Robert L. Lytton, Ph.D., P.E.

Transcription

1 Effects of Trees on Foundations Robert L. Lytton, Ph.D., P.E. Professor, Fred J. Benson Endowed Chair Zachry Department of Civil Engineering Texas A&M University Houston Foundation Performance Association Houston, Texas, December 8, 2010

2 Trees and Tree Characteristics Trees How they affect slab performance and design How they affect drilled shaft performance and design Tree characteristics ti What they need to survive Root zones Water uptake Moisture active zone 2

3 Outline Movement caused by trees Movie of movements caused by trees Design considerations of slabs near trees Design considerations of drilled d shafts near trees Seams of moisture effects Summary 3

4 4

5 Trees What They Need to Survive Water Oxygen Nutrients 5

6 Characteristics of Root Zones Shallow roots Tap roots Root ball Root density 6

7 7

8 8

9 9

10 10

11 11

12 12

13 13

14 14

15 15

16 16

17 17

18 Factors Influencing Evapotranspiration Evapotranspiration Δ ( R ) 900 ( a n G + γ u e ) 2 s e ET T = + Δ + γ ( u ) 2 H 18

19 Water Uptake by Roots Saturated Field capacity Thresh hold Wilting point Dry 19

20 20

21 21

22 Field Capacity (or Lack Thereof) When the gravitational water drains away the soil is at field capacity. Water that remains is held by the soil particles. This water is absorbed by plant roots, or it evaporates. Roots can get water as long as they can overcome the adhesion that holds water to the soil particles. 22

23 Soil Water Balance TAW=the total available soil water in the root zone (mm) RAW= the readily available soil water in the root zone (mm) 23

24 1.0Wateruptakecoeficient SaturatedFieldCapacityThreshholdWiltingPointDrowningAirDryTotalsuction,pF24

25 25

26 Type of plant Location Wilting point (pf) Wilting point (MPa) Trees - U.K U.S Post oak Texas Eucalyptus Australia Eucalyptus Australia Woody plants Burkea africana Africa Ochna pulchra Africa Terminalia sericia Africa Grasses Eragostris pallens Africa Digitaria Africa

27 27

28 28

29 29

30 1.0Wateruptakecoeficient SaturatedFieldCapacityThreshholdWiltingPointDrowningAirDryTotalsuction,pF30

31 After Fatahi et al. 31

32 After Fatahi et al. 32

33 After Fatahi et al. 33

34 After Fatahi et al. 34

35 After Fatahi et al. 35

36 After Fatahi et al. 36

37 After Fatahi et al. 37

38 38

39 Moisture Active Zone Root zone (deepest root fiber) Cemented soil (suction above wilting point) High osmotic suction zone (above wilting point) Water in seams Water table 39

40 Equilibrium Soil Suction vs. TMI 40

41 (-) Suction Ground Surface Wet Season Equilibrium Dry Season Depth 41

42 42

43 43

44 44

45 After D. A. Cameron 45

46 After D. A. Cameron 46

47 After D. A. Cameron 47

48 After D. A. Cameron 48

49 49

50 Design of Slabs Near Trees Find moisture active zone, z m Find edge moisture variation distance, e m 50

51 51

52 After Fatahi et al. 52

53 Design of Drilled Shafts Near Trees Find moisture active zone, z m Determine anchor length below z m m, L A Account for unsymmetric bending Use tensile reinforcing (bond development length) 53

54 DRILLED PIER UPWARD MOVEMENT REINFORCING ANCHOR ZONE 54

55 DRILLED PIER 0 pf pf

56 DRILLED PIER 0 Shear Stress Shear Stress Horizontal Stress Distribution 56

57 DRILLED PIER 0 Axial Tension Deflection Moment

58 Horizontal Swelling Pressure Model Heave mp ( φ ) z tan 45 + /2 = 5-7 ft Z mp = 3-5 ft Joshi and Katti (1980); Komornik (1962); Brackely and Sanders (1992); Symons et al. (1989) Z mp Zone I Move ment active zone Zone II Zero Swell or zone Anch h ( φ ) σ = 1- sin γ t z Zone III Horizontal Active zone ( ) 2 z XZi = zmptan 45 + φ /2 1- H Richards and Kurzeme (1973) 4 times the overburden Joshi and Katti (1980) 42 3 ft, lab Komornik (1962) 55 3 ft, lab 58 Brackely and Sanders (1992) 12 3 ft, field

59 Kim and O Neill (1998) Axial behavior of the pier Test Site Stratigraphy (NGES-UH) Schedule of Rebar and Concrete in Drilled Shaft 59

60 Kim and O Neill (1998) Axial behavior of the pier pf pf 3.1 pf 19 Bar versus Time(1 bar=100 kpa) Uplift Force versus Time 60

61 Kim and O Neill (1998) Axial behavior of the pier Suction (pf) Swelling (in) Horizontal Pressure (psi) Pile Movement (in) Axial Stress (psi) (ft) 6 6 Prediction Measured 6 Prediction Vertical 6 6 Depth Prediction 10 Measured Prediction Measured

62 0 Case Study of Bending Behavior of the Pier Uneven Wetting with Same Initial Condition ( ) Z tan 45 + φ / 2 mp Suction (pf) Swelling (in) Active Zone (ft) Swelling (in) Suction (pf) Depth (ft) NGES-UH Site (Kim and O Neill, 1998) 62

63 Case Study of Bending Behavior of the Pier Uneven Wetting with Same Initial Condition Shear Stress (psi) Horizontal (psi) Horizontal (psi) Shear Stress (psi) NGES-UH Site (Kim and O Neill, 1998) 63

64 Case Study of Bending Behavior of the Pier Uneven Wetting with Same Initial Condition Deflection (in) Lateral Load (lb) Bending Moment (lb-in) Shear Force (lb) Prediction At Rest NGES-UH Site (Kim and O Neill, 1998) 64

65 DRILLED PIER REINFORCEMENT L = 15 L = L = t of Stee el due Friction Stress Percen to Side L = 15 L = 10 L = 15 L = 5 L = 10 L = 10 L = 5 L = 5 L = Temperature and Shrinkage Reinforcement Side Friction Stress, αc, (psf) 65

66 What about Seams of Moisture? 66

67 67

68 68

69 69

70 70

71 71

72 72

73 73

74 74

75 75

76 Answer: Change suction from pf 4.5 (wilting point) to pf 2.5 (higher than field capacity) Horizontal pressure lb/ft2 4 x vertical pressure Enough to cause passive earth pressure 76

77 77

78 78

79 Trees and Tree Characteristics Trees How they affect slab performance and design How they affect drilled shaft performance and design Tree characteristics ti What they need to survive Root zones Water uptake Moisture active zone 79

80 Outline Movement caused by trees Movie of movements caused by trees Design considerations of slabs near trees Design considerations of drilled d shafts near trees Seams of moisture effects Summary 80

81 Effects of Trees on Foundations Robert L. Lytton Professor, Fred J. Benson Endowed Chair Zachry Department of Civil Engineering Texas A&M University Houston Foundation Performance Association Houston, Texas, December 8, 2010