SHEET PILE WALLS. Mehdi Mokhberi Islamic Azad University

Transcription

1 SHEET PILE WALLS Mehdi Mokhberi Islamic Azad University

2 Lateral Support In geotechnical engineering, it is often necessary to prevent lateral soil movements. Tie rod Anchor Sheet pile Cantilever retaining wall Braced excavation Anchored sheet pile 2

3 Lateral Support We have to estimate the lateral soil pressures acting on these structures, to be able to design them. Gravity Retaining wall Soil nailing Reinforced earth wall 3

4 Soil Nailing 4

5 Sheet Pile Sheet piles marked for driving 5

6 Sheet Pile Sheet pile wall 6

7 Sheet Pile During installation Sheet pile wall 7

8 Lateral Support Reinforced earth walls are increasingly becoming popular. geosynthetics 8

9 Lateral Support Crib walls have been used in Queensland. Good drainage & allow plant growth. Looks good. Interlocking stretchers and headers filled with soil 9

10 Active/Passive Earth Pressures - in granular soils Wall moves towards soil A Wall moves away from soil B smooth wall Let s look at the soil elements A and B during the wall movement. 10

11 Active Earth Pressure - in granular soils v = z z h A v z Initially, there is no lateral movement. h = K 0 v = K 0 z As the wall moves away from the soil, v remains the same; and h decreases till failure occurs. Active state 11

12 Active Earth Pressure - in granular soils As the wall moves away from the soil, Initially (K 0 state) Failure (Active state) active earth pressure Decreasing h v 12

13 Active Earth Pressure - in granular soils [ h ] active v WJM Rankine ( ) [ h' ] active KA v' K A sin tan 1 sin 1 2 (45 / 2) Rankine s coefficient of active earth pressure 13

14 Active Earth Pressure - in granular soils Failure plane is at 45 + /2 to horizontal v 45 + /2 h A [ h ] active 90+ v 14

15 Active Earth Pressure - in granular soils h A v z As the wall moves away from the soil, h decreases till failure occurs. h K 0 state Active state wall movement 15

16 Active Earth Pressure - in cohesive soils Follow the same steps as for granular soils. Only difference is that c 0. [ '] K ' 2c h active A v K A Everything else the same as for granular soils. 16

17 Passive Earth Pressure - in granular soils h B v Initially, soil is in K 0 state. As the wall moves towards the soil, v remains the same, and h increases till failure occurs. Passive state 17

18 Passive Earth Pressure - in granular soils As the wall moves towards the soil, Initially (K 0 state) Failure (Active state) passive earth pressure v increasing h 18

19 Passive Earth Pressure - in granular soils v [ h ] passive [ h' ] passive KP v' K P sin tan 1 sin 1 2 (45 / 2) Rankine s coefficient of passive earth pressure 19

20 Passive Earth Pressure - in granular soils Failure plane is at 45 - /2 to horizontal 45 - /2 h A v v 90+ [ h ] passive 20

21 Passive Earth Pressure - in granular soils h B v As the wall moves towards the soil, h increases till failure occurs. h K 0 state Passive state wall movement 21

22 Passive Earth Pressure - in cohesive soils Follow the same steps as for granular soils. Only difference is that c 0. [ '] K ' 2c h passive P v K P Everything else the same as for granular soils. 22

23 Earth Pressure Distribution - in granular soils [ h ] active P A and P P are the resultant active and passive thrusts on the wall [ h ] passive H P A =0.5 K A H 2 h P P =0.5 K P h 2 K P h K A H 23

24 h Passive state Active state K 0 state Wall movement (not to scale)

25 Rankine s Earth Pressure Theory [ '] K ' 2c [ '] K ' 2c h h active passive A P v v K K A P Assumes smooth wall Applicable only on vertical walls 25

26 Retaining Walls - Applications Road Train 26

27 Retaining Walls - Applications highway 27

28 Retaining Walls - Applications High-rise building basement wall 28

29 Gravity Retaining Walls cement mortar cobbles plain concrete or stone masonry They rely on their self weight to support the backfill 29

30 Cantilever Retaining Walls Reinforced; smaller section than gravity walls They act like vertical cantilever, fixed to the ground 30

31 Design of Retaining Wall - in granular soils Block no toe toe Analyse the stability of this rigid body with vertical walls ( Rankine theory valid) W i = weight of block i x i = horizontal distance of centroid of block i from toe 31

32 Safety against sliding along the base F sliding P P { W P A i }. tan soil-concrete friction angle to be greater than P A 2 H P P toe y 1 S 3 R h P P 1 toe P P = 0.5 K P h 2 P A = 0.5 K A H 2 y S 3 R PA

33 Safety against overturning about toe F overturning P P h / 3 P A H/3 { W x i i } to be greater than P A H P P toe y 1 S 3 R h P P 1 toe y S 3 R PA

34 Points to Ponder How does the key help in improving the stability against sliding? Shouldn t we design retaining walls to resist atrest (than active) earth pressures since the thrust on the wall is greater in K 0 state (K 0 > K A )? 34

