toe δ = 5 mm C1 =.63 rt = 1 kpa δ = 3 mm C1 =.7-25 ß =.9 to toe δ = 5 mm ϴ =.3 rt = 3, kpa δ = 3 mm ϴ =.9-1 PCE E LOAD (kn) , 1 PCE , LOAD (kn) 53

54 After data reduction and processing Equivalent Head-down Load-distributions Equivalent Head-down Load-movements LOAD (kn) 4 8 1,2 1,6 3, Pile PCE-2 Compression PCE-2 2 2,5 PCE-7 Max downward movement Max downward movement PCE-7 1,5 Max upward movement 4 6 PCE-2 8 PCE-2 and -7 Shaft 1, 1 5 PCE-2, Toe PCE-7, Toe A TOE LOAD (kn) 3 MOVEMENT (mm) A conventional head-down test would not have provided the reason for the lower capacity of Pile PCE-2 MOVEMENT (mm) LOAD (kn) 2, DEPTH (m) Max upward movement 4 8 1,2 1,6 5 1 PCE PCE-7 B 54

55 The ever so scary N.S.F ghost the drag force Profile of test site and piles. Heröya site. (Bjerrum et al., 1969) 55

56 EFFECTIVE STRESS AND PORE PRESSURE (KPa) PILE LOAD (KN) ,2 1,5 FILL 5 5 σ'z after full dissipation of excess pore pressure 1 Marine Clay 2 Δu Measured distribution 2 σ'z uncoated Start of Bedrock 35 DEPTH (m) DEPTH (m) 1 Distribution calculated from ß=.3 times σ'z for actual excess u Gravel 3 Bitumen coated 35 Notice the distinct Force Equilibrium, the Neutral Plane Distribution of soil stress, excess pore pressure, pile shortening, and load distributions. Heröya site. (Data from Bjerrum et al., 1969). 56

57 Compilation of Norwegian results Drag Force cm cm 57

58 FHWA Project, Keehi Interchange, Honolulu,Hawaii

59 Results from 3 years of monitoring of a 3-m pile, 5-m pile, and a 3-m bitumen coated pile COAT 3/16 inch = 4 mm 59

60 Leung, C.F, Radhakrishnan, R., and Tan Siew Ann (1991) presented a case history on instrumented 28 mm square precast concrete piles driven in marine clay in Singapore Note, the distribution of negative skin friction is linear (down to the beginning of the transition zone) indicating the proportionality to the effective overburden stress TRANSITION ZONE 6

61 Variable load i.e., Live Load LOAD (KN) Two years later (745 days ) ß =.5 Old Silt & Clay Fill 1 DEPTH (m) Live load and drag force cannot coexist! Two months after start (57 days ) Marine Clay Clay Weak Shale Bedrock and Residual Soil 3 61 Data from Leung, Radhakrishnan, and Tan (1991)

62 Endo et al. 1969, presented a very ambitious drag-force study in Japan on four instrumented steel piles during a period of three years. The soils consist of silt and clay on sand. The case history is one of the few that actually also measured settlement

63 Profile of test site and piles Closed-toe, Open-toe, Inclined, and Short Pile (Endo et al., 1969) 63 63

64 Neutral plane = Force Equilibrium = Settlement Equilibrium LOAD (KN) Sandy Silt 5 SETTLEMENT (mm) 1, 1,5 2, 2,5 3, 3, Pile S i l Soil DEPTH (m) C l a y NEUTRAL PLANE t 4 S a n d ( )? 45 5 Open-toe Pile 4 Closed-toe Piles: Inclined and Vertical 45 5 Toe Penetration Closed-toe Piles Load distribution in the three long piles together and settlement of soil and piles measured March days after start. (Data from Endo et al., 1969). 64

65 Toe forces and toe penetrations extracted from the graphs of Endo et al. 2, TOE LOAD (KN) 1,5 Approximation 1, From data in paper TOE PENETRATION (mm) 65

66 Inoue, Y., Tamaoki, K., and Ogai, T., Settlement of building due to pile downdrag. Proc. 9th ICSMFE, Tokyo, July 1-, Vol. 1, pp A Downdrag Case A three-storey building with a foot print of m by 1 m founded on 5 mm diameter open-toe pipe piles driven through sand and silty clay to bearing in a sand layer at about 35 m depth. The piles had more than adequate capacity to carry the building. Two years after construction, the building was noticed to have settled some mm. Measurements during the following two years showed about 2 mm additional settlement. The building was demolished at that time. 66

