Negative skin friction downdrag dragload

1. Negative skin frictiondowndragdragload General Framework and back analysis of a real case Augusto Lucarelli, Derrick Blanksma, Ryan Peterson

2. Terminology and general framework

3. Is it a bearing capacity problem? What happens when Q+QNSF > QP+QPSF ?

4. 0 1 Applied Load Qc at the top of the pile Drag load: the difference between the max axial force and the load at the top of the pile. It is maximum when Qc is zero and goes to zero when geotechnical capacity is reached.

5. So……. • Negative skin friction is a soil-structure interaction problem. • It doesn't change the geotechnical bearing capacity; • It changes the pile stiffness and produces settlements (downdrag); • It changes the axial load distribution along the pile shaft (dragforce). Check structural capacity of the pile. • From a geotechnical point of view, it is a Service Limit State issue. It might be a structural capacity problem at the neutral plan elevation although usually there is no significant bending moment.

6. Let’s work out an example….Drilled Shaft, D=1.0 m; L=30m Settlement profile Ground level 200 mm Layer 1: soft clay tlim = 25 kPa 50 mm -10.0 Layer 2: medium sand tlim = 70 kPa -15.0 -20.0 Pile element Layer 3: dense sand tlim = 110 kPa -30.0 Displacements imposed on the non-liner springs Base: qblim = 5000 kPa The ultimate bearing capacity is around 10 MN

7. Pile-Soil interaction… Pile-soil interaction (along the shaft and at the base) is accounted for by means of non-linear t-z springs. The effect of negative skin friction is evaluated by imposing boundary displacements to the spring. t-z curve at 5 m depth Curve at 5 m depth with negative skin friction 25 tau [kPa] Soil-pile relative displacements 100 -25 t-z curve at 27 m depth Curve at 27 m depth without negative skin friction. It goes through the origin 110 Base curve: displacement at full capacity for drilled piles 0.25-0.30 D qb [kPa] Soil-pile relative displacements -110 Displacements at the base

8. Failure Load: 10000 kN Load settlement curve at the head of the pile Load curve without NSF Load Curve with NSF Axial Load applied at the top [kN] Let’s consider a Service Load of 4000 kN: without negative skin friction the settlement would be around 5 mm. With skin friction the settlement would be around 15 mm. If the last value is not tolerable, the Service Load must be reduced. 4000 2000 40 100 15 5 Displacements [mm] NSF doesn’t effect the ultimate bearing capacity of the pile-soil system. NSF does effect the stiffness of the pile-soil system and axial load distribution along the shaft.

9. Axial force distribution along the pile Axial Force [kN] Axial Force [kN] 2000 4000 2000 0 1000 Qc = 2000 kN Qc = 0.0 10 10 Downdrag force 1800 kN Downdrag force 1700 kN Depth [m] 20 20 Sand Sand

10. Axial Force [kN] Axial Force [kN] 5000 8000 Qc = 5000 kN Qc = 8000 kN 10 10 Downdrag force 600 kN Downdrag force 1200 kN Depth [m] 20 20 Sand Sand

11. Axial Force [kN] Axial Force [kN] 10000 4000 Qc = 8000 Qc = 10000 kN Qc = 2000 Qc = 5000 Qc = 10000 Qc = 0 Downdrag force 0 All NSF has become PSF Neutral plan position 10 10 Depth [m] 20 20 Sand Sand

12. Back analysis of a real case using flac3d Steele County Highway 7 in Owatona Bridge 74551

13. Project Site

15. ShapeAccelArray (SAA) Profile Approximate Length [m] 0 6 12 18 24 30 36 1 25 Final Grade North Abut. South Abut. 0 0 Existing Grade SAA -1 -25 Approximate Displacement [mm] Elevation Location Displacement [in] -2 -50 -3 -75 North Abut. Maximum SAA Deflection = 3.6 in (91 mm) -4 -100 South Abut. 0 20 40 60 80 100 120 Length [ft] SAA Deformation profile measured by the SAA after the bridge deck was placed. N Plan Location

16. SAA Time History 0 0 Surcharge Placement Begins Final Grade Construction Loading Begins Piles Installed North Abut. South Abut. -1 -25 Existing Grade SAA Settlements of the soil around the pile Elevation Location -2 -50 Approximate Displacement [mm] Displacement [in] Construction Ends Surcharge Removal Begins -3 -75 Last Reading North Abut. Time (August, 2010 – June, 2014) -4 -100 South Abut. • Surcharge loads induced roughly 1.7 inches of vertical displacement. • Surcharge removal resulted in 0.9 in of rebound. • Construction loads resulted in a little over 2 in of vertical displacement. SAA N Plan Location

17. Soil Stratigraphy

18. Model Setup: single pile interaction Embedded pile FILL 9 m SOIL 15 m BEDROCK Simplified model: only one embedded pile. The objective is to simulate the local interaction with soil considering the main construction phases.

