The design of soil nailed structures to AS4678

Transcription

1 The design of soil nailed structures to AS4678 Chris Bridges Technical Principal - Geotechnics chris.bridges@smec.com 7 December

2 Presentation Layout Introduction Soil Nailing in AS4678 Design Process 2

3 Soil Nail Main Components HDPE Sheath (for corrosion protection) External centraliser Soil nail head plate (galvanised for corrosion protection) Internal centraliser Outer grout tube Inner grout tube Domed soil nail nut (galvanised for corrosion protection) 32mm diameter deformed soil nail bar (galvanised for corrosion protection) 3

4 Construction

5 Reinforcing the soil

6 How soil nails work R N - resisting forces (nail) R S - resisting forces (soil) R D - disturbing forces Active Zone Passive Zone Figure 2.13 Forces Acting on a Potential Sliding Zone for a Nailed Slope (Phear et al., 2005)

7 How soil nails work Soil nailed wall 7

8 Soil Nailing Is it suitable? 8

9 Advantages & Disadvantages Advantages construction flexibility cost environmental/aesthetic considerations Complex excavation geometry Faster relative to other lateral support systems Permits top-down construction Can be used in situations with limited access Disadvantages temporary conditions (cut must stand vertically) Groundwater clay soils land issues Corrosion ground movement Untried for depths beyond about 25m Not suitable for deep seated failures

10 AS December

11 AS Design Considerations Limit States Strength - Must consider all critical conditions during the excavation/nail installation process Serviceability must include movements during excavation External Stability Internal Stability Design Loads load combinations & factors Design Factors factors on material properties & uncertainty 11

12 AS Structures <15m Face angle <90⁰, >70⁰ Uncomplicated 12

13 AS Informative Guidelines only Appendix C of AS4678 Design principle is similar to Reinforced Soil Walls Appendix C refers back to Appendix B Ground anchors 13

14 Design Process 7 December

15 Design Process 15

16 Design Process 16

17 Step 2 Input parameters 7 December

18 Soil Properties Uncertainty Factors f* = design angle of internal friction of soil (deg.) (= tan -1 (F uf (tanf )); c* = design value of cohesion of soil (kn/m 2 ) (=c F uc ); 18

19 Bar size and hole dia. 19

20 Step 3 - Concept 7 December

21 Nail spacing, length and inclination MODEL 1 No soil nails Planar failure Ref.: Zhang et al, 2001 MODEL 6 Short soil nails L/H = 0.32 Block failure MODEL 8 Long, widely spaced soil nails L/H = 1 Planar failure

22 Nail spacing, length and inclination TESTS 1-4 L/H = 0.67 (incl deg) Closely spaced Two-part wedge: no difference in failure with nail inclination 0-30 deg. TESTS 5-7 L/H = 0.67 & 1.0 Widely spaced at 0 deg inclination Planar failure Ref.: Vucetic et al, 1996

23 F.O.S Deformation (m) F.O.S Deformation (m) Nail spacing, length and inclination Factor of safety for different soil nail inclinations (90 wall face angle) Factor of safety for different L/H ratios (90 wall face angle) f'=36, c =6kPa f'=36, c =6kPa Nail Inclination (deg) L/H Ratio FOS Vertical Deformation FOS Vertical Deformation Horizontal Deformation L Horizontal Deformation Inclination H 23

24 Nail Force (kn) Factor of Safety Settlement (m) Distance from Wall Face (m) End of 5m long nail Nail spacing, length and inclination 10 deg 20 deg 25 deg 30 deg H L Inclination Wall face Nail 1 Nail 2 Nail 3 Effect of soil nail inclination on nail force (Nail 1 is top row of nails) ve +ve Soil Nail Inclination below Horizontal (degrees) Nail 4 Nail 5 Nail 6 FOS 24

25 Closer spaced nails provide greater internal stability Wide spacing of nails can effect internal stability Soil nail spacing should not exceed 1 nail per 6m 2 of hard facing or 1 nail per 2m 2 to 4m 2 of flexible facing otherwise the soil nailed structure will not act as a reinforced mass (CIRIA C637) Max. spacing typically 1-2m Nail Spacing 25

26 Short nails at bottom provides less stability likely pullout Shorter nails at top, longer at bottom provides excellent stability but shorter nails at the top will increase deformations (Shui & Chang, 2005) As L/H increases, deformations decrease no additional effect when L/H>1 Short nail lengths can effect external stability thin gravity wall Length of Nails H (Clouterre) Nail Length Soil nail lengths above 15m should be avoided due to drilling difficulties and greater deformation required to mobilise tensile capacity. 26