67 Inoue 1977 Pile Toe Depth 67

68 Inoue 1977 Settlement between piles in Row 6 and Row 1 from Sep through May 1969 = mm. Slope 1 : 1 (Sep 67 Apr 71) 68

69 Looks bad, but, note, the building foundation is OK. No downdrag resulted because the neutral plane is located in the competent not settling soil. Of course, the pile s have drag forces, but that environmental effect is there be the settlement small or large. Presence of drag force is of no consequence for foundations it is always present, regardless. Singapore Apartments Gue See Sew, 211 Gue & Partners Sdn Bhd 69

70 Office Building placed on long toe-bearing piles in Brisbane, Australia. A later built, light extension was placed on short wood piles. Data Courtesy (27) of Wagstaff Piling Pty. Ltd., Queensland, Australia. 7

71 View toward the roof 71

72 Foundations and underpinning The original piles had a capacity about three times the applied load, but downdrag got the better of them. Luckily, new piles could be installed (driven to the sandstone; for some columns to a four-for-one ratio). $$$$ 72 72

73 Settlement Analysis of Large Pile Groups by the Equivalent Raft Method FILLS, etc. G.W. Equivalent Footing For small groups, start by placingplaced an "Equivalent Raft" at the Location at theof depth of the Neutral the Neutral Plane Plane 2:1 distribution 5:1 The compressibility in this zone must be of soil and pile combined 5:1 Ecombined A pile E pile Asoil E soil A pile Asoil 2:1 distribution Settlement of the piled foundation is caused by the compression of the soil due to increase of effective stress below the neutral plane from external load applied to the piles and, for example, from fills, embankments, loads on adjacent foundations, and lowering of groundwater table. N.B., the above approach goes far beyond the 1948 Terzaghi and Peck suggestion of placing The Equivalent Raft at the lower third depth 73

74 Liquid storage tank in Tessaloniki, Greece (Savvaidis 29) 2 No Settlement Norht-South m diameter, bored piles installed to 42 m depth 1 5 Pile 16-5 Pile 11-1 Pile 7 - Dense, silty sand to 5+ m depth Settlement East-West 74

75 Settlement measured across a tank diameter during a Hydro Test ACROSS THE DIAMETER (m) SETTLEMENT (mm) Curve calculated for a flexible footing located at the pile toe level with parameters fitted to the settlement measured at the tank mid-point (calculations were performed with UniSettle). Fellenius and Ochoa 213;

76 A quote from a textbook *) The net effect negative skin friction is that the pile load capacity [sic!] is reduced and pile settlement increases. The allowable load capacity [sic!] is given as: Q allow Q ult Q neg FS Q neg It could have been worse. Logically, the drag force (Qneg) should have been increased by a factor of safety. But so what! There is so much lack of logic in the approach, anyway. *) Compassion perhaps misdirected compels me not to identify the author 76

77 Do not include the drag force when determining the allowable load! Drag force must neither subtracted from the pile capacity nor from the allowable load ALLOWABLE LOAD - (Fs = 2.5) 5 1, 1,5 2, ALLOWABLE LOAD minus DRAG FORCE CAPACITY 2,5 5 5 DEPTH (m) DEPTH (m) LOAD (kn) Effect of subtracting the drag force 1 1, 1,5 2, CAPACITY 2,5 1 DRAG FORCE 2 5 LOAD (kn) DRAG FORCE 2 HAS INCREASED! If the pile capacity had been reduced with the amount of the drag force before subtracting the drag force from the net capacity, so determined, there would have been no room left for working load! 77

78 The Euro Code The European Community has recently completed EuroCode 7, which is supposed to be adopted by all member states. The EuroCode treats the drag force as a load (an action ) similar to the load from the structure, and requires it to be added to that load and also that it is subtracted from the pile capacity)! Moreover, the shaft resistance in the soil layer that contributes to the drag force is disregarded when determining the pile resistance. That is, when a capacity has been determined to, say, 1, kn and the drag force is expected to be, say, 2 kn (an unrealistically low value), the "usable capacity", i.e., the usable unfactored resistance, is a mere 8 kn. This value is then factored. If resistance factor is.5, the factored resistance is 4 kn. When, as required, the factored drag force is subtracted (applying a drag-force load-factor of, say, 1.5, the amount left to support the factored load from the structure is 1 kn! What salvages the economy of some designs must be that the EuroCode clauses advocate that the designer maintains the faithful approach that the drag force cannot really be that large, can it, please? in determining the magnitude of the drag force. Incredibly, the EuroCode says little on how to calculate settlement of piled foundations and nothing is stated about downdrag! Unfortunately and regrettably, the recently issued US AASHTO LRFD Specs have adopted the EuroCode approach! A few US State DOTs, e.g., Utah, have wisely rejected the AASHTO Specs and apply the Unified Method. 78