19. Embedded pile: lateral interaction with the soil Yield Criteria: effective stress approach FILL is the effective vertical stress kn fnlim fslim factor ….function of soil type, installation method… ks SOIL BEDROCK Linear elastic beam element, EA, EJ…

20. Shear Response along the shaft of the pile P is the perimeter of the pile is unit length along the pile ks

21. (3) SURCHARGE Loading (2) (1) Load induced by backfilling was modeled with a density “ramping” procedure of the back fill. BACKFILL (1) (2) Additional loading (Pile cap, beams, etc…) was simulated by applying an axial force directly to the pile head. (3) Additional surcharge loading after the deck was placed, was simulated by increasing the density in the zones above the pile. SOIL BEDROCK

22. Soil Profile & Properties BACK FILL A) CLAY B) SANDY CLAY C) CLAYEY SAND D) SANDY CLAY E) SAND F) SANDY CLAY G) BEDROCK

23. Settlement 4 cm Calibrated with the SAA

24. Initial Shear Response to Loading BACKFILL SOIL BEDROCK

25. Final Shear Response to Loading BACKFILL SOIL BEDROCK

26. Loading Process

27. Load History Place Fill Load Pile • Case 3: 100 MPa Base Layer Additional Load After Deck Placed

28. Sensitivity analysis: Cases • Five cases were run to simulate the response to changing base layer stiffness • Case 1: The base layer is 10 GPa representing bedrock • Case 2: The base layer is reduced to 1 GPa, representing weathered bedrock • Case 3: The base layer is reduced to 100 MPa, representing gravel or very soft bedrock • Case 4: The base layer is reduced to 10 MPa, representing loose sand • Case 5: The base layer is reduced to 1 MPa, representing soft clay • In all cases, the mean soil modulus was kept constant at 52.5 MPa

29. Axial Force as a function of bedrock stiffness Mean soil modulus = 52.5 MPa Base layer modulus varies by case FILL (Case 5) (Case 4) (Case 3) (Case 2) (Case 1) Ln Ls h = Ln/Ls = 10/15 = 0.67 SOIL Case 3 (100 MPa base layer) correlates fairly well to the strain gage data. BEDROCK

30. Relative displacements Bottom of fill Neutral plane: relative displ. is zero Relative displ. = soil displ. – pile displ.

31. Results from other real cases around the world

32. Results from other real cases around the world

33. Results from other real cases around the world

34. Results from other real cases around the world

35. Results from other real cases around the world

36. Neutral plan position – end bearing in clay

37. Neutral plan position – end bearing in sand & rock In our case Nspt>50….but is an H pile and there is a sand layer just on top of the bedrock….100 Mpa is not a very stiff bedrock.

38. Axial Force and Neutral Plane Position Case 1 Case 2 Case 3 Case 4 Case 5 Axial Force [kN] Axial Force [kN] Axial Force [kN] Axial Force [kN] Axial Force [kN] As the base layer stiffness is decreased, the neutral plane position moves from the bottom of the pile (Case 1) up towards the top (Case 5). Depth [m] Neutral Plane

39. Sensitivity Analysis • Relative stiffness is calculated as the ratio of the mean elastic modulus of the soil to the elastic modulus of the base layer. • The soil modulus was kept constant at 52.5 MPa and the base layer modulus was decreased by an order of magnitude for each case. The initial base layer modulus was 1,000 MPa. • With a relatively stiff base layer, downdrag forces increase, the neutral plane is near the bottom of the pile and pile displacement is minimal. Case 1 Case 2 Case 3 Case 4 Case 5 Case 5 Case 4 Case 5 Case 3 Case 4 Case 2 Case 1 Case 3 Case 2 Case 1

40. Comparison of Axial Forces and Neutral Plane Depth Case 1 Case 1 Case 2 Case 2 Case 3 Case 3 Case 4 Case 4 Case 5 Case 5

41. Considerations Case 1 Case 2 Case 3 • Relative stiffness between the soil and base layer influences the amount of dragload, the axial force distribution and the position of the neutral plane. • At relative stiffness below 0.1 (very stiff base layer), the neutral plane is at the bottom of the pile and maximum possible dragload forces are realized. • At a relative stiffness above 10 (very soft base layer), the neutral plane is near the top of the pile and the drag load forces are minimal. Case 4 Case 5 • Between a relative stiffness of 0.1 and 10, The neutral plane position and drag load forces are functions of several factors. • This region is a transition zone where 1) the drag load increases with increasing base layer stiffness and 2) the position of the neutral plane decreases with increasing base layer stiffness. Case 5 Case 4 Case 3 Case 2 Case 1

42. Downdrag Force Matrix (kN) RS Pile Load (kN) RS is relative stiffness i.e., the ratio of the mean soil modulus to the base layer modulus. Pile Load is applied directly to the top of the pile. The base case is the scenario calibrated to the field data i.e., RS = 0.525 and Pile Load = 550 kN

43. Relative to Max Downdrag Force Matrix RS Pile Load (kN) RS is relative stiffness i.e., the ratio of the mean soil modulus to the base layer modulus. Pile Load is applied directly to the top of the pile. The base case is the scenario calibrated to the field data i.e., RS = 0.525 and Pile Load = 550 kN

44. Downdrag Force Contour High Downdrag force Downdrag force [kN] Base case Low Downdrag force Increasing base stiffness

45. Load test simulation: effect of NSF

46. 250 kN Applied load 500 kN Applied load

47. 750 kN Applied load 1000 kN Applied load

48. 1250 kN Applied Load 1500 kN Applied Load

49. As the axial load applied at the pile head increases, the maximum axial moves toward the pile head vanishing the effect of negative skin friction. All the available friction along the pile becomes positive.

50. Future Developments: pile group