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28 Nail Inclination Nail inclination between 0 and 20 deg does not appear to affect stability >20 deg then rapid increase in wall deformation 28

29 Preliminary Sizing The following relationships were derived by Bruce & Jewell (1987): Length Ratio = Length of soil nail / Excavation height (L/H) Bond Ratio = (Hole Dia. x L) / Nail Spacing Strength Ratio = (Nail Dia.) 2 / Nail Spacing Nail Spacing = horizontal spacing x vertical spacing (m 2 ) Performance Ratio = Outward Movement = d Excavation height H Marl mix of calcium carbonate & clay Morraine glacial debris (silt thru to boulders) 29

30 Step 4 - Analyses 7 December

31 External Stability - Modes of Failure External Stability

32 Elevation (mrl) Stability Analyses Name: Stiff Clay Unit Weight: 21 kn/m³ Cohesion: 5 kpa Phi: 25 Name: Hard Clay Unit Weight: 21 kn/m³ Cohesion: 5 kpa Phi: 28 Name: VLS - LS Siltstone Unit Weight: 22 kn/m³ Cohesion: 22.5 kpa Phi: 30 Name: MS Siltstone Unit Weight: 22 kn/m³ Cohesion: 200 kpa Phi: South Wall Eastern Side Surcharge (Unit Weight): 32 kn/m³ Stiff Clay Hard Clay VLS - LS Siltstone 5 0 MS Siltstone Distance 32

33 Nail Tension Forces 33

34 Modes of Failure Internal Stability

35 Soil Nailing Internal Stability Soil nail pull-out (grout-ground bond) Nail failure (i.e. tensile failure of nail) Pull-out of connection to facing material 35

36 Design pull-out capacity (AS4678) T * = F n.f b..d.l f. u Where:. F n = Structure classification design factor (Table 5.2) = ; F b = Bond reduction factor (Table B2) = 0.7; D = Diameter of grout hole (m); L f = Fixed length (m); u = Design grout/ground resistance (bond stress) (kn/m 2 ). 36

37 Bond Stress Effective stress design methods Pull out tests Empirical values 37

38 Design bond stress (effective stress design method) u = (c* + σ v tanf*) (kn/m 2 ) where: σ v = effective vertical stress at the mid-depth of nail behind the failure surface; f* = design angle of internal friction of soil (deg.); c* = design value of cohesion of soil (kn/m 2 ). 38

39 u = P ult / DL (kpa) Pull-out tests where: P ult = pull out resistance from pullout test (kn) D = diameter of grout hole (m) L = grouted length of soil nail (m) 39

40 Pull-out tests u = P ult / DL ξ ϒ p (kpa) where: P ult = pull out resistance from pullout test (kn) D = diameter of grout hole (m) L = grouted length of soil nail (m) ξ = correction based on test results ϒ p = partial factor (1.25 temp, 1.5 perm) 40

41 Empirical Values Apply a factor on ultimate of Typical values of ultimate bond stress for various materials in the Sydney region are given below: Material Ultimate Bond Stress (kpa) Material Ultimate Bond Stress (kpa) Residual soil Class V Sandstone 150 Class V Shale Class IV Sandstone Class IV Shale 150 See also: FHWA Geotechnical Engineering Circular No. 7 Soil nail walls (2003) CIRIA C637 Soil nailing best practice guidance (2005) Values should be confirmed by pull-out testing. 41

42 Average Bond Stress (kpa) Design pull-out capacity Pull-out resistance is usually assumed to be uniform along the length of the nail. This is reasonable if nails are short and stiff (dia. of bar >20mm, bond length <5m) For more elastic nails (eg. fibre-composite nails) the ave. bond stress reduces when bond length >3m CIRIA Report C Effect of Fixed Length on Bond Stress (Barley and Graham, 1997) Fixed Length (m) GRP 22mm bar GRP Strands Steel 50mm bar Steel 20mm bar Power (GRP 22mm bar) Power (GRP Strands) 42

43 Nail tensile capacity Bar strength (tension capacity) can be determined from equation B4(1) in AS where: T = F k F n F t f p A p (kn) F k = Importance category reduction factor (Table B1) = 0.8 (perm.) (temp); F n = structure classification design factor (Table 5.2, AS4678) = F t = Material reduction factor (Table B2) = 0.9; f p = Tensile strength of nail (kn/m 2 ); A p = Cross sectional area of nail (m 2 ). 43

44 Facing Facing can comprise: Individual plates or pads Mesh Shotcrete facing with steel mesh or steel fibre reinforcement Vegetation Flexible facing provides similar overall stability to rigid facing, but more localised deformation Facing type (based on case studies) face angle 70 = hard face angle 70 = flexible 44