79 Eurocode Guide, Example 7.4 (Bored.3 m diameter pile) Q (unfactored) = 3 KN FILL SOFT CLAY Average unit shaft resistance, rs = 2 KPa 5. m Rs = 94.2 KN; Rs = Qn STIFF SILTY CLAY Average rs = 5 KPa 11.5 m Rs = 543 KN CALCULATION S fq*3 + fn*94 543/fr 1.35* *94 543/ Rt = KN?! (Alternative: If fr = 1.1, the length in the silty clay becomes 12.4 m) "The settlement due to the fill is sufficient to develop maximum negative skin friction in the soft clay ". The Guide states that the two rs-values are from effective stress calculation. The values correlate to soil unit weights of 18 KN/m3 and 19.6 KN/m3, ß-coefficients of.4 in both layers with groundwater table at ground surface, and a stress of 3 KPa from the fill. Note that the example presupposes that the analysis is carried out for the long-term conditions. The Guide states that the neutral plane lies at the interface of the two clay layers. Based on the information given in the example, this cannot be correct. But there is a good deal more wrong with this "design" example. 79

80 Results of analysis using the given numerical values (ß =.4 for both layers and zero toe resistance) LOAD (KN) SETTLEMENT (mm) 8 1? Fs = Maximum Load = 44 KN Qn = 14 KN not 94 KN DEPTH (m) DEPTH (m) 5 1 THE KEY QUESTION: is the settlement acceptable? Rt =? 2 Neutral Plane Toe Movement 2 If the settlement is acceptable, there may be room for shortening the pile or increasing the load. That would raise the location of the neutral plane. Would then the pile settlement still be acceptable? And, would not some toe resistance develop? 8

81 Let's assume that a static loading test has been carried out on the example pile and taken well beyond the the usual maximum movement. (N.B., to assume no toe resistance is too conservative and a small value has therefore been added to the example). Toe resistance is a function of toe movement. Let's also assume a toe response per a q-z function representative for a stiff clay, and a strain softening shaft resistance. Then, the loading test results in the following pile-head load-movement curves. The curve marked "initial" is from a test assumed carried out during the design phase. The curve marked "final" is assumed to have been carried after the consolidation (or most of it) caused by the fill has developed we could have marked it "forensic", i.e., a test undertaken by the lawyers after the piled foundation showed to settle excessively. 8 7 Pile Shortening RULT = 695 KN Head final LOAD (KN) 6 5 Head initial Rt = 6 KN 1 Toe MOVEMENT (mm) Designers do not usually design for the final conditions it is commendable that they did for this example, but the design must also consider the initial conditions. The structure is probably built at the same time as the fill is placed and before the consolidation process had strengthened the soil. I do not think everyone would be comfortable placing a sustained load of 3 KN on piles that plunge at 5 KN (test at initial condition). 81

82 Example of the Unified Design Approach as Applied to a Refinery Structure N (bl/.3m); qt (MPa); 1 2 wp, wn, and wl (%); FILL GW N 5 SAND 1 DEPTH (m) Design for a large refinery expansion was undertaken at a site reclaimed from a lake in the 196s. The natural soils consist of sand deposited on normally consolidated, compressible post glacial lacustrine clay followed by silty clay till on limestone bedrock found at about 25 m to 3 m depth below existing grade. The site will be raised an additional 1.5 m, which will cause long-term settlement. Piled foundations are needed for all structures. Soil Profile Silty SAND m 2 CLAY wp 2 wn wl mr 2 σ' 2 KPa qt m 1 CLAY TILL 25 mr 6 σ' 9 MPa LIMESTONE BEDROCK 3 Fellenius and Ochoa (29) 82

83 Results of a bidirectional-cell test on a 575-mm diameter test pile, a 26 m deep bored cylindrical pile. A 1.5 m thick fill will be placed over the site after construction. Piles are single or in small groups. Results of analysis of test data: Load Distributions LOAD (KN) 1, 2, 3, 4, 5, "Flipped" Bidirectionalcell Test Silt Sand 5 Strain-gage Value 8 DEPTH (m) 1 4 UPWARD Cell 25-2 Clay 2 2 Long-term (af ter the consolidation induced by the Fill is completed). DOWNWARD 3-4 9% , 1,5 LOAD (kn) 2, 2,5 TOE MOVEMENT (mm) MOVEMENT (mm) 6 Till 1 Bidirectional-cell Test 2 q-z extrapolation 3 1, 2, 3, LOAD (KN) 4, 5, 83