35 SHEET PILE WALLS generally used for retaining shallow heights of soil wharfs, key walls temporary structures for excavation (eg. dredging)

36 From McCarthy, 6 th Edition

37 Cantilever Sheet Pile Walls stability depends on passive resistance developed below the lower soil surface failure is by rotation about a point O near bottom of wall Depth of embedment, d Active E.P. d O Passive E.P. fixity

38 a fixing moment is provided by the extra depth below point O to prevent rotation Generally, design is based on a simplified but equivalent approach: the fixity provided previously is replaced by a resisting force, R at point C which is just below point O Passive E.P. Active E.P. d C R

39 Danish Rules a factor of safety, F P is applied to the passive resistance in front of the wall take moments about C to calculate d then, increase d by 20% (empirical) finally, check that the passive resistance provided by the extra 20% is > R, where R = SP H

40 Free Earth Support Passive E.P. A T 45 - /2 tie rod b d a d 45 + /2 Active E.P. anchor wall no fixity Determine the depth of embedment, d by taking moments about A (SM A = 0, apply F P ) Find tension, T from SF H = 0 (don t apply F P ) Constraints: failure wedges must not overlap b > ½d a

41 Standards: AASHTO LRFD Section 11 Abutments, Piers, and Walls

42 AASHTO Section 11 Design specifications for: Conventional gravity/semigravity walls Non-gravity cantilevered walls Anchored walls Mechanically Stabilized Earth (MSE) walls Prefabricated modular walls

43 Common Load Groups for Walls Group DC EV EH (Active) ES LS Strength Ia Strength Ib Service I

44 Load Definitions DC dead load of structural components and attachments EV vertical pressure from dead load of earth fill EH horizontal earth pressure load ES earth surcharge load LS live load surcharge (transient load)

45 Surcharge Loads Earth surcharge AASHTO Section and Live load surcharge AASHTO

46 Conventional Retaining Walls Strength Limit States Sliding Bearing resistance Eccentricity Service Limit States Vertical settlement Lateral wall movement Overall stability

47 External Failure Mechanisms Sliding Failure Overturning Failure Bearing Failure Deep-Seated Sliding Failure

48 1.25 DC 0.90 DC 1.35 EV 1.00 EV Load Factors for Conventional Walls b b 1.50 EHsin(b ) 1.50 EHsin(b ) 1.50 EH 1.50 EH b b 1.00 WA V 1.50 EHcos(b ) 1.00 WA V 1.50 EHcos(b ) 1.00 WA H 1.00 WA H Load Factors for Bearing Resistance Load Factors for Sliding and Eccentricity

49 Conventional Walls - Summary Use resistance factors for spread footings or deep foundations, as appropriate (Section 10.5) Eccentricity limited to: e/b < 0.25 for soil (compare to ASD 0.167) e/b < for rock (compare to ASD 0.25)

50 Non-gravity Cantilevered Walls Strength Limit States Bearing resistance of embedded portion of wall Passive resistance of embedded portion of wall Flexural resistance of wall/facing elements Service Limit States Vertical wall movement Lateral wall movement Overall stability

51 Resistance Factors Bearing Resistance Passive Resistance Flexural Resistance Section Code allows increase in Resistance Factors for temporary walls but specific guidance is not provided

52 ASD Pressure Diagrams Discrete Elements LRFD

53 Non-gravity Cantilevered Walls Below excavation line, multiply by 3b on passive side of wall and 1b on active side of wall for discrete elements Look at forces separately below excavation line on passive side and active side (because different load factors)

54 Non-gravity Cantilevered Walls Factor embedment by 1.2 for continuous wall elements Do not factor embedment for discrete wall elements (conservatism of 3b assumption)

55 Example Cantilevered sheet pile wall retaining a 10- ft deep cut in granular soils Assume 36 ksi yield stress for sheet pile Compare required embedment depth and structural section for ASD and LRFD Load Factor of 1.5 used for EH (active)

56 Example Geometry 10' 125 pcf K a = 0.33 p = 1.5 L L p P p K p = 3 p = 1 A L a P a Factored P a = p * 0.5 * (L+10) 2 * K a * Factored P p = p * 0.5 * L 2 * K p *

57 Example Results Method M max (k-ft) Embedment (ft) Section Modulus (in 3 /ft) ASD (S) (elastic) LRFD (Z) (plastic) Since Z is about 1.15 to 1.20 times S, similar section would be acceptable

58 Anchored Walls Strength Limit States Bearing resistance of embedded portion of wall Passive resistance of embedded portion of wall Flexural resistance of wall/facing elements Ground anchor pullout Tensile resistance of anchor tendon Service Limit States

59 Apparent Earth Pressure Diagrams Based on FHWA-sponsored research Builds upon well-known Terzaghi-Peck envelopes Appropriate for walls built in competent ground where maximum wall height is critical design case Same diagram shape for single or multileveled anchored walls

60 H Recommended AEP for Sands H 1 2 /3 H 1 H 1 2 /3 H 1 T h1 p 2 /3 (H-H 1 ) 1 /3 H T h1 T h2 T hn H n+1 H n H 2 p 2 /3 H n+1 R R p TOTAL 2 3 LOAD H K A γh TOTALLOAD p H H1 3Hn 1 (a) Walls with one level of ground anchors (b) Walls with multiple levels of ground anchors