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46 Vegetated Slope Works July 2001 August 2001 September 2002 April 2003 June 2001

47 Facing 1. FHWA approach 2. FHWA Modified Approach (CIRIA C637) 3. Clouterre Shotcrete can be applied in one of two layers (a temporary layer and a permanent layer). Typical temp thickness is 75mm with a single layer of mesh Shotcrete thickness for a soil nailed wall is generally > 150mm (MRTS03 160mm) Shotcrete thickness for a slope mm (MRTS03 120mm) Shotcrete thickness generally governed by min. cover to steel requirements 47

48 Soil Nailing - Serviceability Serviceability limit state of soil nailed structure is similar to RSW. Remember movement is required to generate resistance. Therefore beware of existing structures on top of proposed cut. Where special concrete facings are used, any excessive deformation of the facing may constitute a serviceability limit state. 48

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50 Soil Nailing - Serviceability 27 November

51 Drainage 51

52 Drainage 52

53 Conclusion AS can be used for soil nailing But: Lack of information and guidance in the standard limits its use Clients specify different standards And 53

54 Thank you 7 December

55 Construction Issues 7 December

56 225m long wall signs of distress during construction

57 Height (m) Original Design Name: General Backfill Model: Mohr-Coulomb Unit Weight: 19 kn/m³ Cohesion: 5 kpa Phi: Pressure (Unit Weight): 20 kn/m³ General Backfill Mudstone Sandstone Claystone Name: Claystone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: 40 Name: Mudstone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: 42 Sandstone Offset (m) Directory: K:\Q LMI\Coffey\Design\Walls\RW08\ 4 Name: Sandstone Model: Mohr-Coulomb Unit Weight: 20 kn/m³ Cohesion: 2 kpa Phi: 42 57

58 Height (m) Revised Design Re-analysed wall with revised ground model and design amended to incorporate same length nails for full height of wall over affected section 1.53 Name: General Backfill Model: Mohr-Coulomb Unit Weight: 19 kn/m³ Cohesion: 5 kpa Phi: Name: Claystone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: Pressure (Unit Weight): 20 kn/m³ General Backfill Mudstone Claystone Uncemented Sandstone Name: Mudstone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: 42 Name: Uncemented Sandstone Model: Mohr-Coulomb Unit Weight: 20 kn/m³ Cohesion: 0 kpa Phi: 35 Sandstone Offset (m) Directory: K:\Q LMI\Coffey\Design\Walls\RW08\ 4 Name: Sandstone Model: Mohr-Coulomb Unit Weight: 20 kn/m³ Cohesion: 2 kpa Phi: 42 58

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60 Remediation But, after wall completed: Wall still showed signs of movement Extra nails 10m in length were recommended along 20m of the unstable section 60

61 Height (m) Remediation Nail Design 1.59 Name: General Backfill Model: Mohr-Coulomb Unit Weight: 19 kn/m³ Cohesion: 5 kpa Phi: Name: Claystone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: Pressure (Unit Weight): 20 kn/m³ General Backfill Mudstone Claystone Uncemented Sandstone Name: Mudstone Model: Mohr-Coulomb Unit Weight: 21 kn/m³ Cohesion: 10 kpa Phi: 42 Name: Uncemented Sandstone Model: Mohr-Coulomb Unit Weight: 20 kn/m³ Cohesion: 0 kpa Phi: 35 Sandstone Offset (m) Directory: K:\Q LMI\Coffey\Design\Walls\RW08\ 4 Name: Sandstone Model: Mohr-Coulomb Unit Weight: 20 kn/m³ Cohesion: 2 kpa Phi: 42 61

62 Spacers of inadequate strength Damage to installed soil nail bars by excavator Lack of experience of contractors staff 62

63 Soil Nail Failures Soil nail facing failure Grout cover cracked and peeled off from nail Ref.: Tan. & Chow, 2004

64 Probed depth = 1590 mm into cement grout Soil Nail Failures ~250mm Cross-section Void within cement grout Probed depth = 320 mm into cement grout 64

65 Nail with no grout Short column of cement grout Plastic sheet 65

66 Soil Nail Failures Slope failure prior to soil nail heads being installed during heavy rain Cause of failure: Ingress of water Assisted by - Upper row of nails too low below crest Wide nail spacing Lack of surface protection Poor construction of nails Ref.: Sun & Tsui, 2005

67 6m 3 failure in Completely Decomposed Tuff Soil Nail Failures

68 Airport Link Construction Issues Soil Nail Walls are not such a great idea in water charged sands and silts (They also can t be expected to have a 2 weeks stand up time) 68

69 Soil Nails and Reinforced Soil 69

70 Excavations Soil Nails 70