84 Distribution of residual force in the pile after installation, but before load is applied to the pile. Distribution of load in the pile immediately after the pile starts to sustain the load from the structure. Qd LOAD (KN) 2, 4, 6, Silt Sand 5 1 Clay DEPTH (m) DEPTH (m) Residual Load Distribution Distribution Residual Force Before Construction bef ore Construction Load Distribution Immediately After Construction Clay 2 2 O-cell O-cell 3 Silt Sand , LOAD (KN) 2, 4, Till 25 Till 3 Other than at the pile toe, the amount of residual force and its distribution are not that interesting 84

85 Long-term load distribution Qd 2, LOAD (KN) 4, 6, Silt Sand 5 DEPTH (m) 1 Clay 2 Long-term Load Distribution O-cell 25 3 The shaft shear is assumed to be fully mobilized. However, the toe resistance value to use is a function of the toe penetration due to downdrag and can only be determined from assessing the soil settlement distribution. Increase of toe load Till 85

86 Force and settlement (downdrag) interactive design. The unified pile design for capacity, drag force, settlement, and downdrag SETTLEMENT (mm) Qd LOAD (KN) 2, 4, Pile Cap Settlement 6, Silt Sand Clay DEPTH (m) DEPTH (m) DEPTH 5 5 Soil Settlement 2 2 O-cell Till 3 3 Pile toe load in the load distribution diagram must match the toe load induced by the toe movement (penetration), which match is achieved by a trialand-error procedure. TOE LOAD (KN) 1, q-z relation 2, 3, 4, 5 1 PILE TOE PENETRATION (mm) 86

87 Force and settlement (downdrag) interactive design. The unified pile design for capacity, drag force, settlement, and downdrag SETTLEMENT (mm) Qd LOAD (KN) 2, 4, Pile Cap Settlement 6, Silt Sand 2 1 Clay DEPTH (m) DEPTH (m) Soil Settlement 2 2 O-cell Till 3 3 Pile toe load in the load distribution diagram must match the toe load induced by the toe movement (penetration), which match is achieved by a trialand-error procedure. TOE LOAD (KN) 1, q-z relation 2, 3, 4, 5 1 PILE TOE PENETRATION (mm) 87

88 The Unified Piled Foundation Design Force and settlement interactive design for capacity, drag force, settlement, and downdrag LOAD (kn) Qd 2, SETTLEMENT (mm) R ULT 4, Pile Cap Settlement 6, Silt Sand 1 DEPTH (m) 1 2 Clay 5 DEPTH (m) 5 1 Soil Settlement Qn Till 3 3 LOAD (kn) LOAD (kn) 5 1, 2, 3, 4, 1, 2, 3, LOAD (kn) 4, 5, 1, q-z relation 2, 3, 4, 5 1 PILE TOE PENETRATION (mm) The design is based on three "knowns": The shaft resistance distribution, the toe load-movement response, and the overall settlement distribution. 88

89 A recent modern application of a piled pad foundation is the foundations for the Rion-Antirion bridge piers (Pecker 24). Another is the foundations of the piers supporting the Golden Ears Bridge in Vancouver, BC (Sampaco et al., Naesgaard et al. 212), pictured below. 89

90 Golden Ears Bridge in Vancouver, BC. 9

91 Piled pad foundation piers supporting the Golden Ears Bridge in Vancouver, BC. BRIDGE DECK Bored piles: (9 mm; 8 m) to provide lateral resistance and Pad FOOTING AND PILE CAP Short bored pile Driven piles: (3 mm; 3 m) piled-pad piles over an about 1 m thick deposit of soft compressible clay Long slender piles 91

92 ShinHo and MyeongJi Housing Project, in the estuary of the Nakdong River, Pusan, Korea Project Managers: Drs. Song Gyo Chung and Sung Ryul Kim, Dong-A University, Busan 92

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96 AIR VIEW (Shinho Site) 96

97 SITE PLAN (SH Site) Silty clay 97

98 CPTU sounding at the location of the Shin-Ho test pile 98

99 The pile alternative investigated was a 6 mm diameter cylinder pile with a 1 mm wall driven closed-toe The questions to resolve in the design were 1. What is the capacity in the different layers? 2. What is the depth to the force equilibrium/settlement equilibrium, i.e., the neutral plane 3. What will be the maximum load in the pile? Is the structural strength adequate? 4. What is the settlement of the pile as a function of the location of the neutral plane? The field tests were designed to answer the questions. The foundation design was per the Unified Design Method. Total savings were about us$3 million over the alternative of steel pipe piles (which included the drag force as an active load). 99

100 Thank you for your attention 1