Design of Deep Foundations for Slope Stabilization

Transcription

1 Design of Deep Foundations for Slope Stabilization J. Erik Loehr, Ph.D., P.E. University of Missouri Annual Kansas City Geotechnical Conference Overland Park, Kansas April 23, 215

2 Stability analysis for reinforced slopes Potential Sliding Surface R R axial R lat Reinforcing Member 2

3 Application drilled shafts drilled shafts Fill Observed sliding surface Stiff Clay 3 after Bruce and Jewell, 1986

4 Application drilled shafts 4 Photo courtesy of Alabama Electric Cooperative

5 Application driven piles 5 Graphic courtesy of Hayward Baker

6 Application driven piles 6 Photo courtesy of Hayward Baker

7 Application - micropiles micropiles anchor 7

8 Application - micropiles Photo courtesy of Schnabel Engineering Photo courtesy of Hayward Baker 8

9 Application soil nails shotcrete facing soil nails Railway sliding surface soil nails 9 after Bruce and Jewell, 1986

10 Application soil nails 1 Photo courtesy of Schnabel Foundation

11 Excluded techniques Deep mix columns Jet grout columns Aggregate columns 11

12 12 Application ground anchors

13 Application ground anchors 13 Photos courtesy of Schnabel Foundation

14 Challenges for predicting resistance Load transfer is complex Deformation required to mobilize resistance Numerous limit states Soil provides both load and resistance Axial and lateral resistance may be incompatible 14

15 Soil movement components Slope Surface Slope Surface axial lat. lat. soil axial Sliding Surface soil lat. axial soil Sliding Surface 15

16 Depth (ft) Lateral load transfer long pile Pile Deflection (in) Lateral Soil Reaction (kip/in) sliding surface sliding surface limit soil pressure l l l l =.1 in =.1 in =.3 in =1. in

17 Depth (ft) Lateral load transfer long pile mode Pile Deflection (in) Bending Moment (kip-in) Shear Force (kip) Lateral Soil Reaction (kip/in) Sliding surface 5 1 sliding surface l l l l =.1 in =.1 in =.3 in =1. in limit soil pressure 17

18 Depth (ft) Lateral load transfer short pile mode Pile Deflection (in) limit soil pressure Lateral Soil Reaction (kip/in) l l l l =.1 in =.5 in =1. in =1.6 in sliding surface

19 Depth (ft) Lateral load transfer short pile mode Pile Deflection (in) Bending Moment (kip-in) Shear Force (kip) Lateral Soil Reaction (kip/in) l l l l =.1 in =.5 in =1. in =1.6 in limit soil pressure sliding surface

20 Depth (ft) Lat. load transfer intermediate mode sliding surface Pile Deflection (in) Lateral Soil Reaction (kip/in) l =.1 in 5 l =.5 in 1 l =1 in limit soil l =3 in pressure 2 4 4

21 Depth (ft) Lat. load transfer intermediate mode Pile Deflection (in) Bending Moment (kip-in) Shear Force (kip) Lateral Soil Reaction (kip/in) l =.1 in 5 l =.5 in 5 l =1 in l =3 in limit soil 5 pressure sliding surface

22 Depth (ft) Axial load transfer Mobilized Axial Load (kip) sliding surface a a a a =.5 in =.1 in =.3 in =.5 in 22

23 Limit states for deep foundations in slopes Soil failure passive (lateral) failure above/below sliding surface pullout (axial) failure above/below sliding surface Structural failure flexural failure shear failure axial failure - compression - tension Serviceability limits Failure of member in bending Slope Surface Relative Movement Reinforcing Member Failed Soil Initial Location Relative Movement Sliding Surface Location after Sliding Sliding Surface Failed Soil Failure of member in Shear Sliding Surface Relative Movement Initial Location Initial Location Sliding Surface Relative Movement 23

24 Lessons Load transfer is complex depends on Soil and pile stiffness Sliding depth Orientation of reinforcement Structural and geotechnical limit states It is dangerous to assume load distribution!! 24

25 Prediction of reinforcement resistance 1. Estimate profile of soil movement 2. Resolve soil movement into axial and lateral components 3. Independently predict mobilization of axial and lateral resistance a. Using p-y analyses for lateral load transfer b. Using t-z analyses for axial load transfer 4. Select appropriate axial and lateral resistance Axial and shear force at sliding depth when first limit state is reached taken to be available resistance for that sliding depth 25

26 Depth (ft) Mobilization of lateral resistance Pile Deformation (in) Mobilized Bending Moment (kip-in) Mobilized Shear Force (kip) clay 1 1 =.1 in =1. in =3. in slide rock