61 LRFD Check on Tensile Breakage Guaranteed Ultimate Tensile Strength (GUTS) Select tendon with: GUTS T n GUTS ΣγiQi

62 Resistance Factors for Ground Anchors Tensile Rupture Mild Steel 0.90 High Strength Steel 0.80 Resistance factors are applied to maximum proof test load For high strength steel, apply resistance factor to GUTS

63 ASD Comparison to ASD Tensile Rupture 0.8 GUTS > 1.33 Design Load (DL = EH + LS) 0.8 GUTS > 1.33 EH LS LRFD GUTS > p EH LS 0.8 GUTS > 1.5 EH LS Maximum proof test load must be at least equal to the factored load

64 Anchor Bond Length L b(min) T n Q a L b = anchor bond length T n = factored anchor load Q a = nominal anchor pullout resistance

65 Nominal Anchor Pullout Resistance Q a d a L b Q a = nominal anchor pullout capacity d = anchor hole diameter a = nominal anchor bond stress L b = anchor bond length

66 Preliminary Evaluation Only Bond stress values in AASHTO should be used for FEASIBILITY evaluation AASHTO values for cohesionless and cohesive soil and rock

67 Presumptive Nominal Bond Stress in Cohesionless Soils Anchor/Soil Type (Grout Pressure) Soil Compactness or SPT Resistance Presumptive Ultimate Bond Stress, n (ksf) Gravity Grouted Anchors (<50 psi) Sand or Sand-Gravel Mixtures Medium Dense to Dense to 2.9 Pressure Grouted Anchors (50 to 400 psi) Fine to Medium Sand Medium to Coarse Sand w/gravel Silty Sands Sandy Gravel Glacial Till Medium Dense to Dense Medium Dense Dense to Very Dense Medium Dense to Dense Dense to Very Dense Dense to to to to to to to 11

68 Resistance Factors Anchor Pullout Cohesionless (Granular) Soils 0.65 (1) Cohesive Soils 0.70 (1) Rock 0.50 (1) Where Proof Tests Preformed 1.00 (2) 1) Using presumptive values for preliminary design only 2) Where proof tests conducted to at least 1.0 times the factored anchor load

69 LRFD/ASD 1.1 Comparison to ASD Anchor Pullout Rock (FS = 3.0, = 0.50) Sand (FS = 2.5, = 0.65) Clay (FS = 2.5, = 0.70) (ASD) L b(min) EH LS 1 FS Dead Load / Live Load L b(min) (LRFD) EH LS

70 Final Anchor Design Section Anchor Pullout Capacity For final design, the contract documents shall require that verification tests or pullout tests on sacrificial anchors in each soil unit be conducted Different than current ASD practice, but intent is not to require, in general, pullout testing

71 Bearing Resistance of Wall Element Assume all vertical loads carried by portion of wall below excavation level Code refers designer to section on spread or deep foundations for analysis methods Resistance factors used are for static capacity evaluation of piles or shafts (i.e., = 0.3 to 0.5 FS ~ 3.0 to 4.5) Resistance factors should be modified to correlate to FS = 2.0 to 2.5 for bearing resistance evaluation

72 MSE Walls Strength Limit States Same external stability checks as for conventional gravity walls Tensile resistance of reinforcement Pullout resistance of reinforcement Structural resistance of face elements and face element connection Service Limits States Same as for conventional gravity walls

73 MSE Walls External Stability

74 MSE Walls Internal Stability Check pullout and tensile resistance at each reinforcement level and compare to maximum factored load, T max

75 Maximum Factored Load Apply factored load to the reinforcements T σ max H = factored horizontal soil stress at reinforcement (ksf) H S S v = vertical spacing of reinforcement v AASHTO

76 AASHTO Factored Horizontal Stresses Factored Horizontal Stress σ H P σ P = load factor (=1.35 for EV) k r = pressure coefficient V k V = pressure due to resultant of gravity forces from soil self weight D H = horizontal stress r Δσ H

77 Reinforcement Tensile Resistance T max T al R c T al = Nominal long-term reinforcement design strength = Resistance factor for tensile resistance AASHTO

78 Resistance Factors for Tensile Resistance Metallic Reinforcement Strip Reinforcement Static loading Combined static/earthquake loading Grid Reinforcement Static loading Combined static/earthquake loading Geosynthetic Reinforcement Static loading Combined static/earthquake loading

79 ASD/LRFD Tensile Breakage Example of Steel Strip Reinforcement T max = h S v ASD T max = p h S v LRFD T max = ( v k r + D h ) S v T al = (0.55 F y A c ) / b T al / T max = 0.55 / 1 = 0.55 T max = 1.35 ( v k r + D h ) S v T al = ( F y A c ) / b with = 0.75 T al / T max = 0.75 / 1.35 = 0.55

80 Other Developments LRFD for Soil Nails NCHRP Draft LRFD Design and Construction Specification for Micropiles

81 ? The End