27 Mobilized Shear Force (kip) Mobilization of lateral resistance Total Slope Movement (in)

28 Depth (ft) Mobilization of axial resistance Mobilized Axial Load (kip) =.1 in =.3 in =.42 in =.5 in clay 3 4 slide rock 5 28

29 Mobilized Axial Force (kip) Mobilization of axial resistance Total Slope Movement (in) 29

30 Sliding Depth (ft) Sliding Depth (ft) Resistance functions (per member) Axial Resisting Force (kip) Lateral Resisting Force (kip) Ultimate < 1. in clay rock Member resistance for individual member

31 Sliding Depth (ft) Sliding Depth (ft) Resistance functions (per lineal foot) Axial Resisting Force (kip/ft) Lateral Resisting Force (kip/ft) spacing = 6-ft clay rock 5 31 Member resistance divided by member spacing

32 4-ft 2-ft -ft -2-ft -4-ft -6-ft -8-ft AEC-Lowman Power Plant Leroy, AEC-Lowman Alabama Power Plant Leroy, 1' 1' 2' Alabama AEC Lowman Power Plant B-11 B-14 B-4 I. Fill IV. dense sand 54-inch diameter shafts B-5 B-6 II. soft to stiff clay III. mixed III. mixed sands sands & clays & clays V. stiff clay VI. stiff VI. to stiff hard to clay hard clay VII. weathered limestone VII. weathered limestone Observed Sliding Surface Reinforcement: 54 #1 bars; 18 #18 bars Permanent steel casing through mixed sands 12-ft length 15-ft c-c staggered spacing MB-9?? 32

33 33 AEC Lowman Power Plant

34 Depth (ft) AEC Design Analyses Pile Deformation (in) Mobilized Bending Moment (kip-ft) Mobilized Shear Force (kip) CL 1 1 SM 2 2 d=.1 in d=.5 in d=1. in d=2. in d=3. in CH LS

35 Mobilized Shear Resistance at Sliding Surface (kips) AEC Design Analyses sliding depth 8-ft 16-ft 24-ft 29-ft 4-ft 47-ft 56-ft Soil Movement (in)

36 Sliding Depth (ft) Sliding Depth (ft) Design resistance AEC Lowman PP Lateral Shaft Resistance (kips) Lateral Resisting Force (kips/ft) soil movement 1 CL 2 moment capacity 2 SM shear capacity 4 CH LS 8 8 shaft spacing = 15-ft 36

37 AEC Lowman PP Completed Shafts 37 Photos courtesy of A.H. Beck Foundation Co.

38 Elevation (ft) Elevation (ft) AEC Lowman PP Observations Bending Moment (kip-ft) Bending Moment (kip-ft) /25/26 9/14/26 11/27/26 CL L-pile L-pile (mod) 1/25/ /27/27 6/28/27 SM /25/ CH installed 7/19/26 LS

39 39 Brown and Chancellor, 1997

40 Depth (ft) Depth (ft) Bending moments Littleville Bending Moment (in-kips) Bending Moment (in-kips) predicted measured (2+7U) measured (1+7U) upslope p mod = predicted measured (2+7U) measured (1+7U) downslope p mod = tot =.39-in 5 tot =.31-in 4

41 Depth, z (ft.) Depth, z (ft.) Axial resistance Littleville Axial Load T, kip (+=tension) Axial Load T, kip (+=tension) upslope =.3 z ult =.6-in predicted downslope =.3 z ult =.6-in measured (2+7U) 5 measured (1+7U) tot =.34-in 5 tot =.24-in predicted measured (2+7U) measured (1+7U) 41

42 42 Large-scale model tests

43 43 Reinforced slope (s/d 2) w/ capping beam

44 p-multiplier Influence of Pile Batter Kubo (1965) Awoshika & Reese (1971) Model Tests 1.5 Reese et al. (26) Recommended for Slopes Batter Angle (degrees) 44

45 p-multiplier p-multiplier Influence of Pile Spacing Upslope Piles = Downslope Piles 1.6 = = = Pile Spacing Ratio, S/d Pile Spacing Ratio, S/d 45

46 Conclusions Predicting resistance for deep foundations used for slope stabilization is complex Current tools provide reasonably practical means to accurately predict resistance Predictions of lateral resistance are generally consistent with field and lab measurements Predictions of axial resistance are sometimes inconsistent with field and lab measurements 46

47 Things to remember It is dangerous to assume load distribution Should not wish resisting forces Should not compute resistance from structural capacity alone Improvement limited by controlling limit state YOU SHOULD NOT PREDICT RESISTANCE BASED SOLELY ON STRUCTURAL CAPACITY!!! Improving one limit state may only make another most critical Improving non-critical limit state provides no benefit 47